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.

Brott i aggregat i obundna granulära material (OGM) under asfaltbeläggningslager påverkar vägbeläggningens prestanda och kan påskynda skador. Därför är det viktigt att kunna bedöma aggregatens brottmotstånd. I denna studie introduceras ett nytt simuleringsramverk kombinerat med en experimentell studie för att förutsäga brott i aggregat i OGM och kvantifiera dess påverkan på OGMs makromekaniska beteende. Det utvecklade ramverket baseras på simuleringar med diskret elementmetod (DEM) och möjliggör utvärdering av brott i aggregat för varierande storleksfördelningar, belastningsförhållanden och aggregattyper. För att säkerställa metodens generella tillämpbarhet har kontaktlagar baserade på kontakmekanik och brottsmodeller utvecklats och integrerats i DEM.För att identifiera materialparametrarna och validera modellen genomförs inneslutna enaxliga kompressionstester på OGM bestående av olika aggregattyper där OGM med olika graderingar testas vid olika kompressionsbelastningar. För OGM bestående av krossade granitaggregat visas att DEM-modellen, som inkluderar en partikelbrottsmodell baserad på Weibulls weakest-link teori, väl fångar de experimentella observationerna. Specifikt fångar DEM-modellen noggrant effekterna av gradering och belastningsmängd på OGMs makromekaniska respons samt på OGMs prestanda i förhållande till aggregatbrott.För att noggrant fånga mekaniken för aggregatbrott för ett brett spektrum av aggregattyper, särskilt för aggregat med marginell kvalitet, har en ny partikelbrottsmodell utvecklats. Denna modell tar hänsyn till både variationen i aggregatens form och den statistiska volymeffekten på variationen av brottkrafter för enskilda aggregat. Den nya brottsmodellens förmåga att fånga brottkrafter hos enskilda aggregat testas genom krossningstester på enskilda partiklar utförda på fyra olika aggregattyper,ivoch den nya modellens predikterbarhet jämförs med två andra vanligt använda brottsmodeller. Resultaten visar att för alla undersökta aggregattyper ger den nya brottsmodellen en bättre överensstämmelse med de experimentella data än de två andra modellerna. Dessutom integreras den nya brottsmodellen i DEM och dess prestanda i förhållande till att fånga aggregatbrott i OGM undersöks på OGM som består av aggregat med marginal kvalitet. Det visas att integrering av den nya brottmodellen i DEM förbättrar noggrannheten i de numeriska förutsägelserna.Vidare utvärderas genomförbarheten av att använda DEM-modellen för att bedöma konsekvenserna av aggregatbrott på OGMs makromekaniska prestanda i termer av elastisk styvhet och beständighet mot permanent deformation. Fokus ligger på OGM innehållande aggregat med marginellt brottmotstånd och genomförbarheten av att använda DEM-modellen för att optimera vägbeläggningens strukturella design för att möjliggöra inkorporering av marginalaggregat utan att kompromissa för mycket med prestandan.De experimentella och numeriska resultaten som presenteras i denna avhandling indikerar att den utvecklade DEM-modellen är ett värdefullt verktyg för att förstå och kvantifiera effekterna av OGM-materialparametrar, såsom aggregattyp och gradering, samt belastningsförhållanden på OGMs prestanda, särskilt med avseende på aggregatkrossning. Det kan konstateras att den utvecklade modellen erbjuder en betydande potential för att optimera val och sammansättning av OGM-material samt den strukturella designen av vägbeläggningen för att minska aggregatkrossning.

Fracture of rock particles is important inmany applications like mining, mineral comminution,unbound granular materials (UGMs) for railwayand road structures. The latter application is themain interest presently, as fracture of rock particles inUGMs affects the UGMs performance and may compromisestructural integrity of a pavement, potentiallyleading to premature road failures. Therefore, it isimportant to assess their resistance to aggregate fractureaccurately. In this study, a new statistical fracturemodel for particle fracture, based on the results ofsingle particle crushing tests, is introduced to investigateaggregate fracture. The proposed model istested for UGMs composed of three different aggregatetypes: brick, granite and a volcanic material andits results are compared with other widely used fractureforce models. The performance of the models isalso investigated by simulating uniaxial monotoniccompression tests on UGMs with different aggregatesize distributions using the Discrete Element Method(DEM) and comparing the results with experiments.Fracture at two different load levels for three differentparticle size distributions are investigated for eachmaterial. One particle size distribution at one loadlevel is used to identify the contact law parametersfor each material, and single particle breakage testare used to identify the fracture force model parameters.The DEM models with a new fracture forcemodel agrees well with the macro-mechanical behaviourobserved in experiments and exhibits the highestdegree of correlation to fracture results obtained fromexperiments.

Adequate aggregate resistance to crushing and abrasion is crucial for good performance of unbound road layers, in particular when incorporating marginal aggregates in road construction. In this study, aggregate crushing in unbound granular materials (UGMs) is investigated experimentally with uniaxial compression tests, and numerically with discrete element method (DEM). A DEM model of UGMs subjected to uniaxial monotonic compression is implemented in a commercial DEM software, incorporating granular mechanics-based particle-contact and breakage laws. Based on comparison with the experimental findings, it is shown that the implemented model captures the UGMs behavior well including aggregate breakage characteristics. The model is used furthermore to evaluate aggregate crushing in UGMs subjected to cyclic loading representative for unbound road layers of low-volume roads. The feasibility of using the developed DEM approach for evaluating the implications of using marginal UGMs in unbound road layers and for optimizing structural design of those layers is discussed.

Aggregate breakage in unbound pavement layers can lead to pavement distresses that affect their functionality and service life. Thus understanding the mechanics and clarifying the factors affecting materials breakage resistance are important for ensuring adequate performance of these layers. In this study, aggregate breakage in unbound granular materials (UGM) is investigated experimentally and numerically. Experimentally, aggregate breakage under uniaxial compression is examined for two UGMs prepared with the same aggregate type but different gradations. To capture the experimentally observed influence of gradation and load magnitude on aggregate breakage, a Discrete Element Method (DEM) model was developed, based on granular mechanics particle contact and failure laws. A simple procedure to identify the contact and failure law parameters from experiments is proposed. With those parameters, the model’s capability of capturing the effect of gradation and loading on the aggregate breakage in UGM is evaluated. Based on comparison with experimental findings, it is shown that the model can capture macro-scale properties of UGM, such as its deformation response under uniaxial compression, as well as the amount of aggregate breakage in the material.