Dr. Frederic Grondin, Professor, Institut de Recherche en Génie Civil et Mécanique, Ecole Centrale Nantes, France

Title: Multiscale Modeling for the Long-Term Creep of Concrete Considering the Interfacial Transition Zone Surrounding Aggregate

Abstract: This work aims to analyse the influence of the weak area surrounding aggregate in concrete, called Interfacial Transition Zone (ITZ), on the long-term creep behaviour of concrete. To simulate the behaviour of concrete, many researchers regard concrete as a two-phase material consisting of mortar matrix and aggregates. However, there is another phase called ITZ which is considered as the weakest zone in concrete and to be responsible for its fragility. ITZ is formed during the mixing process of concrete: before adding water in the mixture, cement grains are in contact with aggregates and it is maintained for some of them after adding water and during the mixing. So, there is a small area around aggregate where the hydration of cement is different from that happening at a certain distance from aggregate. In most cases, this area shows lower mechanical properties due to higher porosities. So, under loading, micro-cracks can appear easier around aggregate in concrete. Also, concrete is a viscoelastic material and shows delayed deformations under a constant load which can causes cracking. The micro-cracks caused by creep can be detrimental to a good state of work of concrete. In civil engineering, the understanding of creep behaviour of concrete is of great interest. For a long period (up to several dozen years), it is impossible to do experimental creep tests and numerical models can be a feasible way. The main theory of the creep of concrete considers that the main hydrate of the cement paste, Calcium Silicate Hydrate (C-S-H), is strongly linked to the viscoelastic behaviour (Bažant and Prasannan, 1988a). The experimental tests showed that it deforms under a constant load and these deformations are restrained by the presence of aggregates, which cause the creation of micro-cracks in the cement paste.