EFFECT OF THE RELEASE OF THE FORMWORKS AND OF A THERMAL SHOCK ON THE TEMPERATURE AND STRAINS GRADIENTS IN NON-REINFORCED CONCRETE

摘要

The effect of the release of the forms at the surface and at the centre of a non-reinforced 700 mm-cube made with four different concretes having water/binder ratio of 0.45; 0.40; 0.35 and 0.30 was studied using vibrating wire gages. The stripping of the forms was done at 24 hours when temperatures at the centre of the cube and behind the 20 mm-thick plywood formwork were maximum. The 0.45; 0.35 and 0.30 cubes were demolded inside the laboratory at 22 ℃. The 0.40 cube was exposed to a second thermal shock just after the release of its formworks in the laboratory when it was exposed to a -6 ℃ outside ambient temperature. At 22 ℃, the removal of the forms was found to increase the microstrain gradient between the surface and the centre of the cube slightly (20 μm/m), but in the case of the exposure to the -6 ℃ temperature, the microstrain value reached 127 μm/m. This instantaneous strain can be high enough to severely microcrack the surface of the concrete. The effect of the thermal shocks recorded can be used to calculate the coefficient of thermal expansion of concrete.