A New Numerical Framework for Aggregate Fracture in Unbound Granular Materials

QC 20250903

Abstract

Aggregate fracture in unbound granular materials (UGM) below asphalt pavement layers affects the pavement performance and may accelerate pavement distresses. Therefore, assessing the fracture resistance of the aggregates is important. In this study, a new numerical framework is introduced combined with an experimental study to predict aggregate fracture in UGMs and quantifying its influence on UGMs macro-mechanical behaviour. The developed framework is based on discrete element method (DEM) and allows to evaluate aggregate fracture for varying gradations, loading conditions and aggregate types. In order to ensure general applicability of the framework, granular mechanics-based contact laws and fracture models are developed and incorporated into DEM.To identify the material parameters and to validate the model, confined uniaxial compression tests are conducted on UGMs composed of different aggregate types where UGMs with different gradation are tested at different compressive loading magnitudes. For UGMs composed of crushed granite aggregates, it is shown that the DEM model, incorporating a particle fracture model based on Weibull weakest link theory captures the experimental observations well. In particular, the DEM model captures accurately the effects of gradation and load magnitude on the macro-mechanical response of UGMs, as well as on UGM performance regarding aggregate fracture.To accurately capture the mechanics of the aggregate fracture across a wide range of aggregate types, particularly for marginal quality aggregates, a new particle fracture model is developed. This model considers both the variability in aggregate shape and the statistical volume effect on fracture force distribution of individual aggregates. The capability of the new fracture model to capture the fracture forces of individual aggregates is tested through single particle crushing tests conducted on four differentiiaggregate types, and its performance is compared with two other widely used fracture models. The results show that, for all examined aggregate types, the new fracture model provides a better fit to the experimental data than the other two models. Furthermore, the new fracture model is incorporated into DEM and its performance with respect to capturing aggregate fracture in UGMs is investigated on UGMs composed of marginal quality aggregates. It is shown that incorporating the new fracture model into the DEM improves the accuracy of computational predictions.Furthermore, the feasibility of using a DEM model to evaluate the implications of aggregate fracture on UGMs macro-mechanical performance in terms of elastic stiffness and permanent deformation resistance is evaluated. The emphasize is given to UGMs containing aggregates with marginal fracture resistance and the feasibility of using the DEM model optimize pavement structural design to allow incorporation of marginal aggregates without excessively compromising performance is evaluated.The experimental and numerical results presented in this thesis indicate that the developed DEM model is a valuable tool for understanding and quantifying the effects of UGM material parameters, such as aggregate type and gradation, and loading conditions on UGM performance, particularly with respect to aggregate crushing. It was found that the developed model offers a significant potential for optimizing UGM material selection and composition as well as structural designs of roads, to mitigate aggregate crushing.

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