气压环境对不同引气剂性能及引气混凝土气孔结构的影响

为研究不同气压环境下引气剂的引气行为特征及其对引气混凝土气孔分布特征的影响，该文通过引气水泥稀浆溶液摇泡试验及硬化混凝土气孔结构分析，获得不同气压环境对烷基磺酸盐类、皂甙类及聚醚类3种类型引气剂的起泡能力、气泡衰减行为及混凝土气孔分布特征的影响规律。结果表明，气压降低导致引气剂引气能力下降，相较于常压（101 kPa），低气压（64 kPa）时3种引气剂引气能力分别下降30.1%（磺酸盐类）、28.1%（皂苷类）及22.0%（聚醚类）。同时，气压降低导致引气气泡寿命缩短，气泡稳定性下降，引气结束15 min时，低压64 kPa下各引气剂的留存气泡体积占比分别为10%（磺酸盐）、17%（皂苷类）及29%（聚醚类），而常压101 kPa时该值分别为25%（磺酸盐）、38%（皂苷类）及49%（聚醚类）。在混凝土硬化过程中，因低气压引气混凝土气泡稳定性较差而导致其含气量损失约0.6%~1.2%，而常压下该值为0.3%~0.5%。低压引气混凝土气孔结构参数要劣于常压引气混凝土，具体表现为单位体积气泡数量减少，气泡间距系数增大且气泡比表面积减小。因此，建议在高原地区采取提高抗冻混凝土含气量设计值、优选并研发高原型引气剂及避免对引气混凝土过分扰动等技术措施，以确保高原地区引气混凝土达到抗冻耐久性所需的含气量值。

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.