Arif Chowdhury

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.

Characterization of aggregate resistance to degradation in stone matrix asphalt mixtures

Stone matrix asphalt (SMA) mixtures rely on stone-on-stone contacts among particles to resist applied forces and permanent deformation. Aggregates in SMA should resist degradation (fracture and abrasion) under high stresses at the contact points. This study uses conventional as well as advanced imaging techniques to evaluate aggregate characteristics and the resistance of aggregates to degradation. Aggregates from different sources and aggregate types with various shape characteristics were used in this study. The micro-Deval test was used to measure aggregate resistance to abrasion. The aggregate imaging system was used to examine the changes in aggregate characteristics caused by abrasion forces in the micro-Deval. The resistance of aggregates to degradation in the SMA was evaluated through the analysis of aggregate gradation before and after compaction with conventional mechanical sieve analysis and nondestructive X-ray computed tomography. The findings of this study led to the development of an approach for evaluating aggregate resistance to degradation in SMA. This approach measures aggregate degradation in regard to abrasion, breakage, and loss of texture.

Influence of Air Voids on Mechanical Properties of Asphalt Mixtures

ABSTRACT Air voids play an important role in influencing the performance of asphalt mixtures. This study utilized several mechanical tests to characterize the influence of air void distributions on mechanical properties and response of asphalt mixtures. The overlay tester, Hamburg Wheel-Tracking Test (HWTT), and a repeated tensile test were used to characterize the resistance to fatigue cracking, permanent deformation, and moisture damage, respectively. The overlay test results showed that specimens with more uniform air void distribution had less variability in terms of resistance to fatigue cracking compared with specimens with less uniform air void distribution. Consequently, recommendations were made to improve air void distribution and reduce the test variability. The HWTT results were found to be insensitive to the air void distribution. The repeated tensile test was used to measure material properties that were required inputs to a fracture-based model for the analysis of moisture damage. The repeated tensile test and model results showed that the detrimental effect of moisture was the highest for specimens with the highest percent of air voids. In addition to the characterization of mechanical properties, this study evaluated the effect of temperature distribution in Superpave Gyratory Compacted (SGC) specimens on air void distribution. It was possible to make some minor changes to the compaction procedure in order to improve the uniformity of temperature distribution and, consequently, enhance the uniformity of air void distribution.

Field Aging of Unmodified Asphalt Binder in Three Texas Long-Term Performance Pavements

Binder hardening in pavements and its impact on pavement durability are becoming better understood. Binders oxidize, harden, and become more susceptible to fatigue and thermal cracking. Data are mounting that binder aging occurs to a significant degree well below the top inch of the pavement, contrary to common belief. Whereas recent data indicate that binders age at constant rates and to a significant depth in pavements, a need exists to obtain the detailed profile for understanding pavement aging as a function of air voids. The extent to which binder oxidation occurs throughout the pavement is an important issue and must be understood and quantified to understand pavement durability better and to design longer-lasting pavements. In this work, two layers in each of three pavements in three Texas districts were evaluated. Pavement measurements included recovered binder properties using a dynamic shear rheometer, size exclusion chromatography, changes in these properties over time due to oxidative aging in the pavement, and pavement total and accessible (interconnected) air voids. Many of the pavement cores were sawed into 0.5-in. layers with the binder and air void properties determined for each layer. These measurements provided an indication of binder aging in Texas pavements and suggest strongly that binders age even inches down into the pavement. It was observed that lower accessible air voids correlated with a lower rate of binder oxidation and hardening.

Evaluation of the Moisture Susceptibility of WMA Technologies

Over the past decade, the use of warm mix asphalt (WMA) for asphalt pavement construction has increased in the United States. However, questions remain about the long-term performance and durability of WMA pavements. One key issue is the moisture susceptibility of WMA pavements. Concerns about WMA moisture susceptibility include the possibility that aggregates will be inadequately dried at lower production temperatures and the fact that several WMA technologies introduce additional moisture in the production process. The objectives of National Cooperative Highway Research Program (NCHRP) Project 9-49 were to (1) assess whether WMA technologies adversely affect the moisture susceptibility of asphalt pavements and (2) develop guidelines for identifying and limiting moisture susceptibility in WMA pavements. The research was conducted through coordinated laboratory and field experiments that investigated the potential for moisture susceptibility in WMA compared to hot mix asphalt (HMA). Design of the experiments was guided by a survey of the state departments of transportation and industry on WMA pavement construction and performance. The survey identified no instances of moisture damage to WMA pavements in service through 2010. This negative finding is supported by the results of recently completed NCHRP Project 9-47A, which conducted intensive evaluations of WMA pavements constructed across the United States between 2006 and 2011. Project 9-49 then focused on development of guidelines for WMA mix design and quality control to identify and minimize any possibility of moisture susceptibility. The laboratory experiments evaluated (1) laboratory-conditioning protocols for WMA before moisture susceptibility testing, (2) the ability of standard test methods to detect moisture susceptibility of WMA, and (3) potential differences in WMA moisture susceptibility measured on laboratory-mixed and -compacted specimens; plant-mixed, laboratory-compacted specimens; and plant-mixed, field-compacted cores. The guidelines are presented in the form of a workflow of conditioning protocols and standard test methods that first assess the potential moisture susceptibility of a WMA mix design or field mixture and then recommend remedies to minimize such susceptibility. Specific test thresholds in the guidelines are based on the results of testing of WMA from field projects in Iowa, Montana, New Mexico, and Texas. This report fully documents the research and includes the following Appendixes: Appendix A, Laboratory Conditioning Experiment; Appendix B, Moisture Conditioning Experiment; Appendix C, Performance Evolution Experiment; Appendix D, Construction Reports and Performance of Field Projects; Appendix E, Mixture Volumetrics; Appendix F, Proposed Draft Revisions to the Appendix to AASHTO R 35; Appendix G, Future Work Plan to Evaluate Moisture Susceptibility of HMA and WMA; and Appendix H, Statistical Results. Appendix F is included herein. Appendixes A—E, G, and H are available on the TRB website.

Relationship of Aggregate Microtexture to Asphalt Pavement Skid Resistance

Aggregate properties are one of the important factors that influence the asphalt pavement skid resistance. This paper presents a detailed analysis of aggregate texture and its relationship to pavement skid resistance. A new method is developed for the evaluation of aggregate resistance to polishing. This method relies on the Micro-Deval test as the mechanism for polishing aggregates and the Aggregate Imaging System (AIMS) for quantifying the change in texture due to polishing. The results show that the Micro-Deval test is an effective method for polishing aggregates within a short time. Also, the AIMS texture analysis is able to rapidly and accurately quantify the influence of polishing on texture. The verification of the new method was achieved through measuring the skid resistance of pavements constructed using three different aggregate sources and three different aggregate gradations. The skid resistance was found to be related not only to average aggregate texture, but also to the texture distribution within an aggregate sample. The developed method can be used in models for predicting the change in asphalt pavement skid resistance as a function of aggregate texture, mixture properties, and environmental conditions.

Development of a Model for Asphalt Pavement Skid Resistance Based on Aggregate Characteristics and Gradation

Many research studies and field experiences have shown a strong relationship between wet-weather accidents and pavement skid resistance. Therefore, measurement methods and models are needed to evaluate the safety level of driving on an asphalt pavement surface during its service life. The objective of this study was to develop a skid resistance prediction model based on measurable quantities such as aggregate shape characteristics, aggregate gradation, aggregate resistance to polishing, and traffic level. To achieve this objective, the skid number (SN) of asphalt pavement sections and traffic data were acquired and analyzed. In addition, statistical analysis was conducted to determine the relationship between different aggregate properties, pavement surface characteristics, and the measured SN values. The aggregate properties were measured using conventional test methods (acid insolubility, magnesium soundness, micro-Deval, and British polish value), and the Aggregate Imaging System (AIMS). The pavement surface characteristics were measured using the dynamic friction tester and circular texture meter. The statistical analysis led to the development of a new model for predicting the asphalt pavement SN as a function of traffic level, initial and terminal aggregate texture values measured using AIMS, and aggregate gradation described using the two-parameter Weibull distribution function.

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.

Predicting Asphalt Mixture Skid Resistance by Aggregate Characteristics and Gradation

The objective of this study was to investigate the influence of aggregate characteristics and gradation on the skid resistance of various asphalt mixtures. Asphalt mixture slabs with different combinations of aggregate sources and gradations were fabricated in the laboratory. These slabs were polished with a wheel-polishing device developed by the National Center for Asphalt Technology. The frictional characteristics of each slab were then measured by the sand patch method, British pendulum, dynamic friction tester, and circular texture meter. Aggregates used in these mixtures were characterized by petrographic analysis, conventional test methods (acid insolubility, magnesium soundness, Micro-Deval, and British polish value), and the aggregate imaging system (AIMS). In addition, the aggregate gradation of each mixture was described by the two-parameter cumulative Weibull distribution function. Statistical analysis of test results led to the development of a function for predicting the International Friction Index, which is a measure of skid resistance of asphalt mixtures, after different intervals of polishing. The parameters of this function were found to be related to (a) initial and terminal aggregate texture measured by using AIMS, (b) rate of change in aggregate texture measured by using AIMS after different polishing intervals in the Micro-Deval, and (c) the Weibull distribution parameters describing aggregate gradation. This function can be useful for estimating the frictional characteristics of an asphalt mixture surface during the mixture design stage.

Comprehensive Evaluation of Compaction of Asphalt Pavements and a Practical Approach for Density Predictions

The compaction of asphalt mixtures has a significant influence on their performance. Insufficient compaction leads to several distresses, such as premature permanent deformation, excessive aging, and moisture damage, even if all desirable mixture design characteristics are met. Currently, no acceptable method relates asphalt pavement density to the compaction pattern. The compaction pattern required to achieve the desired density is established on the basis of experience and trial and error, a process that is time-consuming and expensive. This study involved comprehensive experimental evaluation of several test sections that were constructed with different asphalt mixtures by various compaction methods. Results of these experiments were used to determine the effects of compaction temperature, compaction method, mixture design, and base type on the compactability of asphalt mixtures. In addition, the paper presents a systematic method to determine the compaction pattern required to achieve the desired density of asphalt pavements. This method lends itself to integration in an automated system that can be used by roller operators to estimate density during construction.