Thermal contraction is a key factor in low-temperature cracking, contributing to internal stresses in the bitumen-aggregate composite. Most macromechanical models treat mastic as a continuous material, limiting an in-depth analysis of the component interactions, which is essential for improved material design. This study analyses the low-temperature behaviour of bitumen and mastic containing different basalt filler content using experimental testing and micromechanical finite element modelling (FEM). The model evaluates micromechanical interactions between bitumen and aggregates, with aggregates modelled as spherical particles in the bitumen. Thermal contraction coefficients are predicted via viscoelastic modelling and compared to experimental results. Findings show higher filler content lowers the thermal contraction coefficient while increasing stress concentrations due to the combined thermal properties of bitumen and filler. The micromechanical model aligns well with experimental data, confirming its reliability in predicting stress distribution and thermal behaviour. These insights enhance the understanding of bituminous materials in cold environments.

Permanent deformation in ballast layers is a major contributing factor to the railway track geometry deterioration. In spite of a considerable amount of research on understanding and predicting performance of ballast layers, accurately capturing their settlements remains a challenge. In order to contribute to solving this important issue, a new numerical method for predicting ballast settlements is presented in this paper. This method is based on the finite element (FE) method combined with a constitutive model that captures permanent deformation accumulation in unbound materials under cyclic loading. This allows predicting permanent deformations of large structures and at large number of load cycles in a computationally efficient manner. The developed constitutive model is validated based on triaxial test measurements over wide range of loading conditions. Stress state in ballast layers has been examined with a 3D FE model, for several embankment structures and traffic load magnitudes. The determined stress distributions and loading frequencies were used as an input of the constitutive model to evaluate permanent strains and settlements of ballast layer. The influence of embankment structural designs and traffic loading magnitudes on the ballast layers settlements is examined and the results obtained are compared with the existing empirical performance models.

Low-temperature cracking is one of the most common failures in asphalt pavements, especially in cold regions. Accordingly, considerable amount of research has been performed in order to understand the low-temperature cracking mechanisms and to propose test methods for characterizing and determining cracking performance of bitumen and asphalt mixtures under freezing conditions. The existing test methods, however, require expensive equipment and skilled technicians; they are thus not well suited for routine tests. As a contribution to mitigate this situation, this study intends to investigate experimentally and characterize numerically the low-temperature cracking behavior of bitumen and mastic materials using a refined thermal cracking test method. The proposed method, the annular restrained cold temperature induced cracking (ARCTIC) test, allows to determine the low-temperature cracking properties of the mastic and bitumen with a relatively simple setup. In this paper, finite element (FE) modeling is used for evaluating the effect of test parameters on the temperature, stress and strain gradients induced in the specimen during the test. The ARCTIC test is employed to measure cracking temperatures of two bitumen and two mastic materials. The measurements repeatability is examined and the effect of bitumen type on the thermal cracking potential of bitumen and mastic is evaluated. FE modeling is employed to examine the effect of thermomechanical parameters on thermal cracking performance of the materials and to back-calculate fracture stress and strain from measurements. The results highlight the potential of the proposed test and analysis method for evaluation of low-temperature cracking in bitumen and asphalt mastic.

Asphalt concrete (AC) exhibits significant tension-compression (TC) asymmetry, which is currently not considered in pavement design. This study develops a novel temperature-dependent dual viscoelastic model to quantitatively capture the viscoelastic behaviour of AC. Unlike the conventional viscoelastic constitutive model, the proposed model decomposes strain into tensile and compressive components to characterise AC’s TC asymmetry. Additionally, a systematic modelling framework with intrinsic TC asymmetry is developed for the first time to predict the response of pavement under moving tire load. The results illustrate that implementing the proposed dual viscoelastic model enlarges both the vertical deformation of pavements and the tensile and shear strains in the AC layers, bringing it closer to the realistic scenario compared to the conventional model that only considers compression properties. Furthermore, high temperatures and low vehicular speeds exacerbate the substantial effects of AC’s TC asymmetry on asphalt pavement. This study provides a valuable method to capture AC’s TC asymmetry and predict pavement response more accurately, giving better insight into pavement response and enhancing pavement design and maintenance.

This research provided quantitative evidence on the microstructural transformation due to the phase separation in epoxy-modified bitumen in the domain of epoxy contents from 10 to 90 % by confocal fluorescence microscopy (with the effective pixel size of 21.5 μm) as the systems approached their ultimate state of hardening. The image analysis of phases representing bitumen and epoxy included particle size distributions and spatial arrangement (of discrete droplets) and the local thickness distributions (of percolated structures). The results showed that these systems underwent substantial transformed from a solution of epoxy droplets in bitumen to a quasi-bimodal dispersion of bitumen within an epoxy networked structure, with the catastrophic phase inversion taking place between 40 and 50 % of epoxy. The morphology of phases and their spatial arrangement confirmed that mechanical interactions determined the packing of hyperdispersed droplets (as the systems approached the phase inversion), while the relaxation of surface tensions weakened the interlocking and eased the movability of droplets (far away from the inversion point). This contributed to understand how the microstructure directly resulted from the interfacial tensions between phases. Transformation in the phase inversion zone and the interfacial diffusion, including spectroscopic techniques, were recognised as particularly relevant for further research.

Semi-flexible pavement (SFP) material is a composite comprising cement, coarse aggregates and asphalt mortar, which has complex mechanical properties. Traditional experimental methods struggle to accurately quantify the effect of each phase and their interfaces on the SFP's mechanical properties. Micromechanical modelling based on finite element method offers a promising solution. In this study, a new micromechanical model for SFP is proposed, idealizing the material by representative volume elements. SFP mesostructure is represented as a simplified five element composite consisting of cement, asphalt mortar, aggregate, pore and cement-asphalt mortar interface. Periodic boundary conditions are used to simulate an infinite repetitive structure within a finite computational domain. The resulting model allows evaluating the stiffness and damage resistance of SFP in a computationally efficient manner. This model is utilized to explore the mechanical properties of SFPs and the results are compared with the experimental findings. The results show that the model captures the uniaxial compressive strength and stiffness for all materials examined. The model is further used to evaluate the effect of properties of individual elements of SFP on its stiffness and strength. The feasibility of using the proposed modelling approach to optimize the material design of SFP is discussed.

Low-temperature cracking significantly affects durability of asphalt pavements. This research addresses the role of the mastic phase by experimentally evaluating the low-temperature cracking performance of selected bitumen and mastics with different filler types and contents. Their thermal contraction coefficient (αT), low temperature viscoelasticity, and strength properties are measured using standard tests like the dynamic shear rheometer (DSR) and fracture toughness tests (FTT). An enhanced laboratory technique, the annular restrained cold temperature induced cracking (ARCTIC) test, is employed to study combined thermal and mechanical effects on low-temperature cracking. The alignment of FTT with ARCTIC results highlights a good correlation between these methods for mastics with nearly the same αT. However, the insensitivity of FTT to αT raises concerns about its applicability to materials with significantly different αT, as it may not capture accurately their performance. The ARCTIC test is free of such a problem and even shows a higher sensitivity to the mastic composition. The findings demonstrate that the addition of filler significantly affects the resistance of mastic to low-temperature cracking by altering its αT. Additionally, the filler content, type, and gradation distinctly impact the thermal and mechanical characteristics of mastics, enhancing their ability to withstand lower temperatures more effectively than bitumen. In particular, adding 50 % by volume of different filler types reduces the αT of mastic by 45–60 % and results in 3–10 °C reduction in cracking temperature (Tcr) measured with the ARCTIC test.

The stiffness and failure properties of cement-asphalt mastic-aggregate (C-AM-A) interface are among the most important factors affecting the performance of pouring semi-flexible pavement materials (SFP). Therefore, determining the characteristics of C-AM-A interface are essential for guiding the design of SFP from the perspective of interface enhancement. In this study, the failure characteristics of C-AM-A interfaces are examined experimentally and numerically. Firstly, the effects of temperature and the proportion of cement substituting limestone filler on the bonding strength of C-AM-A interface are analyzed via pull-off tests. Then, an innovative test method based on a three-point bending test of C-AM-A beam is proposed to investigate the influence of test temperature and asphalt mastic type on the fracture characteristics of the C-AM-A interface. Finally, based on the cohesive surface techniques, numerical modeling of C-AM-A beam under three-point bending was applied to study the effect of cohesive parameters on the interfacial fracture characteristics. The results show that the interface bonding strength decreased significantly with the increase of temperature. Using cement as a filler improves the bonding strength, fracture strength, stiffness and fracture energy of C-AM-A interface as compared to the case when limestone filler is used. As the temperature increases from -10 degrees C to 20 degrees C, the failure mode of the C-AM-A interface first alters from adhesive failure to mixed failure mode, and then to cohesive failure. It suggests that enhancing the adhesion of C-AM-A interface is more advantageous for improving the fracture resistance at low temperatures, while increasing the interface cohesion is more important at relatively high temperatures. The laboratory test methods, the numerical model and methodology developed in this study are useful to study the failure behavior of C-AM-A interface and optimize the performance of SFP from the perspective of interface enhancement.

Interlayers in asphalt pavements are potential structural damage initiators. In order to better understand the quantitative role of interlayer parameters, such as surface roughness, binder type, binder content and loading type on interlayer shear strength, this paper focuses on the effects of particle interlock and contact conditions on interlayer strength through experimental and numerical modelling. Experimentally, interlayer shear box strength tests on a model material consisting of stiff binder blended with steel balls are performed with and without normal force confinement. A Discrete Element method model of the test is developed using measurements of the model material for calibrating the contact law and for validating the model. It is shown that this model captures adequately the measured force-displacement response of the specimens. It is thus a feasible starting point for numerically and experimentally studying the role of binder and tack coat regarding interlayer shear strength of real asphalt layers.

Asphalt concrete (AC) exhibits significant tension–compression (TC) asymmetry and aggregate contacts can be one of the critical contributors to this behavior. Nevertheless, the underlying mechanisms are still unclear, and there has been no study to quantify this behavior. To fill the research gap, multiscale characterization and modeling on AC were performed in this study. At the microscale level, nanoindentation tests were conducted to characterize the aggregate contact characteristics in the contact region (CR). The CR was found to have a sandwich-like structure consisting of two interfacial layers, large filler particles, and asphalt mastic. Accordingly, micromechanical models of CR were developed to predict its mechanical behavior in tension and compresison (T&C). The modeling results showed that aggregate contacts significantly increase the compressive modulus, leading to the substantial TC asymmetry of CR. The predicted viscoelastic properties of CR were further applied to the developed mesostructural model of AC. The predicted master curves in T&C showed significant asymmetry and quantitatively agreed with the experimental ones, demonstrating the effectiveness of the adopted modeling approaches. This study is the first study to quantify the asymmetric performance of AC. The outcomes can be applied to evaluate AC's TC asymmetry effects on pavement performance.