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

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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.