M. Emin Kutay

Computational and experimental evaluation of hydraulic conductivity anisotropy in hot-mix asphalt

Moisture damage in asphalt pavements is one of the primary distresses that is associated with the disintegration of the pavement surface, excessive cracking and permanent deformation. Moisture damage is a function of the chemical and physical properties of the mix constituents, and the distribution of the pore structure (microstructure), which affects fluid flow within the pavement. This paper deals with the relationship between the hot-mix asphalt (HMA) microstructure and hydraulic conductivity, which has traditionally been used to characterize the fluid flow in asphalt pavements. Conventional laboratory or field measurements of hydraulic conductivity only provide information about the flow in one direction and do not consider flow in other directions. Numerical modeling of fluid flow within the pores of asphalt pavements is a viable method to characterize the directional distribution of hydraulic conductivity. A three-dimensional lattice Boltzmann (LB) fluid flow model was developed for the simulation of fluid flow in the HMA pore structure. Three-dimensional real pore structures of the specimens were generated using X-ray computed tomography (CT) technique and used as an input in the LB models. The model hydraulic conductivity predictions for different HMA mixtures were validated using laboratory measurements. Analysis of the hydraulic conductivity tensor showed that the HMA specimens exhibited transverse anisotropy in which the horizontal hydraulic conductivity was higher than the vertical hydraulic conductivity. Analysis of X-ray CT images was used to establish the link between fluid flow characteristics and the heterogeneous and anisotropic distributions within the pore structure.

Three-dimensional image processing methods to identify and characterise aggregates in compacted asphalt mixtures

X-ray computed tomography (CT) is a novel tool to quantify the aggregate characteristics in asphalt pavements. This tool can potentially be used in QA, acceptance, design and forensic applications in pavement engineering. However, there have been challenges associated with the processing of the 3D X-ray CT images, including: (1) segmentation of aggregates that are in close proximity and (2) processing noisy or poor contrast images. This paper describes image processing methods to overcome these challenges and describes methods for computation of size, location, contact points and orientation of the aggregates in HMA. Validations of the algorithms as well as example computations of contact points and orientation have been presented. A significant increase in the number of contact points with increasing compaction level and preferred orientation perpendicular to the direction of compaction in the gyratory compactor were some of the findings presented in this paper.

Aggregate structure characterisation of asphalt mixtures using two-dimensional image analysis

In current practice of mixture design, volumetric properties such as voids and binder content along with mechanical properties such as modulus or rutting resistance are used as the main quality indicators. Visualisation is an important tool that has not been widely used in asphalt mixtures. As part of the Reunion Internationale des Laboratoires et Experts des Materiaux activities, the aggregate structure has been identified as a possible important mixture characteristic in need of measuring and quantifying. This paper is a report on part of this effort. Software for processing and analysing two-dimensional images of asphalt concrete mixtures to provide information about the aggregate structure within a mix was developed. Images with accompanying volumetrics and gradation information can be processed with the software and a virtual sieve analysis of aggregates within the image is performed to verify a match with known measured gradations. Once images were successfully processed, analysis is performed to determine the number of contact points between aggregates as well as radial distribution and orientation of each aggregate. Segregation of aggregates within each specimen was also determined. Mixtures with a broad range of variables were compacted in the laboratory, using a number of compaction methods of various countries. In addition, mixtures with various nominal maximum aggregate sizes, aggregate type (limestone or gravel) and design ESALs (E-3 or E-10) were compacted in the US gyratory compactor, using two pressures (600 and 300 kPa) and two temperature levels (120°C and 60°C). Results indicate that the aggregate structure is affected by compaction methods and conditions although volumetrics are very similar. The results show that a fresh look at evaluating the aggregate structure within mixtures is required.

Use of small samples to predict fatigue lives of field cores: Newly developed formulation based on viscoelastic continuum damage theory

Fatigue cracking is one of the major distresses in asphalt pavements. Accurate prediction of fatigue life of asphalt pavements can be extremely important both during the design stage and for prediction of remaining service life of in-service pavements. Traditional fatigue life predictions based on bending beam tests can be costly and time-consuming. The uniaxial push–pull (tension–compression) tests run on cylindrical samples have been a novel alternative. However, the traditional sample size for the push–pull tests may prevent its use for thin in-service pavements. This paper presents the results of a study investigating the possibility of using smaller sample sizes for push–pull tests. The viscoelastic continuum damage (VECD) characteristics of regular and small-size samples are compared, and the difference is observed to be negligible. In addition, a practical fatigue life formulation is derived on the basis of VECD theory. Uniqueness of the derived fatigue life (Nf) equation, differing from previously derived VECD-based Nf equations, stems from the fact that it does not force a certain form of equation to fit the damage characteristic curve. Finally, the differences in fatigue lives of different layers of the field sections at FHWAs accelerated loading facility are investigated.

Evaluation of high RAP-WMA asphalt rubber mixtures

This study focused on evaluating the stiffness, fatigue cracking, reflective cracking, rutting, moisture damage, and workability of asphalt rubber (AR) surface mixtures with reclaimed asphalt pavements (RAP) contents up to 40% with and without a warm mix asphalt (WMA) technology. RAP increased the stiffness of the mixtures; however, WMA mitigated that increase. The addition of RAP had an adverse effect on the resistance to fatigue and reflective cracking of the mixtures. This effect was magnified with the use of WMA regardless if RAP was incorporated in the mixture. Therefore, it was recommended to further investigate the proper drop in temperatures for AR mixtures that incorporate WMA. All mixtures passed the rutting and moisture damage test. The WMA technology improved the workability of the mixtures.

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.

Laboratory Evaluation of Asphalt Binders and Mixtures Containing Polyphosphoric Acid

Four modified asphalt binders were investigated for performance grade, multiple stress creep and recovery (MSCR), mixture dynamic modulus, and mixture fatigue resistance: polyphosphoric acid (PPA) only, PPA plus Elvaloy, styrene–butadiene–styrene (SBS) only, and SBS plus PPA. MSCR data indicated that the binder modified with PPA only had the highest nonrecoverable compliance and lowest percentage of recovery, whereas the binders modified with PPA plus Elvaloy and with SBS plus PPA were best, with the lowest nonrecoverable compliance and highest percentage of recovery, depending on whether the extracted or laboratory binder was evaluated. The dynamic modulus test results illustrated a smaller difference between mixtures, except where the binder modified with PPA plus Elvaloy had a more desirable variation in stiffness (e.g., softer at high frequencies and low temperatures, and slightly stiffer at low frequencies and high temperatures). The fatigue life ranking was different before the data were normalized for controlled strain conditions with the use of viscoelastic continuum damage principles. Without normalization, data from the two SBS-modified mixtures (with and without PPA) had the highest average fatigue life; however, with normalization, the data for mixtures modified with PPA plus Elvaloy exhibited the highest average fatigue life. Implications of the results are that PPA modification strategies can provide adequate resistance to rutting and moisture damage and that modification with PPA only is not the same as (and is statistically less resistant to fatigue cracking than) modification with polymer or with polymer plus PPA. Also, comparable fatigue cracking resistance can be achieved with the use of SBS alone or SBS plus PPA, which uses less polymer in conjunction with PPA.

Estimating Directional Permeability of Hot-Mix Asphalt by Numerical Simulation of Microscale Water Flow

Permeability of hot-mix asphalt (HMA) is an important property that influences asphalts resistance to moisture damage. All current methods used by pavement engineers rely on measuring vertical permeability. Recently, however, it has been shown that horizontal permeability can be much higher than the vertical because of the anisotropic and heterogeneous nature of air void distribution. Laboratory and field measurements of horizontal permeability require sophisticated equipment and detailed procedures that limit the ability to measure this property routinely. This study developed a procedure for estimating the horizontal permeability of asphalt mixes. The procedure requires simple laboratory measurements of vertical permeability and porosity (percent air voids) of sublayers in a specimen. The development of this simple method was supported by numerical simulations of microscale fluid flow in a wide range of HMA mixtures. The simulations helped in understanding the factors that control vertical and horizontal permeabilities. The developed equation and the numerical simulations confirmed that the horizontal permeability is several times higher than the vertical permeability.

Backcalculation of viscoelastic and nonlinear flexible pavement layer properties from falling weight deflections

Appropriate characterisation of individual layer properties is crucial for mechanistic analysis of flexible pavements. Typically in inverse analyses, pavements are modelled as elastic or nonlinear elastic to obtain layer material properties through non-destructive falling weight deflectometer (FWD) testing. In this study, a layered viscoelastic–nonlinear forward model (called LAVAN) was used to develop a genetic algorithm-based backcalculation scheme (called BACKLAVAN). The LAVAN can consider both the viscoelastic behaviour of asphalt concrete (AC) layer and nonlinear elastic behaviour of unbound layers. The BACKLAVAN algorithm uses FWD load-response history at different test temperatures to backcalculate both the (damaged) E(t) and |E*| master curve of AC layers and the linear and nonlinear elastic moduli of unbound layers of in-service pavements. The BACKLAVAN algorithm was validated using two FWD tests run on a long-term pavement performance section. Comparison between the backcalculated and measured results indicates that it should be possible to infer linear viscoelastic properties of AC layer as well as nonlinear elastic properties of unbound layers from FWD tests.