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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

The friction force for aircraft landing is mainly provided by the texture of runway surfaces. The mechanism underlying friction force generation is the energy dissipation of tire rubber materials during random excitation induced by asperities. However, the runway surface texture is deteriorated by cyclic loading and environmental effects during the service life of a runway, leading to loss of braking force and extension of landing distance. Additionally, when an aircraft lands on a wet runway at a high velocity, the hydrodynamic force causes the tires to detach from the runway surface, which is risky and may lead to the loss of aircraft control and runway excursion. Worn-out surfaces along with wet conditions increase the risk of poor control during aircraft landing. Accordingly, this study investigated three types of asphalt runways (SMA-13, AC-13, and OGFC-13). Surface texture deterioration was simulated using a surface texture wear algorithm. Kinematic friction models were established based on the viscoelastic property of rubber materials, power spectrum density, and statistics of surface textures. A finite element model was developed by considering a real rough runway surface and different water film depths (3, 7, and 10 mm). A comparison of hydroplaning speed was conducted between numerical simulation and former experiments. The effects of different factors, such as velocity, wear ratio, runway type, water film depth, and slip ratio, on the skid resistance of the runway were analyzed., QC 20201202

Viscoelastic characterisation of asphalt mixtures is an important component for modelling and performance prediction of flexible pavements. In this study, using spherical indentation testing for measuring the viscoelastic properties of asphalt is explored. Indentation testing may provide an interesting alternative to existing experimental techniques, as it is capable of characterising small material volumes. Thus, it may become a useful tool for the characterisation of thin asphalt layers and for the measurement of binder phase properties in-situ in asphalt mixtures. Spherical indentation tests are performed on two mastic asphalt (MA) mixtures, prepared with different mastic types. The shear relaxation moduli obtained from the indentation tests are compared with the ones measured with seismic and SuperPave Indirect Tensile (IDT) tests. A new statistical analysis methodology is proposed for viscoelastic characterisation of the mastic phase with the indentation tests performed on MA mixtures. The accuracy and sensitivity of the developed method are examined., QC 20211110

Asphalt pavement is subjected to cyclic temperature variations during its service life owing to changes in daily and seasonal climatic conditions. These variations tend to accumulate thermally induced distress leading to initiation and evolution of micro-cracks. The effect of cyclic thermal variations as well as ther-mal incompatibility of mastic and aggregates is of major significance for understanding the behavior of thermally induced damage in pavements. Thermal stress is developed due to differential contraction of mastic relative to aggregates in asphalt concrete at low temperatures. In this paper, low temperature micro-damage initiation in asphalt concrete due to differential thermal contraction is modelled using 2D micro-mechanical volume element. Cohesive zone model (CZM) is adopted to simulate low temper-ature damage initiation at the mastic-aggregate interface (adhesive failure) within the mixtures. A cycle of cooling and heating is applied in the micro-mechanical model in order to capture the effect of thermal damage initiation on the overall stiffness modulus of the mixtures. The results from the model reveal a reduction in stiffness modulus (as compared to the values at similar temperatures within a cycle) after the temperature of-40 degrees C is reached within the applied cyclic cooling and heating. The effects of aggre-gate gradation and binder grade are also monitored by considering four cases of mixtures formed from a combination of two different gradations and two different mastics. Results of the micro-mechanical mod-elling are also compared with experimental observations of comparable mixture types., QC 20210830

QC 20210825

QC 20201117

Aggregate segregation in asphalt mixture is a bothersome engineering issue during pavement construction. The practitioners have some measures to mitigate the segregation potential based on experiences which, however, can only reduce the risk to a certain extent. In this research, the authors aim to contribute to the discussion in a rational non-empirical way, by using novel experimental and numerical techniques. A case study is carried out to investigate the vibration-induced segregation in asphalt mixtures, corresponding to the circumstance arising during material transportation to the construction site. A novel experimental test is conducted for evaluating the segregation characteristics of asphalt mixtures under vertical vibration in laboratory conditions. A numerical investigation based on discrete element method is further performed to study the phenomenon from a micromechanical point of view. The obtained experimental and numerical results indicate that vibratory loading induces aggregate size segregation in asphalt mixtures, and the degree of segregation is influenced profoundly by the adhesive properties of bituminous binders and the aggregate gradation., QC 20200402

Surface wear by studded tires is a primary cause for rutting in portland cement concrete (PCC) pavements, and may, for the pavement sections with high traffic intensities, become a primary factor controlling the length of pavements maintenance intervals. In the present study, PCC pavement field performance with respect to surface wear is examined based on the transverse surface profile measurements performed yearly during first five years of pavements service life. Continuous transverse surface profile measurements have been performed yearly on a 20 km long rigid pavement section in Moscow region, Russia. Based on the measurements, quantitative parameters describing rut depth evolution are obtained and evaluated; the obtained results are discussed in context of previous research findings reported in the literature. The effect of pavement surface wear on the further rutting accumulation rate is examined. The particular attention is also given to the spatial variability of the measured pavement surface wear characteristics, i.e. rut depth and accumulation rate. The implications of the obtained results on the length of rigid pavement maintenance intervals are discussed., QC 20210915

The service life of a road is to great extent controlled by the performance of the unbound layers. Assessing the constitutive behavior of these layers is thus imperative for a sustainable pavement design. Adequate description and measurement of unbound materials resistance to aggregate crushing is an issue, in terms of the measured response coupled to intrinsic properties of the aggregates and unbound materials gradation. In this study, a new Discrete Element Method (DEM)—based model is developed to investigate aggregate damage in unbound road materials. In order to get better insight into micro-mechanics of aggregate crushing, the developed model incorporates granular mechanics-based particle contact and damage laws. By numerical analysis with the DEM, several unbound granular materials have been examined investigating the effect of the materials gradation and aggregates toughness properties on their performance in crushing tests. The capability of the model to capture the effect of unbound material properties on its crushing performance, is evaluated based on comparison with experimental findings., QC 20211002

The flow differences between the particles of asphalt mixtures compacted in the laboratory and in the field have been identified as one of the reasons for the discrepancies between laboratory and field results. In previous studies, the authors developed a simplified test method, the so-called compaction flow test (CFT), for roughly simulating the flow of particles in asphalt mixtures under compacting loads in laboratory. The CFT was used in different studies to examine its capability of revealing the differences between the flow behavior of different asphalt mixtures under various loading modes. The promising results encouraged further development of the CFT by investigating the possible impacts of simplifications and boundary conditions on the results of this test. For this reason, discrete element method (DEM) was utilized to investigate possible impacts of the mold size, geometry of the loading strip as well as the loading rate on the results of the CFT. The results of the simulation indicate that in case of wearing course layers with nominal maximum aggregate size of 11 mm, the length of the CFT mold can be increased from 150 mm to 200–250 mm for reducing flow disturbances from the mold walls. However, since the majority of the flow of asphalt mixture particles is expected to take place within the first 100–150 mm length of the mold, reasonable results can still be obtained even without changing the size of the CFT mold. Moreover, comparing results with different loading strip geometries and loading rates indicates that the current CFT setup still appears to provide consistent results., QC 20191205

QC 20201019

The previous chapters describe numerical models and testing methods designed to simulate, to repeat and to understand the cracking phenomena in asphalt materials and asphalt pavement (or pavements with at least one asphalt layer). This chapter shows some of the most advanced systems to measure the parameters related to cracking. They are classified using a classification grid that considers the most relevant characteristics of the measurement system. The proposed classification method was designed to provide a quick understanding of what data the systems are able to provide and what they can analyze., QC 20181105

In this study, a small-scaled accelerated loading test based on similarity theory and Accelerated Pavement Analyzer was developed to evaluate dowel bars with different materials and cross-sections. Jointed concrete specimen consisting of one dowel was designed as scaled model for the test, and each specimen was subjected to 864 thousand loading cycles. Deflections between jointed slabs were measured with dial indicators, and strains of the dowel bars were monitored with strain gauges. The load transfer efficiency, differential deflection, and dowel-concrete bearing stress for each case were calculated from these measurements. The test results indicated that the effect of the dowel modulus on load transfer efficiency can be characterized based on the similarity model test developed in the study. Moreover, round steel dowel was found to have similar performance to larger FRP dowel, and elliptical dowel can be preferentially considered in practice., QC 20170328