Simulation Model for Indirect Tensile Test of Asphalt Mixtures via Discrete Elements

A discrete element method (DEM) has widely been used to simulate asphalt mixture characteristics, and DEM models can consider the effect of aggregate gradation and interaction between particles. However, proper selection of model parameters is crucial to obtain convincing results from DEM‐based simulations. This paper presents a method to appropriately determine the mechanical parameters to be used in DEM‐based simulation of asphalt concrete mixture. Splitting test specimens are prepared by using asphalt mixture, and the splitting test results are compared with simulation results from two‐dimensional (2D) DEM and three‐dimensional (3D) DEM. Basing on the DEM results, the effects of contact model parameters on the simulation results are analyzed. The slope of the load‐displacement curve at the beginning stage is mainly affected by the stiffness parameters, and the peak load is mainly determined by using the value of the bond strength. The laboratory splitting test of AC‐20 and AC‐13 specimens were performed at different temperatures, namely, −10°C, 0°C, 10°C, and 20°C, and the load‐displacement relationships were plotted. According to the real load‐displacement curve’s slope at the beginning stage and peak load applied, the range of DEM bond model parameters is determined. On the basis of DEM results of the splitting test, the relationships between simulation load‐displacement curve’s characteristics and bond model parameters are fitted. The values of the parameters of the DEM contact bond model at different temperatures are obtained depending on the actual load‐displacement curve’s initial slope and peak load. Lastly the DEM and laboratory test results are compared, which illustrates that the parallel bond model can well simulate the behavior of asphalt mixture.

Fatigue cracking is one of the main distresses affects the service life of asphalt mixture. Recently, polyphosphoric acid (PPA) widely used as a modifier to improve mechanical and rheological characteristics of asphalt binders and mixtures especially at high and low service temperatures. While there are several studies devoted to investigate high and low service temperature of PPA modified asphalt binder and mixture, its fatigue behaviour needs to be studied in more detail. The main objective of this paper is to develop a multiscale heterogeneous numerical finite element (FE) model as well as experimental program to investigate the effect of PPA modification on fatigue behaviour of different bituminous composites from neat asphalt binder to asphalt mixture. The experimental program includes frequency sweep test at test temperature of 25°C by implementing dynamic shear rheometer (DSR) for neat and PPA modified asphalt binder and also indirect tensile fatigue (ITF) at frequency of 1 Hz (0.1 s loading time and 0.9 s rest time) under constant stress of 300 kPa at test temperature of 25°C by using a UTM-25kN. Three different contents of PPA is used for modifying neat asphalt binder including 0.5, 1 and 1.5 wt.% of neat binder to consider the variation of modifier extent on fatigue behaviour of neat asphalt binder and mixture. The mixture specimens were prepared with granite aggregate and asphalt binder of 85/100 penetration grade. The asphalt mixture is modeled as a biphasic medium composed of granular inclusions with linear elastic properties and bituminous matrix exhibiting linear viscoelastic behavior at small strain values. The generalized Maxwell rheological model was used to simulate viscoelastic behavior of the asphalt binder and PPA modified asphalt binder. In order to create the aggregate skeleton of the bituminous composites, a custom software (called “MOA”, French acronym for Random Object Modeler) developed in University of Limoges, has been used to generate random inclusions of various sizes and shapes. The experimental results of asphalt binder and mixture indicated that modification with PPA has a significant effect on improving asphalt mixture’s fatigue life especially in dosage of 1%. On the other hand, results of numerical simulation have shown a good agreement with experimental results.

This study develops a meso-structural modeling approach for asphalt mixtures by integrating computed tomography (CT) technology and the discrete element method (DEM), which accounts for the morphological characteristics of aggregates, asphalt mortar, and voids. The indirect tensile (IDT) tests of SMA-13 asphalt mixtures, a commonly used skeleton-type asphalt mixture for the surface course of asphalt pavements, were numerically simulated using CT-DEM. Through a comparative analysis of the load-displacement curve, the peak load, and the displacements corresponding to the maximum loads from the IDT tests, the accuracy of the simulation results was validated against the experimental results. Based on the simulation results of the IDT tests, the internal force transfer paths were obtained through post-processing, and the force chain system was identified. The crack propagation paths and failure mechanisms during the IDT tests were analyzed. The research results indicate that under the external load of the IDT test, there are primary force chains in both vertical and horizontal directions within the specimen. The interaction between these vertically and horizontally oriented force chains governs the fracture progression of the specimen. During IDT testing, the internal forces within the aggregate skeleton consistently exceed those within the mortar, while interfacial forces at aggregate-mortar contacts maintain intermediate values. Both the aggregate's and mortar's internal forces exhibit strong linear correlations with temperature, with the mortar's internal forces showing a stronger linear relationship with external loading compared to those within the aggregate skeleton. The evolution of internal meso-cracks progresses through three distinct phases. The stable meso-crack growth phase initiates at 10% of the peak load, followed by the accelerated meso-crack growth phase commencing at the peak load. The fracture-affected zone during IDT testing extends symmetrically 20 mm laterally from the specimen centerline. Initial meso-cracks predominantly develop along aggregate-mortar interfaces and void boundaries, while subsequent propagation primarily occurs through interfacial zones near the main fracture path. The microcrack initiation threshold demonstrates dependence on the material's strength and deformation capacity. Furthermore, the aggregate-mortar interfacial transition zone is a critical factor dominating crack resistance.

Predicting crack growth of paving materials under indirect tensile fatigue loads

Characterisation of warm asphalt mixtures with addition of reclaimed asphalt pavement materials

This paper presents the process of virtual prototyping of bulk material preparation devices in mining using numerical simulations of multi-physics phenomena. The discrete element method (DEM), meshless method (MFree), and computational fluid dynamics (CFD) were used in the calculation process. The importance of the extraction process and the practical application of DEM in various industries are discussed. The main contact models between particles and how structural material wear is modelled in DEM are presented. The structure of the computational models in DEM and CFD environments is presented. For the validation of the bulk material computational model, bench tests were carried out to determine the material properties (aggregate: five grades, 0–16 mm; coal concentrate: five grades, 2–32 mm; and so-called raw coal, grade 2–8 mm). The bulk density and angle of natural repose were measured, along with determination of the internal and external friction coefficients. Simulations corresponding to the laboratory tests were carried out. Numerical calculations were carried out for the side chute (results—velocities of the particles, compressive forces in the particles, determination of the wearing process) and for the coke classification line (two lines were assessed according to different aggregate sizes and densities of the bulk material). These multi-physics calculations required a combination of DEM-MFree and DEM-CFD methods. Based on the obtained results, it was possible to evaluate the performance and efficiency of the assessed machines.

The paper aims to simulate the micromechanical behavior of asphalt mixtures during the compaction process using the Discrete Element Method (DEM). The interactions between the components of a Porous Asphalt (PA) mixture were represented using an Elastic Viscoelastic Contact Model (EVCM), which is a user-defined model implemented in EDEM software, developed based on linear elastic and Burger’s viscoelastic constitutive equations. The macroscale parameters of asphalt mortar were characterized using the nonlinear regression analysis of master curves obtained from Dynamic Shear Rheometer (DSR) tests. The verification process of EVCM successfully indicated that the computations trends fall within the range of expected values for the typical asphalt mixture material. Further, a Superpave Gyratory Compaction (SGC) test was carried out and the obtained sample was scanned using X-ray Computed Tomography (X-ray CT) to capture the air void distributions. The DEM was utilized where digital samples were established to simulate the overall process of laboratory and field compaction. The simulation results showed that the model provided a comparable prediction of responses and demonstrated the capability of SGC to fabricate a representative sample. The influence of temperature on the asphalt compaction process was explored and the results implied that temperature decreasing adversely affects the compactability and dramatically increases the demanded compaction efforts which are consistent with the law of viscoelasticity. On the contrary, when the temperature is high, the asphalt binder becomes too fluid and roller loads will simply displace, or “shove” the mat rather than compact it. Tracking the change in the air voids proportion indicates that the motion of aggregates is rather compound. The aggregates flowed vertically downwards in line with the compacting orientation while moved horizontally outwards away from the center. All in all, the findings confirm that the concept is technically practicable, affording the model great potential to help researchers understand the microstructural phases of asphalt mixture during the compaction.

Three digital specimens of asphalt mixtures (AC-13, SMA-13 and OGFC-13) were reconstructed to conduct a virtual simple performance test using the discrete element method. The distribution characteristics of force chains were investigated by a statistical method. The results indicate that it is reasonable and feasible to analyze the mesoscopic responses of asphalt mixtures using digital models. The probability distribution of the normal force chains varies with loading time and the variation laws are consistent at four loading times. And the probability distribution of the shear force chains decays exponentially. Besides, the maximum probability distributions of the normal and shear force chains decrease with increasing timestep. OGFC-13 has the maximum probability distributions of the normal and shear force chains, which are 0.34186 and 0.55884, respectively. The proportions of the “strong” force chains decrease over the loading time, and AC-13 has a maximum proportion of 49.41% for the three asphalt mixtures at four loading times. In addition, the angle distributions of the force chain are mainly near 90°, and the average ratio of the normal contact force to the mean normal contact force increases with increasing loading time at 90°. Finally, the angle distribution proportions in the first and second quadrants are much greater than those in the third and fourth quadrants.

We established a user-defined micromechanical model using discrete element method (DEM) to investigate the cracking behavior of asphalt concrete (AC). Using the “Fish” language provided in the particle flow code in 3-Demensions (PFC3D), the air voids and mastics in asphalt concrete were realistically built as two distinct phases. With the irregular shape of individual aggregate particles modeled using a clump of spheres of different sizes, the three-dimensional (3D) discrete element model was able to account for aggregate gradation and fraction. Laboratory uniaxial complex modulus test and indirect tensile strength test were performed to obtain input material parameters for the numerical simulation. A set of the indirect tensile test were simulated to study the cracking behavior of AC at two levels of temperature, i e, −10 °C and 15 °C. The predicted results of the numerical simulation were compared with laboratory experimental measurements. Results show that the 3D DEM model is able to predict accurately the fracture pattern of different asphalt mixtures. Based on the DEM model, the effects of air void content and aggregate volumetric fraction on the cracking behavior of asphalt concrete were evaluated.

Mesoscopic creep mechanism of asphalt mixture based on discrete element method

Improved discrete element numerical simulation and experiment on low-temperature anti-cracking performance of asphalt mixture based on PFC2D

Basalt fiber (BF) has been proven to be an effective additive for improving the properties of asphalt mixtures. However, the influence of basalt fiber morphology on the properties of asphalt binders and mixtures remains inadequately explored. In this study, chopped basalt fiber (CBF) and flocculent basalt fiber (FBF) were selected to make samples for testing the influence of the two types of basalt fibers on asphalt materials. Fluorescence microscopy was used to obtain the dispersion of fiber in asphalt binders. Then, a temperature sweep test and a multiple stress creep recovery (MSCR) test were carried out to appraise the rheological characteristics of the binder. Moreover, the performance of the fiber-reinforced asphalt mixture was evaluated by a wheel tracking test, a uniaxial penetration test, an indirect tensile asphalt cracking test (IDEAL-CT), a low-temperature bending test, a water-immersion stability test, and a freeze-thaw splitting test. The results indicate that the rheological behavior of asphalt binders could be enhanced by both types of fibers. Notably, FBFs exhibit a larger contact area with asphalt mortar compared to CBFs, resulting in improved resistance to deformation under identical shear conditions. Meanwhile, the performance of the asphalt mixture underwent different levels of enhancement with the incorporation of two morphologies of basalt fiber. Specifically, as for the road property indices with FBFs, the enhancement extent of DS in the wheel tracking test, that of RT in the uniaxial penetration test, that of the CTindex in the IDEAL-CT test, and that of εB in the low-temperature trabecular bending test was 3.1%, 6.8%, 15.1%, and 6.5%, respectively, when compared to the CBF-reinforced mixtures. Compared with CBFs, FBFs significantly enhanced the elasticity and deformation recovery ability of asphalt mixtures, demonstrating greater resistance to high-temperature deformation and a more pronounced effect in delaying the onset of middle- and low-temperature cracking. Additionally, the volume of the air void for asphalt mixtures containing FBFs was lower than that containing CBFs, thereby reducing the likelihood of water damage due to excessive voids. Consequently, the moisture susceptibility enhancement of CBFs to asphalt mixture was not obvious, while FBFs could improve moisture susceptibility by more than 20%. Overall, the impact of basalt fibers with different morphologies on the properties of asphalt pavement materials varies significantly, and the research results may provide reference values for the choice of engineering fibers.

A material testing protocol using asphalt mixture beams in the bending beam rheometer (BBR) is presented in the paper to provide a methodology that can be used by highway agencies in day-to-day quality control and mix design. The objectives of the study are to (1) present an experiment plan that describes the processes of asphalt mixture specimen production and material testing procedures using the BBR instrument and (2) provide future research work with a starting point to develop a specification for using asphalt mixture beams in the BBR to evaluate the low temperature performance of asphalt materials. Several factors that influence the accuracy of a BBR test are taken into consideration. Air void consistency between compacted samples and asphalt mixture beams is evaluated. Statistical approaches are used to determine the number of replicates for a BBR test to be valid. An asphalt pavement construction project in Salt Lake City, Utah, is used to demonstrate the applicability of using asphalt mixture beams in the BBR for in situ quality control of asphalt pavements. Based on testing results, there was an anomaly on three paving days that indicates an irregularity on asphalt materials placed in the field. The paper concludes that the material testing methodology using asphalt mixture beams in the BBR is capable of evaluating low temperature performance of asphalt mixtures. The material testing methodology can quickly settle any dispute and can be adopted by highway agencies to perform in situ quality control of low temperature performance in asphalt pavements.

In Malaysia, road pavements are frequently damaged by the accumulation of water on the surface caused by heavy rain, due to the humid climate. The presence of water within the asphaltic concrete mixture particles may impede their workability. Exposure towards natural phenomenon such as rain can be the main cause related to the presence of water inside the asphalt pavements. To improve road conditions for users, it is necessary to consider alternative road pavement designs. This study proposes using rubber crumb as modifier as its water-impermeable characteristic will be able to enhance the performance of standard asphalt mixtures to reduce asphalt mixtures moisture sensitivity. Subsequently, the Marshall Mix Design method was utilized to conduct Indirect Tensile Strength tests in order to achieve the objectives of this study. A total of 16 samples were prepared for this study, comprising of standard asphaltic combinations as well as asphaltic mixtures with different percentages of crumb rubber (0%, 4%, 6%, and 8%) relative to the overall weight of the asphalt mixtures. In comparison to the typical way of asphaltic mixture, the sample that utilized crumb rubber as a modifier had a substantially greater value of strength based on the Indirect Tensile Strength Test results. As expected from the findings of the study, the presents of water in asphalt mixtures reduces the asphalt mixtures strength and workability. To conclude, incorporating crumb rubber in asphalt mixtures enhances their effectiveness in resisting moisture sensitivity.

Scrap tires are a major part of the global solid waste management problem. In recent years, the problem of waste tires has become very acute that there is an urgent need to find an optimal and efficient way to use scrap tires in asphalt mixtures. The previous studies showed that the utilization of crumb of tires has more effect on asphalt mixture performance, which is represented increasing in the Marshall Stability and increasing crumb of tires causes increasing in Marshall Flow and increasing in air voids content additionally more than original mixes. This paper used crumb of tires in asphalt mixture as a part of aggregate. Three sizes of the grade of crumb tires were used in asphalt mixture No. 4 (4.75 mm), No. 8 (2.36) mm and No. 50 (300µmm). Three percentages of asphalt binder (4, 5 and 6) by weight with three percentages of crumb tires (2, 4 and 8) % by weight also were used with aggregates for preparing asphalt mixture specimens. Asphalt mixture specimens were conducted according to Marshall Methods. Thirty-six specimens were equipped for evaluating Marshall Properties (Marshall Stability and flow, air voids percentage, bulk density, maximum bulk density, and Marshall Stiffness). Indirect Tensile Strength test (ITS) has been applied to obtain the cracking resistance of asphalt mixture using twenty-four specimens, which contains crumb tires. The results showed that the applying of the crumb tires has a significant performance of asphalt mixture by increasing the Marshall stability, flow, air voids, and decreasing bulk density and indirect tensile strength compared to the original mixtures.