Resilient behavior of unbound granular material: A review

When a vehicle moves over a flexible pavement structure, it generates loading and unloading cycles that produce recoverable (resilient) and permanent (plastic) deformations in the granular base and subbase layers, which are made of unbound granular materials (UGMs). The primary parameter used to evaluate the resilient response of UGMs in pavements is the resilient modulus (MR). The MR is widely used in calculating stress-strain states for flexible pavement design and as a control parameter during the construction process. It is also employed to understand the progression of distresses, such as fatigue cracking and rutting. The main objective of this study was to conduct a literature review on the resilient behavior of UGMs. This manuscript presents and describes the MR and the factors that influence it. It also outlines the evolution of the mathematical equations most commonly used to estimate and predict this physical parameter. Conclusions and recommendations for future research are provided at the end of the article. Despite the large amount of research done on the subject, the resilient behavior of UGM has not yet been fully understood. This is since these materials are highly heterogeneous and show nonlinear-anisotropic behavior under different cyclic loading paths and water contents. Likewise, these materials undergo different behaviors depending on their macro and microscopic properties (gradation, density, porosity, texture, mineralogy, particle geometry and orientation, temperature, among others). On the other hand, the main limitation of the mathematical equations is that their parameters are difficult to determine experimentally and are not constants of the material (they are state variables that can change with multiple factors). Additionally, these equations do not consider the boundary conditions to which UGM in pavements are exposed. Moreover, they are obtained from repeated load triaxial (RLT) tests, which cannot simulate the three cyclic stress components (vertical, horizontal, and shear) to which UGMs are subjected in a pavement. In recent years, there has been an increase in studies evaluating the use of recycled aggregates and the effect of temperature (particularly at subzero temperatures), but more research is still needed to reach definitive conclusions.