Dr. V.V.L. Kanta Rao, Principal Scientist, Bridges and Structures Division, CSIR-Central Road Research Institute, New Delhi
Deformed carbon steel reinforcement bar is most vulnerable to corrosion. Corrosion of reinforcement results in cracking of concrete and loss of structural capacity. The corrosion of steel reinforcement in concrete structures, including bridges, is caused due to chlorides and atmospheric carbon dioxide. The concretes with high permeability allow easy passage of the chlorides and carbon dioxide into it to reach steel reinforcement. Various methods are available to prevent/delay the on-set of corrosion of reinforcement. This paper discusses the common methods adopted for prevention of corrosion during construction.
IntroductionCement concrete is a versatile building material and is the second most consumed material in the world, after water. It is made using porland cement, coarse, and fine aggregates, water and one or more admixtures. Concrete is well known for the ease with which it is made, for its ability to be moulded in any form, and its high strength. The microstructure of concrete is such that it has capillary pores to the extent of 28%, which depends upon quality of concrete and the presence of water at the time of mixing of concrete. Concrete made with low w/c ratio is dense with lower amount of pores. These pores are created due to evaporation of excess free water during strengthening of concrete mass, and a concrete made with high w/c ratio will have pores inter connected and extend deep inside the mass of concrete from its surface. More are the inter connected pores, greater is the ease with which water can flow through the concrete (i.e. high permeability).
The durability of concrete is defined as its ability to perform satisfactorily in the given exposure conditions, and it is influenced by the pore structure of the concrete. Inadequate durability has been the cause of premature failures of many concrete structures. The concrete undergoes deterioration due to corrosion of steel reinforcement in concrete, sulphate attack, freeze-thaw attack, alkali-silica reaction, etc. Among them, the corrosion of steel in concrete has been identified as most severe types of attack responsible for deterioration of concrete. The corrosion of steel results in increase in its volume by up to 5-7 times and the resulting expansion of steel exerts tensile stresses in concrete upto about 70 MPa (10000 psi)  which is much less than the tensile strength of concrete. These disruptive forces cause disintegration of concrete through development of cracks, delamination, and cracking leading to loss of integrity of concrete, reduction in load bearing capacity and service life of the bridges.
The deformed carbon steel is most vulnerable to corrosion compared to plain carbon steel. Before 1970's, plain carbon steel reinforcement bars were used in bridge structures. The use of deformed carbon steel bars have come into use since mid-70's. It is not the intention of the author to recommend the use of plain carbon steel bars as reinforcement, but only to highlight the vulnerability of deformed carbon steel bars to corrosion. A concrete bridge built across River Damodar during mid 60's, using plain carbon steel bars as reinforcement, was tested recently by CSIR-CRRI. No distress exhibited as cracks or spalling was noticed on the bridge due to corrosion. Depth of carbonation (i.e. attack of carbon dioxode) was not significant.
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