Study on the void reduction behaviour of porous asphalt pavement based on discrete element method

The objective of this study was to analyse the void reduction behaviour of porous asphalt mixture under load. A three-dimensional discrete element model of porous asphalt mixture based on aggregate gradation and void gradation was built in PFC3D software. The parameter of the model was obtained from creep test. The rutting test was simulated using this discrete element model. And a new method was developed to obtain and analyse the void structure in discrete element model. The simulation results were compared with one of the laboratory test. The comparative analysis indicates that, the discrete element method can be used to simulate the creep response and void reduction behaviour of porous asphalt mixture. Further research shows that porosity, effective porosity, number of connected components and section pores have a good correlation with strain of porous asphalt mixture. With the increase in strain, the proportion of section pores with diameter less than 2 mm increases. In the initial stage of loading, the void reduction is the main reason for rut increment of porous asphalt mixture. In the later stage, the void structure is almost incompressible; the lateral deformation of mixture becomes the domination factor.     

Quantitative distribution characteristics of force chains for asphalt mixtures with three skeleton structures using discrete element method

Granular Matter - The use of glass for pharmaceutical new applications such as high-technology drugs, requires the strictest container inertness. A common theme of paramount importance in glass...     

Micromechanical finite element framework for predicting viscoelastic properties of asphalt mixtures

A micromechanical finite element (FE) framework was developed to predict the viscoelastic properties (complex modulus and creep stiffness) of the asphalt mixtures. The two-dimensional (2D) microstructure of an asphalt mixture was obtained from the scanned image. In the mixture microstructure, irregular aggregates and sand mastic were divided into different subdomains. The FE mesh was generated within each aggregate and mastic subdomain. The aggregate and mastic elements share nodes on the aggregate boundaries for deformation connectivity. Then the viscoelastic mastic with specified properties was incorporated with elastic aggregates to predict the viscoelastic properties of asphalt mixtures. The viscoelastic sand mastic and elastic aggregate properties were inputted into micromechanical FE models. The FE simulation was conducted on a computational sample to predict complex (dynamic) modulus and creep stiffness. The complex modulus predictions have good correlations with laboratory uniaxial compression test under a range of loading frequencies. The creep stiffness prediction over a period of reduced time yields favorable comparison with specimen test data. These comparison results indicate that this micromechanical model is capable of predicting the viscoelastic mixture behavior based on ingredient properties.     

Microscopic investigation of the packing features of soft-rigid particle mixtures using the discrete element method

Soft-rigid mixtures (SRMs) have become popular materials in civil engineering applications for environmental protection because of their outstanding engineering properties. In soft-rigid granular systems, packing features strongly affect shear behavior and directly reflect internal stability. However, the packing features of SRMs have not been previously reported. The aim of this study is to explore the effect of material susceptibility on packing features from a microscopic perspective. First, fifty-three numerical assemblies were established to thoroughly investigate the effects of the size ratio and soft content on several microscopic quantities, e.g., the particle structure, stress network, and local void. Then, the effects of the confining stress and stiffness ratio were analyzed from another eighteen assemblies by six chosen indexes. The results provide microscopic insights into the void structures and stress transmission of SRMs in a packing state.     

Influence of different sources of microstructural heterogeneity on the degradation of asphalt mixtures

The response and degradation of the hot mix asphalt (HMA) materials used in pavement structures are affected by their inherent heterogeneity. The objective of this work is to study the impact of two different sources of HMA heterogeneity in the uncertainty of the mechanical moisture degradation of HMA. The first source of heterogeneity is the spatial variability of the properties of the bulk fine aggregate matrix (FAM) of the mixture, and the second is the location and shape of the coarse aggregate particles. The heterogeneity of the bulk FAM phase was modelled using a random field technique, while that of the coarse aggregates was accounted for by randomly generating realistic probable sets of aggregate particles. Thus, ‘computational replicates’ of HMA microstructures were generated and subjected to moisture diffusion and mechanical loading using a finite element approach. In the mechanical simulations, a non-linear viscoelastic moisture damage constitutive relationship based on continuum damage mechanics theory was selected to characterise the response of the bulk FAM phase. The results show that conducting computational simulations with realistic HMA microstructures that properly capture the heterogeneity of the material is useful to quantify the mean values and dispersion (i.e. uncertainty) associated with the response and degradation of the mixture. This information, which cannot be easily obtained in the field or in the laboratory due to the difficulty of acquiring a sufficient amount of data, is useful to conduct structural reliability analysis and to predict the life cycle behaviour of the material.     

Evaluation of deformation properties of asphalt mixture using aggregate slip test

Asphalt mixture is a multiphase particulate material composed of aggregate, asphalt and filler. The deformation property of asphalt mixture is an external reflection of aggregate slip behaviour. To evaluate the high-temperature deformation properties of asphalt mixture, an aggregate slip device was developed and aggregate slip tests were conducted on five asphalt mixtures for different gradations under different test conditions. Four evaluation parameters, the slip failure load (F_s), the slip failure deformation (D_s), slip modulus parameter (M) and slip energy index (SEI), were obtained according to the load–displacement curves. The relationship between these parameters and rut depth (RD) was analysed. The effects of test temperature and asphalt content on slip resistance of asphalt mixture are studied in this research. The results indicate that the parameter F_s has limitations for large nominal maximum particle-size mixture, and SEI is an effective parameter to evaluate the aggregate slip properties for different nominal maximum particle-size asphalt mixtures. SEI has the strongest relationship to RD, which is the best parameter to evaluate the slip deformation behaviour of asphalt mixture. With the increase in asphalt content, SEI has a peak value and a valley value. When the optimum asphalt content is used in asphalt mixture, aggregate skeleton effect and asphalt cohesive force can both reach a high level, and asphalt mixture has the best deformation resistance.     

基于离散单元法的群楼拆除爆破仿真模拟

基于青岛开发区一商场爆破工程,利用颗粒离散元法建立离散元网格实体模型,用多面体离散元法模拟分析拆除爆破中建筑物的倒塌过程。研发的网格实体模型详细模拟了建筑物从结构局部失稳到整体完全倒塌的整个过程,确定了爆堆轮廓线和爆堆尺寸,直观给出了爆破设计的效果。触地振动、冲击力变化分析结果表明:所给出的爆破方案可达到预期拆除爆破的效果,触地振动符合《爆破安全规程》要求;该模型可优化爆破设计,指导爆破施工,为建筑物拆除爆破的灾害预测与安全评估提供理论依据。     

基于离散单元法的群楼拆除爆破仿真模拟

基于青岛开发区一商场爆破工程,利用颗粒离散元法建立离散元网格实体模型,用多面体离散元法模拟分析拆除爆破中建筑物的倒塌过程。研发的网格实体模型详细模拟了建筑物从结构局部失稳到整体完全倒塌的整个过程,确定了爆堆轮廓线和爆堆尺寸,直观给出了爆破设计的效果。触地振动、冲击力变化分析结果表明:所给出的爆破方案可达到预期拆除爆破的效果,触地振动符合《爆破安全规程》要求;该模型可优化爆破设计,指导爆破施工,为建筑物拆除爆破的灾害预测与安全评估提供理论依据。     

Predicting the tensile relaxation modulus of asphalt mixes based on the mix design and environmental factors

The main objective of this study was to predict the tensile relaxation modulus of asphalt mixes, without having to perform the common relaxation modulus tests, by developing a predictive model based on the mix characteristics, ageing condition, temperature and loading time. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with four replicates. Uniaxial tensile relaxation modulus tests were conducted on the specimens at four temperatures using the trapezoidal loading pattern at a low level of strain. Tensile relaxation modulus master curves of all the experimental combinations were constructed by the sigmoidal model. Statistical analysis of variance and regression analysis was performed on the test data and a predictive model was developed. Finally, the predictive model was verified using a group of measured values other than those used for the development of the model, and it was found that the predicted values correlated well with the measured ones.     

Evaluation of permanent deformation characteristics of unmodified and Polyethylene Terephthalate modified asphalt mixtures using dynamic creep test

One of the major types of plastics that can be found in Municipal Solid Waste (MSW) is Polyethylene Terephthalate (PET) which is a non-biodegradable semi-crystalline thermoplastic polymer, and is considered as polyester material. Generating large amount of waste PET, mainly as bottles, would cause environmental hazards by disposing in landfills. This paper aims to evaluate effects of utilizing waste PET flakes as modifier in asphalt mixture as an alternative solution to overcome the potential risks arise from producing large amount of waste PET as well as evaluating the deformation characteristics of unmodified and PET modified asphalt mixtures. To achieve this aim, different percentages of PET were designated for this investigation, namely: 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1% by weight of aggregate particles, and dynamic creep test was performed at different stress levels (300 kPa and 400 kPa) and temperatures (10 °C, 25 °C and 40 °C). Consequently, Zhou three-stage model was developed. The results showed that permanent deformation characteristics of asphalt mixture were considerably improved by utilization of PET modification, when the permanent strain was remarkably decreased in PET modified mixture compared to the conventional mixture at all stress levels and temperatures. Besides, based on Zhou model, it was concluded that elastic and visco-elastic properties of asphalt mixture were improved by application of PET modification.     

Comparison of solid particle erosion predictions using the dense discrete phase and discrete element models

A numerical procedure involving the dense discrete phase model (DDPM) is used to calculate solid particle erosion. DDPM works in two mechanisms. First, the discrete particles are treated as a pseudofluid, and the interaction among particles is evaluated by solving the governing equations of the pseudofluid. Second, the equivalent pressure of the pseudofluid is applied to a single particle to reflect the blocking effect of high-concentration particles. The numerical procedure is well verified by comparison with the experimental data picked from a direct impact test. In addition, the DDPM predictions are compared with the discrete element model (DEM) predictions in detail. Both methods show that the predicted mass loss caused by sand per unit mass decreases with an increase in sand concentration. DDPM indirectly considers the influence of particle interactions on solid particle erosion, and the predicted erosion contours are more uniform and smoother than the DEM-predicted contours.     

DEM simulation analysis of the improvement in particle discharge flowability using adhesive force distribution models based on admixed particle coating

Particle flowability can be improved by admixing particles smaller than the original particles (main particles). However, the details of the mechanism of this improvement are not yet fully understood. In this study, we used a discrete element method simulation to investigate the effects on the particle flowability of the adhesive force distribution at each contact point based on the admixed particle coating. We used the non-uniform, random, and uniform surface adhesive force distribution models and calculated the discharge flow rates. The non-uniform models had a larger discharge flow rate compared with the other models because the non-uniform adhesive force distribution destabilized the force balance in the bed, and thus destabilized the particle arching structure, which generated discontinuous layers more frequently and improved the flowability. Consequently, in a smaller particle admixing system, the adhesive force distribution at each contact point would help to improve the flowability.     

Effects of air voids variability on the thermo-mechanical response of asphalt mixtures

This paper evaluates the role of the air voids (AV) variability of asphalt mixtures on the thermo-mechanical response of asphalt courses. The methodology presented combines finite element (FE) modelling with a stochastic technique called random fields. This technique is used to generate probable spatial AV distributions in the geometry of compacted asphalt courses. Thus, several asphalt layers with AV contents varying in space were implemented in FEs, and a thermal diffusion process followed by a mechanical loading scheme was simulated. The mechanical response of the asphalt course was made dependent on both the specific AV content and temperature at each point within the layer. It was found that the quality of the compaction process, represented by the dispersion of the AV, strongly impacts the thermal diffusion processes and the uncertainty of the mechanical response of asphalt courses.     

Numerical study of the interfaces of 3D‐printed concrete using discrete element method

3D concrete printing is an additive manufacturing method which reduces the time and improves the efficiency of the construction process. Structural behavior of printed elements is strongly influenced by the properties of the material and the interface surfaces. The printing process creates interface surfaces between layers in the horizontal and vertical directions. The bond strength between layers is the most critical property of printed elements. In this paper, the structural behavior of printed elements is studied using the discrete element method. The material is modelled using discrete particles with bonding between them. A new discrete model of a multilayer geometry is presented to study the behavior of the interfaces of printed concrete. The layers are made up of randomly placed particles to simulate the heterogeneous nature of concrete. The numerical model is developed to simulate the flexural behavior of multilayer specimens. A four‐point flexural test is simulated considering the interface surfaces between layers. This numerical model provides relevant results to improve the behavior of this kind of structural elements. The aim of this work is to provide a discrete element model to predict the mechanical behavior of 3D concrete printed components.     

Influence of particle shape on the shear behavior of superellipsoids by discrete element method in 3D

This paper aims to study the influence of particle shape on the shear strength of superellipsoidal particles by Discrete Element Method (DEM) simulations of triaxial tests in 3D. A total of forty-nine types of equiaxed superellipsoidal particles from three evolution paths have been created. The definition of effective porosity has been proposed. Our findings show that both the particle sphericity and roundness affect the shear strength of the superellipsoidal particle system. Under the mutual impact of initial porosity and particle shape, the simulation results of shear strength and volumetric strains present a trend of initially decreasing and subsequently increasing. The microstructure evolution of superellipsoidal particles during the shearing process is observed microscopically. The anisotropy of fabric reveals the mechanism of effective porosity and sphericity influencing the macroscopic shear strength at the particle scale.     

Evaluation of the discrete element method (DEM) and of the experimental evidence on concrete behaviour under static 3D compression

The authors successfully employed the discrete element method (DEM) in numerical determinations of the response up to and beyond failure of reinforced concrete structures subjected to impact and impulsive loadings in which tensile fracture, which is reliably predicted by DEM models, often controls the dominant failure modes. However, in impact problems when penetration occurs, the reliability of the approach in predictions of the structural response of the 3D compression zone that develops at the tip of the projectile has not yet been explicitly confirmed. In this context, in view of its complexity, the performance of the method is herein assessed and compared with available experimental results in static tests. By means of numerical simulations, it was previously verified that DEM models do predict, but overestimate, the strength increase observed on concrete cubes subjected to static multiaxial compression in relation with the unconfined strength, for confining (lateral) pressures up to about 20% of the unconfined compressive stress. For higher confining stresses, however, the DEM formulation underestimates the compressive strength increase observed in cubic and cylindrical samples, for the reasons examined in the paper, in which limitations of both the numerical predictions and experimental observations are thoroughly discussed.     

Determining a lower boundary of elasticity modulus used in the discrete element method (DEM) in simulation of tumbling mills

Discrete Element Method (DEM) simulations of industrial tumbling mills could involve millions of particles. Even with the considerable increase in the computational power, the simulations still require a large amount of time. Reducing the computational load by selecting a small value for the particle elasticity modulus to increase the time step has become a common approach. As the elasticity modulus decreases, the overlap required to provide the rebound force increases. The appropriate value of overlap is application-dependent and requires a detailed study to ascertain that the accuracy of the results do not adversely affected. In this study, a relationship incorporating particle density and mill diameter was proposed between the elasticity modulus and the interparticle overlap for tumbling mills. The effect of interparticle overlap on the accuracy of the simulated charge shape (i.e. toe and shoulder positions) by DEM was then investigated. A model tumbling mill (100 cm by 21 cm) with a transparent end wall was used to measure the actual charge trajectory by photography. A comparison of the DEM simulations with the model mill charge shape showed that when the overlap was assumed to be lower than the particle radius, the error was negligible. When the interparticle overlap became equal to the particle radius, the lower boundary of elasticity modulus and the maximum simulation speed was achieved. The speed was 102 times of the speed of simulation when an overlap equal to 0.01 of the particle radius was chosen.     

The influence of non-linear elasticity on the determination of Weibull parameters using the fibre bundle tensile test

We address the influence of individual fibre stress–strain non-linearity on the extraction of Weibull-parameters from fibre bundle tensile tests. We extend the statistical theory of fibre bundle strength to include the non-linear elastic behaviour observed in many technically important fibres, e.g. glass-, carbon-, and alumina-fibres. It is shown that neglecting this non-linearity may lead to significant errors in determining the shape and scale parameters of the fibre fracture strength Weibull-distribution. A refinement of the existing extraction technique, accounting for this effect, is presented. The error resulting from neglecting the non-linear behaviour is assessed through a parametric study of the Weibull parameters for different levels of non-linearity. Explicit calculations are performed for two fibres of technical importance, namely Nextel 610™ α-alumina fibre and a T300 carbon fibre.     

Optimization of the coupling parameters and mixing uniformity of multiple organic hydraulic mixtures based on the discrete element method and response surface methodology

To address the mixing uniformity of multiple organic hydraulic (MOH) mixtures in a continuous mixer, three types of mixing parameters and their coupling effects were studied by the discrete element method (DEM) and response surface methodology. To achieve the research goal, only one parameter was selected for each type of parameter, and the corresponding model was established. Numerical simulations and optimization were implemented. A three-level, three-factor Box-Behnken Design method combined with response surface methodology was applied for the numerical design. The influence of the parameters on the mixing uniformity of the mixture was analyzed by analysis of variance (ANOVA). The ANOVA results show that the rotation speed, the installation angle, the filling ratio and the coupling between the rotation speed and the filling ratio have a significant effect on the mixing uniformity of the mixture, that the rotation speed and the filling ratio have the strongest effect on the response, and that of the fitting model of the mixing uniformity can fit the simulation data well. The coupling effect results show that the influence of coupling between the revolution and installation angle on the mixing uniformity is consistent with that between the filling ratio and installation angle and that the coupling effect between the rotation speed and the filling ratio is different. It is also found that the optimal parameter range under one factor is different from that under multivariable coupling. The optimization results show that when the discrete coefficient is the smallest, the optimal combination of the parameters is a revolution of 350 r/min, an installation angle of 25°, and a filling ratio of 70%. The experimental results are consistent with the optimization results, which indicate the correctness of the parameter optimization results.