With the attempt to reduce traffic noise, porous asphalt (PA) mixture is widely used as a wearing course on the highways in the Netherlands. However, due to the open structure, PA mix pavement easily suffers from the loss of individual aggregates from its surface, which is named as ravelling. After the initial ravelling, the damage can rapidly develop into potholes which can significantly reduce the driving safety of the pavement....@en

Crumb rubber modified bitumen (CRMB) can be regarded as a binary composite system in which swollen rubber particles are embedded in the bitumen matrix. Previous study has successfully implemented the micromechanics models in predicting the complex moduli of CRMB binders using more representative constituent parameters. In the regime of master curves, while the micromechanics models used predicted well in the high-frequency range, they underestimated the complex modulus in the low-frequency range. The current study aims to further improve the prediction accuracy of micromechanics models for CRMB by considering the interparticle interactions. To accomplish this goal, a new reinforcement mechanism called chain entanglement effect was introduced to account for the interparticle interaction effect. Results show that the polymer chain entanglement effect accounts for the underestimation of complex modulus and lack of elasticity (overestimation of phase angle) for CRMB at high temperatures/low frequencies. The mechanical properties of bitumen matrix and entangled polymer network can be determined based on the rubber content. The introduction of the entangled polymer network to the generalized self-consistent model significantly improved the prediction accuracy for both complex modulus and phase angle in the whole frequency range. In summary, by incorporating the physio-chemical interaction mechanism into the currently available models, a new dedicated micromechanics model for predicting the mechanical properties of CRMB has been developed. The predicted viscoelastic behaviors can thereafter be used as inputs for an improved mix design.@en

Crumb rubber modified bitumen (CRMB) can be regarded as a binary composite system in which swollen rubber particles are embedded in the bitumen matrix. The current study aims to further improve the prediction accuracy of micromechanical models for CRMB by considering the interparticle interactions. To accomplish this goal, two different strategies were used. Firstly, the (n+1)-phase model was applied to the CRMB system by considering the multilayer properties of swollen rubber particles. Secondly, a new micromechanical scheme called the J-C model was used to account for the interparticle interaction issue. Results show that the (n+1)-phase models slightly increase the prediction accuracy but the underestimation of complex modulus at lower frequencies remains unsolved. The J-C model remedies the underestimation of modulus in the low-frequency range by other models and provides an overall improvement for the relative prediction accuracy by properly addressing the interparticle interactions from the perspective of particle configuration.

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Asphalt mixtures with high porosities (known as porous asphalt (PA) mixes) are becoming a popular choice among road authorities as it provides better skid resistance while also reducing tire-pavement noises. Towards the design and manufacture of PA mix pavement, the evaluation of the mechanical properties of PA mixes is of great importance. To predict the mechanical properties of PA mixes, micromechanical models have been considered as an effective tool. In most research studies, continuum-based micromechanical models, i.e. the Self-consistent model, the Mori-Tanaka model, etc. are widely used to predict the stiffness of asphalt mixtures. However, the limitation of these models is that they cannot describe the characteristics of individual particles and thus they cannot provide accurate predictions. On the other hand, the discrete-based micromechanical model (DBMM) which simulates a granular material as an assembly of bonded particles seems to be a promising alternative. Limited research studies have focused on studying the utilization and the applicability of this model for asphalt mixes. Therefore, this paper aims to propose a framework to use DBMM and to evaluate its performance in estimating a PA mix's stiffness. Based on the obtained results, both the merits and limitations of this model were highlighted.

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Micromechanics, which can be used to relate the properties of a composite to the properties of individual constituents, is considered a good approach to understanding the fundamental mechanisms behind the behavior of asphalt materials. Compared with the semi-empirical and numerical micromechanical models, analytical micromechanical models do not need calibration factors. In addition, they can provide analytical solutions on the basis of a series of assumptions. Using these models, researchers have separated the effects of different stiffening mechanisms (i.e., the volume-filling reinforcement, the physicochemical reinforcement, and the particle-contact reinforcement) for mastic. However, similar research work has not been conducted for asphalt mixtures and, moreover, the characteristics of the particle-contact reinforcement have not been deeply analyzed by researchers. Therefore, this paper aims to understand the stiffness of asphalt mixture through micromechanics. The focus of this study was on porous asphalt mixture where particle-contact reinforcement plays an important role in its behavior. The stiffening effects of different mechanisms were separated using analytical micromechanical models. The effects of temperature/frequency and the properties of the matrix phase on the stiffening effect of the particle-contact reinforcement were analyzed.@en

The mechanical properties of asphalt mixture are always required for the evaluation of the durability of pavements. In order to obtain these properties without conducting expensive laboratory tests and using calibrated empirical models, research studies have been carried out to develop micromechanics-based models. Continuum-based micromechanical models (CBMM), which are developed based on continuum mechanics, have increasingly been utilized to estimate the mechanical properties of asphalt materials based on the fundamental properties of individual constituents. These analytical models are expected to provide reliable predictions without the need for extensive computational facilities. Although the utilization of CBMM has been presented by several past studies, most of the studies do not provide a concise and critical review of these models. Therefore, in this paper, a complete review of CBMM was presented. Commonly used CBMM were introduced in detail and their advantages and disadvantages were discussed and compared. Comprehensive summaries and critical discussions about their current utilization and limitations for predicting the mechanical properties of asphalt materials were given. Further modifications and new development for addressing the limitations were extensively described and discussed. In the end, research challenges were highlighted and future recommendations from different perspectives were proposed.@en

Crumb rubber modified bitumen (CRMB) can be considered as a binary composite system where rubber particles are embedded in the bitumen matrix. The bitumen-rubber interaction process (mainly swelling) significantly changes the mechanical properties of both bitumen and rubber phases. This study aims to predict the complex moduli of CRMB binders with more representative constituent parameters using micromechanical models. To achieve this goal, frequency sweep tests using a dynamic shear rheometer were performed on the liquid phase of CRMB and swollen rubber samples to represent the essential properties of bitumen matrix and rubber inclusion. In addition, the numerical swelling model was developed to estimate the effective volume concentration of rubber after swelling. Results show that the liquid phases of CRMB are stiffer and more elastic than the neat bitumen while the swollen rubber is softer and more viscous than the dry rubber. The effective volume concentration of rubber can increase to 2.126 times as the blend percentage based on the finite element analysis. Using the liquid phase of CRMB binder and swollen rubber properties as the micromechanical model inputs yield more accurate predictions. The used four micromechanical models predict well at higher frequencies while underestimating the complex modulus at lower frequencies.

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In the Netherlands, more than 80% of the highways are surfaced by porous asphalt (PA) mixes. The benefits of using PA mixes include, among others, the reduction of noise and the improvement of skid resistance. However, pavements with PA mixes are known to have a shorter lifetime and higher maintenance costs as compared with traditional dense asphalt mixes. Raveling is one of the most prominent distresses that occur on PA mix pavements. To analyze the raveling distress of a PA mix pavement, the stress and strain fields at the component level are required. Computational models based on finite element methods (FEM), discrete element methods (DEM), or both, can be used to compute local stress and strain fields. However, they require the development of large FEM meshes and large-scale computational facilities. As an alternative, the homogenization technique provides a way to calculate the stress and strain fields at the component level without the need for much computation power. This study aims to propose a new approach to analyze the raveling distress of a PA mix pavement by using the homogenization technique. To demonstrate the application of the proposed approach, a real field-like example was presented. In the real field-like example, the Mori?Tanaka model was used as a homogenization technique. The commonly available pavement analysis tool 3D-MOVE was used to compute the response of the analyzed pavement. In general, it was concluded that the homogenization technique could be a reliable and effective way to analyze the raveling distress of a PA mix pavement.@en

Microwave heating is a promising heating technology for the maintenance, recycling and deicing of pavement structures. Many experimental studies have been conducted to investigate the microwave heating properties of asphalt mixtures in the laboratory. However, very few studies investigated the application of microwave heating on asphalt pavements. This study aims to simulate microwave heating of paving materials using the finite element method. Results show that the developed three-dimensional model, which couples the physics of electromagnetic waves and heat transfer, shows a great potential for optimizing the design of microwave heating prototypes for pavement applications.@en

Oxidative aging is a complex phenomenon in bitumen and its fundamental understanding is needed to optimize paving materials with long-lasting characteristics. This research reports on a diffuse-reaction model for predicting the oxidation of bituminous binders over time and under different conditions. As known, the oxidation of bitumen is affected by the material chemistry, film thickness and temperature. Thus, these factors were considered in this research to simulate the oxidation of a thin bitumen film. Carbon compounds were assumed as the oxidation index of a model bitumen and analyses were performed enabling prediction of chemical compositional changes. In the future, the current model can be used to simulate the actual oxidative aging in (un)modified binders, such as epoxy modified asphalt, presented in a companion paper (Apostolidis et al., Kinetics of Epoxy-Asphalt Oxidation. AM3P).@en

Fundamental models should be developed and utilized in order to facilitate the chemo-mechanical design of modified binder systems for paving applications but not only. Especially, the fact that the incorporation of new chemical substances used as bio-based modifiers or alternative binders is attracting great interest to replace traditional technologies, the development of tools able to provide insight into the various physio-chemical phenomena is crucial. Among other polymer-bitumen interaction phenomena, the dissolution mechanism of polymers in bitumen is a significant aspect that should be considering in order to enhance binder properties through polymer modification. The current research gives emphasis on modelling the mechanism of dissolution for rubbery polymers in bitumen.@en

Micromechanical modelling has been widely used to predict the properties of asphalt mixes. In comparison to numerical micromechanical models, analytical micromechanical models have the benefits of consuming much less time and facilities. The most commonly used analytical micromechanical models are the Eshelby-based micromechanical models. However, without the consideration of particles’ interactions, these models fail to accurately predict the properties of asphalt mixes, especially at high temperatures. In porous asphalt (PA) mixes, due to the formation of an interconnected aggregate network, the particles’ interactions under a compressive loading condition at high temperatures mainly refer to the packing effects. In order to describe the behavior of packed aggregates in PA mixes, Walton’s model which predicts the effective moduli of a pack of spherical particles is possible to be a suitable way. However, to the best of authors’ knowledge, this model has not been utilized for asphalt mixes. Therefore, this paper aims to investigate the application of Walton’s model for predicting the properties of PA mixes at high temperatures.@en

Within the pavement engineering community, porous asphalt (PA) mixes are regarded as mixes capable of reducing noise and improving wet skid resistance. However, these mixes are likely to have the distress of ravelling. In order to analyse the propensity of a given PA mix for ravelling, the homogenisation technique can be considered as an attractive method. Along the line of the homogenisation technique, micromechanical models have been used to predict the stiffness of asphalt mixes. However, it was found that the predicted results were not in good agreement with the experimental values due to the fact that the stiffness of interacted aggregates was not accurately accounted in the models. To deal with this issue, it is important for researchers to study the stiffness of the interacted aggregates network and its role in the behaviour of a given mix. Based on this realisation, this paper provided a methodology to estimate the stiffness of the stone-on-stone skeleton and its role in the overall response of PA mixes. The results showed that the predicted stiffness of the stone-on-stone skeleton is dependent on the loading frequency/temperature and the compaction effort. The frequency response of the stone-on-stone skeleton is similar to that of the mix.@en

Open-graded mixes (OGM) are gaining more attention due to their noise reduction capability and better anti-skiding performance. However, due to the high air voids content, raveling is the most commonly observed distress type on pavements with OGM. In order to analyze the propensity of a given mix for raveling, in the previous work, the authors proposed the homogenization technique. It was notified that although Hirsch model performs quite well, it requires calibrations from laboratory tests. On the other hand, micromechanical modes did not show good agreement with laboratory tests. It was also observed that the micromechanical models could not capture the interactions among packed aggregates, and thus effective modulus was significantly lower than the experimental results at low frequencies. Based on these realizations, Dvorkin’s model which is based on contact theory, is supposed to perform better as notified by past researchers who applied this model for dense asphalt mixtures. To the best of authors’ knowledge, the performance of Dvorkin’s model in the case of OGM has not been studied. The aim of this paper is to demonstrate a procedure to implement Dvorkin’s model and highlight the performance of this model in predicting the effective modulus of OGM. The predicted results by this model were compared with (1) measured values from uniaxial compressive tests and (2) the predictions by micromechanical models. Sensitivity analyses were carried out to find out optimal input parameters.@en

Researchers employ the micromechanical modeling to understand the behavior of a mix at a component level. In the micromechanical modeling the effective properties of a given mix are calculated on the basis of the mechanical and geometrical properties of individual components. It was notified that although numerical models are able to handle complex compositions and almost realistic mix component material properties, it requires dense meshes and thus large-scale computational facilities. On the other hand, micromechanical models with analytical solutions can produce accurate predictions of the mechanical properties of a mix with less computational demands once they are calibrated/validated. Although researchers have carried out much work on evaluating the predictions by micromechanical models for asphalt mixtures, none of the research studies considered the effect of the loading conditions on the performance of these models. Since micromechanical models are insensitive to loading conditions it is considered that the discrepancies between the predictions and experimental results could be associated with the loading conditions. On the basis of these realizations, in this study the predictions of micromechanical models were compared to the effective modulus of dense asphalt mixture and open asphalt mixture under different loading conditions. The main aim of this paper is to investigate the effects of the loading condition and the type of the mix on the performance of micromechanical models.@en

Warm mix asphalt (WMA) technology has been increasingly utilised in rubberised asphalt pavements to reduce the production and compaction temperatures and the incidental fumes and odours. This study aims to investigate the high, intermediate and low-temperature performance of crumb rubber modified asphalt binders containing WMA additives. The asphalt-rubber interactions under various mixing combinations of temperature and time were investigated through both microscopic and mechanical methods to obtain the optimum mixing procedure. The effects of WMA additives (wax-based and chemical-based products) on the binder performance were investigated by multiple stress creep and recovery (MSCR) test, linear amplitude sweep (LAS) test and low-temperature frequency sweep test. Results show that rubberised asphalt binders significantly improve the binder performance of base asphalt at different temperature ranges. The effects of WMA additives on binder performance varied with base asphalt and rubberised asphalt binder. In addition, the nonrecoverable compliance difference was found not suitable to characterise the stress sensitivity of rubberised binders and the difference in the nonrecoverable compliance for an incremental change in applied stress was proved to be a more accurate alternative. For the cyclic LAS test, the failure energy was found to have a strong correlation with the predicted fatigue life using simplified viscoelastic continuum damage analysis and therefore can be considered as a simple indicator for binder fatigue performance ranking. Relaxation modulus and rate derived from low-temperature frequency sweep tests produced comparable results for ranking the low-temperature performance of different binders. It is feasible and promising to use a unified DSR test methodology to characterise the binder performance covering the whole service temperature range.

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ZOAB (Zeer Open Asphalt Beton) is the most widely used asphalt mixture in the Netherlands. As a type of open asphalt mixture, it is known to suffer from raveling distress. In order to analyze the propensity of raveling, micromechanical models are considered effective. However, most of the research work about micromechanical models has focused on dense asphalt mixture and the application of these models on ZOAB mixes has not been paid adequate attention. Therefore, in this research study, the performance of various micromechanical models for predicting mechanical properties of ZOAB was evaluated. The predicted results were compared with the measured values from a dynamic uniaxial compression test. The analysis results showed that none of the applied micromechanical models could obtain acceptable predicted results of the dynamic Young’s modulus and phase angle of ZOAB. On one hand, the Dilute model, the Mori-Tanaka model, the generalized self-consistent model and the Lielens’ model provided lower values of dynamic Young’s modulus and higher values of phase angle, whereas, for the self-consistent model, the predicted results of dynamic Young’s modulus were higher, and the values of phase angle were lower. On the other hand, the shapes of the predicted master curves of both dynamic modulus and phase angle of ZOAB could not match well with the experimental results. The further research on the differential scheme method showed that at lower frequencies the predicted mechanical properties of ZOAB mixes by the applied micromechanical models could not be improved even by following this scheme.

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Within the pavement engineering community, open graded mixes (OGM) are regarded as mixes capable of reducing noise and improving wet skid resistance. However, during their design life, these asphalt mixes are known to suffer from a particular distress type known as raveling. This results in a premature failure of a road network. In order to study the propensity of OGM to raveling, homogenization-based approaches are considered to be accessible and effective. One of the most widely accepted homogenization models for asphalt concrete is proposed by Christensen et al. Several studies related to homogenization techniques have been conducted in the past; however, to the best of the authors’ knowledge not a lot of attention has been paid to the study of OGM by means of homogenization models. The other limitation of the Christensen model is that some parameters are difficult to physically understand. Under the above realization, the objective of the paper is twofold: (1) to propose a modification of the Christensen model for OGM; and (2) to verify the modified model’s capability in predicting the mechanical properties of OGM. In general, it was found that once the proposed factor is calibrated for a given OGM by laboratory tests the obtained results are accurate.@en

Within pavement engineering community, Open Graded (OG) mixes are regarded as mixes capable of reducing noise and improving wet skid resistance. Due to the formation of interconnected aggregate network, aggregate packing plays an important role in the overall response of OG mixes. However, some of the models developed in the past do not consider the effect of aggregate packing in their predictions which leads to unreliable predicted results. Thus, it is important to understand the contribution of packed aggregates in the prediction of mechanical properties of OG mixes. In this paper, a procedure for obtaining the stiffness of packed aggregates in OG mixes was proposed by mean of micromechanics (the Mori-Tanaka model) and it was verified on the basis of laboratory tests. The results showed that the calculated stiffness of packed aggregates is frequency dependent which needs to be considered in the prediction of mechanical properties of OG mixes.@en