Influencing factors of parameters of air-void structure in fresh cement mortars

Abstract

There is a limitation to apply air content in fresh concrete as the only index to pre-control frost resistance of cast-in-place concrete. By using an air void analyzer (AVA), the research investigated the influence laws of factors including water-cement ratio, addition amount of fly ash, vibration time, and time-dependent behaviors on a series of characteristics of fresh cement mortars. These characteristics include content, spacing factor, specific surface area (SSA), and size of air voids. The results demonstrated that when the air content was in the range of 7≈8%, the water-cement ratio insignificantly influenced the air-void structure. As fly ash can adsorb the air void, so its addition substantially reduced the content of large air voids and make the size distribution of voids more uniform. Compared with poker vibration, high-frequency vibration exhibited better effects in more uniformly distributing the air voids in the mixture. As large air voids escaped and split, the air content decreased, which was in favor of reducing the air-void size and the air-void spacing factor; Air-void structure in the mixture underwent complex change over time. On the whole, the air-void structure showed the following change trend: small air voids combined to large ones, air voids escaped, the number of air voids and the air-void SSA declined, and the air-void spacing factor enlarged. Therefore, while preparing concrete in practical engineering, it is particularly necessary to consider the adsorption effect of admixture, the time-delay effect of air-entraining agents, and the time-dependent behaviors of air voids, so as to control air-void parameters of hardened concrete.