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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., QC 20240412

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., QC 20240208

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., QC 20240110

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., QC 20240527

Asphalt concrete (AC) shows significant tension-compression (TC) asymmetry, i.e., different properties in tension and compression (T&C). This asymmetry may profoundly affect AC's performance and deterioration in the field, but limited studies have been performed to quantify this behavior. This study aims to quantitively characterize the global and local mechanical responses of AC in T&C through numerical modeling. To this end, three AC mixtures: the gap-graded SMA10, dense-graded AC20, and open-graded mixtures PA13, were evaluated experimentally and numerically. Digital image processing was used to generate image-based AC models with contact regions (CR), and dynamic simulations were conducted using the steady-state dynamics (SSD) approach. The results indicated that the measured and predicted master curves for AC in T&C qualitatively agree and demonstrate significant asymmetry, with higher moduli but lower phase angles in compression compared to tension. Among the mixtures, PA13 exhibited the most pronounced asymmetry, followed by SMA10 and AC20. Statistical analyses of local stress and strain found that the stress and strain in different phases show significant variations, with more pronounced disparities observed at lower frequencies. Notably, at 10−6 Hz for PA13 in compression, the stress within the aggregate phase exceeded that of the matrix phase by over 250 times, while the strain within the matrix phase surpassed the aggregate phase by more than 600 times. To enhance pavement durability, it is recommended to consider AC's TC asymmetry in pavement design., QC 20231031

Aggregate contacts significantly affect the mechanical behavior of asphalt concrete. However, there still lacks an effective way to account for them in numerical modeling. Therefore, to address this concern, a new virtual-specimen-based modeling approach was developed in this study by simplifying aggregate particles in asphalt concrete as spheres and incorporating aggregate contacts through Contact Region (CR) elements. The complex moduli of both gap-graded and dense-graded mixtures were predicted using this approach and compared with those predicted using the conventional image-based modeling approach and laboratory-measured values. The virtual-specimen modeling revealed that the CR in the gap-graded mixture with a higher proportion of large aggregates can better transmit load among aggregates than in the dense-graded mixture. Both modeling approaches were found to provide good prediction accuracy, but the stress distributions in the virtual-specimen models were more uniform and continuous, leading to better computational convergence and the possibility of nonlinear analysis of asphalt concrete., QC 20230904

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., QC 20230707

Asphalt concrete (AC) is a composite material consisting of binder, aggregates and air voids. The quantitative effect of aggregate-to-aggregate contact on the mechanical performance of AC is an important and complex issue, which has not been fully understood yet. To fill this gap, this study aims to characterize the aggregate contacts in AC and evaluate their effects on the viscoelastic behavior of AC through micromechanical finite element (FE) modeling. To this end, 3D microstructural models were generated through digital image processing (DIP) method and aggregate contacts were captured in the model via contact zone (CZ) elements. A CZ model was proposed and verified by a parametric study to identify the viscoelastic properties of CZ elements, while the viscoelastic properties of matrix phase were determined through laboratory tests. Steady-state dynamic (SSD) analysis was then conducted to investigate the macro-scale viscoelastic response of AC. It was found that the proposed modeling approach captures the measured response accurately. Accounting for aggregate contacts results in higher predicted AC dynamic moduli and lower phase angles, thus improving the agreement between modeling and experimental results. The numerical model developed in this study provides a promising approach for investigating the effect of aggregate contacts on the mechanical performance of AC., QC 20220207

Pouring semi-flexible pavement (SFP) material is a multi-phase composite material with complex damage characteristics and strength mechanism. Therefore, revealing the shear properties and damage characteristics of SFP is essential for understanding the strength formation mechanism of SFP and guiding the design of SFP. Firstly, the effect of porous asphalt mixture (PAM) air void on the shear properties of SFP was analyzed by triaxial compressive tests, and the Mohr-Coulomb theory was applied to analyze the shear strength mechanism of SFP. Then, the pore and crack distribution characteristics of SFP specimens at different loading stages were investigated via X-ray CT. Finally, three damage parameters (D-V, D-C, D-E) were used to describe the damage degree of SFP specimens in different loading stages. The results show that increasing the air void of PAM can effectively improve the triaxial compressive strength and compressive modulus of SFP. The Mohr-Coulomb failure criterion can accurately describe the failure behavior of SFP at 60 C. Increasing the air void of PAM can increase the proportion of interlocking force provided by aggregate-cement contact, which in turn increases the internal friction angle of SFP. When the air void of PAM is 25 %, SFP has the largest proportion of cement -asphalt-aggregate (C-AS-A) interface area, leading to the largest cohesion force of SFP. Moreover, during triaxial compression, C-AS-A interface crack accounts for 63.47 %, which is the main failure mode of SFP. The plane porosity, volume and surface area of 3D pores and cracks of SFP specimens increase with the increase of vertical deformation. The D-E can more accurately characterize the damage status of SFP., QC 20220907

Filling grouting material into the voids of porous asphalt mixture (PAM) is one of the important steps in the construction of pouring semi-flexible pavement (SFP), and the pore characteristics play a dominant role in the slurry permeability performance of PAM. Therefore, understanding the influence of pore characteristics on the slurry permeability performance and revealing the penetration mechanism of grouting materials in the void of PAM are essential for guiding the design of PAM used in SFP and improving the quality and efficiency of the grouting construction. Firstly, the effects of porosity and aggregate size on slurry permeability and water permeability of PAM are experimentally studied. Then, the 3D pore structure parameters, such as porosity, equivalent diameter, coordination number, pore throat dimension and tortuosity are quantitatively and statistically analyzed via X-ray CT and 3D reconstructed model. Finally, the slurry permeability simulation of PAM is conducted. The results show that the porosity and aggregate size have a significant influence on the complexity and spatial anisotropy of the pore structure of PAM. Increasing the porosity and aggregate size can effectively increase the connected porosity ratio, 3D pore size and throat size of PAM, while reduce the number of small pores (D-3d < 1 mm(2)) and isolated pores. Besides, the slurry and water permeability coefficients of PAM are also positively correlated with the porosity and aggregate size. Moreover, to be consistent with the real condition, the slurry permeability coefficient is more suggested as the technical index of PAM to ensure the perfusion effect of PAM. In the simulation, the slurry permeability values in the vertical direction are higher than those in the horizontal direction. The pore characteristics are ranked according to their effect on the seepage of grouting material: connected porosity > pore size and throat size >> pore coordination number (CN) and tortuosity., QC 20220516

The indentation test is a promising technique for the viscoelastic characterisation of asphalt concrete (AC). Indentation measurements are primarily influenced by the material properties in the direct vicinity of the indenter-specimen contact point. Accordingly, it may become a useful alternative for the characterisation of thin asphalt layers as well as for a quasi-non-destructive AC characterisation in the field. In this study, the spherical indentation test is used to measure the linear viscoelastic properties of AC mixtures extracted from a road test section. The measured complex moduli are compared to those obtained by the shear box test and are found to exhibit a linear correlation. The measurements are further analysed using the Gaussian mixture model to assign each indentation test to either aggregate-dominated or mastic-dominated response. The measurements attributed to mastic-dominated response are found to be more sensitive to the temperature and AC's binder properties as compared to the average measurements. Accordingly, the proposed test method may provide a promising tool to measure AC viscoelastic properties and monitor the changes in AC binder phase in a non-destructive manner. A finite element micromechanical model is used to identify a representative scale for the response measured in mastic-dominated tests as well as to quantify the effect of measured properties on the AC damage propensity., QC 20220502