Protective Coatings for RC Structures
Director, Talrak Construction Chemicals Pvt. Ltd., Bangalore
Reinforced concrete is a composite material. Its structural performance is realised only when concrete and steel act in unison during the service life of the structure. The compressive and tensile loads are carried by concrete and steel respectively. Steel protects concrete from cracking under tensile loads and concrete protects steel from corrosion by providing an alkaline environment around it. So long as this happens, the reinforced concrete structures perform satisfactorily. Even though the concrete can be a very strong material, it is also subject to deterioration. Concrete can be porous so that chemicals can penetrate the pores and attack the paste. The paste and aggregate can also be worn down by physical impact and abrasion. Water can penetrate concrete, freeze and expand inside it when the temperature drops, and ultimately weaken the concrete from within. In addition, if the concrete has reinforcing steel bar (rebar) to impart additional strength and other properties, the rebar can corrode if moisture, oxygen and chloride ions penetrate the concrete. Corrosion or rebar contributes to the deterioration of concrete.
Various external hostile environmental substances, such as, water, carbon dioxide, oxygen, chlorides, sulphides and biological organisms are transported from the atmosphere into the concrete and attack steel and concrete in different mechanisms causing premature deterioration of reinforced concrete, challenging its durability and resulting in premature failure of the structures. The entry of the harmful agents can be restricted and or avoided by providing the barrier surface coatings on the concrete.Thus conservation of structural integrity can be achieved during its service life.
There are various mechanisms of concrete deterioration. But the present discussion on the topic of protective coating focuses only on the factors influenced by the atmospheric exposure of reinforced concrete. Among them, the very common ones are carbonation, chloride attack and attack by sulphides.
Protection of concrete should actually begin at the conceptual stage and meticulous strategies be adopted for protecting the concrete from both internal and external environments. Various coating materials and application methods for concrete surface repair and strengthening have been developed. However, selection criterion for these materials has not been established yet. Selection procedures of concrete coating materials must focus on deteriorating mechanisms diagnosed carefully by the conditions of the target structures. For instance, in case of salt damage, repair policy should consider corrosion environment and deteriorating condition to determine symptomatic indications such as (1) removal of permeated chloride ions (2) penetration block of chloride ions moisture, and oxygen (3) derusting of rebar (4) corrosion-control
Corrosion of steel reinforcement in concrete