Influence of cement type and curing on the drying and air permeability of cover concrete

Abstract

Experimental results are presented showing the influence of cement type and curing on the relative humidity, initial weight loss and air permeation in cover concrete that has been exposed to drying at 60% relative humidity for at least 18 months. Three curing periods (1,3 and 28 days) and 17 cements were used, the cements differing mainly with respect to their contents of limestone or ground granulated blastfurnace slag (ggbs). The concretes were all made with a water-cement ratio of 0·59 and the 28 day cube strengths ranged from 26 to 46 MPa. The relative humidity at a mean depth of 25 mm below the exposed sudace reduced to about 80% and 70% after 6 and 18 months of drying respectively. Cement type had little on the reduction of relative humidity with drying time and there was only a marginal retardation of drying rate with increased curing time. The initial four day weight loss, an indicator of moisture transport properties in cover concrete, reduced markedly as the curing period was increased. Increasing the ggbs content of the cement increased the initial four day weight loss with one day curing but reduced it with 28 day curing. The initial four day weight loss generally reduced as the strength at the end of the curing period was increased. The air permeability of cover concrete generally increased with drying time, even beyond 18 months; this effect was more pronounced with concretes that had lower pemzeabilities and appeared to be due to slow drying. On average the air permeability of cover concrete increased significantly with increasing drying time and decreasing curing period, but changes of greater magnitude could arise due to cement type. The compressive strength after curing, the initial four day weight loss and the 18 month air permeability were only broadly related. Concretes with 50% or more of ggbs in the cement tended to have a high air permeability for a given strength eight days after the end of curing or for a given initial four day weight loss.