Experimental investigations on the influence of cover depth and concrete quality on time to cover cracking due to carbonation-induced corrosion of steel in RC structures in an urban, inland environment

This paper reports on a study that was aimed at developing a coherent empirical relationship for the time to cracking of the cover concrete in a reinforced concrete structure, subjected to reinforcing steel corrosion that is induced by carbonation in an inland, urban environment in South Africa. In particular, the study considered the influence of cover depth and the quality of concrete in the cover zone on the extent of corrosion at the time of cover concrete cracking. The results of the study were then used to develop an empirical relationship to estimate the time to cover cracking and the associated extent of reinforcing steel corrosion. Test concretes were prepared using three binder types: plain Portland cement (PC – CEM I 52.5N), 70/30 PC/FA (fly ash) and 50/50 PC/GGBS (ground granulated blastfurnace slag) at relatively high w/b ratios of 0.60 and 0.95. 13.2 mm crushed granite stone and granite crusher sand were used as aggregates. RC prism specimens were prepared using 20 mm diameter high yield deformed steel bars placed at cover depths of 12 mm, 20 mm or 30 mm. Sufficient 100 mm companion concrete cube specimens were prepared for permeability, porosity tests and carbonation depth measurement. The results indicate that, as the cover depth increases, a higher extent of corrosion is required to initiate cracking at the concrete surface. Also, at the same cover depth, the higher w/b ratio concrete, which was more porous, more permeable and had lower tensile strengths, required more steel corrosion to initiate surface cracking. Blended cement concretes required a higher extent of steel corrosion to initiate cover cracking than the PC concretes.