In order to enhance the
quality of concrete structures, cracking in hardening concrete should be
prevented as
far as possible. A traditional way to assess the risk of cracking
at early ages consists of minimising temperature differentials between warm and
cold concrete. In this contribution the results of a recently finished
four-year study on the risk of cracking and crack criteria of
hardening concrete will be presented. The stress development and the tensile stress at
the moment of cracking have been measured in a Temperature Stress Testing
Machine. Different types of concrete and types of cement were considered, i.e.
Portland cement and blast furnace slag cement. Based on the test results a
criterion for cracking at early ages is deduced. A procedure for determination
of stresses in hardening concrete is briefly discussed. In this procedure allowance
is made for the effect of the evolution of the microstructure on
relaxation of
stresses during hydration.