高原低气压环境对引气混凝土含气量及气泡稳定性的影响

该文利用低气压试验箱模拟高原气压环境,试验研究了不同配合比及初始含气量水平下环境气压的降低对引气混凝土含气量及其气泡稳定性的影响。结果表明:与常压相比,环境气压的降低能够显著削弱引气剂的引气能力,当混凝土配合比及引气剂掺量一定时,混凝土含气量随环境气压降低呈线性减少,当环境气压降低至50 k Pa时,混凝土含气量降低约20%~49%。另外,低气压条件下混凝土气泡稳定性变差,具体表现为混凝土含气量经时损失变大,延长振捣时间导致低压混凝土损失更多气泡,二者均使低压下硬化混凝土的气泡间距系数增大,影响混凝土抗冻性。因此,在高原地区应通过采取优选引气剂类型、增大引气剂掺量以及避免过振等技术措施,以确保高原地区引气混凝土含气量能够达到抗冻设计要求。

Effect of low atmospheric pressure of plateau environment on air content and bubble stability of air-entrained concrete

Abstract: The effect of atmospheric pressure reduction of environment on air content and bubble stability of air-entrained concrete were experimentally studied using low-pressure test chamber to stimulate the plateau environment. Three different mixing proportions and three different levels of the initial air content of concretes were prepared for study, and four different common types of air-entraining admixture, saponin, alkyl sulfonate, abietic soap and polyether, were used in the experiments. The results indicated that the change of atmospheric pressure of environment had a significant impact on the performance of air-entraining admixtures. When the mix proportions of concrete and the amount of air-entraining admixtures took a certain value respectively, the air content of fresh concrete decreased linearly with the drop of atmospheric pressure. When the atmospheric pressure was 50 kPa, the air content of fresh concrete fell by roughly 20% to 49%. The higher the air content of fresh concrete mixed in normal atmospheric pressure, the faster the air content of fresh concrete decreased with the drop of atmospheric pressure. Among the four common types of air-entraining admixture used in the experiment, the air-entraining ability of saponin was the least influenced by the drop of atmospheric pressure and the abietic soap was the most. In addition, the bubble stability of fresh concrete mixed under low atmospheric pressure was poor than the one under the normal atmospheric pressure. The air content of the fresh concrete mixed under the low atmospheric pressure decreased more rapidly within 50-55 minutes after finish of the mixing and the spacing factor increased obviously as time went on. Besides, whatever the concrete was mixed under the low or the normal atmospheric pressure, the air content of fresh concrete decreased and the spacing factor of hardened concrete increased with vibration time extended from 0 second to 90 seconds, but it was more obvious within 30 seconds. 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 were recommended: 1) the air-entraining admixture with good air entraining ability and foam stabilization performance should be chosen for the plateau environment, 2) according to different pressure conditions and types of air-entraining admixture, the dosages of air entraining admixture should be increased so that air content of concrete could fully meet the design requirements, and 3) 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.