Analysis of variations in electrical conductivity to assess stability of cement-based materials

With the growing use of highly flowable concrete, it is important to provide a proper assessment of the risk of bleeding and segregation of these materials. The present study was undertaken to evaluate the stability of mortar mixtures by monitoring changes in conductivity along a 350 mm test column during 150 min. In total, 16 mortars with water-binder ratios (w/b) varying from 0.45 to 0.80 prepared with two Type 10 portland cements and different replacements of Class F fly ash were evaluated. The main focus of this paper is to identify comprehensive methodology that can be used to interpret variations in conductivity readings with regard to bleeding, segregation, and overall homogeneity at early age of cement hydration. The analysis of variations in electrical conductivity throughout the sample and as a function of time enables an interpretation of the homogeneity of the plastic suspension. Various stability indexes are derived and related to bleeding and segregation processes that are exacerbated with the increase in w/b, regardless of cement type and degree of fly ash replacement. Derived bleeding, segregation, and homogeneity indexes are shown to be statistically dependant on external bleeding measurements. The segregation and homogeneity indexes are further shown to correlate to physical measured values of variations of density along hardened samples.