热疲劳作用对混凝土力学性能及微观结构的影响

我国西北地区日、年温差大,混凝土经历着温差产生的热疲劳劣化。保持环境湿度恒定,在20 ℃、30 ℃、40 ℃温差下开展两种强度等级的混凝土热疲劳试验,测定其抗压强度、劈裂抗拉强度等宏观性能变化规律;通过超声无损检测技术和压汞试验测定微观结构。结果表明:热疲劳劣化效应明显,随循环温差的增大和循环次数增加,混凝土强度下降明显,C40混凝土下降幅度大于C25混凝土,且劈裂抗拉强度较抗压强度对热疲劳作用更敏感;超声波速呈减小趋势,说明混凝土内部裂隙缺陷增多;同一循环温差下,混凝土的孔隙率、孔隙总体积、平均孔径、中值孔径、最可几孔径随温差循环次数增加而增大,孔隙总表面积减小,孔隙结构表现出粗化的特征且呈劣化的趋势,C40混凝土的孔隙率小于C25混凝土,但其孔隙率相对变化值更大,从微观层面揭示了混凝土在热疲劳作用下强度损伤的内在原因。

Effect of Thermal Fatigue on Mechanical Properties and Microstructure of Concrete

The daily and annual temperature differences in northwest China vary greatly, which leads to thermal fatigue deterioration of concrete. Under the condition of constant environmental humidity, the thermal fatigue tests of concrete with two strength grades were carried out at the temperature difference of 20 ℃, 30 ℃ and 40 ℃ to measure the changes of the macroscopic properties such as compressive strength and splitting tensile strength. The microstructure changes of concrete were measured by ultrasonic nondestructive technology and mercury intrusion porosimetry test. The results show that the thermal fatigue degradation effect is obvious. With the increase of temperature difference and number of temperature difference cycles, the concrete strength decreases obviously. The decrease of C40 concrete is greater than that of C25 concrete, and the splitting tensile strength is more sensitive to thermal fatigue than compressive strength. The ultrasonic velocity decreases, which indicates that the internal defects of concrete increase. The porosity, total pore volume, average pore diameter, medium pore diameter and the most probable pore diameter of concrete increase with the increase of temperature difference and number of cycles, while the total pore surface area decreases, which indicates that the pore structure shows the characteristic of coarsening and a tendency to deteriorate. The porosity of C40 concrete is smaller than that of C25 concrete, but the relative change value of the porosity of C40 is larger, which reveals the internal reason for strength damage of concrete under thermal fatigue from the microscopic level.