Evaluation of the reliability and life span for cement materials by electric property measurements

选取硬化石膏和硅酸盐水泥作为混凝土的简化模型，类比得出了力学强度和介电强度的分布规律。两种强度均采用威布尔统计量进行评估，强度分布均采用双参数威布尔函数。这两个曲线都显示出与半水粉末振动硬化过程中的含水率无关的良好的一致性。在水含量为70wt%的石膏板上，力学强度和介电强度分布曲线均呈两条斜率不同的直线，但两者吻合较好。结果表明，可以用击穿试验得到的介电强度分布来评价硬化石膏或硅酸盐水泥的机械强度分布，硅酸盐水泥浆体或普通水泥的低频阻抗随养护时间的延长而增大，且受氯化钠的影响较大。氯化钠的污染会引起碱集料反应，导致混凝土突然断裂或死亡。在掺活性骨料的水泥净浆中掺入少量的氯化钠，养护3d后，异常点的阻抗增大。经过365天的养护，在掺有活性集料的氯化钠中发现了碱集料反应的证据。通过对人体阻抗的连续监测，可以预见碱集料反应，如果在早期观察到阻抗的异常增大，则可以预测碱集料反应，提出了一种一次获得多个试件强度分布的替代方法。该方法利用试件的弹性模不变性同时增加试件的弯曲应力，并通过电导的变化来监测试件的断裂数量。

Hardened gypsum and portland cement are chosen as a simple model of concrete and analogy beween the mechanical and dielectric strength distributions are found out. Both strength is evaluated in terms of Weibull statistics and the two-parameter Weibull function was adopted for both strength distribution. Both plots shows good coincidence independent of the water content during shaking of hemihydrate powder for hardening. At the hardened gypsum sheets which are made by shaking with 70wt% of water, both plots for mechanical and dielectric strength distribution show two straight lines with different slopes, however, both plots shows in good agreement. From the results, it is concluded that the mechanical strength distribution of hardened gypsum or Portland cement can be evaluated by using the dielectric strength distribution, which is casily obtained by breakdown testing.The low frequency impedance of Portland cement paste or mortal increased with an increasc in curing time and it is strongly affected by the addition of NaCl. The contamination of NaCl would induce the alkaliaggregate reaction ane it induces a sudden fracture of concrete or mortal. When small amount of NaCl is added to cement paste with reactive aggregate (Pyrex glass), the impedance in anomalous increases after 3 days curing. After 365 days curing, the evidence of alkali aggregate reaction is found out at NaCl added mortal with reactive aggregate. By the continuous monitoring of impedance of mortal, the alkali aggregate reaction can be foresee and if the anomalous increase in impedance is observed in early times, the alkali aggregated reaction is apprehensive.An alternative method to obtain a strength distribution of many test pieces at one time is proposed. In this method, bending stress of all subjected samples is simultaneously increased utilizing their elastic modules constancy and the fractured sample number is monitored by the change in electric conductance.