Complex Modulus Of Asphalt Mixtures Research Articles

3D creep behaviour of asphalt mixtures: Experiment and modelling from complex modulus tests

The linear viscoelastic properties of asphalt mixtures can be characterised by the complex modulus or the creep compliance. This paper focused on the creep behaviour of asphalt mixtures and the relationship between the creep behaviour and the complex modulus of asphalt mixtures. 3D complex modulus tests, in which the complex modulus E* and complex Poisson’s ratio ν* were measured, were performed on three asphalt mixtures at six temperatures and five frequencies. The creep tests including loading at fixed constant stress of 0.5 MPa and recovery periods were carried out at three temperatures 15 °C, 30 °C and 60 °C. In addition to the axial strain, the radial strain was also measured in the creep test. The 2S2P1D model was used to simulate the E* and ν* of tested asphalt mixtures and then to calibrate the 3D generalized Kelvin–Voigt model. The evolution of axial and radial strain in the creep – recovery tests were modelled using the creep compliance function obtained from 3D generalized Kelvin–Voigt model. The results showed that the E* and ν* curves generated by 2S2P1D and GKV models fit well with the experimental ones. The transformation from 2S2P1D to generalized Kelvin–Voigt model respected the time–temperature superposition principle. The simulation of 3D creep tests can be established from generalized Kelvin–Voigt model and gives the similar results to experimental data for both loading and recovery period. The evolution of axial and radial strain in the creep – recovery tests respects the time–temperature superposition principle and can be modelled from the E* and ν* of tested asphalt mixtures.

Fabrication and numerical verification of two-dimensional random aggregate virtual specimens for asphalt mixture

This paper proposes a two-dimensional (2D) virtual specimen fabricating method to create 2D virtual specimens that can accurately characterize the microstructure of asphalt mixtures. In this method, the gradation of the 2D aggregate area and the proportion of 2D voids area are calculated by the phase separation method. Random aggregates are generated by using the geometric characteristics database of random aggregates, which is established by the Aggregate Image Measurement System (AIMS). The random aggregate placement angle is determined according to the aggregate angle distribution law. Random aggregates are placed using a method that accurately identifies the scope of the conflicting judgment, and the random voids are generated with image binarization. Four AC-13 2D random aggregate virtual specimens with the same gradation were generated in this research to verify the practicability and effectiveness of the virtual specimen fabricating method. Virtual complex modulus parallel tests were performed on these four virtual specimens in COMSOL finite element software. By comparing the calculated results with the lab test results, it was found that the 2D random aggregate virtual specimens generated by the method proposed in this paper adequately predict the complex modulus of asphalt mixtures. The virtual test is repeatable and provides new ideas and references to creating virtual specimens for asphalt mixtures.

Identification of the viscoelastic properties of an asphalt mixture using a scanning laser Doppler vibrometer

Estimating the master curve of the complex modulus of asphalt mixtures is essential for high quality and sustainable mixture and pavement design. There are multiple standard methods to estimate this master curve using hydraulic-pneumatic testing machines. These methods are complex to perform, need expensive equipment, and have constraints over the geometry of the testing samples. Therefore, the investigation of alternative methodologies to overcome these issues is of importance. In this research, an experimental setup coupled with a back-calculation technique is developed to identify the viscoelastic properties of an asphalt mixture using an optical measurement system. Using this system instead of traditional transducers eliminates the inaccuracies caused due to the attachment of a transducer to the specimen and allows feasible measurements on multiple points of the specimen. The developed method is compared with a standard method and an alternative method based on analytical formulas, and the results exhibit a good level of accuracy at a wide range of frequency and temperature. It is also demonstrated that even though this method can provide the master curve of a specimen with arbitrary geometry, the first natural frequency of the specimen at the highest temperature provides the first data point of the master curve at low frequencies. Therefore, the first natural frequency of the specimen should be considered while selecting the geometry of the test object.

Master Curve Establishment and Complex Modulus Evaluation of SBS-Modified Asphalt Mixture Reinforced with Basalt Fiber Based on Generalized Sigmoidal Model.

Basalt fiber has been proved to be a good modified material for asphalt mixture. The performance of basalt fiber modified asphalt mixture has been widely investigated by extensive researches. However, most studies focused on ordinary static load tests, and less attention was paid to the dynamic mechanical response of asphalt mixture incorporating with basalt fiber. This paper aims to establish the master curve of complex modulus of asphalt mixture incorporating of styrene-butadiene-styrene (SBS) polymer and basalt fiber using the generalized Sigmoidal model. Both loading frequency and temperature were investigated for dynamic mechanical response of asphalt mixture with basalt fiber. In addition, based on the time-temperature superposition principle, the master curves of complex modulus were constructed to reflect the dynamic mechanical response at an extended reduced frequency range at an arbitrary temperature. Results indicated that the generalized Sigmoidal model in this paper could better reflect the dynamic mechanical response accurately with correlation coefficients above 0.97, which is utilized to predict the dynamic mechanical performances accurately. Simultaneously, the modulus values exhibit an increasing trend with loading frequency and decrease versus temperature. However, the phase angle values showed different trends with frequency and temperature.

Improving the Accuracy of Dynamic Modulus Master Curves of Asphalt Mixtures Constructed Using Uniaxial Compressive Creep Tests

Uniaxial monotonic or constant loading tests have been used to efficiently construct master curves of the magnitude and phase angle of the complex modulus of asphalt mixtures. However, when using monotonic or constant tests, the test data fitting model and time-temperature shift factor equation were usually arbitrarily chosen to construct the master curves, which were not validated at all or were validated using only a couple of data points. This study developed a composite test protocol consisting of uniaxial compressive creep tests to construct the master curves and dynamic modulus tests to validate the constructed master curves of the magnitudes of the complex moduli of asphalt mixtures. The loading force and duration were carefully chosen for each test segment to assure that the test specimen was retained in the linear viscoelastic stage during the entire test protocol. Commonly used data fitting models and time-temperature shift factor equations were compared, respectively, when constructing and validating the master curves. The Prony series model was determined to be the best fitting model for the creep strains because it not only provided satisfactory modeling accuracy but also complied with the actual material properties of asphalt mixtures. The Williams–Landel–Ferry (WLF) equation was demonstrated to be the most appropriate time-temperature shift factor equation because the master curve constructed with the WLF equation made the most accurate predictions in a fairly wide range of frequencies. This study refined the master curve construction method for asphalt mixtures using uniaxial monotonic or constant loading tests. With properly selected and validated data fitting model and time-temperature shift factor equation, the constructed master curve had the capability of accurately predicting the magnitude of the complex modulus in a wide range of loading frequencies with an R2 value of approximately 0.97 or higher.

On the impact of the filler on the complex modulus of asphalt mixtures

Mineral fillers have been used in asphalt mixtures from their very beginning. Rapidly, formulators observed that the filler could have a strong influence on the final mixture. Nowadays, mineral fillers are essentially characterised by their ability to stiffen the bitumen, leading to the development of the celebrated “Rigden air voids” RV (EN 1097-4), now the corner stone of the European specifications for fillers in asphalt mixtures. If the way the filler modifies the rheological properties of mastics is now well described, the relationship between mastic and mixture properties remains poorly understood. Therefore, this paper describes a systematic study on the impact of increasing filler contents on asphalt mixtures. Experimental results are presented for two very different asphalt mixture formulas, a regular semi-dense Asphalt Concrete (AC) and an open-graded AC for very thin layers (BBTM). The complex modulus of each formula was measured as a function of filler content with two materials having very different stiffening behaviour, that is, limestone filler (LF) and hydrated lime (HL). For both formulas and regardless of the type of filler, the modulus versus filler content curves exhibited a well-defined maximum. The filler content at which the maximum was found was strongly filler dependent, with HL formulas peaking at 4 wt.% and LF formulas peaking at 10 wt.%. An original interpretation based on reduced variables was proposed, showing that the maximum in modulus occurred for reduced volume fraction of filler close to 66%. This suggests that, before the peak, modulus increased with filler content as a consequence of a stiffer mastic, and after the peak, modulus started to decrease due to poor aggregate coating as a consequence of too-viscous a mastic. As a practical consequence, the studied mix formulas could accept higher filler loadings than currently specified, which would potentially stiffen the mixtures by ∼20%. Therefore, one way to benefit from these findings without changing the specifications would be to use blends of HL and LF. Of course, these findings must be validated with other mixture formulas and raw materials, and similar systematic study on the filler impact on the resistance to fatigue and moisture damage would be needed before implementing the conclusions based on modulus only.

Study of normal and shear material properties for viscoelastic model of asphalt mixture by discrete element method

In this paper, the viscoelastic behavior of asphalt mixture was studied by using discrete element method. The dynamic properties of asphalt mixture were captured by implementing Burger’s contact model. Different ways of taking into account of the normal and shear material properties of asphalt mixtures have been reviewed. Two models, Model I and Model II, with different design parameters were developed and compared. For Model I, Burger’s model parameters in normal and shear direction were calibrated by using laboratory test results from Frequency Sweep Test performed in both normal and shear direction, respectively; while for Model II, the same calibrated parameters in the normal direction were used, but the values for the shear direction were chosen to be equal with the normal direction. The complex modulus of asphalt mixtures were predicted for both optimized models by conducting DE simulation under dynamic strain control loading. A sensitivity study was carried out, where the effects of different design parameters on the dynamic properties of asphalt mixture has been investigated, including the eight parameters of Burger’s model and the friction coefficient.

Prediction of Dynamic Modulus of Asphalt Mixture Based on Viscoelastic Properties of Asphalt Mortar

The objective of this study is to predict the asphalt mixture dynamic modulus using the viscoelastic properties of asphalt mortar. The dynamic viscoelastic characteristics of asphalt mortar and asphalt mixture are tested at different temperatures and loading frequencies using dynamic shear rheometer, such as storage modulus, loss modulus and phase angle. The complex modulus of asphalt mortar, volume parameters and Hirsch model were used to predict complex modulus of asphalt mixture. The experimental measurements of dynamic modulus were employed to compare with the predictions. The result showed this method can predict complex modulus of asphalt mixture very well.

Approach for Quantifying the Effect of Binder Oxidative Aging on the Viscoelastic Properties of Asphalt Mixtures

Because of the noted influence of oxidative aging on mixture properties and pavement performance, it is becoming imperative to have a more complete understanding of the influence of asphalt binder aging on the viscoelastic behavior of asphalt mixtures. This study proposes a new approach to correlating the oxidative aging of asphalt binder for carbonyl functional groups with the viscoelastic behavior of asphalt mixtures for a continuous relaxation spectrum. The asphalt mixture complex modulus and the carbonyl area for the recovered asphalt binder were measured for mixtures subjected to varying durations of long-term aging in the laboratory. The continuous relaxation spectrum was obtained analytically from the 2S2P1D model of complex modulus of asphalt mixture through the inverse Fourier–Laplace transform approach. A consistent horizontal shift in the continuous relaxation spectrum was observed for all mixtures with the increase in aging duration. However, the shape and the amount of shifting of the spectra were mixture dependent. In particular, mixtures with higher asphalt binder absorption exhibited the greatest shift in the continuous spectra for both unmodified and polymer-modified asphalt mixtures. Good correlations were observed between the carbonyl in asphalt binder and the continuous relaxation spectrum parameters of the asphalt mixture. Such relationships should permit the incorporation of long-term oxidative aging directly into the constitutive equation used in pavement response analyses.

A micromechanical based three-dimensional DEM approach to characterize the complex modulus of asphalt mixtures

Asphalt mixture is a typical particulate system whose mechanical performance can be modeled based on the particle interactions (contacts) at microscale. Once calibrated microscopically, the model can be used to predict the macroscopic properties based on the same contact mechanisms. The effect of individual component on the performance of the mixture can also be captured by the calibrated micromechanical model. In this paper, a three-dimensional DEM approach is developed to model the complex modulus of asphalt mixtures. Complex modulus is a crucial viscoelastic material property which describes the stress and strain responses of asphalt mixtures and has been incorporated into the DARWin-ME mechanistic empirical pavement design program to guide pavement design. The model is found to be successful in predicting both the dynamic modulus and the phase angle of a conventional dense graded asphalt mixture over wide ranges of temperatures and frequencies. The nonlinear behavior of the phase angle at high temperature is also captured. The effect of the key model parameters on the dynamic modulus and the phase angle are evaluated which contributes to the fundamental understanding of the material mechanical properties and provides meaningful guidance for the design and modeling of asphalt mixtures.