Stability of Fresh Concrete Mixes

Stability is characterized by the bleeding and segregation tendencies of the concrete using a direct method of measurement based on floatation over carbon tetrachloride. Results were obtained for a range of cement pastes illustrating the relationship between water-cement ratio and initial rate of bleeding and also bleeding capacity. The bleeding capacities per unit volume of cement paste obtained for concrete mixes were much lower than the equivalent values obtained for neat paste. This reduction is partly attributed to internal bleeding that can cause planes of weakness and reduce bond strengths. The segregational instability of a concrete mix results from the matrix having insufficient strength to retain individual aggregate particles in a homogeneous dispersion. This results in honeycombing with a consequent reduction in durability and strength. The direct link between internal cohesion and resistance to segregation was illustrated.