In this paper corrosion rate of steel reinforcements after carbonation of concrete was studied using AC impedance technique and weight loss measurements taking the effects of cement type, water to binder ratio, cover thickness,relative humidity and the like into account. Based on the study a corrosion rate eguation that includes the above factors and the relative humidity of 40 to 95% as variables was proposed by modifying Stern-Geary eguation. Ways of applying the eguation to the inside and outside environments of buildings were discussed, which resulted in a proposal of estimated corrsion rates after carbonation.
The critical amount of corrosion for initiating cracking along reinforcements was studied by wet and dry exposure tests and by RBSM analysis. The experiments showed that the critical amount of corrosion was in the range of 10 to 100 mg/cm 2 and was influenced by water-binder ratio of concrete, cover thickness and bar diameter. The results of the RBSM analysis showed similar tendencies with the experimental ones, when tension softening of concrete and crack development were taken into account. A modifying coefficient, however, was necessary for the critical values obtained by the two methods to coincide. Based on the results, some equations to express the critical amount of corrosion were proposed, taking water-binder ratio, cover thickness and bar diameter into account.
The carbonation rate coefficient of the medium heat Portland cement with fly ash was studied in both accelerated and natural exposure test conditions. The coefficient of the cement was higher than that of ordinary Portland cement (N cement) by 1.5 to 2.0 times under accelerated test conditions when water to binder ratio was 60%, but it was nearly the same with that of N cement under out door exposure test conditions. The cement factor (α2) of the cement was calculated to be 1.1 and the environment related factor (β3) was to be 3.5 according to the carbonation rate coefficient equations by the AIJ durability design recommendation.
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