Ala R. Abbas

Micromechanical analysis of viscoelastic properties of asphalt concretes

A methodology for relating the microstructure of asphalt concretes to their viscoelastic behavior is described. Imaging techniques are used to capture the asphalt concrete microstructure, and the finite element method (FEM) is used to model its stress-strain behavior in the time domain. Aggregates are modeled as linear elastic, and the binder is modeled through mechanistic models as either linear viscoelastic or nonlinear viscoelastic. The binder viscoelastic properties are input into the FEM algorithm by two methods: a built-in viscoelastic function and a user-specified material characterization subroutine. The latter handles non-linearity in an iterative piecewise linear fashion, whereby the mechanistic binder model parameters are updated as a function of the strain level. For each strain level, mechanistic models are fitted to describe binder viscoelastic behavior based on dynamic shear rheometer data. The two approaches used for specifying binder viscoelastic properties into the FEM algorithm were verified by comparing binder response predictions with direct measurements. Finally, the asphalt concrete micro-structure model was verified by comparing FEM predictions of dynamic shear modulus and phase angle with measurements obtained by using a Superpave® shear tester.

Comparison of 2D and 3D image-based aggregate morphological indices

Significant progress has been made over the last two decades in the characterisation of aggregate shape using automated image analysis and processing methods. Aggregate shape characteristics have been quantified using three distinct shape parameters, namely the aggregate form, angularity and surface texture. Several mathematical procedures have been developed to quantify these parameters. For practical reasons, most of these procedures were limited to two-dimensional (2D) and utilised 2D aggregate images. This paper investigates the ability of some of the most widely used 2D aggregate form and angularity indices to describe the shape of five different types of aggregates (natural gravel, basalt, granite, diabase and slate). In addition, it provides a comparison between 2D and three-dimensional (3D) aggregate shape indices developed based on fitting the 3D aggregate shape using spherical harmonic functions.

Effect of Coefficient of Thermal Expansion Test Variability on Concrete Pavement Performance as Predicted by Mechanistic-Empirical Pavement Design Guide

The coefficient of thermal expansion (CTE) of concrete is a property that can affect the performance of the pavement and its service life and is one of the most important inputs in the Mechanistic-Empirical Pavement Design Guide (MEPDG). The CTE can be either estimated or measured in the laboratory. The test method used to determine this property is AASHTO TP 60, still a provisional test method and not yet evaluated for its precision. CTEs of more than 1,800 concrete specimens were measured at the Turner-Fairbank Highway Research Center. The specimens included cylinders that were cast in the laboratory as well as field cores obtained from the Long-Term Pavement Performance pavement sections. Approximately 150 of the specimens were tested individually several times for assessment of repeatability of the test method. An analysis is presented of test differences observed, as is a sensitivity analysis of the CTE test variability on predicted performance based on the MEPDG. The differences in predicted international roughness index (IRI), percent slabs cracked, and faulting due to test variability were determined for concretes with CTEs ranging from 4 to 7 × 10−6 in./in./°F. It was observed that differences in test results may result in significant discrepancies in the predicted IRI, percent slabs cracked, and faulting. Thus, a single test result should not be used as representative of the CTE of a mixture due to the considerable impact of the test variability on the predicted pavement performance. Moreover, the specifications should state the minimum number of tests necessary for the CTE determination and the acceptable test variability.

The Influence of Laboratory Aging Method on the Rheological Properties of Asphalt Binders

Asphalt binders used in asphalt concrete roadway pavements experience aging during construction and subsequently during their service lives. Aging is the combined effect of the evaporation of volatile compounds and the chemical reaction of residual compounds with oxygen. This aging affects the rheological properties of asphalt binders. The SuperpaveTM testing and specification system uses two laboratory procedures for aging of binders prior to measuring their rheological properties, namely the rolling thin film oven (RTFO) and the pressure aging vessel (PAV). These two procedures are used to simulate the aging that takes place during construction and during service, respectively. This paper examines whether the SuperpaveTM prescribed sequence of binder aging procedures (i.e., the RTFO followed by the PAV) is necessary, or whether similar binder rheological properties are obtained using the PAV procedure only. For this purpose, three binders were tested, namely an unmodified PG 64-28, an SBS polymer-modified binder of the same grade, and an SBS polymer-modified PG 76-28. The low temperature and fatigue rheological properties were measured by a bending beam rheometer (BBR) and a dynamic shear rheometer (DSR), respectively. The findings of the study suggest that, with a few exceptions, the rheological properties measured after aging with the RTFO followed by PAV are significantly different than those obtained after PAV aging only.

Wavelet-based characterisation of asphalt pavement surface macro-texture

This paper utilised wavelet analysis to characterise the macro-texture properties of asphalt pavements. The study included Circular Texture Meter (CTMeter) measurements collected from several asphalt pavement sections in the state of Ohio. The asphalt pavements consisted of two mix designs (Superpave and Marshall), three aggregate types (dolomite, limestone, and gravel), and two binder grades (PG 70-22 and PG 64-22). The wavelet approach was used to determine the wavelength ranges and energy content that affect the macro-texture properties of asphalt pavements. In addition, the normalised energy (NE) parameter was utilised to characterise the overall pavement surface macro-texture. The CTMeter data allowed for obtaining six wavelet decomposition levels, namely d1 through d6, with wavelengths up to 56 mm. The analysis revealed that the macro-texture properties of smooth pavement sections are mainly affected by sub-band levels d1 through d4 (i.e. both fine and coarse aggregates), while the macro-texture properties of the rough pavement sections are mainly affected by sub-band levels d3 and d4 (i.e. coarse aggregates). Similar trends in macro-texture properties were observed between NE and the conventional Mean Profile Depth (MPD). However, the variations in the macro-texture properties were better captured using the NE than the MPD. Therefore, it was concluded that the wavelet approach is better suited to characterise the macro-texture properties of asphalt pavements.

Micromechanical Simulation of Asphaltic Materials Using the Discrete Element Method

This paper summarizes the findings of modeling the micromechanical behavior of asphalt mastics and asphalt mixtures under various loading conditions. A commercial discrete element code called Particle Flow Code in 2-Dimensions (PFC2D) is used for this purpose. Asphalt mastics are simulated using an assembly of stiff particles randomly dispersed in a medium of soft particles, representing the aggregate fillers and the asphalt binder, respectively. The stiffening effect of the aggregate fillers on the micromechanical behavior of asphalt mastics is investigated at different filler volume fractions. These results are compared to Dynamic Shear Rheometer (DSR) measurements on actual mastics. These mastic models are used to simulate the micromechanical behavior of hot mix asphalt (HMA) concretes. The behavior of these HMA models was investigated at high and low temperatures under loading conditions similar to those applied in the Simple Performance Test (SPT) and the Indirect Tension Test (IDT), respectively.

Low-Temperature Characterization of Foamed Warm-Mix Asphalt Produced by Water Injection

This study evaluated the low-temperature performance of foamed warmmix asphalt (WMA) produced by water injection and compared it with that of hot-mix asphalt (HMA). Two asphalt binders (PG 70–22 and PG 64–28), two aggregate types (limestone and crushed gravel), and two aggregate gradations [nominal maximum aggregate size (NMAS)] (12.5-mm NMAS and 19.0-mm NMAS) were used in this study. The low-temperature properties of the asphalt binders were measured with the bending beam rheometer, and the low-temperature behavior of the asphalt mixtures was evaluated with the thermal stress restrained specimen test after being subjected to short-term and long-term aging. As expected, the fracture temperatures obtained for the short-term aged specimens were lower than those obtained for the long-term aged specimens. This was the case for both foamed WMA and HMA mixtures. The HMA mixtures exhibited colder fracture temperatures than did the foamed WMA mixtures for the short-term aged specimens, but fracture temperatures comparable to those for the long-term aged specimens. This comparison suggests that the traditional HMA mixtures may have better resistance to low-temperature cracking than foamed WMA does during the initial service life of the asphalt layer, but may have similar resistance to low-temperature cracking at later stages. This study also showed that the low-temperature binder grade had the most significant effect on fracture temperature, whereas the aggregate type had the most significant effect on fracture stress.

Effect of Traffic Load Input Level on Mechanistic-Empirical Pavement Design

This study investigated the effect of the input level of axle load spectra on performance predictions obtained with the Mechanistic–Empirical Pavement Design Guide (MEPDG). Traffic monitoring data collected from 50 weigh-in-motion (WIM) stations distributed across the state of Ohio were analyzed to obtain site-specific and regional axle load spectra. The site-specific axle load spectra were calculated for each axle group (single, tandem, tridem, and quad) and truck class (Classes 4 through 13). The regional axle load spectra were developed on the basis of statewide averages and cluster analysis. The statewide average was calculated with information from all sites, whereas the cluster averages were created on the basis of Class 9 tandem axles. Baseline designs for new flexible pavements were defined in the MEPDG for each of the WIM sites having available data, and site-specific, statewide average, cluster average, and MEPDG default axle load spectra were used to assess the impact of the input level of axle load spectra on the performance of the pavement structure. The statewide average and the cluster averages were found to yield similar predictions of pavement performance, with the cluster averages being slightly closer to the site-specific predictions of pavement performance. In addition, the MEPDG default axle load spectra were found to underestimate the pavement service life significantly and, if used for design purposes, would result in overly conservative pavement layer thicknesses. Therefore, the authors recommend the use of site-specific axle load spectra when possible in the design of flexible pavements. However, if site-specific data are not available, statewide average axle load spectra should be used.

Comparison between fatigue performance of horizontal cores from different asphalt pavement depths and laboratory specimens

This study presents a comparison between the fatigue performance of field cores obtained from different layers (top and bottom) and laboratory produced–laboratory compacted specimens. The field cores were obtained from the Accelerated Pavement Testing (APT) facility at Turner-Fairbank Highway Research Center (TFHRC), located in McLean, Virginia, USA. Uniaxial push-pull (compression-tension) fatigue tests were conducted on samples obtained from the top 50 mm and the bottom 50 mm layers of 100 mm thick asphalt pavements. The samples were obtained by horizontally coring slabs extracted from six APT lanes. The damage characteristic curves (i.e., C versus S curve of the viscoelastic continuum damage (VECD) theory) and the fatigue lives of the field cores were compared with those of the laboratory specimens. The experimental test results and the subsequent analysis findings revealed comparable damage characteristic curves and fatigue lives for the top and bottom field specimens; meanwhile, the field and laboratory specimens exhibited different damage characteristic curves and fatigue lives.

Investigation of Foamed Warm Mix Asphalt Performance Using the MEPDG

In recent years, warm mix asphalt (WMA) produced using foamed asphalt binder has gained popularity in the United States due to its ability to improve workability and compactability of asphalt mixtures produced at lower temperatures. As compared to other WMA technologies, foamed warm mix asphalt does not require the use of costly additives, making it less expensive to produce. In spite of the advantages, several concerns have been raised regarding the performance of this material due to use of lower production temperatures and its impact on aggregate drying and asphalt binder aging. This study aims at evaluating the performance of foamed WMA mixtures in comparison to that of Hot Mix Asphalt (HMA). Two types of aggregates (crushed limestone and natural gravel) and two asphalt binders (neat PG 64-22 and polymer modified PG 70-22M) were used in this study. Dynamic modulus (E*) tests were conducted to evaluate the mechanical properties of the considered mixtures. The new Mechanistic Empirical Pavement Design Guide (MEPDG) software was used to compare the performance of pavement sections construction using foamed WMA and traditional HMA. In addition, it was used to investigate the effect of using foamed WMA on the design thickness of the asphalt layer. The results of this study indicated that the effect of the foamed WMA technology on pavement performance was primarily controlled by the combination of the aggregate and binder types used in the mixture. Such that, for mixtures that had natural gravel the influence of the WMA was dependant on the type of asphalt binder used. For the PG 70-22M binder, the WMA resulted in slightly better performance than HMA. On the other hand, for the neat asphalt binder (PG 64-22), the use of the WMA significantly increased the rutting and International Roughness Index (IRI) of the pavement structure. In addition, for the limestone mixtures, the use of WMA resulted in similar pavement performance to that of HMA. Meanwhile, the use of the WMA yielded much higher design thickness for the gravel mixture with 64-22 binder, while it did not have a significant effect for all other mixtures.