Discrete element modelling of uniaxial constant strain rate tests on asphalt mixtures (original) (raw)
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Finite Elements in Analysis and Design, 2012
This paper concerns with meso-scale finite element (FE) modeling of asphalt mixtures. The proposed FE model is capable of simulating the complex geometry of several types of asphalt mixture in which different gradations of aggregates could be properly modeled. The meso-structure of the mixture is constructed by a novel technique identified in this paper. The main idea of such technique is that the aggregate particles could be randomly packed together into the simulation region, by defining a kind of artificial interaction forces among the particles. After that by the Voronoi tessellation method, the set of the generated discrete grains will alter to space-filling, adjoining polyhedrons with respect to the real geometry so that it were possible to investigate the behavior of mixture by finite element method. Such a model considers the main components of the asphalt mixture consisting of aggregate particles, mastic, interfacial zone and air voids. Moreover, different moving wheel loads with different passing velocities are considered and their effects on the mechanical responses of the asphalt mixture are examined. The FE model gives some better insights into the behavior of mixture's components under moving loads. The responses include vertical displacement of the pavement surface as well as the development of stress in mixture components. The proposed models give results that are in agreement with theoretical predictions and previous studies.
Three-Dimensional Discrete Element Models for Asphalt Mixtures
The main objective of this paper is to develop three-dimensional ͑3D͒ microstructure-based discrete element models of asphalt mixtures to study the dynamic modulus from the stress-strain response under compressive loads. The 3D microstructure of the asphalt mixture was obtained from a number of two-dimensional ͑2D͒ images. In the 2D discrete element model, the aggregate and mastic were simulated with the captured aggregate and mastic images. The 3D models were reconstructed with a number of 2D models. This stress-strain response of the 3D model was computed under the loading cycles. The stress-strain response was used to predict the asphalt mixture's stiffness ͑modulus͒ by using the aggregate and mastic stiffness. The moduli of the 3D models were compared with the experimental measurements. It was found that the 3D discrete element models were able to predict the mixture moduli across a range of temperatures and loading frequencies. The 3D model prediction was found to be better than that of the 2D model. In addition, the effects of different air void percentages and aggregate moduli to the mixture moduli were investigated and discussed.
Studying the Mechanical Behaviour of Asphalt Mixture with the Grid Method
Strain, 2013
The aim of this work is to analyse the strain fields that take place in several asphalt mixture specimens subjected to compression tests. The grid technique is used for this purpose. The main features of this strain measurement technique are first described. The obtained results are then discussed. They show that very strong heterogeneities take place in the strain fields. They are due to the very different mechanical properties of the constituents. Various testing conditions are also investigated because they directly influence the quality of the measurements and the ability of the grid technique to detect small strain amplitudes with a good spatial resolution, this last feature being crucial here because of the very nature of the material under test.
Computational Materials Science, 2007
ABSTRACT The study and development of recycling techniques for pavements is an increasing activity in engineering nowadays. This research line demands a more realistic characterization of the material properties with the aim of simulate the asphalt mixture’s response placed into a multilayered system over granular bases, under dynamic loads, considering also temperature variation or strength reduction for cyclic loads.In order to improve the current formulations, a new viscoplastic model has been developed assuming the strain rate dependency of the material’s response observed in the experimental tests. The strain rate variable affects in a significant way the Young modulus and the viscosity parameter of the model. According to this hypothesis a constitutive equations have been formulated. The mechanical variables involved have been calibrated according to experimental results, developing new expressions for the strain rate dependent parameters. The new viscoplastic model permits us to characterize the material’s response with a few mechanical values, easily obtained from standard laboratory tests. The results obtained show a good approximation to experimental laboratory curves for different rates of loading and temperatures.The model has been applied to simulate the response of a real flexible pavement structure conformed by two asphalt layers over two granular bases, that’s materials with different constitutive behaviors. Experimental tests in the recycled track have been made obtaining the horizontal strain evolution under dynamic load. Different loading rates and temperatures, as well as cracked and continuum pavement responses have been considered in the study. Strains were measured in the interface between the two asphalt layers and simulated using the here proposed model offering a fairly good approximation of the real response observed in the track, although the degree of variation even in the experimental curves is quite high.The results of this study represent a proper base for further developments in structural analysis of pavement layers, considering more complex phenomena, determinant in the long term material’s response, to develop a numerical tool for pavements’ design and lifetime prediction.
Microstructural Simulation of Asphalt Materials: Modeling and Experimental Studies
Journal of Materials in Civil Engineering, 2004
Asphalt concrete is a heterogeneous material composed of aggregates, binder cement, and air voids, and may be described as a cemented particulate system. The load carrying behavior of such a material is strongly related to the local load transfer between aggregate particles, and this is taken as the microstructural response. Simulation of this material behavior was accomplished using a finite element technique, which was constructed to simulate the micromechanical response of the aggregate/binder system. The model incorporated a network of special frame elements with a stiffness matrix developed to predict the load transfer between cemented particles. The stiffness matrix was created from an approximate elasticity solution of the stress and displacement field in a cementation layer between particle pairs. A damage mechanics approach was then incorporated with this solution, and this lead to the construction of a softening model capable of predicting typical global inelastic behaviors found in asphalt materials. This theory was then implemented within the ABAQUS finite element analysis code to conduct simulations of particular laboratory specimens. Experimental verification of the elastic response has included tests on specially prepared cemented particulate systems, which allowed detailed measurement of aggregate displacements and rotations using video imaging and computer analysis. Model simulations compared favorably with these experimental results. Additional simulations including inelastic behavior of laboratory indirect tension tests have been conducted, and while preliminary in nature these results also compared well with experimental data.
A material model of asphalt mixtures based on Monte Carlo simulations
Nucleation and Atmospheric Aerosols, 2019
The paper aims to numerically reflect mineral-asphalt mixture structure by a standard FEM software. Laboratory test results are presented due to bending tests of circular notched elements. The result scatter is relatively high. An attempt was made to form a random aggregate distribution in order to obtain various results corresponding to laboratory tests. The material structure calibration, its homogenization and finite element dimensioning are the issues decisive for the objective mixture description. The representative volume element (RVE) is investigated here, while it does not precisely reflect the material structure it displays relevant global material parameters. The simulation procedure applied here makes it possible to introduce the name of Monte Carlo simulation-based constitutive model.
Constitutive Modeling of Asphalt-Aggregate Mixes with Damage and Healing
Research Thesis, 2006
Asphalt-aggregate mixes are being used throughout the world as a prime construction material for pavements. An asphalt mix is a multiphase heterogeneous material; it is a composite blend of air-voids, asphalt-cement (bitumen) and aggregates of a range of sizes. These materials exhibit extremely complex mechanical behavior that is very difficult to capture and model. Mainly for this reason available pavement-performance models are empirical, as no rigorous constitutive models were yet formulated for asphalt mixes. The motivation underlying this research work was to improve material modeling and characterization techniques for asphalt-aggregate mixes. An up-to-date review of literature revealed that current characterization efforts are limited principally because they deal with material behavior in uniaxial tests and provide essentially one-dimensional models. This dissertation presents the development of a triaxial viscoelastic-viscoplastic constitutive model for asphalt mixes including the effects of damage and healing. The model is confined to the description of pre-peak load response under isothermal conditions. It is based on additive separation of the total strain into viscoelastic and viscoplastic components and provides individual constitutive treatment to each part. The viscoelastic formulation is nonlinear, cross-anisotropic, and characterized by one unique (scalar) time-function. Three nonlinear isotopic effects are modeled: i) damage, i.e. loss of stiffness under load; ii) stiffening, i.e. increase of stiffness under compression conditions, and iii) healing, i.e., a decrease in the level of damage during rest periods. The viscoplastic equations resemble the kinematic-hardening formulations used to describe creep of metals. Internal stress-like variables are used to produce hardening (or softening) in each direction. Neither damage nor healing is included in the viscoplastic model. It should be noted that coupling is introduced between the individual formulations, making the viscoelastic response dependent also on the viscoplastic component. In order to support the development of the constitutive formulation, new experimental procedures were designed and executed using the triaxial apparatus. Creep and recovery test results are presented and analyzed, providing means (also) to calibrate and validate the model for biaxial stress-conditions and one test temperature. Good reproducibility and forecast-ability were obtained in the analyses of versatile test-data for both small and large strain load-cycles; indicating that the model is suitable for simulating the 3D load-response of asphalt-aggregate mixes. The constitutive development in this study constitutes the first attempt to describe the triaxial (viscoelastic-viscoplastic) load-response of asphalt materials including damage and healing. Several aspects of this development were found limited - specifically the ability to rigorously describe the viscoplastic behavior after large rest periods. Further research is needed to try and resolve this limitation and remove some of the other formulation restrictions.
Evaluation of the durability of asphalt mixtures depending on the physical properties of aggregates
Revista de la construcción
This study aims to assess the durability of asphalt mixtures through their performance versus damage by aging and water sensibility, depending on the physical properties of aggregates. Twelve types of asphalt mixtures were analyzed, using 3 types of aggregates (different shape and mineralogy), and 4 types of asphalt binders (2 conventional, 1 high modulus and 1 modified). The characterization of the aggregates is carried out following the protocol proposed by Zingg and the particle index (PI) according to ASTM-D3398. Both parameters have been correlated with the performance of the aggregate in the mixture. The performance of the mixtures is analyzed by the parameters of FENIX test, such as peak tensile load, stiffness rate and deformation capacity in 3 aging conditions and under wet conditions. The water sensitivity was also evaluated according to the UNE-EN12697-12. The results show that the physical characterization of the aggregates through Zingg method is not enough to predict the behavior of the mixtures studied because it considers only the coarse fraction. In turn, the results show that the angularity and surface texture of the fine fraction aggregates, analyzed through the PI parameter, has a significant effect on the performance versus moisture damage and aging of the asphalt mixtures tested.
Sustainability
Optimum stiffness and linear deformation in the unloading phase are fundamental properties of asphalt mixtures required for the durability of flexible pavements. In this research, blends of six different aggregate gradations were used for two base course (BC) and four wearing course (WC) asphalt mixtures. Stability and indirect tensile strength of resulting asphalt mixtures were evaluated to relate to viscoelastic unloading deformation and resilient moduli (instantaneous (MRI) and total (MRT)) at 25 °C using a 40/50 binder for 0.1 and 0.3 s load durations. Results indicated that an increase in coarse aggregate proportion from 48 to 70% for BC has shown a 12% and 14% increase in MRT for 0.1 and 0.3 s load durations, respectively, and an increase in coarse aggregate proportion from 41 to 57.5% for WC has caused a 26% and 20% increase in MRI for 0.1 and 0.3 s load durations, respectively. The same coarse aggregate proportions showed an increase in linear viscoelastic deformation at 0.1...
Investigating the role of aggregate structure in asphalt pavements
Naga Shashidhar, Xiaoxiong Zhong, Aroon V. Shenoy, Ernest J. Bastian Jr, 2000
An approach is presented in which the aggregate structure is taken into account to predict the mechanical response of unbound aggregates and asphalt concrete pavements. This approach could potentially permit scientific analysis of the various aggregate specifications and requirements in the asphalt industry. It has been demonstrated that asphalt concrete behaves as a granular material. The stress patterns within the material differs from the assumptions typically made in continuum models. It has been further shown that modeling the mechanical response of unbound aggregates using discrete element methods could take aggregate structure into consideration. The predictions of such an approach show that different gradations of the aggregate produce different load distributions in the pavements due to differences in aggregate structures.