IntroductionDue to the distinctive climatic characteristics of the Qinghai-Tibet Plateau, such as an extremely frigid climate and significant diurnal temperature fluctuations, concrete structures in this region necessitate substantial freeze-thaw durability. Presently, one of the most efficacious approaches to enhance the freeze-thaw durability of concrete involves incorporating tiny air bubbles into the mixture through the use of air-entraining agents (AEAs). Nevertheless, researchers have discovered that the low atmospheric pressure (LAP) prevalent on the plateau may result in inadequate air content within air-entrained concrete (AEC). Only a limited number of research findings have confirmed that LAP deteriorates the air-void structure of AEC, with relatively few studies investigating its impact on freeze-thaw durability of AEC. Furthermore, there is limited research on the impact of LAP on the distribution of air-void diameters in AEC. Therefore, the air content, air-void structure parameters and freeze-thaw durability of the AEC prepared under normal atmospheric pressure (NAP) and LAP were examined in this study. The primary focus is to investigate the impact of LAP on the distribution of air-voids with varying diameters, thus to further explore how these influences degrade both the air-voids structure parameters and freeze-thaw resistance durability of AEC.MethodsOrdinary Portland cement was used in the experiment. The coarse aggregate was crushed stone and the fine aggregate was river sand. In the experiment, two types of AEAs and a high-efficiency superplasticizer were used to produce the AEC. The experiments were conducted in laboratories located in Lhasa, China (atmospheric pressure values of 64 kPa) and Beijing, China (101 kPa). Three air content design levels (D) were used in this study, namely, 3%, 5%, and 7%. Air content tests, air-void analyzing tests and freeze-thaw durability tests were conducted.Results and discussionCompared to the AEC-N (AEC prepared under NAP), the LAP resulted in a significant increase in the loss of air content of fresh concrete (A_f), with a greater magnitude observed as D increased. The average and maximum losses in air content of hardened concrete (A_h) for AEC-L (AEC prepared under LAP) were approximately 0.5% and 0.7% higher than those for AEC-N, respectively. Furthermore, due to decreased atmospheric pressure, microvoids within AEC experienced a more substantial decrease with increasing D. Notably, air-voids ranging from 0-100 μm exhibited the most pronounced influence. However, increasing D still effectively enhanced the proportion of microvoids within AEC. The proportion of A₁₀₀₀ (the air content in voids with diameters no greater than 1000 μm, the same below ) in A_h showed an overall increasing trend followed by a subsequent decreasing trend as A_h increased, both under LAP and NAP conditions. However, when comparing AEC-N to AEC-L with similar D, the former demonstrated significantly higher proportions of A₁₀₀₀. With similar A_h , the reduction in atmospheric pressure led to a significant decrease in the proportion of microvoids (especially A_(0,100]), a pronounced increase in the proportion of trapped air-voids proportion (A_(1000,4000]), and small changes in the proportion of mesovoids (A_(300,500]) and macrovoids (A_(300,500]) within the AEC. The variation in the proportion of microvoids and trapped air-voids significantly influenced the spacing factor ($\bar{L}$). Specifically, an increase in the air content of microvoids (particularly A_(0,100]), leads to a decreased in the $\bar{L}$ of concrete. Conversely, an increase in the air contentof trapped air-voids resulted in an increase in the $\bar{L}$. However, the proportion of air content in the mesovoids and macrovoids was most weakly correlated with the $\bar{L}$.Concrete subjected to a plateau environment may experience more severe freeze-thaw conditions. when the water-cement ratio was 0.44, the required A₁₀₀₀ under LAP and NAP was about 3.90% and 2.92%, respectively, for achieving a frost resistance durability factor (F_D) of 100%.The analysis presented in this study demonstrates that the maximum allowable F_D in concrete under LAP is around 3.93%, whereas under NAP it is 4.73%. Clearly, despite the deterioration of air void structures caused by LAP, as long as the air content introduced by AEAs in AEC reaches a certain threshold level, the frost resistance durability requirements can still be met. It is suggested that while meeting the mechanical performance requirements of AEC design, there should be an increase in the maximum air content level of fresh concrete by approximately 2.0% compared to plain areas. This adjustment aims to ensure that the frost resistance performance of concrete meets the design requirements.ConclusionsThe LAP caused a more pronounced loss in the air content of hardened AEC-L, and the loss increased as the D increased. The average and maximum loss was about 0.5% and 0.7% higher than those under AEC-N, respectively. Compared to the AEC-N with the same D, a decrease in atmospheric pressure resulted in a reduction in the air content of the microvoids within the AEC-L. This effect was particularly pronounced for air-voids with diameters not exceeding 100 μm, and consequently leading to the deterioration of the air-void structures and a reduction in frost resistance performance of the AEC-L. The variation in the proportion of air content in microvoids and trapped air-voids has a significant impact on the $\bar{L}$ of the AEC. The $\bar{L}$ decreases with an increase in the proportion of air content of microvoids, particularly A_(0,100] and A_(100,200], while it increased with an increase in the proportion of air content of trapped air-voids. The increase in the D of the concrete effectively enhanced the air content proportion of microvoids, thereby enhancing the frost resistance durability of concrete. It is recommended that when designing AEC for plateau areas with severe freeze-thaw environments, the D of fresh concrete should be increased by a maximum of approximately 2.0% compared to plain areas.