Field exposure experiments on the influence of low air pressure environment on the air-void structure of air-entrained concrete and its deterioration mechanism

Field exposure experiments were conducted in Nanjing (101.2 kPa) and Lhasa (65.3 kPa) to comparatively analyze the performance of polyether-based air-entraining agent (AEA-P) and rosin-based air-entraining agent (AEA-R) in low air pressure (LAP) environments. The effects of LAP environments on the air-void structure and pore structure of air-entrained concrete (AEC) with 3 %, 5 %, and 7 % air content were studied using the air-void parameter testing and NMR. Finally, the relative dynamic elastic modulus change of the AEC was investigated in relation to freeze-thaw resistance tests. The results indicate that the bubble stability of the AEA-P is superior to that of the AEA-R. The initial bubble diameter of the AEA-P under LAP conditions is 8–12 % smaller than that of the AEA-R. After 15 minutes of standing, the bubble diameter of the AEA-P is nearly half that of the AEA-R. LAP significantly deteriorates the air-void structure of AEC. These effects are primarily manifested as a reduction in the proportion of air-voids smaller than 200 μm and an increase in the proportion of air-voids larger than 200 μm. Compared to NAP, the proportions of air-voids smaller than 200 μm decreased by 3.8 %, 5.8 %, and 10.2 %, while the proportions of air-voids larger than 200 μm increased by 2.5 %, 3.0 %, and 4.6 % for the three different AEA dosages in LAP. Additionally, LAP reduces the specific surface area of the air-voids, increases the average air-void diameter, and reduces the number of air-voids. Consequently, the air-void spacing factor increases by 23–35 μm compared to normal air pressure (NAP), with the variation ranging from 12.5 % to 28.5 %. Regardless of whether the AEC was molded under LAP or NAP, the total porosity increased with higher air content. Additionally, the number of capillary pores larger than 200 nm was higher in concrete molded under LAP conditions. Under the same air content, the frost resistance of AEC molded in LAP is inferior to that of AEC molded under NAP, after 300 freeze-thaw cycles, the relative dynamic elastic modulus of the AEC decreased by 2.4 %, 3.1 %, and 4.8 % for the different air content levels, respectively. This study offers valuable insights into the freeze-thaw damage mechanisms of concrete in high-altitude cold environments. It also provides measures to enhance the frost-resistant durability of concrete in plateau areas.