Abstract: Based on field exposure experiments conducted in Nanjing (101.2 kPa) and Lhasa (65.3 kPa), the effect of different air contents (3%, 5%, and 7%) on air voids and pore structures of air-entrained concrete under low-pressure environment was studied by air voids parameter test and nuclear magnetic resonance (NMR) techniques. The results show that low pressure significantly deteriorates the air-void structure of air-entrained concrete, primarily manifested by a decrease in the proportion of air voids smaller than 200 μm and an increase in the proportion of those larger than or equal to 200 μm, and the degree of change intensifies as the concrete air content increases. Compared with normal atmospheric pressure conditions, the porosity of three types of air-entrained concrete cast under low-pressure conditions decreases by 8.3%, 9.0%, and 22.1%, respectively, with high air content being most affected by low-pressure conditions. In low-pressure environment, even when the concrete air content is increased by adding a higher dosage of air-entraining agent, the air-void structure parameters still deteriorate, characterized by a reduction in the specific surface area of air voids, an increase in average diameter, and a decrease in the number of air voids, ultimately causing an increase in the pore spacing coefficient. The total porosity of air-entrained concrete cast under both low and normal atmospheric pressures rises as the air content increases. Concrete cast under low pressure conditions contains more capillary pores larger than 200 nm, thereby increasing the total porosity.

Key words: geotechnical engineering; low pressure; concrete; air-void structure; pore structure

摘要： 通过南京（大气压力101.2 kPa）和拉萨（大气压力65.3kPa）两地现场暴露实验，采用气孔参数测试和核磁共振技术，研究了低压环境下不同含气量（3%、 5%、 7%）对引气混凝土气孔及孔结构的影响。结果表明：低气压对引气混凝土的气孔结构具有明显的劣化作用，主要表现为小于200 μm 的气孔比例降低，大于等于200 μm 的气孔比例增加，且变化的程度随混凝土含气量的增加而加剧；较常压条件，低气压条件下成型的3种含气量混凝土的气孔量分别降低了8.3%、9.0%、22.1%，说明高含气量受低气压的影响最大；低气压环境下，即使通过增加引气剂掺量使得混凝土含气量增加，气孔结构参数仍趋劣化，表现为气孔比表面积减小、平均直径增大及气孔数量减少，最终导致气孔间距系数增加；低气压和常压环境下成型的引气混凝土，其总孔隙率均随含气量增加而上升；低压条件下成型的混凝土中，大于200 nm的毛细孔数量更多，导致总孔隙率增加。

关键词: 岩土工程；低气压；混凝土；气孔结构；孔结构