Effect of Cement Type and Micro-cracks on Chloride Penetration in Concrete

Abstract

Chloride-induced corrosion of reinforcing steel rebar is one of the major durability issues affecting reinforced concrete structures (RC). Various models have been developed to assess the durability design with regard to corrosion prevention in chloride-bearing environments, which refer mainly to uncracked concrete. Cracks, however, are frequently present in real RC structures and accelerate corrosion initiation of steel reinforcement, resulting in a shortened service life. This paper presents preliminary results concerning chloride-penetration resistance of different types of uncracked and cracked concrete, made with Ordinary Portland (OPC), Portland-Limestone (PLC) and Pozzolanic cement (PC) and a water/cement ratio of 0.45. Load-induced micro-cracks were obtained with a specifically developed technique. Cracked and uncracked concrete specimens were immersed in a sodium chloride solution for 32 and 90 days. The chloride penetration front was detected on split surfaces, perpendicular to the exposed surface, with a colorimetric technique to evaluate the combined effect of cracks and cement type on the chloride-penetration resistance of different concretes. Results showed that in uncracked conditions cement type strongly affected chloride penetration depth in concrete. A mathematical model was applied to evaluate chloride diffusion coefficient (D) from chloride penetration depth measurements and exposure time. The lowest value of D was found for PC concrete, that can be attributed to a higher pore refinement in the cement paste, five times higher for OPC and nine times higher for PLC. In cracked conditions, an additional penetration of chlorides occurred in correspondence of crack, even for micro-cracks 10-50 μm wide and 8-30 mm deep, leading to a decrease in chloride penetration resistance. The model however did not provide for an estimation of D in correspondence of the crack.

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