Flexural fatigue of high-strength plain and fiber-reinforced concreteinfluence of mesostructure and study of size effect

Abstract

Equal fatigue tests in concrete, meaning those performed on exactly equal or homothetic specimens subjected to the same stress levels, present two problems yet to be solved. On the one hand, the high dispersion of the results, which causes the fatigue life in two apparently identical elements to vary by up to two or more orders of magnitude. On the other hand, the size effect, whereby fatigue strength decreases with increasing element size. The aim of this research work is to study these two issues in depth. Regarding the first one, the influence of the dispersion of concrete mesostructure on the dispersion of fatigue results will be studied; in particular, it will be evaluated whether the stochastic arrangement of fibers (in fiber concrete) or pores (in plain concrete) explains the variability of fatigue life. As for the size effect, fatigue tests will be performed on geometrically similar specimens subjected to the same equivalent stress levels. Macroscopic damage indicators, such as crack opening, will be used to determine the magnitude of the size effect and its variation according to the type of concrete (plain concrete and fiber-reinforced with different fiber contents). The results reveal that the arrangement of fibers and pores varies significantly in apparently identical specimens, and that this partly explains the dispersion of fatigue results. However, the estimation of the fatigue response of concrete from its mesostructure requires complex models, comprising several geometrical parameters of various components. In this work, a methodological approach to the problem is made, proposing mesostructure parameters that are reasonably predictive. With respect to the size effect, it is observed that the presence of fibers reduces the size effect on fatigue life, almost nullifying it. Furthermore, it is concluded that the secondary crack opening rate (dCMOD/dn) is an adequate parameter to explain the fatigue life in general, and the size effect in particular.