Viscoplastic modeling of asphalt mixes with the effects of anisotropy, damage and aggregate characteristics

This paper presents an approach for constitutive modeling of the viscoplastic behavior of asphalt mixes. This approach utilizes an anisotropic non-associated flow rule based on the Drucker–Prager yield surface. The selection of this yield surface is motivated by the field stress paths and material properties associated with permanent deformation at high temperatures. The efficacy of the model is demonstrated by analyzing data from compressive triaxial tests conducted at different confining pressures and strain rates for three different mixes. The model parameters are related to the experimental measurements of aggregate shape characteristics, aggregate surface energy, inherent anisotropic distribution of aggregates, and microstructure damage measured using X-ray computed tomography and image analysis techniques. Establishing the relationship between the model parameters and material properties is important in order to optimize the mix properties, and achieve desirable mix performance.     

Effect of aggregate gradations on properties of porous friction course mixes

This paper presents the study on effect of aggregate gradation on the mix design and performance properties of porous friction course (PFC) mixes. Six aggregate gradations were tested with due consideration to gradations specified for PFC or similar mixes by different agencies around the world. The PFC mixes were characterized for volumetric properties, permeability, unaged and aged abrasion loss, moisture susceptibility, and rutting resistance. The results were statistically analysed to identify the factors that significantly influence the properties of PFC mixes. Findings of the study clearly indicate that the gradations specified by various agencies will have significant effect on the design properties of PFC mixes, thus they are different. It also, helps in framing the Master aggregate gradation band for PFC mixes. Generally, permeability property is considered to be an optional parameter in the design. However, the findings of the present study recommended considering the permeability as one of the prime parameters in the design of PFC mixes.     

Effect of waste plastic bottles on the stiffness and fatigue properties of modified asphalt mixes

Nowadays, the use of recycled waste materials as modifier additives in asphalt mixes could have several economic and environmental benefits. The main purpose of this research was to investigate the effect of waste plastic bottles (Polyethylene Terephthalate (PET)) on the stiffness and specially fatigue properties of asphalt mixes at two different temperatures of 5 and 20 °C. Likewise, the effect of PET was compared to styrene butadiene styrene (SBS) which is a conventional polymer additive which has been vastly used to modify asphalt mixes. Different PET contents (2–10% by weight of bitumen) were added directly to mixture as the method of dry process. Then the resilient modulus and fatigue tests were performed on cylindrical specimens with indirect tensile loading procedure. Overall, the mix stiffness reduced by increasing the PET content. Although stiffness of asphalt mix initially increased by adding lower amount of PET. Based on the results of resilient modulus test, the stiffness of PET modified mix was acceptable and warranted the proper deformation characteristics of these mixes at heavy loading conditions. At both temperatures, PET improved the fatigue behavior of studied mixes. PET modified mixes revealed comparable stiffness and fatigue behavior to SBS at 20 °C. However, at 5 °C the fatigue life of SBS modified mixes was to some extent higher than that of PET modified ones especially at higher strain levels of 200 microstrain.     

Dynamic mechanical characterization of asphalt concrete mixes with modified asphalt binders

The primary objective of this work is to characterize and compare the dynamic mechanical behavior of asphalt concrete mixes with styrene butadiene styrene (SBS) polymer and crumb rubber modified asphalt binders with the behavior of mixes with unmodified viscosity grade asphalt binders. Asphalt binders are characterized for their physical and rheological properties. Simple performance tests like dynamic modulus, dynamic and static creep tests are carried out at varying temperatures and time. Dynamic modulus master curves constructed using numerical optimization technique is used to explain the time and temperature dependency of modified and unmodified asphalt binder mixes. Creep parameters estimated through regression analysis explained the permanent deformation characteristics of asphalt concrete mixes. From the dynamic mechanical characterization studies, it is found that asphalt concrete mixes with SBS polymer modified asphalt binder showed significantly higher values of dynamic modulus and reduced rate of deformation at higher temperatures when compared to asphalt concrete mixes with crumb rubber and unmodified asphalt binders. From the concept of energy dissipation, it is found that SBS polymer modification substantially reduces the energy loss at higher temperatures. Multi-factorial analysis of variance carried out using generalized liner model showed that temperature, frequency and asphalt binder type significant influences the mechanical response of asphalt concrete mixes. The mechanical response of SBS polymer modified asphalt binders are significantly correlated with the rutting resistance of asphalt concrete mixes.     

Effect of aggregate types on the performance of neat and EVA-modified asphalt mixtures

The objective of this study is to determine the effect of bitumen modification with varying percentage of ethylene vinyl acetate (EVA) for preparing the hot-mix asphalt containing various aggregates (marble, granite and quartzite) and compare the results with mixes prepared with neat viscosity grade 30. The selection of aggregates has been done based on their acidic and basic nature. The physical and mechanical properties of EVA-modified bitumen and neat bitumen aggregate mixes were evaluated. Moisture susceptibility tests and wheel tracking test were carried out on the samples. Brookfield viscometer and dynamic shear rheometer were used to determine the mixing and compacting temperatures of neat and modified bitumen. The results indicate that mixes prepared with aggregates that are basic in nature, i.e. with higher calciumcontent, show better bonding with both neat bitumen and EVA-modified bitumen compared with acidic aggregates, i.e. aggregates with higher silica content.     

Laboratory characterisation of asphalt mixes containing RAP and RAS

Due to its economic and environmental benefits, using reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RAS) in new hot-mix asphalt (HMA) has become an integral part of today's asphalt industry. The advantages of using RAP and RAS in HMA are not limited to economic and environmental benefits, and may result in improving a number of mix performance characteristics including rutting and resistance to moisture-induced damage. Despite aforementioned benefits, concerns over premature pavement distresses resulting from using RAP and RAS limit their usage in HMA. Furthermore, because of the lack of mechanistic performance data, use of new mixes containing RAP and RAS remains limited. Therefore, the present study was undertaken to investigate the effects of using different amounts of RAP and RAS on laboratory performance of HMA, and to generate valuable input design parameters for implementation of the mechanistic-empirical pavement design guide (M-EPDG), using local materials. Four types of base course mixes containing 0% RAP, 25% RAP, 40% RAP and 20% RAP+5% RAS, and three types of surface course mixes containing 0% RAP, 25% RAP and 20% RAP+5% RAS were tested. Laboratory tests were conducted to evaluate stiffness, low-temperature cracking, fatigue life, rut and moisture-induced damage potential of the mixes. It was found that dynamic modulus and creep compliance of the asphalt mixes increase and decrease, respectively, with an increase in the amount of RAP and/or RAS used in the mix. Fatigue life was found to increase with increasing RAP content up to 25%, and to decrease when the RAP and/or RAS content exceeded 25%, or when RAS was used in the mix. It should be noted that this conclusion was drawn based on a 15% increment in RAP content. Hamburg wheel tracking (HWT) test results showed increased resistance to rutting and moisture-induced damage, with an increase in the amount of RAP and/or RAS. However, the tensile strength ratio test results were not confirmed by HWT. The findings of this study are expected to be helpful in understanding the effects of using different amounts of RAP and RAS on the performance of asphalt mixes produced using local materials. Furthermore, valuable design input parameters, developed in this study for new mixes containing RAP and RAS, may be used for calibration of the M-EPDG input parameters, with local materials.     

Performance evaluation of rubberised asphalt mixes containing WMA additives

The aim of this research is to investigate moisture susceptibility, rutting resistance and structural response of rubberised asphalt mixtures containing Warm Mix Asphalt (WMA) additives using Tensile Strength Ratio, dynamic creep test, wheel tracking and indirect resilient modulus. Furthermore, impact of WMA additives on compaction was evaluated by air void content of Marshall specimens. Two types of waxes, namely Sasobit and Rheofalt, and an anti-stripping additive, namely Zycotherm, were used to reduce mixing temperature of rubberised mixtures at concentrations of 0, 5, 10 and 15%. It was proved that not only does small amount of the anti-stripper (0.1%) decrease the production temperature effectively, but it also results in the most moisture damages resistance improvement compared to the waxes. Rheofalt, on the other hand, has the most effectiveness in improvement of rutting resistance and resilient modulus. Finally, Sasobit was found the most effective WMA additive regarding compaction effort efficiency.     

Nonlinearly viscoelastic analysis of asphalt mixes subjected to shear loading

This study presents the characterization of the nonlinearly viscoelastic behavior of hot mix asphalt (HMA) at different temperatures and strain levels using Schapery’s model. A recursive-iterative numerical algorithm is generated for the nonlinearly viscoelastic response and implemented in a displacement-based finite element (FE) code. Then, this model is employed to describe experimental frequency sweep measurements of two asphalt mixes with fine and coarse gradations under several combined temperatures and shear strain levels. The frequency sweep measurements are converted to creep responses in the time domain using a phenomenological model (Prony series). The master curve is created for each strain level using the time temperature superposition principle (TTSP) with a reference temperature of 40°C. The linear time-dependent parameters of the Prony series are first determined by fitting a master curve created at the lowest strain level, which in this case is 0.01%. The measurements at strain levels higher than 0.01% are analyzed and used to determine the nonlinear parameters. These parameters are shown to increase with increasing strain levels, while the time–temperature shift function is found to be independent of strain levels. The FE model with the calibrated time-dependent and nonlinear material parameters is used to simulate the creep experimental tests, and reasonable predictions are shown.     

Exposing the nonlinear viscoelastic behavior of asphalt-aggregate mixes

In this study asphalt-aggregate mixes are treated as both viscoelastic and viscoplastic. Following a damage mechanics approach, a nonlinear viscoelastic constitutive formulation is generated from a linear formulation by replacing ‘applied stresses’ with ‘effective viscoelastic stresses’. A non-dimensional scalar entity called ‘relative viscoelastic stiffness’ is introduced; it is defined as the ratio of applied to effective viscoelastic stress and encapsulates different types of nonlinearities. The paper proposes a computational scheme for exposing these nonlinearities by uncovering, through direct analysis of any test data, changes experienced by the ‘relative viscoelastic stiffness’. In general terms, the method is based on simultaneous application of creep and relaxation formulations while preserving the interrelationship between the corresponding time functions. The proposed scheme is demonstrated by analyzing a uniaxial tension test and a uniaxial compression test (separately). Results are presented and discussed, unveiling and contrasting the character of viscoelastic nonlinearities in both cases. A conceptual viewpoint is offered to explain the observations, illustrating the requirements from any candidate constitutive theory.     

Influence of selected WMA additives on viscoelastic behaviour of asphalt mixes and pavements

Warm mix asphalt additives are effective in decreasing production, laying and compaction temperatures of asphalt mixes. However, there are still questions concerning influence of warm mix additives on properties of asphalt mixes and pavement performance. This paper presents results of the comprehensive research of viscoelastic behaviour of asphalt mixes and pavement structures with layers made with warm mix asphalt additives at high temperatures. Two additives of significantly different effects on mixes at higher temperatures were selected for analysis, namely aliphatic synthetic wax produced with the use of Fisher–Tropsch method and formulation of surfactant- based molecules (ionic and non-ionic). Viscoelastic properties of mixes with these two additives and, as a reference mix, with neat unmodified asphalt binder were determined in uniaxial compression with sinusoidal loading using Asphalt Mixture Performance Test. The viscoelastic analysis of pavement structures was performed with use of the VEROAD software and data from laboratory testing. Two different pavement structures were analysed, for light and heavy traffic. The temperature distribution in pavement structure during the hottest summer day in northern Poland in 2012 was taken into account. The model of pavement was loaded with moving wheel at different speeds. The analysis has shown that two tested warm mix additives had different effect on viscoelastic transient response at high temperatures. One of them (Fischer–Tropsch wax) evidently caused an increase in resistance of asphalt mix and pavement structure to loading at high temperature. The second additive (formulation of surfactant-based molecules) slightly reduced resistance of asphalt mix and pavement to loading at high temperatures as compared with the reference mix.     

The effect of crumb rubber modifier on the resistance of asphalt mixes to plastic deformation

This research analyzed the response of bituminous mixes manufactured with rubber to plastic deformation. For this purpose, a set of asphalt mixes containing different percentages of crumb rubber modifier (CRM) added by the dry process as well as the wet process were tested. It also compared the performance of a CRM mix to that of a mix made with high-performance polymer-modified bitumen. The mixes were assessed with the wheel-tracking test and the cyclic triaxial test. Their bearing capacity was also evaluated by determining their stiffness modulus at different temperatures. The results obtained showed that for the dosages and percentages of crumb rubber used, the addition of wet-process and dry-process CRM to asphalt mixes with conventional bitumen increased their resistance to plastic deformation. In fact, the performance of some CRM mixes was superior to that of the mix with high-performance modified bitumen. It also increased their stiffness modulus and creep modulus values and improved their resistance to plastic deformations caused by vehicle traffic loads.     

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 rutting potential for asphalt concrete mixes containing copper slag

Copper slag (CS) is a by-product of the copper extraction process, which can be used as coarse and/or fine aggregate in hot mix asphalt (HMA) pavements. This study used CS as a replacement of the fine aggregate with a percentage of up to 40% by total aggregate weight. The objective of this study was to evaluate the effect of CS on the rutting potential of the asphalt concrete mix using two methods. One method is based on the Dynamic modulus |E*| testing result. Actual pavement temperature data from a test section were used with the developed |E*| master curves. EverStressFE finite element program was used to perform a linear elastic load-deformation analysis for a pavement section and to determine the vertical resilient strain in a 40-mm HMA surface layer. The M-E PDG permanent deformation model was used with and Excel Visual Basic for Applications code to predict the accumulated rutting for different CS mixes for 10 million ESALs. The other method used the data from the flow number (FN) test. Based on the |E*| approach, the results indicated that adding 5% CS in the mix increased the predicted rutting from 0.59 to 0.98 mm at 10 million ESALs (increase by 68%). When 40% CS was used, rutting increased by more than 700% compared with the control mix. After analysing the FN results with the Francken model, the results indicated a decrease in FN as CS content is increased, indicating higher rutting potential. The decrease in FN ranged from 9% for 5% CS to 95% for 40% CS. The mixes containing up to 10% CS satisfied the minimum FN criteria for rutting. A calibration process for the M-E PDG distress prediction models that allows the use of waste and by-product materials such as CS should be considered in the future.     

Investigation of self healing behaviour of asphalt mixes using beam on elastic foundation setup

Self healing of asphalt mixes is known for more than four decades. However, it is a complex phenomenon which depends on the duration of the rest period, temperature, crack size, etc. In order to quantify the self healing behaviour of asphalt mixes, a test setup was proposed in this research using an asphalt beam on an elastic foundation. Within this setup, a notched asphalt beam was glued on a low modulus rubber foundation, and a symmetric monotonic load was applied with loading–unloading–healing–reloading cycles. The rubber foundation was used to avoid permanent deformation and to ensure a controllable healing process. Experimental results indicate that the beam on elastic foundation (BOEF) setup is capable for self healing investigations of asphalt mixes. The healing process was quantified by the recovery of the strength and the recovery of the crack opening displacement. The time, temperature and crack size dependency of the self healing behaviour were observed over the healing periods. Moreover, a self healing model was proposed to decompose the self healing phenomenon observed in the BOEF healing tests. It is shown that at the beginning of the healing period the delayed visco-elastic healing is the main reason of the recovery of the crack opening displacement and the viscous healing is important for healing after longer time/higher temperature.     

Optimisation of hot mix asphalt performance based on aggregate selection

Researchers over the last four decades have identified and demonstrated the effects of aggregate morphological properties (particularly shape, size distribution, angularity and texture) on the mechanical properties of hot mix asphalt (HMA). Rare studies, however, have clearly established the relationships between the aggregate properties and pavement performance. Therefore, they have not provided methods to optimise aggregate properties at the design stage to improve that performance. This study focuses on understanding the effects of aggregate gradation and type on moisture damage resistance of HMA and on pavement performance as indicated by stiffness and rutting. Results show that basalt aggregate achieves higher moisture susceptibility resistance and stiffness than limestone aggregate. Coarser gradation has the highest permanent deformation, while open gradation 2C provides the lowest moisture damage resistance. Furthermore, dense gradation 4C provides the lowest rutting and the highest stripping resistance. It is indicated that suitable selection of aggregate type and gradation can improve pavement performance and reduce the moisture damage problem of HMA.     

Quantification of the inherent uncertainty in the relaxation modulus and creep compliance of asphalt mixes

Advanced material characterization of asphalt concrete is essential for realistic and accurate performance prediction of flexible pavements. However, such characterization requires rigorous testing regimes that involve mechanical testing of a large number of laboratory samples at various conditions and set-ups. Advanced measurement instrumentation in addition to meticulous and accurate data analysis and analytical representation are also of high importance. Such steps as well as the heterogeneous nature of asphalt concrete (AC) constitute major factors of inherent variability. Thus, it is imperative to model and quantify the variability of the needed asphalt material’s properties, mainly the linear viscoelastic response functions such as: relaxation modulus, \(E(t)\), and creep compliance, \(D(t)\). The objective of this paper is to characterize the inherent uncertainty of both \(E(t)\) and \(D(t)\) over the time domain of their master curves. This is achieved through a probabilistic framework using Monte Carlo simulations and First Order approximations, utilizing \(E^{*}\) data for six AC mixes with at least eight replicates per mix. The study shows that the inherent variability, presented by the coefficient of variation (COV), in \(E(t)\) and \(D(t)\) is low at small reduced times, and increases with the increase in reduced time. At small reduced times, the COV in \(E(t)\) and \(D(t)\) are similar in magnitude; however, differences become significant at large reduced times. Additionally, the probability distributions and COVs of \(E(t)\) and \(D(t)\) are mix dependent. Finally, a case study is considered in which the inherent uncertainty in \(D(t)\) is forward propagated to assess the effect of variability on the predicted number of cycles to fatigue failure of an asphalt mix.     

A comparative study of the heterogeneous local mechanical response of two types of asphalt mixes

Two asphalt specimens featuring very different gradations, types of aggregates and binders are investigated in this study. A full-field measurement technique is used for this purpose: the grid method. Displacement and strain fields are captured during compression tests carried out on these specimens. The displacement and strain fields are analyzed and compared in light of the main characteristics of these materials. It is shown that a close relationship exists between gradation and ratio between local and global strain components. The strain recovery that follows the loading phase of the specimens is also analyzed and the difference between their mechanical response at the local level is also highlighted.     

Comparing various fitting models to construct the tensile relaxation modulus master curve of asphalt mixes

This study was to compare the relative ability of seven common fitting models, i.e. Pure Power Law (PPL), Generalized Power Law (GPL), Modified Power Law (MPL), Modified Power Law Series (MPLS), Standard Sigmoid (SS), Generalized Logistic Sigmoid (GLS) and Prony Series (PS), to construct the tensile relaxation modulus master curve of dense graded asphalt mixes. 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 three replicates. Direct tension relaxation modulus tests were conducted on the specimens at four different temperatures using the trapezoidal loading pattern at a low level of input strain. Tensile relaxation modulus master curves were constructed using all the fitting models together with the numerical shifting technique. Finally, both the graphical and statistical comparisons were made among the fitting models, and the best one was found to be PS, followed by MPLS, GLS, MPL, SS, GPL and PPL.     

A micro-damage healing model that improves prediction of fatigue life in asphalt mixes

The focus of the current paper is on the development and validation of a micro-damage healing model that improves the ability of an integrated nonlinear viscoelastic, viscoplastic, and viscodamage constitutive model based on continuum damage mechanics for predicting the fatigue life of asphalt paving mixtures. The model parameters of the continuum-based healing model are related to fundamental material properties. Recursive-iterative and radial return algorithms are used for the numerical implementation of viscoelasticity and viscoplasticity models respectively, whereas the viscodamage and micro-damage healing models are implemented using the concept of the effective undamaged-healed natural configuration. Numerical algorithms are implemented into the well-known finite element code Abaqus via the user material subroutine UMAT. Finally, the model is validated by comparing its predictions with experimental data on an asphalt mix that include repeated creep-recovery tests for different loading times and rest periods in both tension and compression. The significant enhancement of the ability of the constitutive model to predict fatigue life due to inclusion of the micro-damage healing is clearly demonstrated.     

Investigating the effect of filler types on thermodynamic parameters and their relationship with moisture sensitivity of asphalt mixes

Filler plays a significant role in mastic cohesion and adhesion between aggregate–asphalt binder in asphalt mixes. In the majority of research on investigating moisture damage based on thermodynamic concepts, little attention has been given to the role of filler. In the present study, 20 different combinations of asphalt mixes made with 4 filler types (stone powder, hydrated lime, calcium carbonate and portland cement), with two types of asphalt binder (60–70 and 85–100), and two types of aggregate (limestone and granite) were used. Then thermodynamic parameters (with and without considering the effect of filler) were calculated and the relationship between these parameters and test results of moisture sensitivity of asphalt mixes was investigated using statistical analyses. Results obtained by thermodynamic parameters show that only stone powder filler caused an increase in free energy of adhesion between base asphalt binder and aggregates, and other fillers reduced free energy of adhesion. The maximum amount of debonding energy in samples made by asphalt binder 60–70, was related to mastics containing calcium carbonate and hydrated lime fillers, and in asphalt binder 85–100, mastics containing portland cement and calcium carbonate had the maximum amount of debonding energy. However, the minimum amount of debonding energy was related to the mastic containing stone powder. In addition, the results of moisture sensitivity mechanical tests show that samples containing calcium carbonate and hydrated lime fillers had the maximum amount of tensile strength ratio. Finally, the amount of adjusted coefficient of correlation between debonding energy and modified Lottman test results increased from 0.553 in 4 base compounds (without filler) to 0.701 in 16 compounds with filler. The difference in correlation coefficients show the necessity to use the effect of filler on calculating thermodynamic parameters in investigating moisture sensitivity of various asphalt mixes.