The Corrosion of Steel Reinforcement in Carbonated Concrete Under Different Humidity Regimes
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ABSTRACT In this investigation, four concretes (two mix designs and two curing regimes), containing reinforcing bars at covers 11 mm and 20 mm were carbonated fully in 100% CO 2 and 65% relative humidity (RH). They were then placed in two different humidity environments (65% RH and 90% RH). The progress of the corrosion of the samples was followed electrochemically (using the linear polarisation technique) and microscopically (using a scanning electron microscope). Electrochemical monitoring of the corrosion rate showed that corrosion started very soon after placement in the humidity regimes and generally increased with time. The water/cement ratio, the concrete cover of the reinforcement and the curing of the concrete had a significant effect the length of time taken to carbonate the specimens. However, the effect of these variables on the corrosion rate was negligible. In addition, the corrosion rate showed no dependance on the humidity regimes. Microstructural examination of the samples revealed the existence of corrosion products at the steel/concrete interface after 6 months in the humidity regimes. There was more corrosion product in the samples at 90% RH. The samples with the low w/c showed more extensive cracking in the interfacial region. INTRODUCTION With regard to reinforcement corrosion, carbonation of concrete is not generally considered to be as damaging as chloride contamination. However, although the effects of carbonation may take longer to develop, once the carbonation front has reached the steel reinforcement corrosion will proceed if the humidity is high enough. The corrosion of the steel reinforcement results in the cracking and eventual spalling of the cover due to the internal stresses created as a result of the increase in volume associated with the transformation of steel to rust [1]. In carbonated concrete, the corrosion process is thought to be generalised and homogeneous producing, over the long term, a reduction in the cross-sectional area of the steel and a significant amount of oxides which may crack the cover or diffuse through the pores to the surface of the concrete. However, the relationship between the cracking of the concrete cover, the spatial distribution of corrosion and the deposition of the corrosion products are not well understood. Good quality concrete will carbonate more slowly and will have a lower permeability once carbonated. However, once corrosion is initiated, the effect on the corrosion rate is not important. The major factors affecting the quality of the concrete are the w/c ratio, the degree of curing and the reinforcement cover. In this investigation, concrete samples were prepared with two different w/c ratios and cured under two different regimes. After carbonation they were placed in two different humidity 471
Mat. Res. Soc. Symp. Proc. Vol. 370 0 1995 Materials Research Society
followed electrochemically (using the linear polarisation technique) and microscopically (using a scanning electron microscope).
EXPERIMENTAL The samples were cast with w/c rati
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