Characterization of behavior and cracking of a cement paste confined between spherical aggregate particles

In this work, the cement–grain interactions in a pair of 8 mm spherical grains of quartz or calcite, linked by a Portland cement, are analyzed experimentally at a local scale (cement–grain interface) during mechanical testing. The volume of the cement paste, the water/cement ratio and the gap between the grains are constant. Examined samples are conditioned in an atmosphere of constant temperature (21 °C) and high relative humidity (~ 100%) to minimize drying shrinkage. At various stages of hydration, the samples are subjected to tensile or compressive tests, monitored by a high-speed camera. Stiffness and force at rupture are estimated and correlated to the crack initiation and propagation followed through image analysis. The results concern mechanical properties and kinetics of cracking in function of hydration time, for both types of loading and both types of grain. Such local characteristics will be used to propose local interaction laws and to support numerical modeling of the concrete at macroscopic scale.