Hardening concrete : measurments and evaluation of non-elastic deformation and associated restraint stresses

Abstract: If a structural member of hardening concrete is restrained to some degree against expansion and contraction during the heating phase of the hydration process and the subsequent contraction phase, stresses will be induced. The issue of primary interest is whether or not these induced stresses will lead to cracking. Volume changes in hardening concrete are not only ruled by the thermal movements. High Performance Concrete (HPC) is usually based on high binder content mixed with small amounts of water. The low water-to-binder ratio leads to significant autogenous deformations, which has to be included in stress analyses. For water-to-cement ratios below about 0.40, self-desiccation results in a significant drop in the pore humidity even at early ages. In contrast to the moisture flux, self-desiccation is reflected all over the structure, and at restraint conditions this may be a significant contribution to the risk of early age cracking. Models for descpition of autogenous shrinkage and different evaluation techniques how to split between moisture and thermal deformations are presented in the thesis. Laboratory tests and modelling in early age of the stresses caused by restraint volume changes have been performed and discussed for high performance and normal strenght concrete. Evaluated parameters are presented for a great number of concrete mixes. It has been shown that all material data for stress calculations can be taken from material related tests on each individual property without any correction when comparison is done with measured stresses in the relaxation test-frame.