High strength concrete (HSC) is more sensitive to cracking in the time before and during setting than normal concretes are. The intention of the present investigation was to provide more information about the driving forces responsible for this, and which can contribute to the fundamental understanding of the problem. The volume changes active in the period have been identified and discussed regarding the relationship between settlement and shrinkage of HSC. Two concretes were tested, with water to binder ratio of 0.30 and 0.40, respectively. The settlement, shrinkage and pore water pressure were measured for both concretes when exposed to moisture sealed condition or moderate drying, i.e. at 50% RH and no wind. The results indicate that the w/b does not considerably influence plastic shrinkage and settlement at moderate drying. Pore water pressure measurements showed, however, that the capillary tension in the pore water near the top surface developed with a much higher rate in the 0.30 concrete than in the 0.40 concrete. This implies a lower tensile strain capacity at the surface of the 0.30 concrete, and thus, that the tensile strain capacity is a more important parameter than the volume change, in order to explain the high risk of cracking of HSC at very early ages.
展开▼
