Effect of coarse aggregate and water/cement ratio on intrinsic permeability of concrete subject to drying

Intrinsic permeability of two types of concrete to nitrogen gas was tested; each had water/cement ratios of 0.40 and 0.50 and a volume fraction of coarse aggregate of 0.41 ± 0.01. Two coarse aggregates of 10mm single size were used, one a crushed limestone of low porosity, the other a sintered fly ash of high porosity. Test specimens (100mm diameter by 50mm thick) were water cured, then dried, during which weight loss and permeability were measured. All concretes had low permeability but consistently higher weight loss and permeability were found in the lightweight aggregate concrete specimens, indicating that accessible pores became available fairly quickly, despite the good quality of mortar used. Such behaviour may result from too rapid drying of the lightweight concrete test specimens. For a more realistic indication of permeability it is recommended that testing should be done in a laboratory environment that would, ideally, simulate insitu conditions. For similar 28 day strength both types of concrete had comparable permeability.