An experimental study on bleeding and channelling of cement paste and mortar

Existing studies on bleeding are mostly confined to the study of the normal bleeding of cement paste. Channelled bleeding, which may occur under certain conditions during bleeding, has not been as well researched. Further, in most studies, these processes are treated as surface phenomena, and only surface measurements were made. In this study, the bleeding process, including channelling of cement paste and mortar, was investigated. The latter material is a step forward in the study of bleeding of concrete. Besides surface settlement, pore pressure and density profiles were also measured during the experiments to give a better insight into the mechanism of bleeding and channelling. The effect of bleeding and channelling on the strength development in a column was also studied. A difference in strength of about 30% was found between samples from upper and lower parts of a column. However, the effect of channels on the compressive strength is found to be negligible as the area of such channels is small compared to the total area. With these data, a better understanding of the channelling mechanism was achieved. A simple model was then used to estimate when channelling will occur.