Articles of Concrete

High Performance Concrete Admixtures

High Performance Concrete Admixtures for Improving the Properties of Concrete

Pramod Pathak, Director, Multichem Group, Mumbai.

Admixtures are the ingredients in concrete which are other than the hydraulic cementitious material, water, aggregates or fiber reinforcement that are used as ingredients of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that are added to the batch before or during mixing. Admixtures are usually further defined as a non–pozzolanic (does not require calcium hydroxide to react) admixture in the form of a liquid, suspension or water-soluble solid. Some admixtures have been in use for a very long time, such as calcium chloride to provide a cold-weather setting concrete. Others are more recent and represent an area of expanding possibilities for increased performance. Not all admixtures are economical to employ on a particular project.
High Performance Concrete Admixtures for Improving the Properties of Concrete
Also, some characteristics of concrete, such as low absorption, can be achieved simply by consistently adhering to high quality concreting practices.

Water-reducing admixtures improve concrete’s plastic (wet) and hardened properties, while set-controlling admixtures are used in concrete being placed and finished in other than optimum temperatures. Both, when used appropriately, contribute to good concreting practices. Also, both admixtures should meet the requirements of ASTM C 494, (Table 1).

Water-Reducing Admixtures

High Performance Concrete Admixtures for Improving the Properties of Concrete
Water reducers decrease the amount of mixing water required to obtain a given slump. This can result in a reduction of the watercementitious ratio (w/c ratio), which leads to increased strengths and more durable concrete.

Reducing the w/c ratio of concrete has been identified as the most important factor to make durable, high-quality concrete. On the other hand, sometimes the cement content may be lowered while maintaining the original w/c ratio to reduce costs or the heat of hydration for mass concrete pours.

Water-reducing admixtures also reduce segregation and improve the flow ability of the concrete. Therefore, they are commonly used for concrete pumping applications as well.

Water-reducing admixtures typically fall into three groups: low-, medium- and high-range. These groups are based on the range of water reduction for the admixture. The percent of water reduction is relative to the original mix water required to obtain a given slump (Table 2). While all water reducers have similarities, each has an appropriate application for which it is best suited. Table 3 presents a summary of the three types of water-reducing admixtures, their ranges of water reduction and their primary uses. Their effect on air entrainment will vary depending on the chemistry.

How They Work?

When cement comes in contact with water, dissimilar electrical charges at the surface of the cement particles attract one another, which results in flocculation or grouping of the particles. A good portion of the water is absorbed in this process, thereby leading to a cohesive mix and reduced slump.

Water-reducing admixtures essentially neutralize surface charges on solid particles and cause all surfaces to carry like charges. Since particles with like charges repel each other, they reduce locculation of the cement particles and allow for better dispersion. They also reduce the viscosity of the paste, resulting in a greater slump.

High Performance Concrete Admixtures for Improving the Properties of Concrete
Table 4 presents some of the most common basic materials used for each range of water reducer. Other components are also added depending on the requirement of additional properties of concrete. Some water-reducing admixtures have secondary effects or are combined with retarders or accelerators. This will be discussed later.

Effects on Concrete

Water-reducing admixtures are primarily used to reduce the water-cementitious content of concrete, thus increasing strength. In some cases, they can be used to increase the workability or slump of the concrete providing for easier placement. Mid-range water-reducing admixtures were developed to increase the slump beyond the range available with regular water reducers without the excessive retardation that had been known to occur. High-range water reducers, commonly called superplasticizers, were developed for high-strength and high performance concrete applications.

Superplasticizers, e.g., Multiplast Super can take a 3- inch slump concrete to a 9-inch slump without risk of segregation and without compromising its strength. Many precasters can benefit from the use of a superplasticizer, especially because of its improved high early strength development.

All water-reducing admixtures increase strength development as a result of better dispersion of the cement. This increases the exposed surface area of the cement particles, allowing for more complete hydration of the cement.

Set-Controlling Admixtures

Set-controlling admixtures alter the rate of the cement’s hydration and, therefore, the rate of setting (stiffening) of the paste. Coincidentally, they also may affect the hardening or strength gain after the paste has set. Setcontrolling admixtures include retarding and accelerating admixtures.

Retarding Admixtures

These admixtures, Multiplast R slow down the hydration process. They may also reduce the setting time of cement. Retarding admixtures fall into two categories: regular and extended-set. Regular, most commonly referred to as just “retarders,” are used to place concrete in hot climates when long travel times are expected or, in case of emergency, when placement is delayed. They are also commonly used for mass concrete pours to prevent cold joints.

Extended-set control admixtures are those used to delay hydration for many hours or even days. These are usually a twocomponent admixture system. The first component is a retarder (stabilizer) which delays the setting of concrete. The second component is an accelerator (activator) which overcomes the retarder. The concrete typically reaches initial set in a few hours after the activator is applied.

How they work Retarders essentially slow early hydration by reducing the rate at which tricalcium silicate (C3S) reacts with water. Furthermore, retarders slow the growth of calcium hydroxide crystals. Both reactions develop the early setting and strength gain characteristics of paste. The effect remains until the admixture is incorporated into the hydrated material, thereby removing it from the solution and allowing for initial set to occur. The duration of retardation is based on the dose and chemistry of the retarder, cement composition, temperature and the time it was added to the mix.

Accelerators

These admixtures increase the cement’s rate of hydration. Multiplast ACC are designed to increase the rate of hydration of C3S, thereby increasing early strength. There are two types of accelerators: rapid and normal.

Rapid accelerators can set concrete in minutes and are used in shotcreting applications, to make repairs against hydrostatic pressure or when very rapid setting is required. These are typically not used in precast concrete applications.

Standard or normal accelerators are used to speed up construction in cold-weather concreting conditions; however, it is important to note that they are not antifreezing admixtures.

Effect on concrete: Both retarders and accelerators seem to have negligible effects on air entrainment. However, when water-reducing agents are included, such as lignosulfonates, some air may be entrained.

Retarders tend to reduce one-day strengths and usually increase later-age strengths . Retarders may also increase slump loss and cause an early stiffening of the mixture, even though the strength gain has been delayed. Retarders tend to lose their effectiveness as concrete temperature increases. They also tend to increase the plastic shrinkage.

Accelerators typically increase early strengths. However, laterage strengths may be reduced relative to the same concrete without the accelerator. They also tend to increase early-age shrinkage and creep rates, but tests have shown that ultimate values seem to be unaffected.

Combinations

Some admixture chemistries provide for a combination of effects such as water reduction with retardation or acceleration. Advantages of this include reducing the number of admixtures that have to be stored and added to the concrete; less admixture incompatibility; and cost savings. Disadvantages include less flexibility and limited use when an accelerating or retarding effect is not desired. ASTM C 494 lists specifications for these combination admixtures.

NBMCW Febuary 2008

.....

RMC—A Revolution in Production of Con...

RMC—A Revolution in Production of Concrete

Dr. Y.P. Gupta, Material Consultant, Allahabad Bypass Project and Professor of Civil Engineering (Rtd.), MNNIT, Allahabad.

Introduction

Ready mixed concrete, by far the most common form of concrete, accounts for more than half of all concrete consumption. Ready mixed refers to concrete that is batched for delivery from a central mixing plant instead of being mixed on the job site. Each batch of ready-mixed concrete is tailormade according to the specifications of the contractor or concrete mix design and is delivered to the site in green or plastic condition, usually in the cylindrical trucks often known as “Transit mixers.”

Concrete constituents occupy a large space for storage at construction site. Further, the builder has to spend a lot of time and effort to source these materials and test their quality before use. Ready Mixed Concrete (RMC) suppliers take care to collect and store all these materials and supply the required quantity of concrete at the specified time and place so that construction can proceed smoothly. Metropolitan cities are hard-pressed for storage space. Therefore, RMC greatly relieves the space problem.

The real advantage for the construction industry accrues from the quality of the concrete because of the expertise and experience of the batching plant QC Engineer. However, the quality of the structure made using RMC largely depends on close coordination between the supplier of RMC and the builder at the site at all stages starting from ordering concrete to discharging and placing of the concrete. Transit Mixers can drive directly onto the site and can mechanically control the positioning of the discharge chute without the help of contractor's personnel.

History

As early as 1909, concrete was prepared by a horse-drawn mixer that used paddles turned by the cart’s wheels to mix concrete en route to the jobsite. In 1916, Stephen Stepanian of Columbus, Ohio, developed a self-discharging motorized transit mixer that was the predecessor of the modern ready–mixed concrete truck. Development of improved readymixed concrete trucks was developed in the 1920s. During the 1940s, the availability of heavier trucks and better engines allowed mixing drum capacities to increase, which in turn allowed ready-mixed concrete producers to meet the high demand for concrete that developed as a result of World War II.

RMC–A Step Forward and Ideal for Many Jobs

Specification of RMC

The builder should specify the grade (strength) of concrete required for his structure. It is also necessary to specify the minimum cement content and maximum permissible water-cement ratio and the workability in terms of slump value. This will ensure that concrete will have required strength on attaining maturity, workable at the time of placing and will be durable. For special jobs, the type of cement or admixture to be used should also be specified.

Types of RMC

RMC can be classified according to ingredients mixed in concrete. These may be on the basis of Cementitous Material i.e. Flyash is a part of Cement or not and Admixture is used or not. Otherwise, there are two principal categories of ready mixed concrete.
  1. Dry Concrete: All the ingredients are mixed in dry form without mixing water in it. All these materials are sent in rotating drum and measured water quantity is sent in separate Water container. The water is mixed at site when it reaches there.
  2. Green Concrete: All the ingredients are mixed together including the measured water quantity at Concrete Batching Plant itself. They are sent in rotating drum or in transit mixture to the site of concreting.

Code Stipulation

The most important parameter is the time that gets elapsed from the instance of adding water to the placement of concrete. Normally, the concrete has to be placed in about 90–120 minutes or before the rotating drum of transit mixer has made about 300 revolutions. Indian Standard 4926:2003 permits concrete to be discharged from the truck mixer within 120 minutes after loading. It also permits a longer period if suitable retarding admixtures are used or by deliberate chilling.

Mixing Plant

RMC—A Revolution in Production of Concrete
RMC is a specialized material in which the cement aggregates and other ingredients are weigh batched at a plant Figures 1 and 2 in a central mixer or truck mixer, before delivery to the construction site in a condition ready for placing by the builder. Thus, ‘fresh’ concrete is manufactured in a plant away from the construction site and transported within the requisite journey time. The RMC supplier provides two services, firstly one of processing the materials for making fresh concrete and secondly, of transporting a product within a short time.

Sometimes materials such as water and some varieties of admixtures can be transit–mixed (also known as Transit Mixture), that is they can be added to the concrete at the jobsite after it has been batched to ensure that the specified properties are attained before placement. Here materials are batched at a central plant and are completely mixed in the Batching Plant or partially mixed in transit. Transit–mixing keeps the water separate from the cement and aggregates and allows the concrete to be mixed immediately before placement at the construction site (Dry Concrete). This method avoids the problems of premature hardening and slump loss that result from potential delays in transportation or placement of central–mixed concrete. Additionally, transit- mixing allows concrete to be hauled to construction sites further away from the plant. There are several types of RMC plants varying in type of mixing and capacity of concrete production. These plants are generally available in capacities varying from 15 /hour to 200 / hour. A typical RMC plant is shown here.

RMC—A Revolution in Production of Concrete

The Truck Mixer

While ready mixed concrete can be delivered to the point of placement in a variety of ways, the overwhelming majority of it is brought to the construction site in truck–mounted, rotating drum mixers Figure 3. Truck mixers have a revolving drum with the axis inclined to the horizontal. Inside the shell of the mixer drum are pair of blades or fins that wrap in a helical (spiral) configuration from the head to the opening of the drum. This configuration enables the concrete to mix when the drum spins in one direction and causes it to discharge when the direction is reversed.

To load, or charge, raw materials from a transit mix plant or centrally mixed concrete into the truck, the drum must be turned very fast in the charging direction. After the concrete is loaded and mixed, it is normally hauled to the job site with the drum turning at a speed of less than 2 rpm. The maximum number of revolutions the drum may rotate before delivery is about 300.

Transportation of Concrete

Central–mixed concrete is completely mixed at the plant then transported in a truck or transit mixer or agitator truck. Freshly mixed concrete may be transported in a open dump truck if the jobsite is near the plant or very low slump is required like for pavement quality concrete used in road construction. Slight agitation of the concrete during transit prevents segregation of the materials and reduces the amount of slump loss.

Site Preparation

A fully loaded transit mixer weighs approximately 25 Tons. Hence prior checking of good access to the site of discharge of concrete from transit mixture is essential. This will avoid problems of delay on the day of concreting.

Quality Assurance

For this a sample of concrete must be taken out of Transit Mixture (as shown in Figure 4) to measure the workability by taking the slump. Samples are also taken for determining actual compressive strength of concrete. Three cubes of size 150x150x150 are made on site of this concrete from every or alternate transit mixture depending upon the total quantity of concrete ordered. Samples should be taken from different parts of the load.

Handling and Placing

Efficient use of RMC depends upon a rapid turnaround of truck mixers and proper facilities for rapid discharge and placing of concrete. With proper access and site facilities, the modern truck mixers can position it and discharge the full load in 15 to 30 minutes. They represent a potential delivery rate of nearly 30 m3 per hour. The concrete arrives with the ordered workability and hence no extra water should be added at the site. Concrete that does not arrive within the tolerance limit of ordered workability may be rejected or if permitted, it can be altered by mixing a small dose of Admixture, after judging the condition of concrete.

Ready-mixed concrete is often remixed once it arrives at the jobsite to ensure that the proper slump is obtained. However, concrete that has been remixed tends to set more rapidly than concrete mixed only once. The builder often handles the concrete with only a few manual laborers. Continuous handling methods such as mobile pump and conveyor system help in increasing the turnover. It is best to discharge the concrete from the truck mixture as close as possible to the place where it is required. Concrete can be discharged directly from the truck through chutes or it can be pumped by static or Mobile Pump as shown in Figure 5 at the construction pouring point.

Advantages of Using Ready Mixed Concrete
  • Ready Mixed Concrete can ensure quality because of the expertise and experience of RMC plant Technical Staff.
  • There is no botheration of ordering materials like Aggregate, Sand, Cement etc an find place to store them. Then arrange for site mixing machine.
  • Ready-mixed concrete is particularly advantageous when small quantities of concrete or intermittent placing of concrete are required.
  • Ready-mixed concrete is also ideal for large jobs where space Figure 4: Taking Sample for Testing is limited and there is little room for a mixing plant and aggregate stockpiles.
RMC—A Revolution in Production of Concrete

Conclusion
  • RMC is ‘Fresh’ Concrete manufactured in a plant away from the construction site and transported within the stipulated time to the site.
  • Concrete arrives with the ordered specifications. Do not add water at the site.
  • Modern Truck Mixers can discharge the full load in 15 To 30 Minutes.
  • Concrete can be discharged directly from the truck through chutes or it can be pumped by static or mobile pump at the pouring point.

NBMCW May 2008

.....

Designing Reinforced Concrete Structure...

Designing Reinforced Concrete Structures for Long Life Span

Dr. Rakesh Kumar, Scientist and Dr. Ram Kumar, HoD, Bridges and Structures Division, Central Road Research Institute, New Delhi.

Innovation in construction industry is highly linked with development of advanced construction materials. In the recent two–three decades lot of research relating to how to enhance the life of reinforced concrete structures has been carried out. As a result of which—it has been possible to design structures having service life span of more than 100 years. This article discusses some aspects of possibility for designing reinforced concrete structures for a very long life.

Introduction

Designing Reinforced Concrete Structures for Long Life Span
Designing Reinforced Concrete Structures for Long Life Span
The presence of heavy reinforcement i.e. a high degree of congestion of reinforcement in structural elements significantly hampers concrete placement and its quality due to lack of proper compaction. Adequate compaction of such sections by proper means is essential for durability assurance and often depends on the crew’s ability to ensure it. Inadequate compaction of concrete in such structural elements can lead to surface and structural defects and inadequate bond development with the reinforcement. Durability of reinforced concrete structures is mainly dependent on the quality of the concrete, quality of reinforcing steel, cover depth of reinforcement, compaction and curing of concrete and finally quality management of the construction practices. Notably, the serviceability and the safety of concrete structures have been the prime concern of the structural engineers. The serviceability limit of concrete structures is primarily governed by the extent of damage resulting from daily service loads and various deterioration processes, which might be active throughout the structure’s life. Durability problems in concrete structures may be due to several causes such as errors in design or carelessness in detailing, use of inferior construction materials, inadequate quality control, poor workmanship, heterogeneity of the materials etc. Durability affecting features of concrete structures are observed in the form of cracking, spalling (Figure. 1), corrosion of reinforcing steel bars (Figure. 2), loss of mass (Figrue. 3) and loss of strength. The cause of concrete deterioration can be physical, chemical and in most cases, a combination of both. The net effect of concrete deterioration processes is to weaken the integrity of the complex microstructure of concrete. The low porosity and dense microstructure of concrete significantly reduce many sources of its deterioration. In concrete, cement paste is the primary active constituent. Therefore, the mechanical properties and performance of concrete is largely determined by the properties of the cement paste. Microstructure characteristics of concrete such as its porosity, pore size distribution, properties of transition zone, and connectivity of pores, govern almost all the gas and liquid transport phenomena through the concrete [1-5]. Therefore, the rate at which a concrete structure may deteriorate is mainly depend on the permeability of the concrete as well as how the concrete is placed, compacted, cured, and allowed to sustain load, cover depth and quality of cover concrete. Contact with, or the presence of certain aggressive chemical ions, such as chlorides, sulphides, acids, carbon dioxide, and even water, causes the deterioration of the concrete. Such deterioration involves either leaching of material from the surface by a dissolution mechanism or by expansion of material inside the concrete. Exposure conditions vary over a wide range including hot and dry desert ambient air, wind, and rain or snow. Higher ambient air temperature may accelerate the chemical reaction of concrete leading to faster deterioration. Furthermore, the concrete quality degradation mechanism may be either a physical effect such as shrinkage, creep, erosion, and similar factors, or a chemical reaction such as sulphate attack, reinforcement corrosion, alkali-silica reaction, carbonation, freezing and thawing, and other similar factors. Designer should throughly understand the interaction of concrete with both exposure environment and service loads.

The broad categories of factors, which determine the durability of a concrete structure, are design, material properties, and construction practice. Errors in design or carelessness in detailing may lead to cracking, leading to premature demise of useful life of a concrete structure. Long-term durability of concrete in civil infrastructures such as road and bridges can be achieved if the construction materials quality, structural detailing and dimensioning, and concreting works are appropriately performed. It is well recognized that the quality of concrete in structures and defects induced at early age due to various reasons are main factors for the long-term durability of concrete. These deterioration processes can be physical, chemical or mechanical or combination of them.

Among various foresaid factors, cracking due to shrinkage, poor workmanship, environmental factors, and over load/overstress initiate the process to reduce concrete durability. Such concrete cracking which cannot be eliminated but can be minimized provides path for the ingress of water/moisture, air to allow reinforcement corrosion to start. Therefore, there is a need for quality management for concrete placement, compaction, and curing. Also reinforcement should be such that it has “sufficient” cover depth protecting the reinforcing bars from deeper and wider cracks; and/or, reinforcement which does not corrode or would corrode only to predetermined minimum amount. Innovation in construction is highly linked with development of advance construction materials and technology. There are materials and technology available to ensure construction of long-life structures.

Mechanism for Enhancing Durability

The fundamental fact that properties of material originate from its internal structure is also valid for concrete as well as steel. The principle of modifying internal structure suitably has been used in developing a number of metals, composites, and other materials [6]. Improvement of durability of concrete has remained an active research area for concrete technologist for many years. As a result of continuous effort for enhancing durability of concrete structures, high-performance concrete (HPC) and selfcompacting concrete (SCC) have been developed. Improved properties of high-performance concrete are due to the modification of its microstructure. The modification is significantly dependent on the reaction mechanism among the ingredients of concrete, physical process, and curing. Chemical and mineral admixtures augment the reaction mechanism. In high-performance concrete, commonly used admixtures are silica fume [7, 8] and fly ash [9-11]. These materials improve the microstructure of concrete by pozzolanic action as well as a filler effect. Better performance of high-performance concrete is primarily due to refinement of the pore structure of the concrete particularly at the transition zone [7, 11]. Even the proven technology of high-performance concrete can enable the structures to double its useful lifespan in comparison with engineered structures constructed with conventional concrete technology [12].

A water-to-cement ratio (w/c) of 0.4 by mass is required for complete hydration of all the cement particles and for hydration products to fill all the space originally occupied by the mixing water [12]. If the w/c is higher than 0.4 by mass, even if all the cement particles hydrate, there will always be some residual original mixing water-filled spaces that can hold freezable water. If w/c is lower than 0.4 by mass, some of the cement will always remain unhydrated; but, in theory, all of the mixing water-filled spaces could be filled. However, the amount of water that goes into chemical combination with Portland cement is equal to about w/c of 0.2 by mass. The additional Amount of water, i.e., 0.2 w/c by mass [12] is needed to fill gel pores. This extra water must be available if the hydration product is to be formed. On the other hand, the development of superplasticizers has revolutionized technology and has made it possible to make workable and/or very workable concrete with very low water-to-cementitious ratio even less than 0.2 [13-15]. Such concrete not only achieve highstrength but also possess improved durability.

The use of some mineral admixtures, such as coal fly ashes and other pozzolans, work as a filler in addition to contributing pozzolanic activity and fill the spaces occupied by water in capillary pores and make them discontinuous. As a consequence of this, the morphology of hydrated cement changes which favorably affect most of the mechanical properties of concrete in comparison with conventional concrete [4, 7, 10, 16].

Highly Durable Concrete Structures

A greater understanding of concrete behavior at microstructure level and performance under different aggressive conditions has improved the confidence of concrete technologists to think about highly durable concrete lasting for 1000 years. Recently some efforts have been made for designing highly specialized structures, such as bridges, tunnels, and tall structures, for a lifespan of a century or more [17- 19]. Most recently, Mehta and Langley [20] designed an unreinforced, monolith concrete foundation consisting of two parallel slabs, to last for 1000 years. They used high-volume Class F fly ash concrete in the construction of the foundation. The slabs were built with HVFA concrete mixture containing 240 lb/ yd3 of Class F fly ash and 180 lb/ yd3 of portland cement. The petrographic examination of oneyear- old test slab, that was cast and cured under the similar conditions, has shown crack-free nature of the HVFA concrete [21].

At present, this seems to be achievable for concrete without reinforcement to predict/speculate on a 1000-year life. In-depth understanding of microstructural behavior of concrete, and possibility for improvement of it, to overcome shortcomings that cause reduction in durability of concrete, by the use of chemical and mineral admixtures, has given the basis to concrete technologist to think for design of highly durable concrete structures that should last for several centuries. For such structures the following items should be clearly understood and implemented.
  • Quality management of material, methods, and testing.
  • Manage all design and construction aspects to ensure the structural integrity.
  • Designer should have adequate knowledge of material properties such as strength, creep, shrinkage, etc., of concrete and their affect on cracking of the concrete.
  • Design adequate depth of cover for the reinforcing steel.
  • Use of fly ash and/or other pozzolonic materials instead of ordinary portland cement only.
  • Use of high-quality aggregates free from deleterious compounds for preventing alkali-aggregate reactivity, and similar actions. Aggregates should also have proven reliability.
  • Concrete, from its proportions, mixing, methods of construction, (compacting and curing), should be given careful attention so that an adequately dense concrete, with full compaction and a desirable pore system may be ensured.
  • Adequate cover for the reinforcement ensuring highquality compaction and curing of the concrete. High-performance & self-compacting concrete may help in minimizing the potential of corrosion of reinforcement and deterioration of concrete due to poor quality of cover.
  • Corrosion resistant steel, steel coated with corrosion resistance layer such as cementitious material slurry, stainless steel, or other types of newer steel, may be used.
  • Concrete should be carefully tested and quality managed to meet long-term tests such as water and air permeability, shrinkage, creep, freezing and thawing, chloride-ion penetration by ponding and chloride diffusivity.
  • Prediction of life of structures based on corrosion rate of reinforcement.

Conclusion

The possibility for design of reinforced concrete structures for a very long lifespan of several years exist without a proven method (by calculation or experiments). The improved microstructure of concrete by judicious use of mineral admixtures, such as flyash, silica fume, and other pozzolans, as well as new generation of chemical admixtures, have given hope for the RC structures for life span of more than 100 years. Concrete structures for a very long lifespan need materials of high-quality and also comprehensive knowledge about concrete properties and their effects on design aspects of the structure, and a new generation of steel reinforcement.

Acknowledgment

The approval of Dr Vikram Kumar, Director, Central Road Research Institute, Mathura Road, New Delhi to publish the work is acknowledged.

References
  • Neville, A. M. and Brooks, J. J. (1990). “Concrete technology.” ELBS Edition, Logman Singapore, Publishers (Pte) Ltd.
  • Soroka, I. (1979). “Portland Cement Paste and Concrete.” McMillan Press limited, London, UK.
  • Brandt, A. M. (1995). “Cementbased composites: Materials, Mechanical Properties, and Performance.” E & FN SPON, U. K.
  • Mehta, P. K. (1996). “Concrete: structure properties and materials.” Prentice~Hal, Inc., Englewood Cliffs, New Jersey.
  • Kumar, R. (1997). “Strength and permeation quality of concrete through mercury intrusion porosimetry.” Ph.D. thesis, Department of Civil Engineering, Indian Institute of Technology Delhi, New Delhi, India.
  • Shackelford, J. F. (1992). “Introduction to material science for engineers.” 3rd Edition, Maxwell Macmillan International addition, London, UK
  • Mehta, P. K., and Aitcin, P. C. (1990). “Principles underlying production of high-performance concrete.” ASTM Cement, Concrete, and Aggregates, 12(2), Winter 1990, pp. 70-78.
  • Malhotra, V. M. and Ramezanianpour, A. A. (1994). “ Fly Ash in Concrete,” 2nd Edition, ,CANMET, Ottawa, Canada.
  • Naik, T. R., Singh, S. S., and Mohammad M. (1995). “Properties of high performance concrete incorporating large amounts of high-lime flyash,” International Journal of Construction and Building Materials, l 9(6), 195-204, Butterworth-Heineman, England.
  • Wesche K. (1991). “Fly Ash in Concrete’s Properties and Performance.” Report of Technical Committee 67-FAB, use of flyash in building, E & FN SPON, Chapman & Hall, U.K.
  • Naik, T. R. (1997). “Concrete Durability as influenced by density and/or porosity.” Proceedings of the Cement and Concrete Institute of Mexico Symposium, World of Concrete – Mexico, Guadalajara, Mexico, June 4-7, 1997.
  • Mather, B., and Hime, W. G. (2002). “Amount of water required for complete hydration of Portland cement.” ACI Concrete International, 24(6), 56-58.
  • Feylessoufi, A., Villiéras., F., Michot, L. J., De Donato, P., Cases, J. M., and Richard, P. (1996). “Water, environmental, and nano-structural network in a reactive powder concrete.” Cement and Concrete Composite, 18(1), 23-29.
  • Richard, P., and Cheyrezy, M. (1999). “Composition of reactive powder concrete.” Cement and Concrete Research, 25(7), 1501- 1511.
  • Khayat, K. H., Hu., C., and Laye, J. M. (2002). “Importance of aggregate packing density on workability of self-compacting concrete,” Proceedings, First North American Conference on the Design and Use of Self- Consolidating Concrete, Center for Advanced Cement-Based Materials, North–western University, Evanston, IL, U.S.A., November 12-13, 2002, pp. 53- 62.
  • Malhotra, V. M. (1995). “Fly Ash, Blast-Furnace-Slag, Silica Fume, and Highly Reactive Metakaolin,” Proceedings, Seminar On Recent Advances in Concrete Technology, UWM Center for By-Products Utilization, University of Wisconsin-Milwaukee USA, Proceedings Complied by Tarun R. Naik and Henry J. Kolbeck.
  • Dunaszegi, L. (1998). “Highperformance concrete in the confederation bridges.” ACI Concrete International 20(4), 43- 48.
  • Holley, J. J., Thomas, M. D. A., Hopkins, D. S., Cail, K. M., and Lanctot, M.–c. (1999). “Custom HPC mixtures for Challenging bridge design.” Concrete International 21(9), 43-48.
  • Langley, W. S., Gilmour, R. A., Turnham, J. Forbes, G., and Mostert, T. (1997). “Quality management plan for the confederation bridge.” Proceedings, Third CANMET/ACI International Symposium on Advances in Concrete Technology, Auckland, New Zealand, August 24-27, 1997, ACI Special Publication SP-171, pp. 73-96, American Concrete Institute, Formington Hills, Michigan, Ed. V. M. Malhotra.
  • Mehta, P. K., and Langley, W. S. (2000). “Monolith foundation: Built to last 1000 years.” ACI Concrete International, 22(7), 27- 32.
  • Asselanis, J., and Mehta, P. K. (2001). “Microstructure of concrete from a crack-free structure designed to last a thousand years.” Proceedings, Third CANMET/ACI International Symposium on Sustainable Development of Cement and Concrete, San Francisco, U.S.A., September 16-19, 2001, ACI Special Publication SP-202, pp. 349-358, American Concrete Institute, Formington Hills, Michigan, Ed. V. M. Malhotra.

NBMCW December 2007

.....

Integral Watertight Concrete Structures...

Integral Watertight Concrete Structures- An Insight

Upen Patel, Marketing Manager, BASF Construction Chemicals (India) Pvt. Ltd. Mumbai

Preface

Since the Stone Age mankind has struggled to keep the structures watertight, Even today’s struggle is on to deploy one or other kind of waterproofing system to achieve total water tightness of structures. In spite of good construction practices at site and using branded products, engineers struggle to keep structures watertight. So far the approach has been based on achieving watertight tanking around the structure to guard entry of water in the structure; less attention and focus has been on getting integral water tightness (within the structure). This article explains the main sources of leakages in the structure, and using two technological advancements to enable integral water tightness to structure with a live case study.

Sources of Leakage

There are three main sources of water leakage in concrete structures:
  • Construction Joints
  • Cracks
  • Porous media

Construction Joints

Large structures of concrete are cast in number of sections. The dividing lines between two sections are the joints between already harden concrete and freshly poured concrete, with continuity of reinforcements. At these joints fresh concrete shrinks and creates a fine crack. These cracks are normally 0.1 – 0. 3 mm in width and are passages for water to pass through the structure.

Cracks

Integral Watertight Concrete Structures- An Insight
Due to various reasons such as excessive segregation of concrete mix, high water cement ratios, movements, settlements, rapid variations in ambient temperatures, early de-stripping, etc… concrete structures develop cracks during the construction stage and these cracks are some time deeper and can easily transport water from one side to another.

Porous Concrete Media

Concrete has heterogeneous matrix, a mixture of binding paste and fillers. While mixing the ingredients and placement of fresh concrete, concrete entraps air and the same is attempted to remove by compaction using mechanical means. Achieving uniform compacting throughout the volume of freshly placed concrete is very difficult to achieve. In adequate compaction results in to the air voids. Theoretically, concrete requires only 23 – 25% water by weight of cement for the chemical hydration of the cement. While actual concrete in practice contains 40 – 50% of water by the weight of cement. The extra water is provided to achieve desired workability and easy of placement. This extra water leaves the concrete mass during the hydration reaction resulting in to the formation of pores. An interconnected series of such pores are popularly known as capillary pores. These capillaries make concrete porous. Besides these two features concrete also contains hydration pores which are formed due to volume changes of hydration products and are normally filled with loose lime, one of end product of the hydration reaction. These hydration pores promotes diffusion of corrosive agents in the concrete.

Integral Watertight Concrete Structures- An Insight

The Integral Watertight Solution

To check the porosity of concrete and leakages through the joints & cracks, integral watertight concept is gaining popularity. The concept is a combination of two systems:
  • Watertight joints using reinjectable hoses
  • Watertight concrete mass by deploying self–compacting concrete concept

Re-injectable Hoses

Integral Watertight Concrete Structures- An Insight
The re-injectable hoses are made up of PVC plastic core which enables toughness to the hose. The core has injection channel in the centre, which connects to openings at regular distance in all four directions. The openings are guarded with neoprene seals.
Integral Watertight Concrete Structures- An Insight
The hose is placed at the central line of the construction joints using clips and the ends are connected to nonperforated hose with termination in near-surface mounted junction boxes. After the casting and destripping of the concrete cover of the junction box is located and marked for future operations. Each length of the hose is first injected with water to assess the leakages at the construction joint. Then injected using water–based lowviscosity, re-swellable, vinylacrylate injection resin. The injection resin pressurized the soft neoprene seals and squeeze out around seals to the openings and travels around the hose and to the crack of the construction joints and other cracks which connects to the construction joints. In the next stage of operation, injection pressure is release and the hose is applied with the vacuum. The neoprene seals now gains original size and seals the openings and prevents the suction of resin from outside of the hose to inside. Also all the resin from the central channel is sucked out and then the hose is rinse using water under low pressure re-circulation stage. Now the resin in the cracks has set and forms an effective seal for passage of water in the future. The hose can be injected with water to verify the effectiveness of the injection. If leakages are noticed then the hose is re-injected with the resin once again. Overall the re-swellable acrylate resin and injection hose provides following main benefits:
  • Re-injectable hose – permanent access to the construction joint
  • In-build QA system–Test the effectiveness by injecting with water
  • Re-swelling injection resin– swells up to 2.5 time in volume to maintain tight seal even in the case of movements in the cracks

Watertight Self– compacting Concrete

To obtain proper, robust self– compacting concrete, it is important to include all of following components in the mix. These components enhance the performance of the fresh concrete as mentioned below:
  • Hyperplasticiser–which is based on PCE polymer and have 30 – 40% water reduction capabilities
  • Viscosity Modifying Agent – Improve the shear resistance and thickens the paste to achieve effective segregation resistance
  • Pozzolans – Facilitate increase in the paste volume without increasing the temperature of concrete and enables segregation resistance.
Integral Watertight Concrete Structures- An Insight
Hence, to achieve a robust mix of self–compacting concrete, it is must that all of these three ingredients are present and are properly included to maximize the benefits they can offer on the hardened properties of the mix.

From the water tightness and durability aspects—these three ingredients enable the following benefits:

Overall by carefully implementing a proper self– compacting mix, achieving watertight concrete mass is possible. Also in the case of large projects, developed mix can be tested for permeability to standards such as DIN 1048 and that can be one of the acceptance criteria. While in the case of smaller projects such special self– compacting mix can be supplied by Ready mix producers who can design and control the ingredients.

Following case history of Hercules Harbor in Monaco enables us an insight in one of such successful implementation of this integral watertight concrete concept.

Case History– Hercules Harbor

Client–Gouvernement of Monaco Location of site–Algeciras Spain Contractor–DRAGADOS BEC V Engineering–DORIS Engineering France R & D–Institute Francais du Petriole Norwegian Technical Institute and many others Technology Supplier–BASF CC Spain (BETTOR MBT)

New studies were made in the 1980‘s to protect and extend the existing harbor. The depth of the sea–bed of 55 meters did not allow conventional construction. Further the Government decided to minimise the impact for the environment during construction. Based upon studies made in France and Norway the Monaco Government decided to build a prefabricated “ semi floating seawal l” a technique common in the offshore oil industry.

Integral Watertight Concrete Structures- An Insight

The structure was 352 metres long, 44 m wide and about 35 m tall; its design required 2,900 MT of steel cables for stressing and 45,000 of concrete!

As the structure cannot be constructed in-situ and also near by area was not available for precasting yard, Engineers from DORIS Engineering decided to construct in a dry-dock, 1200 km away in Spain. As such long structure cannot be transported on the ship; it required to be floated in sea and to be transported by towing. This required total watertightness of the structure and all the joints it has.

Any leakage at joint or within the concrete mass would make it sunk below the ocean.

Based upon the construction of TROLLE, an off shore platform built in Norway, DORIS Engineering recommended the use of Self–compacting Concrete. To secure the construction joints, Masterflex 900, the re-injectable hose was specified allowing testing joints and injecting and re-injecting with resin where necessary.

After the construction all the joints were injected using Masterflex 801, water based reswelling resin and tested for watertightness.

No form of external waterproofing treatment was carried out for this structure. No membrane or no accidental drill and grouting were implemented.

Finally, the structure was towed in the sea as it can be carried on the ship and was positioned at the final location in 2004.

Conclusion

Integral Watertight Concrete Structures- An Insight
The new age technologies and quest of civil engineers have lead to solutions which are long lasting. Such implementations make these technologies time tested and real for the rest of the world to get inspired and to implement.


NBMCW September 2007

.....

Effect of Usage of Admixture in Concret...

Mr. Hasan Rizvi, Asst. General Manager–Business Development, CICO Technologies Limited, New Delhi

Concrete consists of cement, sand, aggregate and water. Anything other than these if added in concrete either before or during mixing to alter the properties to our desired requirement are termed as admixtures. The use of admixtures offers certain beneficial effects to concrete like improved workability, acceleration or retardation of setting time, reduce water cement ratio, and so on.

There are two basic types of admixtures available: chemical & mineral. Admixtures like flyash, silicate fume, slag comes in the category of mineral admixtures. They are added to concrete to enhance the workability, improve resistance to thermal cracking and alkali–aggregate reaction and to enable reduction in cement content.

Flyash is fine residue left after combustion of ground or powdered coal. They are all generally finer than cement and consist mainly of glassy–spherical particles as well as residues of hematite and magnetite, char and some crystalline phases formed during cooling. The use of flyash in concrete makes the mix economical, and improves the workability, reduces segregation, bleeding and reduced heat of hydration but also provides ecological benefits.

Silica fume, which is also known as microsilica. It is obtained as a byproduct during the production of silicon and ferrosilicon alloys. The particle size of silica fume is 100 times smaller than cement particles i.e. its fine as cigarette smoke. Its a highly effective pozzolanic material, which improves the properties of concrete such as improved compressive strength, bond strength, abrasion resistance, dense concrete that results in protection of reinforcement against corrosion.

Chemical admixtures are added to concrete in very small amounts mainly for air entrainment, reduction of water or cement content, plasticizing of fresh concrete mixtures or to control the setting time of concrete. These admixtures can be broadly catagorised as superplasticizers, accelerators, retarders, water reducers and air entraining admixtures.

Superplasticizers are added to reduce the water requirement by 15 to 20% without affecting the workability leading to a high strength and dense concrete. Superplasticizers are liner polymers containing sulfonic acid groups attached to the polymer at regular intervals. The commercial formulation can be sulfonated melamine–formaldehyde conden- sates, sulfonated naphthalene formaldehyde condensates, and modified lignosulfonates, polycar- boxylate derivatives. The main purpose of superplasticizers is to produce a flowing concrete with very high slump 175 to 200 mm which can be used effectively in densely reinforced structures, the increased slump of concrete depends upon dosage, type & time of super– plasticizers (it's better to add it before concrete is placed.), water cement ratio, nature and amount of cement.

Accelerators are added to reduce the setting time of concrete thus helping early removal of forms and are also used in cold weather concreting. Calcium chloride is the most commonly used accelerator for concreting. The use of calcium chloride in reinforced concrete can promote corrosion activity of steel reinforcement. As people are getting aware so there is a growing interest in using chloride free accelerator.

Retarders are added to increase the setting time by slowing down the hydration of cement. They are preferred in places of high temperature concreting. Retarders consist of organic & inorganic agents. Organic retarders include unrefined calcium, sodium & ammonia salts lignosulfonic acids, hydrocarboxylic acids & carbohydrates. Inorganic retardants include oxides of lead, zinc, phosphate and magnesium salts. Most retarders also act as water reducers. They are called water-reducing retarders. Thus resulting in greater compressive strength due to low water cement ratio.

Water reducing admixtures are added to concrete to achieve certain workability (slump) at low water cement ratio. A concrete with specified strength at lower cement content thus saving on the cement. Water reducers are mostly used in hot weather concreting and to aid pumping. Water reducer plasticizers are hygroscopic powder, which can entrain air into concrete.

Air entraining admixtures entrain small air bubbles in concrete. These air bubbles act as rollers thus improving the workability and are also very effective in freeze-thaw cycles as they provide a cushioning effect on the expanding water in the concreting in cold climate.

Air entraining admixtures are compatible with most admixtures, care should be taken to prevent them from coming in contact during mixing.

Generally, the effectiveness of both the types of plasticisers are dependent on the ambient temperature condition and thus in summer the amount of plasticiser to be used to cater for the same degree of increase in plasticity can be more than the quantity to be used in winter.

Change in normal setting time within some fixed requirement also makes the production dependent on others chemicals and as such plasticisers with different nomenclatures are available in the market.

CICO Technologies Limited, an Indian ISO 9001: 2000 Company with 75 years backing produces a range of plasticising admixtures for concrete.

A number of RMC companies are using CICO admixtures. some modifications are required at the time of trials. modification in the Plasticisers can fulfill the requirements of any particular client.

Conclusion

It can be seen that proper use of admixtures offers certain beneficial effects to concrete including improved quality, acceleration or retardation of setting time, enhanced frost & sulphate resistance improves workability.

NBMCW December 2008

.....

Self Compacting Concrete for Rafts and ...

Self-Compacting Concrete

Raajesh Ladhad, Consultant, Structural Concepts, N.G. Joshi, Concrete Consultant, Siddappa, A. Hasbi, President & CEO, Corniche India Pvt Ltd, Mumbai

Self–compacting concrete can be defined as a category of High Performance Concrete that hasexcellent deformability in the fresh state and high resistance to segregation and can be placed and compacted under its self– weight without applying vibration. This method of placing concrete actually started in Japan in 1998 for the walls of a large LNG tank belonging to Osaka Gas Company, where the requirement was to place concrete amidst congested reinforcement. A number of projects since then have been executed around the world with a variety of application needs and requirements.

Development of Self– Compacting Concrete

For several years, the problem of the durability of concrete structures was a major topic of interest in the concrete industry. To make durable concrete structures, sufficient compaction of concrete by skilled workers is required. However, the gradual reduction in the number of skilled workers in the developed world, led to similar reduction in the quality of construction work. One solution for the achievement of durable concrete structures independent of the quality of construction work is the employment of self-compacting concrete, which can be compacted in every corner of a formwork, purely by means of its own weight and without the need of vibrating compaction. The necessity of this type of concrete was proposed by Okamura in 1986. Studies to develop self-compacting concrete, including fundamental study on the workability of concrete, were carried out by various researchers and today one would find more than thousand technical articles that have been written on this topic.

The use of self–compacting concrete in actual structures has gradually increased. The main reasons for the employment of self-compacting concrete can be summarized as follows:
  • Shortening of construction period
  • To assure compaction in the structure: especially in confined zones where vibrating compaction is difficult.
  • To eliminate noise due to vibration: effective especially in new structures amidst densely populated areas.
That means the current condition of self-compacting concrete is a ‘Special concrete’ rather than a traditional concrete.

A typical application example of self–compacting concrete is the two anchorages of Akashi–Kaikya Bridge opened in April 1998, a suspension bridge with the longest span in the world. The volume of the cast concrete in the two anchorages amounted to 290,000 M3. A new construction system, which makes full use of the performance of self– compacting concrete, was introduced for this. The concrete was mixed at the batching plant beside the site, and was pumped out of the plant. It was transported 200 mts through pipes to the casting site, where the pipes were arranged in rows 3 to 5 mts apart.

The concrete was cast from gate valves located at 5 meter intervals along the pipes. These valves were automatically controlled so that a surface level of the cast concrete could be maintained. In the final analysis, the use of self–compacting concrete shortened the anchorage construction period by 20% from 2.5 to 2 years.

The use of self–compacting concrete in India was actually a matter of academic interest in the initial stages. This was the case owing to the higher initial cost compared to the traditional method of concreting. But as many construction companies repeatedly found themselves in situations of time constraints and placement difficulties, the method of self compacting concrete started flourishing in Indian Sub-continent too.

Inherent Problems in Rafts & Basements

Self-Compacting Concrete
One of the areas of big concern for contractors and consultants is the raft foundation and basement of the structure. The problems are multifold when it comes to those structures that are built in areas where the water table is quite high.

Such structures are plagued by leakage of water into the basements and contractors generally spend sleepless nights in waterproofing them, which sometimes could even prove to be more expensive than the concrete itself.

The possibility of leakages in a structure generally occur due to flaws and cracks that develop in concrete as a result of shrinkage, thermal gradient, construction joints etc in concrete. Though careful design and detailing, material selection is done and exposure conditions are thought of, the water tightness of the structure still depends upon the construction practice, and the skillfulness of the worker at the site. Therefore it is of prior importance that the concrete is made independent of the worker and appropriate quality control measures are maintained. One aspect of making the body of the concrete water tight is to produce High Performance concrete, where certain durability tests are done and the quality of concrete ensured. A little improvement on the HPC is Self–compacting Concrete (SCC).

SCC therefore is a concrete which flows like honey under its own weight and when put into forms of any shape it fills it completely, parllely maintaining its homogeneity. It even goes around reinforcements and needs no vibration for compaction. Its rheology is very different from all other concretes; it has a moderate viscosity and an extremely high cohesion. Sometimes, we come across foundations where the thickness of raft is of the order of 1–4mts and the huge caging (reinforcements) becomes a big challenge to place the concrete and to ensure vibration. In such cases, the best option is to go for a self– compacting concrete, which ensures better compaction without any voids and also eases the placing method, adding to the saving in construction time.

SCC for Rafts & Retaining Walls for Kesar Solitaire

Kesar Solitaire
The Kesar Solitaire is a prestigious project being developed by M/s. Kesar Group, when completed would be a landmark on the Palm Beach Road in Navi Mumbai. This commercial complex is situated just next to the creek and the water table is about 1 meter below the ground. On examining the water, it was found to be highly saline and the chloride content in water was more than 7000 ppm. Another issue at the site was the upcoming monsoons, the basement needed to be completed before the start of the monsoons, which otherwise would have flooded the site. It was therefore decided to go for an M 40 grade SCC for the triple basement structure, which was 10 meters below the ground level.

The SCC mix was designed with “Corniche SF” brand Silica Fume to take care of the chloride ion permeability of the structure due to the saline water table. An RCPT value of less than 1000 coulombs according to ASTM C 1202 and water permeability of max 50 mm according to DIN 1048 was specified, so as to ensure the durability of the raft concrete. The mix actually contained higher binder content and therefore the water demand was high.
Self-Compacting Concrete
To ensure thorough dispersion of silica fume and flyash, a PCE based admixture was used. Glenium SKY 584 is a PCE based admixture based on the concept of Total Performance Control which not only helps in dispersion of fines but also allows longer slump retention. The concrete was produced in an RMC plant (M/s. L&T Ready Mix Concrete) and it typically took more than 60 minutes to reach the site. Glenium SKY 584, helped in achieving the retention of over 60 minutes, with a slump flow of more than 600 mm at the site. In order to improve the viscosity of the concrete, Glenium Stream 2, a viscosity modifying admixture was also added to the concrete.

The retaining walls, at the periphery of the structure, poised an entirely different issue. The thickness of the wall was 300 mm and after erecting the form work it was a challenge by itself to place and vibrate the concrete. It was therefore decided to use M 40 grade SCC so that even 3 meter pour (free fall) of concrete is possible. The use of SCC in retaining walls actually helped in achieving a homogeneous concrete void of any honey combs. The strengths achieved were more than 50 MPa at 28 days as the water binder ratio was maintained at 0.33. The mix design and the strengths achieved are given here:

Conclusion

The use of SCC has been found to be useful particularly in rafts and basements where the concrete has to withstand higher water table pressure and in places where the thickness of raft is more than 1 meter. In places, where the reinforcement is dense or the concrete has to be placed is not so easily accessible areas such as retaining walls, use of SCC is a boon to civil engineers. Use of SCC has also been found to be suitable in cutting down the construction time for structures. Further, some innovative uses have been found in rehabilitation and repair works as well.

While the material cost of SCC has generally been higher than conventional concrete, benefits such as reduction of construction time, reduction of labor and ultimately the durability of the structure more than compensates the cost. Being a unique solution for placing concrete in difficult areas, the absence of vibration and noise is the other advantage.

NBMCW November 2008

.....

Reinforced Concrete:High-Strength Steel...

Durability Characteristics of High-Strength Steel Fiber Reinforced Concrete

Durability Characteristics of High-Strength Steel Fiber Reinforced Concrete

Ramadoss, P, Research Scholar and Nagamani, K. Professor, Structural Engineering Division, Department of Civil Engineering, Anna University, Chennai

The combined use of silica fume having pozzolanic reaction and filler effect, and super-plasticizer will improve the interface of the materials, thereby enhancing the strength of the concrete, can lead to economical high performance concrete (HPC) with enhanced durability. Addition of steel fibers in to the silica fume concrete alters their brittle character in to a more ductile one, and improves the mechanical properties. An experimental investigation was carried out to assess the durability performance of high-strength steel fiber reinforced concrete (HSFRC) for water to cement material ratios (w/cm) ratios ranging from 0.4 to 0.25. The variation in concrete strength from 55.6 to 86.5 MPa was achieved by varying the water-to-cementitious materials ratios of the mix from 0.40 to 0.25, and silica fume replacement at 5% and 10%. Crimped steel fibers of fiber volume fractions, Vf= 0.5%, 1.0% and 1.5%, with an aspect ratio of 80 were used in this study. Test results indicated that inclusion of steel fibers in to silica fume concrete improves the compressive strength by 13% and concrete mixes are having the better durability performance.

Introduction

Cement-based materials such as concrete have long been used for the construction of civil infrastructure. However, the deterioration of civil infrastructure all over the world, has led to the realization that cement-based materials must be improved in terms of their engineering property and durability. The use of admixture as silica fume-a highly pozzolanic mineral, is a relatively effective way in improving these properties. ACI 318-95 has recently revised, specifying that structural concrete should have high durability, as presented by water to binder ratio (W/B). A concrete structure is said to be durable if it withstands the conditions for which it has been designed, without deterioration, over the past years. The term durability characterizes the resistance of concrete to a variety of physical and chemical attacks due to either internal or external causes.

Balaguru and Shah 1992, and ACI Committee 544 (ACI 544.1R-96) have reported that the addition of steel fibers in concrete matrix improves all engineering properties of concrete such as flexural strength, tensile strength, compressive strength, and toughness.

HPC is achieved by using super-plasticizer to reduce water-binder ratio and by using supplementary cementing materials (SCM) such as silica fume (CSF), which usually combines high–strength with high durability. Silica fume concrete has been reported to possess lower water permeability (ACI 226-1987). Steel fiber reinforced concrete (SFRC) is a cement-based composite material reinforced with randomly distributed discrete steel fibers of small diameter. It contains pozzolans and admixtures commonly used with conventional concrete. The demand for HSC/HPC has been growing at an ever-increasing rate over the past years, which lead to the design of smaller sections. Reduction in mass is also important for the economical design of earthquake resistant structures (ACI 363-92).
Durability Characteristics of High-Strength Steel Fiber Reinforced Concrete
Due to the inherent brittleness of HSC/HPC, it lowers its post-peak portion of the stress-strain diagram almost vanishes or descends steeply (ACI 363-92).This inverse relation between the strength and ductility is a serious drawback for the use of HSC/HPC and acompromise to this drawback can be obtained by the addition of discontinuous short steel fibers of small diameter in to the concrete matrix. Bharatkumar et al. (2001) have studied the durability characteristics such as water absorption (%), void content (%), coefficient of absorption and sorptivity, of fly ash concrete, found to improve by reduction of w/b ratio and further improved by addition of cement replacement materials (CRM). Chang et al. (2001) have studied the durability properties of HPC, found to improve by reduction of w/b ratio and further improved by addition of cement replacement material. But for the writers’ knowledge durability studies are very limited on the HSFRC. Designing concrete for low permeability results in restricted access of water or solutions from external sources. While permeability of good concrete can be difficult to measure, the study of porosity or air content, coefficient of absorption and water sorption can provide some qualitative insight in to permeability. In this study, the durability performance of high-strength steel fiber reinforced concrete with compressive strength ranging from 55.6 - 86.5 MPa was examined.

This paper presents an experimental investigation on thedurability properties such as water absorption, coefficient of absorption, air (void) content and sorptivity of HPC with w/cm ratios ranging from 0.25 to 0.4 and silica fume replacement at 5%, 10%, and 15%, and studies the effect of inclusion of crimped steel fibers (volume fractions Vf = 0.5 %, 1% and 1.5 %) on these properties.

Experimental Program

Materials, Mixture proportions, and Preparation of specimens

Ordinary Portland cement-53 grade having 28-day compressive strength of 53.5MPa, satisfying the requirements of IS: 12269–1987 and condensed silica fume (Grade 920-D) contained 88.7% of SiO2,having specific surface area of 23000 m2/kg, a specific gravity of 2.25, fineness by residue on 45micron of 2% were used. The chemical composition of OPC and CSF are given in Table 1a &1b respectively.

Durability Characteristics of High-Strength Steel Fiber Reinforced Concrete
Fine aggregate: Locally available river sand passing 4.75mm IS sieve, conforming to grading zone-II of IS: 383-1978 was used. It has fineness modulus of 2.65, a specific gravity of 2.63 and water absorption of 0.98 % @ 24 hrs.

Coarse aggregate: Crushed blue granite stone with 12.5mm maximum size, conforming to IS: 383-1978 was used. The characteristics of coarse aggregate are:

Specific gravity = 2.70; Fineness modulus = 6.0; Water absorption= 0.65 % @24hrs.

Super-plasticizer: Sulphonated naphthalene formaldehyde (SNF) condensate as HRWR admixture conforming to ASTM Type F (ASTM C494) and IS: 9103-1999 was used. Specific gravity of SNF = 1.20.

Fibers conforming to ASTM A820-01 have been used, are crimped steel fibers of diameter =0.45 mm and length = 36 mm, giving an aspect ratio of 80, ultimate tensile strength (fu) = 910 MPa and elastic modulus (Ef) = 200 GPa.

Mixtures were proportioned using guidelines and specifications given in ACI 211.1–1991 and ACI 211.4R–93, recommended guide– lines of ACI 544-1993. Mixture proportions used in the test program are summarized in Table 2. This aspect of work was carried out elsewhere (Ramadoss and Nagamani 2006). Water present in super-plasticizer is excluded in calculating the water to cementitious materials ratio. For each water-cementitious materials ratio, 6 fibrous concrete mixes were prepared with three fiber volume fractions, Vf= 0.5%, 1% and 1.5 % by volume of concrete (39, 78 and 117.5 kg/m3). Super-plasticizer with dosage range of 1.75 % to 2.75 % by weight of cementitious materials has been used to maintain the adequate workability of plain andiber reinforced concrete.

Concrete was mixed using a tilting drum type mixer and specimens were cast using steel moulds, compacted with table vibrator. For each mix at least three 150mm x300mm cylinders and three 100 x 100 x 500mm prisms were prepared. Specimens were demoulded 24 hours after casting and water cured at 27±2oC until the age of testing at 28 days. All the specimens were cured in the same curing tank to maintain uniform curing for all the specimens.

Compressive Strength Test

Compressive strength tests were carried out according to IS: 516-1979 [16] standards using 150 mm cubes loaded uniaxially. The tests were done in a servo-controlled compressive testing machine by applying load at the rate of 14 MPa /min. Minimum of three specimens were tested to assess the average strength.

Durability Studies

The water absorption test was performed according to ASTM C 642 [21] and air (void) content, coefficient of absorption and sorptivity were also evaluated based on the water absorption test.

Coefficient of Absorption

Powers (1968) suggested the use of co-efficient of absorption as a measure of the permeability of water in to the hardened concrete. This is measured by the rate of intake (capillary absorption) of water by dry concrete for the period of 60 minutes.

Sorptivity

Sorptivity (water sorption) measures the rate of penetration of water in to the pores of concrete by capillary suction. When the cumulative volume of water penetrated per unit surface area of exposure is plotted against the square root of time of exposure, the resulting graph could be approximated by a straight line passing through the origin. The slope of this straight line is considered as a measure of rate of movement of water through the capillary pores and is called Sorptivity (Hall 1993). This test evaluates the quality of concrete based on surface pores of the concrete specimens.

Test Procedure

Durability Characteristics of High-Strength Steel Fiber Reinforced Concrete
Immediately after the immersion of the cube in to water the start time of the test was recorded; at intervals of 30, 60, 90, and 120 minutes after the start of the test, the specimen was removed from the water tub; after the surplus water was wiped of with tissue papers, it was weighed to the nearest 0.01gm and then returned to the tub. The relationship between water sorption and time was determined by Power (1968) as

i = S t1/2---(1)

Where S [in mm/min1/2] is the water sorptivity of the concrete and i [in mm] is the cumulative absorbed volume per unit area of in flow for duration of time‘t’

Results and Discussion

Compressive Strength

Compressive strengths for various mixes are given in Table 3. The 28-day strengths obtained vary from 55.6 to 86.5 MPa depending upon the w/cm ratio, binder content and fiber volume fraction in percent. Table 3 shows that the addition of fiber volume fraction from 0.5% to 1.5% increases the compressive strength by about 13 percent of plain concrete.

Durability Characteristics

For the series of high-strength fiber reinforced concrete mixes contained silica fume having w/cm ratios varying from 0.40 to 0.25, water permeability was unmeasurable.

Water Absorption and Air (void) Content

The results of water absorption and air content are presented in Table 3. Water absorption and Air content for concrete mixes investigated after 28-day are found to be in the range of 0.83 – 2.0% and 2.02 – 4.80% respectively. According to the CEB report of Concrete Society, United Kingdom (1989), concrete quality is classified as good if the saturated water absorption is around 3%. This indicates that the performance of the mixes developed is considered to be good from the point of view of the water absorption. Results show that water absorption and air content reduce as the w/cm ratio reduces and also increasing of SCM. Bharatkumar et al. (2001) obtained water absorption in the order of 4.91– 3.51 for fly ash concrete and% void from 11.98–8.47, which compare well with present results.

Coefficient of Absorption

The values of coefficient of absorption obtained are given in Table 3. For the concrete mixes investigated after 28 days, it is in the range 1.35x10-10– 0.632x10-10m2/sec. It is seen that there is reduction in the value of coefficient of absorption as the w/cm ratio reduces. Bharatkumar et al. (2001) have reported the coefficient in the range of 3.01x10-10– 0.89 x10-10m2/ sec for flyash concrete (HPC), which compares well with present results.

Sorptivity

Results of water sorption for various HSFRC mixes are given in Table 3 and are in the range of 0.0916 – 0.0427 mm/”min (< 0.77mm/”min) indicated that durability performance of concrete containing silica fume is excellent (Navy 1997). Taywood Engineering limited (1993) has suggested that good quality concrete has the sorptivity value less than 0.1mm/”min. Past findings (MacCarter et. al. 1992; Martys and Farraris 1997) indicated that sorptivity can be correlated to permeability, as is a function of porosity, pore diameter, and continuity of pores within the concrete matrix. Figure 1 shows the typical sorptivity plot for HSFRC at 28-days. Bharatkumar et al. (2001) have reported that sorptivity of fly ash concrete (HPC) is in the range of 0.0883 – 0.0627 mm/min 0.5(1.14 x10-5–0.81 x10-5m/s 0.5) which compares well with the present results. It is clear that concrete sorption is closely related to surface pores of the concrete paste. The surface pores reduce as C-S-H gel developed by the addition of silica fume, as a result which decreases the water sorption and hence the durability performance of mixes improves.

Sea Water Resistance

Durability Characteristics of High-Strength Steel Fiber Reinforced Concrete
For studying the resistance of HSFRC in sea water, cubes of 150 mm side were weighed and immersed in sea water prepared artificially, (3 % of sodium chloride by weight of water) for 45 days continuously and then taken out and weighed. The percentage loss in weight and the reduction in compressive strength were evaluated and given in Table 3. The maximum loss in compressive strength obtained was about 3.84 % for non-fiber concrete and 2.53% for fiber concrete, indicating that HSFRC mixes have better and higher resistance against sea water. No cracks were formed on the surface of the specimens. This indicated that the reaction between the CaOH and NaCl solution is reduced due to the reduction of harmful lime content in the concrete matrix as CSF reacted with lime present in the paste matrix.

Acid Resistance

For studying the resistance of HSFRC to acids, 150mm side cubes were weighed and immersed in sulphuric acid solution (containing 1% of sulphuric acid by weight of water) for 45 days continuously and then taken out and weighed. The percentage loss in weight and the reduction in compressive strength were evaluated and given in Table 3. The maximum loss in compressive strength obtained was found to be about 4.51% for non-fiber concrete and 4.42% for fiber concrete, which show that SFRC mixes are less attacked by acid. Less deterioration effect was noticed in the case of SFRC. From the tests, it could be observed that SF concrete/ SFRC greatly enhance the durability in aggressive environments.

Conclusion

Based on the above experimental investigation, the following conclusions can be drawn:
  • It is observed from the results of water absorption and air content that quality of concrete mixes is good and show that water absorption and air content reduce as the w/cm ratio reduces and also increasing of SCM.
  • Coefficient of absorption of HSFRC mixes is found to be of the order 10-10m2/sec. and is found to reduce with reduction in w/cm ratio of the mix.
  • Sorptivity of HSFRC mixes is found to be of the order 10 -5­m/ min.0.5shows quality of concrete mixes is superior and therefore, HSFRC is a very less permeable concrete. The above two characteristics are comparable to the reported values of flyash based HPC. It is observed from the test results of sorptivity test that the quality of concrete mixes is superior and therefore, HSFRC is a less permeable concrete.
  • HSFRCs exhibit better performance against the attack of sulphuric acid and sea water.
  • The use of silica fume and low w/cm ratio resulted in particularly impermeable concrete.

References

  • ACI Committee 318-1995 Building code requirements for structural concrete, ACI 318-95, American Concrete Institute, Detroit
  • Balaguru, N., and Shah, S.P. 1992 Fiber reinforced concrete composites, McGraw Hill International edition, New York.
  • ACI Committee 544-2006 State-of-the-art report on fiber reinforced concrete, ACI 544.1R- 82, American Concrete Institute, Detroit.
  • ACI Committee 363-1992 State-of-the-art report on high-strengthconcrete, ACI 363-1992, American Concrete Institute, Detroit
  • Bharatkumar, B.H., Narayanan, R., Raghuprasad, B.K., and Ramamoortht, D.S. 2001 “Mix proportioning of high-performance concrete,” Cement and Concrete Composites, 23, 71-80.
  • Chang, P.K., Peng,Y.N., and H wang, C.L. 2001 “A design consideration for durability of high-performance concrete,” Cement and Concrete Composites, 23, 375-380.
  • ACI Committee 211-1999 Standard practice for selecting proportions for normal, heavy weight and mass concrete, ACI 211.1-91 ACI Manual of concrete practice.
  • ACI Committee 211-1999 Guide for selecting proportions for High strength concrete with Portland cement and Flyash, ACI 211.4R-93 ACI Manual of concrete practice.
  • ACI Committee 544–2006 Guide for specifying, mixing, placing and finishing steel fiber reinforced concrete, ACI 544.3R- 95, American Concrete Institute, Detroit.
  • Ramadoss, P., and Nagamani, K. 2006 “Investigations on the tensile strength of high-performance fiber reinforced concrete using statistical methods,” Computers and Concrete- An International Journal, 3(6), 389-400.
  • IS: 516-1979, Indian standard methods of tests for strength of concrete, BIS 2002 Bureau of Indian Standards, New Delhi, India.
  • American Society for Testing and Materials 1990 Standard test method for specific garavity, water absorption and unit weight of hardened concrete, ASTM Stand Concrete Aggregate, 4, 318-9.
  • Power, T.C. 1968 Properties of fresh concrete, New York.
  • Hall, C., 1993 “Water sorptivity of mortars and concrete- A review,” Magazine of Concrete Research, 419(147), 51-61.
  • Nawy, E.G. 1997 Concrete Construction Engineering Hand Book. CRC Press, Boca Raton, New York
  • Taywood engineering limited, 1993 Correspondence to MBT (Singapore) pvt.Ltd Australia,
  • McCarter, W.J., Ezirim, H. and Emerson, M.1992 “Absorption of water and chloride in to concrete,” Magazine of Concrete Research, 44 (158), 31-37.
  • Martys, N.S., and Ferraris, C.F. 1997 “Capillary transport in mortars and concrete,” Cement and Concrete Research, 27 (5, 747-760.

NBMCW November 2008

.....

Load Capacity: Concrete Types And How T...

Concrete Types And How They Behave

Jens Lützow, MEVA Schalungs-Systeme, Haiterbach Germany

Accurately calculating and predicting the pressure that poured concrete will exert on the formwork, is one of the most important tasks when planning on-site concrete works. Sure knowledge of the expected loads will enable the site to choose the best building technique and the appropriate technical equipment.

Flowable and Self-compacting Concrete: Pressure Behavior

The introduction of new concrete types with additives and different setting patterns underlines the challenge of predicting the load to be expected when pouring them– especially since, up to now–there was little definitive experience to draw on. Methods of calculating concrete pressure used hitherto do not take the new concrete types into account and cannot be safely used when pouring them. Yet, with architectural structures becoming slimmer and finer and fair-faced concrete finish increasingly being demanded, it is precisely this new type of concrete that is being used increasingly. The uncertainty about the chemical interaction between additives and their possible influence on the hydration progress and final setting is immense.

Old Calculation Methods Endanger Economics and Safety

Concrete Types And How They Behave
Up to now, fresh concrete pressure was often determined using the German DIN Standard 18218 dating from 1980 or the English CIRIA Report (Construction Industry Research and Information Association). Both assume a basic concrete recipe consisting of three simple components as used in the 70´s and whose characteristics are set out in the DIN 1045 and DIN 1048 German Standards. Concrete recipes being used predominantly in Europe no longer behave that way: slump and setting behavior are distinctly different.

Self-compacting concrete, used especially for difficult geometries to pour into heavily reinforced, slim structures, exert higher pressures on the formwork. Drawing on laboratory tests, a general rule of thumb was followed: take hydrostatic pressure to determine the load capacity of the formwork. That seemed to put everyone on the safe side. This is why the formwork was often set out for too heavy a load.

To fill this gap in competence thus became an urgent matter, since safety on the site demands knowing what kind of pressure the concrete poured will exert. Without proven and tested methods of doing so, it would be only a matter of time before something could go seriously wrong with threat to life and limb during construction. The task was to extend the DIN standard 18218 (1980) to include concrete consistencies F5, F6 and SCC.

Going Safe: New Methods Proved in Practice

MEVA´s engineers were the driving force in updating the DIN 18218 and developing appropriately simple methods of safely calculating concrete pressure. The theoretical formulae were tested in practice and brought on-site for final verification. The results and insights have now found their way into the revised DIN 18218 and provide a secure basis for any construction engineer to work with.

Concrete Types And How They Behave
MEVA stands for sophisticated formwork systems, continuous product development and comprehensive services. Since its foundation 35 years ago MEVA has launched numerous innovations, and is now among the leading, globally operating companies in formwork technology. MEVA provides a dense distribution network with branch offices, subsidiaries on different continents and specialized dealer. This offers contractors all over the world competent advice and backup for the planning and construction of sophisticated buildings.

What´s New?

A calculation of the concrete pressure according to DIN 18218 is now possible for concrete consistency in all categories from F1 to F5 and F6 as well as for self-compacting concrete. The new calculation method is based on the Proske-Schuon-Theory. This approach takes into consideration the setting behavior of concrete, which depends on consistency and temperature. So, the site can determine maximum pour rate from simple diagrams. All concrete types are depicted according to maximum fresh concrete pressure and the corresponding maximum pour rate. A distinction is made between final setting at 5 and 15 hours.

Concrete Types And How They Behave
The revised DIN 18218 takes into account the increasing use of self-compacting and flowable concrete mixtures on today´s building sites. The new, simple-to-use diagrams enable the site to safely determine the concrete load to be expected and thus choose the appropriate formwork to handle it. The advantage is twofold: assuming too much concrete pressure would be waste of money and material; assuming to low a concrete pressure could endanger the safety of everyone on site.

Formwork Concept based on new Calculation Method: Heights up to 32m achieved using flowable concrete.

Site Report from Testbed Project for Airbus Jet Engines

One of the most challenging projects on which to test and prove the new concrete pressure calculation method was the construction of the Testbed for Rolls-Royce engines in Arnstadt near Erfurt in Germany. Around 200 turbofans of the Trent 500, 700 and 900 series that power the new Airbus A 300 series will be tested, maintained and overhauled in the facility every year. These engines power the Airbus A330, A340-500/600 and A380. The construction plans place exceptional demands on the formwork solution and the concrete finish.

Strict Requirements: Low-Porosity Finish

The test stand for jet engines comprises four segments. The engines are brought into the testbed through a large, hermetically closed gate. For simulation purposes, air is drawn in through a 23m high intake funnel. Behind the engines, exhaust fumes are discharged through an exhaust chamber, 11.80m high and from there through a 32m high exhaust funnel to the outside. Extreme air pressure and speed demand exceptionally low porosity in the concrete finish. The smallest particles on the wall surface would in effect turn into little missiles as a result of the extreme pressure.

Therefore, the inside walls need to have a hermetically sealed surface corresponding to category 4 of the classification for fair-faced concrete published by the Deutscher Betonverein (German Concrete Association) [edition: August 2004].

Unique for Site-mixed Concrete: Mammut 350 takes concrete pressure of up to 11 m high pours in one go

Mammut 350 is a sophisticated system with a load capacity of 100kN/m2. Strong enough to build 75 cm thick walls with a pouring rate of 1 m/h - in pours of up to 11m and a total wall height of 32m. It is unique for on-site concreting to pour such wall heights with flowable concrete. Thanks to the large-size panels 350/250 a surface of 8.75m2 can be assembled with just one panel.

This represented a great advantage for the Arnstadt jobsite in assembling the formwork for the large wall segments of upto 290m2. Two panels stacked vertically achieve the required 7m high formwork units with only one horizontal joint, or with two joints when 11m high. The absolute symmetry of the panels offers a perfect tie hole and joint pattern.

Concrete Types And How They Behave

Concrete recipe and pressure

The heavily reinforced walls require a suitable concrete and therefore theconstruction parties developed a recipe. Finally, a CEM III concrete with a low water-cement-ratio was chosen, which had to fulfil the requirements of a concrete consistency of F5 to F6 according to the new DIN standard 1045. As a consequence, flowability was optimized by using a polycarboxylate-ether (PCE).

Concrete Types And How They Behave

Comparison of slump [a] acc. to DIN1045­2 (2001) and acc. to the former standards DIN18218 (1980) or DIN1045 (1978).

New calculation method

Concrete Types And How They Behave
A calculation of the concrete pressure according to DIN 18218 is no longer possible for a concrete consistency of F5 to F6 as it is used on this project. However, MEVA is a member in the study group Fresh Concrete Pressure of “Flowable Concrete”, which, in 2006, published a DAfStb- eport of the same name (DAfStb = German Committee for Reinforced Concrete ) . In the context of this study, and with the involvement of academics from different universities and experts from other companies, a new calculation method was developed which is based on the Proske-Schuon-Theory. This approach takes into consideration the setting behavior of concrete, which depends on consistency and temperature.

On the testbed project in Arnstadt, the reliability of this academic formula was verified under on-site conditions. A number of similar building segments were poured under comparable external circumstances. The concrete pressure was gauged during the placing of concrete.

Concrete Types And How They Behave
Under these circumstances, the calculation according to DIN 18218 led to a fresh concrete pressure of 40 to 45 kN/m2.

For the PCE­modified concrete used here, the rate of pouring was determined to be 1 m/h and the final setting to becompleted after 9 hours. According to the Proske-Schuon-Formula this results in a fresh concrete pressure of 100 kN/m2. Actual measurements showed pressures between 85 and 100 kN/m2.

The time curve of the fresh concrete pressure on the one hand confirms the DAfStb-report, and, on the other hand, it underlines the necessity to revise the so far accepted DIN standard 18218, which in this case would have ended in disaster if applied uncritically.

Mammut 350: Sufficient pressure reserves

The high daytime temperatures required adding a retarder to the concrete, which resulted in the final setting occurring only after 15 hours. Nevertheless, the Mammut 350 has sufficient reserves to cope with load peaks of 100 kN/m2.

Concrete Types And How They Behave

Static analysis confirms new calculation method

The test stand for jet engines in Arnstadt is the first jobsite where several cycles were poured with identical heights and with the same concrete recipe. This allowed the engineers to analyze the concrete pressure curve. The results are astounding, since the on-site values and those arising from the Proske-Schuon-Formula vary only by +/- 5 %. After 15 pours the Proske-Schuon-Theory is thus validated not only in theory, but in on-site practice also. As a result, the progress report was verified in its first phase and the results will form the basis of the extended and revised DIN 18218.

NBMCW November 2008

.....

Carbonation A Durability Threat for Con...

Carbonation A Durability Threat for Concrete

Jayobroto Burman Roy, General Manager, ATS Infrastructure Ltd. Noida

Introduction

The term “Carbonation” of concrete means the chemical reaction between carbon dioxide in the air and hydration products of the cement. The process includes:
  1. Diffusion of CO2 in the gaseous phase into the concrete pores,
  2. Its dissolution in the aqueous film of these pores,
  3. The dissolution of solid Ca(OH)2in the water of the pores,
  4. The diffusion of dissolved Ca(OH)2in pore water and its reaction with the dissolved CO2,
Final result of several steps through which the calcium carbonate is formed may simply be described by the following reaction which is assumed to be irreversible.

Ca2+(aq) + 2(OH¯)(aq) + CO2(aq) CaCO3(s) + H2O (Eq.1.1)

Effect of Carbonation

When Ca(OH)2is removed from the paste, hydrated CSH, C3S and C2S will also carbonate.

3CaO·SiO2·3H2O+3CO2 3CaCO3+2SiO2+3H2O (Eq. 1.2)

Factors affecting Concrete Carbonation

The rate of carbonation depends on porosity (for CO2to Diffuse) & moisture content of the concrete (for dissolution of solid Ca(OH)2). The diffusivity of CO2 depends upon the pore system of hardened concrete and the exposure condition. The pore system of concrete depends upon the type and the content of binder, water/binder ratio, and the degree of hydration. Thus, the main factors affecting concrete carbonation are:
  1. Pore system of Hardened Concrete which in turn depends upon w/c ratio, type of binder, and degree of hydration,
  2. Relative humidity (for dissolution of Ca(OH)2),
  3. The concentration of CO2.

    Optimal conditions for carbonation occur at a RH of 50% (range 40–90%).
    • If RH <40%, CO2 cannot dissolve,
    • If RH >90%, diffusion of carbon dioxide will be inhibited by the water that has filled the pores and hence CO2 cannot enter the concrete.
    • The most dangerous range of relative humidity for carbonation is 40% to 80%, since the carbonation reaction calls for the presence of water, while under higher atmospheric humidity the diffusion of carbon dioxide will be inhibited by the water that has filled the pores.

Carbonation and Concrete Durability

Understanding Corrosion

Corrosion is an electrochemical process involving the flow of charges (electrons and ions). At active sites on the bar, called anodes, iron atoms lose electrons and move into the surrounding concrete as ferrous ions. This process is called a half-cell oxidation reaction, or the anodic reaction, and is represented as: (Figure 1.2).

Schematic Diagram of Corrosion
2Fe’ -> 2Fe2++ 4e- (Eq. 1.3)

2H2O + O2 + 4e-’ -> 4OH- (Eq. 1.4)

2Fe2++ 4OH-’ -> 2Fe(OH)2 (Eq. 3.5)

This initial precipitated hydroxide tends to react further with oxygen to form higher oxides. The increases in volume as the reaction products react further with dissolved oxygen leads to internal stress within the concrete that may be sufficient to cause cracking and swelling of the concrete cover.

Consequence of Carbonation

  • Carbonation results in a decrease of the porosity making the carbonated paste stronger. Carbonation is therefore an advantage in non-reinforced concrete.
  • However, main consequence of carbonation is the drop in the pH of the pore solution in the concrete from the standard values between 12.5 and 13.5, to a value below 9 in the fully carbonated zones, so that the passive layer that usually covers and protects the reinforcing steel from corrosion becomes unstable.
Carbonation A Durability Threat for Concrete

Once this layer is destroyed rusting of iron bars and subsequent expansion of the concrete takes place and durability of concrete decreases. Hence Carbonation is harmful for reinforced concrete.

Methods to Measure Carbonation

Extent of Carbonation is measured in two ways:
  • First way is to measure the concentration of CO2 absorbed by the concrete specimen.
  • Second way is to carbonate the specimen in (a) natural or (b) laboratory environment conditions and then break it and spray a pH indicator to know the extent of Carbonation.
Both methods are described in detail on next page.

IR Spectrum Analysis

Schematic Diagram of pH
The carbonation set-up consists of a close loop in which a mixture of air and carbon-dioxide could be introduced at a certain RH. Due to the carbonation reaction, an amount of CO2 molecules will be immobilized reducing the concentration of carbon dioxide in the circulating gas mixture. The CO2 concentration in the gas is measured using an IR absorption device. A pump is used to circulate the gas while also temperature and RH are measured.

Using pH Indicators

Carbonation Depth vs. Compressive strength
In this method, first concrete specimen is kept in an open environment for a number of years or in Carbonation Chamber for a number of months. Generally, conditions of 70% CO2, 50% Relative Humidity, and 20-22ºC is maintained in a carbonation chamber, fig. 1.4(a). Then sample is broken and is sprayed with a pH indicator. Popularly a standard solution of 1% phenolphthalein in 70% ethyl alcohol is used. In the noncarbonated region with pH values above 9.2, the phenolphthalein indicator turns purple-red; and in the carbonated portion with pH less than 9.2, the solution remained colorless. Figure 1.4(a), (b).

Literature Review

Figure 1.6 shows a compilation of carbonation depth by various researchers. It was found that most of the researches were confined to a particular zone of compressive strength. Very less data is found concrete above 60 MPa concrete strength.

Figure 1.5 shows the collection of various carbonation depth verses strength. It is noted that most of the research is done on concrete with strength below 60 MPa. The value and variation of data after this is negligible. Figure 1.6 shows the results of rapid carbonation. It shows slightly higher depth but follows similar trend. Figure 1.7 shows a critical analysis of Figure 1.5. Table 1.1 shows a few sets of distinct data derived from Figure. 1.5

Classification of the Data

Following conclusions can be arrived:
  1. Carbonation Depth shows a good variation and possibly a good method to judge durability upto 60 MPa.
  2. After 60 Mpa Compressive Strength, the values of normal carbonation depth are insignificant and it is difficult to access the durability of concrete.
  3. Based on the table no 1.1, following bound distribution can be made:
    Carbonation Depth vs. Compressive strength
    1. Normal concrete is distributed over whole bound,
    2. Concrete with one pozollonic material is lying in middle and lower bound,
    3. Concrete with mixed pozollonic materials is lying in lowerbound.
  4. The plot shows that concretes with mixed pozollonic materials (till 30%) are more reliable w.r.t. carbonation depth as compared to normal concrete or concretes with only one pozollonic material.
Results of Carbonation Depths

Numerical Prediction Method

Jiang et. al. presented a good numerical modeling as presented below for normal and high volume flyash:

The model for predicting HVFA concrete carbonation is in good agreement with the test results obtained in an accelerated carbonation apparatus. It can be used to predict the evolution of carbonation depth with time. However, more tests are required to confirm these preliminary observations. From this study, it can also be seen that at a given binder content, the carbonation depth of HVFA concrete is greater than OPC concrete. The increase of curing period can improve the carbonation behavior of HVFA concrete. The carbonation depth of HVFA concrete of appropriate mix proportion can approach that of OPC concrete, and meet the requirements of structural concrete.

Experiments

Carbonation Depth Results Analysis of Carbonation Results

Carbonation Depth

Carbonation Depth vs. Compressive strength
Relationshp between carbonated depth vs. Strength are plotted here.

The values of carbonation depth have been plotted against design strength in Figure 1.13. It has been divided into 6 parts. Each part is the line representing for a fixed replacement level of fly ash and silica fume. It is a comprehensive graph and it can be seen very clearly that as strength increases the carbonation depth values are decreasing.

If we assume that carbonation depth are directly related to durability of concrete, i.e. higher the carbonation depth lower will be durability, following conclusion can be given.
  1. The effect of 15% replacement by Fly Ash is not clear. Carbonation depth has increased for M25, M30, and M40 but decreased for M20 and M35. In Figure 4.8 if we delete point ‘X’ then we may conclude that 15% Flyash replacement increases carbonation depth and will therefore decreases durability. In this experiment samples were water-cured for 28 days only. What would have happened if samples were water-cured for 56 days? Will it result in better durability of concrete with 15% replacement by Flyash?
  2. 6% replacement by Silica Fume showed significant decrease in Carbonation Depth, hence is more durable.
  3. 15% Flyash and 6% silica Fume replacement shows further fall in carbonation depth values, hence it is even more durable.
  4. Due to less available data not much can be said about concrete with 30%FA and 6%SF replacement or 40%FA and 6% FA replacements, but it is clear that concrete made by double replacement of cement generally shows better durability property as compared to single replacement or no replacement.
  5. Double replacement of 40% fly– ash and 6% silica fume shows least carbonation. However, exact replacement levels which would give maximum durability without going for a higher strength concrete is cannot be predicted.
  6. The argument that “pozzolana decreases conc. of Ca(OH)2 and CO2 has to react with less Ca(OH)2 and hence carbonated front should move faster inside concrete” seems void. Except for 15% Flyash replacement, all other pozzolana/flyash concretes have shown lower carbonation.
  7. Variation of the graph is very-very similar to as predicted after Literature Review (see Figure1.5-8)

Conclusion

Comparision of Carbonation
In this discussion, the mechanism of carbonation along with the adapted experimental methods are presented. Phenolphthalein method of detecting depth of carbonation is found to be the most popular method. A lot of experimental and imperical model to estimate carbonation (Jiang et al.) have been reported in literature.

Carbonation data from various literature have been plotted for comparative study and excellent co-relation where noticed. Indeed Jiang et al. model must have simulated all these results. It was noticed that a lot of data exists under 60 MPa concrete. Very less data exist for concrete above 60 MPa. This is because above 60 MPa, the depth of carbonatin is insignificant.

NBMCW November 2008

.....

Prestressed Concrete in Building: Adva...

Prestressed Concrete in Building

Partha Pratim Roy, B. E. (Civil), M. E. (Structure), General Manager (Technical), ADAPT International Pvt. Ltd.

Prestressed Concrete

Prestressed concrete is a method for overcoming concrete's natural weakness in tension. Prestressing tendons (generally of high tensile steel cable or rods) are used which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to self–weight and gravity loads. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebar, and inside poured concrete.

Prestressing can be accomplished in two ways: pre-tensioned concrete and bonded or unbounded post-tensioned concrete.

Pre-tensioned Concrete

Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction.

However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line. Thus, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size. Pre-tensioned elements may be balcony elements, lintels, floor slabs, beams or foundation piles. An innovative bridge construction method using pre-stressing is described in stressed ribbon bridge.

Bonded Post-Tensioned Concrete

Typical Layout of Bonded System
Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element. A set of tendons are fished through the duct and the concrete is poured.

Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete.

The duct is then grouted to protect the tendons from corrosion. This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure.

Post-stressing is also used in the construction of various bridges, both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge.The advantages of this system over unbonded post-tensioning are:
  1. Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
  2. Tendons can easily be 'weaved' allowing a more efficient design approach.
  3. Higher ultimate strength due to bond generated between the strand and concrete.
  4. No long term issues with maintaining the integrity of the anchor/dead end.

Unbonded Post-Tensioned Concrete

Typical Layout of Unbounded System
Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab.

The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). The advantages of this system over bonded post-tensioning are:
  1. The ability to individually adjust cables based on poor field conditions, e.g., shifting a group of 4 cables around an opening by placing 2 to either side).
  2. The procedure of post-stress grouting is eliminated.
  3. The ability to de-stress the tendons before attempting repair work.

Post-Tensioning in Building Structures

Market Factors Favoring the Post-tensioning System

Followings are the market factors, which favor implementing Post-Tensioning system in Building structures:
Dubai Pearl Project
  • Longer spans
  • Unique designs: irregular shapes
  • Shorter construction cycles
  • Cost reduction
  • Shorter floor-to-floor heights
  • Superior structural performance

Direct Cost Reduction

Beach Tower, Sharjah, UAE

Direct Cost Comparision between RC and PT Systems

Cost structure of RC vs. PT Slabs
Post-tensioning offers direct cost reduction over conventionally reinforced slabs primarily by reducing concrete and rebar material quantities as well as rebar installation labor. Typically, savings between 10%–20% in direct cost are achieved.

Followings are the factors which contribute to direct cost reduction:
  • Less concrete material
  • Reduction in slab thickness reduces total building height and cost
  • Less rebar
  • Less labor cost for installation of material
  • Reduced material handling
  • Simplified formwork leads to less labor cost
  • Rapid reuse of formwork leads to less formwork on jobsite
As a rule, the break even mark between conventional and prestressed solutions is approx. 7m spans.

In a typical slab with spans over 7 meters, the net savings in material cost can range between 10%–20% of original RC alternative. A typical comparative cost structure is shown on the next page:

Material represents 60% of direct cost of a post-tensioning system. Cost structure of PT System is shown there.

Improved Construction Efficiency

Construction Cycle
Since post-tensioned slabs are designed to carry their own weight at time of stressing, they can significantly improve construction efficiency and deliver an additional 5%-10% of indirect savings.

Following factors contribute to improved construction efficiency:
  • Shorter construction cycles
  • Less material handling and impact on other trades
  • Simpler slab soffit–less beams and drop caps/panels
  • Quicker removal of shoring gives more access to lower slabs
Typical 5-Day Construction Cycle schedule for 800-1,000 m2 of slab is shown below. 3-day cycle is also achievable with early strength concrete and industrial formwork.

Superior Structural Performance

The prestressing in post-tensioned slabs takes optimal advantage of tendon, rebar and concrete properties to deliver an economical structural system.

Factors contributing to superior structural performance are listed here:
  • Use of high-strength materials
  • Deflection control
  • Longer spans are achieved
  • Crack control and water-tightness
  • Reduced floor-to-floor height
  • Lighter structure requires lighter lateral load resisting system
  • Economy in column and footing design
  • Reduced noise transmission compared to RC
  • Lower total cost of ownership (maintenance) compared to RC alternatives

Typical Quantities

Post-Tensioning and rebar rates vary greatly depending on span configuration and loading. Compared to other countries, PT projects in US are designed with less loading and lower PT and rebar rates.

Bonded System

Layout of Original Design

Layout of Alternate Design
US values (1 kN/m2 SDL & 2.5 kN/m2LL)
  • 3 – 4 kg/m2 of PT
  • 5 kg/m2 of Rebar
With higher loading (3 kN/m2SDL & 3 kN/m2LL)
  • 3.5 – 5 kg/m2of PT
  • 7 – 9 kg/m2of Rebar

Unbonded System

US values (1 kN/m2SDL & 2.5 kN/m2LL)
  • 3.75 kg/m2of PT
  • 6 kg/m2of Rebar

Case Study of Value Engineering

Legend Plaza, Dubai, UAE is designed using ADAPT-Floor Pro (www.adaptsoft.com). Salient features of this project are listed here:

Project Parameters

  • Gross Floor Area–Superstructure: 72,000 m2
  • Typical Floor Gross Area: 11,000 m2
  • Total Floors: 7
  • Typical Floor Slab Spans: 10 m max / 8 m avg.

Type and Location

  • Type of Structure: High-end Residential
  • Location: Dubai, UAE
  • Construction Date: Aug 2004 – Jan 2005

Project Team

  • Prime Structural Engineers: Adnan Saffarini
  • Contractor: SBG
  • Client: Private Investment
  • PT Supplier: Freyssinet Gulf
  • PT Value Engineers: ADAPT Corporation

Design Criteria

  • Design Code: BS-8110
  • Concrete Compressive Strength Fcu: 40 MPa
  • Reinforcement Yield stress:460MPa
  • Superimposed Dead Load: 6 kN/m2
  • Design Live Load: 2 kN/m2

Original Design

  • Original Floor System: Hourdy Slab System
  • Depth of Floor System: 380 mm
  • Boundary Beams: Yes

Alternative Design offered by Freyssinet & ADAPT

  • Post-tensioned Floor System: 2-way Flat Plate
  • Depth of Slab:220 mm
  • Boundary Beams: None

Benefits

  • 70 % less rebar
  • 13 % less concrete
  • Elimination of all Hourdy Blocks
  • Unified structural slab system
  • Beams & drop caps were deleted, simplifying slab installation
  • 25% less formwork
  • 3 months shorter construction program
  • 15% savings in site overhead and plant
Value engineering saved Contractor over 1 million US$.

References

  • Wikimedia Foundation, Inc., (http://en.wikipedia.org)
  • ADAPT Corporation (http://www.adaptsoft.com)
  • Interested readers can contact the author at intl@adaptsoft.com

NBMCW October 2008

.....

Conventional Vis–a–Vis Mineral Admi...

Conventional Vis–a–Vis Mineral Admixed Concrete for Cement Concrete Pavements Construction

Conventional Vis–a–Vis Mineral Admixed Concrete

Dr Rakesh Kumar, Scientist and Dr Renu Mathur, Scientist &amp; HoD, Rigid Pavements Division, Central Road Research Institute (CRRI) New Delhi.

This article presents a comparison between conventional concrete and mineral admixed concrete to be used in the construction of cement concrete pavements. Mineral admixtures such as flyash, rice-husk ash and silica fume were used in this study. These admixtures were selected to arrive at an economical way for enhancing flexural strength of concrete without significantly affecting compressive strength. Ten concrete mixes were used in this investigation. The performance of concrete mixes was evaluated at fresh state as well as hardened state. Hardened state concrete properties such as compressive strength and flexural strength were evaluated. The study suggests that mineral admixed concrete shows higher flexural strength in comparison with conventional concrete. It also reveals that flexural strength can be drastically enhanced by using 5 to 10 percent of flyash, rice husk ash and silica fume as partial replacement of cement. However, from economical point of view replacement of 10 per– cent cement by mass with flyash is a better option for enhancing flexural strength of control concrete to be used in the construction of cement concrete pavements.

Introduction

The design of concrete pavements (also known as rigid pavements) is based on the flexural tensile strength of concrete as these pavements fail due to bending stresses. The flexural strength (sometimes called the modulus of rupture) of concrete is determined by use of beam specimens under either three-point or center-point loading following any standard test procedure.

Physical & mechanical properties of cement

The third-point loading is preferred because in the middle third of the span, the specimen is subjected to pure moment which is not possible in the center-point loading device. Hence, the center-point loading test gives results about 15 percent higher in comparison to three-point loading test. Depending on axle load, present traffic, design life, fatigue consumption etc., the average 28-day flexural strength of at least 3.5 MPa is commonly encountered in practice. This range of flexural strength can be achieved by using concrete of M 35 and above grade depending on its constituent ingredients. In recent past, a lot of research was carried out throughout globe for improving the performance of concrete in terms of its rheology, strength and durability qualities1,2. Consequently, several new constituents such as mineral and chemical admixture of concrete were used to meet the specific need of strength and durability of concrete. The use of mineral admixture in combination with chemical admixture has allowed the concrete technologists to tailor the concrete for many specific requirements. Among the several types of mineral admixture available, the most common are industrial by-products, such as silica fume, fly– ash, blast furnace slag, limestone powder and rice-husk ash etc. Use of these mineral admixtures improve the particle packing density, rheological properties in fresh state and mechanical properties including flexural strength and durability of cementitious system3-9. Amongst the mineral admixtures, silica fume seems to be the most useful for the development of very high strength concretes and/or high performance concrete. Various advantages of using silica fume, flyash and rice husk ash in concrete need not to be overemphasized. Therefore, in this study the relative performance with respect to compressive and flexural strength development of conventional vis-à-vis mineral admixed concretes containing ASTM Class F flyash, Silica fume, and rice-husk ash, for the construction of cement concrete pavements has been reported.

Physical & mechanical properties of Flyash

Experimental Study Materials

Properties of silica fume
Ordinary Portland cement with properties as given in Table 1 was used throughout this experimental investigation. The cement met the requirements of the Indian Standard specification IS: 8112-198910.

The flyash (FA) used in this study was obtained from a thermal power plant near Delhi and was evaluated as per ASTM C 618 requirements.11 Table2 shows some of the important physical and chemical properties of the flyash.
Properties of rice husk ash

Grading of sand
This flyash met the ASTM C 618 requirements. It is therefore, concluded that the flyash may perform satisfactorily in cement concrete.

Silica fume (SF) used in the experimental work had specific gravity of 2.18 and Blaine specific surface was around 450 m2/kg. Some important properties of silica fume used are presented in Table 3.

The rice husk ash (RHA) used was obtained from a commercial plant located at Tharsguda, Orissa, India. Its specific gravity and bulk density were 2.06 and 718 kg/m3respectively. The particles of rice husk ash were finer than 45mm. The various properties of rice husk ash are given in Table 4.

Throughout the study, the same land-quarried local sand was used. The specific gravity of the sand was 2.62 and its water absorption was 0.73%. The bulk density of the fine aggregate was 1615 kg/m3. The fineness modulus of the sand was 2.4. The sand was evaluated for the water absorption, specific gravityand grading as per the procedure given in IS: 383-197012. The data on grading of sand is reported in Table5. 20 mm (maximum nominal size) graded crushed stone was used as the coarse aggregate for this study. The specific gravity of he coarse aggregate was 2.62. Its water absorption was 0.27%.
Grading data of coarse aggregates
The bulk density of the coarse aggregate was 1610 kg/m3. The sieve analysis data on the coarse aggregate both for d" 20 mm and d" 12.5 mm are given in Table 6.

A new generation carboxylic ether polymer based super plasticizer was used as a high-range water reducing agent for this study.

Mix Proportions

Ten concrete mixes were used for this study whose mix proportions are summarized in Table 7. This includes one controlled conventional concrete i.e. a mix without mineral admixture and nine minerals admixed concrete mixes. The mineral admixtures used were Class F flyash, silica fume and rice husk ash. Three concrete mixes using each of the mineral admixtures were manufactured. Admixed concrete mixes were made with 5%, 8%, and 10% replacement of cement in conventional concrete with these mineral admixtures.
Properties of rice husk ash
The replacement level was limited to 10% with a notion to achieve 28-day compressive strength similar to conventional concrete and flexural strength at least about 4.0 MPa. The water-to-cementitious material ratio was kept around 0.42 and dosages of superplasticizer were varied in range of 0.2% to 0.4% of binder's mass to achieve a slump value of 75±5 mm. It is worth to mention hat the slump specifications are different for the cement concrete road construction for fixed form paving and slipform paving. Hence, the range of slump value is so selected to fit the requirements of both forms of the paving. The mixing procedure adopted was as described below. All the ingredients except superplasticizer were mixed in dry state for few seconds in a tilted drum type concrete mixer, then ¾ of total required water was added and the mix was further mixed for a couple of minutes. The superplasticer was mixed in remaining 1/4th water and added to the mix in the final stage of mixing. The mixture was mixed for another two to three minutes before evaluating its properties at fresh state.

Sample Preparation

For compression testing, 150 × 150 × 150-mm3 cubes and for flexural tensile strength prism specimens with dimensions 100 × 100 × 500-mm3 were prepared from all concrete mixes. All the specimens were demoulded after 24 hours of casting and curing in the steel mould. Thereafter, the demoulded specimens were marked for identifications and kept submerged in a curing tank till the age of testing.

Mix proportions of concrete

Fresh properties of concrete mixes

Flexural strength of concrete mixes

Results and Discussion Properties of Fresh Concrete

The results obtained on the fresh state properties i.e. slump, concrete temperature and concrete unit weight are presented in Table 8. The targeted slump was 75 ± 5 mm for all the concrete mixes. Additions of silica fume as well as rice husk ash slightly decreased the slump value in comparison of control concrete. Therefore, the same was brought within the range required by increasing the amount of more water reducing agent (Table 7). This indicates that addition of silica fume and rice husk ash decreases the slump of the concrete. However, in the case of FA concrete the slump value increased marginally. Based, on the results given on the slump values in Table 8 and the amount of super plasticizer required to maintain those value Table 7, it is obvious that replacement of cement by SF and RHA reduces the slump of the fresh concrete which is mainly due to increased fineness of the SF and RHA particles in comparison of cement particles.
Three-point loading
The data on concrete temperature presented in Table 8 indicates that the replacement of cement by RH & silica fume is similar. In general, an insignificant effect on concrete temperature can be noticed. A close look, of the Table 8 indicates that the mineral admixed concrete shows higher unit weight than the controlled one, which is due to micro filling effect of the mineral admixture. However, it can be noticed that the increase in replacement levels have insignificant effect on unit weight of concrete containing mineral admixture, which may be due to the fact of close range of replacement, levels.

Mechanical Properties

Conventional Vis–a–Vis Mineral Admixed Concrete
The most important mechanical properties of concrete for the use in pavement construction is flexural strength. In addition to it, the compressive strength of concretes was determined for their comparison. The results have been discussed in the following sections.

Compressive Strength

The compressive strength of concrete mixes was determined as per the standard procedure described IS: 51613 at 7 and 28 days. Average result obtained on triplicate specimens was used for the reporting. Figures 2-4 illustrate the development of compressive strength of concrete mixes containing 5%, 8% and 10% of mineral admixtures i.e. silica fume (SF), rice husk ash (RHA) and Class F flyash in comparison of controlled one, respectively. It is obvious from the Figure 2 that the concrete containing silica fume, out performs other concrete mixes including control one in term of compressive strength development. Similar trends for strength development can be observed in the cases of replacement levels of cement with mineral admixtures under nvestigation i.e. 8% and 10% (Figures 3-4). It can further be seen that concrete containing fly– ash (FA) develops least compressive strength. However, in the replacement levels used in the study, it shows strength nearly at par with control concrete. The insignificant reduction in strength of concrete containing flyash in comparison with control one may be due to the lower level of replacement of cement with flyash.

Strength development of concrete mixes
Strength development of concrete mixes
Figure 2: Strength development of concrete mixes containing 5% of mineral admixture
Figure 3: Strength development of concrete mixes containing 8% of mineral admixture

Further, the effect of levels of replacement of cement with the mineral admixture on strength development is shown in Figures 5-7. Figure 5, 6, and 7 show the development of strength of concrete containing silica fume, rice husk ash and flyash, respectively along with control concrete. From Figure 5, it can be observed that the increase in the level of replacement of cement by silica fume increases the compressive strength of concrete; however, the increase in replacement level from 8% to 10% is not much significant. Therefore, optimum level of silica fume seems to be 8% in concrete. In the case of concrete containing rice husk ash (Figure 6) the replacement level has not much significant effect on the strength gain. But, in the case of flyash concrete (Figure 7) no significant reduction in strength in comparison with control concrete can be seen with increase of replacement levels from 5% to 10%.

Strength development of concrete mixes
Strength development of concrete mixes
Figure 4: Strength development of concrete mixes containing 10% of mineral admixture
Figure 5: Effect of replacement levels of SF on strength of concrete mixes


Strength development of concrete mixes
Strength development of concrete mixes
Figure 6: Effect of replacement levels of RHA on strength of concrete mixes
Figure 7: Effect of replacement levels of FA on strength of concrete mixes

Flexural Strength

The determination of flexural tensile strength or modulus of rupture is essential to estimate the stress at which the concrete member may crack. Its knowledge is useful in the design of pavement slabs and airfield runway as flexural tension is critical in these cases. For this, specimens of standard dimension of 100 mm × 100 mm × 500 mm were used. The specimen was placed in the Universal testing machine such that the load was applied to the upper most surface as cast in the mould. The test was conducted at the age of 28 days as per the standard procedure described in IS: 51613. The average result obtained on three specimens was taken as the representative flexural strength of the concrete. Table 9 presents the flexural strength of concrete. It is obvious from the Table 9 that concrete mix containing SF outperform all other concretes including controlled concrete in flexural strengths development at all the levels of replacement. Concrete mix containing RHA performs next to concrete mix containing SF. Concrete mix containing FA also shows improvement over flexural strength in comparison of control concrete mix. The increase in flexural strength of concrete containing admixtures is mainly due to the densification of transition zone of concrete and overall improvement in the homogeneity of concrete in comparison with controlled concrete. Therefore, the easiest economical way for enhancing flexural strength of conventional concrete up to 20 percent of 28-day value is incorporation of good quality of flyash in it.

Conclusion

Conventional Vis–a–Vis Mineral Admixed Concrete
The following important conclusions can be drawn from this experimental study:
  • Replacement of cement by SF and RHA reduces the slump of the fresh concrete in comparison of control concrete.
  • The increase of replacement levels of silica fume from 8% to 10% has not much significant effect on the development of compressive strength.
  • The replacement levels of rice husk ash have no significant effect on strength development and perform at par with control concrete.
  • The replacement levels of flyash have no significant effect on strength development and it performs nearly at par with control concrete.
  • Concrete mix containing mineral admixture such as flyash, silica fume and rice husk ash develops higher flexural strength than the conventional concrete without compromising compressive strength.
  • The most economicalconcrete mix with improved flexural strength for the construction of rigid pavements be produced by using 370 kg/m3 of cement and 40 kg/m3of flyash.

Acknowledgments

The kind permission of the Director, Central Road Research Institute, Mathura Road, New Delhi – India to publish this research work is highly acknowledged.

References

  • Nehdi M, Mindess S & Aitcin PC, Rheology of high-performance concrete: effect of ultrafine particles, Cement and Concrete Research, 28 (1988) 687.
  • Feng N, Shi Y & Hao T, Influence of ultrafine powder on the fluidity and strength of cement paste, Advances in Cement Research, 12 (2002) 89.
  • Mehta PK, Advancement in concrete technology, ACI Concrete International, 21 (1999) 69.
  • Aitcin PC & Neville A, High performance concrete demystified, ACI Concrete International, 15 (1993) 21.
  • Aitcin PC, Influence of condensed silica fume on the properties of fresh and hardned concrete; Condensed Silica Fume, Edited by PC Aitcin (Les Presses Del University de Sherbrooke, Quesec), 1983, 25.
  • Buil M, Paillere AM & Roussell B, High strength mortars containing condensed silica fume, Cement and Concrete Research, 14 (1984) 693.
  • Mehta PK, Condensed silica fume, Concrete Technology & Design, Vol.3, Cement Replacement Materials (Ed. R. N. Swamy) Surrey University press, 2000.
  • Obla KH, Hill R, Thomas MDA & Hooton RD, Properties of concrete containing ultrafine flyash, Proceedings of Fifth CANMET/ACI International Conference on Durability of Concrete, supplementary volume, Chairperson: Mohan Malhotra, Barcelona, Spain, Farmington Hill, MI 2001, 141.
  • Mehta PK & Pirtz D, Use of rice husk ash to reduce temperature in high-strength mass concrete, Journal of American Concrete Institute.75 (1978) 60.
  • Indian Standard Method of Physical Tests for Hydraulic Cement, IS:4031, 1988, Bureau of Indian Standards, New Delhi.
  • Annual Book of ASTM Standards, Concrete and Aggregates, Vol. 04.02, ASTM, Philadelphia, PA, 1999.
  • Indian standard specifications for coarse and fine aggregates from natural sources for concrete, IS: 383-1970, Bureau of Indian Standards, New Delhi.
  • Indian standard code of methods of tests for strength of concrete, IS:516-1959, Bureau of Indian Standards, New Delhi.

NBMCW September 2008

.....

Shear Strengthening of RC Beam by Exter...

Shear Strengthening of RC Beams

M.C. Sundarraja, M.E, and S. Rajamohan, M.E., Ph.D Associate Professor in Civil Engineering, Thiagarajar College of Engineering, Madurai

In recent years, several studies have been conducted to investigate the flexural strengthening of reinforced concrete (RC) members with fiber reinforced composite fabrics. Shear failure of RC members is disastrous and occurs with no advance warning of distress. In order to take full advantage of the ductility of an RC member, it is desirable to ensure that flexure rather than shear governs ultimate strength. A large number of structures constructed in the past have been found to be deficient in shear and needs strengthening. Deficiencies can be due to many factors such as insufficient shear reinforcement due to design errors or use of outdated codes, reduction in steel area due to corrosion, increase in demand of service load and construction defects. The main objective of this research work is to gain better understanding about the shear behavior of RC beams strengthened externally with bonded flexible glass fiber reinforced polymer (GFRP) strips on beam web. The present study encompasses some of the important parameters such as spacing of strips on shear span and inclination of GFRP strips both for retrofitted and rehabilitated concrete beams. Also the research work aims to study the failure modes, efficiency, strength gain, and deformability of strengthened beams.

Introduction

The use of reinforced concrete as a structural building material is well established in modern engineering design and construction. From earlier days, many materials have been employed as reinforcing agents for concrete including wood, steel, bamboo, and a number of synthetically manufactured composite materials. Recently, the use of high strength fiber-reinforced polymer (FRP) materials has gained acceptance as structural reinforcement for concrete. The FRP materials were initially developed for aerospace applications in the year 1940. There is an urgent need for a concrete reinforcement material that has high tensile strength and non-corrosive in saline environment. Since FRP has both these properties, research works have been done to use the material as primary reinforcement for concrete.

For each type of degradation there are various possible methods using new materials. However, engineers must exercise caution in applying new technologies to historic structures. There is a long history of misapplication of new materials, which have often caused more damage to historic buildings over long run. For example, in the repair of masonry monuments, preservation architects today view standard interventions of the 20th century, such as the use of Portland cement and reinforced concrete, as outdated and harmful to the historic masonry fabric. Engineers must take a long-term view and must consider the whole life design of an intervention, including the reversibility and the future repair of each intervention.

For historically significant structures, engineers must justify the use of materials that differ from the original fabric. In cases where materials decay regularly, it is generally accepted to replace by the same kind of material for the repair. The Eiffel Tower is a classic example of this kind of replacement. Every iron element of the tower are replaced at least once during its lifetime. In Gothic buildings, masonry pinnacles are replaced approximately every 200 years, using the same source of stone each time, and so is the case of King’s College Chapel. In such cases, there is no question of using new materials to repair elements which have decayed. It is more important to use the same materials and the same technology to maintain the structure, even if new materials have better mechanical properties.

New materials, such as FRP, are more appropriate in cases where there is a lack of strength in a historic structure. For example, FRP may be used to compensate for local decay of a single element in a historic timber roof truss. Similarly, a historic metal bridge may require strengthening to carry greater traffic loads. Because such strengthening methods may be irreversible in historic structures, engineers should carefully consider all the options before choosing the materials for repair.

The seismic resistance of historic masonry buildings is a special scenario to consider. Most earthquake engineers feel that historic buildings do not have sufficient ductility to resist a major seismic event. Many engineers would propose that a structure needs to be strengthened to improve its seismic resistance, but our knowledge of the seismic response of masonry buildings is limited at present. In the repair of historic monuments, FRP may offer some advantages including high strength and improved durability over metallic reinforcing. Such materials may also minimize aesthetic impacts in comparison to other materials. The long term stability of FRP in various moisture and temperature conditions is not tested and there is a need for additional research on the long-term compatibility of FRP with traditional materials.

FRP is a composite of two material groups: (1) reinforcing fibers; and (2) polymer resin matrix. Reinforcing fibers are generally made from glass, carbon, aramid, or its combinations. This study emphasizes the use of glass fiber reinforced polymer (GFRP) reinforcement exclusively. The material properties and failure response of FRP differ significantly from those of steel. A design methodology is required that reflects the material specific characteristics of the FRP reinforcement. To prevent a sudden brittle tensile failure of the FRP reinforcement, beams must be designed to be over reinforced relative to the balanced strain condition at ultimate (Nanni 1993). FRP has much potential as longitudinal reinforcement in concrete structures susceptible to reinforcement corrosion and stressed primarily in bending. Examples of such structural components include bridge decks, footings, floor slabs, and wall type structures (abutments, stems, and wing walls). In these members, flexural strength is essentially provided by the longitudinal reinforcement, and shear strength is provided by the concrete alone because of the lack of transverse (shear) reinforcement.

Several research studies have been done in upgrading rein forced concrete (RC) elements using fiber reinforced plastic (FRP) plates instead of steel-plates. Most of them deals with RC beams and slabs strengthened for flexure by bonding FRP plates or strips to their soffits. Recently, several researchers studied external shear strengthening of RC beams using FRP sheets. Shear strengthening of RC Beams by externally bonded side CFRP sheets (O.Chaallal, M.J.Nollet and D.Perraton) gave results showing the feasibility of using epoxy bonded strips to restore or increase the load carrying capacity in shear of RC beams. This method also reduce substantially shear cracking.

The overall objective this R&D work is to study failure mode, shear strengthening effect on ultimate force and load deflection behaviour of RC beams bonded externally with GFRP strips. Results of experimental studies on 1000mm long beam models strengthened with externally bonded GFRP strips are presented and discussed.

Scope of the Investigation

The purpose of this paper is to provide experimental data on the response of RC beams strengthened in shear using bidirectional GFRP fabrics. In this study, the RC beams were strengthened in shear with epoxy bonded GFRP sheets attached on the vertical sides in the shear region of the beam. Three sets of 1000mm long RC beams (five in total) having cross sectional dimensions of 100mm x 150mm were considered. The beam’s height was selected on the basis of shear strengthening with special regards given to anchorage of side strips; it therefore resulted in relatively stiff beam. The first set of beam was designed at full strength in shear and considered as control beam. The second and third set of beams were under-designed in shear, but were strengthened in shear to achieve the same shear strength as control beam using externally bonded side GFRP strips. The side strips were placed perpendicular to the beam’s longitudinal axis for two beams and at 1350 with respect to the beam’s longitudinal axis for the two beams of third set. All beams of these three sets had the same flexural reinforcing steel ratio.

The effect of shear strengthening is discussed with the help of experimental results. In this, the failure modes, efficiency, strength gain and deformability of strengthened beams were studied and analyzed.

Experimental Program Materials

Ingredients

Ordinary Portland cement (OPC)– 53 grade (Birla-super) was used for the investigation. It was tested for its physical properties in accordance with Indian Standard specifications. The fine aggregate used in this investigation was clean river sand, passing through 4.75mm sieve with specific gravity of 2.63. The grading zone of fine aggregate was zone II as per Indian Standard specifications. Machine crushed Blue Granite broken stone angular in shape was used as coarse aggregate. The maximum size of coarse aggregate was 20mm and specific gravity of 2.78. Ordinary clean portable water free from suspended particles and chemical substances was used for both mixing and curing of concrete.

Reinforcement

The longitudinal reinforcements used were deformed, hotrolled,high-yield strength bars of 10mm and 8mm diameter. The stirrups were made from mild steel bars with 6mm diameter. The yield strength of steel reinforcements used in this experimental program was determined by performing the standard tensile test on the three specimens of each bar. The average yield stresses of steel bars obtained were 390 N/mm2, 375 N/ mm2 and 240 N/mm2 for 10mm, 8mm and 6mm diameter respectively.

Concrete

For concrete, the maximum aggregate size used was 20 mm. The concrete mix proportion designed by IS method to achieve the strength of 20 N/mm2 and was 1:1.68:3.46 by weight. The designed water cement ratio was 0.55. Three cube specimens were cast and tested at the time of beam test (at the age of 28 days) to determine the compressive strength of concrete. The average compressive strength of the concrete was 29.11 N/mm2.

FRP Laminates

Glass Fiber

Properties of Glass Fiber
Glass Fiber composites are among the oldest and least expensive of all composites. E-glass is the most common type of glass fiber used in resin matrix composite structures and was used in this investigation. The principal advantages of E-glass are low cost, high tensile and impact strengths and high chemical resistance. The disadvantages of E-glass, compared to other structural fibers are lower modulus, lower fatigue resistance and higher fiber self-abrasion characteristics. In general, fiber composites behave linearly elastic to failure. The properties of the Glass Fiber supplied by the manufacturer are summarized in the Table 1.

Epoxy Resin

Properties of Glass Fiber
The success of the strengthening technique critically depends on the performance of the epoxy resin used. Numerous types of epoxy resins with a wide range of mechanical properties are commercially available. These epoxy resins are generally two part systems, a resin and a hardener. The resin and hardener are used in this study is Araldite GY 257 and Hardener HY 840 respectively. The properties of epoxy resin and hardener supplied by the manufacturer are summarized in Table 2.

Bonding Procedure

Bonding of Glass fiber fabric to beam

Before bonding the composite fabric onto the concrete surface, the shear region of concrete surface was made rough using a coarse sand paper texture and cleaned with an air blower to remove all dirt and debris. Once the surface was prepared to the required standard, the epoxy resin was mixed in accordance with manufacturer ’ s instructions. Mixing was carried out in a metal container (Araldite GY 257 – 100 parts by weight and Hardener HY 840 - 50 parts by weight) and was continued until the mixture was in uniform colour. When this was completed and the fabrics had been cut to size, the epoxy resin was applied to the concrete surface.

The composite fabric was then placed on top of epoxy resin coating and the resin was squeezed through the roving of the fabric with plastic laminating roller. Air bubbles entrapped at the epoxy/concrete or epoxy/fabric interface were to be eliminated. During hardening of the epoxy, a constant uniform pressure was applied on the composite fabric surface in order to extrude the excess epoxy resin and to ensure good contact between the epoxy, the concrete and the fabric. This operation was carried out at room temperature. Concrete beams strengthened with glass fiber fabric were cured for 24 hours at room temperature before testing.

Experimental Setup

A Two-point loading system was adopted for the tests. At the end of each load increment, deflection, ultimate load, type of failure etc., were carefully observed and recorded.

Results and Discussions

Properties of Glass Fiber
The test results were based on the value of load-deflection behavior. The load versus deflection curves are shown in Figure 1 to 5. A summary of the experimental results is presented in the Table 3 & 4. This includes loads and deflection at ultimate stages as well as modes of failure. Here ultimate stage is defined as the stage of loading beyond which the beam would not sustain additional deformation at the same load intensity.

In the following sections, results are discussed for each set of the study in terms of load deflection, cracking behavior and model failure.

In the case of retrofitted beams by EB-GFRP strips, initial cracking occurred at 37.5 kN for control beam, at 45 kN for the retrofitted beam with inclined strips and at 55 kN for retrofitted beam with vertical strips. After cracking, the stiffness of all models dropped but the reduction in stiffness was more pronounced in the control beams than in the retrofitted beams. Also, from the stiffness enhancement point of view, vertical strips were slightly more efficient than inclined strips.

Cracking in control beams started at mid span with vertical flexural cracks. Later, diagonal cracks developed and widened around 60 kN, introducing shear failure. In the retrofitted beam with vertical strips, failure was due to shear cracking, which produced a severe delamination along the middle two strips. The diagonal strips had forced the diagonal crack to bend, whereas the vertical strips inhibited the propagation of diagonal shear cracks.

Deflection Chart of Rehabilitated RC Beam

In the case of rehabilitated beams by EB-GFRP strips, initial cracking occurred at 37.5 kN for control beam, at55 kN for the rehabilitated beam with inclined strips and at 55 kN for rehabilitated beam with vertical strips. From the stiffness enhancement point of view, vertical strips were slightly more efficient than diagonal strips. In the rehabilitated beam with vertical strips, failure was due toshear cracking. In the case of rehabilitated beams with inclined strips, the failure was due to shear diagonal cracking & flexural cracks. The diagonal strips had forced the diagonal crack to bend, whereas the vertical strips inhibited the propagation of shear diagonal cracks.

Conclusion

Based on the investigation the following conclusions were made:
Flexural Cracks in Control Beam
  • For the retrofitted beam with vertical strips, mode of failure was shear cracking, which produces a severe delamination along the middle two strips
  • For the rehabilitated beam with vertical strips mode of failure was shear cracking
  • In the case of rehabilitated beams with inclined strips, the failure was due to shear diagonal cracking and flexural cracks
  • The diagonal strips had forced the diagonal crack to bend, whereas the vertical strips inhibited the propagation of shear diagonal cracks
  • From the stiffness enhancement point of view, vertical strips were slightly more efficient than diagonal strips
  • It is easier to maintain a relatively uniform thickness of epoxy resin through out the bonding length.
  • Restoring or upgrading beam shear strength using FRP side strips can result in increased shear strength and stiffness with substantial reduction in the shear cracking. Restoring beam shear strength using GFRP is a highly effective technique
  • Vertical side strips outperformed diagonal side strips for shear strengthening in terms of crack propagation, stiffness and shear strength.
Shear Cracks in Rehabilitated RC Beam

References

  • Antonio Nanni.(2001), “Relevant applications of FRP composites in concrete structures,” Proc., CCC 2001, Composites in construction, Porto, Portugal, Oct. 10-12, pp.661-670
  • A.Ghani Razaqpur, Burkan O. Isgor, S. Greenaway and Alistair Selley (2004), “Concrete Contribution to the Shear Resistance of Fiber Reinforced Polymer Reinforced Concrete Membersm,” Journal of Composites for Construction ASCE., pp452-460.
  • Bilal S. Hamad, Amal Y. Hage Ali and Mohamad H. Harajli (2005), “Effect of Fiber-Reinforced Polymer Confinement on Bond Strength of Reinforcement in Beam Anchorage Specimens,” Journal of composites for construction ASCE, pp 44-51.
  • Bimal Babu Adhikary and Hiroshi Mutsuyoshi (2004), “Behavior of Concrete Beams Strengthened in Shear with Carbon-Fiber Sheets,” Journal of Composites for Construction ASCE, pp258-264.
  • Chung-Yue Wang, Chien-Chih Shih, Shao-Chih Hong and Wei- Chih Hwang 2004), “Rehabilitation of Cracked and Corroded Reinforced Concrete Beams with Fiber-Reinforced Plastic Patches,” Journal of composites for construction ASCE, May-Jun, pp 219-228
  • Joseph R. Yost, Shawn P. Gross,and David W. Dinehart (2001), “Shear strength of normal strength concrete beams reinforced with Deformed GFRP Bars,” Journal of composites for construction pp 268-275.
  • Khalifa, A., Tumialan, G., Nanni, A. and Belarbi, A. (1999), “Shear strengthening of continuous RC beams using externally bonded CFRP sheets,” SP-188, American Concrete Institute, Proc., 4thInternational symposium on FRP for reinforcement of concrete structures (FRPRCS4), Baltimore, MD, Nov., pp. 995-1008.
  • Laura De Lorenzis, Antonio Nanni (2004), “International workshop on preservation of historical structures with FRP composites,” National Science Foundation (NSF) Arlington, VA 22230 USA,
  • O-Chaallal, M.J Nollet, D.Perraton (1998), “Shear strengthening of RC beams by externally bondedside CFRP strips,” Journal of composites for construction ASCE pp.111-113.
  • Pedro R. Salom, Janos Gergelyand David T. Young (2004), “Torsional Strengthening of Spandrel Beams with Fiber- Reinforced Polymer Laminates,” Journal of composites for construction ASCE, Mar-Apr ,pp 157-162
  • Tamer El Maaddawy1and Khaled Soudki (2005), “Carbon-Fiber- Reinforced Polymer Repair to Extend Service Life of Corroded Reinforced Concrete Beams,” Journal of composites for construction ASCE, pp187-194.
  • Theofanis D. Krevaikas and Thanasis C. Triantafillou (2005), “Masonry Confinement with Fiber- Reinforced Polymers,” Journal of composites for construction ASCE, Mar-Apr, pp 128-134
  • Zhichao Zhang and Cheng-TzuThomas Hsu (2005), “Shear strengthening of reinforced concrete beams using Carbon- Fiber-Reinforced Polymer Laminates,” Journal of composites for construction ASCE, pp 158-169.

NBMCW October 2008

.....

Realistic Cable Modeling in Prestressed...

Importance of Realistic Cable Models in the Design of Prestressed Concrete Structures

Importance of Realistic Cable Models in the Design of Prestressed Concrete Structures

Saleem Akhtar, Dr.K.K.Pathak, Dr. S.S.Bhadauria, Dept. Civil Engg., UIT, RGTU Bhopal CSD Group, AMPRI (CSIR) Bhopal.

Concrete is perhaps the most important construction material due to its various added advantages. The major problem with concrete is that it is very weak in tension. To strengthen the concrete to withstand tensile stresses, reinforced and prestressed concrete have been developed. In prestressed concrete, cable layout plays vital role in reducing tension from the concrete. Due to curvature cable exerts forces on the concrete to counterbalance the forces causing tension. Cables are laid as a continuous curve but for analysis purpose, they are modeled by some mathematical curve. The most important of them is the discrete parabolic modeling in different spans. Parabolic modeling simplifies the analysis to large extent but in this process cable geometry at the intermediate supports becomes discontinuous. To have more realistic models, we have used reverse parabolas and B-spline curves for cable modeling. These curves not only ensure the smooth profile but also provide realistic stress value. In this study, on comparing the results of these three different models, it is found that there is significant variation in stress values at intermediate supports, which clearly indicates the importance of realistic cable modeling.

Introduction

Spline curve
While analysing prestressed concrete beams in the text books 1-4, the prestressing cable is assumed to traverse parabolic profile. The reason being, for parabolic profile, curvature becomes constant and cable force can berepresented as an equivalent uniformly distributed load acting in the opposite direction to the working loads. Although, parabolic assumption simplifies analysis, thecable profile becomes discont–inuous at intermediate supports. Actual cable profile is like a smooth spline. In this study, prestressed concrete beams are analysed for parabolic, reverse parabolas and B-spline profiles (Figure 1). Using these methodologies, two prestressed concrete beams have been analyzed. Results are compared in terms of stresses and deflections at various sections of the beam. It is observed that cable profile plays vital role on the stresses and deflections hence it should be properly accounted for analysis and design purposes.

Cable Models

In this study, following three different types of cable modeling i.e. parabolic, reverse parabolas, and B-spline are considered.

(a) Parabolic Modeling

A general parabola may be defined as y = ax2 . . . . (1)

The approximate constant curvature of this curve may be defined as- K » d2y/dx2 = 2a . . . . (2)

In continuous prestressed concrete structures where cable profile changes its curvature several times, each time a new parabola has to be defined. This is accounted through load balancing approach, analytically and STADD software, numerically.

(b) Reverse Parabolic Modeling

This method was proposed by Lin and Burn4. In this reverse parabolas are used at the support to maintain smooth– ness of the cable. Due to fitting of multi parabolas and mathematical complexities in maintaining the continuity at the juncture of two parabolas, this approach is quite involved. Since this models the true profile, results are quite close to the exact ones.

(c) B-Spline Modeling

A B-spline is a typical curve of the CAD philosophy5-6. It models a smooth curve between the given ordinates. Braibant and Fleury7, Pourazady et al8, Ghoddosian9 etc., have used this curve in shape optimization problems. When a B-spline curve is used the geometrical regularities are automatically taken care of. Following are few important properties of B-spline curve.

(1) The curve exhibits the variation diminishing properties. Thus the curve does not oscillate about any straight line more often than its defining polygon.

(2) The curve generally follows the shape of the defining polygon.

(3) The curve is transformed by transforming the defining polygonal vertices.

(4). The curve lies between the convex hull of its defining polygon (Figure 2).

(5). The order of the resulting curve can be changed without changing the number of defining polygon vertices (Figure 3).

The curvature of B spline curve, K, in a finite element is calculated using vector calculus as described in next section.

Finite Element Formulation

In this study, linear elastic analysis of prestressed concrete structures is carried out. Nine node Lagrangean element and three node curved bar element have been used to model the concrete and the cable. Cable is assumed to be embedded in the concrete. The radius of curvature R in the element is given by- R = 1/K . . . . (3)

Forces transferred by the cable to concrete element is shown in Figure 4. Assume curvature between 1&2 and 2&3 as equal to the curvature at two Gauss points. Let the length of the cable between 1-2 and 2-3 be approximated as-

Importance of Realistic Cable Models in the Design of Prestressed Concrete Structures

. . . . (4)

where (x1 ,y1), (x2 ,y2) and (x3 ,y3) are the co-ordinates of 1,2 and 3.

The tension variation in the bar element can be expressed by isoparametric interpolation as-

Tn=å3i=1Ti Nci . . . . . (5)

Nci’s are shape functions of the cable element.

Using the formulae of vector calculus, normal and tangential forces Pn and Pt are calculated in the following manner.

where Tn is the tension in the cable, T is the tangent vector, r is the local co-ordinate and R is the radius of curvature.

The resultant of these forces is given by-

Importance of Realistic Cable Models in the Design of Prestressed Concrete Structures

. . . . . (8)

Their equivalent finite element nodal forces are worked out using virtual work theorem. Perfect bond between cable and the concrete has been assumed. The detailed formulation of this can be found in Ref.10. Using these formulations, a FE software PRES2D has been developed in FORTRAN program–ming language.

Numerical Examples

Spline curve
To compare the effects of parabolic, reverse parabolas and B-spline modeling of the prestressing cable, two PC beams are analysed using PRES2D software. Following data are used for the analysis purpose:
  1. Wobble coefficient = 1x10-5
  2. Young’s modulus (concrete) = 2x104 N/mm2
  3. Young’s modulus (steel) = 2x105 N/mm2
  4. Density (concrete) = 2500 Kg/m3
  5. Poisson’s ratio = 0.15
  6. Cross sectional area of cable = 1600 mm2 Negligible friction is considered in this study.

(a) Two Span Beam with Uniformly Distributed Loading

A two span beam of 30 m length and 300mm x 750mm cross section with 23 KN/m uniformly distributed load (including self– weight) and prestressing force of 938 KN has been analyzed by load balancing method with idealized parabolic profile, Lin’s method [Lin and Burns (1995)] with actual cable profile considering two parabolic segments at midspan and two reversed parabolic segments at the support and proposed B-spline model. The idealized parabolic model and realistic model is shown in Figure 5(a) & (b). Parabolic model is accounted through load balancing approach analytically and through STADD software numerically. For FE analysis using B-spline model, the beam is discretised into 20 plane stress nine noded elements making total of 123 nodes as shown in Figure 5(c). Stresses at various locations due to different approaches are given in Table 1. Negative sign indicates compressive stresses. Deflections at middle of span are given in Table 2. It can be seen that conventional parabola approach results are in close match with STAAD results while the results of Lin’s approach are in good match with B-spline approach; particularly at the supports. It can also be observed that the top fibre stresses are compressive while the bottom stresses are more compressive in case of Lin’s method when compared to conventional parabolic layout, which is also observed from B-spline results. Deflection at mid span is less in case of Lin’s cable layout as well as B-spline cable profile compared to conventional approach (Table 2). It should be noted that design based on parabolic model would be erroneous as it results in lesser tensile stresses than the actual ones. The analysis considering two parabolic segments at mid span and two reversed parabolic segments at support, is very complex. The proposed approach using B-spline model of the cable overcomes these difficulties.

(b) Two Span Beam with Concentrated Loading

Spline curve
A two span beam of 24 m length and cross section of 300mm x 600mm with centrally applied concentrated loads of 150 KN and prestressing force of 1200 KN is taken up for comparision (Figure 6a). The beam is analyzed using load balancing method with two parabolas, Lin’s approach using 6 parabolic segments, STAAD software and by our approach using PRES2D software. For FE analysis the beam is divided into 20 nine nodded plane stress elements making a total of 123 nodes as shown in Figure 6b. Stresses and deflections due to different approaches are given in Table 3 and Table 4. It can be observed that stresses obtained om parabolic modeling and STAAD Pro software are close because both use same formulation. Realistic cable modeling by B-spline gives stresses comparable to Lin’s approach. But stress analysis by 6 parabolic segments model is very complex as the very modeling of these parabolas istedious. Designbased on B-spline model will be more accurate. It can be observed that tensile stresses due to Lin’s approach are on higher side. Hence design based on it will be conservative. Deflections at arious sections due to different approaches come out to be approximately the same.

Conclusion

In this study, a novel approach to analyse prestressed concrete beams is presented. Cables are modeled as B-spline. To make it user–friendly, this approach is coded in a 2D FE software where concrete is modeled by 9 node Lagrangian and cable by 3 node curved bar elements. Using this software two representative problems are analysed and results obtained from various methods are compared. It is observed that realistic cable modeling should be carried out to obtain true stresses and deflections for design considerations. Stresses are found to be more sensitive to the cable model than the deflections.

References

  • Hurst MK. Prestressed Concrete Design, First Edition,Chapman and Hall, London (1988).
  • Gilbert RI. Time Effects in Concrete Structures. Elsevier New York (1988).
  • Jain AK. Reinforced Concrete- Limit State Design. Nem Chand & Bros. Roorkee, (1989).
  • Lin TY, Burns NH. Design ofprestressed concrete structures, Third Edition, John Wiley and Sons. Inc.,New York (1982)
  • Qing SB, Liu Ding Yuan. Computational Geometry-Curve and Surface Modeling. Academic Press London (1989).
  • Rogers DF, Adams JA. Mathematical Elements for Computers Graphics. Second Edition McGraw Hill New York (1990).
  • Braibant V, Fleury C. Shape optimal design using B-splines. Comp. Meth. Appl. Meth. and Engg, 44,247-267, (1984).
  • Pourazady M, Fu Z. An integrated approach to structural shape optimization. Computers & Structures, 60(2), 274-289, (1996).
  • Ghoddosian A. Improved Zero Order Techniques for Structural Shape Optimization Using FEM. Ph.D Dissertation Applied Mechanics Dept. IIT Delhi (1998).
  • Pathak, K.K., Gradientless shape optimization of concrete structures using artificial neural networks, Ph.D Thesis, Deptt. of Applied Mechanics, IIT Delhi, March 2001.

NBMCW August 2008

.....

Effect of Temperature on rubber latex m...

Effect of High Temperature on the Addition of Three Industrial Wastes as Pozzolanas and Rubber Latex on the Properties of Concrete

rubber latex modified tertiary blended concrete

Dr. K. B. Prakash, Professor of Civil Engineering, B.S.Krishnamurthy, Professor of Civil Engineering, K.L. Society’s, Gogte Institute of Technology, Belgaum

Extensive research work for decades is in progress throughout the globe in concrete technology in finding alternative materials which can partially or fully replace ordinary Portland cement (OPC) and which can also meet the requirements of strength and durability aspects. Amongst the many alternatives materials tried as partial cement replacement materials, the strength, workability and durability performance of industrial by products like flyash, blast furnace slag, silica fume, metakaoline, rice husk ash, etc., now termed as supplementary cementitious materials (SCM) are quite promising. Subsequently, these have led to the development of binary, ternary and tertiary blended concretes depending on the number of SCM and their combinations used as partial cement replacement materials. Binary blends contain one SCM, ternary blends contain combination of any two SCM and tertiary blends contain combination of any three SCM respectively. In this paper, an experimental investigation is carried out to assess the compressive strength of rubber latex modified tertiary blended concretes subjected to elevated temperatures of 1000C and 2000C for 24 hours duration. From the experimental investigation, it can be concluded that rubber latex modified tertiary blended concretes subjected to 1000C or 2000C with 30% replacement of cement by tertiary blends (FA+SF+BFS) in proportions of (10+10+10) show a better resistance to temperature.

Introduction

Human safety in case of fire is one of the major considerations in the design of buildings. It is extremely necessary to have a complete knowledge about the behavior of all construction materials before using them in the structural elements[1]. Concrete is a non-homogeneous material consisting of hardened cement paste and aggregates. With an increase in temperature, cracking is initiated due to thermal incompatibilities between the aggregates and the hardened cement paste[2].

Developments in 1990s have seen a marked increase in the number of structures involving the first time heating of concrete. These include nuclear reactor pressure vessels, storage tanks for hot crude oil and hot water, coal gasification and liquefaction vessels which are subjected to elevated temperatures of up to 2500C. The extensive use of concrete as a structural material in all the above mentioned structures necessitated the need to study the behavior of concrete at high temperature and its durability for the required needs.[3]

A number of research workers in various countries have investigated the effect of elevated temperature on the residual strength of concrete. In most of these investigations the main variable, as far as heating regime is concerned, has been the maximum level of temperature to which the concrete is subjected, the value generally ranging from 1000C–9000C[3]. Research work has also been carried out to evaluate the performance at elevated temperature on blended cement concretes containing supplementary cementitious materials (SCM) as partial replacement to ordinary Portland cement. These SCM include industrial by products like flyash [4], silica fume [5], ground granulated blast furnace slag [6], metakaoline[7] etc.

Research works have indicated that the fire resistance of concrete is highly dependent on its constituent materials, particularly the pozzolanas. Experimental investigations also indicate that the addition of silica fume highly densifies the pore structure of concrete, which can result in explosive spalling due to the build up of pore pressure by steam. It is also investigated that the addition of flyash or ground granulated blast furnace slag enhances the fire resistance of concrete. It is reported that the compressive strength of flyash concrete retained higher strengths than the pure OPC concrete at higher temperature upto 6500C[1]. Research work has been carried out on three high strength concrete mixes incorporating silica fume, flyash and ground granulated blast furnace slag independently subjecting to a maximum temperature of 9000C and tested under loaded conditions. It is observed that ground granulated blast furnace slag concrete showed the best performance followed by flyash and silica fume concretes. Similarly, research work has also been carried out to evaluate the performance of metakaoline concrete at elevated temperature upto 8000C. The research findings has indicated that after an increase in compressive strength at 2000C, the metakaoline concrete suffered a more severe loss of compressive strength and permeability related durability than the corresponding silica fume, flyash and OPC concretes at higher temperatures.

Research Significance

Not much published research work is available on tertiary blended concretes containing rubber latex and subjected to elevated temperature. Hence this experimental investigation was carried out to investigate the performance of rubber latex modified tertiary blended concretes subjected to elevated temperature of 1000C and 200 C.

Experimental Programme

rubber latex modified tertiary blended concrete
In this paper, an attempt is made to study the effect of elevated temperature on the properties of rubber latex modified tertiary blended concrete. In the experimentation, three supplementary cementitious materials viz. flyash (FA), silica fume (SF) and blast furnace slag (BFS) are used in different proportions to replace the cement. The proportions of (FA+SF+BFS) adopted are (0+0+0), (5+10+15), (10+10+10), (15+5+10), (20+5+5), (25+2.5+2.5), and (30+0+0) respectively. 53 grade OPC was used in the experimentation. Coarse aggregates of 12mm and down size having a specific gravity of 2.95 and locally available sand with a specific gravity of 2.59 and falling in Zone-II were used. To impart workability a sulphonated naphthalene polymer based superplasticizer was used at the rate of 1% by weight of cement and was obtained from Fosroc company. The flyash used in the experimentation was obtained from Thermal Power Plant, Raichur (Karnataka) having chemical composition as shown in Table 1.

rubber latex modified tertiary blended concrete
The silica fume was obtained from Elkem laboratories, Navi Mumbai possessing chemical composition as shown in Table 2. The blast furnace slag was obtained from Steel Authority of India Ltd., Bhadravati (Karnataka) having chemical composition as shown in Table 3. The rubber latex was obtained from Rubber Board, Kottayam (Kerala). It was centrifuged rubber latex with 60% dry rubber content (DRC).

The mix design was carried out for M25 grade concrete as per IS: 10262 -1982[8] which yielded a proportion of 1:1.23:2.41 with a w/ c ratio of 0.45.

The cement, sand and coarse aggregates were weighed according to the proportion of 1:1.23:2.41 and dry mixed. Before mixing, 30% of cement was replaced by (flyash + silica fume + blast furnace slag), according to the proportions such as (0+0+0), (5+10+15), (10+10+10), (15+5+10), (20+5+5), (25+2.5+2.5), and (30+0+0) respectively. The required amount of water was added to this dry mix and intimately mixed. The calculated quantity of superplasticizer was now added and mixed thoroughly. After this, 2% rubber latex (by weight of cement) was added to the mix and the entire concrete was agitated thoroughly to get a homogeneous mix. Then the mix was placed layer by layer in the moulds to cast the specimens. The specimens were prepared both by hand compaction as well by imparting vibrations through vibrating Table. The specimens were finished smooth and kept under wet gunny bags for 24 hours after which they were cured for 28 days. After 28 days curing, the specimens were placed in the heating chambers.

rubber latex modified tertiary blended concrete
For assessing compressive strengths of reference mix and rubber latex modified tertiary blended concretes at elevated temperatures, standard cube specimens of 150 x 150 x 150mm were cast. All the specimens were placed in the heating chambers for 24 hours and subjected to elevated temperatures of 1000C and 2000C respectively. After 24 hours, all the specimens were removed and cooled to room temperature and then tested for the compressive strength as per IS:519-1959[9].

Experimental Results

rubber latex modified tertiary blended concrete
Table 4 and table 5 show the results of the effect of rubber latex modified tertiary blended concretes with different proportions of (FA+SF+BFS) subjected to elevated temperature of 1000C and 2000C respectively. Figure 1 and Figure 2 show the variation of compressive strength in the form of bar graphs.

Observations and Discussions

It is observed that the rubber latex modified tertiary blended concrete with (10+10+10) proportions of (FA+SF+BFS) subjected to elevated temperature of 1000C show a higher compressive strength as compared with reference mix subjected to elevated temperatures of 1000C. The percentage increase the compressive strength as compared to reference mix is about 16.00%.

It is also observed that the rubber latex modified tertiary blended concrete with (10+10+10) proportions of (FA+SF+BFS) and subjected to elevated temperature of 2000C show a higher compressive strength as compared to reference mix subjected to elevated temperatures of 2000C. The percentage increase in the compressive strength as compared to reference mix is about 15.79%.

The increase in the compressive strength may be due to the fact that the rubber latex modified tertiary blended concrete with (10+10+10) proportion of (FA+SF+BFS), changes the morphological structure of concrete which can resist the temperature in a better way. The flyash and blast furnace slag added in concrete may accommodate the pressure created by temperature rise with minute air bubbles and silica fume added imparting the strength.

Effect of Temperature on rubber latex modified tertiary blended concrete

Conclusion

It can be concluded that 30% replacement of cement by (10+10+10) proportions of (FA+SF+BFS) in rubber latex modified tertiary blended concrete subjected to elevated temperature of 1000C or 2000C for 24 hours show a better resistance to temperature as compared to other proportions.

Acknowledgment

The authors would like to express their deep sense of gratitude and sincere thanks to the Board of Management and Principals of both the engineering colleges for all the encouragement and cooperation given in conducting the experimental investigation. The authors also sincerely thank all the research contributors whose research publications have been referred in the preparation of this paper.

References

  • Chi-Sun Poon, Salman Azar, Mike Anson, Yuk-Lung Wong, “Performance of metakaolin concrete at elevated temperature,” Cement & Concrete Composites 25(2003) pp.83-89.
  • Xu Y, Wong Y.L, Poon C.S and Anson M. “Influence of PFA on cracking of concrete and cement paste after exposure to high temperature,” Cement and Concrete Research, 33(2003) pp. 2009-2016.
  • Potha Raju M, Janaki Rao A, “Effect of temperature on residual compressive strength of flyash concrete,” The Indian Concrete Journal, May 2001, pp.347-350.
  • Potha Raju M, Shobha M and Rambabu K. “Flexural strength of flyash concrete under elevated temperature,” Magazine of Concrete Research, 2004, 56. No.2, March, pp.83-88.
  • Saad M, Abo-El-Enein S.A, Hanna G.B and Kotkata M.F. “Effect of Temperature on Physical and Mechanical Properties of Concrete containing Silica Fume,” Cement and Concrete Research, Vol.26, No.5, 1996, pp.669-675.
  • Ramlochan T, Thomas M.D.A and Hooton R.D. “The effect of pozzolans and slag on the expansion of mortars cured at elevated temperatures,” Cement and Concrete Research, 34(2004),pp. 1341-1356.
  • Bai J, Wild S. “Investigation of the temperature change and heat evolution of mortar containing PFA and metakaolin,” Cement and Concrete Composites, 24(2002), pp.201-209.
  • IS:10262-1980: “Recommended guide lines for mix design.”
  • IS:519-1959: “ Method of testing for concrete strength.”

NBMCW October 2008

.....

Strength Properties of Steel Fibre Rein...

An Investigation on the Strength Properties of Steel Fibre Reinforced Concrete Produced with Glass Powder as Pozzolana

Dr. K.B. Prakash, Professor, B.R. Patagundi, Asst. Professor, K.L.E.Society’s College of Engineering & Technology, Belgaum.

It has been estimated that several million tones of waste glasses are generated annually worldwide. The key sources of waste glasses are waste containers, window glasses, window screen, medicinal bottles, liquor bottles, tube lights, bulbs, electronic equipment etc. Only a part of this waste glass can be used in recycling. The remaining waste glass cannot be used for any purposes. But recently the research has shown that the waste glass can be effectively used in concrete either as glass aggregate (as fine aggregate or as coarse aggregate) or as a glass pozzolana. The waste glass when grounded to a very fine powder shows some pozzolanic properties. Therefore, the glass powder to some extent replaces the cement and contributes for the strength development. In this experimentation, an attempt has been made to study the characteristic strength properties of steel fibre reinforced concrete produced by replacing the cement by waste glass powder in various percentages like 0%, 10%, 20%, 30%, and 40%.

Introduction

Almost every industry produce waste irrespective of the nature of their products. The effective disposal of these wastes is a challenging task ahead. In olden days, such wastes were used as land fill materials for the low-lying areas. Waste generation and its disposal in landfill sites are unsustainable. The industrial wastes like flyash, silica fume, blast furnace slag etc. and other wastes like solid waste, waste plastics, waste glass, waste tiles and other agricultural wastes are causing the environmental pollution in one or the other way. The efficient safe disposal or efficient recycling is one of the challenging tasks ahead of engineers.

The concrete industry, to some extent is making use of many of these industrial wastes effectively in the production of concrete. For example the use of industrial wastes like flyash, silica fume and blast furnace slag in concrete can act as pozzolana and replace a part of cement. The pozzolanic reaction adds to the strength of concrete and also results in savings of cement. Thus nowadays the cement industries are making use of flyash, silica fume, and blast furnace slag as pozzolanas to replace a part of cement.1

It has been estimated that several million tones of waste glasses are generated annually worldwide2 The key sources of waste glasses are waste glass containers, window glasses, & glass screen, medicinal bottles, liquor bottles, tube lights, bulbs,electronic equipment etc. Only a part of this waste glass can be used in recycling. The remaining waste glass cannot be used for any purposes. But recently the research has shown that the waste glass can be effectively used in concrete either as glass aggregate (as fine aggregate or as coarse aggregate) or as a glass pozzolana.3 The waste glass when grounded to a very fine powder shows some pozzolanic properties. Therefore, the glass powder to some extent can replace the cement and also contribute for the strength development.

Post consumer and other waste glass types are a major component of the solid waste stream in many countries and most is currently landfilled.3 Alternatively, waste glass could be used as a concrete aggregate, either as a direct replacement for normal concrete aggregates (low value) or as an exposed, decorative aggregate in architectural concrete products (highvalue). Expansive alkali silica reactions (ASR) can occur between glass particles and cement paste, particularly in moist conditions and with high alkali cements. This reaction is not confined to glass aggregates but can occur when ever aggregates contain reactive silica. However, it is now fairly well accepted that by controlling the reactive silica, cement alkali level and moisture, the reaction can be reduced or totally mitigated.4,6

Finley ground glass has the appropriate chemical composition to react with alkalis in cement (pozzolanic reaction) and from cementitious products. The pozzolanic properties are likely to be derived from the high SiO2 content of glass. Powdered glass used in combination with Portland cement contributes to strength development.5 Various suppres– sants can minimize ASR of glass aggregate concrete. Pulverised fuel ash (Pfa) and metakaolin (MK) can completely eliminate ASR.2

Experimental Programe

Oxide Content for Waste Glass
In this experimentation, an attempt is made to study the characteristic strength properties of steel fibre reinforced concrete produced by replacing the cement by waste glass powder in various percentages like 0%, 10%, 20%,30%, and 40%.

In the experimentation 43 grade OPC, locally available sand and coarse aggregates were used. The specific gravity of sand was found to be 2.61 and was Zone II sand.The specific gravity of coarse aggregate was found to be 2.92. The coarse aggregate used were of 12mm and down graded size. To impart workability, a super plasticizer Conplast SP 430 was used at the rate of 0.7% by weight of cemntitious materials. The glass powder was obtained by crushing waste glass pieces in a cone crusher mill. The glass powder passing through 600 micron was used for the experimentation. The chemical composition of glass powder is shown in Table 1. The different percentage replacement of cement by waste glass powder used in experimentation were 0%, 10%, 20%, 30%, and 40%.

The mix design was carried out for M20 grade of concrete by IS 10262:19827, which yielded a mix proportion of 1:1.29:3.11 with a water cement ratio of 0.44.

The specimens were prepared according to the mix proportion and by replacing cement by glass powder in different proportion. The entire mix was dry mixed by adding1% steel fibres (by volume fraction) and then water was added (w/c = 0.44) along with super plasticizer at a dosage of 0.7% (by weight of cement). The entire mix was homogeneously mixed and specimens were cast.

Strength Test
To find out the compressive strength, specimens of dimensions 150X150X150mm were cast and Tested under compressive testing machine of capacity 2000 KN as per IS 516:19598. To find out tensile strength, the cylindrical specimens of dimensions 150mm diameter and 300mm length were cast. Split tensile strength was obtained by testing the specimens on CTM of capacity 2000 KN as per IS 5816:19999. To find out the flexural strength, the specimens of dimensions 100mm x 100mm x 500mm were cast. Two point loading was adopted on an effective span of 400mm and tested as per IS 516:19598. For impact strength test, the specimens were of dimension 150 mm dia and 60 mm height. Drop weight test was adopted for testing impact specimen. They were kept in Shrudder’s impact testing machine and the hammer weighing 4.54 kg was dropped from a height of 457 mm. Number of blows required tocause first crack and final crack were noted down. Impact energy is calculated by the following formula.

Impact energy = WhN (N-m)

Where,

W=Weight of ball in N = 45.4N
h=Height of fall in metres = 0.457m
N= Number of blows required to cause first crack or final failure as the case may be.

Strength of SFRC

Test Results

Tables 2, 3, 4, 5 and Fig 1, 2, 3, 4 show respectively the compressive strength, tensile strength, flexural strength and impact strength test results of steel fibre reinforced concrete when the cement is replaced by waste glass powder in different proportions.

Discussions on Test Results

Glass Powder and Cement before mixing

Mixing of Glass Powder and Cement
It has been observed that the higher compressive strength for SFRC can be obtained when 20% cement is replaced by glass powder. The percentage increase in compressive strength for 20% replacement by waste glass powder is found to be 5.77%. It is observed that the compressive strength of SFRCincreases linearly from 0% replacement to 20% replacement of cement by waste glass powder and thereafter it decreases continuously. At 30% and 40% replacement of cement by waste glass powder, it is observed that the compressive strength strikes lower than the reference mix.

Similar trends are observed for tensile strength, flexural strength and impact strength. The percentage increase in tensile strength, flexural strength and impact strength for 20% replacement of cement by waste glass powder are found to be 15.95%, 17.37%, and 51.59% respectively.

This may be due to the fact that 20% replacement of cement by glass powder may give rise to maximum pozzolanic reaction and it may act as strong filler material, thus contributing towards more strength.

Conclusions

The following conclusions may be drawn based on experimental observations.
  • Steel fibre reinforced concrete produced by replacing 20% cement by waste glass powder gives higher compressive strength, tensile strength, flexural strength and impact strength.
  • In general, waste glass powder can be used effectively as pozzolana in steel fibre reinforced concrete tosave the consumption of cement.

Acknowledgment

The authors would like to thank the management authorities of KLES’s College of Engineering and Technology, Belgaum for their kindsupport. The authors also thank Principal Dr. S.C. Pilli and HOD, Civil Engineering Department Dr.V.V.Karajinnifor giving all the encouragement needed which kept the enthusiasmalive.

References

  • Suryawanshi C.S., “Use of industrial and domestic waste in concrete”, Civil Engineering & Construction Review, Feb 1999.
  • Byars E. A, Morales B and Zhu H. Y., “Waste glass as concrete aggregate and pozzolana – laboratory and industrial projects”, Concrete, Vol.38, No.1, January 2004, pp 41-44.
  • Byars E A, Morales B and Zhu H Y, “Conglasscrete I”, www.wrap.org.uk.
  • Baxter S. Jin W and Meyer C, “Glasscrete–concrete with glass aggregate” ACI Materials Journal, Mar -April, 2000, pp 208-213.
  • Albert Tang, Ravindra Dhir, Tom Dyer, and Yongjun, “Towards maximizing the value andsustainable use of glass”,Concrete Journal, Volume 38, January 2004, pp.38-40.
  • Byars E A, Morales B, and Zhu H Y, “Conglasscrete II”, www.wrap.org.uk.
  • IS 10262:1982, “Recommended guidelines of concrete mix design”.
  • IS 516:1959, “Method of tests for strength of concrete”.
  • IS 5816:1999, “Splitting tensile strength of concrete method of test.”

NBMCW August 2008

.....

Concrete Suitable for Higher Temperatur...

Concrete Suitable for Higher Temperature

J. Prasad, Associate Professor; A.K. Ahuja, Associate Professor, and M. Shahiq Khan, Deptt. of Civil Engg., Indian Instt of Technology (IITR), Roorkee.

Concrete structures need repair and rehabilitation many a time as they get damaged due to excessive stresses due to unforeseen forces caused by man-made or natural disasters. But there are few situations in practice, where concrete is expected to show signs of damage such as cracks during its service period itself. If precautions are not taken while designing or constructing such concrete structures, they may also demand repair and rehabilitation work. However, with careful design and construction such situations can be avoided. Present paper enlists and recommends various prevention measures which should be incorporated while designing a concrete mix to be used for structural elements which will be subjected to high temperature exposure, thus causing damage during its design life.

Introduction

Pozzolanic concretes are used extensively throughout the world; the oil, gas, nuclear and power industries are among the major users. The applications of such concretes are increasing day by day due to their superior structural performance, environmental friendliness, and energy-conserving implications [1]. Although the concrete in such structures is always provided with adequate insulation, nevertheless, it is difficult to inspect the insulation and there is always a possibility of its deterioration. Also, problems with the cooling system in nuclear power plants may lead to hazardous situations where the concrete gets overheated. The exposure of concrete to elevated temperatures has a direct effect on its compressive, tensile and flexural strength [2, 3]. One of the main reasons for loss of strength in concrete at elevated temperature is the formation of cracks between cement paste and aggregate because of thermal incompatibility between the two ingredients [3]. Present paper reports the results of a study on compressive strength of pozzolanic concrete as well as ordinary concrete after being heated to elevated temperature up to 2000C. Since cyclic temperature changes occur in nuclear reactors and can lead to further decrease in concrete strength [4], the experimental program is expanded to also include the effect of cyclic heating on the strength of concrete.

Experimental Programme

The effect of high temperature on the compressive strength of concrete has been thoroughly explored in the present study. The strength of ordinary and pozzolanic concretes incorpo– rating silica fume and ground granulated blast furnace slag at 2000C temperature for heating cycle ranging between 0 to 28 days are compared. For this purpose, different concrete mixes are obtained mixing blast furnace slag and silica fume with 43-Grade Ordinary Portland Cement (OPC). To start with, various tests are conducted on cement to determine its physical properties as per recommendations of IS: 4031-1999 and the test results are listed in Table 1.

While making concrete, water– cement ratio is maintained as 0.45 by weight and cement content as 396 kg/m3 in all the cases. Various concrete mixes are prepared by replacing Ordinary Portland Cement (OPC) 43-Grade with silica fume and ground granulated blast furnace slag (GGBFS) at a 5%, 10% and 20% by weight of cement.

Physical Properties of Ordinary Portland Cement

GGBFS is processed from granulated slag, a cementitious material that consists of same constituents, which are also present in cement. It has more than 90% glass content or calcium alumino silicates. The physical and chemical properties of GGBFS and micro-silica are given in Table 2.

Physical & Chemical Properties of GGBFS and Micro Silica

The physical and chemical characteristics of both micro-silica and GGBFS satisfy the requirement as given in IS: 15388-2003 for micro-silica and IS: 12089-1987 for GGBFS.

The cube specimens of 150 mm size are cast in steel moulds in three layers and compacted on a vibrating table. A total of seventy–two cubes are prepared. The specimens are kept in the moulds for 24 hours, after which they are de-moulded and placed in a water curing tank for 28 days. The specimens are then dried and heated in an electric oven with a maximum temperature of 2000C for 7, 14, 21 and 28 heating cycles and tested under compression to failure after cooling. Each heating cycle consists of a heating period of 8 hours after the oven temperature reaches 2000C and subsequent cooling period of 12 hours as shown in Figure 1.

Experimental Results

To understand the behavior of heated concrete specimens under varying conditions considered in the present investigation, the results obtained are represented in graphical form as given below.

Time-Temperature curve

Compressive Strength of Silica Fume Concrete

The failure load under compression resisted by an average of the test concrete cubes in the same batch is considered for the calculation of compressive strength. The compressive strength of tested specimens is shown plotted against the heating cycles in Figure 2. It is observed that silica fume concrete gives better result in comparison to plain concrete for all heating cycles. It is also noticed that the maximum compressive strength occurs at 7- day heating cycles for all cases of concrete. Compressive strength of 10% and 20% silica fume concrete (on an average) is 38% and 28% respectively higher than plain concrete. It is due to the fact that higher percentage of silica fumes with same water–cement ratio increases porosity inside the concrete and also porosity increases with increasing temperature. Hence, it is better to use lower percentage of silica fume i.e. around 10% only to achieve better compressive strength when concrete is exposed to heating cycles with highest temperature of around 2000C.

Compressive Strength of GGBFS Concrete

Compressive strength of Slag Concrete
As in the case of silica fume concrete, ground granulated blast furnace slag (GGBFS) concrete also gives better result in comparison to ordinary concrete for all heating cycles as shown in Figure 3.

It is observed that the compressive strength of ordinary and 20% GGBFS concrete is maximum at 7-day heating cycles after which it decreases, whereas in 5% and 10% GGBFS concrete, strength increases as the heating cycle increases. Compressive strength of all percentage slag concretes are almost equal at 7-day heating cycles with their values around 21% higher than that of ordinary concrete. At 28-days heating cycle, whereas 5% and 10% GGBFS concretes result in 32% and 36% higher strength respectively, compressive strength of 20% ground granulated blast furnace slag concrete is only around 10% more than that of plain concrete. Hence, 10% GGBFS is the optimum quantity of slag for replacement of cement by weight for concrete which is to be exposed to elevated temperature up to 2000C for different cycles.

Comparison of Strength of Concrete

Concrete Suitable for Higher Temperature
Figure 4 compares the performance of 10% pozzolanic concrete with ordinary concrete. Comparison for20% replacement of cement by pozzolanic material by weight is shown in Figure 5. It is observed that whereas both types of pozzolanic concrete show better compressive strength than ordinary concrete, silica fume concrete shows better results than slag concrete.

Conclusions

The following conclusions are drawn from the experimental investigation reported here in.
  1. The pozzolanic concretes show better performance at elevated temperature for different heating cycles than ordinary concrete.
  2. Compressive strength of 10% and 20% silica fume concrete on an average is 38% and 28% respectively higher than plain concrete.
  3. Compressive strength of all percentage slag concretes are almost equal at 7-day heating cycles with their values around 21% higher than that of ordinary concrete.
  4. At 28-days heating cycle, whereas 5% and 10% GGBFS concretes result in 32% and 36% higher strength respectively, compressive strength of 20% ground granulated blast furnace slag concrete is only around 10% more than that of plain concrete.
  5. Both silica fume based and ground granulated blast furnace slag based concretes can safely be used for construction of structural elements which are expected to be subjected to higher temperature during their service life. Whereas both types of pozzolanic concrete show better compressive strength than ordinary concrete, silica fume concrete shows better results than slag concrete.

References

  • Mehta, P.K., “Advancements in Concrete Technology,” Concr. Int. 96(4) (1999), pp. 69-76.
  • Blundel, R., Diamond, C. and Browne, R., “The Properties of Concrete Subjected to Elevated
  • Temperatures,” Construction Industry Research and Information Association, Report No. 9, London, (1976), pp. 78-87.
  • Ghosh, S. and Nasser, K.W., “Effect of High Temperature and Pressure on Strength and Elasticity of Lignite Flyash and Silica Fume Concrete,” ACI Material Journal, V. 93, No.1 (1996), pp. 51-60.
  • Bertero, V. and Polivka, M., “Influence of Thermal Exposure on Mechanical Characteristics of Concrete,” Proceedings of International Seminar on Concrete for Nuclear Reactors. ACI SP-34 (1972), pp. 505-531.
  • India Standards, IS: 4031-1999, “Indian Standard Methods of Physical Tests for Hydraulic Cement,” Bureau of India Standards, New Delhi.
  • India Standards, IS: 8112-1989, “Specification for 43 grade Ordinary Portland Cement,” Bureau of India Standards, New Delhi.
  • India Standards, IS: 1 2 0 8 9 - 1 9 8 7 , “Specification for Granulated Slag for Manufacture of Portland Slag Cement,” Bureau of India Standards, New Delhi.

NBMCW June 2008

.....

Comparative Studies of Different Cement...

S.K. Agarwal and L.P. Singh, Scientists, EST Division, Central Building Research Institute, Roorkee

The compressive strength of four cements (Three PPC and one OPC) was determined according to different international standard, BIS, BS, ASTM and Sri Lanka. The 28 days strength of one PPC is 53 MPa, the other two shows 48MPa when tested as per BIS. The 43 grade OPC also shows 50 MPa at 28 days. However, when tested as per BS & ASTM methods, the 28 days strength is approx 32.5 Mpa and 27 MPa in all the cases.

Introduction

In the last two decades, the quality of cement has improved considerably, due to improvement in the cement manufacturing technology. Cement conforming to 43 grade or 53 grade are giving much higher compressive strength as recommended in respective BIS codes 8112 and 12269 [1,2]. Different countries have their own standards for testing cement. Different international standards have fixed consistency (Srilanka=0.40, BS-12=0.400, ASTM =0.485). However, according to BIS normal consistency varies 0.27 to 0.33 depending upon the fineness of cement. In BIS method normal consistency has to be determined on daily bases. One study involving 53 grade OPC has been done [3].

Purpose of the present study is to compare compressive strength of different cement as per different standards, so that it can be mentioned when cement is exported to other countries. Further, the study will also help in minimizing errors and time saving BIS method once accepted by BIS. A fixed value of normal consistency can be introduced like other standards. Further export of these cements will be easy once these cements have been tested as per different standards. In the present study four cements (3 Portland Pozzolana and 1 OPC) was taken. The brands of cements are not disclosed.

Materials

Cement

Three portland pozzolana cements and ordinary portland cement was purchased from the local market. Their physical and chemical properties are given in Table I. The results have been compared with BIS 1489-91[4].

Comparative Studies of Different Cements with Different Standards

Consistency

Normal consistencies of all the four cements were determined as per BIS [5].

Casting of Cubes

70 mm cubes were cast as per BIS. After 24 hours, cubes were demolded and kept in water at 27+20C. Cubes were also cast at 0.400 and 0.485 normal consistencies according to BS and ASTM methods.

Compressive Strength

Compressive Strength of cube was determined at 1, 3, 7 and 28 days as per BIS methods [6]. The results are summarized in Table 2.

Result and Discussion

Comparative Studies of Different Cements with Different Standards
It is clear from Table 2 that Compressive Strength of all the four Cements is different at 1, 3, 7 and 28 days, when tested as per different codes of practice existing in different countries.

Comparing Strength data of 1, 3 and 7 day for three blended Cements, Cement-I is more than Cement 2 and 3. The higher earlier strength can be attributed to fineness of cement I. It is known that the short term activity depends on the specific surface area of the pozzolona, where as the long-term activity depends on the chemical and mineralogical composition of pozzolana. The pozzolana activity indices are highly correlated with the specific surface areas of flyashes. Further, fineness of flyashes is one of the most important physical properties affecting pozzolanic reactivity.

At early ages fineness plays an important role in the strength of mortars via a micro–filler or a cement dispersion effect or both.

In the present case, the surface area of Cement I is more compare to cement 2 and 3 showing there by higher initial strength compare to cement 2. This can be attributed to higher CaO content and less IR. However, the surface area of both the cements (2 and 3) is nearly same. The 28 days strength of C1, C2 & C3 according to BIS corresponds to 53 MPa and 48 MPa. However, when compared to BS or ASTM standards these cements are approx 32.5 MPa and 27 MPa. This is approx 35% and 40% less compare to BIS values.

The present study indicates that Indian cements are showing much lower strength, when tested as per BS/SriLanka and ASTM methods. Since blended cements especially portland pozzalana cement is available in abundance in the country with 20-30% pozzolana content. In order to ensure high performance of concrete in marine/ aggressive environment by using PPC, the proportions of cementitious materials should be much more than those generally used in blended cement in our country.

This study is important from the point of view for cement manufacturing units exporting cement as per survey by 2010, 60% cement will be blended. Their characterization will help in export.

At early ages fineness plays an important role in the strength of mortars via a micro – filler or a Cement dispersion effect or both.

In the present case, the surface area of Cement I is more compare to Cement 2 and 3. Showing there by higher initial strength compare to Cement 2.this can be other build to higher Cao Content and less IR.However, the surface area of both the cement is nearly same. The 28 day strength of C1, C2 & C3.according to BIS corresponds to 53 MPa as 43 Mpa. However, when compared to BS or ASTM standard these Cements are approx 32.5Mpa and 27 MPa. This approx 35% and 40% less compare to BIS.

The present study indicates that Indian Cements are showing much lower strength, when tested as per BS/Srilanka and ASTM method. Since blended Cements especially portland pozzalana Content. In order to ensure high performance of concrete In order to ensure high performance of concrete in marine/ aggressive environment by using PPC the proportions of cementitious materials should be much more than those generally used in blended Cement in our country.

This study is important from the point of view for Cement manufacturing units exporting Cement.

Conclusion

It is concluded from the present study that Cement claming 53 Mpa strength for PPC according to BIS are showing approx 32.5 and 27 Mpa when tested as per BS/ASTM method.

Acknowledgment

This paper is part of ongoing R&D work in Central Building Research Institute and is published with the permission of Director, CBRI, Roorkee, India.

References

  • Specification for 43 grade Ordinary Portland Cement BIS 8112 (2005). Manak Bhavan New Delhi.
  • Specification for 53 grade Ordinary Portland Cement BIS 8112 (2005). Manak Bhavan New Delhi.
  • Dordi, C.M., Concrete for the Millennium Advance in Cement Technology Ed. S.N. Ghosh Tech Books International p.421 (2002).
  • Specification for Portland Pozzolana Cement, BIS 1489 (Part I) Feb 2004, Manak Bhavan New Delhi
  • Specification for physical test for hydraulic Cement BIS 4031 (Part 4); Determination of consistency of standard Cement Paste. (2005), Manak Bhavan, New Delhi.
  • Specification for Physical Test for hydraulic Cement BIS 4031 (Part 6)–Determination of Compressive Strength (2005). Manak Bhavan, New Delhi.

NBMCW October 2008

.....

Behavior of Prestressed Concrete Beams ...

Urmil V. Dave, Associate Professor, and Kinjal H. Trambadia, M. Tech Student Civil Engineering Department, Institute of Technology, Nirma University Ahmedabad

Comparative behavior of Prestressed concrete (PSC) beams subjected to two point loadings in terms of failure load, deflection and failure modes is evaluated. Effect of Glass Fiber Reinforced Polymer (GFRP) strengthening on PSC beams before and after first cracking is measured. Experiment includes testing of twelve simply supported PSC beams having cross-section 150 mm x 200 mm with effective span of 3.0 meter. Four unwrapped PSC beams, four PSC beams wrapped by GFRP after initial loading up to first crack and four un cracked PSC beams strengthened using GFRP are tested up to failure.

Four different wrapping patterns are executed on beams. For (2L/7) & (2L/ 6) span loadings, wrapping of full length at bottom and up to 1/3rd of depth is provided, forming a U-shape around the beam cross-section. For (2L/4) span loading, wrapping of full length at bottom and up to 1/3rd of vertical depth is provided and extra wrapping near the supports is provided. For (2L/3) span loading, U shape wrapping is provided near the supports, for full depth.

It is observed that in (2L/7) & (2L/6) span loadings, compared to unwrapped PSC beams, the FRP wrapping along longitudinal direction, reduces deflections and increases the load carrying capacity for wrapped PSC beams. In (2L/4) span loading, combination of vertical and horizontal GFRP sheets, together with a proper epoxy adhesion, lead to increase the ultimate load carrying capacity for wrapped PSC beams. In (2L/3) span loading, presence of vertical GFRP sheets near support reduces the shear effects considerably and increase load carrying capacity.

Introduction

Modern structural engineering tends to progress toward more economical structures through gradually improved methods of design and the use of higher strength materials. Such developments are particularly important in the field of reinforced concrete; the limiting features of ordinary reinforced concrete have been largely over come by the development of prestressed concrete. A Prestressed concrete member can be defined as one in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from the given external loading are counteracted to a desired degree. Concrete is basically a compressive material. Prestress applies a precompression to the member that reduces or eliminates undesirable tensile stresses that would otherwise be present. Cracking under service loads can be minimized or even avoided entirely. Deflections may be limited to an acceptable value.

Design of PSC Beams

PSC beams are designed using limit state theory using IS-1343 (1984). All beams have same cross-section of 150 x 200 mm with effective length of 3 m. Requirement of reinforcement is evaluated by applying prestressing force of 85.47 kN in case of all the beams. Six high tensile 4mm Φ prestressing wires and 8mm Φ @ 300 c/c stirrups are used as lateral reinforcement in beams. Two no of 10mm Φ bar at top and bottom respectively is used as non prestressing reinforcement. Specimen detailing is shown in Figure 1.

Behavior of Prestressed Concrete

Development of Prestressing Facilities

Prestressing system exclusively developed for this research work is shown in Figure 2, Screw jack is provided for prestresing of HTS wire and for fixing of HTS wires, Wire locking barrel is sed. The screw jack along with moving anchor block is fixed to the jacking end side (stretching end). The steel plate mould is fabricated and put into place with bolted end plates having holes for pretensioning wires to pass through.

Materials

Selected proportion for M45 grade concrete mix is cement: sand: grit: kapchi: water 1:1.36:2.21:0.55:0.37. 4 mm HTS wires are used for prestressing Tension test for the wire is performed on UTM and its ultimate strength is found 2350 N/mm2. Figure 3 shows the load deformation relationship for HTS wires. GFRP laminate is used for wrapping on beams.

Four PSC beams are tested up to first cracking. They are then strengthened using Glass Fiber Reinforced Polymer (GFRP) laminates and after tested upto failure. Four PSC control beams immediately after the required curing and strengthened using GFRP laminates are then tested. Different types of strengthening arrangement are provided based upon different span loadings. Figure 3 shows the different type of strengthening arrangements provided.

Testing of Beams

Behavior of Prestressed Concrete
Total 12 PSC beams are cast and tested by applying two point loading. Four PSC beams are tested with four loading positions mentioned as below. Four PSC beams are tested up to first cracking, after that provided with wrapping and are tested up to failures which are denoted by PSCWFC1, PSCWFC2, PSCWFC3 & PSCWFC4 respectively. Four un cracked PSC beams are wrapped and tested, who are denoted by PSCW1, PSCW2, PSCW3 & PSCW4 respectively. Four different types of loading positions i.e. 2L/7, 2L/6, 2L/4 & 2L/3 span loadings respectively are employed for testing of beams and are denoted with notations 1, 2, 3 & 4 respectively. As shown above 1, 2, 3 & 4 generally written after category of specimen exhibit the loading span employed for that category of specimen.

Results and Discussions

The overall comparison is obtained between the experimental observations and theoretical calculations and results for Failure Load and Load-Displacement for different span loadings in case of all the beams are given in Table 1. Basic beam theory is applied for calculating deflection and failure load in case of all the beams.

Behavior of Prestressed Concrete
Behavior of Prestressed Concrete

Failure Load

Table1 as well as Figure 6 give the comparison of experimental and theoretical values of failure loads for all the beams. Load carrying capacity of unwrapped PSC beams is observed significantly less compared to the wrapped PSC beams. Higher load carrying capacity is observed in case of un cracked wrapped PSC beams compared to PSC beams wrapped after loading up to first visible crack. The experimentally observed failure loads are higher than the capacity of the beam evaluated theoretically. Table 1 gives the percentage increase in failure load in case of all the wrapped PSC beams compared to unwrapped PSC beams tested at different span loadings obtained experimentally. The percentage increased in failure load for PSCWFC and PSCW beams are 17.24%, 16.67%, 15.78% and 25%, and 20.68%, 30%, 39.47% and 40.91% respectively as compared to unwrapped PSC beams for different span loadings as shown in table 1.

Load-deflection

Maximum displacements at the time of failure load in case of all the specimens are given in Table 1. These are observed at the centre of the beams during the testing by using LVDT (Linear variable displacement transducer). Figures 7 a), 7 b), 7 c) & 7 d) represent the load– displacement relationship for all the beams tested at 2L/7, 2L/6, 2L/4 and 2L/ 3 loading spans respectively. From Figures. 7 a) to 7 d), it can be seen that as the span loading increases, the tendency of the beams to deflect under lesser loading decreases. This is primarily due to the reduction in the moment caused by the point loads. It is apparently shown by Figures that the load carrying capacity of the beam increases as the loading span increases. But actually, it is the reduction in the moment that causes the beam to deflect lesser as the span increases. Because of the increase in loading span, the load required inducing same amount of moment increases, and hence for the same loading the deflection observed is lesser than the load for the smaller loading span.

Behavior of Prestressed Concrete

Failure Mode

Catastrophic failure i.e. failure with large sound is observed in wrapped beams during the experiments due to the sudden failure of the fibers in tension. Specimens also fail due to the de-bonding of the fibers from the concrete surface. Figures . 8 a), 8 b), 8 c), & 8 d) show mechanism behind failure of the PSC beam wrapped after first crack. Here load is given in the form of push, due to which bottom fibers are in tension. Due to this, bottom fibers of the beam get de-bonded in tension as shown in Figures. 8 a) & 8 b) respectively, as the tension carrying capacity of the wrapping system is more than its bonding capacity.

Concluding Remarks

  • In PSC beams, due to the high tensile strength tendons present in the tension zone and are bonded well with the high strength concrete of grade M45, the pretension in the tendons prevents the widening of the cracks at the earlier stages.
  • The highest failure load is observed in wrapped uncracked PSC beams compared to other beams. This is due to the transfer of stresses from concrete to the fibres through bonding, thus preventing the beam to fail even after the strain in concrete crosses the allowable limit. The wrapping acts as an external reinforcement and takes complete tensile load after the concrete fails to transfer effectively the stresses to the tendons/ reinforcement.
  • Theoretical failure load is higher compared to experimentally observed failure load due to assumption of linear stress distribution in the side reinforcement. But in reality very few amount of side fibers at extreme bottom faces take part during the loading. Parabolic stress distribution is generally observed in case of the fibers.
  • The wrapping also helps the PSC beams to take up more shear load compared to unwrapped PSC beams. This is clear from the fact that the beams loaded with higher span loadings in anticipation of shear failure have actually failed in flexure.
  • Failure modes observed in all unwrapped PSC specimens are closely matching with expected flexural & shear failure. De-bonding of bottom fibers and tension failure of side fibers are observed as failure mode in case of all wrapped beams.
  • Also in case of wrapped beams, the failure takes place primarily due to bond failure i.e. de-bonding between concrete surface and FRP and/or due to tension failure of the FRP sheets. This avoids the local failure or failure of the concrete in compression.

References

  • Patrick X. W. Zou, “Flexure Behavior and Deformability of Fiber Reinforced Polymer Prestressed Concrete Beams,” Journal of composites for construction, Nov. 2003, Vol. 7, No. 4, g. 275-284
  • S.K. Padmarajaiah & Ananth Ramaswamy, “Flexural strength predictions of steel fiber reinforced high-strength Concrete in fully/partially prestressed beam specimens,” Journal of Cement and Concrete Composites, May 2004, Vol.26, No. 4, Pg.275-290.
  • Sydney Furlan Junior, “Prestressed fiber reinforced concrete beams with reduced ratios of shear Reinforcement,” Journal of Cement and Concrete Composites, Nov. 1999,Vol. 21, No. 3, Pg. 213-221.
  • N. F. Grace, G. A. Sayad & A.K. Soliman, “Strengthening reinforced concrete beams using fiber reinforced polymer Laminates” ACI Structural Journal, Oct. 1999, Vol. 96(5)
  • Thanasis C. Triantafillou, “Shear strengthening of reinforced concrete beams using epoxy-bonded FRP composites,” ACI Structural Journal, Mar. 1998, Vol.95, No.2.
  • Raafat EI-hacha & Mark F. Green, “Flexural behavior of concrete beams strengthened with prestressed carbon fibre reinforced polymer sheets subjected to sustained loading and low temperature,” Canadian Journal of Civil Engineering, Apr. 2004, NRC research lab., Pg.239-252.
  • Hakan Nordin & Bjorn Taljsten, “Concrete Beams strengthened with Prestressed near surface mounted CFRP.” Journal of Composites for construction, Jan.-Feb. 2004, Vol.10 No. 1.
  • Kan Nordin, “Flexural Strengthening of Concrete Structures with Prestressed Near Surface Mounted CFRP Rods,” Department of Civil Engineering, Lulea University of Technology, May 2003.
  • N Krishna Raju, “Prestressed Concrete,” Third Edition Tata McGraw-Hill Publishing Company Limited, New Delhi, 1995.
  • Anthony J. Wolanski, B. S. “Flexural behavior of Reinforced and Prestressed concrete beams using Finite element analysis,” Marquette University, May 2004.
  • IS: 1343-1980, “Code of Practice for Prestressed Concrete,” first revision, Bureau of Indian Standards, New Delhi, 1981.

NBMCW January 2008

.....

Self Compacting Cocrete

Self Compacting Cocrete

Dr. S.C. Maiti, Ex–Joint Director, National Council for Cement &amp; Building Materials, New Delhi. Raj K Agarwal, Managing Director, Marketing &amp; Transit (India) Pvt. Ltd, New Delhi.

Introduction

Concrete mixtures having high workability and high cohesiveness will be self–compacting concrete. The self–compacting concrete (SCC) is defined as a flowing concrete that can be transported without any segregation and placed without the use of vibrators to construct concrete structures free of honeycombs. Initially such concrete was developed by Japanese researchers. For such concrete which is specially required for heavily reinforced sections, a viscosity modifying agent (VMA) is required along with a polycarboxylic ether (PCE) based superplastisizer. Because of high fluidity, SCC requires higher fines content, in order to resist bleeding and segregation. Natural fine aggregate together with manufactured sand and mineral admixture {flyash or ground granulated blast furnace slag (ggbs) or silica fume} provide higher fines contents in the concrete mix. A cohesive SCC is thus produced in order to flow steadily in the heavily reinforced concrete sections, without any segregation & bleeding.

Materials and Mix Proportions

Besides cement, water and aggregates, the necessary ingredients for producing SCC are superplasticizers (PCE based), viscosity–modifying agents and mineral admixtures e.g. flyash, ground granulated blast furnace slag & silica fume. The proportion of fine aggregates required is higher, may be around 55% and the corresponding proportion of coarse aggregate (generally of smaller size, say 10 or 12 mm maximum size) will be around 45%. The mineral admixtures and fine sand (manufactured sand) are required to make the highworkability concrete mix cohesive.

Typical concrete mix proportions for high strength (74.5 MPa at 28 days) SCC used by Gettu & others (from Spain) (1) are as follows:
  • Cement (OPC-53 grade) = 428 Kg/ m3
  • Water = 188 l/ m3
  • Flyash (2935 cm2 / gm) = 257 Kg / m3
  • Superplasticizer (vinyl copolymer) = 7.9 Kg / m3
  • Sand (crushed limestone ) (0-5mm) = 788 Kg / m3
  • Coarse aggregate (gravel) (5-12mm) = 736 Kg / m3
The water / binder ratio of the concrete mix is 0.27. A look at the materials & mix proportions indicate use of smaller size coarse aggregate (12mm maximum size) & the shape is rounded, being gravel aggregate. In fact, crushed gravel will be a better option in order to obtain high- workability and highstrength SCC.

The workability measured for the above mix is “ slump flow” of 48cm. In our country, still we are carrying out the usual slump test even for high–workability concrete mix. The “flow test” as specified in IS 9103 (2) can be conducted for testing such high–workability concrete mix, but the “slump flow test” will be better than the “flow test,” as no lifting (15 times in 15 seconds) of concrete is necessary, as the SCC is a flowing concrete mixture.

Vachhani and others (3) used SCC in the prestigious Delhi Metro construction. Concrete mix proportions for M-35 grade of SCC are as follows :
  • Cement = 330 Kg / m3
  • Water = 163 l / m3
  • Flyash = 150 Kg / m3
  • Superplasticizer = 3.12 l / m3
  • VMA ( glenium stream 2 ) = 1.3 l / m3
  • Retarder ( Pozzolith 300 R) = 0.99 l / m3
  • Sand = 917 Kg / m3
  • Coarse aggregate:
  • 20mm maximum size = 455 Kg / m3
  • 10 mm maximum size = 309 Kg / m3
Vachhani and others (3) highlighted the mechanism of self compaction, which is based on :
  1. Large quantity of “fines” (500 to 650 Kg / m3),
  2. Use of high –range water – reducing superplasticizers (with water- reduction of 25%), and
  3. The use of Viscosity Modifying Admixtures.
“Fines” includes cement, flyash and the part of sand of size less than 0.125 mm. This together with water & chemical admixtures constitute the paste in the concrete mix. The paste makes the concrete mix cohesive and controls the segregation–resistance of the mix. The polycarboxylic ether–based superplasticizer ( presently being imported) generally provides water–reduction of the order of 30-40 % in the concrete mix. The VMA improves the segregation–resistance of the mix without changing the fluidity or workability. The retarder in the concrete mix controls the workability–retention, which is specially important in hot climate.

Fresh Self–Compacting Concrete

The characteristics of fresh SCC are fully described by the following properties :
  1. Filling ability–ability to completely fill all the spaces in the formwork,
  2. Passing ability–ability to flow around reinforcement, and
  3. Segregation resistance–ability to resist segregation of materials during transportation and placing.
Consequently new test methods have been developed to test SCC in the fresh state. The “filling ability” is tested by “ slump flow” and “ V funnel,“ the “passing ability” is tested by “L- Box“ and“ U–Box“ and the segregation–resistance is tested by “ V- funnel”

Hardened Self–Compacting Concrete

The properties and characteristics of hardened SCC do not greatly differ from those of normal concrete, except that SCC can not be used in mass concrete construction using bigger size aggregates, say 75mm or 150mm sizes. Because such concrete always needs to be compacted with needle vibrators, in order to compact thoroughly in the forms.

Any required compressive strength of SCC can be achieved. Vachhani & others (3) obtained 28- day compressive strength of 44-49 MPa in the above–mentioned concrete mix proportions for M-35 grade concrete, for the Delhi Metro construction.

The high – strength SCC can be called “ High–performance concrete,” as such concrete has denser microstructure with lower inherent “porosity” and “permeability,” because of lower water- cementitious materials ratios and use of mineral admixtures in concrete.

Concrete Mix Proportioning Approach

The Self–Compacting Concrete, because of its high–workability and cohesiveness, generally needs higher fines content and lower size (10 or 12 mm maximum size) of coarse aggregate. Smoother and rounded or semi- rounded (may be crushed gravel) coarse aggregate will develop cohesiveness in the concrete mix. Bapat’s (4) suggestion is good. Flakiness & elongation indices of coarse aggregate should be less than 15% each. Large quantity of fines is also required–500 to 650 Kg/m3 of concrete, & therefore crushed stone fine aggregate is also required along with natural fine aggregate. Flyash has also been used as an essential ingredient of SCC. In India, 30 to 50% flyash has been used in SCC. Originally Japanese people (5) suggested water–powder ratio between 0.90 & 1.1 (by volume). But it is the paste that controls the segregation of the concrete mix. The powder & the paste includes finer ( less than 0.125mm) part of the fine aggregate. Vachhani (3) & Bapat (4) used about 35 to 36% paste to produce self compacting concrete. The viscosity modifying agent also controls the segregation– resistance of the concrete mix.They are generally starch, cellulose & gum–based. Preferable & satisfactory VMA is “Welan Gum.” The quantity of such VMA required in SCC is very less, about 0.1% by weight of cementitious materials.

Prof P.K. Mehta (6) included “Welangum,” silica fumes & ultrafine colloidal silica under the list of VMA. Gum or cellulose– based material is capable of modifying the viscosity of SCC, but the silica fume may not be able to modify the viscosity of concrete. Subramanian and Chattopadhyay (7) observed that micro silica at an appropriate dosage may be beneficial in reducing the dosage of “Welan gum.”

The following mix proportioning steps for SCC can be followed.
  • The target 28-day compressive strength of concrete can be calculated first based on standard deviation value used for the specified grade of concrete.
  • The water–cementitious materials ratio can be decided based on the target 28–day compressive strength of concrete. This can be in the range of 0.30 – 0.50, 0.30 for a 28 day compressive strength of about 90 MPa, while 0.50 for a 28 day compressive strength of about 30 MPa .
  • For the high – workability concrete mix, the water content of concrete will be in the range of 180 – 190 l/m3 of concrete.
  • The maximum size of aggregate for SCC is more or less fixed at 10 or 12 or 16 mm.
  • The sand (natural + manufactured) content can be kept at about 55% & the coarse aggregate content can be about 45%, by weight of total aggregate.
  • The superplasticizer required is PCE–based and about 1% by weight of total cementitious material. The cementitious material includes ordinary Portland cement, flyash /ggbs & silica fume (in case of high strength concrete). For normal strength concrete (say from M-25 to M-50), no silica fume will be required, but about 20 to 30 % good quality flyash will be required. If ggbs is used in place of flyash, its percentage can be 40 to 50 %, by weight of total cementitious material. For high strength concrete of M-60 to M-80, about 10% silica fume will be required instead of flyash or ggbs. The dosage of super plsticizer & the viscosity modifying agent can be fixed based on one or two trial mixes in a laboratory.
  • With the above details in hand, concrete mix proportions for any grade of SCC can be arrived at.

Conclusions

The self compacting concrete, a high workability cohesive concrete mix needs polycarboxylic ether–based superplasticizer and a viscosity modifying agent.

The proportion of fine materials in the concrete mix is also higher than that of normal concrete mixes. Therefore, in addition to natural fine aggregate, manufactured sand and mineral admixture eg flyash, ggbs or silica fume is also to be used.The percentage of fine aggregate is around 55%, while that of coarse aggregate is around 45%, by weight of total aggregate. Smaller size of coarse aggregate (10,12 or 16mm maximum size) having soother surface texture (rounded or crushed gravel) is required for concrete to flow smoothly in the formwork. For normal “standard” concrete grades of M-25 to M-50, about 20 to 30 % flyash or 40 to 50 % ggbs can be used, whereas for high strength self – compacting concrete of grades M-60 to M-80, 10 % silica fume will be required.

References

  • Gettu,R, Izquierdo, J, Gomes, P.C.C & Josa, A. Development of high – strength self- compacting concrete with flyash: a four – step experimental methodology. 27th conference on OUR WORLD IN CONCRETE & STRUCTURES : 29 – 30 August 2002, Singapore, pp.217 – 224.
  • IS 9103. Specification for concrete admixtures. Bureau of Indian Standards, New Delhi.
  • Vachhani, S.R, Chaudary, R & Jha, S.M. Innovative use of self compacting concrete in Metro construction. I.C I Journal, Vol. 5, No 3, Oct – Dec 2004, pp.27 -32.
  • Bapat, S.G, Kulkarni, S.B & Bandekar, K.S. Self- compacting concrete in nuclear power plant construction. I.C.I Journal, Vol-6, No 3, Oct- Dec 2005, pp- 37- 40.
  • Okamura,H, Ozawa,K & Ouchi,M. Selfcompacting concrete. Structural Concrete, Vol-1, No1, March 2000.
  • Mehta,P.K & Monteiro,P.J.M. Concrete-Microstructure, Properties & Materials. Third edition, 2006, Tata McGraw –Hill Publishing Co Ltd, New Delhi, p.478.
  • Subramanian, S & Chattopadhyay, D. Experiments for mix proportioning of Self – compacting concrete. The Indian Concrete Journal, Jan 2002, pp. 13 – 20.

NBMCW November 2007

.....

Engineering of Self Compacting Concrete

Subrato Chowdhury, &amp; Sandeep Kadam, UltraTech Cement Limited, Andheri (East), Mumbai
Engineering of Self Compacting Concrete

Cementitious material is the lifeline of modern infrastructure. Increasing demand for concrete in newer applications leads to engineer the properties of concrete at fresh and hardened state.

One of the most important performance criteria for concrete is the fluidity at fresh state. Appropriate fresh state properties are achieved by engineering suitably the theology of concrete. Such engineering is achieved by incorporating chemical & mineral admixtures into cementitious system. The development of self-compacting concrete is primarily achieved by designing the appropriate theology using different cementitious system, admixtures, etc.

Self-compacting (or consolidating) concrete (SCC) is a particular concrete mix which has a special performance requirement of self–consolidation or compaction at the time placement. However, at the hardened state, there is not much difference in terms of mechanical properties and durability between SCC and other type of concrete mixes viz. high performance concrete (HPC), normal strength concrete (NSC), etc.

The important aspects of achieving the functional requirements (filling ability, passing ability and resistance to segregation) of SCC are related with:
  • Appropriate characterization of ingredients
  • Mix proportion
  • Mixing method
  • Placement
This paper would discuss the effect of characteristics of individual ingredients, different approaches for mix proportioning and the mixing method on the overall performance of the SCC mix in fresh state, especially on its theology. The effect of method of placement, especially in terms of the pressure exerted on the formwork will also be discussed.

Introduction

Concrete is a suspension of aggregates in cement paste (1). A suspension is self-flowing if it flows under its own weight. Additionally, it is to ensure–uniform suspension of solid particles during casting and thereafter until setting (2). The above perspective induces the definition of self-compacting (or, consolidating) concrete (SCC), as a concrete mix, which in fresh state, has the ability to fill the formwork and encapsulate reinforcing bars only through the action of gravity i.e. self-weight at the time of placement without any external energy inputs from vibrators, tampering or similar actions and with maintained homogeneity at the time of placement (3). SCC can be used in most application where traditional vibrated concrete, such as conventional normal strength concrete (NSC), high performance concrete (HPC) is used. Two principal advantages of SCC are improved homogeneity of fresh concrete that leads to more durable concrete at hardened state as well as higher productivity in terms of pouring of concrete, and improvement in working condition and less noise pollution (4, 5).

The difference between the SCC and vibrated concrete exists in the performance requirements during fresh state; not much in terms of properties at harden state such as strength, durability. SCC is engineered to fill all the space within the formwork passing through the reinforcements or other obstruction without segregation. This attributes to three important functional requirements related to workability of the concrete mix: filling ability, resistance to segregation, and passing ability (3). Filling ability is the high fluidity and deformability to ensure adequate flow under selfweight. Resistance to segregation is the ability of the particle suspension (in fresh state) to maintain homogeneity throughout the mixing, transportation, and placement process. Passing ability is the ability to pass obstacles, narrow opening and closely spaced reinforcement bar without getting blocked by interlocking of aggregate particles (3). Filling ability and passing ability of a fresh concrete mix depend on its fluidity and resistance of segregation on the homogeneity. Additionally, the paste or mortar has to deform well too. The yield stress and plastic viscosity generally characterizes such theological behavior of fresh concrete mix. Fluidity is inversely proportional to the yield stress, while plastic viscosity has direct proportionality on homogeneity. Contact and collision between aggregates as well as the interparticle friction increase with the decreases in relative distance between aggregates particles in the concrete mix, resulting in the blockage of aggregate particles (6). Limiting coarse aggregate volume increases inter-particle separation and reduces the inter-particle friction and collisions resulting in minimization of the blockage leading to improvement in passing ability.

The increase of paste volume with emphasis to low water powder ratio (w/p) in presence of compatible chemical admixtures further strengthens the fluidity and helps in attaining homogeneity. Adequate homogeneity improves viscosity of the mix, which in turn enhances the segregation resistance. An optimum balance between fluidity and viscosity is the key to achieve efficient selfcompacting characteristics of the concrete mix at fresh state. In SCC, the powder contains binder component consisting of ordinary Portland cement (OPC), mineral admixtures like flyash along with/ without filler material like limestone powder, dolomite etc. To achieve moderate plastic viscosity and low yield value, multiple chemical admixtures are required. Special chemical admixture like viscosity modifier admixture (VMA) is used for controlling the viscosity of the mix and superplasticizer for lowering the yield stress. In addition, the characteristics of fine and coarse aggregates play very important role on the yield stress of the mix.

Overview of SCC

The work on SCC had started in 1988 in Tokyo University, Japan. The Japanese concept spread through Asia and to Europe around 1993 (7). This concept is well accepted in USA now. A few points are important with regard to engineering of structures using SCC mix to satisfy the intended specification.

These are:
  1. characterization of the ingredients
  2. mix proportion technique to achieve desired characteristics
  3. mixing method
  4. effect of method of placement, especially the pressure exerted on formwork.
The above points are deliberated in the following sections of the paper.

Characterization

Ingredient characterization exhibits different aspects depending upon the background of the users. These concepts range from that of the scientist, who thinks of it in atomic terms, to that of the concrete technologists, who thinks of it in terms of properties of concrete in fresh and harden state, procedure of construction and quality assurance, etc. Characterization of an ingredient deal with those features of the material like composition, structure, etc that are significant for a particular preparation, study of properties or use etc. The three basic functional requirements of SCC mix at fresh state, i.e. filling ability, passing ability and resistance to segregation could be assessed in terms of the theological characteristic like yield stress and plastic viscosity.

An appropriate ingredient characterization helps to achieve the performance behavior of SCC at both fresh and hardened states. General-purpose Ordinary Portland Cement (OPC) is suitable to be the main cementitious constituent for SCC. It is also well–established that compatibility between superplasticizer and OPC plays important role on the rheological characteristic of mortar. Certain chemical compounds of OPC clinker such as alkali (Na2O, K2O); sulphate (SO3) has significant influence on such compatibility (8). The presence of mineral admixtures has a definite role on the performance of paste, especially format ion of micro mortar. The micro-mortar formations is involved with all particles below the size of 125¼, chemical admixture and water (6). Flyash is commonly used mineral admixture in SCC. Particle size distribution of flyash, chemistry of flyash and presence of un-burnt coal particles has enough impact on fluidity and deformability of mortar for SCC (10). The bulk solid volume of the fly ash also has significant impact on the rheology. Low lime content flyash improves the fluidity of the paste (9). The flow value increased as the bulk solid volume of flyash is increased (9, 10). High belite content OPCwas used at the initial years of SCC without any application of VMA (3). However, high alite content high strength OPC may be desirable for achieving high strength SCC along with appropriate replacement level of OPC by mineral admixture.

The compatibility of multiplechemical admixtures present along with mineral admixtures needs a serious attention towards satisfactory performance of rheological properties as well as hydration kinetics that has bearing on hardened properties.

The fine aggregate is one of the major components of paste formations. Well-graded fine aggregate is desirable. The size of coarse aggregate in SCC is 5 to 20 mm. However, the size of the aggregate is decided based on the size of the opening such a spacing of reinforcement bar (1). Larger the aggregate size more the driving force for flow would be required. Blocking will occur if the maximum size of the aggregate is large as well as the content of the larger size aggregates is high. Crushed stone aggregates require more paste volume for nonblockage criteria compared with the natural gravels. Higher packing density of aggregates reduces demand of superplasticizer (1, 2). Extensive works on characterization of ingredients like OPC, fly ash and fine aggregates for SCC were carried out by authors and are published elsewhere. (8,10,11,12).

Ingredients for self-compacting concrete shall satisfy the respective codal specifications. Findings of the works, on characterization of ingredients, carried out by authors are summerised here.

Ordinary Portland Cement

Clinkers may have different levels of alkali and sulphate concentrations, but the corresponding OPC shows fairly the same levels of sulphate owing to addition of gypsum during grinding process. Alkali and sulphate content of the clinker not that of cement binder, has influence on the rheology of mortar for SCC.

Initial flowability and viscosity of mortar mixes are not influenced by the alkali and sulphate content of the clinkers irrespective of the dosages of flyash. The initial flowability decreases and viscosity increase with elapsed time for all the cement replacement levels and types of OPC.

Low sulphate content of clinker increases the flow ability and reduces viscosity irrespective of alkali content. Alkali content of clinkers has similar trend of effect on flowability and viscosity but this influence is not as significant as that of sulphate. OPC from low sulphate bearing clinkers and cement replacement level of 50% and above by flyash is vulnerable to the risk of segregation. Low sulphate content increases the filling ability of concrete mixes.

Flyash

The flow of the mortar is affected adversely with flyashes having higher percentage of particle size above 90ì, and the mortar becomes unfit for the purpose. The flow is enhanced with fly ashes having higher percentage of particle size below 45ì.

Flyash with high lime and sulphate content is not suitable for producing SCC as it decreases the flow and increases the viscosity; non-cohesiveness of the mortar is also increased significantly. Flyash with higher LOI, i.e. the higher carbon content, is not a suitable mineral admixture for SCC mortar. It affects the rheology adversely making the mix highly viscose aswell as non-cohesive.

Higher quantity of fly ash could result in adjustment of chemical admixture to lower dosages for achieving appropriate flow and viscosity of mix. Flyash of appropriate characteristics acts as flow enhancing and viscosity reducing agent in SCC mortar. Increase in flyash quantity neutralizes the negative impact of high sulphate and high alkali content of OPC clinker as well as the size fraction of fine aggregates on the rheology of mixes. Though quantity of flyash does not significantly influence the initial spread and viscosity of mixes, its increase in value helps in retention of higher spread diameter and lower viscosity.

Fine Aggregate

Initial viscosity of mortar mixes is influenced by the size fraction of fine aggregate. The finer fraction of sand reduces flowability and increases viscosity of mortar mix. Lower quantity of fines in fine aggregate accentuates the possibility of segregation.Ingredients characterized and found suitable by mortar rheology experiments are suitable for selfcompacting concrete.

Mix Proportioning Method

A number of methods for proportioning SCC mix have been developed over the years with primary attention to produce satisfactory self–compacting properties but with less attention to the properties at hardened state. Most of the methods those are presently available may have some inherent limitations, either in terms of ingredients for which they have been shown to be suitable or in terms of the range of concretes that can be produced. These methods are of varying complexity and may require wide range of information on the effect of each ingredients on the mechanics of SCC mixes. In general, the SCC mix proportioning methods consider volume as the key parameter because of the importance of the need to fill over the voids in between the aggregate particles by the paste.

Different mix-proportioning methods can be grouped in having two categories of approaches. The basic steps of first category are determination of quantity of coarse aggregate, and then deriving appropriate quality of mortar compatible for SCC mix. While in the second approach, the suitable mortar mix is first proportioned and then quantity of coarse aggregates is determined. The mixes proportioned by both these categories can further be subdivided in to three types; powder type, VMA type and mixed type. In first type cement content is very high, mineral admixture content is very low to none and no VMA is used. The second type method results in almost equal quantity of cement and mineral admixture, and high quantity of VMA is required for maintaining homogeneity of the mix though superplasticizer requirement comes down significantly compared to the first type. Mineral admixture content in the third type mix is about one–third of the powder content and a lower quantity of VMA is used (13).

Okumara and Ozawa of University of Tokyo developed most probably the first method of SCC mix proportion in 1995 [3, 14]. Their method is also known as general method. This is a step-by-step method in which VMA is not used. First the quantity of coarse aggregate, per unit volume of concrete mix, is set at 50% of the dry rodded weight. The required mortar volume is determined taking into consideration the air content in the mix. The fine aggregate content is worked out about 50% of the resulting mortar volume. The water/powder ratio and superplasticizer dosages of the mortar are adjusted until the minimum relative flow area of 5 and relative flow rate between 0.9 – 1.1 are achieved using mortar spread and V–funnel test respectively [3]. The mix proportion thus arrived at is tested for selfcompactability by concrete funnel test and slump flow test. The mix is considered satisfactory from selfcompatibility consideration if it exhibits slump flow of 650mm and relative flow rate between 0.5 and 1.0. This method is applicable to a limited range of Japanese materials; 5-20 mm sized coarse aggregates, fine aggregates of size less than 5mm, and high belite Portland cement. The air-entraining agent was used. Criterion related to concrete strength is not included in this mix proportioning method. This method falls under first category and produces only powder type mix.

Bui, et al [15] introduced a new approach for the proportioning of SCC that essentially falls under the second category and can produce combined and VMA type mixes. The approach is based on the paste rheology model, which is built on the combination of the criteria of minimum apparent viscosity, minimum flow and optimum flow viscosity ratio. The effect of aggregate properties and content has been considered to develop a new paste model for SCC. The model developed by testing wide range of concrete composition also provides a basis for quality control and further development of mineral and chemical admixtures. Polycarboxylate based superpla–sticizer was used and a viscosity modifying agent was used in some mixes. Relationship between viscosity and flowability of paste, with aggregate spacing were developed using average aggregate diameter 5.675 mm.

For different paste volume, water binder ratio, cement content, flyash content, admixtures, the flow of each paste were plotted against viscosity. The limits for segregation and low deformability zone were also plotted. Bui et al defined three zones for mix proportion with the help of these plots. One extreme zone is segregation zone in which flow is very high and viscosity is low. Other extreme zone is low deformation zone where viscosity is high and flow is low. The satisfactory zone falls in between these two extreme zones. The paste rheology, which is falling within the satisfactory zone, was considered appropriate for the purpose of selfcompatibility. Subsequently, the unit volume was achieved by addition of aggregates into the paste without any additional adjustment.

Mixing Method

The ingredients of vibrated HPC mix and SCC mix are similar except for VMA. The HPC mix is manufactured adopting multistage mixing method. It has been observed that mixing method has significant influence on the properties of the concrete mix both in hardened and fresh state (16, 17). Hardly any information is available in this respect for SCC mix.

Form Pressure

SCC results in higher form pressure because of its extreme fluidity showing nearly Newtonian behavior (18). The method as well as rate of casting dominates the form pressure (19). The traditional vibrated concrete results in lower form pressure than SCC having same casting rate. The correlation between form pressure and casting rate is relatively linear. When concrete is placed using pump and if the pumping is done from bottom it creates more anchor pressure than that when pumping from top (18). The anchor force due to pump filling from bottom doubles than that when filling from top, the reason is that the pressure from pump adds to the pressure of concrete. The relation between concrete pressure and optimal rate of pouring calls for further study to establish their inter-relation (20).

Leemann and C. Hoffmann investigated the pressure exerted by SCC on formwork both at laboratory scale and at field (20). They studied the formwork pressure caused by SCC with varying workability and conventional concrete filling the formwork from top in the laboratory and the pressure of SCC pumped into the formwork at its base was determined in a field study. The studies conclude that the maximum pressure of filled into a formwork from top is dependent on the casting speed and rate of the continuous pressure decrease of the SCC already cast. SCC pumped into the formwork a tits base can locally surpass hydrostatic pressure.

Concluding Remarks

SCC mix engineering starts with balancing between high fluidity and high segregation resistance to achieve appropriate self-compacting properties, hardened state properties as well as optimized behavior of the suspension within the formwork. Meticulous selection and characterization of locally available ingredients are the key to engineer the rheology of SCC.
  • The constituents of the material have significant impact on the concrete rheology and hydration kinetics of SCC mix. The approach for characterization of SCC mix leading to defined acceptance criteria needs further work.
  • The characterization in terms of physical and chemical properties of ingredients of powder, aggregates and their influence on the behavior of SCC is essential.
  • Selection of appropriate chemical admixtures, its dosages, its chemical compatibility with powder are issues to be addressed further.
  • A detailed investigation on the effect of curing regime on the properties of SCC at hardened state needs further investigation.
  • The form pressure in SCC is few folds more and different compared with vibrated concrete. More work is needed to under stand the relation between pump and concrete pressures.
  • Few of the areas like adjustment for mix proportioning procedure, use of local aggregates, mixing methodology, online controlling of rheology, prediction of strength and durability, need to be looked into.

References

  • M.A. Rahman, M. Nehdi, “Rheology of Cement Pastes using Various Accessories,” First North American Conference on Design and Use of Self Consolidating Concrete. (November 2002), pp. 49-53.
  • K. H. Khayat, Chong Hu, Jean- Michel Laye, “Importance of Aggregate packing Density on Workability of Self-consolidating Concrete,” First North American Conference on Design and Use of Self–Consolidating Concrete. (November 2002), pp. 55-62.
  • A. Skarendahl, O. Petersson, “Self-compacting Concrete-State – of–the–art report 174-SCC,” RILEM Technical Committee, France, Report 23. (2000)
  • Kamal H. Khayat, “Holistic Approach,” First North American Conference on Design and Use of Self–consolidating Concrete. (November 2002) pp. 9.
  • K. H. Khayat, “Stability of Self compacting Concrete, Advantage and Potential Application,” First International RILEM Symposium on Self–compacting Concrete, Stockholm, Sweden. (September 1999) , p p. 143–152.
  • Peter Billberg, “Mix Design Model for Self-compacting Concrete,” First North American Conference on Design and Use of Self Consolidating Concrete. (November 2002), p p.63-68
  • Preface, Third International Symposium on Self–compacting Concrete, Reykjavik, Iceland. (August 2003).
  • P.C.Basu, P. P. Biswas, S. Chowdhury, A. K. Ghoshdast idar, P.D. Narkar, “Influence of Components of Portland Cement on Rheology of Mortar for Self- Compacting Concrete,” Second North American Conference on the Design and Use of Self– compacting Concrete , Illinois, Chicago, USA. (October-November 2005).
  • Pipat Termkhajornkit, Toyoharu Nawa, Hiroshi Ohnuma, “Role of Flyash and Naphthalene Sulfonated Superplasticizer on Fluidity of Paste,” First North American Conference on Design and Use of Self–Consolidating Concrete. (November 2002), p p. 43-44.
  • P.C. Basu, S. Saraswati, S. Chowdhury, “Effect of Different Fly Ashes on Rheology of Mortar for Self-compacting Concrete,” Second North American Conference on the Design and Use of Self–compacting Concrete, Illinois, Chicago, USA. (October-November 2005).
  • P.C. Basu, S. Chowdhury, “Influence of Minor Constituents of Portland Cement on Rheology of Mortar for Self–Compacting Concrete,” Proceeding of The Structural Engineering Convention), Indian Institute of Science, Bangalore. (2005), pp. 209-219.
  • P. C. Basu, S. Chowdhury, “Impact of Fine Aggregate Particle Size on Mortar Rheology for SCC,” The Indian Concrete Journal, Volume 81. (January 2007), pp. 1-8.
  • Ouchi Masahiro, Nakamura Sadaaki, Osterberg Thomas, Hauberg Svenerik, “Application of Selfcompacting Concrete in Japan, Europe and United States,” Sweden. (2005), pp. 1-1 8.
  • Okamura H, Ozawa K, “Mix Design for Self-compacting Concrete,” Concrete Library of JSCE 25. (1995), pp . 107-120.
  • V. K. Bui, S.P. Shah, K. Akkaya, “A New Approach in Mix Design of Self-consolidating Concrete,” First North American Conference on Design and Use of Self– consolidating Concrete. (November 2002), pp. 69-74.
  • P. C. Basu, S. Saraswati, “Durability of High Performance Concrete: An Overview and Related Issues,” Proceedings of International Symposium on Advances in Concrete through Science an Engineering, Evanston, Illinois, USA. (March 2004).
  • M. Kakizaki, H-Edahiro, T.Tochigi and T. Nikki, “Effects of mixing method on mechanical properties and pore structures of ultra high strength concrete.” SP 132-54.
  • Wolfgang Brameshuber, Stephan Uebachs, “Investigations on the Form Pressure Using Self compacting Concrete,” Third International Symposium on Self–compacting Concrete, Reykjavik, Iceland. (August 2003), p p. 281-287.
  • Peter Billberg, “Form Pressure Generated by Self-compacting Concrete,” Third International Symposium on Self-compacting Concrete, Iceland. (August 20 03), pp. 271-280.
  • Andreas Leemann, Cathleen Hoffmann, “Pressure of Self Compacting Concrete on Formwork,” Third International Symposium on Self–compacting Concrete, Iceland, (August 2003), pp. 288-298.

Acknowledgment

The article has been reproduced from the SEWC’07 proceeding with the kind permission from the SEWC organisers.

NBMCW July 2008

.....

High Performance Concrete - Novel Appro...

Dr. S. K. Manjrekar, Chairman and Managing Director, Sunanda Speciality Coatings Pvt. Ltd, Mumbai.

High Performance Concrete Novel Approach with a Pre-blended Material
The development of high performance concrete is a giant step in making concrete a high-tech material with enhanced characteristics and durability. High performance concrete is an engineered concrete obtained through a careful selection and proportioning of its constituents. The concrete is made with the same basic ingredients but has a totally different microstructure than ordinary concrete. The low water/binder ratio of high performance concrete, that is its universal characteristic, results in a very dense microstructure having a very fine and more or less well connected capillary system. high performance concrete’s dense microstructure make the migration of aggressive ions more difficult, consequently high performance concrete are more durable when exposed to aggressive environmental conditions. This fact has been endorsed by a case study of the use of specially formulated HPC in an aggressive chemical environment at a fertilizer plant in Gujarat.

What is High Performance Concrete?

The concrete that was known as high-strength concrete in late seventies is now referred to as high performance concrete because it has been found to be much more than simply strong.

The Strategic Highway Research Programme (SHRP) is a $150,000.00 product-driven research program under the Federal-aid highway program in U.S.A. SHRP was developed in partnership with the State Departments of Transportation, American Association of State Highway and Transportation (AASHTO), Transportation Research Board (TRB), industry, and the Federal Highway Administration (FHWA).

SHRP defined HPC as :
  1. Concrete with a maximum water-cementitious ratio (W/C) of 0.35
  2. Concrete with a minimum durability factor of 80%, asdetermined by ASTM C 666
  3. Concrete with a minimum strength criteria of either
  4. - 21 Mpa within 4 hours after placement (Very Early Strength, VES),
  5. - 34 MPa within 24 hours (High Early Strength, HES), or
  6. - 69 MPa within 28 days (Very High Strength, VHS)
High performance concrete can hence be defined as an engineered concrete with low water/binder concrete with an optimized aggregate/binder ratio to control its dimensional stability and which receive an adequate water curing.

Water/Cement or Water/Binder Ratio

Both expressions were deliberately used above, either singly or together, to reflect the fact that the cementitious component of high performance concrete can be cement alone or any combination of cement with supplementary cementitious materials, such as, slag, flyash, silica fume, metakaolin, rice husk ash, and fillers such as limestone. Ternary systems are increasingly used to take advantage of the synergy of supplementary cementitious materials to improve concrete properties in the fresh and hardened states and to make high performance concrete more economical.

Despite the fact that most high performance concrete mixtures contain at least one supplementary cementitious material, which should favor the use of more general expression water/binder ratio, the water/ binder and water/cement ratios should be alongside each other. This is because most of the supplementary cementitious materials that go into high performance concrete are not as reactive as portland cement, which means that most of the early properties of high performance concrete can be linked to its water/ cement ratio while its long-term properties are rather linked to its water/binder ratio.

Concrete compressive strength is closely related to the density of the hardened matrix. High performance concrete has also taught us that the coarse aggregate can be the weakest link in concrete when the strength of hydrated cement paste is drastically increased by lowering the water/binder ratio. In such cases, concrete failure can start to develop within the coarse aggregate itself. As a consequence, there can be exceptions to the water/binder ratio law when dealing with high performance concrete. In some areas, decreasing the water/binder ratio below a certain level is not practical because the strength of the high performance concrete will not significantly exceed the aggregate’s compressive strength. When the concrete’s compressive strength is limited by the coarse aggregate, the only way to get higher strength is to use a stronger aggregate.

High performance Concrete: A Composite Material

Standard concrete can be characterized solely by its compressive strength because that can directly be linked to the cement paste’s water/cement ratio, which still is the best indicator of paste porosity. Most of concrete’s useful mechanical characteristics can be linked to concrete compressive strength with simple empirical formulas. This is the case with elastic modulus and the modulus of rupture (flexural strength), because the hydrated cement paste and the transition zone around coarse-aggregate particles constitute the weakest links in concrete. The aggregate component (especially the coarse aggregate) contributes little to the mechanical properties of ordinary concrete. As the strength of the hydrated cement paste increases in high performance concrete, the transition zone between the coarse aggregate and the hydrated cement paste practically disappears. Since there is proper stress transfer under these conditions, high performance concrete behaves like a true composite material.1

Making HPC

High performance concrete can not be made by a casual approach. Each ingredient viz : cement, supplementary cementitious materials, sand, course aggregates, superplasticizer, and the other admixtures must be carefully selected and checked, because their individual characteristics significantly affect the properties of the final product.

Particular attention must be paid to water content. Even seemingly insignificant volumes of water present in the aggregates or admixtures must be accounted for. Compressive strengths from 50 to 75 MPa can usually be achieved easily with most cements.2

Concrete Shrinkage

If water curing is essential to develop the potential strength of cement in plain concrete, early water curing is crucial for high performance concrete in order to avoid the rapid development of autogenous shrinkage and tocontrol concrete dimensional stability, as explained below.

Cement paste hydration is accompanied by an absolute volume contraction that creates a very fine pore network within the hydrated cement paste. This network drains water from coarse capillaries, which start to dry out if no external water is supplied. Therefore, if no drying is occurring and if no external water is added during curing, the coarse capillaries will be empty of water as hydration progresses, just as though the concrete was drying. This phenomenon is called selfdesiccation. The difference between drying and selfdesiccation is that, when concrete dries, water evaporates to the atmosphere, while during selfdesiccation, water stays within concrete means it only migrates towards the very fine pores created by the volumetric contraction of the cement paste.

In ordinary concrete with a high water/cement ratio greater than 0.50, for example, there is little cement and more water than is required to fully hydrate the cement particles present. A large amount of this water is contained in well connected large capillaries, in ordinary concrete. This means that the hydrated cement paste does not shrink at all when selfdesiccation develops.

In the case of high performance concrete with a water/binder ratio of 0.30 or less, significantly more cement and less mixing water have been used, so that the capillary network that developed within the fresh paste is essentially composed of fine capillaries. When self-desiccation starts to develop as soon as hydration begins, the menisci rapidly develop in small capillaries if no external water is added. Since many cement grains start to hydrate simultaneously in high performance concrete, the drying of very fine capillaries, can generate high tensile stresses that shrink the hydrated cement paste. This early shrinkage is referred to as autogenous shrinkage. Of course, autogenous shrinkage is as large as the drying shrinkage observed in ordinary concrete when these two types of drying develop in capillaries of the same diameter.

But, if there is an external supply of water, the capillaries do not dry out as long as they are connected to this external source of water. The result is that no menisci, no tensile stress, and no autogenous shrinkage develops within the high performance concrete.

Therefore, an essential difference between ordinary concrete and high performance concrete is that ordinary concrete exhibits no autogenous shrinkage whether or not it is water-cured, whereas high performance concrete can experience significant autogenous shrinkage if it is not water-cured during the hydration process. Autogenous shrinkage will not develop in high performance concrete if the capillaries are interconnected and have access to external water. When the continuity of the capillary system is broken, then and only then, will autogenous shrinkage start to develop within the hydrated cement paste of a high performance concrete.

High performance concrete must be cured quite differently from ordinary concrete because of the difference in shrinkage behavior described above. If HPC is not water-cured immediately following placement or finishing, it is prone to develop severe plastic shrinkage because it is not protected by bleed water, and later on develops severe autogenous shrinkage due to rapid hydration reaction. While curing membranes provide adequate protection for ordinary concrete (which is not subject to autogenous shrinkage), they can only help to prevent the development of plastic shrinkage in high performance concrete. They have no value in inhibiting autogenous shrinkage. Therefore, the most critical curing period for any HPC runs from placement or finishing up to 2 or 3 days later. During this time, the most critical period is usually from 12 to 36 hours. In fact, the short time during which efficient water curing must be applied to HPC can be considered a significant advantage over ordinary concrete. Those who specify and use HPC must be aware of the dramatic consequences of skipping early water curing. Initiating water curing after 24 hours is too late because, most of the time, a great deal of autogenous shrinkage will already have occurred and, by this time, the microstructure will already be so compact that any external water will have little chance of penetrating very deep into the concrete.

Water ponding, whenever possible, or fogging are the best ways to cure HPC; one of these two methods must be applied as soon as possible immediately following placement or finishing.

The water curing can be stopped after 7 days because most of the cement at the surface of concrete will have hydrated and any further water curing will have little effect on the development of autogenous shrinkage due to compactness of the HPC microstructure. Moreover, after 7 days of water curing, HPC experiences little drying shrinkagedue to the compactness of its microstructure and because autogenous shrinkage will have already dried out the coarse capillaries pores. Even then, the best thing to do is to paint HPC with an sealing agent so that the last remaining drops of water in the concrete can hydrate more cement particles. There is no real advantage to paint a very porous concrete since it is impossible to obtain an absolutely impermeable coating; painting HPC, however, is easier and more effective.

Durability of HPC

The durability of a material in a particular environment can only be established by time. Based on years of experience with ordinary concrete, we can safely assume that high performance concrete is more durable than ordinary concrete. Indeed, the experience gained with ordinary concrete has taught us that concrete durability is mainly governed by concrete impermeability and the harshness of the environment.

A specially designed high performance, selfleveling, nonshrink pre-blended high performance concrete was formulated and was put into use against the aggressive chemical environments at a fertilizer plant in Gujarat – Gujarat Narmada Fertilizers Ltd. (GNFC).

This pre-blended high performance concrete was specially formulated to meet the MES & VES proportion as defined in the SHRP programme.

GNFC is a world largest single stream manufacturer of ammonia and urea. Subsequently, for diversification various products viz. Ammonium Nitro-phosphate (ANP), Calcium Ammonium Nitrate (CAN) etc. were added. CAN is a physical mixture of ammonium nitrate and lime mixed at a particular temperature to form granules. As the mixture is not a chemical reaction, it results into availability of free lime in CAN granules. Lime is inert and remains in dormant condition as far as effect on concrete structure is concerned, but CAN which is available in free form in the CAN granules, reacts with hydration products of concrete and deteriorates concrete. CAN also reacts with reinforcement present in RC member and causes corrosion.

Problem

The signs of damages/ deterioration on concrete particularly in CAN plant were first observed in the form of cracks on edges of RC member which started widening within a span of 6 to 8 months. Concrete in cover portion started sounding hollow which would ultimately result into debonding. As such, this type of failure in RC members can be due to many reasons but one observation which narrowed down the probabilities was observation of watery droplets around these members. The droplets were chemically analysed and they were found to be containing CAN. It was found on further investigation that CAN is highly hygroscopic and hence it would attract moisture from atmosphere and form watery layer all around the surface on which CAN is present. In addition to this hollow sound and cracks, diminishing of cement slurry and erosion like failure was also observed. Coarse aggregates could be seen on the surface of RC member. These are the signs of medium corrosion of RC members wherein cracks, loss of external finish, leaching of liquid and progressive reduction in strength etc. would occur

These observations were immediately followed by the signs of corrosion wherein debonding and spalling of concrete, corrosion of reinforcement and disintegration of concrete by dissolution of cement slurry were observed.

Conventional Solution

Two alternatives were initially decided to be implemented. One was to build up the thickness of damaged/removed concrete by concrete of higher grade after water washing of exposed surface of beam, application of good bond coat and repairing of the reinforcement bars by welding was carried out. Second method was to build up the thickness with epoxy screed after similar preparations. In first method, the thickness was to be built up by pouring concrete after providing suitable shuttering and in second method, thickness was to build up in layers. Both the alternatives were tried but they failed. First method failed earlier as compared to second method. Additionally, huge wastage was observed in second method which made the second method uneconomical.

Limitations of Conventional Solution

High Performance Concrete Novel Approach with a Pre-blended Material
  1. Repairing was carried out in running plant where airborne CAN dust and humidity were present.
  2. CAN, present in core of concrete which was not apparently visible and hence it was not removed.
  3. Immediate coating of repaired surface, stopping the breathing of repair mortar and implemented.
  4. Stresses resulting in debond of the repair mortar Unapproachability to surface ofcut outs due to their covering by ducts/equipment which enclosed a part of surface and could not be approached and repaired.
  5. Minor vibrations transmitted from the equipment during repairing activity.
  6. Repair system limitation was that the repairing was in layers each of 25 mm thickness which sandwiched CAN dust in-between every layer and did not allow to establish a proper inter layer bond.

Innovative Solution

All these advertise were examined jointly with the representatives of GNFC. Considering the overall view of the problems as well the limitations involved the job demanded a robust, fast setting, non-shrink, impermeable high performance concrete. POLYCRETE is a high strength, fast setting and non shrink specially formulated HPC. Besides combining all the above properties it also is free flowing and self levelling. Its application procedures are much simple than conventional methods.

Based on all the above properties and parameters coupled with ease in application and fast strengths which will delay CAN particles from depositions again POLYCRETE was suitably selected for the project.

Repair procedures were suggested which included a Low viscosity Bonding agent and shear keys as per design requirements.

Present Scenario

High Performance Concrete Novel Approach with a Pre-blended Material
This system has been applied in end of September, 1997. Repaired area was continuously observed and there are no signs of deterioration observed since then. Strength of the repaired mass was measured in December 1997 and it was found to be around 650 Kg/cm2. This system has ensured that the repaired portion has an excellent mechanical strength and all chances of its getting debonded from original surface are eliminated.

References

  1. Baalbaki, W., Benmokrane, B., Chaallal, 0., Aitcin, P.-C., (Sept.- Oct. 199) “Influence of Coarse Aggregate on Elastic Properties of high performance concrete,” ACI Materials Journal, Vol. 88, No. 5, pp. 499-503.
  2. Aitcin, P.-C., (1993) “Durable Concrete–Current Practice and Future Trends,” ACI SP-144, pp. 83-104.
  3. Nilsen, A.U., Aitcin, P.-C.(Vol. 14, No. 1, Summer, 1992), “Properties of High-Strength Concrete Containing Light-, Normal–and Heavyweight Aggregate,” Cement, Concrete and Aggregates, pp. 8-12.
  4. Lessard, M., Dallaire, E., Blouin, D., Aitcin, P.-C (Sept. 1994)., “High Performance Concrete Speeds Reconstruction of McDonald’s,” Concrete International, Vol. 16, No. 9, pp. 47-50.
  5. Aitcin, P.-C., Neville, A.M., Acker, P., (Sept., 1997) “The Various Types of Shrinkage Deformation in Concrete: An Integrated View,” to be published in Concrete International, Whiting, D., “In-Situ Measurements of the Permeability of Concrete to Chloride Ions,” ACI SP-82 1984, pp. 501-524.
  6. Kreijger, P.C. (1987), “Ecological properties of Building Materials,” Materials and Structures, Vol. 20, pp. 248-254.

NBMCW October 2007

.....

Concrete Containing More Than Two Admix...

Effect of Sustained Elevated Temperature on the Properties of Concrete Containing More Than Two Admixtures

Concrete Containing More Than Two Admixtures
D. K. Kulkarni, Selection Grade Lecturer, Civil Engineering Department, Rajarambapu Institute of Technology, Rajaramnagar, Islampur. Dr. K.B. Prakash,
Portland pozzolana cement and locally available sand and aggregates were used
Professor Civil Engineering Department K. L. E Society's College of Engineering & Technology, Belgaum.

Concrete is a material often used in the construction of highrise buildings. In case of unexpected fire, the concrete elements such as columns, beams, etc. will be subjected to extreme temperatures and needs assessment of their performance after fire. Hence, it is important to understand the changes in the concrete properties due to extreme temperature exposures.

In this paper, an attempt is made to find out the effect of sustained elevated temperature on the properties of concrete containing more than two admixtures. The following combinations of admixtures are used in this experimentation work.
  • Superplasticiser + Air Entraining Agent + Accelerator
  • Superplasticiser + Air Entraining Agent + Retarder
  • Superplasticiser + Air Entraining Agent + Waterproofing Compound
  • Superplasticiser + Air Entraining Agent + Shrinkage Reducing Admixture
  • Superplasticiser + Air Entraining Agent + Viscosity Modifying Admixture

The tests are conducted to evaluate the strength characteristics of concrete like compressive strength, tensile strength, flexural strength, and impact strength of concrete when it is subjected to a temperature of 600°C for 6 hours.

Introduction

One of the greatest advantages of concrete as a building material is its remarkable resistance to fire. The distress in concrete due to fire manifests in the form of cracking and spalling of the concrete surface1. Concrete though not a refractory material is incombustible and has good fire resistant properties2. The property of concrete to resist the fire reduces damage in a concrete structure whenever there is an accidental fire. In most of the cases the concrete remains intact with minor damages only. The reason being low thermal conductivity of concrete at high temperature and hence limiting the depth of penetration of fire damage. But when the concrete is subjected to high temperature for long duration, the deterioration of concrete takes place3. Concrete has been widely used as construction materials in buildings and other industrial structures for a long time. The recent technological advances have extended its use to special applications like aircraft engine test cells, tube jet runways, nuclear reactor vessels and missile launching pads, which have to endure higher tempratures4.

Chemical admixtures play a key role in the production of concrete with enhanced performance also known as High Performance Concrete or HPC. In conjunction with mineral additives, such as silica fume, chemical admixtures have enabled major improvements in many of the properties of concrete, particularly, compressive strength and durability.

Now-a-days the concrete is called upon for the use in various tricky situations and the concrete has to show a resistive nature for all the special situations for which it is used. In such circumstances, it becomes necessary to use two or more than two admixtures simultaneously in concrete.

Experimental Programme

The main aim of this experimentation work is to find the effect of sustained elevated temperature on the properties of concrete containing more than two admixtures. The following combinations of admixtures have been selected for the studies on concrete:

Concrete Containing More Than Two Admixtures
  • Superplasticiser + Air Entraining Agent + Accelerator (S+AEA+A)
  • Superplasticiser + Air Entraining Agent + Retarder (S+AEA+R)
  • Superplasticiser +Air Entraining Agent + Waterproofing Compound (S+AEA+W)
  • Superplasticiser +Air Entraining Agent + Shrinkage Reducing Admixture (S+AEA+SRA)
  • Superplasticiser +Air Entraining Agent + Viscosity Modifying Admixture (S+AEA+VMA)

Portland pozzolana cement and locally available sand and aggregates were used in the experimentation. The specific gravity of fine and coarse aggregate was 2.66 and 2.85 respectively. The experiments were conducted on a mix proportion of 1: 1.26:2.51 with w/c = 0.41 which corresponds to M20 grade of concrete. The admixtures and their chemical content and dosages used in the experimentation are shown in Table 1.

The fine aggregate, cement and coarse aggregates were dry mixed in a mixer for 60 seconds. The required quantity of fibers and hybrid fibers were added into the dry mix and again the entire mass is mixed homogeneously for another 60 seconds. At this stage approximately 80% of calculated quantity of water (w/c = 0.41) was added into the dry mix and agitated for 3 minutes. Now the superplasticiser was added in the remaining 20% water and this liquid was added to the concrete. The concrete was mixed again in the mixer, after which the remaining two more admixtures were added and homogeneously mixed. This homogeneous concrete mass was poured into the moulds which were kept on the vibrating table. The concrete was consolidated in three layers by using just the required vibration time needed for a good compaction. After consolidation the top surface was finished smooth and covered with wet gunny bags. After 12 hours, the specimens were demoulded and transferred to the curing tank wherein they were allowed to cure for 28 days.

For compressive strength test, the cubes of dimensions 150 X 150 X 150 mm were cast and were tested under compression testing machine as per I S 516-19595. For tensile strength test, the cylinders of diameter 100 mm and length 200 mm were cast and were tested under compressive testing machine as per I S 5816- 19996. For flexural strength test the beams of dimensions 100 X 100 X 500 mm were cast and were tested on an effective span of 400 mm with two point loading as per I S 516-19595. For impact test four different test methods are referred in the literature7. Drop weight method being the simple method, was adopted to find the impact energy. Impact strength specimens were of dimensions 250 X 250 X 30 mm. A steel ball weighing 13.03 N was dropped from a height of 1 m on the centre point, which was kept on the floor. Number of blows required to cause first crack and final failure were noted down. From these number of blows, the impact energy was calculated as under. Impact energy = w h N (N-m)

Where w = Weight of steel ball = 13.03 N

h = Height of drop = 1 m

N = Number of blows required for first crack or final failure as the case may be.

After 28 days of curing, the specimens were transferred to the electric furnace wherein they were maintained at 6000 C for 6 hours. After 6 hours they were cooled to room temperature and then tested for their respective strengths.

Test Results

Table 2 gives the compressive strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of compressive strength w.r.t. reference mix. The variation of compressive strength is depicted in the form of graph as shown in Figure 1.

Table 3 gives the tensile strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of tensile strength w.r.t. reference mix. The variation of tensile strength is depicted in the form of graph as shown in Figure 2.

Table 4 gives the flexural strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of flexural strength w.r.t. reference mix. The variation of flexural strength is depicted in the form of graph as shown in Figure 3.

Table 5 gives the impact strength test results of concrete with different combinations of admixtures. It also gives percentage increase or decrease of impact strength w.r.t. reference mix. The variation of impact strength is depicted in the form of graph as shown in Figure 4.

Discussion on Test Results

Concrete Containing More Than Two Admixtures

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum compressive strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), ( S + A E A + W ) , (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least compressive strength. The percentage increase in the compressive strength of the above said combinations w.r.t. reference mix are respectively 45.07%, 32.65%, 25.07%, 15.76%, and 7.48%.

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum tensile strength when subjected to 600°C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least tensile strength. The percentage increase in the tensile strength of the above said combinations w.r.t. reference mix are respectively 55.35%, 53.02%, 51.63%, 47.91%, and 13.48%.

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum flexural strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least flexural strength. The percentage increase in the flexural strength of the above said combinations w.r.t. reference mix are respectively 111.03%, 77.93%, 35.17%, 30.34%, and 9.65%.

It has been observed that the concrete produced from the combination of admixtures (S+AEA+R) show maximum impact strength when subjected to 6000C for 6 hours. This is followed by the combination of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). The reference mix without any combination of admixtures shows the least impact strength. The percentage increase in the impact strength of the above said combinations w.r.t. reference mix are respectively 77.77%, 55.56%, 44.43%, 33.33%, and 11.10%.

This may be due to the fact that the addition of combination of admixtures induce more workability thus making the compaction a perfect one. This makes the concrete more dense which is ultimately responsible for increase in the strengths. The addition of AEA creates small air bubbles in the concrete. These induced air bubbles can resist the expansion of concrete due to temperature.

Conclusions

It can be concluded that the combinations of admixtures used in the experimentation such as (S+AEA+R), (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA), do not have any compatibility problems either with respect to the properties of fresh concrete or hardened concrete. It can also be concluded that the maximum strength of concrete can be obtained with the combination of admixtures (S+AEA+R) when subjected to 6000C for 6 hours. This is followed by the combinations of admixtures (S+AEA+A), (S+AEA+W), (S+AEA+SRA), and (S+AEA+VMA). Hence it can be recommended to use any combinations of admixtures on the site to suite the situations.

Acknowledgment

The authors would like to thank Dr.(Mrs) S. S. Kulkarni, Principal, RIT, Sakharale and Dr.S.C.Pilli, Principal, KLE Society's College of Engg. & Technology, Belgaum for giving all the encouragement needed which kept our enthusiasm alive. Thanks are also due to the management authorities and others who constantly boosted our morale by giving us all the help required. Thanks are also due to authorities of MBT Pvt.Ltd(Degussa) Mumbai India for supplying the required admixtures.

References

  • Lakshmipathy M and Balachandar M, "Studies on the effects of elevated temperature on the properties of high strength concrete containing supplementary cementatious materials," Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp. 539-554
  • Balamurugan P and Perumal P, "Effect of thermoshock on bond strength of HPC, "Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp. 555-556
  • Sashidhar C, Sudarsana Rao H, Ramana N.V and Vaishali Gorpade, "Studies on SIFCON subjected to elevated temperature," Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9, 2005, SRMIST, Chennai, India. pp.567-576
  • Anbuvelan K, Dinesh M, Kumaravel K, Thiyagarajan A and Sureshkumar N, "Sustained elevated temperature effects on post peak flexural strength of high strength concrete containing polypropylene fibers," Proceedings of the International Conference on recent advances in concrete and construction technology, Dec 7-9,2005, SRMIST, Chennai, India. pp. 577-590
  • I S : 516-1959 "Methods of tests for strength of concrete," Bureau of Indian Standards, New-Delhi.
  • I S : 5816-1999 "Splitting tensile strength of concrete method of test," Bureau of Indian Standards, New-Delhi
  • Balsubramanain, K. et al, "Impact resistance of steel fiber reinforced concrete," The Indian concrete Journal, May 1996, (pp 257-262).

NBMCW February 2008

.....