Abstract
In this work, we presents an experimental study to characterize the mechanical properties at the local scale of the cement paste-aggregate interface during hydration and during chemical degradation by leaching. It is generally accepted that the microstructure of the interface between the aggregates and the cement paste is characterized by a higher porosity than that of cement paste. This makes this zone a weak point in the concrete. The particular properties of this zone strongly influence the mechanical behavior of concrete. In this work, the mechanical properties of the cement paste-aggregate composite, are experimentally studied. The experimental tests are performed on composites at classical aggregate scale (one centimeter of section). These composite are composed by and Portland cement paste and aggregates. The cement paste is prepared with a water/cement ratio of 0.5. The shape of the prepared composites makes them convenient to characterize the cement paste-aggregate interface. At different stages of hydration, the composites are subjected to compression tests, direct tensile tests or shear tests by means of specific devices. The same tests were carried out on the cement paste in order to compare with composites results. The study of the mechanical properties at local scale revealed that the cement paste-aggregate interface is the weakest zone in the composite. An experimental protocol was developed to study the effects of calcium leaching mechanism at the cement paste-aggregate interface on the mechanical properties of the concrete. As the process of leaching with the deionised water occurs very slowly, the experimental study in the laboratory is accelerated by replacing the water by ammonium nitrate solution. To quantify the development and kinetics of degradation at the cemented bond, the concrete leaching fronts are characterized at different levels of degradation by using phenolphthalein. Local mechanical tests (compression, tensile and shear) are performed on composite and cement paste at different stages of degradation. The experimental results show that there is a leaching effect on the alteration of the mechanical properties at the cement paste-aggregate interface.