Articles of Concrete

Deterioration Mechanisms of Reinforced ...

Sanjeev Kumar Verma, Research Scholar, Civil Engineering Dept., Univ. Institute of Technology, Rajiv Gandhi Technological Univ., Bhopal, Sudhir Singh Bhadauria, Director, G.S. Institute of Technology and Science, Indore, Madhya Pradesh, India, and Saleem Akhtar, Professor and Head, Civil Engineering Dept., Univ. Institute of Technology, Rajiv Gandhi Technological Univ., Bhopal.

Introduction

Failure of structures was almost non-existent in the past. Structures were generally known for their durability, soundness, and stability. But Due to lack of performance of building structures in last few decades, there has been a growing interest in the field of durability and service life of structures.

Degradation and deterioration of structures caused by physical and chemical damage results in the decrease in performance with time, physical damage occurs due to fire, abrasion or expansion and contraction stresses while chemical damage occurs due to harsh environment. Lack of durability of concrete structures or initiation of cracking has been caused mainly due to exposure to harsh environment, which results in degradation of structures as shown in fig. 1.

Reinforced Concrete Structures
Figure 1: degraded concrete structures

Several researchers have performed studies to indentify the causes of deterioration of RC structures, Wang and Liu (2010) identified change in bond strength, loss of concrete cover in tensile zone and/or reduction of concrete cover in compressive zone as the major cause of deterioration of RC structures. Crack growth due to corrosion products expansion has been considered as an important factor for the durability of structures by Benin et al. (2010). According to Mitra et al. (2010) repair and maintenance planning of concrete structures is based on the conditional states of concrete categorized by the assessment of conditions such as rusting and cracking, delamination, loss in steel section, workmanship, carbonation and chloride contents. From the analysis performed by Bastidas-Arteaga et al. (2008), the failure probability of bridge girders depends highly on the corrosion rates, surface chloride concentration and the traffic frequency. Ganjidoost et al. (2010) studied the sustainability and durability of concrete structures in corrosive environmental conditions mostly in marine environments; they proposed that W/C ratio must be between 0.3 and 0.5 to resist against corrosion and permeability. According to Song and Saraswathy (2007) corrosion of rebars is the major deterioration process, and reviewed the methods for monitoring the corrosion of reinforced concrete structures including electrical, electrochemical, harmonic, ultrasonic pulse velocity, X-ray and visual methods. Melchers et al. (2007) considered the influence of corrosion and its initiation coupled with applied loads, and impact of internal damage for determining structural deterioration of R.C. Beams under saline environment corrosion. Berto et al. (2007) proposed that main effects of corrosion are bond deterioration, reduction in steel cross sections, cover spalling, and concrete damage or cracks. Song and Kwon (2007) considered carbonation as the major cause of deterioration and found during carbonation process permeability of concrete changes due to change in capillary porosity. Durham et al. (2007) inspected several precast concrete bridges to identify the causes of deterioration, information collected includes concrete deterioration, environmental humidity, reinforcing steel corrosion, asphalt wearing surface, drainage and bridge site photos. And concluded that deterioration was mainly due to longitudinal cracks (caused corrosion of steel bars), and flexure cracks (overloading of live-loads). Chong and Low (2005) analyzed the defects in construction facilities at both construction and occupancy (after 2 to 6 years) and found that main causes of defects are design, workmanship, material, lack of protection and maintenance. According to Amleh and Mirza (2004) chloride content, quality of concrete cover and electrical resistivity of concrete have significant effects on the rebar corrosion. Dias and Jayanandana (2003) performed experiments to measure depth of carbonation, concrete cover, chloride content and sulfate attack, for the assessment of durability. Karokouzian et al. (2003) studied several swimming pools and found that ASR as the main cause of cracking and deterioration. According to Snathanam et al.(2001) sulfate attack is one of the major cause of deterioration and estimating the remaining service life of structures exposed to sulfate attack is important in order to develop repair and maintenance schedule.

From the above literature it has been recognized that, Strength as well as environmental conditions to which structure is exposed over is also important for the service life of RC structures. So, it is important to understand various deterioration mechanisms of RC structures. Major deterioration mechanisms of RC Structures identified are:

NBMCW June 2013

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Review on Corrosion Inhibitors in Concr...

M.N.Bajad, Research Scholar, Dr. C.D.Modhera, Professor and
Dr.A.K.Desai, Professor, Department of Applied Mechanics, S.V.N.I.T, Surat, (Gujarat)

Corrosion Inhibitors
Corrosion of reinforcing steel represents the most widespread form of deterioration of concrete structures resulting in significant costs for repair and replacement worldwide. Despite the huge demand, a simple, cheap and reliable technique that either protects the steel in concrete from corrosion or at least lowers its corrosion rate is still lacking. The concrete repair industry has developed novel techniques that are claimed to prevent the steel from corrosion and/or to restore the protective character of the cover concrete by introducing corrosion inhibitors into the carbonated or chloride contaminated concrete. Inhibitors, chemical substances that prevent or retard corrosion, are applied as concrete admixtures or as surface applied liquids both for preventive or for restorative applications. A short review on literature results regarding the performance of the most frequently used inhibitors for steel in concrete in laboratory and in field tests is given, in particular two inorganic inhibitors, calcium nitrite (DCI) and MFP, and several organic inhibitors, the migrating inhibitors (MCI or SIKA) and an organic corrosion-inhibiting admixture (OCI) are addressed. The problem of transport of inhibitors into concrete is discussed. A critical review of corrosion inhibitors to be used on reinforced concrete structures regarding concentration dependence, durability and measurement and control of the inhibitor action is presented in this paper.

NBMCW April 2013

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Effect of Friction on Cable Profile Des...

Pradeep Purohit, Associate Professor, Department of Civil Engineering, SATI, Vidisha
Saleem Akhtar, Prof and Head, Department of Civil Engineering UIT (RGPV) Bhopal
K.K.Pathak, Professor, Civil and Environmental Engineering, NITTTR, Bhopal

Introduction

Cable layout design is an important task for the realistic design of prestressed concrete structures. Due to friction, effective prestress force along the span of beam decreases, hence cable layout design should be carried out considering the realistic forces. Some of the prominent reported work in the area of cable layout design are as follows.

Akhtar et.al (2008) carried FEA analysis of prestressed concrete beams using B-spline cable profile for non friction conditions. Brandt(1989) and Kirsch (1973,93) have carried out cable layout optimization using mathematical programming methods. Lin (1963) presented load balancing method for design and analysis of prestressed concrete structures using straight and parabolic tendon profile. Lounis et.al (1993) carried out multi objective optimization of prestressed concrete beam and bridge girder. Kuyucular (1991) obtained optimum cable profile of prestressed concrete slabs using elastic theory and finite element method. Pathak et.al (2004) presented analysis of prestress concrete beam considering different cable models. Utrilla et.al (1997) and Quiroga et.al (1991) obtained cable layout in bridge decks using linear and non-linear programming methods. Li et.al (2009) obtained equivalent load and loss of tendon force using cubic spline profile.

In the cable layout design it is necessary to estimate frictional prestress losses to find out optimum value of tendon-force required for design and to be applied at the time of prestressing, some of the prominent researches in this field are as follows:

NBMCW April 2013

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Hybrid Fibre Reinforced Concrete – A ...

S. K. Singh, Principal Scientist, Ajay Chourasia, Principal Scientist
M. M. Dalbehera Scientist, and S. K. Bhattacharyya, Director, CSIR, Central Building Research Institute, Roorkee.

Introduction

Concrete is a relatively brittle material and develops micro cracks even during curing and initial stages of strength development whereas reinforcement of concrete with randomly distributed short fibres improves the initiation and propagation of cracks by improving overall properties of the conventional concrete. Fiber reinforcement is commonly used to provide enhanced toughness and ductility to brittle cementitious matrix. The reinforcement of concrete with a single type of fiber may improve the desired properties to a limited level, whereas combination of two or more types of fibres if used in optimal way in concrete achieves better engineering properties due to positive synergetic effect. This includes combination of different kinds of fibres with different shapes, dimensions, strengths and modulus to concrete matrices. The hybrid fibre utilizes the individual qualities of its constituent fibres to undermine the inherent limitations of their counterpart. To control micro cracks in concrete, microfibers are used having diameters (10-40m compared to 500m), while to control macro cracks, macro fibres are used to resists propagation and crack opening. The Fig.1 presents comparison of typical load- deflection curves for conventional FRC and hybrid FRC.

Comparison of Conventional and Hybrid FRC
Fig1: Comparison of Conventional and Hybrid FRC

The hybrid fibre reinforced concrete, is effective in arresting cracks both at macro and micro levels. This is due to positive interaction between the fibres and resulting hybrid performance exceeds the sum of individual fibre performances. This phenomenon is termed as "Positive Synergy Effect". The various hybrids are based on fibre constitutive response, fibre dimensions, fiber function as described below:
  • Hybrids Based on Fibre Constitutive Response: Where one type of fibre is stronger and stiffer and provides reasonable first crack strength and ultimate strength, while the other type of fibre is more ductile or can readily undergo considerable slippage to provide better toughness and strain capacity in the post crack zone.
  • Hybrids Based on Fibre Dimensions: Where one type of fibre is smaller (micro) and provides micro-crack control at early stage and delays coalescence. This leads to a higher tensile strength of the composite. Other fibre is larger and is intended to arrest the propagation of macro-cracks and therefore, resulting a substantial improvement in the fracture toughness of the composite.
  • "Hybrids Based on Fiber Function: Where one type of fibre is intended to improve the fresh and early age properties such as ease of production and controlling plastic shrinkage, the other fibre leads to enhanced mechanical properties."
The most commonly used fibers are steel, carbon, asbestos etc. (high modulus fibers) and polyester, polypropylene (low modulus fibers). Properties of some fibers are listed in Table 1.

NBMCW January 2013

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Properties of Concrete by Replacement o...

Vinayak R.Supekar, Student, PGP-ACM. Civil (C&M), NICMAR, GOA
Popat D.Kumbhar, Associate Professor, Department of Civil Engineering
Rajarambapu Institute of Technology, Rajaramnagar, Sakharale.

Introduction

Concrete is the most widely used material of construction all over the world. A huge quantity of concrete is consumed by global construction industry. In India, the conventional concrete is mostly produced by using natural sand obtained from the riverbeds as fine aggregate. The advantage of natural sand is that the particles are cubical or rounded with smooth surface texture. The grading of natural sand is always not ideal. It depends upon place to place. Being cubical, rounded and smooth textured, it gives good workability. One of the important ingredients of conventional concrete is natural sand or river sand. However, due to the increased use of concrete in almost all types of construction works, the demand of natural or river sand has been increased. The infrastructure development such as express highway projects, power projects and industrial developments have started in a big way now. Available natural sand is getting depleted and also it is becoming costly [1]. Thus, to meet these increased demands of construction industry, excessive quarrying of sand from river beds is taking place causing the shortage of natural sand. This scarcity of natural sand due to such heavy demands in growing construction activities have forced engineers to find a suitable substitute. One of the cheapest and the easiest ways of getting substitute for natural sand is by crushing natural stone to get artificial sand of desired size and grade [2]. The use of artificial sand will conserve the natural resources for sustainable development of the concrete in construction industry [3].

Artificial sand is a process controlled crushed fine aggregate produced from quarried stone by crushing or grinding and classification to obtain a controlled gradation product that completely passes the 4.75mm sieve. Artificial sand generally contain more angular particles with rough surface textures and flatter face than natural sand that are more rounded as a result of weathering. Over the time some investigations have shown that angular particles, rough surface of artificial sand influences the workability and finish ability in fresh concrete. The artificial sand have to satisfy the technical requisites such as workability, strength and durability of concrete and hence it has become necessary to study these properties in order to check the suitability and appropriate replacement level of artificial sand in comparison with the natural sand for producing concretes in an economical way [4, 5].

In the present study, an attempt has been made to experimentally study the strength of concrete cubes and cracking patterns of concrete slab panels by replacing the natural sand with artificial sand at various replacement levels of 20%, 40%, 60% and 100%. The results have shown that the natural sand can be replaced with the artificial sand upto a maximum replacement level of 60% in order to produce concrete of satisfactory strength. The results have also indicated that concrete slab panels showed minimum area of cracks on its surfaces thus improving the durability property.

Experimental Work

Materials Used
The properties of various materials used in making the concrete (M20) are discussed in the following sections.

NBMCW November 2012

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Chemically Activated Blended Cements as...

S.K. Agarwal, V. Sood and L.P. Singh, EST Division, CSIR-Central Building Research Institute, Roorkee

Introduction

Sustainable cements with clinker factor of 0.5 or lower can be made using high volume of supplementary cementitious materials like fly ash, slag, metakaolin, silica fume and natural pozzolanas [1-2]. Since these cements are more suitable for producing highly durable concrete products and thus can be called sustainable cements. If these cements are judiciously blended with proper selection of admixtures, mixture proportioning and curing can noticeably improve the durability of concrete [3-4].

These cements are slower in setting and hardening compared to ordinary portland cement. Further these mineral additives reduce the early strength of concrete [5-10]. To compensate the low early strength chemical activation of cement has been employed. Though thermal and mechanical activation is also possible but are not usually employed due to certain limitations. Calcium chloride is most popular and cheapest activator available for accelerating setting behaviour of cement. However, considering corrosion effect on reinforcement it is not used in the concrete. Other available admixtures such as nitrites, nitrates, aluminates, calcium formate and triethanolamine have been reported [10-13].

In the present study, sodium sulfate and superplasticizer has been used to compensate loss of early strength. This paper presents the optimization of cement by replacing it with slag and fly ash of first field by activating with sodium sulfate and superplasticizer. The study will further help in reducing replacement of ordinary portland cement by these cementitious materials and in turn reducing clinker factor.

Experimental Investigation

Materials
Ordinary portland cement of 43 grade conforming to BIS 8112-2005 [14], Fly ash of first field collected from thermal power plant near Delhi and slag were used for preparation of cement cubes. The chemical composition of these cementitious materials used in the present study is given in table 1. Technical grade sodium sulfate was used as chemical activator. Superplasticizer (SP) based on sulphonated naphthalene formaldehyde condensate (SNF) conforming to BIS 9103 (2004) was used in the present study.

Table1- Chemical/Physical composition of Cementitious Materials
Composition,% Cement Fly ash Slag
SiO2 20.5 60.24 33.95
Al2O3 3.8 25.14 10.53
Fe2O3 2.6 4.75 1.25
CaO 60.5 3.10 40.40
MgO 3.2   8.65
Chloride content     0.03
SO3 2.5 1.35 0.10
CaO+MgO+ SiO2 __ __ 83.0
SiO2 + Al2O3 + Fe2O3 __ 90.13 __
LOI 1.0 1.32 0.54
Fineness cm2/gm 3100 2850 4020

The x-ray of the materials used in the present study is shown in figure 2-4.

NBMCW November 2012

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Geopolymer Concrete: Opportunities, Lim...

Dr Rakesh Kumar, Principal Scientist and Dr. Renu Mathur, Sr. Principal Scientist Rigid Pavements Division, CSIR - Central Road Research Institute (CRRI), New Delhi

Introduction

Portland cement concrete is the key construction material for all the development activities across the world. The concrete industry is the single largest consumer of the natural resources i.e. rocks, sand, and water. Each one of the constituent material of concrete has some adverse effects on the environment1. The manufacturing of the main constituent of concrete i.e. Portland cement is responsible for emission of about 6% of the total global anthropogenic carbon dioxide gas that is the key greenhouse gas held responsible for the global warming and climate change2-3. Compounded with the faster rates of depletion of materials needed for the manufacturing of Portland cement and good quality aggregates, concrete industries give rise to sustainability issues. On the other hand, there are ample possibilities for recycling of suitable industrial by-product materials for their high-volume high-end utilization in the concrete industry4. In light of the above facts, wide-spread innovative researches are being carried out all over the Globe for the high-valued utilization of suitable industrial by-products for the high-volume or complete replacement of the Portland cement from the concrete. Geo-polymer concrete (GPC) is a recent attempt in this direction. Natural or/and artificial pozzolans when combined with suitable alkaline activators yields to geopolymer which is used for binding aggregates to produce concrete for a wide range of applications. Concrete manufactured in this way is popularly known as geopolymer concrete. In last few decades, considerable interest has been generated in this concrete, particularly in light of today's burning issues i.e. sustainability and reduction of green house gas emissions from cement-based industries, responsible for global warming. The technology of geopolymerisation is not new and has been used in the construction of the Pyramids at Giza and other ancient civilizations structures5-6.

NBMCW November 2012

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Fly ash Utilization: Part I - Understan...

Geetha Seshadri, Gunjan Suri, Pranshu Chhabra Kapoor and Rakesh Kumar Khandal, Shriram Institute for Industrial Research, Delhi.

Introduction

With the success stories of research and development work conducted on fly ash, so far, the generation of fly ash is no longer being considered as an issue of concern or fly ash as an undesirable waste product. In fact, now fly ash is being considered as a useful resource for various industrial applications.

The successful experiences of utilization of fly ash in large volumes in value-added applications in the developed world have resulted in increasing the level of utilization of fly ash to the extent that it matches well with the generation of fly ash. Moreover, the life-cycle of products based on fly ash has been well understood as well as appreciated as a result of which fly ash based products are being taken as green materials.

Inspite of all this happening elsewhere, the scenario in developing countries is quite different from that in the developed world. Till the other day, there existed a kind of resistance in accepting fly ash as a useful material for different applications in the developing countries. With the time, the only change that has happened pertains to the perception of the people who have started accepting fly ash as a useful resource material. Even though, the exploitation of the potential of fly ash for different industrial applications has yet to attain desired levels, however, the utilization of fly ash in large volumes as a valuable resource is just a question of lime.

Because of the fact that the requirement of electricity has been rising and most of it would be generated from coal, the generation of fly ash would always be rising especially in developing countries like India. The quantities of fly ash generated during 2009 has touched a figure of 110 million tonnes. The estimates for the year 2030 indicate the production to cross 10 billion tonnes. Such large volumes of fly ash need to be disposed off in an environment-friendly manner to avoid the burden on the environment and to eliminate the adverse impacts due to the stocking of fly ash in ponds, on the ecology. The utilization levels for the year 2009 have been to the extent of nearly 50% of the fly ash generated. To achieve a utilization level of almost 100% of the fly ash generated, a lot of efforts are needed especially in applications where fly ash can be consumed in large volumes. This has to be done not just for a given year but for year-after-year on a sustainable basis to dispose-off the ever increasing quantities of fly ash with time.

The experiences so far have been good and their success is worth emulating. The utilization of fly ash, so far, has largely been in applications where fly ash is used either as a replacement for the conventionally used materials or as an additive in certain applications to benefit from the synergistic or advantageous attributes of fly ash. The applications where fly ash can be used to produce highly valuable materials have yet to become a reality. There are several bottlenecks at various stages. Unless such bottlenecks are removed, the goal of converting all of the fly ash into useful and valuable materials will remain far-fetched.

The present paper touches upon some of the important aspects essential for exploitation of fly ash. Not only the areas where fly ash can become a resource material are discussed but the reasons as to why it has not been happening in several other areas are also highlighted. Further, the path forward leading to the exploitation of fly ash in high volumes and at the same time high value applications has also been suggested.

NBMCW October 2012

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Properties of Fibre Reinforced Concrete...

Rattandeep Singh, M.Tech (St.), Deepak Gupta, M. Tech (St.) Asso. Professor, Punjab Agricultural University, Ludhiana. (Pb)

Introduction

Concrete is the basic engineering material used in most of the civil engineering structures, it is the premier construction material used widely across the world in most types of engineering works. According to research, concrete stands second to water on volume consumption basis Concrete is a composite material made up of cement, fine aggregates, coarse aggregates and water mixed in desired proportion based on strength requirement. Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which binds the other components together, eventually creating a robust stone-like material. Concrete is used more than any other man-made material in the world. Concrete like other engineering materials needs to be designed for properties like strength, durability and workability. Concrete mix design is the science of deciding relative proportions of ingredients of concrete, to achieve the desired properties of concrete. Use of mineral admixtures like fly ash, slag, metakaolin and steel fibre have revolutionized the concrete technology by increasing its strength and durability of by many folds. The concrete in which steel reinforcement bars, plates or fibres have been incorporated to strengthen a material is called reinforced concrete.

Now a days, construction and demolition industry is one of the country’s largest waste producers. The waste produced by demolition can be minimized and could be utilized again. Concrete recycling is an increasingly common method of disposing of dismantled concrete structures which was once routinely shipped to landfills for disposal. But recycling is now increasing due to improved environmental awareness, government laws and economic benefits. In India, about 14.5 MT of solid wastes are generated annually from construction industries, which include waste sand, gravel, bitumen concrete bricks, masonry, & concrete. However, some quantity of such waste material is being recycled and utilized in building materials. Most of the waste materials produced by demolished structures disposed off by dumping them in land fill. Dumping of wastes on land is causing shortage of dumping place in urban areas. Therefore, it is necessary to start recycling and re-use of demolition concrete waste to save environment. Concrete recycling gains importance because it protects natural resources and eliminates the need for disposal by using the readily available concrete as an aggregate source for new concrete or other applications. Large-scale recycling of demolished concrete will help conserve natural resources, and solve a growing waste-disposal crisis. The future for recycled aggregates will be driven by reduced landfill availability, greater product acceptance, continuing government recycling mandates, and the continuing decay of a large stock of existing infrastructure, as well as by the demands of a healthy economy.

The term "fly ash" is often used to describe any fine paniculate material precipitated from the stack gases of industrial furnaces burning solid fuels like coal. The characteristics and properties of different fly ashes depend on the nature of the fuel and the size of furnace used. Fine grade fly ash has acquired considerable importance in the building materials sector. Disposal of fly ash not only causes air and ground water pollution but also requires huge land area. It is estimated that 72 to 75 thermal power stations across country are presently producing more than 90 Million Tonne of fly ash per annum. For disposal of 400 to 500 MT of ash generation about 100 to 125 acres of waste land is required to dump fly ash. But now a days fly ash can be utilized in many ways in civil engineering works and it can save up to 15 to 25 % of construction cost.

Fibre-reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It also increases speed of construction and in some cases may even eliminate the need for conventional reinforcement. It contains short discrete fibres that are uniformly distributed and randomly oriented. Fibres include steel fibres, glass fibres, synthetic fibres and natural fibres. Within these different fibres that character of fibre-reinforced concrete changes with varying concretes, fibre materials, geometries, distribution, orientation and densities. Fibre-reinforced normal concrete are mostly used for on-ground floors and pavements, but can be considered for a wide range of construction parts. Fibres are usually used in concrete to control cracking due to both plastic shrinkage and drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibres produce greater impact, abrasion and shatter resistance in concrete. Steel is the strongest commonly-available fibre, and come in different lengths and shapes. Steel fibres can only be used on surfaces that can tolerate or avoid corrosion and rust stains.

It is expected that this study involving combination of waste materials i.e. recycled course aggregates with full replacement of fresh course aggregates and fly ash with partially replacement of cement and also addition of steel fibre to concrete, will be useful to the existing construction methodology.

NBMCW October 2012

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Study of Bond Strength of Concrete Usin...

Paratibha Aggarwal, Associate Professor, Babita Saini, Assistant Professor, Sarvesh Tripathi, M.Tech student, Deptt. of Civil Engg., N.I.T. Kurukshetra

Introduction

Concrete has been the leading building material since it was first used and is bound to maintain its significant role in the upcoming future due to its durability, maintenance free service life, adaptability to any shape and size, wide range of structural properties plus cost effectiveness. The concrete is the most important construction material which is manufactured at the site. It is the composite product obtained by mixing cement, water and an inert matrix of sand and gravel or crushed stone. It undergoes a number of operations such as transportation, placing, compaction and curing. The distinguishing property of concrete is the ability to harden under water. The ingredients can be classified into two groups namely active and inactive. The active group consists of cement and water, whereas the inactive group consists of fine and coarse aggregates. The inactive group is sometimes also called inert matrix. Concrete has high compressive strength but its tensile strength is very low. In situations where tensile stresses are developed the concrete is strengthened by using steel bars or short randomly distributed fibres forming a composite material called reinforced cement concrete (RCC) or fibre reinforced concrete. The resistance of concrete to the slipping of reinforcing bars embedded in concrete is called bond strength. The bond strength is provided by adhesion of hardened cement paste and by the friction between concrete and reinforcement. It is also affected by the shrinkage of concrete relative to steel. On an average bond strength is taken approximately as 10% of the compressive strength. The roughness of the steel surface, water, the chemical composition of cement and steel bar diameter are the factors that affect the bond strength of concrete. In pull-out tests on plain bars, the maximum load generally represents the bond strength that can be developed between the concrete and steel. With plain bars the maximum load is not very different from the load at the first visible slip, but in the case of the deformed bar, the maximum load may correspond to a large slip which may not in fact be obtained in practice before other types of failure occur. The load shall be applied to the reinforcing bar at a rate not greater than 2250 kg/mm, or at no-load speed of the testing machine head of not greater than 1.25 mm/min, depending on the type of testing machine used and the means provided for ascertaining or controlling speeds. The maximum load for each type of failure shall be recorded. The new replaces the old and same follows with the buildings. Older buildings require reconstruction for better and higher economic gains and on account of obsolescence on structural or functional grounds and also due to the damages inflicted on them by natural disasters and wars. The rate of demolition showed an upward trend which in turn increased the dumping costs due to unavailability of appropriate sites nearby. Thus efficient use of the demolished concrete would reduce the costs and definitely lead to conservation of the invaluable non-renewable sources of energy and hence must be given due importance. The demolished concrete could be used as aggregate for concrete resulting in large consumption of the material. Recycling is the act of processing the used material for use in creating new product. The usage of natural aggregate is getting more intense with the development in infrastructure area. In order to reduce the usage of natural aggregate, recycled aggregate can be used as the replacement materials. Recycled aggregate are comprised crushed, graded inorganic particles obtained from the materials that have been used in the constructions and demolition debris. These materials are generally from buildings, roads, bridges, and sometimes even from catastrophes, such as wars and earthquakes.

NBMCW October 2012

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Self-compacting Concrete with Silica F...

Dr. D.K. Kulkarni, Professor; Dr. S.B.Vanakudare, Professor & Head Civil Engineering Department, S.D.M.College of Engineering, Dharwad

Introduction

In today’s world, more attention is paid to healthy and pollutant-free environment, so that proper utilization of waste produced by various industries is of prime importance. Recently, a trend has developed to use GGBS, silica fume, fly ash and metakaolin in the construction industry1.

With the beginning of the twenty-first century, we are entering into an area of sustainable development, since it will not be possible, in the future, to consider technological aspects only, without giving equal importance to the ecological balance in the planet earth. From this point of view, cement and concrete industries can be considered to be environment-friendly since a large amount of waste materials, such as fly ash, blast furnace slag, metakaolin and silica fume in cement and concrete industries is helpful for both manufacturing cementitious products with improved properties and for reducing the disposal of waste materials2.

From waste to wealth is the concept with which concrete technologists, civil engineers and researchers are constantly working in this direction in exploring the properties of almost all the materials available both as natural and artificial resources, which can partially replace cement. Research work has shown that certain materials in nature possess pozzolanic property. Pozzolanic materials possess little or no cementitious value within themselves but when come in contact with moisture and calcium, hydroxide exhibits cementitious properties3.

With the advent of the portland cement concrete cult in India in the early part of the twentieth century, there was close adoption of the technological practices of the west; however the local materials had to be adopted to the local market conditions and the native sentiments had to be fully respected. This lead to preferred development of blended cements over the alternative practice of using cement substitute materials in site-mixed concrete, which was limited to large mass concreting project sites only4.

The use of pozzolanic and cementitious materials in the cement and concrete industry has risen sharply during the last fifty years as a mineral admixture. The reasons behind it are many. Cement requires high energy during its manufacturing process whereas fly ash, blast furnace slag, metakaolin and silica fume are the industrial by-products that are easily available and require little or not much of processing having inherent cementitious properties5.

NBMCW October 2012

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Materials and Methods for High Performa...

High Performance Concrete

Sunny Surlaker, MC-Bauchemie India Private Limited, Mumbai

Introduction

High Performance Concrete (HPC) should have at least one property like High strength, High durability, Acid Resistance, Self-compaction, Low permeability to water, Chemicals or other aggressive media, as compared to normal concrete, to qualify as HPC.

Material technology has evolved concrete today into an engineered material with several new constituents. The concrete today is tailor made for specific applications and it contains several different materials like PFA, GGBSF, Microsilica, Metakaolin, Colloidal Silica and several other Binders, Fillers and Pozzolanic materials. The development of specifying the concretes as per its performance requirements rather than the constituents and ingredients in concrete has opened innumerable opportunities for producer and user of concrete to design concrete as per specific requirements.

As always, durability would be the prime consideration as most structures cannot be easily replaced or repaired. Depending upon the appropriate selection of exposure classes, specialized specifications for concrete mix designs would need to evolve. The mix designs are getting relatively complex on account of interaction of several materials and calls for expertise in concrete technology. HPC will soon become the norm considering its special properties and low cost maintenance strategies.

High Performance Concrete (HPC)

There are no unified definitions for High Performance Concretes (HPC) and different Institutions and experts in concrete technology defineit differently. The American Concrete Institute defines High Performance Concrete as "Concrete that meets special performance and uniformity requirements that cannot always be obtained by using conventional ingredients, normal mixing procedure and typical curing practices."

With the requirements, we place on concrete construction today in terms of durability, speed of construction, strength, long workability, most concretes today qualify as High Performance Concretes (HPC). It should be remembered that HPC is not only a function of the concrete, but is a system composed of the latest generation of materials (OPC, PFA, Aluminosilicates/Microsilica and PCE Admixtures), state-of-the-art mix design practices, use of the latest technology to place and finish it and effective curing. Only when every step of the process above is given due diligence, will the resulting concrete end up as high performance concrete.

Based on the experience of the author and studies worldwide, it is seen and proved that HPC can only be made with the best materials and practices available. General Experience shows that the composition of HPC differs substantially from normal concretes.

NBMCW October 2012

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Effect of Fly Ash on Recycled Aggregate...

Use of recycled aggregate in concrete can be useful for environmental protection and economical terms. Recycled aggregates are the materials for the future. Again fly ash is also a by product from various industries. Hence a study was needed to arrive at various combinations of both of these materials. Hence a popular concrete mix 0.5:1:1:2 was used in this study for experimental purpose. Fly ash confirming to IS 3812:2003 was used as a part replacement of cement. Fly ash was replaced by cement in % of 0%, 10%, 20%, 30% and 40%. Again natural aggregates were replaced by recycled coarse aggregates (RCA) in % of 0%, 10%, 20%, 30% 50% and 100%. Thus total thirty different trial mixes were prepared and the effects of RCA with fly ash, on fresh and hardended properties have been determined.

D. N. Parekh, Research Scholar, and
Dr. C. D. Modhera, Professor, Applied Mechanics Department; SVNIT; Surat.

Introduction

Globally, the concrete industry consumes large quantities of natural resources, which are becoming insufficient to meet the increasing demands. At the same time, large number of old buildings 8 other structures have reached the end of their service life and are being demolished, resulting in generation of demolished concrete. Some of this concrete waste is used as backfill material, and much being sent to landfills. Recycling concrete by using it as replacement to new aggregate in concrete could reduce concrete waste and conserve natural sources of aggregate. In the last two decades, a variety of recycling methods for construction and demolition wastes (CDW) have been explored and are in well developed stage. It is known as recycled aggregate (RA). BS EN Standards [1,2] recommends for recycled concrete to be used in secondary structural members of relatively low grades, e.g. curbs, paving blocks and ground bearing floor slabs.

Review of Literature

Cho and Yeo (2004) [3] found that, due to the high water absorption of the recycled aggregate, a higher slump loss was observed when compared to that of natural aggregate concrete. Dhir et al. (1999) [4,5] showed that the compressive strength of concrete prepared with 100% coarse and 50% fine recycled aggregates was between 20 and 30% lower than that of the corresponding natural aggregate concrete. However, the reduction in strength can be minimized if the mixing procedure is modified (Otsuki et al. 2003; Tam et al. 2005) [6,7]. Further, Olorunsogo and Padayachee (2002) [8] found that the water absorptivity of concrete prepared with 100% recycled aggregate was higher than that of natural aggregate concrete at the curing age of 28 days. Abou-Zeid et al. (2005) [9] reported that recycled aggregate concrete exhibited higher water permeability and lower resistance to chloride ion penetration compared to conventional concrete. Salem et al. (2003) [10] showed that recycled aggregate concrete had a lower resistance to freezing and thawing compared to natural agg. concrete. Otsuki et al. (2003) [6] reported that the carbonation resistance of recycled aggregate concrete was inferior compared to that of natural aggregate concrete. Finally, the drying shrinkage and creep of recycled aggregate concrete was found to be higher than those of natural aggregate concrete (Tavakoli and Soroushian 1996; Gomez-Soberon 2003) [11, 12].

Fly ash is known to be a good pozzolanic material and has been used to increase the ultimate compressive strength and workability of fresh concrete (Mehta 1985) [13]. Naik and Ramme (1989) [14] produced concrete mixes containing large quantities of fly ash which achieved compressive strengths of 21 and 28 MPa within 28 days.

Experimental Programme

Following experimental programme was taken up on different materials as per respective Indian Standards. Utmost care was taken for preservation of materials, to the testing of samples and taking readings.

All materials, i.e. fly ash, cement, aggregates etc were tested as per respective IS. Both the normal and recycled aggregates used in this study were in an air-dried state. A constant effective water/binder ratio (w/b) was maintained at 0.5 (or effective water of 266.7 kg/m3) for all concrete mixtures. Since the recycled aggregates had greater water absorption than normal aggregates, they required more water to maintain the slump of fresh concrete from 50 to 100 mm.

Now, Mix type i. e. Mix A prepared with constant proportions of 0.5:1:1:2. Cement content in this mix has been replaced by fly ash in 10%, 20%, 30% and 40%. Natural aggregates (NA) were replaced by Recycled Aggregate (RA) of 0%, 10%, 20%, 30%, 50%, and 100%. Detail proportions with mix designations are given below in Table 1.

Table 1 Mix Proportions A with fly ash
DesignationWaterCementFly AshFine AggregateCoarse Aggregate
RecycledNatural
AF00R000.510102
AF00R100.51010.21.8
AF00R200.51010.41.6
AF00R300.51010.61.4
AF00R500.510111
AF00R1000.510120
AF10R000.50.90.1102
AF10R100.50.90.110.21.8
AF10R200.50.90.110.41.6
AF10R300.50.90.110.61.4
AF10R500.50.90.1111
AF10R1000.50.90.1120
AF20R000.50.80.2102
AF20R100.50.80.210.21.8
AF20R200.50.80.210.41.6
AF20R300.50.80.210.61.4
AF20R500.50.80.2111
AF20R1000.50.80.2120
AF30R000.50.70.3102
AF30R100.50.70.310.21.8
AF30R200.50.70.310.41.6
AF30R300.50.70.310.61.4
AF30R500.50.70.3111
AF30R1000.50.70.3120
AF40R000.50.60.4102
AF40R100.50.60.410.21.8
AF40R200.50.60.410.41.6
AF40R300.50.60.410.61.4
AF40R500.50.60.4111
AF40R1000.50.60.4120

These mixes were prepared tested in laboratory for fresh and hardended properties of RAC. Comparisons for the different results are prepared with graphs.

Materials Testing

Fly ash and Cement
The fly ash used here was in accordance with its IS 3812 (Part 1): 2003 [15]. Chemical requirem- ents and physical requirements of fly ash were in accordance with table 1 and table 2 of IS 3812 (Part 1): 2003 [15] respectively.

An OPC 53 grade conforming to IS 12269 – 1987 [16] was used throughout the study for concrete production. The samples of cement were taken in accordance with the requirements of standard IS: 3535-1986[17] and tested by using the relevant specification of IS [18-27]. All results were shown in Table 2 with their respective IS requirements for comparisons.

Table 2 : Results of Cement
NoType of TestResultsIS - Limits
1.Type of CementOPC 53 Grade-
2.Fineness Test8%<10%
3.Normal Consistency29%-
4.Soundness Test.4.9 mm< 10 mm
5.Initial Setting time85 Minutes>30 min
6.Final Setting time.
198 Minutes
<600 min

7.

Compressive Strength Test27.87MPa (3 – Days)>27 MPa
37.66MPa (7 – Days)>37 MPa
54.73MPa (28 – Days)>53 MPa

Fine and Coarse Aggregates
Figure 1
Figure 1: Grading Curve of Fine and Coarse Aggregate
Local river sand with a fineness modulus of 3.04 was used as a fine aggregate. Crushed limestone with a maximum size of 20 mm was used as coarse aggregate. Jaw crusher was used for preparation of recycled aggregates and opening for size has been decided by considering the Fineness Modulus (FM) of NCA. In this study, NCA and RCA were used with same FM to overcome aggregate size effect in concrete. Samples for any testing of aggregate were prepared in accordance with IS: 2430 – 1995[22]. Sieve Analysis and other testes of aggregates were carried out as per the guidelines given by respective IS [18] and detail grading chart is shown below in Fig 1.

Elongation and flakiness index tests were carried out in accordance with IS: 2386 (PART I) – 1997[19], while specific gravity, apparent specific gravity, water absorption and bulk density were determined using IS: 2386 (PART III) – 1997[20]. Results were shown in Table 3.

Table 3 :Basic properties of aggregates
NCARANFA
Specific Gravity2.842.352.70
Apparent Sp. Gravity2.772.312. 67
Water Absorption0.89%7.57%0.80%
Bulk Density (kg/m3)149012901480
Apparent Density (kg/m3)282025102680
Flakiness Value (%)14.4 %25.33 %-
Elongation Value (%)16.45 %21.50 %-

Table 4 : Mechanical properties of Aggregates
NCARA
Impact Value (%)8.88 %15.03 %
Crushing Value (%)14.04 %25.52 %
Abrasion Value (%)15.58 %26.70 %
IS Limit (For Road) (%)

30.00 %

IS Limit (For Buildings) (%)
  1. % (50% for abrasion)

Table 5 : Workability for Mix A with different % of FA and RCA (in slump value)
Fly ash mixDifferent % of RA
010203050100
AF00135125110857560
AF10135130115908575
AF20140135120958580
AF301451351201009080
AF4015514512510510090

Mechanical properties of aggregates were determined by using IS: 2386 (Part IV) – 1997[21] and shown in table 4 below. It shows that all aggregates results were within prescribed IS limits.

Concrete Testing
Sampling and analysis of concrete have been carried out as per IS: 1199 – 1959[18]. The quantities of cement, fly ash, silica fume, each size of aggregate, and water for each batch was determined by weight, to an accuracy of 0.1 percent of the total weight of the batch. Fresh properties of concrete (i.e. workability) was measured with help of slump cone test and test was carried out in accordance with IS:1199 – 1959[18]. The cubical moulds were used 150x150x150 mm while cylindrical mould was of 150 mm diameter and 300 mm height and both conforming to IS: 10086-1982[32]. All strength tests were carried out in accordance with IS: 516 – 1963 [33], while split tensile strength of concrete was measured in accordance with IS: 5816 – 1999[30].

Results and Discussions

Fresh Properties of Concrete
Fly ash is spherical, and when used in concrete typically increases slump or reduces the required mixing water. However, results in figure 2 indicates that the slump of fresh concretes containing recycled aggregates and fly ash increased slightly with increased fly ash replacement. For example, AF00, AF10, AF20, AF30 and AF40 with 100% replacement of recycled aggregate had initial slumps of 60, 75, 80, 80 and 90 mm respectively. The slightly greater slump of fresh concrete containing greater fly ash replacement could have been caused by a greater volume of paste (ground fly ash had a much lower specific gravity than Portland cement), leading to reduced aggregate particle interference and enhanced concrete workability. In addition, perhaps not all of the fly ash particles were ground and sufficient spherical particles exist to help lubricate the mixture. This result was similar to Ravina (1984)[34] who found that fly ash can reduce slump loss when it is partially used to replace cement.

Ratio of different quantities i.e. fct/fck , fcr/fck or E/fck gives clear idea regarding relationship of both quantities with % of recycled coarse aggregates. Again similar pattern of results were observed for all graphs.

Figure 2Figure 3
Figure 2: Workability for Mix A with different % of FA and RCAFigure 3: fck7 for Mix A with different % of FA and RCA (MPa)

Figure 4Figure 5
Figure 4: fck for Mix A with different % of FA and RCA (MPa)Figure 5: Ratio of fct/fck for Mix A with different % of FA and RCA

Figure 6Figure 7
Figure 6: Ratio of fcr/fck for Mix A with different % of FA and RCAFigure 7: Ratio of E/fck for Mix A with different % of FA and RCA

Table 6 : fck7 for Mix A with different % of FA and RCA (MPa)
Fly ash mixesDifferent % of RCA
010203050100
AF0028.427.9527.1726.3423.8721.25
AF1027.7527.126.3324.922.520.2
AF2026.525.8524.8823.5621.8919.41
AF3025.0724.2523.522.3320.618.12
AF4023.222.521.7520.6619.116.15

Table 7 : fck for Mix A with different % of FA and RCA (MPa)
Fly ash mixesDifferent % of RCA
010203050100
AF0040.7140.3339.6737.533.630
AF1040.4139.1637.7535.6732.4629.5
AF2039.2337.535.6734.0231.8127.95
AF3033.232.2131.1430.4628.8825.37
AF4030.129.8928.4927.5826.1122.83

Table 8 : fct for Mix A with different % of FA and RCA (MPa)
Fly ash mixesDifferent % of RCA
010203050100
AF004.624.594.544.153.522.97
AF104.614.524.213.893.22.81
AF204.414.123.783.453.022.55
AF304.254.013.73.53.22.6
AF404.23.933.63.383.042.5

Table 9 : fcr for Mix A with different % of FA and RCA (MPa)
Fly ash mixesDifferent % of RCA
010203050100
AF004.624.594.544.153.522.97
AF104.614.524.213.893.22.81
AF204.414.123.783.453.022.55
AF304.254.013.73.53.22.6
AF404.23.933.63.383.042.5

Table 10 : E for Mix A with different % of FA and RCA (MPa)
Fly ash mixesDifferent % of RCA
010203050100
AF00267292664626500260072507324143
AF10266642638626065255772478624008
AF20264012600725577251772461923580
AF30249732472224445242662383922831
AF40241702411423731234762305122042

Conclusions

Following conclusions are drawn.
  • 10% of RA and 10% of fly ash is giving the same results as that of 100% NA and 0% fly ash.
  • Workability for fresh concrete was increased with increase of incorporation of fly ash. Though due to incorporation of RA workability is going on decreasing with increase of % of RA
  • Compressive strength of mix A prepared with 100% RA and 40% fly ash are lower than 100% NA and 0% fly ash concrete, by nearly 44%.
  • Ratio of fct/fck or fcr/fck is giving clear idea regarding change of values for 40% fly ash incorporation.
It can be generally concluded that 10% of RA with 10% of fly ash might give better result than 10% RA and 0% fly ash for workability point of view.

Acknowledgement

Research reported in this research was supported by Gujarat Council of Science and Technology (GujCOST), Department of Science and Technology; Govt of India. This support is greatly acknowledged.

References

  • BS 8500-1 Concrete. Complementary British standard to BS EN 206-1. Method of specifying and guidance for the specifier. BSI; 2006.
  • BS 8500-2 Concrete. Complementary British standard to BS EN 206-1. Specification for constituent materials and concrete. BSI; 2006.
  • Cho, Y. H., and Yeo, S. H. (2004). "Appli- cation of recycled waste aggregate to lean concrete subbase in highway pavement." Can. J. Civ. Eng.,31(6), 1101–1108.
  • Dhir, R. K., Limbachiya, M. C., and Leelawat, T. (1999). "Suitability of recycled concrete aggregate for use in BS 5328 designated mixes." Proc. Inst. Civ. Eng., Struct. Build., 134(4), 257–274.
  • Dhir, R. K., Munday, J. G. L., and Ong, L. T. (1986). "Investigations of the engineering properties of OPC/pulverized-fuel ash concrete: De- formation properties." Struct. Eng., 64B (2), 36–42.
  • Otsuki, N., Miyazat, S., and Yodsudjai,W. (2003). "Influence of recycled aggregate on interfacial transition zone, strength, chloride penetration, and carbonation of concrete." J. Mater. Civ. Eng., 15(5), 443–451.
  • Ravindrajah R S, Loo Y H and Tam C T. (2005) Strength evaluation of recycled aggregate concrete by in-situ tests. Mat Stru; 21(4):289–95.
  • Olorunsogo, F. T., and Padayachee, N. (2002). "Performance of recycled aggregate concrete monitored by durability indexes." Cem. Concr. Res.,32(2), 179–185.
  • Abou-Zeid, M. N., Shenouda, M. N., McCabe, S., and El-Tawil F. A. (2005). "Reincarnation of concrete." Conc. Int.,27 (2), 53–59.
  • Salem, R. M., Burdette, E. G., and Jack- son, N. M. (2003). "Resistance to freezing and thawing of recycled aggregate con- crete." ACI Mater. J., 100 (3), 216–221.
  • Tavakoli, M., and Soroushian, P. (1996). "Drying shrinkage behavior of recycled aggregate concrete." Conc. Int., 18 (11), 58–61.
  • Gomez-Soberon, J. M. V. (2003). "Relation- ship between gas absorption and the shrinkage and creep of recycled aggregate concrete." Cem Conc Res,25(2), 42–48.
  • Mehta, P. K. (1985). "Influence of fly ash characteristics on the strength of Portland-fly ash mixture." Cem. Concr. Res.,15 (4), 669–674.
  • Naik, T. R., and Ramme, B. W. (1989). "High-strength concrete containing large quantities of fly ash." ACI Mater. J.,86 (2), 111–116.
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  • IS: 12269 – 1987 Specifications for 53 – grade ordinary Portland cement.
  • IS: 3535 – 1986 Method of Sampling hydraulic cements – First Revision.
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  • IS: 2386 – 1997 (PART III) Method of Test for Aggregate for Concrete – Specific Gravity, Density, Voids, Absorption and Bulking.
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  • IS: 4031 – 1988 (Part – 4) Methods of physical tests for hydraulic cement – Determination of consistency of standard cement paste (First Revision)
  • IS: 4031 - 1988 (Part-5) Methods of physical tests for hydraulic cement - Determination of Initial and Final Setting Times (First Revision)
  • IS: 4031 – 1988 (Part – 6) Method of Physical Tests for Hydraulic Cement - Determination of Compressive Strength of hydraulic cement other than masonry cement.
  • IS: 5513 - l976 Specification for Vicat Apparatus.
  • IS: 5514 – 196 Specification for apparatus used in Le-Chatelier test.
  • IS: 5816 – 1999 Splitting tensile strength of concrete – Method of Test.
  • IS: 10080 – 1982 Specification for vibrating machine for casting standard cement mortar cubes.
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NBMCW August 2012

.....

Applications of High Performance Concre...

Jitendra Thakur, Dy. Chief Engineer, Jaypee (Hydro Div.); Noida

General

Hydropower Structures are constructed to tap the untamed water resources and generate useful Electrical Energy for the benefit of mankind. The quantity of concrete consumed in making various components of hydropower structures is enormous. A few of its components viz. glacis of Spillway, Diversion Tunnel, Head Race Tunnel, Silt Flushing Tunnel, Tail race Tunnels etc. are required to be lined/coated with High Performance Concrete so that their performance in handling high velocities of water and huge quantities of silt is enhanced. The use of High Performance concrete has resulted into lesser repairs, on one hand, and increased durability, on the other hand.

High Performance Concrete

The high Performance Concrete is defined as the concrete that cannot be made by conventional methods of manufacturing concrete. The Definition of High Performance Concrete, applicable to Hydropower Structures, may differ with that of the definition used for High Performance Concrete used for Infrastructure say Bridges wherein High Performance Concrete is required for Early High Strength, greater span, reduced depths of members etc. but in case of Hydropower Structures, High Performance Concrete is required basically to have higher resistance against abrasive, erosive and cavitational action of moving water. Structures are lined/coated with High Performance Concrete to enhance their performance.

Three Gorges DamKrishnaraja Sagar Dam
Three Gorges Dam (China – 2009) built on River YangtzeKrishnaraja Sagar Dam (Karnataka - 1924) built on river Kaveri

During sixties Dams were built in Masonary. The structures were bulky and were lesser stressed. A few exmples are Nagarjuna Sagar Dam (Andhra Pradesh), Krishnaraja Sagar Dam (KN) Gandhi Sagar Dam (MP) Jawai Dam (RJ). They were built in Masonary and are performing nicely. The openings (gates) to allow flood to pass were greater in number and velocities at spillway were lesser.

Tata HEP
Tata HEP (Bhutan – 2007) built on river Wangchu
With more innovation in the field of materials and equally more understanding of loads, the factor of safety kept for the uncertainties is decreasing. Today, the gate sizes are bigger than yester years, less in numbers; tunnels are smaller in size, resulting into increased values of stresses. The construction time is also reducing and putting more pressure on quality aspects of the construction. A few recently completed schemes are Karcham HEP (1000 MW), Tala HEP (1020MW), Baghlihar HEP(450MW) etc.

Concrete is becoming more like scientific material rather than the material which Sir John Smeaton had developed in 1756 by adding pebbles as a coarse aggregates and brick powder into cement. This was later modified by Sir Joseph Aspdin in 1824, who created Cement by burning ground limestone and clay together.

Today's concrete, with the help of innovative peripherals, has developed following capacities:

(i) it can remain in liquid state for a longer duration,
(ii) it can be prepared with very small quantity of water, and
(iii) it can also gulp waste materials having pozzalanic reactivity like flyash, Ground Granulated Blast furnace Slag, Silica fume, Rice Husk ash etc. to give itself strength and increased life.

The addition of mineral admixtures, while manufacturing concrete, tends to densify the transition zone between aggregates and matrix in the structure of the concrete. This makes the transition zone more stronger and more energy is required to break the transition zone, which generally is weakest zone and responsible for failure of concrete. Thus, the resistance against abrasion increases. Addition of Mineral Admixture, lesser quantity of water and optimum quantity of super-plasticizer makes concrete more durable as compared to conventional concrete.

Spillway Structure

The flood water is allowed to pass the Dam through its spillway. The quantum of Discharge depends upon Probable Maximum Flood to be passed. The velocity of water passing at this structure is quite high. The Glacis of Spillway tends to bear velocities to the tune of 25 to 60m/s. This structure is most vulnerable from erosion point of view. Considerable damage to stilling basin is generally caused by eddy currents coupled with high velocity flow and impact due to debris. The galcis, stilling basin, bucket etc. suffer huge damages when flood is allowed to pass through it.

Tehri Spillway Structure

Three Gorges Dam China
Three Gorges Dam (China – 2009) built on River Yangtze
Tehri HEP is located in Uttaranchal. The spillway is designed for and inflow flood of about 15,500 cumecs. The flood while passing the spillway, negotiates a head of about 200, causing its velocity to be of the order of 55-60m/s. Such velocities are very high and can easily erode any concrete surface. The main consideration in designing the mix for glacis is to achieve lowest abrasion loss and resistance to impact.

The prime aim of designing concrete of glacis is to have lower loss of material against erosive action of water laden with heavy silts. High Performance Concrete (HPC) is considered necessary for guarding against cavitations damages and for taking care of possible damages due to abrasion. It was found that the increase in quantity of mineral admixture ie. Microsilica along with other aspects of Mix Design, the resistance against abrasion improved considerably from 1.33% for M30 to 0.94% for M70. The mix proportion adopted is given in Table 1:

Table 1 : Concrete Mixes for Tehri Dam Project
Mix IngredientsRecommended Grades of concrete and Quality of Mix Ingredients in kg/m3
M30M50M60M70
Jaypee Cement317340365380
Microsilica343738
CA-1 ( 20mm)684696687680
CA-2 ( 10mm)456464458454
FA-1 ( Crushed stones)266271268265
FA-2 ( Natural sand)494503496491
Water143142140142
Superplasticizer (% of binder)1221.75
W/B ratio0.380.350.34
W/c ratio0.450.420.380.37
Initial slump175180230185
Slump after 45 min.120110160120
Compressive strength results
3 days20.8929.3343.2951.16
7 days28.1343.8256.4960.85
28 days35.7356.1868.2375.4
abrasion loss after 72 hr (%)
High speed (3360 rpm)6.263.062.882.75
slow speed (1100 rpm)1.331.11.050.94

Chukha Spillway Structure

Chukha HEP is 336 MW Project and is located in Bhutan. In this Project, M20A20 grade concrete was used alongwith rails in spillway and the project was commissioned during 1986. The erosion of concrete between rails was found to vary from 10mm to 150mm. Repairs of spillway glacis were thereafter carried out by filling the cavities on top layer of concrete with High performance concrete and was found to perform satisfactorily.

Damaged Observed in SpillwayDamaged Observed in Stilling Basin
Damages Observed in SpillwayDamages Observed in Stilling Basin

Kinzua Dam Stilling Basin, USA

Damages to stilling basins have occurred even in advanced countries. Typical amongst these is the stilling basin of Kinzua Dam on the Allegheny River in Western Pennsylvania (USA). It was put to operation initially in 1967, but experienced severe abrasion /erosion damages. The basin was repaired in 1973-74 using steel fiber reinforced concrete overlay. Deterioration continued to the extent that repairs were again necessary in 1983. Thereafter, repairs were carried out with high performance concrete made with micro silica.

The performance of this reconstruction work has been regularly inspected. Previous repairs using conventional concrete had a life far less than one year, the micro silica concrete exhibited very little wear (Table 2).

Table 2 :
Mix Ingredients
Cement, Type I/Il386 kg/m3
Silica Fume Slurry (46% dry powder)156
Water80
Silica fume70
Admixtures6
Coarse Aggregate,19 mm,SSD 971
Fine Aggregate,SSD 824
Water (added as batch water)50
w/cm0.28
Fresh Properties
Average air Content:3.2 %
Average slump:250 mm
Average unit weight:2444 kg/m3
Silica fume by cement mass:18%
Core compressive strength
7 days, 28 days, 90 days test results from Contractor72.9, 89.1, 107.0
28 days, 90 days test results from Owner94.6, 103.2

Tala Dam

Tata HEP - Spillway Glacis
Tala HEP- Spillway Glacis
Tala HEP is built on River Wangkha in Bhutan. The scheme generates 1020 MW of power. The spillway glacis was constructed with M50A40 grade with microsilica fume for taking care of possible damages due to abrasion. The abrasion loss for mix without micro silica was 5.57% at 72 hrs. while it was 3.76% for M50 grade concrete and further lower 2.84% for M70 concrete. The cement content was reduced from 460 kg to 400 kg by addition of about 4% silicafume. This indicates that a considerable improvement in abrasion resistance can be achieved by the use of micro silica. The chloride permeability also decreased considerably due to addition of mineral admixture. Thus, making concrete more durable. The mix proportion adopted is given in Table 3.

Power Intake

Intake is the first structure of the water conductor system. The water passes from Intake to HRT and thereafter to the Power House for the generation of Electricity. The velocity at this structure is about 4-5 m/s along with some silt. The silt gets flushed in the Desilting Chamber before entering into HRT.

Tala Power Intake

The ternary system has been used in producing M30 A40 grade pumpable concrete for use in transition portion from rectangular to horse shoe shape in intake structure. The mix proportion adopted is given in Table 4.

By adding about 7-8% micro silica, abrasion resistance of concrete is improved. The micro silica in concrete is being used mainly for improving abrasion resistance of concrete. In intake structure, the micro silica is used in M30 A40 grade concrete for 5m transition portion from rectangular to horseshoe shape, down stream of service gate. As this portion of concrete structure is likely to be subjected to higher abrasive forces due to high velocity of water, use of micro silica was considered necessary. The pumped concrete with a slump of about 150 mm was produced using 7.5 % micro silica by weight of cement (PSC).

Table 3 : Concrete Mixes for Tala Dam Project
Mix IngredientsRecommended Grades of concrete and Quality of Mix Ingredients in kg/m3
M30M50M60M70
Slag Cement460400400400
Silicafume0404040
CA-1 ( 20mm)693788830830
CA-2 ( 10mm)520544564577
FA (River sand)520544475520
Water207150135108
Superplasticizer (% of binder)01.751.251.95
W/B ratio0.450.340.30.245
Compressive strength results
3 days17.7827.0742.1343.82
7 days26.048.0456.3162.58
28 days38.3359.7469.2378.18
Abrasion loss after 72 hr6.713.763.302.84
Water Permeability (depth of Penetration)90nilnilNil
Chloride Permeability RCPT (Coloumbs)2635237173125

Table 4 : Concrete Mix design for Power Intake
IngredientsCementMicrosilicaFACA 40mmCA 20mmCA 10mmWaterSuperplaticizer
Quantity (kg/m3)400305876203342381756.02

Head Race Tunnel

Head race Tunnel is one of the key elements in deciding the cost of a Hydropower Scheme. The smaller is the diameter of Head Race Tunnel, the economical is the project. The length of HRT is generally in kilometers. The Tala HRT is 23 km long. Karcham HRT is 17.2 km long. As the diameter of the HRT is decreased, the velocity of flow increases, resulting into erosion and cavitation problems. It becomes utmost important to control the quality of HRT lining so that repairs are less and performance is better.

Damaged Observed Head Race TunnelDamaged Observed Diversion Tunnel
Damages observed in Head Race TunnelDamages observed in Diversion Tunnel

Tala Head Race Tunnel

Tala HRT is 6.8m in diameter and is 23km long. The concrete mix design of M20A20 grade was used in HRT lining. The mix was designed initially with a cement content of 375 kg/m3 (OPC) and was further revised with a cement content of 350 kg/m3 (OPC) in addition to micro silica content of 25 kg/m3. However, due to extensive cracking in concrete lining and alkali silica reactive aggregates with OPC, use of PSC without microsilica was resorted to. The cement content in concrete lining was optimised to avoid cracking in concrete due to heat of hydration of cement. The mix design with different cement content by using OPC and PSC are given in Table 5:

Table 5 : Mix design of M20A20 grade concrete for HRT lining
Sr. No.Cement Content (kg/m3)Type of CementWaterFA20 mm CA10 mm CASuperplaticizer (kg/m3)Remark
1375OPC-43G1847795994004.5Kerb, overt & Invert stage II
2350+25MSOPC-43G1907026374255.63Kerb, overt & Invert stage II
3350PSC178.57276544365.25Kerb, overt & Invert stage II
4340PSC1707034594084.08Invert stage I
5380PSC190746-9906.08Overt
6300PSC1567784334204.20Invert stage I
7325PSC1697916774374.55Kerb, overt & Invert stage II

Common cause of cracking in concrete is due to restrained drying shrinkage. It is caused by loss of moisture from the cement paste in the mix. Due to excessive overbreaks, tensile stresses are caused by differential shrinkage between the surface and the interior concrete. The large shrinkage causes cracks at the surface and then it penetrates deeper into the concrete with time. Cracks are also caused by differential thermal stresses. It was found that addition of microsilica does not help every time. Ground Granulated slag was found to perform better in this case.

Diversion Tunnel

Diversion Tunnel is one of the initial structures constructed at the Hydropower Power Dam site. The non-monsoon flood is diverted through this structure so as to enable the construction of Main Dam. The erosion problems of Diversion Tunnel are similar to the Head Race Tunnel. Huge damages are observed at the DT when non-monsoon flood is passed through it. Use of High Performance concrete tends to improve performance of these structures, but due to excessive heavy sized of boulders and silt etc., damages are generally observed in this structure.

Conclusion

With increasing innovation in materials and mechanization, the civil engineering structures, in general and hydropower structures, in particular, are becoming more durable and thereby less repairs are required to be carried out. Better results shall be expected with the understanding of the behaviour of High Performance Concrete by people engaged in its manufacturing, placing and finishing.

Curing shall have to be more scientific and rigorous. In case of hot climates, curing shall start immediately after placing High Performance concrete. Firstly, by fogging and thereafter by wetting. As high performance concrete has virtually no water available for bleeding. Any delay in curing shall have permanent damage to the concrete, resulting into decreased line and becoming vulnerable as far as durability aspects are concerned. In case of cold climates, loss of heat from the body of concrete shall be protected by applying insulation like plastic sheets, thermocol etc. With improved site procedures, the site test results shall approach more towards laboratory results and therefore, concrete shall have both strength (to resist loads) and stamina (to fight with the distress).

References

  1. Relevant Websites
  2. Papers of various authors published in International Conference on 'Accelerated Construction of Hydropower Structures' held in Bhutan

NBMCW August 2012

.....

Concrete Mix Design with Portland Pozzo...

The Indian standard IS:456-2000 recommends the use of Portland pozzolana cement (blended cement) as well as mineral admixtures for concrete mixes provided that there are satisfactory data on their suitability, such as performance test on concrete containing them. In the present paper concrete mix design with blended cement is presented based on relevant latest I.S. codes.

Dr. S. K. Dubey, Professor (structures), Deptt. of Civil Engineering., M.A.N.I.T., Bhopal

Introduction

Concrete is a widely used structural material consisting essentially of a binder and a mineral filler. It has the unique distinction of being the only construction material actually manufactured on the site, or in a RMC plant whereas other materials are merely shaped to use at the worksite.

Concrete has become an indispensable construction material. In the present scenario concrete has crossed the stage of four component system i.e. cement, water, coarse aggregate and fine aggregate. It can be combination of more number of ingredients such as fly ash, ground granulated blast furnace slag, silica fume, rice husk ash and admixtures etc. are generally used in concrete production in practice which depends upon the requirement and the availability of the expertise.

The fundamental requirement of a concrete mix is that it should be satisfactory both in the fresh as well as in the hardened state, possessing certain minimum desirable properties like workability, strength and durability. Besides these requirements it is essential that the concrete mix is prepared as economically as possible by using the least possible amount of cement content per unit volume of concrete, with due regard to the strength and durability requirements. Since concrete is produced by mixing several discrete material, the number of variables governing the choice of mix design are necessarily large. However continuous research work in this field by various investigators, has helped to identify the major parameters controlling the proportions of ingredients in the mix.

Principle of Mix Design

The basic principle which is generally used for mix design for proportioning mixes is Abram's law for strength development. According to this law, for any given conditions of test the strength of workable concrete mix is dependant only on the water-cement ratio. Lesser the water-cement ratio in a workable mix greater will be its strength. From practical considerations compressive strength is taken as an index of acceptability. Mix proportioning is normally carried out for a specific characteristic compressive strength requirements ensuring that the mix so proportioned should satisfy the workability requirements without segregation and bleeding of concrete.

Data For Mix Design

The following data are required for mix design:
  • Grade designation
  • Type of cement
  • Maximum nominal size of aggregate
  • Minimum cement content
  • Maximum water-cement ratio
  • Workability
  • Maximum cement content
  • Any admixture used
  • Exposure conditions (as per IS: 456)
  • Method of transportation and placing of concrete

Mix Design Procedure

  • Target mean strength

  • Ft = fck + 1.65 × S

    Where,

    Ft = Target mean strength at 28 days in N/mm2

    fck = Characteristic compressive strength at 28 days in N/mm2

    S = Standard deviation

  • Selection of water-cement ratio

  • Table 3: Environmental Exposure Conditions (Clauses 8.2.1 and 35.3.2
    SI. No.EnvironmentExposure Conditions
    i)MildConcrete surfaces protected against weather or aggressive conditions, except those situated in coastal area.
    ii)ModerateConcrete surfaces sheltered from severe rain or freezing whilst wet Concrete exposed to condensation and rain Concrete continuously under water Concrete in contact or buried under non-aggressive soil/ground water Concrete surfaces sheltered from saturated salt air in coastal area
    iii)SevereConcrete surfaces, exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation.
    Concrete completely immersed in sea water Concrete exposed to coastal environment
    iv)Very severeConcrete surfaces exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wet Concrete in contact with or buried under aggressive sub-soil/ground water
    v)ExtremeSurface of members in tidal zone Members in direct contact with liquid/solid aggressive chemicals

    This ratio should be selected based upon the relationship between target mean compressive strength of concrete and Compressive strength of cement. IS: 10262-1982 fig. 2 gives the values of water-cement ratio for various cements. The water-cement ratio can also be taken from table (5) of IS:456-2000 for particular environmental exposure conditions as starting point. The supplementary cementitious material that is mineral admixtures shall also be considered in water-cement ratio calculations.

    The above selected water-cement ratio should be checked against limiting water–cement ratio for the requirement s of durability and the lower of the two will be adopted.

    Table 5: Minimum Cement Content, Maximum Water-Cement Ratio and Minimum Grade of Concrete for Different Exposures with Normal Weight Aggregates of 20 mm Nominal Maximum Size
    Clauses 6.1.2, 8.2.4.1 and 9.1.2)
    SI. No.ExposurePlain ConcreteReinforced Concrete
    Minimum Cement Content kg/m3Maximum Free Water-cement RatioMinimum Grade of ConreteMinimum Cement Content kg/m3Maximum Free Water-cement RatioMinimum Grade of Conrete
    (1)(2)(3)(4)(5)(6)(7)(8)
    i)Mild2200.60-3000.55M 20
    ii)Moderate2400.60M 153000.50M 25
    iii)Severe2500.50M 203200.45M 30
    iv)Very severe2600.45M 203400.45M 35
    v)Extreme2800.40M 253600.40M 40
    Notes
    1. Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions mentioned in 5.2. The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolona and slag specified in IS 1489 (Part 1) and IS 455 respectively.

    2. Minimum grade for plain concrete under mild exposure condition is not specified.

  • Selection of water content

    The water content i.e. the quantity of maximum mixing water per unit volume of concrete may be determined from the following table:

    Table: Maximum water content per cubic meter of concrete
    S.No.Nominal maximum size of agg.Maximum water content
    1.10 mm208 kg
    2.20 mm186 kg
    3.40 mm165 kg
    The above table is for angular coarse aggregates and for 25 to 50mm slump range.

    Note:
    • The water estimates can be reduced by approximately 10 kg for sub-angular aggregates, 20 kg for gravel with some crushed particles & 25 kg for rounded gravel to produce same workability.
    • For desired workability (other than 25 to 50mm slump range) the required water content may be increased by about 3 percent for every additional 25mm slump or alternatively by use of chemical admixtures.
    • Water reducing admixtures usually decrease water content by 5 to 10 percent and 20 percent and above respectively at appropriate dosages.

  • Cementitious material content

  • This may be calculated from the free water-cement ratio and the quantity of water per unit volume of concrete. The cementitious material content so calculated shall be checked for minimum content for the durability requirements and the greater of the two values adopted. The maximum cement content shall be as per IS: 456-2000.

  • Proportioning of coarse aggregate content

    For a water-cement ratio of 0.5 approximate values of aggregate volume is given in the following table.
    Table: Volume of coarse aggregate per unit volume of total Aggregate for different Zones of Fine Aggregate
    S, No.Nominal Maximum Size of AggregateVolume of Coarse Aggregate* per unit Volume of Total Aggregate for Different Zones of Fine Aggregate
    mmZone IVZone IIIZone IIZone I
    123456
    i)100.500.480.460.44
    ii)200.660.640.620.6
    iii)400.750.730.710.69
    *Volumes are based on aggregates in saturated surface dry condition.

    Note:
    • Adjust volume of coarse aggregate for decrease in w/c ratio by 0.05 the increase in coarse aggregate by 1%.
    • For more workable concrete such as pumped concrete etc the above estimated coarse aggregate content may be reduced by up to 10%.

  • Determination of Fine Aggregate Content:

  • This is obtained by finding out the absolute volume of cementitious material; water and the chemical admixture by dividing their masses to their specific gravity, multiplying by 1/1000 subtract the results of this summation from unit volume.

    The above obtained values are distributed into coarse and fine aggregate fractions by volume in accordance with coarse aggregate fractions.

    The coarse and fine aggregates contents are then determined by multiplying with their respective specific gravities and multiplying by 1000.

  • Trial Mixes

  • The trial mixes shall be made and cubes be tested, if any discrepancies may be observed during concrete making it should be taken into considerations and more trial mixes be prepared and finally the mix which provides sufficient information, including the relationship between compressive strength, water cement ratio and slump, from which the mix proportions for field trials may be arrived at. The concrete for field trials shall be produced by methods of actual concrete production.

  • Example

  • Grade of DesignationM-25
    Type of CementPPC confirming to Is1489 – Part I
    Maximum Nominal Size of Aggregate20 mm
    Minimum Cement Content300 kg
    Maximum water cement ratio0.5
    Workability60 mm slump
    Type of AggregateCrushed Angular aggregate
    Maximum Cement Content450 kg/m3
    Chemical AdmixturesFosroc (as supplied)

    Sieve Analysis of Coarse Aggregate
    IS sieve NoAnalysis of Coarse Aggregate Fraction, % passingPercentage of Different FractionsRemarks
    IIIIIICombined
    30%70%100%
    20mm8110024.37094.3Conforming to Table: 2 of IS383
    10mm0.847.2432.933.14
    4.75mm04.403.083.08
    2.36mm-----

    IS sieve NoPercentage passingRemarks
    4.75mm100Conforming to Zone IV Table 4 of IS383
    2.36mm98.5
    1.18mm96
    600 µm85
    300 µm3
    150 µm0

    Target Mean Strength Ft = fck + 1.65 S

    = 25 + 1.65 * 4.0

    = 31.6 N/mm2

    The mix proportions obtained for above materials are:

    Water : Cement : Sand : Aggregate = 184.5 lits: 410 kg: 606 kg : 1176 kg

    w/c ratio = 0.45, 1 : 1.478 : 2.868

    Admixture dose = 0.25 % of weight of cement in terms of volume of admixture.

    Cube strength after 7 days = 24.0 N/mm2, 20.88 N/mm2, 22.22 N/mm2

    Average Value = 22.36 N/mm2

    Cube strength after 28 days = 31.11 N/mm2, 34.66 N/mm2, 31.11 N/mm2

    Average Value = 32.29 N/mm2

Conclusions

The concrete mix produced with blended cements (PPC Cement) possesses nearly same quality as of OPC except with adjustments in water cement ratio as such the heat of hydration of PPC is lower than OPC the cement content obtained is slightly higher for blended cements. Hence, it is concluded that with proper quality control and supervision at site, the effectiveness of concrete produced with blended cements will be increased. It will also satisfy the requirements of workability, strength and durability.

References

  • IS:456-2000 (fourth revision)code of practice for plain and reinforced concrete structures, BIS New Delhi
  • IS:10262-1982, Recommended guideline for conc. mix design, BIS, Feb 1983 , BIS New Delhi.
  • IS:10262-2009, Concrete mix proportioning – Guide lines July, 2009 , BIS New Delhi.
  • IS:383 – 1970, Specification for coarse and Fine aggregates from natural sources for concrete BIS New Delhi.
  • Shetty, M. S., Concrete Technology Theory and Practice, 2005, S Chand & Co. New Delhi.
  • Roy Sabyasachi, Pumped concrete with admixtures, Civil Engineering & Construction Review, 2004 pp 51-54.
  • N. Krishnaraju, Design of Concrete Mixes, CBS publishers, Delhi, 1993
.

NBMCW June 2012

.....

Influence of Marble Powder Mortar and C...

Influence of Marble Powder on Mechanical Properties of Mortar and Concrete Mix

This paper aims to focus on the possibilities of using waste materials from differentmanufacturing activities in the preparation of innovative mortar and concrete. The use of waste marble powder (dust) was proposed in partial replacement of cement, for the production of Mortar and Concrete Mix. In particular, tests were conducted on the mortars and concrete mix cured for different times in order to determine their workability, flexural as well as compressive strength. Partial replacement of cement by varying percentage of marble powder reveals that increased waste marble powder (WMP) ratio result in increased workability and compressive strengths of the mortar and concrete.

Er. Tanpreet Singh and Er. Anil Kumar Nanda, Associate Professor & Head Surya School of Engg. & Tech. Surya World, Rajpura

Introduction

Blended cements based on the partial replacement of Portland cement clinker (PC) by wastes have been the subject of many investigations in recent years. The use of the replacement materials offer cost reduction, energy savings, arguably superior products, and fewer hazards in the environment. These materials participate in the hydraulic reactions, contributing significantly to the composition and microstructure of hydrated product. In building industry, Marble has been commonly used for various purposes like flooring, cladding etc., as a building material since the ancient times. The industry's disposal of the marble powder material, consisting of very fine powder, today constitutes one of the environmental problems around the world. In India, marble dust is settled by sedimentation and then dumped away which results in environmental pollution, in addition to forming dust in summer and threatening both agriculture and public health. Therefore, utilization of the marble dust in various industrial sectors especially the construction, agriculture, glass and paper industries would help to protect the environment. Some attempts have been made to find and assess the possibilities of using waste marble powder in mortars and concretes and results about strength and workability were compared with control samples of conventional cement sand mortar/concrete.

Methodology

To carry out the proposed study cubes of mortar (1:3) with varying partial replacement of cement with the same amount of WMP were cast and tested at three different intervals of 7days. Their results were compared with those of standard (1:3) mortar and concrete cubes.Detail of mortar mix has been shown in Table 1.

Table 1: Mortar Mix (1:3) Proportion when cement was replaced.
%Replacement 0% 5% 10% 15% 20%
Cement 1200 1140 1080 1020 960
Sand 3600 3600 3600 3600 3600
Marble Powder 0 60 120 180 240
Water 528 528 528 528 528
W/C ratio .44 .46 .49 .51 .53

In the same way cube specimens and beams samples of M35 grade of concrete have been tested in laboratory for which each percentage of marble powder i.e. 0%, 5%, 10%, 15% and 20%. Three properties of concrete namely workability, compressive strength and flexural strength have been selected for study and evaluated according to IS: 1199-1959 and IS: 516-1959 respectively. Before initiating the test properties of materials were determined according to respective IS codes. The properties are shown in Table 2.

Table 2: Material Properties
Material Fineness/Fineness Modulus Specific Gravity
Cement 0.225 (m2/g) 3.12
Fine Aggregate 2.60 2.71
Course Aggregate 2.96 2.85
Marble Powder 1.5 (m2/g) 2.67
Consistency of cement is 30%.
Initial and final setting time of cement is 127 minutes and 420 minutes respectively.
Fine aggregate conforms to zone III as per IS: 383 —1970.

Mix Design

Based on the Indian Standard (IS: 10262-1982), design mix for M35 grade of concrete was prepared by partially replacing fine aggregate with five different percentages by weight of marble granules (0%, 5%, 10%, 15%, and 20%). The mix proportion for M35 Grades of concrete with varying percentage of marble granules is presented in Table 3.

Table 3: Mix Proportion for Concrete Mix
Mix Materials Mix Proportion
M35 %Marble Sand (kg) Waste Marble (kg) C:w:fa:Ca:Wm
0 635 0 1:0.40:1.59:2.91:0.00
5 603.25 31.75 1:0.40:1.51:2.91:0.08
10 571.5 63.5 1:0.40:1.42: 2.91:0.158
15 539.75 95.25 1:0.40:1.35: 2.91:0.238
20 508 127 1:0.40:1.27: 2.91:0.318
Cement content and coarse aggregate is 400 kg and 1165 kg respectively while W/C is 0.40 for each mix proportion.

Results and Discussions

Mortar Mix

28 days Compressive Strength Test on Mortar

Test results tor every specimen were shown below in Table 4. It is observed here that with increase of WMP (replacing cement) the strength falls when the WMP is 15% or 20%.

Table 4: Compressive strength for Mortar Mix (N/mm2).
Curing Days 5% 10% 15% 20%
28 days 39.6 37.1 35.2 34.6

Mortar Mix

Concrete Mix

Workability

It is observed here that degree of workability is medium as per IS 456-2000. The slump values of the concretes obtained from waste marble granules mix gave negligible effect as compared to normal concrete mix as shown in Table 5.

Table 5: Slump Value for Concrete Mix
% Replacement Slump value (mm)
0 55
5 57
10 63
15 65
20 66

Slump Value

Compressive Strength

The test results are also presented in Table 6. By increasing the waste marble granules the compressive strength values of concrete tends to increase at each curing age. This trend can be attributed to the fact that marble granules possess cementing properties. It is also as much effective in enhancing cohesiveness due to lower fineness modulus of the marble powder or granules both. Furthermore, the mean strength of concrete mixes with marble granules was 5-10% higher than the reference concretes. However, there is a slight decrease in compressive strength value concrete mix when 20% marble granule is used as compared with that of 15% marble granule mix.

Table 6: Compressive Strength for Concrete Mix (N/mm2).
Days 0% 5% 10% 15% 20%
7 days 31.3 32 34.5 33.7 32.9
14 days 34.8 36.1 38.3 35.4 34.7
28 days 39.2 40.8 42.9 41.1 38.8

Compressive Strength

Flexural Strength Test

The flexural strength calculations are done as per IS: 516-1959. The results of the flexural strength tests for the waste marble powder mix concrete are shown in Table 7. The results show that the flexural strength of waste marble mix concrete increases with the increase of thewaste marble ratio in these mixtures. This trend can again be attributed to the fact that marblegranules possess cementing properties.

Table 7: Flexural Strength for Concrete Mix (N/mm2).
Curing Days 0% 5% 10% 15% 20%
28 days 4.9 5.1 5.3 5.5 5.4

Flexural Strength

Conclusions

Mortar Mix

Marble powder is partially replaced in cement by weight; there is a marked reduction in compressive strength values of mortar mix with increasing marble powder content when compared with control sample at each curing age.

Marble Mix Concrete

The slump values of the concretes obtained from waste marble granules mix gave negligible effect as compared to normal concrete mix. Degree of workability is medium conforming to IS: 456 – 2000. The mean strength of all concrete mixes with marble granules was 5-10% higher than the references concrete conforming to IS: 456-2000. The flexural strength of waste marble mix concrete increases with the increase of the waste marble ratio in these mixtures.

References

  • Agarwal, S. K., and D. Gulati. 2006. Utilization of industrial wastes and unprocessed micro-fillers for making cost effective mortars. Construction and Building Materials, 20: 999–1004.
  • BaharDemirel, (2010), "The effect of the using waste marble dust as fine sand on the mechanical properties of the concrete," International Journal of the Physical Sciences, 5 (9), pp 1372-1380.
  • Hameed M S and Sekar A S S (2009) Properties of Green Concrete containing Quarry Rock Dust and Marble Sludge powder as a fine aggregate ARPN J Engineering Applied Sciences4: 83-89.
  • HanifiBinici, Hasan Kaplan and SalihYilmaz, (2007), "Influence of marble and limestone dusts as additives on some mechanical properties of concrete," Scientific Research and Essay, 2(9), pp 372379.
  • IS:383-1970, Specification for Coarse and Fine Aggregate from Natural Sources for Concrete—Bureau of Indian Standards, New Delhi.
  • IS:10262-1982 Recommended Guidelines for Concrete Mix Design—Bureau of Indian Standards, New Delhi.
  • IS: 456-2000, Plain and Reinforced Concrete—Code of Practice—Bureau of Indian Standards, New Delhi.
  • IS: 516-1959, Methods of Tests for Strength of Concrete—Bureau of Indian Standards, New Delhi.
  • IS: 8112-1989, 43 Grade Ordinary Portland cement—Specification, Bureau of Indian Standards, New Delhi.
  • Mehta, P. K. 2002. Greening of the Concrete Industry for Sustainable Development, Concr. Int., 24: (7): 23-27.
  • Topçu, I. B., T. Bilir, and T. Uygunoglu. 2009. Effect of waste marble dust content as filler on properties of self-compacting concrete. Construction and Building Materials, 23: 1947–1953.

NBMCW June 2012

.....

Concrete Cloth - Its Uses and Applicati...

Prof. K.Srinivas and Prof. Ravinder, Asst. Professors, National Institute of Construction Management and Research, Hyderabad.

Introduction

Worldwide there is increasing demand for construction and construction materials, for that concrete is the most extensively used material in construction. These days concrete is being used for so many purposes in many different adverse conditions.

Concrete is the mixture of the following ingredients,
  1. Cement
  2. Fine aggregate
  3. Coarse aggregate &
  4. Water
Concrete is a freshly mixed material, which can be moulded into required shape. There are many advantages of concrete, but there is one drawback is that, it is not flexible, when it is hardened. To overcome through this drawback of concrete. a new construction material was developed by British Engineering Company called Concrete Canvas.

Concrete cloth (CC) is a unique proprietary material. It has a very wide range of applications throughout the building & civil engineering industry. Concrete cloth is a flexible; cement impregnated fabric that hardens when hydrated to form a thin, durable, water & fire proof concrete layer. CC allows concrete construction without the need for plant or mixing equipment. Simply position the canvas & just add water. CC has a design life of above 10 years and is significantly quicker and less expensive to install compared to conventional concrete.

Concrete Cloth
Figure 1: Concrete Cloth Section

CC consists of a 3- dimensio- nal fiber matrix containing a specially formulated dry Concrete mix. A PVC backing on one surface of the cloth ensures the material is completely waterproof, while hydrophilic fibers (Polyethylene and Polypropylene yarns) on the opposite surface aid hydration by drawing water into the mixture. The material can be hydrated either by spraying or by being fully immersed in water. It can be easily nailed, stapled through or coated with an adhesive for easy attachment to other surfaces. Once set, the fibers reinforce the concrete, preventing crack propagation & providing a safe plastic failure mode. CC is available in 3-thicknesses; CC5, CC8 & CC13, which are 5, 8 & 13 mm thick respectively & it is as shown in Fig.1.

Literature Review

General
Concrete Cloth is a flexible, cement impregnated fabric that hardens on hydration to form a thin, durable, waterproof and fire resistant layer.

History of Concrete Cloth
The British Army just placed a sizeable order for an innovative new material that combines the flexibility of fabric with the structural performance of concrete. Unlike anything else on the market, this revolutionary technology enables the use of concrete in a completely new way. The product, called Concrete Cloth, was developed by a British engineering company called Concrete Canvas. It will soon be used to enhance frontline defenses in Afghanistan.

The story behind its inception is somewhat unusual. Four years ago, we entered a competition run by the British Cement Association. At the time, we had no idea that our entry for a rapidly deployable emergency shelter would result in the launch of our own technology development company. Our research has now included trips to disaster zones around the world, including Uganda and New Orleans.

Concrete Cloth
Figure 2: CC used to sandbag protection in defenses

Four years later, the concept has matured into a technology that has applications far beyond emergency shelter. Following development funded through a combination of private equity investment and grants, the company relocated to a dedicated production site in South Wales, UK, where we have begun volume production of Concrete Cloth and Concrete Canvas Shelters.

The British Army quickly saw potential uses for this new material and started trials using Concrete Cloth as a method of reinforcing sandbag defenses. This solution, shown in Fig 2, reduces degradation of sandbag walls in harsh environments such as Afghanistan, where the combination of wind, sand, and extreme temperatures mean frequent repairs to frontline defenses. In addition, damage is caused by incoming fire and outgoing muzzle flash. Concrete Cloth is completely fireproof and has performed very well during range trials where it was tested with small-and transported medium-caliber weapons. The material comes in 10 m (33 ft) rolls to eliminate the need for heavy lifting equipment and plant machinery. This is a big advantage when operating in remote areas where most supplies have to be by helicopter. The material is then simply unrolled over the sandbag wall, secured using battens, and sprayed with water. A durable and hard wearing surface is produced within 24 hours. Key to the success of the material is the fibers that form a reinforcing matrix within the Concrete Cloth. These provide a stable failure mode, absorb energy, and help maintain the structural integrity of the concrete when impacted. A ballistic projectile will pass through the cloth, but crack propagation is limited. The sand in the sandbag is therefore retained within the concrete shell. In contrast, standard sandbag cloth will typically tear, and the fill is lost very quickly.

In January 2008, a small quantity was used on the frontline in Afghanistan to validate its performance in the field. As a result of these trials, the UK Ministry of Defense has just awarded Concrete Canvas a contract to supply 5500 m2 (6600 yd2) to the frontline.

Deployable Shelters
The original concept for Concrete Canvas was to create rapidly deployable hardened shelters that required only water and air for construction. The key was the use of inflation to create a surface that was optimized for compressive loading. This allowed thin-walled concrete structures that are both robust and lightweight. The shelter, such as the one shown in Fig 3, is deployed in the following four stages:

Concrete Cloth
Figure 3: CC Shelter

Delivery-the shelter is supplied folded and sealed in a sack. The 16 m2 (19 yd2) variant is light enough to transport in a pickup truck or light aircraft;

Inflation-once delivered, an electric fan is activated to inflate the inner PVC liner and lift the structure until it is self-supporting. The shelter is then pegged down with ground anchors around the base;

Hydration-the shelter is sprayed with water. Hydration is aided by the fiber matrix, which wicks water into the mixture; and

concrete cloth

Setting-the Concrete Cloth cures in the shape of the inflated inner PVC liner. The structure is ready to use 24 hours later. Access holes allow the installation of services such as water, power, air conditioning, and heating units. The structures are designed as part of a modular system. Units can be easily linked together, allowing the space to be tailored to the application. If required, they can be demolished using basic tools. The thin-walled structure has a very low mass, leaving little material for disposal.

The University of Bath in Bath, UK, has conducted finite element analysis of the shelters, showing that the structures can withstand a high distributed compressive load. This allows sandbags, earth, or snow to be piled on top-giving the shelters excellent thermal properties and protection against shrapnel, blasts, and small arms fire.

Concrete Cloth Concrete Cloth
Figure 4: CC Bulk Rolls Figure 5: CC Batched Rolls

Concrete Canvas Shelters are specified to withstand 0.75 m (2.5 ft) of wet sand on the sides (sufficient to stop 7.62 mm [.30 calibers] rounds) and 0.5 m on the roof (to protect against shell fragments).

Methodology

General
Concrete Canvas (CC) is a flexible; cement impregnated fabric that hardens when hydrated to form a thin, durable, water and fire proof concrete layer. The following data provides useful information for installers, customers and specifies of CC. It provides an overview of useful data and techniques that can be used across a wide range of applications.

CC Specification
CC Types
There are 3 CC types available with the following indicative specifications:

Bulk Rolls / Batched Rolls
CC is available in two standard roll sizes; bulk rolls or smaller batched rolls. The quantity per roll differs between the CC types. Bulk rolls weigh 1.6T and are supplied on 6 inch cardboard tubes which can be hung from a spreader beam and unrolled using suitable plant equipment (see below). Bulk rolls provide the fastest method of laying CC and have the additional advantage of reducing the number of joints required. Batched rolls are supplied on 3 inch cardboard cores with carry handles designed as a 2 to 4 man lift. All CC thicknesses can be supplied batched to custom lengths for a small additional charge.

Examples of CC Applications
Some examples of applications for the different CC types are given in Table 2.

concrete cloth

CC Material Properties
Strength

concrete cloth
concrete cloth
Very high early strength is a fundamental characteristic of CC. Typical strengths and physical characteristics are as follows:

Compressive testing based on ASTM C473 – 07
- 10 day compressive failure stress (MPa) 40
- 10 day compressive Young’s modulus (MPa) 1500

Bending tests based on BS EN 12467:2004
- 10 day bending failure stress (MPa) 3.4
- 10 day bending Young’s modulus (MPa) 180

Abrasion Resistance (ASTM C1353-8)
- CC lost 60% less weight than marble over 1000 cycles.

Tensile Test
Abrasion Resistance (DIN 52108)
- Similar to twice that of OPC Max 0.10 gm/cm2

CBR Puncture Resistance EN ISO 12236: 2007 (CC8 & CC13 only)
- Min. Push-through force 2.69kN
- Max. Deflection at Peak 38mm

Resistance to Imposed Loads on Vehicle Traffic Areas

EN 1991-1-1:2002 (CC8 & CC13 only)
- Category G compliant
- Gross weight of 2 axle vehicle 30 to 160kN
- Uniformly distributed load not exceeding 5kN/sq m

Physical Properties
Initial Set≥ 120 min
Final Set.≤ 240 min.

Method of Hydration
CC can be hydrated using saline or non saline water. The minimum ratio of water to CC is 1:2 by weight. CC cannot be over hydrated so an excess is recommended. The recommended methods are: In a hot/arid environment, re-wet the material 2 - 4 hours after the initial hydration.

Concrete Cloth
Figure 6: Spray The Fiber Surface With Water Until It Feels Wet To Touch For Several Minutes After Spraying.

Immersion: Immerse CC in water for a minimum of 90 seconds.

Spraying: Spray the dry CC with water until it is saturated. Do not use a direct jet of pressurized water as this may wash a channel in the material and create a weakened area

Re-spray the CC again after 1 hour if:
  • Installing CC5
  • Installing CC on a steep or vertical surface
  • Installing in warm climates
Cutting of CC
A ‘snap off’ type disposable blade is the most suitable tool for cutting CC before it is hydrated or set. When cutting dry CC, a 20mm allowance should be left from the cut edge due to lost fill. This can be avoided by wetting the CC prior to cutting.CC can also be cut using handheld self sharpening powered disc cutters.

Cutting Set CC
Set CC can be cut as with conventional concrete, with angle grinders, construction disc cutters or good quality tile cutters.

CC Mechanical Fixing

There are a large number of mechanical fixings that are suitable for use with Concrete Canvas. Some of these fixings can be used in conjunction with the non-mechanical joining methods described later in this to improve the mechanical strength or water proofing properties of joints.

Staples
The versatility of CC means that a wide range of manual, electric or gas powered staplers are suitable for attaching CC to soft substrates such as wooden boarding for building cladding. Commercially available hand staplers are suitable for fixing 2 layers of CC together where a small amount of compression force is required.

Concrete Cloth Concrete Cloth
Figure 7: Snap off Blade Figure 8: 3.5 Disc cutter

Concrete Cloth Concrete Cloth
Figure 9: 3.6 Angle Girder Figure 10: Electric Stapler

Concrete Cloth Concrete Cloth
Figure 11: Standard Nail Attached to CC Figure 12: Self Tapping Screw

Nails
Standard nails can be used to attach CC. Alternatively, a power tool such as the Hilti nail gun, provides a quick and effective method of securing CC to hard surfaces such as concrete or rock. This may be appropriate where CC is being used to recondition an existing concrete surface or for spall lining in mining applications. It is important to ensure that the nail is used with at least a 15mm washer to ensure the head does not penetrate through the surface of the Canvas.

Screws
Self tapping screws provide a quick and readily available means of attaching CC to a substrate or to itself. Typical applications include sandbag reinforcement or covering existing wooden or steel structures.

Alternatively large thread ‘self drive’ screws provide an excellent method of securing overlapped layers of CC together. This is ideal where a good mechanical joint is required, for applications such as slope stabilization, dust suppression or vehicle track way.

Hog Rings
Hog-rings are available in a wide range of sizes and can be applied to Concrete Canvas using pneumatic, electrical or hand powered tools. They provide a rapid means of securing CC either to itself or onto a wire mesh substrate such as gabion baskets. Hog-rings can also be used to attach adjacent sheets of CC together by laying the material face to face (fiber side together) and then hog-ringing at intervals along the edge. The spacing will vary depending on the level of mechanical fixing required.

If a more water proof seal is required, a Bitumen tape can be applied over the hog-rings, onto the PVC backing, using a blow-torch. Using this method, large panels of Concrete Canvas can be prefabricated with relative ease.

Concrete Cloth
Figure 13: Hog Ring Used to Attach CC

Concrete Cloth
Figure 14: Bitumen tape over the hog-rings Photo 3.12 Medium gauge wire

Medium gauge wire can be used as a simple alternative to hog-rings where plant equipment is not available. The end of the wire should be cut to a sharp point to aid penetration through the CC layers.

Pegging
Pegging is recommended for ground surfacing applications such as ditch lining, slope stabilization or erosion control. Typically pegs are specified every 2m for most applications, but this will vary depending on the ground conditions and application. Pegs should be used at joints where possible to secure adjacent layers together. Pegs are available directly from Concrete Canvas Ltd. in a range of sizes which are suitable for use with CC. The peg must have a sufficiently sharp point to penetrate the surface of the Canvas.

Concrete Cloth Concrete Cloth
Figure 15: Peg Used to Join CC Figure 16: Recommended Minimum Coverage

CC Non- Mechanical Fixing
Simple CC Overlap & Simple CC Overlap (Screwed)Overlapping Concrete Canvas is the simplest method of joining 2 layers together. This is appropriate for the majority of ditch lining, erosion control and ground surfacing applications. Overlapped joints should be compressed along the entire length while the material sets to ensure there are no voids between layers. This can be done using screws, sandbags, water weights, loose fill, staples etc. Overlapping CC will provide a moderate level of water proofing and is suitable for drainage applications. We recommend overlapping cut edges by a minimum of 100mm and sealed edges by a minimum of 50mm.

CC Bonding Sealant
Concrete Canvas can be joined and sealed by applying a bonding sealant between overlapping layers. Concrete Canvas Ltd. can provide a recommended sealant which bonds to both the PVC backing and fiber surface of Concrete Canvas. A minimum bead size of 6mm should be applied along the length of the joint and the two layers firmly pushed together. The sealant works in both wet and dry conditions so can be applied before hydration or immediately after. The bonding sealant will fully cure in 24 hours.

Concrete Cloth Concrete Cloth
Figure 17: CC8 over lapped joint with CC Grout Figure 18: Ditch lining

Concrete Cloth Concrete Cloth
Figure 19: Slope Protection Figure 20: Pipeline Protection

Concrete Cloth Concrete Cloth
Figure 21: Ground Resurfacing Figure 22: Mining Applications

CC Jointing Grout
Concrete Canvas can be joined and sealed by overlapping layers and applying a grouting compound along the joint. Concrete Canvas Ltd. can provide a bespoke cementious grout called CC Jointing Grout for use in drainage and ditch lining applications. CC Jointing Grout is based on the same cementitious mix as Concrete Canvas to create a homogenous joint. The use of other grouting compounds may retard setting time and reduce set strength. Grout should be applied after hydrating the two overlapped layers. Ensure that the bottom layer, including the overlapped area is properly hydrated. CC Jointing Grout has similar setting characteristics to CC and initial set takes place after two hours. After applying the grout, continue to hydrate as per CC hydration instructions, ensuring not to apply jets of water directly onto the joint to avoid washout of the grout.

Finishing
The fiber surface of CC can be easily painted once set using standard exterior masonry paint. This provides a quick and simple method of improving the aesthetic appearance of CC. Alternatively, Concrete Canvas Ltd. can recommend a range of copolymer concrete surface treatments which can provide a coloured uniform finish as well as hydrophobic protection against staining and organic growth. Fire retardant paints have also been shown to be effective where thermal performance is critical.

A white cement blend of Concrete Canvas is available on request subject to minimum volumes. This blend provides a brilliant white uniform finish and is most commonly used for architectural applications.

Findings

Applications of CC
Ditch lining
CC can be rapidly unrolled to form ditch or tank lining. It is significantly quicker and less expensive to install than conventional concrete ditch lining and requires no specialist plant equipment. The 30m ditch shown below was lined in 45min.

Slope Protection
CC can be used as slope stabilization and other erosion control applications such as temporary and permanent slope protection, retaining walls, boulder fences, low level bunds and river bank and dam revetments.

Pipeline Protection
CC can be used as a coating for overland or underwater pipeline protection, providing a superior tough rock shield. In remote areas it can be used to coat steel pipe on site without expensive wet concrete application plants. CC will set underwater and provide negative buoyancy.

Ground Resurfacing
CC can be secured with ground anchors to rapidly create a concrete surface for flooring, pedestrian walk-ways or dust suppression. CC8 and CC13 have been tested to EN 1991-1-1:2002 (Resistance to Imposed Loads on Vehicle Traffic Areas)

Mining Applications
CC can be used as an alternative to poured or sprayed concrete or as a quick way of erecting strong permanent or temporary blast and vent structures and spall lining. CC has been successfully tried in Mpumalanga, South Africa.

Concrete Cloth Concrete Cloth
Figure 23: Bund Lining Figure 24: Sandbag Reinforcement

Concrete Cloth Concrete Cloth
Figure 25: Gabion Reinforcement / Capping Figure 26: Dust Suppression

Bund Lining
Earth containment bunds can be quickly lined with CC to provide an efficient, chemically resistant alternative to concrete walling.

Sandbag Reinforcement
CC has been proven to prevent the degradation of sandbags from sustained incoming fire, outgoing muzzle fly ash and environmental exposure. A sandbag wall protected by CC withstood 900 rounds of 7.62 NATO, fired by a GPMG LR at a range of 100m. There is currently over 5500sqm of CC being used by the British Army in Afghanistan.

Gabion Reinforcement / Capping
CC can be used to cap or repair gabion walls to provide long-term protection and prevent FOD (Foreign Object Damage). Covering gabions with CC also prevents water ingress which can cause slump, whilst protecting the geo-textile membrane from the effects of UV degradation.

Dust SuppressionCC5 was developed as a result of in-theatre feedback, for use as a dust suppression surface around Helicopter Landing Sites. Benefits include: speed of installation, durability, and good coverage (CC) will conform to the underlying ground conditions).

Advantages of CC
  • Rapid-the material can be hydrated by either spraying it or fully immersing it in water. Once hydrated, it remains workable for 2 hours and hardens to 80% of its final strength within 24 hours. These times can be reduced by adding accelerants into the dry mixture at the point of manufacture;

  • Easy to use-dry Concrete Cloth can be cut or tailored using simple hand tools such as utility knives. The PVC side can be supplied with an adhesive backing and the fibrous side bonds well to concrete or brick surfaces when set. It can be easily repaired or upgraded using existing cement products;

  • Flexible-Concrete Cloth can be easily nailed through before setting. It has good drape characteristics, allowing it to take the shape of complex surfaces including those with double curvature;

  • Strong-the fiber reinforcement acts to prevent cracking, absorbs energy from impacts, and provides a stable failure mode;

  • Fireproof-Concrete Cloth is a ceramic-based material and will not burn;

  • Waterproof-the PVC backing on one surface ensures that Concrete Cloth is completely waterproof;

  • Adaptable-Concrete Cloth is currently supplied on 1.2 m (4 ft) wide rolls but can be manufactured with a roll width of up to 5 m (16.4 ft). The cloth can be produced in a range of thicknesses from 5 to 20 mm (0.2 to 0.8 in.); and

  • Durable-Concrete Cloth is chemically resistant and will not degrade in ultraviolet light.

  • Environmentally Friendly
CC is a low mass, low carbon technology which uses up to 95% less material than conventional concrete for many applications. It has minimal impact on the local ecology due to its limited alkaline reserve and very low wash rate.

concrete cloth

Limitations of CC
  • CC cannot be over hydrated and an excess of water is always recommended.

  • Do not jet high pressure water directly onto the CC as this may wash a channel in the material..

  • CC has a working time of 1-2 hours after hydration. So do not move CC once it has begun to set.

  • Working time will be reduced in hot climates.

  • If CC is not fully saturated, the set may be delayed and strength reduced.
Cost comparison of CC
Engineers Incorporated Ltd (EIL) was commissioned by Concrete Canvas Ltd (CCL) to prepare a comparison of costs for lining an open, trapezoidal ditch 900 x 900 x 900mm, 500m in length.

concrete cloth

concrete cloth

The comparison for construction costs requested, were:-
  • In Situ concrete lining, average thickness 150mm.

  • Precast Concrete Paving Slabs, laid on sand / cement screed.

  • Sprayed Concrete (Gunite) average thickness 100mm with mesh.

  • Concrete Canvas CC8.
We have provided a rate build up for each of the options under consideration.Below is a summary of the results:-

The above rates assume that the initial ditch excavation to form the trapezoidal shape is complete prior to commencement of lining and therefore has been excluded in the costs.

The above rates assume that the site has tarmac access for pouring in-situ concrete, delivery of sprayed concrete and paving slabs.

Analysis of Rates

Conclusion

Concrete cloth (CC) is a unique proprietary material. It is a time & material saving technique. It is very easy to place & handle. Concrete cloth is a flexible; cement impregnated fabric that hardens when hydrated to form a thin, durable, water & fire proof concrete layer. CC allows concrete construction without the need for plant or mixing equipment. Simply position the canvas & just add water. CC has a design life of 10 years and is significantly quicker and less expensive to install compared to conventional concrete. It is specially used, where the workmanship is very difficult. It is specially used in emergency works such as in military.

concrete cloth

concrete cloth

References

  • www.concretecanvas.co.uk
  • www.concreteinternational.com
  • The Journal For Science, Engineering And Technology In Wales, issue 62, winter 2009
  • www.buildingtomorrow.co.uk

NBMCW May 2012

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Resistance of Concrete Containing Waste...

Resistance of Concrete Containing Waste Glass Powder Against MgSO4 Attack

Most soils contain some sulphate in the form of calcium, sodium, potassium and magnesium. They occur in soil or ground water. Because of solubility of calcium sulphate is low, ground water contain more of other sulphates and less of calcium sulphate. Ammonium sulphate is frequently present in agricultural soil and water from the use of fertilizers or from sewage and industrial effluents. This study presents investigation on the effect of sulphate attack on the properties of concrete. Decomposing of waste glass possesses major problem because glass is non-biodegradable, remains in our environment and do not decompose easily by itself therefore do not have significant environmental and social impact could result in serious impact after disposal. Concrete produced by replacing cement with waste glass powder (GP) in different proportion has been studied. Higher resistance to sulphate attack was obtained when 20% cement was replaced by waste glass.

M. N. Bajad, Research Scholar, S.V.N.I.T, Surat, C. D. Modhera, Professor, S.V.N.I.T, Surat and A. K. Desai, Associate Professor, S.V.N.I.T, Surat.

Introduction

Glass is a rigid liquid i.e. super cooled liquid, static, not solid, not a gas but does not change molecularly between melting and solidification in to a desired shape. Glass is one of the most versatile substances on earth used in many applications and in a wide variety of forms. Glass occurs naturally when rock high in silicates melt at high temperature and cool before they can form a crystalline structure. Obsidian or volcanic glass is a well known example of naturally occurring glass. When manufactured by human's the glass is a mixture of silica, sand, lime and other materials. The elements of glass are heated to 9820 Celsius. Heat can return the glass to a liquid and workable form, making it easy to reuse and recycle.

Table 1 and Figure 1,2 shows the chemical composition of the cementing materials. The particle size distribution of the glass powder and cement are shown in figure 3

Table 1: Chemical composition of cementing materials
Composition (% by mass)/ property Cement Glass powder
Silica (SiO2) 20.2 72.5
Alumina (Al2O3) 4.7 0.4
Iron oxide (Fe2O3) 3.0 0.2
Calcium oxide (CaO) 61.9 9.7
Magnesium oxide (MgO) 2.6 3.3
Sodium oxide (Na2O) 0.19 13.7
Potassium oxide (K2O) 0.82 0.1
Sulphur trioxide (SO3) 3.9 -
Loss of ignition 1.9 0.36
Fineness % passing (sieve size) 97.4(45 µm) 80 (45 µm)
Unit weight,Kg/m3 3150 2579
Specific gravity 3.15 2.58

Resistance of Concrete Containing Waste Glass Powder
Figure 1: Chemical composition of cement

Resistance of Concrete Containing Waste Glass Powder
Figure 2: Chemical composition of glass powder

Resistance of Concrete Containing Waste Glass Powder
Figure 3: Particle Size Distributions of Cementitious Materials

Resistance of Concrete Containing Waste Glass Powder
Figure 4: Mechanism of sulfate attack
Sulphates reacts chemically with the product of hydration (hydrated lime and hydrated calcium aluminates in the cement paste to form calcium sulphate and calcium sulfo aluminates) are called ettringite. These new crystals occupy empty space (Figure 4) and as they continue to form, they cause expansion, disruption, loss of bond between the cement paste and aggregate because paste expansion produces a small gap around small aggregate particles and a bigger gap around larger particles as shown in figure 5, which result in micro cracks and these cracks may be responsible for reduction in strength or damaging the concrete (Figure 6) by changing the chemical nature of the cement paste and of the mechanical properties of the concrete.

Resistance of Concrete Containing Waste Glass Powder
Figure 5: Paste expansion produces a small gap around small aggregate particles and a bigger gap around larger particle
Resistance of Concrete Containing Waste Glass Powder
Figure 6: Cracking of concrete due to sulphate attack

The Chemistry of Sulphate Attack

The end result of sulphate attack can be excessive expansion, delaminating, cracking, and loss of strength. The degree to which this attack can occur depends on water penetration, the sulphate salt and its concentration and type (e.g. .sodium or magnesium), the means by which the salt develops in the concrete (e.g. is it rising and drying causing crystallization), and the chemistry of the binder present in the concrete. These processes and factors have been the subject of intense study across the world in recent years and outcomes are reported widely.

It can be summerarised that at a concentration of about 0.2% sulphate content in the ground water, concrete may suffer sulphate attack; that magnesium sulphate can be more aggressive than sodium; and that there are three key chemical reactions between sulphate ions and hardened cement pastes. The reactions are: recrystallisation of ettringite; formation of gypsum; and decalcification of the main cementitious phase (C-S-H).

In the presence of the calcium hydroxide formed in cement paste, when the latter comes in contact with sulphate ions, the alumina containing hydrates are converted to the high sulphate from ettringite. These etteringte crystals grow, expand, or swell by mechanisms, which are still the subject of controversy among researchers. While there is agreement that most (but not all) ettringites will expand in this formation, the exact cause are not agreed.

The formation of gypsum as a result of cat ion exchange reactions is also capable of causing expansion but is normally linked to loss of mass and strength. The decalcification of the C-S-H has not received as much discussion as the other two types of sulphate attack, but can be just as important, particularly where the sulphate solution is lower in PH (i.e. more acidic. This particular reaction, with more gypsum formation, leads to both strength loss and expansion. This is a particular situation in which blended cements with lower initial calcium/silica (C/S) ratios in the C-S-H gel are shown to be less susceptible to this type of attack.

Chemical process

The sulphate ion + hydrated calcium aluminates and/or the calcium hydroxide components of hardened cement paste + water = ettringite (calcium sulphoaluminate hydrate)

C3A.Cs.H18 + 2CH +2s+12H = C3A.3Cs.H32

C3A.CH.H18 + 2CH +3s + 11H = C3A.3Cs.H32

The sulphate ion + hydrated calcium aluminates and/or the calcium hydroxide components of hardened cement paste + water = gypsum (calcium sulphate hydrate)

Na2SO4+Ca(OH)2 +2H2O = CaSO4.2H2O +2NaOH

MgSO4 + Ca(OH)2 + 2H2O = CaSO4.2H2O + MgOH

Research Significance

waste glass contain high silica (SiO2) i.e.72%. Waste glass when ground to a very fine powder (600 micron) SiO2 react with alkalis in cement (pozzolanic reaction) and form cementitious product that help contribute to the strength development and durability.

when concrete contain waste glass powder it gives higher percentage of C2S,Low C3A,C4AF,C3S/C2S Content which result in production of less heat of hydration and offers greater resistance to the attack.

It has been estimated that several million tons of waste glass is generated annually worldwide due to rapid growth of population, improvement in the standard of living, industrialization and urbanization. Hence utilization of waste glass has become a critical issue worldwide. The key sources of waste glasses are waste containers, window glasses, and windscreen, medicinal bottles, liquor bottles, tube lights, bulbs, electronic equipment, etc.

Recycling, disposal and decomposing of waste glass possesses major problems for municipalities everywhere, and this problem can be greatly eliminated by re-using waste glass as a cement replacement in concrete. Moreover, there is a limit on the availability of natural aggregate and minerals used for making cement, and it is necessary to reduce energy consumption and emission of carbon dioxide resulting from construction processes, solution of this problem are sought thought usages of waste glass as partial replacement of Portland cement. Replacing cement by pozzolanic material like waste glass powder in concrete, not only increases the strength but also reduces the unit weight.

Recycling of waste glass may affect respiratory system if breath in pollutants. Case-local residents at merceds Arumbula claimed that the neighborhood and kids have developed asthma once the plant was built in their community

Disposal of waste glass degrade communities living condition and harmful to human health because lactates and gas releases from the landfill site

Sulphate reacts with product of hydration causes expansion. Therefore an experimental investigation in developing concrete containing waste glass powder is very important

The Methodology and Investigations

Experimental Programme

The purpose of this investigation was to evaluate the effect of partial replacement of cement by waste glass powder (GP) on strength of concrete specimens. The experimental parameters and their levels were chosen according. Experimental programme plan is shown in Figure 7.

Resistance of Concrete Containing Waste Glass Powder
Figure 7: Plan of Experimental programme

Constituent Materials

Cement


Ordinary Portland Cement (OPC) 43 grade confirming to IS 8112

Aggregate

Locally available sand and coarse aggregates were used in this experiment. The sand used was Zone II, with specific gravity of 2.62. Specific gravity of coarse aggregate was 2.93. Coarse aggregate used 20 mm and down size.

Admixture

To impart workability to the mix, a superplasticiser from a reputed company was used with the dosage of 2% by weight of cement

Supplementary Cementitious Materials

The glass powder was obtained by crushing waste glass pieces in a cone crusher mill. The 600-micron passing fraction was used for the experimentation

Mix Proportions and experimental factors

Mix design carried out to form M20 grade of concrete by IS 10262: 2009 yielded a mix proportion of 1:2.35:4.47 with water cement ratio of 0.50. Nine different mixes (M1,M2,M3,M4,M5,M6,M7,M8,M9)were prepared using cement replaced by waste glass powder (GP) at varying percentages of 0, 5, 10, 15, 20, 25, 30, 35 and 40.

Casting

Fifty four number of Specimens size 150 x 150 x 150 mm and Twenty seven number of specimens of dimensions 150 x150x 700 mm were cast according to the mix proportion and by replacing cement with glass powder (GP) in different proportion

Preparation of Solution and Caution

A 5% MgSO4 solution has five grams of magnesium sulphate dissolved in 100 ml solution.

Procedure Weigh 5 gram of magnesium sulphate & pour it into a graduated cylinder or volumetric flask containing about 80 ml of water. Once the magnesium sulphate has dissolved completely add water to bring the volume up to final 100 ml.

Caution-Do not simply measure 100 ml of water and add 5 gram of magnesium sulphate. This will introduce error because adding the solid will change the final volume of the solution and change the final percentage.

Curing of Specimens

To find out the effect of water (concrete without subjected to attack), the specimens were immersed in a 100% H2O solution (water) for 7, 28, and 90 days

To find out the effect of Sulphate attack, the specimens were immersed in a 5% MgSo4 solution for 7, 28, and 90 days.

Testing

To find out the strength, specimens were tested at 7, 28 and 90 days using a compression testing machine (CTM) of capacity 2000KN in accordance with the provisions of the Indian Standard specification IS 516:1959

Test Results

Test results are presented graphically and in tubular forms and have been discussed under different categories.

Workability

Table 2 and Figure 8 shows the results of workability of concrete with cement replacement by glass powder in various percentages ranging from 5% to 40% in increments of 5% (0%, 5%, 10%, 15%, 20%,25%, 30%,35% and 40%). It seems workability of concrete decreases as the glass content increases.

Table 2: Overall result of slump of concrete
Mix Designation Percentage replacement of cement by glass powder Slump
(mm)
Percentage increase or decrease with respect  to reference mix
M1 0(Ref.mix) 100 -
M2 05 94 -6
M3 10 91 -9
M4 15 88 -12
M5 20 82 -18
M6 25 76 -24
M7 30 73 -27
M8 35 72 -28
M9 40 66 -34

Resistance of Concrete Containing Waste Glass Powder
Figure 8: Variation of slump of concrete with cement replacement by glass powder
Density

Table 3 and Figure 9 shows the results of density of concrete at 28 days with cement replacement by glass powder in various percentages. It seems unit weight of concrete decreases as the glass content increases.

Table 3: Overall, result of density of concrete
Mix Designation Percentage replacement of cement by glass powder Density
(at 28 Day in kg/m3)
Percentage decrease in density with respect to reference mix
M1 0(Ref.Mix) 2408 ----------
M2 05 2396 0.49
M3 10 2379 1.2
M4 15 2369 1.61
M5 20 2354 2.24
M6 25 2335 3.03
M7 30 2315 3.86
M8 35 2303 4.36
M9 40 2283 5.19

Resistance of Concrete Containing Waste Glass Powder
Figure 9: Variation of density of concrete with cement replacement by glass powder
Strength

(a) Compressive Strength

As expected, the compressive strength increased with increasing curing time as shown in table 4,8 and in figure 10,14. Table 5,6,7 and Figure 11,12,13 shows overall results of compressive strength with and without subjecting to sulphate attack with cement replacement by glass powder for 7 days, 28 days, and 90 days.

Table 4:Overall results of development of compressive strength in concrete without subjecting to attack with age
Age, days Compressive strength, MPa
0%
GP
5% GP 10% GP 15% GP 20% GP 25% GP 30% GP 35% GP 40% GP
7 21.05 22.28 23.27 24.86 27.30 23.72 17.62 16.04 12.93
28 27.05 28.58 29.77 31.56 33.50 30.52 24.22 22.44 19.03
90 27.33 28.87 30.08 31.85 33.86 30.82 24.44 22.72 19.25

Resistance of Concrete Containing Waste Glass Powder
Figure 10: Variation of compressive strength development in concrete without subjecting to chloride attack with age
Table 5:Overall results of compressive strength with and without subjecting to sulphate attack for 07 days.
Mix Designation Percentage replacement of cement by glass powder
(%)
Concrete without subjecting to Sulphate attack Concrete subjected to Sulphate attack for 07days Percentage decrease of compressive strength when subjected to Sulphate attack
Compressive Strength (MPa) Percentage increase or decrease in compressive strength w.r t. ref.mix. Compressive Strength (MPa) Percentage increase or decrease in compressive strength w.r t. ref.mix.
M1 0(Ref.mix) 21.05 ------- 20.62 -------- 2.04
M2 05 22.28 +6 21.83 +6 2.01
M3 10 23.27 +11 22.57 +9 3.00
M4 15 24.86 +18 24.36 +18 2.01
M5 20 27.3 +30 26.75 +30 2.01
M6 25 23.72 +13 23.00 +12 3.03
M7 30 17.62 -17 17.09 -17 3.00
M8 35 16.04 -24 15.71 -24 2.05
M9 40 12.93 -39 12.67 -39 2.01

Resistance of Concrete Containing Waste Glass Powder
Figure 11: Variation of compressive strength of concrete with cement replacement by glass powder and when subjected to Sulphate attack for 7 days
Table 6: Overall results of compressive strength with and without subjecting to sulphate attack for 28 days.
Mix Designation Percentage replacement of cement by glass powder Concrete without subjecting to Sulphate attack Concrete subjected to Sulphate attack for 28 days Percentage decrease of compressive strength when subjected to Sulphate attack
Compressive Strength (MPa) Percentage increase or decrease in compressive strength w.r t. ref.mix. Compressive Strength (MPa) Percentage increase or decrease in compressive strength w.r t. ref.mix.
M1 0(Ref.mix) 27.05 ------ 25.45 -------- 5.91
M2 05 28.58 +6 27.15 +7 5.00
M3 10 29.77 +10 28.10 +10 5.61
M4 15 31.56 +17 30.00 +18 4.94
M5 20 33.50 +24 31.85 +25 4.92
M6 25 30.52 +13 29.01 +14 4.94
M7 30 24.22 -10 23.20 -9 4.21
M8 35 22.44 -17 21.55 -15 3.96
M9 40 19.03 -30 18.25 -28 4.1

Resistance of Concrete Containing Waste Glass Powder
Figure 12: Variation of compressive strength of concrete with cement replacement by glass powder and when subjected to Sulphate attack for 28 day
Table 7:Overall results of compressive strength with and without subjecting to sulphate attack for 90 days
Mix Designation Percentage replacement of cement by glass powder Concrete without subjecting to sulphate attack Concrete subjected to sulphate attack Percentage decrease of compressive strength when subjected to sulphate attack
Compressive Strength (MPa) Percentage increase or decrease in compressive strength w.r t. ref.mix. Compressive Strength (MPa) Percentage increase or decrease in compressive strength w.r t. ref.mix.
M1 0(Ref.mix) 27.33 ------ 22.80 ------- 16.57
M2 05 28.87 +6 24.22 +6 16.10
M3 10 30.08 +10 25.65 +11 14.72
M4 15 31.85 +17 27.18 +19 14.66
M5 20 33.86 +24 28.86 +27 14.76
M6 25 30.82 +13 26.30 +15 14.66
M7 30 24.44 -11 21.60 -5 11.62
M8 35 22.72 -17 19.70 -14 13.29
M9 40 19.25 -30 16.88 -26 12.31

Resistance of Concrete Containing Waste Glass Powder
Figure 13: Variation of compressive strength of concrete with cement replacement by glass powder and when subjected to Sulphate attack for 90 days
Table 8: Overall results of development of compressive strength in concrete subjected to sulphate attack with age
Age, days Compressive strength, MPa
0%
GP
5% GP 10% GP 15% GP 20% GP 25% GP 30% GP 35% GP 40% GP
7 20.62 21.83 22.57 24.36 26.75 23.00 17.09 15.71 12.67
28 25.45 27.15 28.10 30.00 31.85 29.01 23.20 21.55 18.25
90 22.80 24.22 25.65 27.18 28.86 26.30 21.60 19.70 16.88

Resistance of Concrete Containing Waste Glass Powder
Figure 14: Variation of compressive strength development in concrete subjected to sulphate attack with age
Sulphate attack lowered the compressive strength of concrete between 2 to 4 % in 7 days, 3 to 6% in 28 days and 11 to 17% in 90 days

Compressive strength of concrete with 20% cement replacement by glass powder in the sulphate attack experiment showed a higher value by 30%, 25%, 27% compared to control concrete for 7 days, 28 days and 90 days respectively.

(b) Flexural Strength

Table 9, 10 and figure 15, 16 shows the result of Variation of flexural strength of concrete with cement replacement by glass powder for 7, 28 and 90 days. It seems flexural strength of concrete with 20% cement replacement by glass powder showed a higher value by 22%, 20%, 17% compared to control concrete for 7 days, 28 days and 90 days respectively

Table 9: Overall results of flexural strength of concrete with cement replacement by glass powder
Mix Designation Percentage  replacement of cement by glass powder Flexural strength
(N/mm2)
[07days]
Percentage  increase or decrease in flexural strength with respect to reference mix Flexural strength
(N/mm2)
[28 days]
Percentage  increase or decrease in flexural strength with respect to reference mix Flexural strength
(N/mm2)
[90 days]
Percentage  increase or decrease in flexural strength with respect to reference mix
M1 0(Ref.mix) 2.40 - 3.50 - 3.60 ------
M2 05 2.45 +2 3.62 +4 3.64 +2
M3 10 2.78 +16 3.78 +8 3.82 +7
M4 15 2.85 +19 3.95 +13 4.00 +12
M5 20 3.05 +22 4.17 +20 4.21 +17
M6 25 2.90 +21 4.00 +15 4.05 +13
M7 30 2.82 +18 3.90 +12 3.92 +9
M8 35 2.42 +1 3.57 +2 3.60 0
M9 40 2.32 -4 3.41 -3 3.45 -5

Resistance of Concrete Containing Waste Glass Powder
Figure 15: Variation of flexural strength of concrete with cement replacement by glass powder for 7, 28 and 90 days


Table 10: Overall results of development of flexural  strength in concrete with age.
Age, days Flexural strength, MPa
0%
GP
5% GP 10% GP 15% GP 20% GP 25% GP 30% GP 35% GP 40% GP
7 2.40 2.45 2.78 2.85 3.05 2.90 2.82 2.42 2.32
28 3.50 3.62 3.78 3.95 4.17 4.00 3.90 3.57 3.41
90 3.60 3.64 3.82 4.00 4.21 4.05 3.92 3.60 3.45

Resistance of Concrete Containing Waste Glass Powder
Figure 16: Variation of flexural strength development in concrete with age

Discussion on Test Results

Workability

Workability decreases as the glass content increased (i.e. cement content decreased) due to reduction of fineness modulus of cementatious material, less quantity of cement paste is available for providing lubricating effect per unit surface area of aggregate, and hence mobility of aggregate is restrained.

Density

Unit weight of concrete without waste glass is higher than with waste glass. Such a difference was attributive to the fact that the specific gravity of waste glass i.e. 2.58 is much lower than specific gravity of cement i.e. 3.15

Strength

An increasing trend in strength was observed with increasing replacement of cement with glass powder up to 20%. The highest percentage increase in the compr- essive strength was about 30% and flexural strength was about 22% at 20% replacement level. When the cement replacement level was increased beyond 20%, the compressive strength decreased.

The increase in strength up to 20% replacement of cement by glass powder may be due to the pozzolanic reaction of glass powder. Waste glass when ground to a very fine powder, SiO2 react chemically with alkalis in cement and form cementitious product that help contribute to the strength development. Also it may be due to the glass powder effectively filling the voids and giving rise to a dense concrete microstructure as a result waste glass powder offers resistance against expansive forces caused by sulphates and penetration of sulphates ion into the concrete mass. However, beyond 20%, the dilution effect takes over and the strength starts to drop. Thus it can be concluded that 20% was the optimum level for replacement of cement with glass powder.

The strength improvement at early curing ages was slow due to pore filling effect. Waste glass powder(GP) initially acts like a pore filler and only later, after 7-10 days, its hydration liberates sufficient amount of lime for starting the secondary pozzolanic reaction.This reaction leads to more quantity of C-S-H gel getting formed.

Conclusions

Based on experimental observa- tions, the following conclusions are drawn:
  1. Higher strength was achieved when 20% cement was replaced by glass powder in concrete.
  2. The density of concrete reduces with the increase in the percentage of replacement of cement by glass powder.
  3. The workability decreased as the glass content increasedvUse of super plasticizer was found to be necessary to maintain workability with restricted water cement ratio.
  4. Considering the strength criteria, the replacement of cement by glass powder is feasible.
  5. It is recommended that the utilization of waste glass powder in concrete as cement replacement is possible.
  6. Strength properties were affected when concrete produced by replacing cement by glass powder was subjected to attack.
  7. Waste glass powder in appropriate proportions could be used to resist sach attack.

Acknowledgements

The authors would like to thank the authorities of S.V.N.I.T. Surat for their kind support. The valuable suggestions, efforts and timely help extended by one and all in concrete discipline are gratefully acknowledged. Sincere gratitude is extended to all the authors whose publications provided us directional information from time to time. The cooperation and help received from the scientific and technical staff of advanced materials laboratory in the preparation of this paper are gratefully acknowledged.

References

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  • Yixin Shao, Thibaut Lefort,Shylesh Moras and Damian Rodriguez, (2000), "Studies on concrete containing ground waste glass," Cement and Concrete Research, 30,pp91-100.
  • Suryvanshi.C.S. (1999) "Use of industrial and domestic waste in concrete," Civil Engineering and Construction Review, 26, pp26-31.
  • Byars E.A.,Morales.B. And Zhu H.Y., (2004) "Waste glass as concrete aggregate and pozzolana-laboratory and industrial projects," Concrete, 38, pp41-44.
  • Baxer.s, Jin W and Meyer C., (2000) "Glasscrete-Concrete with glass aggregate, ACI Materials journal, pp208-213.
  • Tang Albert, Dhir Ravindra, Dyer, Tom and Yongjun, (2004), "Towards maximizing the value and sustainable use of glass," Concrete Journal, 38,pp38-40.
  • Seung Bum Park, Bong chum Lee and Jeong Hwan Kim., (2004), "Studies on mechanical properties of concrete containing waste glass aggregate," Cement and concrete Research, 34, pp 2181-2189.
  • Omer ozkand and Isa Yuksel, (2008), "Studies on mortars containing waste bottle glass andindustrialby-products," Construction and Building Materials, 22, pp1288-1298.
  • Nathan Schwarz, Hieu cam and Narayanan Neithallath, (2008), "Influence of a fine glass powder on the durability characteristics of concrete and its comparison to fly ash," Cement and Concrete Composites, 30, pp486-496.
  • Chen C.H..Huang R.,Wu J.K.and Yang C.C., (2006), "Waste E-glass particles used in cementitious mixtures," Cement and concrete Research, 36, pp449-456.
  • Her-Yung Wang, (2009), "A Study of the effects of LCD glass sand on the properties of concrete," Waste Management, 29, pp335-341.
  • Jitendra A.Jain and Narayanan Neithalath, (2010), "Chloride transport in fly ash and glass powder modified concrets-Infludence of test methods on microstructure," Cement and Concrete Composites, 32, pp148-156.
  • Federico L.M. and Chidiac S.E., (2009), "Waste glass as Supplementary Cementitious material in concrete-Critical review of treatment methods" Cement and Concrete Composites, 31, pp606-610.
  • Bang R.S.,Pateriya I.K. and Chitalange M.R., (2009), "Use of pond ash as fine aggregate-Experimental Study," New Construction Materials, pp48-51.
  • Manjit Singh, Mridul Garg and K.K.Somani., (2006), "Experimental investigations in developing low cost masonry cement from industrial wastes", The Indian Concrete Journal, pp31-36.
  • Guerrero A.,Hernandez M.S. and Goni.S, (2000) "The role of the fly ash pozzolanic activity in Simulated Sulphate radioactive liquid waste." Waste Management, 20, pp51-58
  • Seung-Bum Park and Bong-Chum Lee, (2004), "Studies on expansion properties in mortar containing waste glass and fibers" Cements and Concrete Research, 34, pp1145-1152.
  • P.T.Santosh Kumar, (2009), "Combined influence of sand and water cement ratio on the compressive strength of concrete," The Indian Concrete Journal,pp9-14
  • Caijun Shi,Yanzhhhong Wu,Chris Riefler, and Hugh Wang, (2005), "Characteristics and pozzolanic reactivity of glass powders," Cement and Concrete Research, 35, pp 987-993.
  • Bashar Taha and Ghassan Nounu, (2008), "Using Lithium nitrate and pozzolanic glass powder in concrete as ASR Suppressors," Cement and Concrete Composites, 30, pp 497-505.
  • Mukesh C.Limbachiya, (2009), "Bulk engineering and durability properties of washed glass sand concrete," Construction and Building Materials, 23,pp1078-1083.
  • Andrea Saccani and Maria chiara Bignozzi,(2010), "ASR expansion behavior of recycled glass fine aggregates in concrete," Cement and Concrete Research, 40, pp531-536.
  • Rachida Idir,Martin Cyr and Arezki Tagnit-Hamou., (2010), "Use of fine glass as ASR inhibitor in glass aggregate mortars," Construction and Building Materials, 24, pp1309-131
  • V.Ducman, A.Mladenovic and J.S.Suput (2002), "Lightweight aggregate based on waste glass and its alkali-Silica reactivity," Cement and Concrete Research, 32, pp223-226.
  • Her-Yung Wang and Wen-Liang Huang, (2010), "A Study on the properties of fresh self- consolidating glass concrete (SCGC)," Construction and Building Materials, 24, pp619-624.
  • Mohamad J.Terro, (2006), "Properties of concrete made with recycled crushed glass at elevated temperatures," Building and Environment, 41, pp633-639.
  • Ahmad Shayan and Aimin Xu,, (2006), "Performance of glass powder as a pozzolanic material in concrete: A field trial on concrete slabs," Cement and Concrete Research, 36,pp547-468
  • M.Shahul Hameed and A.S.S.Sekar, (2009), "Quarry dust as replacement of fine aggregates in concrete," New construction Materials, pp52-56.
  • Ilker Bekir Topcu and Mehmet Canbaz, (2004), "Properties of Concrete Containing Waste glass," Cement and Concrete Research, 34, pp267-274.
  • Bashar Taha and Ghassan Nonu, (2008), "Properties of Concrete contains mixed colour waste recycled glass as sand and cement replacement," Construction and Building Materials, 22,pp731-720
  • Mageswari M and Vidivelli B. (2010) "The Use of sheet glass powder as fine aggregate replacement in concrete," The open Civil Engineering journal, 4, pp65-71
  • Narayanan Neithalath, (2011), "An Overview of the benefits of using glass powder as a partial cement replacement material in concretes," The Indian concrete journal, pp9-18
  • Turgut P.,.Yahlizade E.S., (2009), "Research in to concrete blocks with waste glass," International Journal of civil and Environmental Engineering,4,pp 203-209
  • Zdenek P.Bazant,Goangseup Zi, Meyyer, (2000), "Fracture Mechanics of ASR in concretes with waste glass particles of Different Sizes," Journal of Engineering Mechanics,pp226-232
  • Bashar Taha,Ghassan Nounu, (.2009), "Utilizing waste Recycled Glass as Sand/Cement replacement in concrete, "Journal of materials in civil Engineering, pp709-721
  • Meyer C, Egosi N, Andela C, (2001). "Concrete with waste glass as aggregate," Proceedings of the International symposium concrete Technology unit of ASCE and University of Dundee, pp37-45
  • Shetty M.S., (2006). "Concrete Technology Theory and Practice" S.Chand and Company Ltd., New Delhi,
  • Gambhir M.L, (2006) "Concrete Technology," Tata McGraw-Hill Publishing Company Limited, New Delhi,
  • Sood H, Mittal L.N., Kulkarni, P.D. (2003)" Laboratory Manual on Concrete Technology," CBS Publishers and Distributors, New Delhi,
  • Methods of tests for strength of concrete, IS 516:1959, Bureau of Indian Standards, New Delhi
  • Methods of sampling and analysis of concrete, IS 1199: 1959, Bureaue of Indian Standards, New Delhi
  • Methods of test for determination of water soluble chlorides in concrete admixtures, IS 6925: 1973, Bureau of Indian Standards, New Delhi
  • Method of making, curing and determining compressive strength cures concrete test specimens, IS 9013:1978, Bureau of Indian Standards, New Delhi
  • Specification for apparatus for flexural testing of concrete, IS 9399: 1979, Bureau of Indian Standards, New Delhi
  • Handbook on Design of reinforced concrete to IS 456:1978, SP: 16 (S&T):1980, Bureau of Indian Standards, New Delhi
  • Handbook on concrete mixes (Amendment No.1), SP: 23 (S&T): 1982, Bureau of Indian Standards, New Delhi
  • Explanatory handbook on Indian standard code of practice for plain and reinforced concrete(IS 456:1978), SP: 24(S&T): 1983, Bureau of Indian Standards, New Delhi
  • Methods for analysis of concrete (Cement content, sulphate content and alkali contents), BS 1881: Part 124:1988, Bureau of British Standards
  • Indian Standard code of practice for plain and Reinforced Concrete, IS456:2000, Bureau of Indian Standards, New Delhi.
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  • Recommended guidelines of concrete mix design, IS 10262:2009, Bureau of Indian Standards, New Delhi

NBMCW May 2012

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Use of Manufactured Sand in Concrete an...

G Sreenivasa, General Manager (Business Development), UltraTech Cement Limited Bangalore

Introduction

Natural or River sand are weathered and worn out particles of rocks and are of various grades or sizes depending upon the amount of wearing. Now-a-days good sand is not readily available, it is transported from a long distance. Those resources are also exhausting very rapidly. So it is a need of the time to find some substitute to natural river sand.

The artificial sand produced by proper machines can be a better substitute to river sand. The sand must be of proper gradation (it should have particles from 150 microns to 4.75 mm in proper proportion).

When fine particles are in proper proportion, the sand will have fewer voids. The cement quantity required will be less. Such sand will be more economical. Demand for manufactured fine aggregates for making concrete is increasing day by day as river sand cannot meet the rising demand of construction sector. Natural river sand takes millions of years to form and is not repleneshible.

Because of its limited supply, the cost of Natural River sand has sky rocketed and its consistent supply cannot be guaranteed. Under this circu‎mstances use of manufactured sand becomes inevitable.

River sand in many parts of the country is not graded properly and has excessive silt and organic impurities and these can be detrimental to durability of steel in concrete whereas manufactured sand has no silt or organic impurities

Manufactured Sand
Manufactured sand River sand

However, many people in India have doubts about quality of concrete / mortars when manufactured or artificial sand are used. Manufactured sand have been regularly used to make quality concrete for decades in India and abroad.

Pune - Mumbai expressway was completely built using artificial/manufactured sand.

Issues with Manufactured Sand
  1. The Civil engineers, Architects, Builders, and Contractors agree that the river sand, which is available today, is deficient in many respect. It does content very high silt fine particles (as in case of Filter sand).
  2. Presence of other impurities such as coal, bones, shells, mica and silt etc makes it inferior for the use in cement concrete. The decay of these materials, due to weathering effect, shortens the life of the concrete.
  3. Now-a-days, the Government have put ban on lifting sand from River bed.
  4. Transportation of sand damages the roads.
  5. Removing sand from river bed impact the environment, as water table goes deeper & ultimately dry.
General Requirements of Manufactured Sand
  1. All the sand particles should have higher crushing strength.
  2. The surface texture of the particles should be smooth.
  3. The edges of the particles should be grounded.
  4. The ratio of fines below 600 microns in sand should not be less than 30%.
  5. There should not be any organic impurities
  6. Silt in sand should not be more than 2%, for crushed sand.
  7. In manufactured sand the permissible limit of fines below 75 microns shall not exceed 15%.
Crushing, Screening, and Washing

Manufactured Sand Manufactured Sand
Rotopactor( Sand Making machine) Vertical Shaft Impactor

Manufacturing of Sand process involves three stages, crushing of stones in to aggregates by VSI, then fed to Rotopactor to crush aggregates into sand to required grain sizes (as fines). Screening is done to eliminate dust particles and Washing of sand eliminates very fine particles present within. The end product will satisfy all the requirements of IS:383 and can be used in Concrete & construction. The VSI Plants are available capacity up-to 400Ton Per Hour(TPH).

Manufactured Sand (M Sand)

Manufactured Sand
VSI Crushed Sand -Cubical Jaw crushed sand -Flaky

Only, sand manufactured by VSI crusher/Rotopactor is cubical and angular in shape. Sand made by other types of machines is flaky, which is troublesome in working. The Jaw crushers, are generally used for crushing stones in to metal/aggregates. Manufactured sand from jaw crusher, cone crusher, roll crusher often contain higher percentage of dust and have flaky particle.

IS Code Provisions

BIS Guidelines IS: 383-1970 for selection and testing of Coarse and Fine aggregates available. Generally, Sand is classified as Zone I, Zone II, Zone III and Zone IV (i.e. Coarser to Finer). There is sieve designation for each zone. Gradation is made in accord with the usage of the sand. There are testing sieves, consists of 4.75mm, 2.36mm, 1.183mm, 600microns, 300 microns, 150 microns and a pan

Typical Sieve analysis: Comparison of River & Manufactured Sand
IS Sieve % of passing(River Sand) % of passing (Manufactured Sand ) Zone II (As per IS:383)
4.75mm 100 100 90-100
2.36mm 99.7 90.7 75-100
1.18mm 89 66.2 55-90
600micron 60.9 39.8 35-59
300micron 17.7 25.5 8-30
150micron 3.1 9.9 0-20
75micron Max 3 Max 15 Max 15
Zone II Zone II
Zone IIZone IINote: The gradation of manufactured sand can be controlled at crushing plant

Technical specification – comparison between Manufactured and River sand
Sl No Property River sand Manufactured sand Remarks
1 Shape Spherical particle Cubical particle Good
2 Gradation Cannot be controlled Can be controlled
3 Particle passing 75micron Presence of silt shall be less than 3%(IS:383-1970)reaffirmed 2007 Presence of dust particle shall be less than 15% Limit 3% for uncrushed & limit 15% for crushed sand
4 Silt and Organic impurities Present (Retard the setting & Compressive Strength) Absent Limit of 5% for Uncrushed & 2% for Crushed sand
5 Specific gravity 2.3 – 2.7 2.5 – 2.9 May vary
6 Water absorption 1.5 - 3% 2 – 4% Limit 2%
7 Ability to hold surface moisture Up-to 7% Up-to 10%
8 Grading zone(FM) Zone II and III
FM 2.2 -2.8
Zone II
FM 2.6 – 3.0
Recommends Zone II for Mass Concrete
9 Soundness(Sodium sulphate-ss & Magnesium sulphate -ms) (5 cycles) Relatively less sound (Ex. >5) Relatively sound
(Ex. <5)
Limit 10% ss and 15% ms
10 Alkali Silica Reactivity 0.002 -0.01 0.001- 0.008 Limit 0.1%expansio

Behaviour of Manufactured & River Sand when used in Concrete:
Sl No Property River sand Manufactured sand Remedies
1 Workability & its retention Good & Good retention Less & Less retention Control of fines & apply water absorption correction, use of plasticisers
2 Setting Normal Comparatively faster Apply water absorption correction, use retarders
3 Compressive strength Normal Marginally higher As shown above
4 Permeability Poor Very poor
5 Cracks Nil Tend to surface crack Early curing & protection of fresh concrete

Cost comparison of Manufactured and River sand:
Sl no Location- Bangalore City River sand Artificial sand Remarks
1 Market rate Rs 1100 per MT Rs 600 per MT 50% Cheaper
2 In Concrete - Rs per Cu‎m Rs 770 – 880 Rs 420 – 480 Saving of Rs 350-400 per cu‎m
3 In Mortar(1:5) for 100kgs Rs 198 Rs 156 20% less

Typical Compressive Strength of Concrete: The following results show the behavior of manufactured sand and riverbed sand when used in concrete:

  • With using Riverbed Sand: (All proportions are by weight)
    • Cement -50 Kg
    • River Sand -75 Kg
    • Agg. 20 mm- 75 Kg
    • Agg. 12 mm -37.5 Kg
    • Water -19 ltrs
    Compressive strength achieved after 7 - days curing …….44.1MPa

  • With using Artificial Sand : (All proportion are by weight)
    • Cement -50 Kg
    • Artificial Sand - 70 Kg
    • Agg. 20 mm - 80 Kg
    • Agg. 12 mm - 35 Kg
    • Water - 19 ltrs
    Compressive strength achieved after 7 -days curing …….46.8MPa

Vastu Aspects of River sand

Now-a-days, Vastu Shastra is more popular, consults Vastu by many people while constructing a house. As per Vastu Shastra, the building material must be free from traces of human or animal body. The river sand contains bones of human beings and animals. The shells are also a kind of bone. It is not easy to take out all such things present in the river sand. Hence, the best solution for this is to use artificial/crushed sand of good quality for human well being.

Environmental Impact

The River sand lifting from river bed, impact the environment in many ways:
  • Due to digging of the sand from river bed reduces the water head, so less percolation of rain water in ground, which result in lower ground water level.
  • The roots of the tree may not be able to get water.
  • The rainwater flowing in the river contents more impurities.
  • Erosion of nearby land due to excess sand lifting
  • Disturbance due to digging for sand & lifting, Destroys the flora & fauna in surrounding areas
  • The connecting village roads will get badly damaged due to over- loading of trucks, hence, roads become problem to road users and also become accidents prone
  • Diminishing of Natural Rivers or river beds, not available for future generations.
Conclusion
  • Considering, the acute shortage of river sand, huge short coming on quality of river sand, high cost, greater impact on road damages and environmental effects, The Construction Industry shall start using the manufactured sand to full extent as alternative, reduce the impacts on environment by not using the river sand.
  • The Local Authorities/PWD/ Govt, shall encourage the use of Manufactured sand in Public Construction Works, if possible, shall make mandatory to use Manufactured sand wherever available with immediate effect.
  • The Govt. Shall come out with, Policy on Sand – encourage the industry people to set up more no of Sand crushing Units across the all Districts, States to meet the sand requirements of the Construction Industry.
References

www.vsicrushers.com

www.robosilicon.com

ICOMAT Report

NBMCW April 2012

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Durable Cost Effective Building Materia...

India produces large quantity of waste by-products from various agro industrial processes. Among these waste, utilization of phosphogypsum, fluorogypsum, slag, fly ash, lime sludges, etc. is paramount due to their enormous availability and the pollution of the ground water likely to be caused by their unplanned dumping on the fertile land. The utilization of waste gypsum particularly phosphogypsum (PG) and the fluorogypsum (FG) is significant. Unlike PG, the utilization of FG, a waste of hydrofluoric acid industry is also important as it too creates pollution of the atmosphere and the ground water. Investigations accomplished at the CBRI showed that useful cementitious binder can be produced by blending small quantity of Ca(OH)2 and chemical additives with the FG followed by fine attrition. Data showed that a plaster/binder of low water demand, high compressive strength and low water absorption can be produced. The hydration of binder as evaluated by differential thermal analysis (DTA) show rapid conversion of anhydrite phase into dihydrate gypsum. The addition of 15-20% waste lime sludge may add economy to the new binder. The FG binder is suitable for making building bricks, flooring tiles and plastering which may be considered a new concept to the effective use and disposal of FG waste to accrue economy to the HF industry as well as to the Nation. Cost wise the FG binder is cheaper than the lime and cement binders. The technology of high strength plaster from FG and its use in making flooring tiles has been commercialized.

Dr. Manjit Singh,Former Scientist ‘G” (Director Grade Scientist) & Head, Environmental S&T & Clay Products Divisions, Central Building Research Institute, Roorkee (India) Advisor/Consultant to Gypsum & Cement Industries

Introduction

Huge quantity of wastes are being produced from the industries like metallurgy, petrochemicals, fertilizers, paper and pulp making per annum in India. From the building materials angle, the utilization of fly ash, slags, phosphogypsum, fluorogypsum, press mud, red mud etc. is paramount. Over 6.5 million tonnes of by-product gypsum are produced annually from several phosphatic and hydro-fluoric acid industries in India, Singh1. The waste of phosphoric acid industry popularly called as phosphogypsum is available to great extent. Fluorogypsum a waste of hydrofluoric acid manufacture has been studied for use in supersulphated cement or binding agents as reported by Taneja, et.al 2 , Swanski 3.

In China Republic, similar cementitious binders based on fluorogypsum were developed by Yan et.al.4-5. The use of fluorogypsum which is available to the extent of slightly over 1.0 million tonne per annum is significant. Fluorogypsum contains impurities of fluoride and free acidity. These impurities particularly free acidity may interfere with the setting and strength development of plaster/building components. Since the utilization of fluorogypsum is limited, thus, there is a disposal problem of this waste and it has become essential to suitably utilize the material in value added building materials.

Durable Cost Effective Building Materials from Waste Fluorogypsum Durable Cost Effective Building Materials from Waste Fluorogypsum
Figure 1: DTA of fluorogypsum anhydrite Figure 2: SEM of unpurified fluorogypsum

Researches 6-9 have shown that water-resistant binding materials can be produced from phosphogypsum, slag, cement, fly ash and lime sludge. In this paper, investigations undertaken to characterize and to beneficiate fluorogypsum to make high strength binding/cementitious plaster have been presented. Effect of various chemical activators/additives on setting, strength, water absorption, porosity, etc. of the fluoro plaster/binder has been studied. The hydration and microstructure properties of the binder are reported. The suitability of fluorogypsum plaster for making building bricks and blocks containing various admixtures and its use in plastering work has been discussed. The economy of making the plaster is also indicated.

Materials & Methods

The sample of fluorogypsum was collected from M/s Navin Fluorine International, Bhestan, Gujarat, India. It was analyzed for chemical composition as per IS: 128810 and as per standard test procedures cited by Scott, et.al.11-12. The fluorogypsum contained Fluoride 1.32%, SiO2 + insoluble in HCl 0.65%, Al2O3 + Fe2O3 0.65%, CaO 41.19%, MgO Tr., SO3 56.10 % and Loss on ignition 0.61% and pH 5.0. Data showed that fluorogypsum possess high purity i.e. CaSO4.2H2O besides fluoride as the major impurity. The low pH value shows presence of free acidity. Minor earthly impurities of SiO2 and Al2O3 + Fe2O3 have also been identified.

The chemical activators of laboratory grades ranging from sulphates to chloride of alkali and alkaline earth hydroxides were used to activate hydration of fluoroanhydrite. Differential thermal analysis (DTA) (Stanton Red croft, UK) and Scanning electron microscopy (SEM LEO 438VP) of the raw gypsum and hydrated plaster were studied.

DTA (Fig. 1) shows endotherm and exotherm at 140° and 250°C due to the inversion of poorly weathered anhydrite into hemihydrate (CaSO4.½ H2O) and anhydrite (CaSO4 (II)) peaks. The SEM of fluorogypsum sample (Fig. 2) shows majority of crystals in fluorogypsum are anhedral to subhedral platy prismatic interspersed with lath in the jumble form. Twinning of some of the crystals may also be recorded.

Purification of Fluorogypsum

As the fluorogypsum contains free acidity which may corrode the grinding media (balls etc.) and the lining of the ball mill and sometimes the plaster/ binder may become hygroscopic, it is therefore, essential to neutralize the acidity to get the suitable product. With this objective, effect of addition of dry hydrated lime (Ca (OH)2) was studied on the pH of fluorogypsum. It was found that at 0.8 -1.0% addition of (Ca(OH)2) to the fluorogypsum, a neutral pH value of 7.0 was obtained. The SEM of purified fluorogypsum is shown in Figure 3. It can be seen that gypsum crystals are euhedral platy prismatic and lath shaped in nature without agglomeration/jumbling showing absence of any foreign impurity.

Preparation of Gypsum Binder / Plaster

Durable Cost Effective Building Materials from Waste Fluorogypsum
Figure 3: SEM of purified fluorogypsum
The fluorogypsum dried at 42 ± 2°C, was ground in the ball mill to a fineness of 98-99% passing through 90 micron (I.S. 9 sieve) sieve. The ground material was then blended with different chemical activators for 1 hour in the blender / powder mixer to get a uniform product. The binder was tested and evaluated as per references13-14. The water absorption and porosity of fluorogypsum plaster was examined by immersing the 2.5 cm x 2.5 cm x 2.5 cm cubes of the binder/plaster (28 days cured) in water for a period of 2h, 8h and 24 hrs. The porosity of cubes was evaluated by multiplying the water absorption with bulk density of the hydrated plaster.

Effect of addition of lime sludge obtained from M/s Rashtriya Chemicals & Fertilizers, Mumbai, (chemical composition (by wt.%) - P2O5 2.01, F 0.12, Na2O + K2O 0.026, organic matter 0.06, SiO2 +insoluble in HCl 1.50, CaO 50.50, Al2O3 + Fe2O3 0.026, MgO 0.64, SO3 0.98, LOI 44.50) was examined on the properties like compressive strength, bulk density, water absorption and porosity of fluorogypsum plaster/binder on the basis of 2.5cm x 2.5cm x 2.5cm cubes for the period of 3, 7 and 28 days.

Preparation of Bricks and Plastering Studies

The effect of various materials such as saw dust, rice husk, exfoliated vermiculite etc. on the properties of fluorogypsum binder was studied to arrive at optimum mix composition for casting bricks. 5cm x 5cm x 5cm cubes were cast at the workable consistency for the properties like compressive strength, bulk density, water absorption and the porosity. Based on cube data, large size bricks (19cm x 9cm x 9cm) were cast by hand molding.

Preparation of Building Blocks & Flooring Tiles

Based on 5 cm cube data (previously optimized on 5 cm x 5 cm x 5cm cube strength), 40 x 20 x 10 cm blocks were cast using anhydrite plaster previously produced plus optimized chemical activator (such as sod. Sulphate or sod. sulphate + ferrous sulphate) with washed saw dust and foamed slag separately. The foamed slag (a by-product of steel plants) of size 6 mm and down having density 300 - 400 kg/m3 may be used to develop insulative blocks.

The flooring tiles of sizes 200 mm x 200 mm x 20 mm and 300 mm x 300 mm x 20 mm or larger size are cast by vibration moulding of the mix containing fluoroanhydrite powder with different pigments, catalyzed (0.5 - 1.5%)), a small quantity of glass fibre and coloured stone chips at normal consistency. The fly ash and red mud industrial solid wastes are also added to the anhydrite mixes for moulding these tiles. These tiles after demoulding are cured in high humidity (over 90%), dried at 42 ± 2°C and tested for properties such as flexural strength, compressive strength, water absorption, wear resistance and porosity as per IS:1237-1980, specification for cement and concrete tiles.

Plastering of Brick Wall

To find out suitability of fluorogypsum binder for use in internal plastering, mortars of mix proportions 1:1, 1:2 an 1:3, by volume were prepared at mason consistency to plaster the burnt clay brick wall. Mortar mixes 1:1, !:2 and 1:3, binder-sand in 12mm thickness were applied over the internal brick wall. The fineness modulus of the sand was kept at 1.91 (50:50 Badarpur and Ranipur river sand). The finish coat of 3 mm of neat binder was applied over 9 mm of 1:2 , binder-sand under coat. Before applying binder-sand plaster, the brick wall was well watered so that mortar water may not get evaporated before the mortar was set. The plastered patches were examined for their various characteristics after 24 hours and onward.

Durable Cost Effective Building Materials from Waste Fluorogypsum

Properties of Fluorogypsum Binder/Plaster

Durable Cost Effective Building Materials from Waste Fluorogypsum
Figure 4: DTA of hardened fluorogypsum plaste
The physical properties of fluorogypsum plaster activated by the chemical activators are reported in Table 1. Data show that with the use of (NH4)2SO4 activator, the setting time of fluorogypsum plaster was beyond the maximum specified limit of 6.0 hrs as per ASTM C 61-50. At the same time, the compressive strength was also much less. However, with the use of combined chemical activators i.e. Ca(OH)2 – CaCl2 – Na2SO4, the setting time was accelerated and was much within the limit and the rate of strength development was even quite high at 3 days of hydration. The hydration of fluorogypsum plaster/binder was supplemented by DTA. The thermo grams are shown in Fig. 4. It can be seen that intensity of endotherms at 140-150°C,190-200°C and exotherms at 360-370°C were increased due to dehydration of gypsum and inversion of CaSO4(III) in to β-CaSO4. The intensity of endotherms and exotherms found to be increased with curing period indicating increase in gypsum formation. The fluorogypsum has been found sound in nature.

The hardening of fluorogypsum is due to increase in its solubility and the rate of dissolution of the anhydrite and due to increase in the rate of nuclei formation. In fact, the chemical activators added to the anhydrite plaster react with the CaSO4 ions and form transient double salt. The colloidal particles of the activator concentrate on the surface of CaSO4 molecules and establish the potential centers around which crystallization sets in when solution becomes supersaturated.

CaSO4            H2O CaSO4. Activatorions —> Ca2+ + SO42+
(fluoroanhydrite)——>
                                     Activators
Activatorions   H2O CaSO4.2H2O (Gypsum)
                       ——>

Durable Cost Effective Building Materials from Waste Fluorogypsum

Water Absorption and Porosity of Fluorogypsum Binder/Plaster

The results of water absorption and the porosity of fluorogypsum binder is shown in Table 2. Data show that fluorogypsum binders produced with the chemical activators Ca(OH)2 – CaCl2 – Na2SO4 possess lower water absorption and the porosity values than the (NH4)2SO4 activator. On the basis of strength development, water absorption and porosity properties of the fluorogypsum binder, the addition of chemical activators i.e. Ca(OH)2 – CaCl2 – Na2 SO4 (3.0 – 0.5 – 0.5 by wt.%) were selected for further studies.

Effect of Lime Sludge on the Properties of Fluorogypsum Binder

The addition of lime sludge from the paper industry on the properties of flourogypsum binder containing activators (Ca(OH)2:CaCl2:Na2SO4) is shown in Table 3. The trend of results show an increase in consistency and decrease in strength values with the addition of lime sludge.

However, the attainment of strength is quite high. Data showed that with increase in lime sludge addition, the water absorption and the porosity values were increased with the enhancement of immersion period. At 20.0% addition of the lime sludge, maximum water absorption (21.90%, 23.60%, 26.40% at 2hr, 8hr, 24 hr) and the porosity values (40.60, 43.70, 48.90 at 2hr., 8hr., 24 hr.) were attained. These studies suggest that fluorogypsum plaster may be partly replaced with the lime sludge to economize the use of such binder.

Development of Bricks from Fluorogypsum Binder/Plaster

Burnt clay bricks are essential ingredients for providing shelter to the millions, and is most popular because it can be adopted for any size or shape of construction. Brick constitutes about 13% of the total cost of the building materials required for construction of moderate house. The burnt clay brick industry as it exists today, is not able to meet the demand of a the modern construction agencies which require bricks of higher strength, better shape and of lower water absorption. The escalating cost of energy and unprecedented pressure on activities have undoubtedly caused great setback to the production and quality of the bricks. It is, therefore, imperative that an alternative material is required to bridge the gap which is environment–friendly and is acceptable to the construction agencies. It would therefore be useful to consider certain features of technological development in this country with particular reference to the current thinking on the need to use waste materials. Thus, utilization of fluorogypsum binder for making building bricks can be considered a new and useful preposition in building sector.

Durable Cost Effective Building Materials from Waste Fluorogypsum

The efforts were therefore, made to use fluorogypsum waste for making building bricks. The effect of various materials such as saw dust, rice husk, exfoliated vermiculite etc. on the properties of fluorogypsum binder was studied to arrive at optimum mix composition for casting bricks. 5cm x 5cm x 5cm cubes were cast at the workable consistency for the compressive strength, bulk density, water absorption, and the porosity.

Effect of Saw Dust and Rice Husk

Durable Cost Effective Building Materials from Waste Fluorogypsum
Figure 5: Building bricks cast from fluorogypsum binder/plaster using saw dust, rice husk and exfoliated vermiculite
The effect of saw dust and rice husk on the compressive strength and bulk density of the fluorogypsum binder are listed in Table 4. It can be seen that with the addition of saw dust and the rice husk to the fluorogypsum binder, the compressive strength and the bulk density values were reduced. In case of saw dust, the fall in strength was comparatively less than the addition of rice husk. However, the decrease in the bulk density was much more with the addition of rice husk than the saw dust. In view of reduction in the density values, the manufacture of lightweight bricks can be contemplated.

Durable Cost Effective Building Materials from Waste Fluorogypsum

Durable Cost Effective Building Materials from Waste Fluorogypsum

The effect of saw dust and rice husk on the water absorption and the porosity of the fluorogypsum binder was studied. Data showed that the strength values are higher in case of addition of rice husk than the addition of saw dust. This may be attributed to the organic impurities particularly sugars present in the saw dust. However, there is decrease in bulk density of the plaster with increase in saw dust and rice husk. With the addition of 10% saw dust to the plaster, the water absorption was found to be 8.75%, 9.42% and 9.98%, while the porosity was 15.58, 16.78 and 17.78 at 3, 7 and 28 days of curing. In case of addition of 5% rice husk, the water absorption of the plaster was 9.38%, 10.35% and 11.46% and the porosity was 17.40, 19.20 and 19.40 at 3, 7 and 28 days respectively.

Effect of Addition of Exfoliated Vermiculite on the Properties of Fluorogypsum Binder

The effect of addition of exfoliated vermiculite on the compressive strength and the bulk density of fluorogypsum are reported in Table 5. Data show that with the increase in vermiculite content, the compressive strength and the bulk density are reduced. However, there is an increase in the strength and density values with the increase in curing period. It can be noted that the density can be further reduced by increasing the vermiculite content but the cost of the composition may also be increased. At 10% addition of vermiculite, the water absorption was 17.64%, 18.18% and 20.45% while the porosity was 30.51, 32.54 and 36.61. However, at 10.0% addition of vermiculite, adequate strength and the density values are achieved.

Preparation of Full Size Bricks

On the basis of properties obtained by the addition of an optimum quantities of saw dust (10%), rice husk (5%) and the exfoliated vermiculite addition (10%) to the fluorogypsum binder, the full size bricks (19 x 9 x 9 cm) were cast at normal consistency. These bricks were tested for physical appearance, compressive strength, water absorption and efflorescence as per IS: 3495 (Part 1)15. The properties of bricks are listed in Table 6. The strength and water absorption values complied with the properties of IS:1289416 except those bricks prepared with rice husk. Typical photograph of binder–saw dust, binder–rice husk, binder–exfoliated vermiculite and binder-lime sludge bricks are shown in Fig. 5.

Durable Cost Effective Building Materials from Waste Fluorogypsum

Suitability of Fluorogypsum Binder in Plastering

Durable Cost Effective Building Materials from Waste Fluorogypsum Durable Cost Effective Building Materials from Waste Fluorogypsum
Figure 1: DTA of fluorogypsum anhydrite Figure 2: SEM of unpurified fluorogypsum

The suitability studies never carried out following the proceed explained in this paper earlier. It was found that plaster patches developed adequate strength and hardness after 24 hours of application and further continued. The texture of the plaster was smooth and hard and showed good adhesion with the bricks.

Building Blocks & Flooring Tiles from FG Properties of Building Blocks

The properties of building blocks produced from FG are listed in Table 7. It can be seen that on using 10% saw dust and 40% of foamed slag, the density is lowered and the strength data comply with the minimum strength of 2.0 MPa specified in IS:2849-1983, a specification for non-load bearing gypsum partition blocks (solid and hollow type). These blocks are mainly suitable for internal non-load bearing partition walls or for inner leaf of cavity wall construction. The blocks should not be used under damp conditions as they are liable to suffer deterioration as their strength is seriously reduced.

Durable Cost Effective Building Materials from Waste Fluorogypsum

Durable Cost Effective Building Materials from Waste Fluorogypsum

According to Mydall et.al.17, the vitreous materials have far lower thermal conductivity than crystalline material. Since foamed slag used in making of blocks is a vitreous material, therefore, 300 x 300 x 30 mm size boards may be cast using the above composition. The boards after curing for 28 days are dried and subjected to the thermal conductivity test as per heat flow method specified in ASTM (518-1971), specification for method of thermal conductivity of building insulating materials by heating flow method. The results are given in Table 8. The data show that the blocks produced using foamed slag have lower thermal conductivity than the conventional building materials like light weight concrete and common burnt clay bricks.

The strength of blocks is much higher than the minimum specified value of 50 kg/cm2 (5 MPa) IS:3590-1976, specification for load-bearing light weight concrete blocks. Hence, the blocks produced with neat plaster at normal consistency are rightly suitable for load-bearing internal partition walls. The typical photographs of blocks made using (1) 10% saw dust, (b) 40% foamed slag and (c) neat plaster is shown in Fig. 6.

Properties of Flooring Tiles

The properties of flooring tiles moulded from polymerized and activated FG are listed in Table 9. It can be seen that the phosphogyp-sum flooring tiles complied with the requirements of flexural strength, water absorption and wear resistance as given in IS: 1237-1980.

Durable Cost Effective Building Materials from Waste Fluorogypsum

The tiles produced using fly ash or red mud in place of pigments also complied with the standard requirements. These tiles are suitable for use in flooring for general purposes for places where light loads are taken up by the floor such as office buildings, schools, colleges, hospitals and residential buildings. A typical photograph of flooring tiles produced out of anhydrite binder is shown in Fig. 7.

Durable Cost Effective Building Materials from Waste Fluorogypsum

Cost Economics of the Flooring Tiles

The cost of a plant of capacity 100 m2/day FG tile may cost the capital investment of 30-35 lakhs. The major equipment required for the plant may be blender, ball mill/pulverizer, vibro press, demoulding plates, casting moulds, powder mixers, driers, curing chamber, etc. These plant & machinery are easily available in Indian market. Some machineries are the proprietary items. An Indian Patent entiled ‘A Novel High Strength Plaster Composition and Flooring Tiles Made Therefrom’, Patent No. 696/Del/2000 by Dr.Manjit Singh & Dr. Mridul Garg has been claimed.

Conclusion

High strength plaster can be developed from fluorogypsum waste. A mixture of Chemical activators like Ca(OH)2, CaCl2 and Na2SO4 has been finalized as the strengthening compound to invert waste anhydrite into strong gypsum matrix at faster pace. The addition of 15-20% lime sludge to the binder can economize the production of fluorogypsum plaster without sacrificing the strength value. Building bricks can be produced by adding an optimum quantity of saw dust (10%), rice husk (5%) or exfoliated vermiculite (15-20%) to the fluorogypsum binder. The fluorogypsum on admixing with river sand, lime sludge and the exfoliated vermiculite (in optimum proportion) is suitable for plastering (Finish & undercoat) over the internal brick wall. The fluorogypsum plaster/binder may replace different type of cement/lime based putties available in the market. The fluorogypsum plaster is also suitable for making building blocks when mixed with washed saw dust and foamed slag for insulative purpose and light weight partitions in normal as well as multistoried buildings.

References

  • Singh M, From waste to wealth- developing potential construction materials from industrial wastes, (Edited by Prof.P.C.Trivedi),Avishkar Publishers, Distributers, Jaipur, pp 36-62, 2006.
  • Taneja C A, Malhotra SK, Supersulphated cement from waste anhydrite, Res & Ind.,19 :51-52,1974.
  • Swanski A, Fluorogypsum binder, Chem Abstract, 98 :1660 3 j, 1983.
  • Yan P, You Y, Studies on the binder of fly ash-fluorgypsum-cement, Cem. Conc. Res., 28(1):135-140,1998.
  • Yan P, Yang W Y, Xiao Q, You Y, The microstructure and properties of the binder of flyash fluorogypsum-portland cement. Cem. Conc. Res., 29(3):349-354, 1999.
  • Gnyra B, Aitcin P C, Preparation from waste fluoroanhydrite of cementitious powders suitable for casting into dense and / or foamed anhydrite panels, Chem. Abstract, 95:137 299k, 1981.
  • Singh M, Garg M, Gypsum binders and fibre reinforced gypsum products, The Ind Concr Jour, 8: 387-392, 1989.
  • Singh M, Garg M, Investigations of a durable gypsum binder for building materials, Constr Bld. Mater. 6 :52-56, 1992.
  • Valentini, Santaro L, Hydration of granulated blast furnace slag in the presence of phospho-gypsum, Thermichemica Acta, 78 :101-112, 1984.
  • IS:1288-1983, Specification for methods of chemical analysis of mineral gypsum, Bureau of Indian Standards, New Delhi,1983.
  • Scott W W and Furman N H, Standard Methods of Chemical Analysis, V Edition (London), 1 (1952) 214-216, 1052.
  • Vogel A I, A Text Book of Quantitative Inorganic Analysis, 3rd Edition (London), Longmans & Green Co., 575, 1960.
  • IS : 2542 (Part 1)-1978, Methods of test for gypsum building plaster, excluding premixed plaster Bureau of Indian Standards, New Delhi, 1978.
  • ASTM C 61-50, Specification for Keen’e cement, 1990.
  • IS:3495 (Part 1)-1976, Methods of Test for Burnt Clay Building Bricks: Parts 1&2, Determination of compressive strength and Water absorption, 1976.
  • IS:12894 - 1990, Specifications for Fly ash-Lime Bricks, Bureau of Indian Standards, New Delhi, 1990.
  • Mydall P, Diamant RME, Vitreous slag as a raw material for the precast concrete industry, Lime Gravel, 48 (4) : 72-75, 1973.

NBMCW March 2012

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Revolutionary Quick Setting, Self-curin...

Revolutionary Quick Setting, Self-curing Medium Density Solution for Concrete Repair from Chowgule Construction Technologies

Solution for Concrete Repair
Easy to use, factory packed, self-curing, achieves high compressive strength —> 3 N / mm2 in 4 hrs and can be painted over, 24 hrs after application-this is MortaRep Super Strong a polymer modified multi purpose mortar for concrete and brick surfaces which can be easily applied on horizontal, vertical, and over head surfaces in a single layer and up to a thickness of 50 mm.

Scenarios such as water seepage in mineral substrates, leads to carbonation and deterioration of the structure over a period of time, this is a natural enemy to the durability of any structure. Captain Bond offers products, which protect the structure against such seepages. MortaRep SuperStrong is a new introduction from Chowgule Construction Technologies Pvt Ltd (CCTPL).

It combines - a) Anti-corrosive coat, b) Key coat for steel and concrete, c) Waterproof and non shrink mortar, and d) Concrete or plaster finish – all in a single product.

Solution for Concrete Repair
Traditionally, repair of concrete substrates is done using the Polymer Modified Mortar method. This process involves removal of the old/weak concrete areas, treating the exposed steel - removal of rust and application of anticorrosive coating, and building up the removed concrete with site prepared sand/cement/polymer mortar in layers of 20 mm each, interspaced with curing. This process can take up to 7 days before the area is handed over for further use.

With MortaRep SuperStrong, once the damaged concrete is removed and the exposed steel is cleaned, the pre-mixed and factory packed product is mixed with water - 5 ltrs. to 25 kgs of powder product, and then applied directly onto the substrate to be repaired. Once installed, it is allowed to selfcure for 24 hours and then can be over painted to provide a finished surface. A coat of CrystalFlex 2K SuperStrong powder is recommended over weak substrates before the application of MortaRep SuperStrong.

Its unique properties include high early compressive strength—4 hrs. => 3N / mm2, 7 days = 20 N / mm2, 28 days = 35 N / mm2, Flexural strength – 7 days = 3 N / mm2, 28 days = 7 N / mm2, Specific Gravity of 1.2 gms / ltr.

In India, the CCTPL range of products has been introduced were years before. The company is respected for its professionalism in our primary markets of Maharashtra, Goa, Tamil Nadu, Andhra Pradesh, Karnataka and Kerala. It has achieved excellent results in complicated cases to the fullest satisfaction of our customers.

CCTPL is specialized in waterproofing and concrete repair products for over a decade. CCTPL is a part of the 97-year-old Chowgule Group of companies, which has its presence in shipping, logistics, and automotives in addition to construction chemicals. CCTPL has collaboration with KAUBIT Aktiengesellschaft for waterproofing and repair products, Schedetal AG for ECB and TPO membranes, OttoChemie for sealants and ABP for Epoxy and PU-based floor coatings. We are trusted by leading architects, consultants and developers in our areas of operations.

NBMCW February 2012

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In-Place Concrete Strength and Temperat...

John Gnaedinger, President, Con-Cure Corporation Ballwin, Missouri USA

One of the most important questions on any concrete construction project is "What is the strength of that concrete right now?" To answer that, contractors usually break concrete test samples (cylinders, cubes or beams) by crushing and measuring the force required. However, all that really tells you is the strength of the sample-the strength of the in-place concrete is still unknown, and yet that is what is truly needed.

In-Place Concrete Strength and Temperature: The Science of Maturity
Engineers assume test samples lag behind the in-place concrete in strength due to the difference in the rate of strength gain between the small test sample and the much more massive slab or wall. Because greater mass generates higher curing temperatures, and because concrete gains strength faster at warmer temperatures, concrete test samples typically gain strength more slowly than the concrete they are supposedly emulating, and are generally assumed to be a conservative measure of the concrete strength. The engineering community has relied on this assumption for the entire history of concrete construction, but there are many examples of jobsite disasters where this assumption proved to have deadly consequences. The Willow Island Cooling Tower Collapse in West Virginia (USA) in 1978 is a classic case where 51 workers died because the in-place concrete didn’t match the test samples. The 2009 Vedanta/Balco chimney tower collapse (shown in photo) in Korba, India (more than 40 dead) is another example.

Most of the time, relying on test samples to determine early deshuttering or post-tensioning means that concrete projects are needlessly delayed. Knowing in-place strength is important so these critical construction operations can take place as soon as it is possible, safely. Today, new technology and testing methods can improve jobsite safety by eliminating any guesswork, and as a bonus, dramatically improve scheduling for critical construction operations.

Since 1987, ASTM has recognized The Maturity Method as a means of non-destructively estimating in-place concrete strength (C1074). In the USA and Europe, virtually every standards body and concrete organization now recognizes concrete maturity testing as a valid and critically important concrete assessment tool for bridges, roads, structures, highrises, precast concrete, or any other significant concrete project.

Conducting this test is quite straightforward. One simply needs to first create a calibration strength curve based on the specific concrete mix design being used, and then monitor and record concrete temperatures in-place. The calibration process uses standard concrete test samples (cylinders, beams or cubes), which are cured under controlled conditions and crushed at specific ages. The strength and ages are plotted, and a calibration curve is generated. Each mix design must be calibrated separately-the method is highly mix-specific. Furthermore, concrete delivered to the jobsite must be "substantially similar" to the concrete tested in the lab. Once the system is calibrated, the in-place concrete is then instrumented with sensors and recording devices. The recorded temperature history of the in-place concrete (Figure 1) is applied to a formula, and an estimate of the in-place strength is obtained (Figure 2).

In-Place Concrete Strength and Temperature: The Science of Maturity
Figure 1: Temperature history graph showing one day of temperature history for a typical section of fresh concrete.

Implementing concrete maturity monitoring on any time-sensitive concrete project brings many benefits. The main benefits include:
  • Early-age strength estimation to determine the soonest that time critical construction tasks can safely commence, such as formwork stripping (deshuttering), post-tensioning, reshoring, etc. For certain types of construction, as well as cold-weather projects, this might allow stressing of PT tendons and deshuttering one or two days sooner.
  • Dramatically improving safety by helping prevent premature deshuttering or stressing PT tendons because many areas of a structure can be monitored at once.
  • Optimizing concrete mix designs using maturity allows producers to create mixes that meet all structural requirements while at the same time keeping overdesign to a minimum, thus lowering costs and reducing cement factors which can cut down on unwanted slab shrinkage problems.
  • "Back-up" for faulty test samples is one big benefit cited by experienced users. When an entire construction project is unnecessarily delayed because crews are waiting for test samples to reach strength (even though the structure itself is already there), suddenly the "cost" of maturity monitoring is tiny compared to the potential losses. This is known as "cheap insurance."
In-Place Concrete Strength and Temperature: The Science of Maturity
Figure 2. Maturity (strength) "Equivalent Age" graph showing equivalent maturity of 3.3 days (32.4Mpa) after applying only 1 day of temperature history for a typical section of fresh concrete. The in-place concrete has the same strength as concrete cured in the laboratory for 3.3 days (at 22°C).

Maturity systems typically pay for themselves in a remarkably short time period. For example, one contractor reported that they saved $10 (US) per cubic yard of concrete on a 50,000CY project because they were able to optimize their mix designs using less cement while still obtaining ideal results. That’s a savings of $500,000 (US) on the concrete mix designs alone! Considering that their initial investment in the maturity equipment was less than $12,000 (US), that is a very substantial return on the investment, even before the project was completed.

In-Place Concrete Strength and Temperature: The Science of Maturity
Another metric used to determine ROI is "time saved." For a parking garage, the sooner the owner can open the doors, the sooner he can make money, so rapid construction often carries large incentives. In one recent case, the contractor was able to earn a $400,000 (US) early-completion bonus because they saved one to two days in waiting time for every concrete placement. There were 37 concrete placements, and they saved a minimum of one day for each pour, sometimes more. They were able to complete the project 40 days sooner than planned, with an early-completion bonus of $10,000 (US) per day. The client’s investment in the maturity system was under $10,000. Another way to look at it: They earned $390,000 simply using a maturity system on one project.

In-Place Concrete Strength and Temperature: The Science of Maturity
Commercially-available maturity systems can be of great benefit, since all the math is done automatically. One such system, the ZoneCure® System (shown here) from Con-Cure Corporation (USA), features reusable sensors (so the cost-per-test is kept low), and it can even transmit concrete status wirelessly to your email. Imagine pouring concrete and getting an email when it has reached the desired strength! The ZoneCure® system is available in India and all of Southeast Asia through agreement with Samhitha Innovations (Bangalore).

In-Place Concrete Strength and Temperature: The Science of Maturity
The ZoneCure® system consists of small, reusable sensors that are embedded in fresh concrete at any desired location. In the photo shown here, the sensor is placed inside a sleeve, which is then tied to a concrete support element. The sensor lead wires are routed to the outside of the concrete through the formwork, and the lead is attached to a battery-operated transmitter/receiver/recorder (also generically called a "maturity meter"). ZoneCure® maturity meters (one is shown below monitoring a precast beam) are unique in that they also form a "mesh network" at the jobsite, allowing them to pass data wirelessly to each other, via "hopping" technology. This means the meters can be widely distributed across a jobsite and they will still be able to communicate with the base station wirelessly, so all the concrete temperature and strength information can be observed live, instantly and in real time.

ZoneCure® system costs are determined by the needs of the individual client, and are mainly dependent on the number of maturity meters required. A small system can cost less than $5,000(US), and the system is infinitely scalable for any size project or operation. Even the largest precast concrete producers in the world can equip their entire plant with a highly sophisticated system consisting of multiple wireless "zones" for less than $30,000. There is no jobsite the ZoneCure® System cannot handle.

In-Place Concrete Strength and Temperature: The Science of Maturity

Keep in mind, though, that any maturity system, large or small, will return great dividends to the user because of the dramatic improvements in efficiency, reduction in schedule, and improved safety.

Markets Served:

  • Cast-in-place concrete, primarily elevated decks
    • For projects where a user would have to gain access to areas where returning to simply download the data to a handheld reader would pose a safety risk or where the time required to access that location is significant.
    • For example, any highrise is a prime candidate. It can take more than an hour for one person to leave the trailer, ride a lift or climb to the floor being monitored, download the data, climb or ride back down to the trailer and then store the data to the PC. With ZoneCure, the data is always present on the desktop, allowing instant data analysis and project feedback.
    • Clearly, any Post-tensioned project is a prime candidate.
  • Tilt-up projects
  • Tall walls
    • When monitoring tall structures, keeping personnel on the ground is always best. With ZoneCure, the data is beamed to the site trailer, eliminating the need to access the sensor directly.
  • Precast concrete plants
    • Due to the staggering number of test samples that must be taken at any precast plant, the benefits of the ZoneCure system for precast are immediately apparent:
      • Save time by monitoring pieces instantly without having to collect and break test samples.
      • Saving even a few hours of curing time for each piece saves a tremendous amount of curing time. Saving curing time means they can flip their forms faster.
      • Eliminating the reliance on test samples can lead to dramatically lower cement costs, since the focus turns to the strength of the piece rather than the strength of the cylinder.
  • Concrete batch plants (QC)
    • Progressive concrete producers use maturity to optimize their mixes and target-market proprietary mix designs for specific performance in the field.
    • Our clients use the system in their labs for QC, plus use the system in the field to assess in-place mix performance under varying ambient conditions, using the data to further enhance their mixes for the optimal cost/performance balance. This allows producers to both save money on mixes by cutting back on compulsory overdesign, and improve profit margins by promoting their engineered mix performance based on in-situ strength targets.
  • Concrete testing labs
    • Labs use the ZoneCure System in much the same way a contractor does: They save time and money by monitoring the concrete performance remotely.
  • Cement plants (QC)
    • Cement plants use the ZoneCure system in their labs to monitor quality. Since the data is always available to the lab personnel, they are able to make faster decisions on production and product performance.

NBMCW February 2012

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Causes for Accelerated Structural Deter...

Dr. Y. P. Gupta, Advisor, Naini Bridge Information Centre; COWI - DIPL Consortium, Allahabad

Introduction

In India, we come across many old buildings needing major repairs or go early in to a state of dilapidation condition to make them unfit for occupation. However, if a building has given about 25 to 30 years of service without much maintenance or major repair, then it is reasonable to expect that it would need some structural repair soon. The main cause for this is weathering and ageing effect or inadequate maintenance and care. However, generally at an age of less than 10 years; many poorly designed and/or constructed buildings are found to be in a very bad structural and general health condition needing major structural repairs. This premature deterioration is largely due to poor construction or inappropriate design and / or neglect of timely repairs.

The premature deterioration of structure is an economic burden not only on owners but also to the municipalities’ and nation as a whole. The longer life of structures enables better utilization of natural resources. Reduction in waste due to demolition enhances energy conservation. Such philosophy will be in harmony with the environment and help in achieving sustainability of system. It would also lead to benchmark standards of maintenance and upkeep of buildings. Ofcourse the buildings/structures should also be appropriately designed for resistance to natural disasters like earthquakes, cyclones, floods etc.

Stages of Construction of New Building/Structures

  1. A concept plan
  2. Selection of appropriate materials and methodology of construction
  3. Freezing assumptions in structural design
  4. Selection of trained manpower and machinery
  5. Execution of the project
Proper understanding of the probable causes of poor performance or faster deterioration of buildings is essential, so that precautionary measures can be taken by owners. The details of some these aspects are given here.

Expected Service Life of Structures

There is very little literature available on the subject of expected service life of structures. The lifespan of RCC generally is taken as 100 years. However, there are some expected as well as prevalent conventions about design life span, which are given here:
  • Monumental Structures like temple, mosque or church etc     - 500 to 1000 years
  • Steel Bridges, Steel Building or similar structures     - 100 to 150 years
  • Concrete bridges or Highrise building or stone bridges etc     - 100 years
  • residential houses or general office/commercial buildings etc     - 60 to 80 years
  • Concrete pavements     - 30 to 35 years
  • Bituminous pavements     - 8 to10 years

Conception and Design Process

In Architectural and structural design, the following stages are normally considered:
  1. Concept plan of project to satisfy its functional needs,
  2. Architectural planning and design
  3. Detailed structural design
  4. Preparation of working/execution drawings
  5. Preparation of specifications and tender conditions for execution
  6. Modifications (if any) during construction phase.
Conception of structure / building depends on the functional need and local environment rather than purely on architectural/structural considerations. Though, the basic structural frame will share relatively small proportions (about 45%) of the total cost of the project, yet the designer has to give due emphasis on its proper planning, design and specifications of structural frame work. However, these have to conform to local environment and locally available construction resources e.g. materials, labour and plant & machinery.

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 1: Excessive strain & poor concreting at beam column joint
The functional utility as well as aesthetics of building are important in the architectural design process. But, durability is the most important in structural design and laying down specifications for construction for achieving service life of structure. Thus, all three aspects (viz architectural, structural and construction) are important. Yet, many a times, there are clashes between the three. Slender RCC columns, provided from architectural considerations, especially at the ground-floor level, where the trend is to make the ground floor area reserved for parking and access, but these became critical for earthquake resistance and could be a cause for slow disintegration within five to ten years. RCC columns could be observed to split, mainly due to corrosion of the steel bars and the resultant cracking. Corrosion crack in concrete is cancerous, if not treated in time, and will spread to cause the concrete structures to crumble and collapse. So, it is necessary to provide shear walls in the ground floor at suitable places in the design process itself.

The design of slender columns, shear walls and beams are provided to look slim and to increase carpet area but could become problems during construction and may not ensure the correct alignment and proper compaction of concrete. Many times columns are reduced in thickness well below the minimum 225/300mm to result in congestion of steel bars, with probability of exposure due to inadequate cover after de-shuttering. For such structure, appropriate coordinated efforts between architect, structural designer and construction team are needed.

In structural designs, emphasis is given on stress and strains on concrete and steel with a suitable factor of safety. Structures are designed on the basis of LIMIT STATE philosophy which was adopted in 1978 in India. An approach with respect to performance of structures like cracks, deflections and minimum cement content for durability and control of maximum w/c ratio are considered as per IS:456-2000 in the design. The IS code lays down following factors for durability of concrete for adoption during design / construction stages:
  1. Environment or exposure condition,
  2. Cover to embedded steel,
  3. Type and quality of constituent materials,
  4. Cement content and water/ cement ratio,
  5. Workmanship, and
  6. Shape and size of the Structural member.

Exposure to highly aggressive environment

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 2: Poor quality of concrete and in grace of moisture leading to corrosion
Relatively new but severely distressed buildings are observed in the locations, close to sea or creek. Corrosion of reinforcement and distress in concrete in the structural members (columns, beams, slabs) in some of them could be alarming. It is mainly due to high concentration of chlorides and/or sulphates in the ground water and saline environment around the structure. Moisture from soil could rise. The absence of damp proof course at the DPC level could allow the dampness to rise quite high. This combined with substandard materials and workmanship could get further aggravate deterioration process. Thus, extra care must be taken in materials selection and have proper control on quality of concrete in structures, close to sea.

Selection of Materials of Construction

Materials for construction depend on nature of building/structure. In India and world over, reinforced concrete is extensively used for all type of structures including residential, highrise buildings, commercial and institutional, recreational & religious type of buildings etc. Load bearing buildings are commonly used for residential purposes with one or two storeys. Steel structures were limited to bridges and industrial & warehouse type buildings. Composite types of buildings are still new to India. Thus there can be large variety of materials which can be used in construction depending upon type of structure, location and budget allowed. The following are the main construction materials:
  • Concrete (Plain, Reinforced, or Pre-stressed)
  • Steel of all grades: Fe 250, Fe 415, Fe500 and high tensile steel
  • Bricks Work
  • Stones
  • Timber
  • Fiber Reinforced Concrete
  • Composite materials ferrocem- ent, FRp etc.
  • Different type of plastics and polymers like Acrylic etc

Reinforced Cement Concrete

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 3: Sulphate attack on pipe pedestal
Concrete came into existence in last 200 years or so and it was expected that concrete should behave like stone structures. But with the introduction of steel reinforcement in concrete, its structural utility has been enhanced for taking tensile loads. Hence, reinforced cement concrete has now become one of the most important materials of construction all over the world. Thus, reinforced cement concrete has become so important that without this, there is no major building in India or anywhere in world.

However, because of its heterogeneous character, the durability of structure has assumed much importance in last three decades. Durability is affected because of poor quality of its constituent materials and workmanship could lead to early deterioration. Chlorides may also be present in these materials which lead to early and faster corrosion of reinforcement. Poor quality of concrete materials also affects quality of concrete:
  • Quality of cement
  • Type and quality of course aggregates
  • Fineness of sand and silt contents therein
  • Concrete mix design etc.
Quality of other associated construction materials should also be looked into:
  1. Poor burnt clay Bricks (its crushing strength should be more than 3.5 MPa),
  2. Weathered Stone
  3. Rusted Steel

Construction

Concrete could become a treacherous construction material, if not manufactured correctly and not compacted fully. It may show low strength and high permeability. Though, it does not show signs of immediate weakness, but only after about five to ten years of construction, (depending upon the environmental conditions), signs of deterioration become visible. Therefore, for executing good construction at site, we need:
  • Good quality of construction material
  • Appropriate methodology
  • Trained manpower and
  • Appropriate machinery.
Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 4: Early corrosion due to inadequate cement content in concrete
Engineers in India are well trained in the academic and technical institutions in theories. Our engineers and construction managers can match their counter- parts in the developed countries in producing sophisticated design calculations and drawings, including the use of computers so that our structures are designed safe, elegant and slender. But how about executing them to specifications at the construction site? In India, concrete structures are made at construction site itself and hence, quality largely depends on artisans like mason, bar bender etc. The construction industry has virtually no practical training facilities to enable our basic artisans to produce good, cohesive, workable and durable concrete economically. They are either not trained or do not make efforts in the correct way of concreting like placing and compacting (vibrating) the concrete to obtain a dense and impervious watertight concrete which protects the reinforcement against corrosion. Result is poor quality and unsound structures.

Even after the construction supervision by site staff, corrosion of the steel bars and spalling/splitting of concrete continues at an early stage, leading eventually many times to the total collapse of the building. Generally, in India owners feel that the consulting fees to an Architect or a consulting Engineer is a waste of money and they feel happier, if the Architects and Engineers extend their services free of cost. It is recommended that site construction services (Architects and Engineers) should be engaged on payment and the owner must utilize their full expertise and experience. The site engineers should be paid by the owner, as is done all over the world. Then only site engineers shall be answerable for quality to the owner of structure. Therefore, to get good construction, we must have trained manpower and quality supervision throughout the construction period by a competent, experienced and strict Resident Engineer / site supervisor.

Causes of Early Deterioration of Concrete Structures

Newly constructed RCC structures are failing in a fraction of its design life span. Therefore, the causes of premature deterioration in relatively new buildings are different as compared to those for old buildings. Hence, the approach for the repair (or restoration) of such buildings should be quite different from that of old buildings. The process of repair should start with a thorough visual survey, followed by non-destructive testing of the structural elements and chemical tests on concrete and ground water. Generally, the main reason is poor or incorrect design and/or poor quality of materials. There may be several other reasons also as described below.

Poor workmanship: Need for certified Artisans & quality of construction

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 5: Shrinkage cracks in concrete pavement due to concreting in hot weather
Substandard workmanship in RCC can be in the form of honey combing (insufficient compaction of concrete) or inadequate cover to reinforcement (improper placement of bars) or both. These lead to early corrosion of reinforcement especially in thin structural member. In masonry and plaster, poor workmanship could be in the form of loosely fitted masonry joints (within walls or between external walls and beams / columns), poor lines and levels and hollow plaster etc. They lead to excessive seepage. Some of these deficiencies may become evident only after the full loading of structure has been put to use like 5 to 7 years. Therefore, construction workers employed should be trained artisans, who know the job well. To get trained manpower, there should be training schools to cater to the need of artisans like bar bender, concreting mason etc. Training schools should be opened and look into the following:
  • Construction workers to be trained and certified for a number of trades e.g. concrete, bar bending, masonry/plaster, carpenter, etc.
  • Assess the availability and demand of the certified personnel and then fill the gap with certified trained construction personnel
  • Run simple & short training courses with minimal loss of wages for personnel on job
  • Practical sessions with hands on experience on works itself
Further, necessary steps must be taken at construction site which calls for increase in number and proper mix of knowledge, skills and attitudes.

Effect of Climate

Climate plays a significant role in the decay of structure. Prolonged exposure to polluted environment and acid rain can deteriorate concrete or dissolve bricks and will also corrode embedded or exposed metal ties and fastenings. High levels of moisture and excessive fluctuations in heating and cooling can promote the movement of soluble salts. Salt movement is characterized by patches of white crystals in the surface of walls and can cause considerable damage. Frost can also contribute to the decay due to freezing and expansion of embedded water and resultant cracking of surface concrete.

Inadequate cement quantity

IS:456-2000 has laid down from durability considerations, the minimum cement content in concrete, irrespective of strength depending upon the exposure conditions to which the structure. Many design and construction engineers overlook this codal requirement and even unscrupulous contractors use less cement than that specified. The minimum quantity of cement is needed not only to coat the fine and coarse aggregate particles but also to fill the voids between the aggregate particles and to provide a thicker film of cement grout for easy workability. Thus, the aggregate particles slide over each other, during compaction of concrete.

Excessive water cement ratio

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 6: Beam member with inadequate cover
For concrete to be durable there is a maximum w/c ratio specified for mixing concrete, as well as to give proper workability and concrete strength. Again the IS 456:2000 has laid down upper limits of water cement ratio. Normally, it needs about 11.5 liters of water (say w/c = 0.23) per 50 kg bag of cement for hydration process. However, the concrete will be stiff and un-compactable with this quantity of water. Therefore, additional water and super-plasticizer is added to make the concrete workable. This extra water (not needed for Hydration) eventually evaporates and leaves minute capillary pores which permit the ingress of moisture and pollutants which lead to slow corrosion of steel bars and ultimate disintegration of the concrete. The objective is to add only the water as per concrete mix design.

Many a times, the concrete is made at site and contractors use more than the permissible water to make concrete workable because of following:
  • The contractors may not have or like to use vibrators.
  • Masons want fluid concrete for easy placing and compaction of the concrete.
  • The sand with excessive silt (clay) demands more water to maintain workability (high slump) desired by the workers.
  • The coarse aggregates (stone metal) are excessively flaky instead of cubical, due to bad crushers. The extra surface area of the flaky stone particles, demand extra water to maintain easy workability desired by workers.
  • Concreting during the hot weather in day time at temperature more than 35°C without any precautions (cooling of concrete ingredients by shading or using ice flakes.) accelerates the rate of hydration (setting) of the concrete. The concrete becomes stiff and unworkable, demanding excess water in the concrete. If this condition exists then, it is preferable to do concreting at night time when it is cooler.
The shuttering and staging is too flimsy and rickety as it may result in the collapse of the formwork and false work or have excessive leakage of cement slurry/ grout through the big gaps in the shuttering.

Inadequate concrete cover

The black smiths who fix reinforcement bars are neither trained to bend the bars accurately nor to fix them effectively to ensure that the specified cover is left between bars and the formwork (shuttering). Quite often, not only the bars themselves touch form work but also the binding wire loose ends and the steel bars are seen at the surface of the concrete and they are subjected to early carbonation of concrete.

Honeycombed or Un-vibrated concrete

Honeycombed concrete is a major source of weakness in concrete and cause of safety concern especially in multi-storied buildings, due to inadequate vibration/compaction in columns, walls, beams and slabs. Use of form vibrator is essential for narrow walls, partitions and architectural fins.

Cold joints or bad construction joints

Most construction site personnel do not plan properly the sequence of pouring concrete to minimize the number of construction joints. They do not take adequate precautions to eliminate cold joints. A cold joint is a joint where fresh concrete is placed against a previous un-compacted concrete which has already hardened due to lapse of concrete setting time. Therefore, the fresh concrete will not homogeneously merge with the older concrete.

Alkali-Aggregate Reactivity

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 7: Poor Concreting in column
Sometimes, chemical reaction occurs between reactive siliceous minerals or carbonates, present in aggregate and the alkaline hydroxides derived from hydrated cement. The result of reaction is formation of alkali-silicate gel of ’unlimited swelling’ type. These reactions are called i) Alkali-silica reaction, and ii) Alkali-carbonate reaction. Because the gel is confined by the surrounding cement paste, so the internal pressure develops and causes cracking and disruption of concrete. Under most conditions, this very slow reaction causes excessive expansion and cracking of concrete after few years. Aggregates containing particular varieties of silica are susceptible to attack by alkalis (Na2O and K2O) originating from cement, admixtures or other sources, producing an expansive reaction.

Aggregates petro-graphically known reactive type or aggregates which, on the basis of past history or laboratory experiments are suspected to have reactive tendency are needed to be avoided in concrete or used only with cements of low alkalis [not more than 0.6 percent as sodium oxide (Na2O)]. Use of pozzolanic cement and certain pozzolanic admixtures may be helpful in controlling alkali aggregate reaction. Alternately use non-reactive aggregate from alternate sources.

Initially rust steel bars

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 8: Water tank: Less cover in soffit of slab and week concrete
Often steel bars are stored in open areas, exposed to rain and atmospheric moisture resulting in rusting of them. The corrosion process starts rapidly in the presence of moisture (especially in coastal areas). The steel bars are rarely wire-brushed and cleaned thoroughly before being placed in shuttering prior to concreting. In other cases, due to suspension of work due to reasons whatsoever, structural frame remains exposed to sun, rain and misuse for a long duration. Such prolonged exposure to weather can cause rusting of rebars and carbonation to adversely affecting durability of the building frame.

Congested reinforcement bars

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 9: Less cover in slab bottom and cold joint
Sometimes design engineer provides too many steel bars in the narrow and slender RCC columns, walls or beams which lead to practically no cover in concrete or even space for inserting a needle vibrator to ensure full compaction. This results in honeycombed concrete. Through honeycombing, the moisture and atmospheric pollution enters the steel bars and thus starting the corrosion process.

Porous cover blocks

The cover blocks are invariably made at site with no attention to the correct mix proportion or the specified water cement ratio. If the main concrete is of M30 grade, then the cover blocks too should be of the same grade and should be dense and impervious. The cover blocks are usually fixed to the steel bars at about one meter centers and if they are porous they become the starting source of decay of concrete as they permit the ingress of moisture which corrodes the steel. If dense concrete cover blocks cannot be made then it is preferable to use plastic cover blocks which are now available.

Effect of Cracking on the Life or Durability of Structure

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 10: Poor quality of concrete in cover and ingress of moisture
A good understanding of cracks in concrete will help us avoid failures of concrete on one hand and avoidable worries and expenditure on repairs on the other hand. Cracks in concrete are rarely symptoms of disease by itself. Cracks in concrete are more prevalent than imagination. But not all the cracks are dangerous. Cracks could lead to any of the following effects:
  • Reduce loading capacity of structure
  • Progressive failure (Cracks propagate at smaller stress than that required to initiate it).
  • Loss of appearance
  • Leakages (effects serviceability)
  • Apprehension of failure in mind (Psychological)
  • All above reasons reduce durability of concrete. Some typical cases of cracks are shown in figures above.

Controlling cracks

  • Better Concrete mix design.
    • Least possible w/c ratio.
    • Optimum quantity of cement content.
    • Use mineral admixtures or have better fineness of cement.
  • Have friendly environmental conditions like wind, ambient temperature, and moisture etc at the time of concreting at site.
  • Have dense concrete
  • Use low heat or pozzolanic cement in mass concreting.

Conclusion

Repair / restoration of the main structural members should be accorded highest priority while starting the repair / rehabilitation work or allocating funds for the work (which is neglected sometimes). Many concrete structures start showing signs of distress within 5 to 10 years however, these could be due to poor concreting or wrong placement of reinforcement. Therefore, it will be desirable if the soundness of structure is assessed during or soon after the construction of building is over. Thus wherever the structure is week, it may be strengthened.

Causes for Accelerated Structural Deterioration of Reinforced Concrete
Figure 11: Initially rusted reinforcement bars and less cover in soffit of slabs

Requirements for Construction Engineers/Workers

To achieve good concrete construction at site or to have good concrete, it is necessary that manpower (both engineer & skilled worker level) must be properly trained and certified in relevant type of work. For such training, there must be training centers and ITI’s or vocational training schools at large number of locations.

Reference

  • Gupta Y. P. "Use of MALWA Recycled Aggregate in Concrete Construction: a Need of Society for Better Environment," Journal of Indian Concrete Institute, Vol. 10, No. 4, Jan 2010.
  • Performance of Structures-Real Estate August 1989.

NBMCW February 2012

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Effect of Mixing Methods in Behavior of...

R. Preetha, GVVSR Kishore, Scientific Officer, C. S Pillai, Associate Director, Indira Gandhi Centre For Atomic Research, A. K. Laharia, ADDL. Chief Engineer(c), M. Umashankar, Engineer in Charge Quality Assurance Civil, Nuclear Power Corporation Ltd.

General

Fly ash is a good pozzolanic mineral admixture, which can replace large quantity of cement in the concrete. The positive effects of using fly ash in concrete are better quality of concrete, eco-friendly and preservation of resources. Construction of power plants require huge quantity of concrete and use of fly ash, as pozzolanic material, can improve the durability of structure, thereby enhancing the safety of plants and also economy in the life cycle cost of the plant.

Properties of concrete whether strength or durability depends on constituents, mixing, placing, compaction, curing etc. To get a holistic view on behavior of flyash concrete to be used in major construction of safety structures, study of mixing methods were also included. Mixing method is the specified order in which ingredients of concrete are introduced in the mixer along with the stages of addition, if any, and time of agitation at each stage. Ingredients of Fly Ash based concrete are added into the mixer in different stages and in every stage, it is agitated for a specified period of time.

Mixing is important to achieve desirable homogeneity and performance of concrete mix. Success of a mixing method depends on optimal combination of mixing energy, time, and mixing sequence. However, studying of mixing method of concrete is difficult because there has been no consensus on evaluation criteria for quality of concrete mixing. The material and mixing systems usually mutually interact. It was expected that mixing methods have influence on property of concrete. A study was conducted to assess the impact of mixing method on the fresh state properties, strength and durability of the concrete. Laboratory mixer of 10 x 7 tilting drum types was used for this study.

Identification of Mixing Methods

A multistage mixing sequence with varying time of agitation at each stage was adopted for the study. The details of these mixing methods are given in Table 1.

Effect of Mixing Methods in Behavior of Flyash Concrete

Mixing method I & III are three stage mixing sequence while mixing method II, IV and V are two stage mixing sequence. Coarse Aggregate, Fine Aggregate and Cement was always loaded in the mixer in the first stage itself. Variation was made for Flyash, quantity of water and chemical admixture in the different mixing methods. Mixing method IV and V were identical in terms of loading sequence, the only difference being the time of agitation in second stage. Except mixing method IV, total mixing time in all the methods was maintained as 270 seconds. (4.5 minutes.)

Identification of Mixes

Mix proportions of M50, M35 and M20 grade of concrete with 40%, 50% and 60% cement replacement levels (CRL) respectively for carrying out the study on mixing methods are given in Table 2.

Effect of Mixing Methods in Behavior of Flyash Concrete

Results and Discussion

The properties of fresh and hardened state of concrete mixes produced by different mixing methods for M50, M35 and M20 grade identified mixes are given in Table 3. The results for slump are plotted in figure 1.The compressive strength results of mixes at 14, 28 and 56 days produced by different mixing methods for grade M50, M35 and M20 are given is Figure 2. The RCPT result of these mixes at 28 days and 56 days respectively are given in Figure 3.

Effect of Mixing Methods in Behavior of Flyash Concrete

Effect of Mixing Methods in Behavior of Flyash Concrete
Figure 1: Slump for concrete mixes produced by different mixing methods

Effect of Mixing Methods in Behavior of Flyash Concrete
Figure 2: Compressive strength of concrete mixes produced by different mixing methods

Effect of Mixing Methods in Behavior of Flyash Concrete
Figure 3: RCPT results for concrete mixes produced by different mixing methods

From the mixing method trials for TPC candidate mixes, it is seen that the mixing method IV with duration of 210 seconds was not giving satisfactory results for strength and durability tests hence ruled out. Among the other four, though mixing method of I &V were found to be giving good results when strength and durability tests are considered. Mixing sequence V was found to be satisfactory when fresh, hardened properties and durability aspects are considered together.

Summary

As expected, the mixing method and sequencing has noticeable influence on the behavior of the concrete mixes both in fresh and hardened states. Mixing method, V was finalized for further studies on fly ash concrete, as it is a two stage mixing, and results were satisfactory when fresh, hardened properties and durability aspects are considered together. The optimum mixing method for a given batching plant may be developed before start of the work. The duration of the mixing time corresponding to 270 seconds mixing time for laboratory mixer has to be calculated for equivalent mixing time for automatic batching plant, keeping the mixing energy same.

Acknowledgment

Authors are grateful to the management of NPCIL for financing the project "Development of FLY Ash Concrete Suitable for NPP Structures," BHAVINI for providing the facilities and AERB, NPCIL for manpower and guidance.

NBMCW January 2012

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Civil Engineering | Properties of Concr...

A View of Concrete Technologies and Required Related Research for Materials of Construction and Their Testing Methods

Dr. Y. P. Gupta, Consultant, Allahabad Bypass Project; BCEOM-LASA JV Chairman, ICI UP Centre Professor (RTD.) Civil Engineering, MNNIT, Allahabad

Introduction

In the past 60 years, significant changes have taken place in the type, properties of concrete and its constituent materials. During the 1940’s to 2000’s substantial basic research was conducted in the United States and many other Countries which produced a thorough understanding of the properties of concrete materials, such as cement and aggregates, and the effect of these materials on the green and hardened properties of concrete. Material standards and specifications, concrete mix design and ingredient proportions, test procedures, and construction techniques were developed extensively on the basis of this knowledge.

In recent years, Construction Industry has been placing strong emphasis on high-strength and high-performance concrete and on shorter construction times. In response to this challenge, research has been focused on producing changes in the properties of the basic ingredients of concrete, such as cement, and on developing new ingredients to achieve better-quality, higher-strength, and more-durable concrete.

Admixtures: Needs and Challenges in Concrete Technologies

For several generations, concrete admixtures have been developed with the aim of altering a wide range of green and hardened concrete properties to achieve high-early-strength and high-performance concrete. Use of admixtures has allowed a dramatic reduction in the water-cementitious materials ratio (w/cm) in the concrete mix, which in turn has resulted in higher-strength and more-durable concrete. Significant research has also been done on the development and use of cementitious and pozzolanic materials, such as fly ash, silica fume and slag to replace or supplement the cement content in the concrete mixture. These materials have significantly improved the durability of concrete by reducing its permeability.

Today, it is quite common for admixtures and cementitious / pozzolanic materials to be included in concrete in addition to the standard concrete ingredients. Such complex concrete mixtures are significantly different from the simple Concrete mixes produced in the 1960’s to 2000’s in India. Yet many specifications and construction practices developed in accordance with basic research of the 1950s are still being applied to today’s concrete materials and construction industry, especially on small scale projects.

In addition, there are still unresolved problems and many unanswered questions associated with today’s concrete. For example, excessive shrinkage and shrinkage cracking are being observed in many of the high-performance and high-strength concrete. These unintended consequences impact the durability of the concrete and thus tend to defeat the purpose of using such concrete mixes. Another important set of issues with today’s concrete relates to the timing, duration, and type of curing, and the balance between curing time and speed of construction.

Still another issue is the knowledge gap among many practitioners with regard to the properties of individual concrete ingredients, how the various ingredients interact in the concrete mix, and how to arrive at the optimum mixture for the type of application and level of exposure to adverse environments. An effective technology transfer plan is needed to convey to practitioner’s state-of-the art information and the latest research findings on materials and concrete properties should be informed to field Engineers & implemented.

Testing Methods

Current testing methods for concrete and its ingredients are another challenging issue. Some of these methods are simple but time-consuming and tend to slow the pace of construction. New or improved tests for determining the properties of concrete and its materials need to be developed like Concrete strength is known after 28 days which is too long. These test methods should combine speed, accuracy, and precision. Technologies from other fields, such as medicine or the military action that can be non-intrusive should be considered in determining the concrete strength.

Research for New Concepts

The challenge to the research community in this millennium is to promote and develop a thorough and comprehensive understanding of the properties of concrete and of its multiple ingredients. This challenge can be met through a well-planned basic research program. This program should include the development of new and improved methods for testing concrete and its materials. Another program should focus on the best and most effective means of transferring the knowledge and methods thus developed to concrete practitioners for implementation. The following are some specific directions which these programs might take.

a) Cement
Many changes have occurred in the sources and production of cement, including the raw materials, fuel used and the grinding of the clinker. Today’s cements are much finer than those of the 1950’s and 1980’s. Research in the basic properties of cement is needed to evaluate the effect of such properties as fineness, chemical composition on the heat of hydration, and on shrinkage characteristics.

b) Admixtures and Pozzolanic / Cementitious Materials
There is a need to evaluate all the properties of the various admixtures and cementitious or pozzolanic materials. Issues associated with the use of these materials in concrete, including setting time, plastic and hardened concrete shrinkage, and the need for extensive curing should be investigated. The research should produce ready to use tables of the types and dosage or proportion of these materials in concrete, and the specific level of performance and strength achieved with each. The research should also focus on developing a new family of admixtures that would improve the tensile strength of concrete and facilitate the fast construction of concrete structures. For example, new admixtures now being produced help in the self-compaction of concrete in structures. This must be simplified.

c) Curing Materials
The industry has moved away from moist curing toward the use of curing compounds which are more convenient to use. However, the use of high cement content, higher fineness of cement, silica fume, and low basic research and emerging Technologies related to concrete w/cm ratio has made the concrete more prone to shrinkage and thermal cracking. Curing compounds are not effective in preventing shrinkage or cracking. New curing compounds are needed not only to prevent evaporation, but also to replenish lost concrete mix water. For example, the curing compound might include chemicals that could condense ambient moisture on the concrete surface to provide much needed moisture. Further, Concrete ingredients like aggregates, Admixtures are to be developed which can help in self- curing of Concrete without the use of water or curing compound and not giving rise to shrinkage cracking.

d) Fibers for use in Concrete
Apart from cement, water, aggregate and admixture; different types of fibers are also developed. Fiber Reinforced Concrete has a very high resistance to abrasion and impact loading that means it has good ductility similar to mild steel. It also has a higher tensile strength compared to normal concrete and better abrasion resistance. Apart from high strength, it also has high performance & fracture energy. Following are some of the various types of fibers, which may be used in FRC.
  • Carbon Fibers
  • Steel Fibers
  • Glass Fibers
  • Polypropylene Fibers etc.
Polypropylene Fibers have become very popular these days. Thus concrete mix in green stage should be such that fibers are not collected in pockets and they are well dispersed in the entire concrete matrix. Studies for use of Fiber reinforcement in concrete must be done extensively. This should also be done for Composite concrete Construction using normal concrete and fiber reinforced concrete.

e) Tests For Concrete

1. Tests for Green Concrete
Tests for green concrete properties such as slump, air content, and unit weight have been useful in controlling the quality and consistency of concrete mixtures. However, it can be expected that more emphasis will be placed on shorter construction duration on the roads, bridges, and airports. The present tests for plastic concrete tend to cause delays in construction. New technology is needed to enable testing of the workability, air content, and unit weight of mixtures in a non-intrusive manner. For example, a non-intrusive device similar to a radar gun could be developed for measuring the concrete workability from the concrete chute itself during its discharge.

2. Tests for Hardened Concrete
A better means of predicting the strength and durability of concrete is needed. Tests based on the hydration process, rate of strength development, and other physical and chemical indicators should be developed for predicting the ultimate strength and permeability / durability of concrete. The availability of such tests would allow better optimization of the concrete mixture with respect to the types and proportions of its ingredients. In addition, the concept of 28-day strength may become obsolete as an acceptance requirement. Concrete mixtures of the future may reach their ultimate strength in less than 7 days. This accelerated development of strength may alter the microstructure of the concrete. Research is needed to better understand the physical and chemical properties of hydration process and its related compounds, as well as the extent of micro-cracking and volume change in the mortar matrix.

3. Tests for Permeability
Advances have been made in measuring the permeability of concrete to better predict its durability. Nonetheless, existing devices either are too slow or provide an indirect measure of concrete permeability. Thus a fast, accurate, and repeatable device for determining the permeability of concrete is needed. A procedure should also be developed for predicting the durability of concrete from the automatic analysis of permeability data.

Technology Transfer To Field

Good-quality research in concrete technology and its constituent materials is being conducted at many places. This research is generating new information and technologies. However, effective means of transferring the research findings and products from the research phase to application in field are needed. Many practitioners do not attend conferences, workshops, or meetings. These practitioners often do not receive full information on the properties of new materials and how these materials, individually or collectively, affect the strength, durability, and volume change of the concrete. A detailed plan for transferring the knowledge and new products resulting from completed research in concrete technology and its materials should be developed and implemented. The Concrete making materials & Design handbooks and Internet information should be the centerpiece of such plan.

Summary

This millennium brings challenges and opportunities for research on the basic properties of concrete and its materials. New non-intrusive devices and other test methods should be devised to allow faster, more accurate testing of concrete materials and construction procedures. Performance based specifications should be developed for concrete materials and construction aspects in field. Appropriate tests should be designed to assess compliance with the requirements. An effective technology transfer plan should be developed to translate research results and new products for implementation by practitioners or Field Engineers.

NBMCW December 2011

.....

Polypropylene Fiber Reinforced Concrete...

The capability of durable structure to resist weathering action, chemical attack, abrasion and other degradation processes during its service life with the minimal maintenance is equally important as the capacity of a structure to resist the loads applied on it. Although concrete offers many advantages regarding mechanical characteristics and economic aspects of the construction, the brittle behavior of the material remains a larger handicap for the seismic and other applications where flexible behaviour is essentially required. Recently, however the development of polypropylene fiber-reinforced concrete (PFRC) has provided a technical basis for improving these deficiencies. This paper presents an overview of the effect of polypropylene (PP) fibers on various properties of concrete in fresh and hardened state such as compressive strength, tensile strength, flexural strength, workability, bond strength, fracture properties, creep strain, impact and chloride penetration. The role of fibers in crack prevention has also been discussed.

S. K. Singh, Scientist, Structural Engineering Division, Central Building Research Institute, Roorkee & Honorary Secretary Institute of Engineers, Roorkee

Introduction

Ceramics were the first engineering materials known to mankind and they still constitute the most used materials in terms of weight [1, 2]. Hydraulic cements and cement-based composites including concretes are the main ceramic-based materials. Concrete offers many advantages in the application due to its improved mechanical characteristics, low permeability and higher resistance against chemical and mechanical attacks. Although concrete behavior is governed significantly by its compressive strength, the tensile strength is important with respect to the appearance and durability of concrete. The tensile strength of concrete is relatively much lower. Therefore, fibers are generally introduced to enhance its flexural tensile strength, crack arresting system and post cracking ductile behaviour of basic matrix.

Concrete modification by using polymeric materials has been studied for the past four decades [3]. In general, the reinforcement of brittle building materials with fibers has been known from ancient period such as putting straw into the mud for housing walls or reinforcing mortar using animal hair etc. Many materials like jute, bamboo, coconut, rice husk, cane bagasse, and sawdust as well as synthetic materials such as polyvinyl alcohol, polypropylene (PP), polyethylene, polyamides etc. have also been used for reinforcing the concrete [4,5,6,7,8]. Research and development into new fiber reinforced concrete is going on today as well.

Polypropylene fibers were first suggested as an admixture to concrete in 1965 for the construction of blast resistant buildings for the US Corps of Engineers. The fiber has subsequently been improved further and at present it is used either as short discontinuous fibrillated material for production of fiber reinforced concrete or a continuous mat for production of thin sheet components. Since then the use of these fibers has increased tremendously in construction of structures because addition of fibers in concrete improves the toughness, flexural strength, tensile strength and impact strength as well as failure mode of concrete. Polypropylene twine is cheap, abundantly available, and like all manmade fibers of a consistent quality.

Properties of Polypropylene Fibers

The raw material of polypropylene is derived from monomeric C3H6 which is purely hydrocarbon. Its mode of polymerization, its high molecular weight and the way it is processed into fibers combine to give polypropylene fibers very useful properties as explained below [9]:
  • There is a sterically regular atomic arrangement in the polymer molecule and high crystallinity. Due to regular structure, it is known as isotactic polypropylene.
  • Chemical inertness makes the fibers resistant to most chemicals. Any chemical that will not attack the concrete constituents will have no effect on the fiber either. On contact with more aggressive chemicals, the concrete will always deteriorate first.
  • The hydrophobic surface not being wet by cement paste helps to prevent chopped fibers from balling effect during mixing like other fibers.
  • The water demand is nil for polypropylene fibers.
  • The orientation leaves the film weak in the lateral direction which facilitates fibrillations. The cement matrix can therefore penetrate in the mesh structure between the individual fibrils and create a mechanical bond between matrix and fiber.
Polypropylene Fiber Reinforced Concrete Polypropylene Fiber Reinforced Concrete
Figure 1: monofilament fiber Figure 2: Fibrillated fiber
The fibers are manufactured either by the pulling wire procedure with circular cross section or by extruding the plastic film with rectangular cross-section. They appear either as fibrillated bundles, mono filament or microfilaments as shown in Fig. 1 & 2. The properties of these three types of PP fibers are given in Table 1 [10]. The fibrillated polypropylene fibers are formed by expansion of a plastic film, which is separated into strips and then slit. The fiber bundles are cut into specified lengths and fibrillated. In monofilament fibers, the addition of buttons at the ends of the fiber increases the pull out load. Further, the maximum load and stress transfer could also be achieved by twisting fibers [11].

Role of Fibers

Cracks play an important role as they change concrete structures into permeable elements and consequently with a high risk of corrosion. Cracks not only reduce the quality of concrete and make it aesthetically unacceptable but also make structures out of service. If these cracks do not exceed a certain width, they are neither harmful to a structure nor to its serviceability. Therefore, it is important to reduce the crack width and this can be achieved by adding polypropylene fibers to concrete [13]. The bridging of cracks by the addition of PP fibers has been shown in Fig 3.

Thus addition of fibers in cement concrete matrix bridges these cracks and restrains them from further opening. In order to achieve more deflection in the beam, additional forces and energies are required to pull out or fracture the fibres. This process, apart from preserving the integrity of concrete, improves the load-carrying capacity of structural member beyond cracking. This improvement creates a long post-peak descending portion in the load deflection curve as shown in Fig 4 [12]. Reinforcing steel bars in concrete have the same beneficial effect because they act as long continuous fibres. Short discontinuous fibres have the advantage, however, of being uniformly mixed and dispersed throughout the concrete.

The major reasons for crack formation are Plastic shrinkage, Plastic settlement, Freeze thaw damage, Fire damage etc.

Plastic shrinkage: It occurs when surface water evaporates before the bleed water reaches the surface. Polypropylene fibers reduce the plastic shrinkage crack area due to their flexibility and ability to conform to form. The addition of 0.1% by volume of fibers is found effective in reducing the extent of cracking by a factor of 5-10. The extent of crack reduction is proportional to the fiber content in the concrete.

Table 1: Properties of various types of polypropylene fibers
Fiber type Length (mm) Diameter (mm) Tensile strength (MPa) Modulus of elasticity (GPa) Specific surface (m2/kg) Density (kg/cm3)
monofilament 30-50 0.30-0.35 547-658 3.50-7.50 91 0.9
microfilament 12-20 0.05-0.20 330-414 3.70-5.50 225 0.91
Fibrillated 19-40 0.20-0.30 500-750 5.00-10.00 58 0.95

Polypropylene Fiber Reinforced Concrete
Figure 3: Bridging of crack using Polypropylene fibers Figure 4: Typical load-elongation response in tension of FRC.

Plastic settlement: High rate of bleeding and settlement combined with restraint to settlement (e.g. by reinforcing bars) leads to settlement cracking. In case of PFRC, fibers are uniformly distributed. Fibers are flexible so they cause negligible restraint to settlement of aggregates.

Freeze thaw damage: Small addition of polypropylene fibers in concrete reduces the flow of water through the concrete matrix by preventing the transmission of water through the normal modes of ingress, e.g. capillaries, pore structure, etc. The implications of these qualities in concrete with polypropylene fiber additions are that cement hydration will be improved, separation of aggregate will be reduced and the flow of water through concrete that causes deterioration from freeze/ thaw action and rebar corrosion will be reduced, creating an environment in which enhanced durability may take place.

Polypropylene Fiber Reinforced Concrete
Spalling of homogenous structure of Concrete due to insufficient capillary pores Developed explosion channels due to melting of PP fibers
Figure 5: Flowing out of steam pressure through the melted PP fibers in the case of fire

Fire damage: Heat penetrates the concrete resulting in desorption of moisture in outer layer. Moisture vapors flow back towards the cold interior and are reabsorbed into voids. Water and vapor accumulate in the interior thereby increasing the vapor pressure rapidly causing cracks and spalling in the concrete. In case of PFRC, the fibers melt at 160oC creating voids in the concrete. The vapor pressure is released in newly formed voids and explosive spalling is significantly reduced as shown in fig 5[14].

Properties of PP Fiber Reinforced Concrete

Before mixing the concrete, the fiber length, amount and design mix variables are adjusted to prevent the fibers from balling. Good FRC mixes usually contain a high mortar volume as compared to conventional concrete mixes. The aspect ratio for the fibers are usually restricted between 100 and 200 since fibers which are too long tend to "ball" in the mix and create workability problems. As a rule, fibers are generally randomly distributed in the concrete; however, placing of concrete should be in such a manner that the fibers become aligned in the direction of applied stress which will result in even greater tensile and flexural strengths. There should be sufficient compaction so that the fresh concrete flows satisfactorily and the PP fibers are uniformly dispersed in the mixture. The fibers should not float to the surface nor sink to the bottom in the fresh concrete. Chemical admixtures are added to fiber-reinforced concrete mixes primarily to increase the workability of the mix. Air-entraining agents and water-reducing admixtu- res are usually added to mixes with a fine aggregate content of 50% or more. Superplasticizers, when added to fiber-reinforced concrete, can lower water: cement ratios, and improve the strength, volumetric stability and handling characteristics of the wet mix. The properties of PFRC with various fiber volume % are shown in Table 2.

Table 2 Mechanical Properties of Polypropylene Fiber Reinforced Concrete
No Concrete mix Fibers Vf % fcu (MPa) ft (MPa) fs (MPa) Slump (mm) Ref.
w/c Cement (kg/m3) CA (kg/m3) FA (kg/m3) Admixture Specimen shape Type l/d
1. 0.49 390 (OPC) 1000 (10mm) 640 Super plasticizer (Fosroc 430) Cylinder, Cubes & Prism Fibrillated
(20mm long & 0.29mm dia)
69 0 0.10 0.30 17.2 14.1
12.6
1.08 1.72 1.34 4.5
2.5
3.0
100 – 120 [15]
2. 0.45 360 (OPC) 1100 (20mm) 647 - Prism Micro filament
(19mm long & 0.048mm dia)
396 0 0.045 0.082 0.128 - -
-
-
2.24 2.33
2.40
2.43
4.01 3.76 4.01 4.22 - -
-
-
[10]
3. 0.45 360 (OPC) 1100 (20mm) 647 - Prism Mono filament
(30mm long & 0.55 mm dia)
55 1.0 1.2 1.4 - -
-
2.50
2.68
2.70
5.36 5.47 5.51 -
-
-
[10]
4. 0.48 418 (OPC) 724 (25mm) 998 - Cylinder Mono filament 56 0 1.0 1.5 35.03
35.42
30.74
2.23 3.21 3.21 - - - 102
38
7
[16]
5. 0.40 372 OPC + 28 SF 1140
(20mm)
750 Super-plasticizer Prism Mono filament 200 0 0.5 56.10
56.10
4.10 4.40 5.21
5.61
100
80
[29]
6. 0.50 383
(PPC)
1162
(20mm)
572 - Cylinder, Cubes & Prism Graded Fibrillated (12mm ~ 24mm) NR 0 0.1 0.2 0.3 35.23
39.50
41.00
48.00
3.54 4.42 4.88 4.95 5.23
5.47
5.65
6.35
- -
-
-
[25]
7. 0.44 430
(PPC)
1154
(20mm)
540 - Cylinder, Cubes & Prism Graded Fibrillated
12mm ~ 24mm)
NR 0 0.1 0.2 0.3 41.22
49.78
50.22
52.00
3.72 4.53 4.67 4.75 5.35
5.99
6.12
6.29
- -
-
-
[25]
8. 0.39 498
(PPC)
1136
(20mm)
503 NIL Cylinder, Cubes & Prism Graded Fibrillated
12mm ~ 24mm)
NR 0 0.1 0.2 0.3 46.15
50.67
55.33
57.11
3.89 4.88 5.09 5.52 5.56
5.70
6. 40
6.84
- -
-
-
[25]
9. 0.30 567
(OPC)
630 1050 Superplasticizer
(Paric FP300U)
Cylinder Fibrillated
(6 mm long & 0.06 mm dia)
100 0 0.25 0.50 81.60
60.80 60.00
4.40
4.10
4.30
- -
-
400-600 [23]
10. 0.30 567
(OPC)
630 1050 Superplasticizer
(Paric FP300U)
Cylinder Fibrillated(30mm long& 0.06mm di) 500 0.25 0.50 71.90
59.40
5.40
4.70
- - 400-600 [23]
11. 0.36 314OPC+56 Fly ash 1268 (20mm) 713 Super plasticizer Cylinder, Cubes & Prism -
Mono filament
Mesh Type
-
700
150
0 0.10 0.10 38.20
37.60
37.20
-
-
-
4.80
5.10
5.40
73 55
45
[30]
12. 0.40 415 1120
(20mm)
740 - Cubes & Cylinder Fibrillated 126 0
0.1
0.2
0.3
38.0
34.5
42.0
41.4
4.00
4.40
5.00
5.15
- -
-
-
20
20
15
10
[28]
Where: Vf - volume fraction of fiber; fcu - compressive strength; ft - tensile strength and fs - flexural strength, SF- Silica fume

Polypropylene fibers are used in two different ways to reinforce cementitious matrices. One application is in thin sheet components in which polypropylene provides the primary reinforcement. Its volume content is relatively high exceeding 5%, in order to obtain both strengthening and toughening. In other application the volume content of the polypropylene is low, less than 0.3% by volume, and it is intended to act mainly as secondary reinforcement for crack control, but not for structural load bearing applications [11]. The performance and influence of the polypropylene fibers in the fresh and hardened concrete is different and therefore these two topics are treated separately.

Effects on Fresh Concrete
The main parameter, which is often used to determine the workability of fresh concrete, is the slump test. The slump value depends mainly on the water absorption and porosity of the aggregates, water content in the mixture, amount of the aggregate and fine material in the mixture, shape of the aggregates and surface characteristics of the constituents in the mixture. The slump values decrease significantly with the addition of polypropylene fibers as shown in Table 3. The concrete mixture becomes rather clingy resulting in increasing of the adhesion and cohesiveness of fresh concrete. During mixing the movement of aggregates shears the fibrillated fibers apart, so that they open into a network of linked fiber filaments and individual fibers. These fibers anchor mechanically to the cement paste because of their large specific surface area. The concrete mixture with polypropylene fibers results in the fewer rate of bleeding and segregation as compared to plain concrete. This is because the fibers hold the concrete together and thus slow down the settlement of aggregates. Due to its high tensile and pull-out strength, the PP fibers even reduce the early plastic shrinkage cracking by enhancing the tensile capacity of fresh concrete to resist the tensile stresses caused by the typical volume changes. The fibers also distribute these tensile stresses more evenly throughout the concrete. As the plastic shrinkage cracking decreases, the number of cracks in the concrete under loading is reduced, due to decrease in cracks from the existing shrinkage cracks. If shrinkage cracks are still formed, the fibers bridge these cracks, reducing at the same time their length and width. Moreover, as the rate of bleeding decreases, the use of polypropylene fibers may accelerate the time to initial and final set of the concrete as this led to a slower rate of drying in the concrete [14].

Table 3: Effect of polypropylene fibers on concrete slump [18]
(mm)
Initial slump
(mm)
Final slump
(mm)
Fiber length
(mm)
90
130
170
127
1245
114
76
70
120
48
53
64
51
51
30
51
51
19

Effects on Hardened Concrete
The addition of polypropylene fibres in the concrete did not significantly affect the compressive strength and the modulus of elasticity but they do increase the tensile strength. Splitting tensile strength of PFRC approx ranges from 9% to 13% of its compressive strength. Addition of PP fibers in concrete increases the splitting tensile strength by approx 20% to 50% [16].

Compressive strength: The compression strength of concrete is a vital parameter as it decides the other parameters like tension, flexure etc. The effect of polypropylene fiber on the compressive strength of concrete has been discussed in many literatures and observed that polypropylene fiber either decreases or increases the compressive strength of concrete, but overall effect is negligible in many cases. In fact, the effect of a low volume of polypropylene fiber on the compressive strength of concrete may be concealed by the experimental error.

Flexural tensile strength: The flexural tensile strength increases with increase in volume fraction of fiber. It is also observed that there was increase in strength for with the increase in aspect ratio of fibre.

Polypropylene Fiber Reinforced Concrete Polypropylene Fiber Reinforced Concrete
Figure 6: Fracture shape of plain concrete Figure 7: Fracture shape of PFR concrete
Bond strength: It is necessary that there should be a good bond between the fiber and the matrix. If the critical fiber volume for strengthening has been reached then it is possible to achieve multiple cracking. This is a desirable situation because it changes a basically brittle material with a single fracture surface to fracture into a pseudo ductile material which can absorb transient minor overload and shocks with little visible damage. So the aim is to produce large number of multiple cracks at as close spacing as possible so that the crack widths are very small, almost invisible to naked eye so that the rate at which aggressive materials can penetrate the matrix is reduced. High bond strength helps to give close crack spacing but it is also essential that the fibers should give sufficient ductility to absorb impacts. But in terms of physiochemical adhesion there is no bond between the fiber and the cement gel. The use of chopped and twisted fibrillated polypropylene fibers with their open structure has partially remedied the lack of interfacial adhesion by making use of wedge action at the slightly open fiber ends and also by mechanical bonding through fibrillation. The general pull out loads of twisted fibrillated fibers [20, 21] may range from 300-500N for commonly used staples but the accurate calculation of bond strength is complicated by a lack of knowledge of the surface area of fiber in contact with the paste. It is observed that in damaged products and in broken specimens, usually fiber breaks instead of fiber pull out [9].

Fracture Properties: The failure behaviour of high-strength concretes is effectively improved by the use of fibers. The typical shear bond rupture due to strain localization could be avoided (fig. 6). Instead of this, a large number of the longitudinal cracking, which was predominantly oriented in the direction parallel or sub-parallel to the external compressive stresses, was formed at the entire concrete specimens as shown in fig7.

Creep and shrinkage properties of concrete: Fibers reduce creep strain, which is defined as the time-dependent deformation of concrete under a constant stress. Compressive creep values, however, may be only 10 to 20% of those for normal concrete. Shrinkage of concrete, which is caused by the withdrawal of water from concrete during drying, is also reduced by fibers. The shrinkage, creep and total time dependent deformation of various PFRC mixes along with non fibrous concrete mix are presented in fig 8[15]. The reduction in shrinkage due to the presence of fibers is expected from number of viewpoints. First, the fibers do not exhibit any shrinkage, thus reducing overall shrinkage of the mix. In addition the fibers have a role in retaining the water in the concrete mix upto a certain limit which helps to delay the shrinkage. Therefore addition of fibers to the concrete mixes is always advantageous in reducing shrinkage deformation.

Polypropylene Fiber Reinforced Concrete Polypropylene Fiber Reinforced Concrete
Figure 8: Time dependent deformation of polypropylene fibers Figure10: Effect of polypropylene fibers on impact resistance of concrete

Flexural impact properties: The number of blows required to develop the first visible crack on the beam’s lower surface is defined as the initial-crack impact number (Ncr). Failure impact number Nf is defined as the number at which one main macro-crack develops from bottom to top of the beam. Impact ductility index is defined as the ratio of failure impact number to initial crack impact number, which can be used to present the flexural impact ductility.

J=Nf / Ncr

where J is impact ductility index, which for plain concrete is 1. The flexural impact test results are shown in table 6 by researcher[10]. The impact resistance for concretes with various volume fractions of fibrillated polypropylene fibers has been shown in figure 10. The results indicate that significant improvement in impact resistance of concrete can be achieved with relatively low volume fraction of polypropylene fibers.

Table 6: Impact properties of fiber reinforced concrete
Type of mix Vf % Average Impact number Average failure
Impact number
Impact ductility index
Control 0 25.8 26.8 1.04
Microfilament 0.05
0.095
0.14
34.7
28.6
38.1
46.5
30.4
40.1
1.34
1.06
1.05
Monofilament 1
1.2
1.4
68.9
70.7
62.8
224.2
712.7
831
3.26
10.08
13.23

Chloride penetration: Besides improved mechanical properties due to inclusion of fiber, chloride penetration is also reduced substantially by the presence of fibers depending upon its orientation. Antoni [17] studied the effect of chloride penetration and found that the effect is insignificant for shorter fiber due to the random orientation of short fibers as compare to long fibers. Further, the chloride movement into concrete is reduced significantly by the presence of fiber as the interfacial transition zone in the direction perpendicular to the chloride penetration whereas fiber provides easier path for the chloride to migrate in direction along the fiber.

Obstacles in Use of PFRC

Although PP fibers are gaining wide applications in many fields, there is still need for improvement in some properties. A major fire will leave the concrete with additional porosity equal to the volume of fibers incorporated in the concrete usually in the order of 0.3 to 1.5% by volumetric fraction. In respect of monofilament fibers, the poor bond between fiber and matrix results in a low pull out strength. The PP fibers are also attacked by sunlight and oxygen, however surrounding concrete in PFRC protects the fibers so well that this shortcoming is not significant. Further, sometimes the fibers function as initiator of the micro cracking because of their low modulus of elasticity as compared to the cement matrix. Thus mechanical bond with the cement matrix is also low. The fibers cause the enhancement of the pores volume of concrete by creating more micro-defects in the cement matrix.

Conclusion

Innovations in engineering design and construction, which often call for new building materials, have made polypropylene fiber-reinforced concrete applications. In the past several years, an increasing number of constructions have been taken place with concrete containing polypropylene fibres such as foundation piles, prestressed piles, piers, highways, industrial floors, bridge decks, facing panels, flotation units for walkways, heavyweight coatings for underwater pipe etc. This has also been used for controlling shrinkage & temperature cracking.

Due to enhance performances and effective cost-benefit ratio, the use of polypropylene fibers is often recommended for concrete structures recently. PFRC is easy to place, compact, finish, pump and it reduces the rebound effect in sprayed concrete applications by increasing cohesiveness of wet concrete. Being wholly synthetic there is no corrosion risk. PFRC shows improved impact resistance as compared to conventionally reinforced brittle concrete. The use of PFRC provides a safer working environment and improves abrasion resistance in concrete floors by controlling the bleeding while the concrete is in plastic stage. The possibility of increased tensile strength and impact resistance offers potential reductions in the weight and thickness of structural components and should also reduce the damage resulting from shipping and handling.

Acknowledgment
The author wishes to express his sincere thanks to Ms Sonal Dhanvijay & Ms Vedanti Ganwir of Visvesvaraya National Institute of Technology, Nagpur for their valuable help in preparing this paper.

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NBMCW December 2011

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