CHLORIDE PENETRATION RESISTANCE OF CONCRETE

Abstract

Use of fly ash is beneficial in reinforced and mass concrete for reducing the heat of hydration and improving the pore structure. Its use is especially beneficial from the durability point of view when concrete is exposed to severe exposure condition such as seawater and ground water containing salts. Concrete structures exposed to such environment are attacked by the chloride and sulfate ions. It has been shown that due to chloride attack, the reinforcing bars in the structure become vulnerable to corrosion. This work is focused on the improvement in resistance to chloride penetration due to incorporation of fly ash in concrete. The compressive strength development in concretes containing fly ash has also been studied up to a cured age of 120 days for different fly ash replacement levels. The studies have been conducted on sixteen mixes of similar workability with four water binder ratios of 0.35, 0.4, 0.45 and 0.5 and three replacements of OPC by fly ash of 50, 60 and 70% along with control OPC concrete without fly ash. The chloride penetration studies have been carried out using the conventional immersion test for exposure ages of 6, 12 and 17 weeks. The specimens were exposed to 3.5 and 7.0% sodium chloride solutions by continuous immersion. The results of the study show that though there is slow initial strength development in fly ash concretes, they continue to gain strength till a later age. There is also great improvement in the resistance to chloride penetration due to incorporation of fly Ash, which increases with increase in the replacement level. Improvement in resistance to chloride penetration is obtained by pore refinement, even if the compressive strength is adversely affected at larger levels of replacement. Though at lower ages the results are inconclusive the difference in resistance to chloride penetration is considerably high at higher exposure durations. Key words: Reinforced concrete; fly ash; compressive strength; bath curing; corrosion; diffusion; diffusion coefficient; pseudo equilibrium surface concentration. iii