工程水泥基复合材料高温损伤超声特性

为了研究工程水泥基复合材料(ECC)在高温作用后的损伤机理及超声特性，对不同温度(20，105，250，400，600和800℃)作用后的ECC试件进行超声波和抗压强度测试.结果表明:超声波通过高温作用后ECC的波形幅值、主频幅值、能量和ECC抗压强度的变化趋势相同；不同温度作用后40~50kHz频段的能量占比均最大，约为0~300kHz总能量的15%；在20~250℃作用后声速变化不大，温度高于400℃后声速随温度升高而降低.原因是高温后自由水汽化、水化产物分解和骨料物性改变，导致孔隙蒸汽压力升高和骨料界面损伤，ECC内部裂纹产生并扩展.扫描电镜测试结果表明，PVA纤维随温度升高发生软化、熔化和汽化，ECC基体中产生空隙和孔道并与裂纹连通形成网络，有利于释放孔隙蒸汽压力，减弱ECC高温损伤.研究表明超声特性可以有效反映ECC的高温损伤演化过程.

Ultrasonic Characteristics of Thermal-Damaged Engineered Cementitious Composites

To study the damage mechanism and ultrasonic properties of thermal-damaged engineered cementitious composites(ECC)， ultrasonic monitoring and compression tests were performed on ECC specimens exposed to high temperatures(20， 105， 250， 400， 600 and 800℃). Results indicate that the changes in waveform amplitude， main frequency amplitude， energy， and compressive strength of thermal-damaged ECC are the same; the energy ratio in the 40~50kHz band is the largest， which is about 15% of the total energy in the 0~300kHz band. The wave velocities of ECC specimens heated from 20 to 250 ℃ show no significant changes， and decrease when heated above 400℃. The reason can be relate to free water vaporizes， hydration products decompose， and aggregate physical properties change at high temperature， which leads to the increasing of pore vapor pressure and damaging to the aggregate interfaces， thus the internal cracks in the ECC are generated and expanded. Scanning electron microscopy test results indicate that PVA fiber softens， melts， and vaporizes with increasing temperature. Voids and pores are generated in the ECC matrix and connect with cracks forming into networks， which are beneficial to the releasing of the pore vapor pressure and reducing the thermal damage of ECC. Results also show that the ultrasonic characteristics can effectively reflect the thermal damage evolution process in the ECC.