Effect of entrained air voids on the microstructure and mass transport properties of concrete

The effects of entrained air on microstructure and transport properties of concrete with up to 11.5% air at different w/c ratios, curing and conditioning regimes were investigated. It was found that air voids disrupt the packing of cement and increase the heterogeneity of the microstructure. The width of the affected interface is around 30 μm. Gaseous diffusivity and permeability are increased by up to a factor of 2–3 at the highest air contents. This effect is similar to that due to increasing w/c ratio from 0.35 to 0.50 when samples are conditioned at 52% r.h or 50 °C. The effect on sorptivity is less consistent, while the effect on electrical conductivity is influenced by the moisture condition of the air voids. It is estimated that every 1% increase in air content increases transport by 10% or decreases it by 4%, depending on whether the air voids act as conductors or insulators.