Reduction of concrete sorptivity with age through carbonation

Ordinary Portland cement (OPC) concrete specimens that had been air-dried for 3.5 years were found to have an increase in weight and reduction in sorptivity, with weight and sorptivity changes being greater for specimens with higher original sorptivity. Since this was attributed to carbonation of the surface zone, the porosity differences between end slices and the specimen interior in cylindrical specimens from another test series (of 4 years of age) were measured, together with the depths of carbonation. It was found from sensitivity analyses performed on a neural network model that the porosity difference can be attributed, in increasing order of importance, to (i) depth of carbonation, (ii) original sorptivity (which reflects both the quality of the mix and the efficiency of curing), and (iii) whether the end slice was a top or bottom one (reflecting the direction of casting and compaction). Tests on existing structures also showed that sorptivity decreased with age and also that (carbonated) surface sorptivities were lower than interior sorptivities, further confirming that the reduction of sorptivity with age is due to surface carbonation and that this carbonation occurs in and benefits surfaces with poorer initial quality more. This implies that carbonation could have mutually compensatory effects on some aspects of concrete durability.