Joe W Button

UNIFIED IMAGING APPROACH FOR MEASURING AGGREGATE ANGULARITY AND TEXTURE

The effect of fine aggregate morphologic properties on the behavior of asphalt mixtures is well recognized. However, current procedures for measuring fine aggregate properties are at best indirect indicators. The indirect nature of current measurements has led to inconsistency in predicting the extent to which the measured properties influence pavement performance. This paper presents 2 independent methods that integrate several aspects of image-analysis techniques for quantifying angularity and texture of fine aggregates. The first technique relies on the change of aggregate shape as it is subjected to erosion-dilation morphologic operations. The second technique uses the change of an object perimeter with its shape. Analysis shows that the 2 methods are able to capture aggregate shape at 2 different scales. Angularity is captured using low-resolution images, whereas surface texture is captured using high-resolution images. These techniques have potential benefit in quantifying effects of texture and angularity on asphalt pavement performance.

Implications of Experimental Measurements and Analyses of the Internal Structure of Hot-Mix Asphalt

This study summarizes the experimental and analysis methods used in characterizing and quantifying the internal structure of hot-mix asphalt (HMA) paving mixtures. The implications of the internal structure analysis for the design, compaction, and performance of HMA are also discussed. The methods can be divided into two categories: volumetric analysis methods and imaging methods. The volumetric analysis methods rely on bulk measurements of the percentage of air voids in certain aggregate sizes or in the whole mix as indicators of packing. These concepts are currently used in the Superpaver design system for densegraded HMA, stone matrix asphalt design, and the Bailey method for selection of aggregate gradations in HMA. The imaging methods quantify the distribution of the aggregate skeleton, voids in the mineral aggregate, and air voids by analyzing images of the internal structure. These images can be captured two-dimensionally with a simple setup of a microscope connected to a camera or three-dimensionally with an X-ray computed tomography system.

FINE-AGGREGATE ANGULARITY: AUTOMATED IMAGE ANALYSIS APPROACH

Angularity is one of the important aggregate properties contributing to the permanent deformation resistance of asphalt mixtures. Therefore, methods that are able to rapidly and accurately describe aggregate angularity are valuable in the design process of asphalt mixtures. Two computer-automated procedures, which make use of the advances in digital-image processing, to quantify fine aggregate angularity, are presented. The first method relies on the concepts of the erosion-dilation techniques. This consists of subjecting the aggregate surface to a smoothing effect that causes the angularity elements to disappear from the image. Then, the area lost as a result of the smoothing effect is calculated and used to quantify angularity. The second method is based on the fractal approach. Image-analysis techniques are used to measure the fractal length of aggregate boundary. The fractal length increases with aggregate angularity. The proposed imaging techniques are used to capture the aggregate angularity of 23 sand samples that represent a wide range of materials. The results are compared with visual analysis and indirect methods of measuring fine-aggregate angularity, such as the uncompacted air voids, and the angle of internal friction of aggregate mass. In general, the results indicate much promise for measuring aggregate properties using automated imaging technologies.

Guidelines for Using Geosynthetics with Hot-Mix Asphalt Overlays to Reduce Reflective Cracking

This paper provides complete guidelines for using geosynthetics with hot-mix asphalt overlays to reduce reflective cracking. Definitions of the various types of geosynthetics that are commercially available along with some of their advantages and disadvantages are also provided. These guidelines address the following: when to consider a geosynthetic product, how to select and store geosynthetics, cost considerations, pavement design with a geosynthetic, overlay construction with a geosynthetic, construction inspection, and potential construction problems.

EVALUATION OF SIMPLE PERFORMANCE TESTS ON HOT-MIX ASPHALT MIXTURES FROM SOUTH CENTRAL UNITED STATES

Since development of the Superpaver mix design procedure under the Strategic Highway Research Program about a decade ago, there has been a need to develop some type of a simple physical test to complement the Level 1 volumetric mixture design procedure. NCHRP Project 9-19 recognized the dynamic modulus test along with the flow time and flow number tests as the top three candidates for a simple performance test that could identify mixtures susceptible to permanent deformation. A critical evaluation of these three tests is presented along with the Super-pave shear test-frequency sweep at constant height with the asphalt pavement analyzer (APA) as the torture test to identify mixes susceptible to permanent deformation. The Hamburg wheel-tracking device (HWTD) also was used on selected mixtures. Special laboratory hot-mix asphalt mixes designed to exhibit low dynamic modulus but high recovery of strains were included in this evaluation. Results indicated that flow number value and flow time slope correlated better with laboratory rutting (APA and HWTD) than dynamic modulus. Flow number value, flow time slope, E*/sin Φ at 1 Hz, flow number slope, and flow time value were among the best five correlations with both the APA and the HWTD rut depths.

Effects of Temperature on Interface Shear Strength of Emulsified Tack Coats and Its Relationship to Rheological Properties

The objective of this study was to investigate tack coat interface shear bond characteristics at various temperatures and to relate tack coat characteristics in the field to the rheological test results in the laboratory. Interface shear strength (ISS) of two emulsified asphalt tack coats (CRS-1 and trackless) was evaluated at temperatures ranging from −10°C to 60°C. Two emulsified tack coats were applied on full-scale pavement test sections at application rates of 0.14, 0.28, and 0.70 L/m2. Cores were then extracted from the full-scale test sites to be tested in the laboratory while simulating different field conditions. Tests were conducted with a newly developed direct shear device. The binder grade for the residue of CRS-1 emulsion was PG 58-28. The high temperature grade for the residue of the trackless emulsion was PG 82. Trackless material was brittle at a low temperature, and its low temperature PG grade could not be determined. Within the evaluated temperature range, the ISS of the tacked interface increased with the decrease in temperature. Generally, bonding performance, as measured by the ISS, of the trackless emulsion was superior to CRS-1 emulsion, specifically at temperatures greater than 40°C. In addition, results show that interface shear strength for both tack coat materials at each application rate increased with an increase of the binder rheology parameter G*/sin δ. This relationship can be used to establish a laboratory design threshold for this parameter, to ensure that the selected application rate and tack coat material will perform adequately in the field.

Effects of Pavement Surface Type and Sample Preparation Method on Tack Coat Interface Shear Strength

The objective of this study was to quantify the effects of tack coat type, tack coat application rate, and surface type (i.e., hot-mix asphalt versus portland cement concrete) including milled versus unmilled surfaces on the interface shear strength based on full-scale test application. The variation of interface shear strength between field- and laboratory-prepared samples was also investigated. To achieve this objective, five types of tack coat materials were applied at three application rates on four types of surfaces at the Pavement Research Facility site of the Louisiana Transportation Research Center. Samples were cored from the constructed test lanes, and the interface shear strength was measured using the Louisiana Interface Shear Strength Tester. Results of this study showed that a direct relationship was observed between the roughness of the existing surface and the developed shear strength at the interface. A small amount of water seemed to negatively affect interface shear strength with PG 64-22 used as a tack coat material. However, the effect of surface wetness on interface shear strength was less evident for emulsion-based tack coat materials. Laboratory-prepared samples grossly overestimated the interface shear strength when compared with field-extracted cores. While a decreasing trend was observed in the laboratory, an increasing trend in the measured interface shear strength was observed in the field.

Production, Placement, and Performance Evaluation of Warm Mix Asphalt in Texas

This paper documents the first warm mix asphalt field trial placed by the Texas Department of Transportation in 2006 using the emulsion based technology (Evotherm) developed by MeadWestvaco. The production, laydown, and compaction characteristics of warm mix were compared with a conventional hot mix asphalt control using a standard mixture design. Field-mixed, lab compacted samples were evaluated for production density, moisture susceptibility and cracking resistance. Field cores were obtained after one month and one year of service and evaluated using Hamburg Wheel-Tracking Tests and the TTI Overlay test. Cores were also evaluated using X-Ray computed tomography to determine the distribution of air voids versus depth in the compacted sample. Field performance of the warm mix is comparable to the hot mix control section after two years of service.

Using Pseudostrain Damage Theory to Characterize Reinforcing Benefits of Geosynthetic Materials in Asphalt Concrete Overlays

Reflective cracking is one of the more serious distresses associated with existing hot-mix asphalt (HMA) or portland concrete cement pavements overlaid with a thin bituminous layer. Preventive maintenance techniques have included incorporating geosynthetic materials (defined here as grids, fabrics, or composites) into the pavement structure. These materials have exhibited varying degrees of success, and their use within a particular agency has been based primarily on local experience or a willingness to try a product that appears to have merit. A methodology is described that was used to compare the relative effectiveness of six commercially available geosynthetic materials in reducing the severity or delaying the appearance of reflective cracking in HMA overlay. Each geosynthetic material was incorporated into compacted HMA specimens and tested to failure. Engineering fracture mechanics and pseudostrain energy concepts based on the elastic-viscoelastic correspondence principle were used and demonstrated to be appropriate and efficient in characterizing the fatigue damage process. By considering the effects of the geosynthetic products on the loading and unloading paths of the HMA specimens, a new concept was developed and termed the reinforcing factor, R. The use of this value allows the industry to characterize the relative reinforcing benefits of geosynthetic materials in reducing reflective cracking in HMA overlays. A crack speed index was then derived to summarize the complex interactions of the material properties. In general, grids and composites performed better than fabrics, which in turn performed better than a thin tacked surface as compared with unreinforced specimens. Design equations were developed between the fracture properties of the geosyntheticmixture system and the relaxation modulus properties of the HMA, which can be used in forward-calculating design methods to predict the rate of crack growth and support the design of an HMA overlay to resist reflective cracking. To calibrate the design equations, comparative field test pavements were constructed in three regions of Texas (Amarillo, Waco, and McAllen) using each geosynthetic material. These pavements will be monitored over the next 4 years.

EFFECTS OF SUPERPAVE RESTRICTED ZONE ON PERMANENT DEFORMATION

The purpose of this study is to evaluate the restricted zone effect using four different aggregates: crushed granite, crushed limestone, crushed river gravel, and a mixture of crushed river gravel as coarse aggregate with natural fines. As the restricted zone is a component of Superpave, the blends prepared met most of the Superpave criteria, except the restricted zone in selected mixtures and fine aggregate angularity in three mixtures. Each type of aggregate was used for mixture design of three gradations: above, through, and below the restricted zone. The twelve mixtures designed were tested in the laboratory to evaluate their relative resistance to permanent deformation. Four types of tests were performed using Superpave equipment: simple shear at constant height, frequency sweep at constant height, repeated shear at constant stress ratio, and repeated shear at constant height. Rutting resistance of the mixtures was measured using the Asphalt Pavement Analyzer. Researchers found that there is no relationship between the restricted zone and permanent deformation when crushed aggregates are used in the mixture design. Superpave mixtures with gradations below the restricted zone were generally most susceptible to permanent deformation while mixtures above the restricted zone were least susceptible to permanent deformation. Recommendations include elimination of the restricted zone from hot mix asphalt design specifications.