Abstract
The effect of low atmospheric pressure of the environment on the air content and bubble stability of air-entrained concrete was investigated in Beijing and Lhasa. The results indicate that the reduction of atmospheric pressure can weaken the air-entraining capability of air-entraining agents (AEAs). The air content of fresh concrete decreased by 9%–39% when the atmospheric pressure dropped to 64 kPa. The bubble stability of concrete mixed at a low atmospheric pressure becomes worse. Within 50–55 min after mixing, the air content of concrete mixed at a low atmospheric pressure decreases greatly, and the void spacing factor increases obviously. The concrete mixed at a low atmospheric pressure will lose more air content when vibration time increases, leading to the decrease of air content and the increase of the spacing factor, which are more significant than the concrete mixed at normal atmospheric pressure. On the basis of the experiment results in this study, the type of AEAs must be carefully selected, and the vibration time must be strictly controlled to ensure that the air content of concrete will meet the design requirements in low atmospheric pressure areas.
Highlights
Introduction e QinghaiTibet Plateau is over 4,500 m above the sea level on average, and its average annual temperature is below 0°C; freeze-thaw damage is the main failure mode of many concrete structures in this area [1, 2]
E above test results are similar to the conclusion based on the low atmospheric environment chamber mentioned in the literature [15], that is, low pressure will weaken the airentraining capability of air-entraining agents (AEAs), resulting in difficulties in air entraining. is phenomenon is more obvious in concrete with large slump or high air content. is finding may be related to the difficulty of suppressing the bubble overflow when the concrete viscosity is small or the poor stability of the bubbles when the air content is high. e specific reasons still need to be further studied
According to the test results (Table 5), saponin and polyether were the two types of AEAs whose air-entraining capability was less affected by the varying of atmospheric pressure. us, they were used in the studies on time-dependent behavior of the air content and Parameter Type Specific surface Specific gravity Fineness modulus 28-day compressive strength (MPa)
Summary
E effect of low atmospheric pressure of the environment on the air content and bubble stability of air-entrained concrete was investigated in Beijing and Lhasa. e results indicate that the reduction of atmospheric pressure can weaken the air-entraining capability of air-entraining agents (AEAs). e air content of fresh concrete decreased by 9%–39% when the atmospheric pressure dropped to 64 kPa. e bubble stability of concrete mixed at a low atmospheric pressure becomes worse. On the basis of the experiment results in this study, the type of AEAs must be carefully selected, and the vibration time must be strictly controlled to ensure that the air content of concrete will meet the design requirements in low atmospheric pressure areas. The above research results provide a useful reference for the preparation of air-entrained concrete in low-pressure environments, studies on the influence of LAP on AEAs and the performance of air-entrained concrete remain few. (ii) Part 2: two types of AEA, whose air-entraining capability was less affected by LAP based on the experimental results from Part 1, were utilized to produce the concrete with initial air content of 7% under Beijing (101 kPa) and Lhasa (64 kPa). The air content of fresh concrete and the air void analysis of hardened concrete were tested according to the standard for the test method of performance on ordinary fresh concrete [18] and ASTM C457 [19], respectively
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