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Received Nov 7, 2017; Accepted Jan 31, 2018
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1. Introduction
The altitude of China’s Tibet region is higher than 3000 m. The significant climatic environment of high-altitude area is low pressure, large temperature difference between day and night, dry and windy due to high and complex terrain, high-altitude atmospheric circulation, and solar and other strong radiation along with various other factors. In plateau climate conditions, air entraining concrete is used in hydropower, transportation, and civil engineering projects to resist the frequent freeze-thaw cycle to avoid freezing and other induced disease, thereby ensuring the safe operation of the building [1].
The AEA can introduce a large number of homogeneously distributed air micropores and, consequently, improve the mixing workability, antipermeability of concrete performance, and frost resistance [2, 3]. The factors of ambient temperature, humidity, and air pressure affect the air entraining concrete performance besides the factors of raw materials, proportion of mixture, and construction method. The mechanism of atmospheric pressure affects relatively the concrete performance. Therefore, the corresponding test could not be easily carried out, and the relevant study is less reported. Mielenz et al. suggested that existing pressure gradient between the big bubbles and the small bubbles makes the transfer of air possible between the bubbles. The transfer of air leads to change in the volume of air in pastes over the period of time [4]. Ley et al. proposed that the diameter of bubbles becomes smaller or even breaks up [5]. This changes the pore volume and distribution in fresh concrete by increasing the pressure around the pores. At the same time, the liquid behavior of different AEAs is different. According to Zhu et al. [6], the air content of the concrete shows decreasing tendency with the decreasing atmosphere pressure. Li and Fu suggested that the air content of concrete shows linear decrease with the decreasing atmosphere pressure. The higher the initial air content, the greater the decreasing rate of the air content with the ambient air pressure is found. The greater the slump of concrete is, the stronger the ability to resist reduction in the...