疲劳荷载对蒸养混凝土电阻率的影响研究

Effect of fatigue load on resistivity of steam cured concrete

地铁工程中,预制混凝土构件主要承受弯曲疲劳荷载,导致混凝土产生疲劳损伤,进而影响混凝土的绝缘性能(电阻率)。通过电阻率的变化也可以表征疲劳荷载作用下混凝土的损伤程度。为研究弯曲疲劳荷载对蒸养混凝土电阻率的影响,测试不同加载频率、荷载幅值与饱水度下混凝土电阻率的变化,同时结合超声波测试技术探究电阻率与疲劳损伤变量之间的关系,分析疲劳荷载对混凝土毛细吸水作用下电阻率的影响。试验结果表明,对混凝土疲劳加载36万次后,不同工况下混凝土的电阻率降低30%左右,且混凝土的电阻率在疲劳加载过程中均呈现出先快速下降而后缓慢下降的两阶段变化趋势;当疲劳加载频率较高时(15 Hz),高饱水度混凝土的电阻率较其余工况短时间内迅速下降,这可能是高加载频率与高饱水度共同作用下,形成的较大孔隙水压力加速了微裂纹的形成及扩展;基于麦克斯韦两相电导理论推导建立混凝土电阻率与疲劳损伤变量之间的关系模型,该模型与试验数据拟合效果较好,表明电阻率法可用于混凝土疲劳损伤的定量评估。此外,疲劳荷载作用后,混凝土的毛细吸水质量与相对电阻率(混凝土毛细吸水后与毛细吸水试验前的电阻率之比)降低幅度均有明显增加。

In Subway Engineering,precast concrete components mainly bear bending fatigue load,leading to concrete fatigue damage and affecting its insulation performance(resistivity).On the contrary,the change of resistivity can also be used to characterize the damage degree of concrete under fatigue load.To study the effect of bending fatigue load on the resistivity of steam cured concrete,the changes of concrete resistivity under different loading frequencies,load amplitude and water saturation were tested,and the relationship between resistivity and fatigue damage variables was studied using ultrasonic technology.Finally,the effect of fatigue load on the resistivity of concrete under capillary water absorption was analyzed.The test results show that after 360000 times of fatigue loading on concrete,the resistivity of concrete under different working conditions decreases by about 30%,and shows a two-stage decreasing trend of rapid decline first and then slow decline in the process of fatigue loading.When the fatigue loading frequency is high(15 Hz),the resistivity of highly watersaturated concrete decreases rapidly within short time compared to other working conditions,which is likely due to the large pore water pressure formed under the joint action of high loading frequency and high water saturation accelerates the formation and propagation of microcracks.Based on Maxwell’s two-phase conductivity theory,a relationship model between resistivity and fatigue damage variables of concrete is established.The model fits well with the experimental data,indicating that the resistivity method can be used for the quantitative evaluation of concrete fatigue damage.In addition,after fatigue loading,the capillary water absorption quality and the reduction range of relative resistivity(ratio of resistivity of concrete after capillary water absorption to before capillary water absorption test)of concrete increase significantly.