Eyad Masad

Comparing finite element and constitutive modelling techniques for predicting rutting of asphalt pavements

This paper focuses on a comprehensive evaluation of the effects of different finite element (FE) modelling techniques and material constitutive models on predicting rutting in asphalt pavements under repeated loading conditions. Different simplified 2D and more realistic 3D loading techniques are simulated and compared for predicting asphalt rutting. This study also evaluates and compares the rutting performance predictions using different material constitutive behaviours such as viscoelastic–viscoplastic, elasto-viscoplastic and coupled viscoelastic, viscoplastic and viscodamage behaviours. The simulations show that the assumption of the equivalency between a pulse loading and an equivalent loading, which are commonly used as simplified loading assumptions for predicting rutting, is reasonable for viscoelastic–viscoplastic and elasto-viscoplastic constitutive behaviours. However, these loading assumptions and material constitutive models overestimate rutting as damage grows. Results show that the 2D plane strain FE simulations significantly overestimate rutting as compared with the rutting performance predictions from more realistic 3D FE simulations.

Microstructural properties of warm mix asphalt before and after laboratory-simulated long-term ageing

This paper presents microstructural analysis of two original and their respective warm mix asphalt (WMA) binders, obtained with the addition of Advera® and Sasobit®. The binders were analysed before and after long-term ageing with the aid of atomic force microscopy (AFM). The AFM images highlighted the effect of ageing using rolling thin film oven (RTFO) followed by pressure ageing vessel (PAV) on the binders’ microstructure. Ageing of original binders generally increased the size of the “bee” structures. The ageing effect on the microstructure morphology of WMA binders was found to be negligible. However, ageing could affect the mechanical properties without morphology variations. The high variability inside each sample analysed makes the analysis complex. Sasobit® changed the original binders’ microstructure considerably, while Advera® did not affect it. For the original and Advera® binders, 24 hours after sample preparation, the dispersed phase expanded, and this effect was generally more pronounced for unaged than for aged binders. This could indicate a lower mobility of the species forming microstructures in aged samples. The reduced mobility of these species during heat treatment also affects the microstructure of the samples. A hypothesis on the development of “bee” structures starting from dendritic structures was proposed.

Evaluation of Aggregate Characteristics Affecting HMA Concrete Performance

This report documents the outcomes of the ICAR study on the Evaluation of Aggregate Characteristics Affecting HMA Concrete Performance. This study was conducted with support from the Federal Highway Administration (FHWA) program on Simulation, Imaging, and Mechanics of Asphalt Pavements at Texas A&M University. The first outcome includes assessment of HMA sensitivity to aggregate shape characteristics. Aggregate shape is characterized through detailed measurements of angularity, form, and texture using the Aggregate Imaging System (AIMS). The shape characteristics are presented in terms of the distribution of the property in an aggregate sample rather than an average index of this property. The second outcome of this study is the development of a viscoplastic model for permanent deformation. The model accounts for the aggregate structure in the mix, which is related to the shape properties measured using AIMS. The model capabilities are demonstrated through matching the results of testing various mixes from the Accelerated Loading Facility (ALF) of the FHWA using the triaxial creep and strength tests. In addition, the model is used to predict the response of mixes that include aggregates with different shape characteristics in order to develop relationships between the model parameters and aggregate shape characteristics. As part of the model development, an experiment was conducted to capture and characterize damage evolution in HMA due to permanent deformation. HMA specimens were loaded using a triaxial compression setup to four predefined strain levels at three confining pressures. Consequently, image analysis techniques were used to analyze damage distribution. The results of the damage experiment supported the damage evolution function proposed in the viscoplastic model.

Effect of treatment temperature on the microstructure of asphalt binders: insights on the development of dispersed domains.

This paper offers important insights on the development of the microstructure in asphalt binders as a function of the treatment temperature. Different treatment temperatures are useful to understand how dispersed domains form when different driving energies for the mobility of molecular species are provided. Small and flat dispersed domains, with average diameter between 0.02 and 0.70 μm, were detected on the surface of two binders at room temperature, and these domains were observed to grow with an increase in treatment temperature (up to over 2 μm). Bee‐like structures started to appear after treatment at or above 100°C. Moreover, the effect of the binder thickness on its microstructure at room temperature and at higher treatment temperatures was investigated and is discussed in this paper. At room temperature, the average size of the dispersed domains increased as the binder thickness decreased. A hypothesis that conciliates current theories on the origin and development of dispersed domains is proposed. Small dispersed domains (average diameter around 0.02 μm) are present in the bulk of the binder, whereas larger domains and bee‐like structures develop on the surface, following heat treatment or mechanical disturbance that reduces the film thickness. Molecular mobility and association are the key factors in the development of binder microstructure.

Three-dimensional microstructural modelling of coupled moisture–mechanical response of asphalt concrete

Three-dimensional (3D) microstructural representation of asphalt concrete subjected to moisture diffusion and mechanical loading is simulated and analysed. The continuum moisture–mechanical damage mechanics framework and the moisture damage constitutive relationship developed by the authors are used in this study to couple the detrimental effects of the mechanical loading and moisture diffusion on the complex response of asphalt concrete. A 3D finite element (FE) microstructural representation of a typical asphalt concrete is used for these simulations. The 3D microstructure is reconstructed from slices of two-dimensional X-ray computed tomography images that consist of the matrix and the aggregates. Results show that the generated 3D FE microstructure along with the coupled moisture–mechanical constitutive relationship can be effectively used to simulate the overall thermo-hygro-mechanical response of asphalt concrete. The analyses provide insight into the impact of the microstructure on the overall response of asphalt concrete.

Evolution of the microstructure of unmodified and polymer modified asphalt binders with aging in an accelerated weathering tester

This paper presents findings on the evolution of the surface microstructure of two asphalt binders, one unmodified and one polymer modified, directly exposed to aging agents with increasing durations. The aging is performed using an accelerated weathering tester, where ultraviolet radiation, oxygen and an increased temperature are applied to the asphalt binder surface. Ultraviolet and dark cycles, which simulated the succession of day and night, alternated during the aging process, and also the temperature varied, which corresponded to typical summer day and night temperatures registered in the state of Qatar. Direct aging of an exposed binder surface is more effective in showing microstructural modifications than previously applied protocols, which involved the heat treatment of binders previously aged with standardized methods. With the new protocol, any molecular rearrangements in the binder surface after aging induced by the heat treatment is prevented. Optical photos show the rippling and degradation of the binder surface due to aging. Microstructure images obtained by means of atomic force microscopy show gradual alteration of the surface due to aging. The original relatively flat microstructure was substituted with a profoundly different microstructure, which significantly protrudes from the surface, and is characterized by various shapes, such as rods, round structures and finally ‘flower’ or ‘leaf’ structures.

Evaluation of ageing in asphalt cores using low-field nuclear magnetic resonance

This paper introduces an innovative methodology for estimating the ageing of asphalt concrete cores without extracting the binder. Asphalt concrete samples at different ageing stages (unaged, 3-month and 6-month aged) and with different percent air voids (4%, 7% and 10%) were analysed with low-field nuclear magnetic resonance (NMR). The transverse relaxation time T2 and relative hydrogen index (RHI) obtained from NMR measurements were related to the viscosity of the asphalt binder. The samples were analysed during cooling from 70°C to room temperature, showing increase in viscosity with decreasing temperature. There was a clear trend indicating higher viscosities in samples that were aged for a longer period and samples with higher percent air voids. The RHI and T2 values obtained from low-field NMR measurements and the viscosity data calculated from measurements using a dynamic shear rheometer were correlated to develop a model that relates viscosity with RHI.

Modelling moisture-mechanical damage in asphalt mixtures using random microstructures and a continuum damage formulation

This paper presents a computational modelling approach to evaluate moisture damage in hot mix asphalt (HMA) materials. The approach combines a methodology to randomly generate HMA microstructures and a finite element (FE) formulation that uses a coupled mechanical-moisture continuum damage constitutive relationship. Probable two-dimensional HMA microstructures were randomly generated and treated as independent replicates. These replicates were used as the model geometry in FEs and they were computationally subjected to a one-year moisture-conditioning regime. During this time, a mechanical load was periodically applied. The mean and dispersion values related to the evolution of moisture and mechanical damage were identified, and the effects of varying the air void content and the moisture diffusion coefficient of the asphalt matrix were assessed. The results demonstrate that the modelling approach is a viable tool to probabilistically quantify the impact of physical and volumetric properties of HMA mixtures on their susceptibility to undergo moisture-mechanical damage.

Implementation of mechanistic-empirical pavement analysis in the State of Qatar

The State of Qatar is experiencing tremendous growth in infrastructure including road network and highways. The current methods used in design of asphalt pavements in the State of Qatar are empirical and might not be suitable for the design of long-lasting pavements. Given the significant increase in traffic, road authorities in the State of Qatar have been considering the use of mechanistic-empirical methods in the design and analysis of asphalt pavements. This study documents the results of a study in which the mechanistic-empirical pavement design guide (M-E PDG) software was used in the design of asphalt pavements with input parameters that were carefully selected to reresent local materials and climatic conditions. The selection of material properties was based primarily on specifications and design guides in the State of Qatar and on published literature about these materials. The mechanistic-empirical method was also used to assess the benefits of adopting the concepts of perpetual pavement design and also to compare the performance of pavement structures in which various bitumen grades, granular bases and chemically stabilised sub-base were used. A life-cycle cost analysis was carried out to determine the design with the highest net present value among the various options investigated. It is expected that the outcomes of this study would promote the use of mechanistic-empirical methods in the State of Qatar and the region. Inevitably, this will require significant efforts to calibrate material and damage prediction models used in the M-E PDG for more accurate representation of material properties and measured pavements performance.

Incorporating the heterogeneity of asphalt mixtures in flexible pavements subjected to moisture diffusion

Moisture diffusion is one of the mechanisms by which water can reach the microstructure of asphalt mixtures and negatively affect their performance. In this work, a finite element model and computational simulations were used to study the response of asphalt pavement layers subjected to the coupled effects of moisture diffusion and mechanical loading. The model considered that the evolution of water-related diffusion processes in these materials is influenced by the microstructure of the mixture, particularly by the air void (AV) phase. Thus, a stochastic technique was used to incorporate the heterogeneity of the AV structure in compacted asphalt layers, which also permitted to account for the variability of the physical and mechanical properties that result from the heterogeneous void structure. The results demonstrate that moisture diffusion has a significant influence on the macromechanical response of asphalt pavement layers, especially during the initial months of service. In addition, the results clearly demonstrate that including the heterogeneity of material properties is valuable to quantify the uncertainty associated with these coupled processes.