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• 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]

To achieve long-lasting, high-strength, and sustainable asphalt pavement, this study prepared a hybrid fiber asphalt mixture using micron-scale calcium sulfate whiskers (CSW) and millimeter-scale basalt fibers (BF). The response surface method was employed to optimize the mix design, develop a prediction model, and verify road performance. The preparation process was refined based on the optimal mix design. The findings revealed that the best overall road performance was obtained with 5 % CSW content, 6 mm BF length, and 0.32 % BF content. The mixture demonstrated optimal performance with CSW wet mixing and BF dry mixing. Additionally, testing the fiber asphalt mixture at the mixture level was found to be crucial, as results can vary from those obtained at the mastic level. This study provides an in-depth analysis of the hybrid fiber asphalt mixture's action mechanisms in material optimization, performance verification, and preparation processes, offering valuable insights for related research and engineering applications of hybrid fibers. • A high-strength hybrid fiber asphalt mixture was prepared. • Fiber-modified asphalt requires mixture-level testing. • Optimal results were achieved with 0.32 % BF and 5 % CSW. [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 DEM model was established by randomization methods. • The material random field and initial defects were introduced to characterize the heterogeneous. • Nanoindentation test was adopted to capture the actual micromechanical distribution of mortar. • A half-normal distribution was used to construct the material random field of mortar. • Discrete Fracture Network was utilized to characterize the microscopic initial defects. To investigate the mesoscopic fracture of heterogeneous cement-treated base (CTB) materials, a mesoscale numerical model was established through discrete element method (DEM) and randomization algorithm. The proposed model was composed of aggregate, mortar, interface transition zone (ITZ), pores and initial defects. Combined with nano-indentation test, a half-normal distribution function was used to characterize the micromechanical heterogeneity of mortar and ITZ. A discrete fracture network (DFN) was used to characterize the initial microscopic defects in mortar hardening. Then the heterogeneous DEM model and DFN were integrated, and mechanical properties of CTB and failure mechanism were investigated on meso-level. The influence of material random field and microscopic initial defects on cracking behavior was further studied. The simulation results showed that the failure of CTB was mostly due to the accumulation of damage at mesoscopic level. Material heterogeneity results in stress concentration and induces microcracks. Initial defect was negative to structural strength but would increase the allowable failure deformation. The random particle model combined with microscopic initial defects could better characterize the mesoscopic cracking process of CTB materials. The present research provided a new numerical method for investigating the progressive failure process and the failure mechanism of cement-treated composites. [ABSTRACT FROM AUTHOR]

• Nanoindentation was conducted to evaluate the micro-scale modulus of mortar and ITZ of CTB. • SEM and EDS tests were adopted to study the heterogeneous characteristics of CTB. • The thickness of ITZ was estimated for CTB with a service period of 2 years. • The difference of micro-properties between CTB and cement concrete was analyzed. Compared with ordinary cement concrete, cement-treated base (CTB) material consumes much less cement and water, and its mechanical properties are also significantly different. The macroscopic properties of CTB are determined by microstructure. In order to understand the mechanical performance of in-service CTB, it is of great importance to investigate its microscopic structure and properties. In the study, the microscopic mechanical, structural, and chemical characteristics of the mortar and interfacial transition zone (ITZ) of in-service CTB were investigated through micro-scale characterizations. The results indicated the aggregate-mortar interface of in-service CTB is much weaker than that of concrete. A large number of micro-cracks and micro-pores were distributed in the ITZs and mortar. The micro-scale modulus of ITZ was approximately 85.6% of the modulus of the mortar and showed high variance. More compacted cement paste structure is beneficial to improve the mechanical properties of CTB. [ABSTRACT FROM AUTHOR]

This study prepared foamed waste oil-activated rubberized asphalt (ORA) by activating crumb rubber (CR) with waste vegetable oil (WVO) to address the issue of undesirable foaming effect. Optimal foaming parameters for different rubberized asphalt were determined based on foaming characteristic experiments. Furthermore, rheological properties, fatigue resistance, and adhesion performance of various unfoamed or foamed rubberized asphalt were evaluated through dynamic shear rheometer, bending beam rheometer, pull-out tests. Finally, the analytic hierarchy process was utilized to select the optimum foamed rubberized asphalt. The results show that foaming pressure has a great influence on rubberized asphalt foaming effect, and is recommended at 300 kPa. Compared to conventional rubberized asphalt whether foamed or not, with the increase of activation degree, low-temperature performance, and fatigue resistance of ORA are significantly ameliorated, and the adhesion property of ORA first increases and then decreases, but detrimental to high-temperature performance. The foaming process and foaming agent addition are conducive to low-temperature performance but unfavorable to fatigue resistance and high-temperature property, and have little effect on adhesion performance. ORA (mass ratio of WVO to CR is 1:5) with foaming agent is determined as the optimum selection. • The foaming effect of rubberized asphalt is greatly affected by the foaming pressure. • Foamed ORA is prepared to address the undesirable foaming effect of rubberized asphalt. • CR activation can effectively enhance the foaming effect, fatigue resistance and low-temperature performance of foamed RMA. [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]

Emulsified asphalt cold recycled technology has received great attention for its low carbon and environmental protection. There have been many researches on the performance improvement of emulsified asphalt cold recycling mixture (CRME) and its achievements, but there lacks systematic sorting. Based on this, this study summarizes the related research from the performance formation mechanism and influencing factors. Firstly, the demulsification mechanism of emulsified asphalt and the performance formation mechanism of CRME are summarized. Secondly, the influence of asphalt material (type and content), aggregate type (new aggregate, RAP), gradation, additives (regenerant, cement, fiber, etc.), water consumption and fabrication methods on CRME performance is analyzed. Finally, the future development of CRME is prospected. In the future, the fusion characteristics of old and new asphalt on RAP surface can be studied from multi-scale perspective. Besides, we can combine genome to construct CRME material library and improve the preparation technology standard of CRME pavement. • The development status of emulsified asphalt cold recycling technology was reviewed. • The influence of different factors on the strength of recycled mixture was analyzed. • The optimal dosage range of different influencing factors was summarized. • The development of emulsified asphalt cold recycling technology was prospected. [ABSTRACT FROM AUTHOR]

• Representative models of waste cooking oil at different stages are determined. • When the triglyceride is converted to free acid, the positive electrostatic potential, and cohesive energy density increase. • After chemical treatment, waste cooking oil can improve rejuvenation effects on fraction free volume and diffusion characteristics of aged asphalt. Waste cooking oil (WCO) has received more and more attention as an asphalt rejuvenator. The properties of WCO itself will change with the use of time, affecting its rejuvenation effect on aged asphalt. This paper explored the property changes of WCO with the cooking process and the rejuvenation effect on aged asphalt through molecular dynamics (MD) simulations and density functional theory (DFT) calculations. First, the representative molecules of WCO at different stages were determined. The polarity, basic properties, and solubility parameters of WCO at different stages were discussed. Finally, the thermodynamics properties, binding energy, free volume, and diffusion characteristics of WCO rejuvenated asphalt at different stages were compared. With the cooking process, the triglyceride (TG) is converted into fatty acid (FA), positive maximum electrostatic potential and electrostatic interaction increase, FA has stronger adsorption capacity leads to the reduction of model energy. After chemical treatment, the dipole moment and cohesive energy density of treated fatty acid (TFA) decrease. WCO at different stages can restore asphalt properties, and the surface free energy of asphalt can be better restored by FA. WCO at different stages mainly affects non-bond energy. Compared with FA, the incorporation of TFA has a better rejuvenation effect on the free volume fraction and diffusion coefficient of aged asphalt. The diffusion coefficients in asphalt are respectively: TFA > TG > FA. [ABSTRACT FROM AUTHOR]

• A 2D meso-scale crack propagation model of CTB was developed through FEM with cohesive elements. • CTB was assumed as a three-phase material composed of aggregate, cement mortar and ITZ. • DIP and vectorization processing were utilized to create a 2D heterogeneous numerical model. • Zero-thickness cohesive elements with traction-separation laws were used. • Nano-indentation tests and inverse analysis were employed to determine parameters. Although much attention had been paid to the study of macroscopic performance of Cement Treated Base (CTB), the meso-scale study of this heterogeneous and quasi-brittle material is rather limited. In this paper, a two-dimensional (2D) meso-scale crack propagation model of CTB assumed as a three-phase material composed of aggregate, cement mortar and interface transitional zones (ITZ) was developed through Finite Element method (FEM) with cohesive elements. Digital image processing (DIP) methods and vectorization processing were utilized to create a 2D heterogeneous numerical fracture model using the digital image obtained through scanning. The zero-thickness cohesive elements with traction-separation laws were inserted into both the cement mortar and the aggregate–cement interfaces to simulate the potential micro-crack in the cohesive zone model (CZM). Nano-indentation tests and inverse analysis were employed to determine the material parameters of model. Different cohesive parameters were obtained for the crack propagation of FE models to study the meso-scale material properties. Analysis results showed that the load-displacement curves, crack geometry, and tensile strength based on the numerical simulation agreed with those obtained from laboratory tests. [ABSTRACT FROM AUTHOR]