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Fatigue failure of cement-stabilized macadam (CSM) base has always been a concern in highway construction. The microscopic initial defect is an important factor affecting its fatigue performance. The objective of this paper is to investigate the fatigue cracking of CSM materials under cyclic loading through numerical simulation and to analyze the influence of microscopic initial defects. The mesoscale random particle model was established using the discrete element method (DEM). The discrete fracture network (DFN) was used to characterize the microscopic initial defects in cement mortar. A parallel bond evolution method was also developed to simulate the time-dependent fatigue damage under cyclic loading. Then virtual semicircular bending (SCB) fatigue tests were carried out to reproduce the mesoscopic fatigue cracking. The results show that the established mesoscale model can accurately simulate the evolution of microcracks. The fatigue damage presents nonlinear accumulation, and the interface is a weak area of fatigue failure. Through optimizing the cracking propagation path, initial defects induce the penetration crack and reduce the fatigue life. The large-size defects should be more concerned. Fractures with a radius greater than 0.15mm should be controlled within the density range of 20 m/m2, which is beneficial to improve the anti-fatigue performance of CSM. [ABSTRACT FROM AUTHOR]

• Fatigue damage under cyclic loading was simulated based on the parallel bonding evolution model. • The fatigue damage model was established with residual strength variables. • A residual fatigue-life prediction model was proposed based on strength degradation. Fatigue damage is the process of material fatigue failure caused by cyclic loading, and the development of damage degrades the material properties. This paper aims to investigate the fatigue degradation law of cement-treated base (CTB) materials using a discrete element method (DEM) simulation technique. A fatigue evolution model was adopted to characterize the time-dependent fatigue damage under cyclic loading. Virtual semi-circular bending (SCB) fatigue and residual strength tests at different fatigue stages were performed on mesoscale DEM model to evaluate the fatigue damage of CTB material. With residual strength variable, the fatigue damage model was established to reveal the accumulation of damage. The results show that the residual strength evolution nearly appears an exponential correlation with the fatigue stages. In the first stage, the strength degradation is stable and appears as approximately linear. At about 80% of the fatigue life, the degradation rate is significantly accelerated. The residual strength degradation rate is related to the initial stress level and decreases with the increase of stress ratio. At the same fatigue life stage, the greater residual strength attenuation will be generated with a lower load level. The fatigue damage evolution model indicates the fatigue damage accumulation approximately linearly in the early stage and rapidly in the late stage. The late fatigue stage has the main contribution to fatigue failure. The degradation law demonstrates the lower the load level is, the more the damage accumulation at the same fatigue life stage will be, and the corresponding critical damage is greater. Moreover, based on the strength degradation law, a prediction model is established to evaluate the residual fatigue life of pavement base material. [ABSTRACT FROM AUTHOR]

As a sustainable solution, embedding rubber inclusions in the semi-rigid pavement base course can ensure the consumption of a large amount of waste tire pollution and improve the mechanical properties of cement-treated base materials. The rubber modification has a potential effect on reducing the possibility of pavement reflective cracks caused by base cracking. This paper aims to investigate the meso-cracking characteristics of rubberized cement-stabilized aggregate (RCSA) mixtures and to quantitatively evaluate the influence of rubber inclusion content and particle size through numerical simulation. The rubber particle size of 0.6–4.75 mm was selected. The virtual RCSA samples were prepared by replacing similar-size aggregates with rubber particles based on the reference mixtures without rubber. Five volumetric replacement contents (20%, 40%, 60%, 80% and 100%) of three gradation ranges (0.6–1.18 mm, 1.18–2.36 mm, and 2.36–4.75 mm) were adopted. An indirect diametric tensile (IDT) simulation test was adopted to obtain the investigated performance of IDT strength, static stiffness, toughness and toughness index. The simulation results show that the incorporation of rubber inclusion has a negative effect on the IDT strength of the RCSA sample. However, material extensibility and post-peak toughness are improved and stiffness is reduced with increasing rubber replacement percentage. The optimal rubber content of the investigated mixtures should be controlled within 80% of the corresponding aggregate gradation volume, which is conducive to controlling the reduction of the overall strength and the play of the extensibility. The strength reduction and ductility improvement of modified concrete are positively correlated with the particle size of rubber inclusion. The modified RCSA mixture with a smaller rubber particle size could ensure a higher IDT strength and extensibility. For the investigated sample, the best comprehensive modification effect can be obtained using the finer rubber substitute with a particle size of 0.6–1.18 mm. The rubber inclusion acts similarly to a cavity defect, which will result in fracture preferring to occur adjacent to the rubber aggregate. Ignoring the adverse effect of strength, the practical implication of rubber incorporation is to improve the ductile behaviour of the RCSA mixture and reduce base cracking. The present numerical simulation methods have the potential to investigate the meso-cracking mechanism of RCSA. It also provides a theoretical guidance for the gradation design of rubberized mixtures. [ABSTRACT FROM AUTHOR]

• Integrated with the laboratory NSCB fatigue test, a FDEM was established. • The DEM model and loading system of NSCB were constructed using software PFC5.0. • Experiments were conducted to obtain the model parameters through an inverse analysis. • The decay-rate law was adding into the LPBM and implemented in the DEM model. • The PBEM was used to capture the fatigue behavior and cracking process in CTB. Fatigue failure of the cement-treated base (CTB) is always paid more attention by highway constructors. Meso-scale cracking evolution plays an essential role in the research of fatigue deterioration mechanism, which is hard to be characterized using experimental techniques alone. The objective of this paper is to investigate the fatigue behaviour of CTB materials under cyclic loading through the discrete element method (DEM). A parallel bond evolution model (PBEM) was developed to simulate the time-dependent fatigue damage. Integrated with the PBEM, a fatigue damage evolution model (FDEM) was established and implemented in the Particle Flow Code (PFC5.0) to describe the fatigue decay behaviour under cyclic loading. The fatigue modelling parameters were further calibrated by the notched semicircular bending (NSCB) strength and fatigue test. Then a series of virtual NSCB fatigue tests were conducted to study the decay law of CTB materials. The simulation results indicate that the fatigue damage accumulates in a nonlinear manner under cyclic loading and the steady crack growth is the main stage of fatigue failure. With the increase of stress ratio, the stable crack growth stage is shortened obviously. The established PBEM is capable of capturing the fatigue behaviour and cracking process of CTB materials. It provides a reliable numerical technique for modelling and investigating fatigue damage of cement-treated base materials. [ABSTRACT FROM AUTHOR]

• Mesoscale heterogeneous CTB model was established by discrete element method. • DIP method was adopted to capture the actual aggregate size and distribution. • Randomization method was used to randomly generate the mesoscale CTB specimen. • The effect of aggregate gradation and material properties on cracking were analyzed. The fracture of Cement Treated Base (CTB) is greatly influenced by its mesoscale structure and characteristics. The Discrete Element Method (DEM) was adopted to build a mesoscale Semicircular Bending (SCB) model of CTB. The DEM models were built based on two methods including the Digital Image Process (DIP) of actual aggregate distribution and randomization algorithm to generate aggregates. The mesoscale heterogeneity of the model consisted of aggregates, cement mortar, Interface Transition Zone (ITZ) and air voids. Analysis results showed that the DEM models based on both DIP and randomization with the Linear Parallel Bond Model (LPBM) presented satisfying results, compared with laboratory tests. It was found that the main cracks occurred along ITZ between mortar and aggregates. The gradation and distribution of aggregate, and air voids have all significant influence on the fracture resistance and the crack propagation of the CTB. The lime-ash treated material simulation results showed relatively higher fracture toughness over strength ratio. [ABSTRACT FROM AUTHOR]