Effects of Carbonation on Corrosion Rate of Reinforcing Steel in Different Concrete and Repair Materials

Abstract

Reinforced concrete with different concrete mix proportions, i.e. binder types or w/b ratio, would provide different quality to protect the reinforcing steel from corrosion. When carbonation occurred, corrosion of steel embedded in concrete can be initiated. This paper reports effects of carbonation on electrochemical properties of steel embedded in concrete with different mix proportions as w/b ratio of 0.4 and 0.6, fly ash added up to 30% by weight of binder, and also in six repair materials. All samples was exposed to accelerated carbonation (4% CO₂, 50 ± 5% relative humidity (RH), and 40 ⁰C temperature) and laboratory environment (0.04% CO₂, 75 ± 5% RH, and 28 ⁰C). The electrical resistivity was monitored by using four-point Wenner probe. The Linear polarization resistance (LPR) was used to characterize the corrosion rate of embedded steel at different exposure time. The carbonation depth of specimens was also tested by using the phenolphthalein indicator. The void contents of repair material specimens were also determined. The results showed that the electrical resistivity of concretes and repair materials increased along with an increase of carbonation depth. However, in case of fly ash concrete, the electrical resistivity decreased at longer exposure period in accelerated carbonation due to decomposition of C-S-H by carbonation. It was also found that the corrosion rate of steel embedded in concrete and repair materials increased as an increase of carbonation depth, even the carbonation depth was less than the covering depth. Steel embedded in ordinary Portland cement (OPC) concrete or low w/b concrete shows lower corrosion rate due to higher pH of concrete. The guideline for evaluation of corrosion initiation and severity due to carbonation is also proposed.