MICRO STRUCTURAL ORIGIN OF THE APPARENT THERMAL TRANSIENT CREEP OF CONCRETE AT HIGH TEMPERATURE

This research has been conducted within the framework of the study of concrete behaviour at high temperature. The aim of this study is to model the thermal damage mechanisms of concrete and to investigate the behaviour of concrete specimens when they are simultaneously subjected to a constant compressive load and to high temperature. A new approach, the multiphase Digital Concrete Model and the damage model MODEV, both implemented on finite-element software SYMPHONIE, has been used to model the thermal damage of concrete. In this approach, 2 main categories of concrete thermal damage mechanisms are considered:- Mechanical thermal damage as a result of restrained thermal strain on a macroscopic and mesoscopic scale: This damage can be due, on one hand, to the temperature gradient or the boundary conditions, or even the geometric shape on a macroscopic scale. It can be due, on other hand, to the differential expansion between the cement paste and the aggregates on a mesoscopic scale.- Purely thermal damage as a result of the different chemical transformations, which occur mainly in cement paste: dehydration, important mass loss beyond 120°C, and other chemical transformations. In order to study the behaviour of concrete specimens when they are subjected such loading, numerical simulations of the experimental tests carried out by Holst [4] have been performed. The “Digital Concrete” model is used to scrutinise the structural mesoscopic effects and to quantify the damage induced. Material and thermal characteristics are randomly distributed accordingly to the material mixture: granulates, sand and cement paste. Theses simulations show that the « transient thermal strain», noticed during experimental tests [5]), is a structural effect and is due to heterogeneous nature of concrete at mesoscopic scale