Response Of Asphalt Concrete Research Articles

Numerical study of the aggregate contact effect on the complex modulus of asphalt concrete

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.

Use of angle of repose of aggregates as an indicator of asphalt concrete properties

Morphological characteristics of aggregates play a significant role in overall response of asphalt concrete under mechanical loading. Due to its simplicity, minimal resources, and less testing time, Angle of Repose (AoR) has been used by various researchers to characterize morphological characteristics of aggregate. This work presents the results of an experimental study to develop interrelationship between AoR and macroscopic response of Asphalt Concrete (AC). For this purpose, (i) aggregates were artificially smoothened through abrasion to different levels of texture/smoothness, and (ii) AoR was measured. Three AC mixtures were prepared using these processed aggregates and were tested for volumetric, strength and flow properties. A strong positive correlation was found between AoR and Marshall parameters (stability, Marshall quotient, retained stability, retained Marshall quotient). A weak negative correlation was found between AoR and Marshall flow. This can be attributed to interparticle resistance developed during compaction and mechanical loading. Thus, it can be concluded that AoR can be effectively used to capture changes in aggregate morphology and AC response.

Interrelationship between uncompacted void content of aggregates and asphalt concrete properties

ABSTRACTIt is a well known fact that morphological characteristics of aggregates influence the mechanical response of asphalt concrete (AC). Uncompacted void content (UVC) of aggregate is often related to their morphological characteristics. A detailed experimental study was performed to explore relationship between UVC and mechanical response of AC. Initially, aggregates were artificially smoothened using Los Angles abrasion testing machine. UVC was then evaluated using blended aggregates at five levels of smoothness/texture. Increasing aggregate smoothness resulted in decreased UVC values. Higher UVC was obtained with a finer gradation compared to coarse gradation. Three AC mixtures prepared using these smoothened aggregates were tested for their strength and flow properties. A linear, strong positive correlation was found between UVC and Marshall parameters (stability, Marshall quotient). This can be attributed to interparticle resistance developed during mechanical loading. Parabolic relations were found between UVC and retained Marshall parameters. The retained Marshall parameters was found to be a function of UVC. It is concluded that UVC can be effectively used to capture changes in aggregate morphology and AC response.

Analysis of transient viscoelastic response of asphalt concrete using frequency domain approach

The analysis of transient linear viscoelastic response of asphalt concrete (AC) is important for engineering applications. The traditional transient response of AC is analyzed in the time domain by performing complicated convolution integral. The frequency domain approach allows one to determine the transient responses by performing simple multiplication instead of the complicated convolution integral, and it does not require the time derivative of the input excitation, and thus, the approach could greatly reduce the analysis complexity. This study investigated the frequency domain approach in calculating the transient response by utilizing the discrete Fourier transform technique. The accuracy and effectiveness of the frequency domain approach were verified by comparing the analytical and calculated responses for the standard 3-parameter Maxwell model and by comparing the time and frequency domain solutions for AC. The effect of aliasing of the frequency domain approach can effectively reduce by selecting a small sampling interval for the time domain excitation function. A sampling interval is acceptable as long as the amplitude of the Fourier transformed excitation is close to 0 more than half of the sampling rate. The results show that the frequency domain approach provides a simple and accurate way to perform linear viscoelastic analysis of AC.

Cyclic triaxial tests on bituminous mixtures

In the framework of a study on rutting of bituminous layers, thermo-controlled triaxial tests under constant confining pressure on homogeneous asphalt concrete specimen have been performed. These tests are cyclic sinusoidal tests, with the aim to reproduce in some way the loading and unloading phases due to traffic on roads. The influences of confinement and load frequency have been studied. Tests have highlighted that confinement notably decreases the strain rate as well as the level of strain reached at the end of the test. The tests did not allow to clearly observe the influence of frequency on the response of the material, due to problems of repeatability. In order to avoid such problems, another test procedure was adopted. It consisted in changing either confinement or frequency during the test, with the purpose to see an evolution of strain rate, and to establish correlations. These tests have clearly shown the influence of cyclic loading and the relative influence of loading frequency on the response of asphalt concrete. The main objective of this triaxial programme is to develop a numerical model, based on rheological laws, able to predict the behaviour of bituminous layers.

Microstructural modeling of asphalt concrete using a coupled moisture–mechanical constitutive relationship

The coupled effect of moisture diffusion and mechanical loading on the microstructure of asphalt concrete is studied. The traditional Continuum Damage Mechanics (CDM) framework is modified to model detrimental effects of moisture and mechanical loading. Adhesive/cohesive moisture-induced damage constitutive relationships are proposed to describe the time-dependent degradation of material properties due to moisture. X-ray two-dimensional (2D) computed tomography-imaging technique is used to construct finite element (FE) microstructural representation of a typical dense-graded asphalt concrete. After being calibrated against pull-off experiments, the proposed moisture-induced damage constitutive relationship, which is coupled to thermo-viscoelastic–viscoplastic–viscodamage mechanisms, is used to simulate the microstructure of asphalt concrete. Simulation results demonstrate that the generated 2D FE microstructural representation along with the coupled moisture–mechanical constitutive relationship can be effectively used to model the overall thermo-hygro-mechanical response of asphalt concrete.

Laboratory investigation of asphalt concrete dynamic modulus testing on the criteria of meeting linear viscoelastic requirements

In this study, asphalt concrete (AC) dynamic modulus testing is evaluated for meeting linear viscoelasticity (LVE) requirements. The evaluation is performed by performing stress sweep dynamic modulus testing at selected test frequencies. Three well-known linearity principles, namely superposition principle, proportionality principle and 5% stiffness reduction principle, are implemented to classify the AC response as a LVE. Analysis of the stress sweep dynamic modulus test results using these principles consistently showed that the stress and strain linearity limits for the studied mix are 155, 414, 506, 772, 470 and 462 kPa and 55, 69, 84, 99, 36 and 50 micro-strain each for loading frequency of 0.1, 0.5, 1, 5, 10 and 25 Hz, respectively. The current dynamic modulus testing standard, American Association of State Highway and Transportation Officials (AASHTO) TP-62, recommends maintaining test strains and stresses below 150 micro-strain and 700 kPa to make sure AC dynamic modulus testing is preformed within LVE range. However, these recommended limiting stress and strain values are considerably higher than the results found in this study. Moreover, the softening rate at which the dynamic modulus decreases is found to be approximately the same over the entire stress sweep range, which indicates the dependency of dynamic modulus on stress. This indicates that there is a significant chance of conducting the dynamic modulus test at stress and stain levels beyond the LVE region.

Characterization of Microdamage Healing in Asphalt Concrete with a Smeared Continuum Damage Approach

The fatigue of asphalt concrete (AC) mixtures is a major source of distress in pavement structures. In most laboratory fatigue studies, testing is carried out by means of continuously repeating load cycles. However, under real traffic conditions, AC is subjected to a succession of load pulses as the traffic passes. Between these loads the material is not subjected to external forces and undergoes changes related to relaxation and healing, which together act to increase the overall fatigue life relative to what is observed under continuous loading. In the study reported in this paper, the effects of a rest period on the fatigue response in uniaxial, on-specimen, displacement-controlled loading was studied at 4.48C, 21.18C, and 388C. Loading consisted of a sinusoidal pulse–rest history with rest periods of 1, 5, or 10 s. Mixtures were used with the same aggregate structure and constituent materials but different asphalt and air void (AV) contents. It was found that the introduction of even short rest periods substantially improved the fatigue response of AC. A viscoelastic damage model for smeared healing was derived and characterized with the measured data. This model was compared with a damage-only continuum formulation and used to identify the impacts of healing, apart from any viscoelastic relaxation effects. It was found that greater AC and lower AV content yielded more favorable healing behaviors. The findings also suggested differences in the active healing mechanisms in pulse–rest and block–rest loading, typically used to study healing in AC. These differences are discussed.

Constitutive Modeling of Cyclic Viscoplastic Response of Asphalt Concrete

Rutting (permanent deformation) is the most important distress that asphalt concrete pavements are prone to at high temperatures. However, available classical viscoplastic (VP) theories are incapable of properly predicting permanent deformation of asphalt concrete materials at high temperatures subjected to cyclic loading conditions. A physically based VP hardening relaxation model is proposed to overcome this issue. This model considers the changes in the microstructure that occur during the rest period and cause the induced hardening stress to relax and recover. This mechanism affects the VP properties of asphalt concrete before and after the rest period is applied. A memory surface is introduced as the general criterion for the evolution of the hardening relaxation mechanism. The memory surface possesses a state variable memorizing the history of the VP deformation during the loading history. The proposed VP hardening relaxation model is coupled with Schapery's viscoelastic model and the classical Perzyna's VP model and is validated against extensive experimental data, including repeated creep and recovery tests at different stress levels, loading times, and rest periods. It is shown that the proposed model reasonably predicts the cyclic VP response of asphalt concrete materials at high temperatures.

First Observation of Blending-Zone Morphology at Interface of Reclaimed Asphalt Binder and Virgin Bitumen

One of the challenges in designing recycled asphalt mixtures with a high amount of reclaimed asphalt pavement (RAP) is estimating the blending degree between RAP binder and added virgin bitumen. The extent of blending is crucial because asphalt concrete response is influenced by the final binder properties. This paper focuses on the evaluation of interaction and extent of blending between RAP binder and virgin bitumen by studying the microstructures of the blending zone with atomic force microscopy (AFM). AFM is used to probe the change of microstructural properties from a RAP binder and virgin bitumen to the blending zone of these two. Averaged microstructural properties have been observed in thin-film blends of RAP binder and pure bitumen. The morphology of the blending zone (spatial extent of about 50 μm) exhibits domains of a wide range of microstructure sizes (160 nm to 2.07 μm) and can be considered to be a completely blended new material that has been observed directly for the first time. The fully blended binder properties are found to be between those of the two individual binders, as could be inferred from the averaged microstructural properties derived from AFM images of the blending zone. This finding is also consistent with the results of mechanical tests by dynamic shear rheometer on the same materials. Finally, a design formula is proposed that relates the spatial dimensions of the blending zone to temperature and mixing time, which will eventually allow the results of this study to be extended from small-length scales up to the engineering level.

Experiment and Research of Using Fiber Bragg Grating to Monitor the Dynamic Response of Asphalt Concrete

Based on the fiber Bragg grating sensing technology, a FBG sensor is designed to monitor the dynamic response of asphalt concrete; the sensitivity coefficient of FBG sensor is 1.28pm/με. Through the static load test, the correlation between strain and wavelength variation is 0.1797µε/pm, after second packaged and embedded in SMA-13 asphalt concrete. The instantaneous impact on the road of high-speed vehicles is simulated by using drop hammer. The results are shown that the Sensor can satisfy the requirements of practical applications and succeed in monitoring the dynamic response of asphalt concrete. The monitored signal can reflect the viscoelastic-plastic deformation law of asphalt concrete. The sensor can be used to monitor the dynamic response of asphalt concrete.

Application of a viscoelastoplastic continuum damage tensile model to asphalt mixes in Sweden

ABSTRACT The objective of the research presented herein was to apply a viscoelastoplastic continuum damage (VEPCD) model on three Swedish mixes. The procedure includes the theoretical models and the supporting experimental testing protocols necessary for predicting responses of asphalt mixtures subjected to uniaxial tension loading. The model encompasses the elastic, viscoelastic, plastic and viscoplastic components of asphalt concrete behavior. Addressed are some of the major factors affecting asphalt concrete response, such as rate of loading, loading time, stress/strain amplitude, temperature, and damage. The modeling strategy is based on developing separate models for strain components and then integrating those models to form a viscoelastoplastic continuum damage (VEPCD) model. The developed model accurately predicts responses up to localization when microcracks start to coalesce and grow. However, once major macrocracks develop and propagate, the currently developed model ceases to predict responses accurately. The model was also used to evaluate and compare fatigue and rutting potential of the mixtures.

3D Finite Element Model for Asphalt Concrete Response Simulation

ABSTRACT An extensive experimental, analytical, and numerical investigation on the response of asphalt concrete is currently in progress at Delft University of Technology. The objectives of this Asphalt Concrete Response (ACRe) project are: (a) the formulation and finite element implementation of a three-dimensional, strain-rate sensitive, temperature- and loading history-dependent constitutive model, and (b) the development of the necessary experimental set-ups, testing procedures, and data analysis methods for determination of the model parameters. These objectives are strongly interrelated: on the one hand, the model dictates what should be measured in a test, while on the other hand, the response observed in the tests sets the requirements for the model. As a result, model development/verification and experimental testing have been progressing in parallel throughout the project. In this contribution both the finite element and the experimental aspects of the project will be presented. The constitutive...