Effect of silica fume on the plastic shrinkage and pore water pressure of high-strength concretes

High-strength concrete is more sensitive to cracking before and during setting than normal concrete is. The intention with the present investigation was to provide more information about the parameters responsible for this, and which can contribute to the fundamental understanding of the problem, and in particular of the influence of silica fume. Four concretes were tested, all with water to binder ratio of 0.40, and with 0, 5, 10 and 15% silica fume. The settlement, shrinkage and pore water pressure were measured for all concretes when exposed to moisture sealed condition or 50% RH. The results indicate that the silica fume content does not significantly influence plastic shrinkage and settlement at these conditions. Another important observation was an immediate respond on external drying, which involves a low tensile strain capacity. The strain capacity seems to depend on the measured pore water pressure, which reflects the tensile stress in the pore water. The pore water pressure close to the exposed surface became negative less than half an hour of exposure at 50% RH. The effect of silica fume content was not pronounced except for the one with 15% which showed a significantly higher rate of pressure decrease than the other concretes.