Influence of atmospheric pressure on performance of air entraining agent and air void structures of air-entrained concrete

In In order to investigate the air-entraining behavior of air-entraining agents under different atmospheric pressure environment and the influence of atmospheric pressure on the air void structures of air-entrained concrete, the foaming ability of three kinds of air-entraining agents (Alkyl sulfonate, Saponin and Polyether), the attenuation law of bubble volume changing with time and the air void structures of air-entrained hardened concrete mixed under different atmospheric pressure environment were obtained by the air-entraining cement slurry bubble test and the air void structures analysis of hardened concrete respectively. The results indicated that the low atmospheric pressure environment weakened the initial air-entraining ability of air-entraining agents. Compared with the normal atmospheric pressure of 101 kPa, the air entraining capacity of the three types of air-entraining agents decreased by 30.1% (Alkyl sulfonate), 28.1% (Saponin) and 22.0% (Polyether) respectively, when the atmospheric pressure was 64 kPa. The life cycle of the bubble produced under low atmospheric pressure environment was obviously shorter than that under normal atmospheric pressure environment. After the air-entraining cement slurry bubble test were finished 15 min, the volume of retained air bubbles for each air-entraining agent at the low atmospheric pressure of 64 kPa were 10% (Alkyl sulfonate) and 17% (Saponin) and 29% (Polyether) respectively. And these values were 25% (Alkyl sulfonate), 38% (Saponin) and 49% (Polyether) when the atmospheric pressure was 101 kPa. These results indicated that the air-entraining ability of Polyether and Saponin were less influenced by the drop of atmospheric pressure and the Alkyl sulfonate was the most. Comparing the air content between the fresh concrete and the hardened concrete, it was found that during the hardening of concrete, the loss of air content of concrete produced under low atmospheric pressure environment was 0.6%-1.2% while the value was 0.3%-0.5% for the concrete produced under normal atmospheric pressure environment. The results also illustrated that the bubble stability under low atmospheric pressure environment was weakened which resulted in a large loss of air content. When the amount of air-entraining agent was constant, the number of bubbles of air-entrained concrete mixed under low atmospheric pressure environment was significantly reduced due to the weakened air-entraining ability and the spacing factor was also increased. In addition, the percentage of large air bubble in the air-entrained concrete casted under low atmospheric pressure environment was increased, which resulted in the specific surface area of air bubbles, and it was significantly smaller than that of the air-entrained concrete casted under normal atmospheric pressure environment. When the air content of air-entrained concrete mixed under low and normal atmospheric pressure environment respectively were close through adjusting the amounts of air-entraining agents, the results of air void structures analysis of hardened concrete also showed that the spacing factor of air void was much larger, the number of bubbles in unit volume of concrete was less, and the specific surface area of bubble was larger in the air-entrained concrete mixed under low atmospheric pressure environment. Therefore, in order to meet the design requirements of air content of air-entrained concrete for different constructions in the plateau regions, the following measures are recommended as follows: 1) The air-entraining admixture with good air entraining ability and foam stabilization performance should be chosen for the plateau environment; 2) During the construction in the plateau regions, the vibration time should be reduced with the precondition of the impaction of concrete if possible to avoid air content loss.