Pro­tec­tive Coat­ings for RC Struc­tures

NBM&CW - - PROTECTIVE COATINGS -

Di­rec­tor, Tal­rak Con­struc­tion Chem­i­cals Pvt. Ltd., Ban­ga­lore

In­tro­duc­tion

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.

De­te­ri­o­ra­tion Mech­a­nism

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.

Pro­tec­tive Coat­ings

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

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