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