青藏高原低气压环境下钢管混凝土的核心混凝土密实性评估方法研究

为研究高海拔低气压条件对混凝土抗压强度与超声波速相关性的影响，分别在高海拔地区(西藏山南)与低海拔地区(广西南宁)进行了不同水灰比混凝土试件的制备与强度、超声波速测试，研究表明，低气压条件下7～56d龄期混凝土的抗压强度比标准气压条件下相同配合比混凝土高出约5.8%～38.2%；低气压条件下混凝土的超声波速低于标准气压条件下相同强度混凝土，不同气压下混凝土超声波速与强度存在不同的线性关系，并在此基础上提出了不同气压下混凝土超声波速与强度的相关关系模型。通过压汞试验(MIP)及数值模拟分析，揭示了混凝土超声波速受微观结构中固体超声波速及孔隙率两因素共同影响的机理，建立了不同气压下混凝土固体超声波速的计算模型。基于混凝土超声波速与强度的相关性及不同灌注工艺的密实程度，提出了不同气压条件下钢管混凝土的核心混凝土密实性评估方法。

Methodology on evaluating the compactness of core concrete in CFST serving under low atmospheric pressure over the Qinghai-Tibet Plateau

This study aims to investigate the effect of low atmospheric pressure on the correlation between concrete compressive strength and penetrating ultrasonic pulse velocity (UPV). Concrete specimens were cast and tested with varying water-to-cement ratio, respectively at the high-altitude (Shannan, Tibet Autonomous Region) and the low-altitude regions (Nanning, Guangxi Autonomous Region). The results show that, concrete specimens made under low atmospheric pressure exhibited 5.8%～38.2% higher compressive strength at 7～56d than the one produced under normal atmospheric pressure. The UPV penetrating through concrete made under low atmospheric pressure was lower as compared to the series of standard atmospheric pressure. Besides, it was found that there existed varying linear correlation between the UPV and the strength of concrete when produced under different atmospheric pressure levels. Given this, a correlation model was established to capture the relationship between UPV, concrete strength and atmospheric pressure. Together with mercury intrusion porosimetry (MIP) test, the numerical simulation revealed that the global UPV penetrating through concrete was dependent upon the UPV at the solid phase and porosity. Thereafter, a computational model was proposed to determine the UPV for solid phase of concrete when producing under varying atmospheric pressure. Eventually, based on the correlation between the UPV and concrete strength alongside the compactness degree, a method to evaluate the compactness of core concrete in concrete filled steel tube (CFST) serving under different atmospheric pressure conditions is proposed.