Sammendrag
It is well known that horizontal surfaces of high strength concrete (HSC) are susceptible to cracking in the time before and during setting. It appears that there are two critical time periods for such cracking: An early period ranging from placing to an hour or two, and a late period that covers the setting time and early hardening. The latter is considered here. In this period, the cracking risk may be assessed from the ratio between strain (the volume change) and tensile strain capacity, or from the ratio between stress generated from the volume change at restraint, and the tensile strength. All these parameters were measured. It is shown that relatively moderate volume changes of HSC-surfaces caused by drying plus cooling, generate restraining stress that may exceed the tensile strength, and thus, generate failure. The results indicate that crack risk assessment from stress/strength is more reliable than using strain/strain capacity. The starting point of tensile strength evolution of hardened concrete seems to coincide fairly well with the initial setting time. Also, the tensile strain capacity reaches a low level at this point, but continues descending for another hour or more.