Designing Reinforced Concrete Structures for Long Life Span

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

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

Introduction

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

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

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

Mechanism for Enhancing Durability

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

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

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

Highly Durable Concrete Structures

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

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

Conclusion

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

Acknowledgment

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

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NBMCW December 2007