Permanent deformation of unbound granular materials: A review

Several studies have been conducted in the pavement area to try to understand the elastoplastic behavior of unbound granular materials (UGMs) under cyclic loading. This paper presents a literature review on the permanent deformation phenomenon of UGMs when they form granular layers in flexible pavement structures. The factors affecting the resistance to permanent deformation are presented and described, and the evolution of the mathematical equations most used to predict the deformation in these materials under cyclic loading is shown. Conclusions and recommendations for future studies are presented at the end of the manuscript. Despite the substantial number of researches carried out on the subject, the elastoplastic behavior of UGMs has not yet been fully understood. These heterogeneous and anisotropic materials change their behavior mainly with stress magnitude and water content, but they are also strongly dependent on the way factors such as gradation, density, porosity, morphology, chemical and mineralogical composition, temperature, among others, interact. Generally, most of the mathematical equations that try to predict the accumulation of permanent deformation in UGMs are empirical, based on repeated load triaxial (RLT) tests, they cannot simulate the three components of cyclic stress to which a UGM is subjected in a pavement, their parameters are difficult to determine experimentally and do not show physical meaning (they are obtained by regression). In recent years, studies to evaluate the use of recycled aggregates and the effect of temperature (mainly at low temperatures) have increased. Likewise, the use of various technological tools such as artificial neural network (ANN) modeling, and so forth, has increased.