Evolution of the mechanical behaviour of a high performance self-compacting concrete under drying

Mechanical behaviour of cement-based materials is strongly influenced by drying. In order to evaluate only drying effects on mechanical behaviour, it is necessary to conduct the studies on mature materials, i.e. materials for which the strength increase due to hydration has already reached stabilization with an adequate curing regime. Previous studies on two mature mortars with large water-to-cement (w/c) ratios of 0.5 and 0.8 showed that, during drying, there is the occurrence of a competitive effect which controls the evolution of the mechanical behaviour. The competitive effect occurs between the increase in material strengthening due to capillary suction and hygral gradients and drying induced micro-cracking due to material heterogeneities and differential shrinkage. This effect depends on the quality of the binder paste. Therefore, a high performance self-compacting concrete, i.e. a high performance material, with water-to-binder (w/o) ratio of 0.41 is cast to study the evolution of the mechanical behaviour under drying conditions. Uniaxial compression and bending tests are carried out on samples subjected to air-drying (21 +/- 1 degrees C and 60 +/- 5% R.H.) and/or oven-drying (60 degrees C and 10% R.H.). The results reveal that the competitive effect also has a great influence on the mechanical behaviour of high performance materials. The uniaxial compressive strength of the high performance self-compacting concrete increases while its bending strength decreases with air-drying. This shows the higher sensitivity of bending strength to the drying induced micro-cracking. In the case of accelerated drying in the oven, the drying induced micro-cracking strongly affects the uniaxial compressive strength, Young's modulus and bending strength in spite of a higher paste quality leading to lower porosity. The maximum size of the aggregates seems to play an important role in this deterioration of mechanical properties, especially in the case of accelerated drying. Drying induced micro-cracking is made clearly visible by hydraulic oil impregnation. (C) 2010 Elsevier Ltd. All rights reserved.