On the measurement of free deformation of early age cement paste and concrete

Autogenous deformation and thermal dilation produce stresses, which may lead to cracking in early age concrete subjected to external restraint. To quantify the two types of "free deformation" by measurements is a prerequisite to fundamental understanding, as well as to formulate numerical models for use in stress calculations. However, results reported in the literature reveal large deviations and also inconsistencies between different measuring methods. The present paper discusses free deformation measurements and show that different types of measuring errors are involved, where, for instance, reabsorption of bleed water is an important one, and a standard test procedure should therefore describe how to handle the effect of bleeding. Furthermore, it is possible to obtain fairly good reproducibility within one laboratory using the same test rig, whereas a Round-Robin test program showed that it is far more difficult to produce similar results from different laboratories measuring on the same concrete. The implication is that measuring errors were present. Hence, there is a need for more calibration work and better control of test rig behavior. (C) 2003 Elsevier Ltd. All rights reserved.