Characterisation of air-void systems in concrete

The air-void system in both fresh and hardened states for different concrete mixes was measured and characterised using the methods presented in ASTM C231 and ASTM C457 respectively. The mix variables included two water/binder (w/b) ratios (0·33 and 0·45), one replacement level of Portland cement with slag cement (50%) and a range of air contents achieved through the use of different air-entraining agents. Consistent results were obtained on duplicate specimens using the two testing procedures. The cumulative size distribution of the air voids estimated from chord length measurement was satisfactorily represented by an S-shaped function from which the differential distribution can be derived. The importance of air-void fineness on the spacing was clearly illustrated by the small variation in Powers' spacing factor when large air bubbles were included.