Effect of water–binder ratio and fly ash on the homogeneity of concrete

In this paper, effect of water–binder ratio and fly ash on the homogeneity of concrete are investigated. The image analysis, segregation degree, compressive strength and microhardness are used as the evaluation of homogeneity. The results demonstrate that the image analysis method can effectively describe the homogeneity of concrete. Meanwhile, the segregation degree exhibits a linear relationship to the reduction ratio of compressive strength of concrete. It also can be concluded that the lower water–binder ratio and the addition of fly ash can effectively improve the homogeneity in ITZ of concrete. On the other hand, microhardness testing indicates that the top of aggregate has the highest microhardness and the narrowest width of ITZ, and the interior of concrete performs better than the surface. The reason is that both the segregation at macro-level and the asymmetrical distribution of the water content around the single aggregate particle lead to the obviously difference in the formation of hydration products and the compactness in ITZ of hardened concrete, which can be illustrated by SEM analysis. Thus, the homogeneity of concrete can be evaluated by the combination of image analysis, segregation degree, compressive strength and microhardness.