Effects of micro-environmental climate on the carbonation depth and the pH value in fly ash concrete

Whereas carbonation is a major reason of steel corrosion in atmospheric environment, it is important to understand the depth of carbonation front and its rate of propagation towards the embedded reinforcement steel. In this study, for ordinary concrete and for fly ash concrete with 15%, 30% and 45% fly ash replacement ratios by mass, the depth of fully carbonated zone and the pH value were measured according to a rapid carbonation test of concrete under artificial climatic conditions. By considering pozzolanic reactions of fly ash and carbonation reactions in concrete, the effects of fly ash content and micro-environmental climates (temperature and relative humidity) were analyzed in terms of the depth of fully carbonated zone and the thickness of partially carbonated zone. Results show that the depth of carbonation increased gradually with the age of carbonation but at a decreasing carbonation rate. With an increase in fly ash replacement, the depth of fully carbonated zone and the thickness of partially carbonated zone both increased. Especially for fly ash concrete incorporating 45% fly ash (FA45), the depth of fully carbonated zone and the thickness of partially carbonated zone were approximately two to three times of those of the control mixture (FA0). Micro-environment temperature had little effect on the thickness of partially carbonated zone, however, the higher the micro-environment temperature, the deeper was the fully carbonated zone. The depth of fully carbonated zone and the thickness of partially carbonated zone decreased with an increase in the relative humidity of the micro-environment.