混凝土硫酸盐腐蚀损伤的声波与声发射变化特征及机理

试验模拟干湿循环作用下混凝土受10%(质量分数)硫酸钠溶液侵蚀的腐蚀环境,测试和分析硫酸盐不同侵蚀时期混凝土单轴压缩试验时波速和声发射的变化特征.采用环境扫描电镜和能谱仪进行微观观测并结合X射线衍射测试手段分析受蚀混凝土的损伤机理.结果表明:受侵蚀60 d和80 d的试件加载初期会有较明显的压密阶段,试件受硫酸盐侵蚀和干湿循环作用愈久,加载中波速急剧下降的突变点愈提前;受蚀40 d以上的试件加载中声发射事件活跃区间较集中,在腐蚀产生的缺陷和薄弱位置容易出现应力集中和能量集中释放,声发射事件数量急剧上升的突变点提前.通过数学模型以声发射累积振铃计数为损伤变量建立损伤模型可以表征混凝土中环境腐蚀、荷载及损伤之间的作用关系.腐蚀阶段钙矾石与石膏的膨胀作用和硫酸钠的结晶压在试件内部形成微破损,受蚀混凝土表现出不同宏观性能. 

Ultrasonic velocity and acoustic emission properties of concrete eroded by sulfate and its damage mechanism

Simulation experiment was performed on the performance of concrete exposed to sulfate attack and dry-wet cycles. Changes in ultrasonic velocity and acoustic emission activities for concrete at different erosion periods were tested by uniaxial compression test. The damage mechanism was analyzed by environmental scanning electron microscopy (ESEM), energy-dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). The results show that at the beginning of loading for concrete after 60 d and 80 d erosion, the specimen exhibits a significant compaction phase. The longer the concrete is exposed to sulfate attack and dry-wet cycles, the earlier the sudden drop in ultrasonic velocity occurs. For concrete after 40 d erosion, the active zone of acoustic emission is narrow during the loading process. The problems of stress concentration and a sudden release of energy tend to occur at the weak position caused by erosion in concrete, and a sharp increase in acoustic emission events appears in advance under sulfate attack. By means of a mathematical model, the damage model of concrete was established based on the cumulative ring-down count of acoustic emission as a variable, which reveals the relations of sulfate attack, load and damage for concrete in complex underground environments. During erosion time, the expansions caused by ettringite and gypsum and the swelling of sodium sulfate crystallization result in the evolution of micro-destruction, leading to the different macroscopic properties of corroded concrete.