Jon Epps

Nevada's Approach to Pavement Management

The Nevada Department of Transportation has a fully implemented pavement management system (PMS) at the network level and an extensive pavement evaluation system at the project level. Both systems use actual field performance data that are collected annually from pavement sections throughout the state. The PMS is being further developed to incorporate performance modeling, life-cycle cost analysis, and network optimization processes. Once fully developed the system will allow the engineer to select the best rehabilitation and maintenance alternative that can carry the pavement over the entire analysis period. The project-level evaluation process includes the use of nondestructive testing data and condition surveys from the PMS. The overall systems are summarized at both levels, and the individual steps that are involved within each system are described.

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.

EVALUATION SYSTEMS FOR CRUMB RUBBER MODIFIED BINDERS AND MIXTURES

The use of crumb rubber modified (CRM) asphalt mixtures has been increasing steadily. The reasons for this increase in use are twofold: the Intermodal Surface Transportation Efficiency Act of 1991 mandate and the potential of better performance. The technology of testing, design, and evaluation of CRM binders and mixtures has not yet caught up. Therefore, there is a need for establishing standard procedures for the design and evaluation of CRM mixtures. The outcome of a recent research project sponsored by the Nevada Department of Transportation (NDOT) is presented. The binder testing system concentrated on the newly developed Strategic Highway Research Program (SHRP) performance grading system and its applicability to CRM binders. It was concluded that the standard SHRP system will not work for CRM binders containing coarse rubber particles. The plate and cup system will have to be used for such CRM binders. However, the plate and cup system cannot adequately replace the bending beam rheometer for low-temperature testing. In the case of CRM mixtures, a modified Hveem mix design procedure was developed and validated on an actual NDOT project. Temperature susceptibility, moisture sensitivity, rutting resistance, and low-temperature cracking of CRM mixtures also are presented.

THERMAL CRACKING MODELS FOR AC AND MODIFIED AC MIXES IN ALASKA

Low-temperature cracking is a major distress mode in Alaskan pavements because of the extreme temperature conditions—which range, in some instances, from about −50°C in winter to more than 40°C in summer. The use of asphalt modifiers in Alaskan pavements occurred over the past 15 years. These modifiers include Styrene-Butadiene-Styrene polymers, Styrene-Butadiene-Rubber polymers, ULTRAPAVE, and CRM [both the dry process (PlusRide) and the wet process]. Field observations and laboratory studies in Alaska and elsewhere indicate that the use of these modifiers would improve the low-temperature cracking resistance of pavements. The degree to which these modifiers provide beneficial effects for Alaskan pavements needs to be evaluated. The objectives of this research were (1) To characterize asphalt and polymer modified asphalt from a number of selected sites using Superpave PG grading system and to conduct thermal stress restrained specimen tests (TSRST) and Superpave IDT laboratory tests on field specimens; (2) To compare low-temperature cracking performance using field surveys; (3) To verify the applicability of the Superpave thermal cracking model (TCMODEL) and other available models for predicting low temperature cracking; and (4) To recommend guidelines for predicting minimum pavement temperatures in Alaska. Results of this study indicate, in general, significant improvement in low-temperature cracking resistance when polymer modifiers are used. Comparisons between predicted and observed low-temperature cracking using available crack propagation models, including Superpave TCMODEL, were poor. An improved regression model was developed using minimum air temperature, TSRST fracture temperature and strength, and pavement age to fit the observed field data for both conventional and polymer modified sections.

DEVELOPMENT OF PERFORMANCE MODELS BASED ON DEPARTMENT OF TRANSPORTATION PAVEMENT MANAGEMENT SYSTEM DATA

The Nevada Department of Transportation (NDOT) is developing a network optimization system (NOS). The objective of the system is to evaluate various alternatives and to recommend the most cost-effective rehabilitation treatment to be used on the various sections of the state highway system. The NOS consists of several subsystems, including performance models, life-cycle cost analysis, and network optimization. Performance models have been developed for the rehabilitation treatments most commonly used by NDOT: overlay and mill/overlay. The performance models were developed on the basis of the pavement management system data that have been collected by NDOT for the last 15 years. The models were generated through statistical analyses that related the present serviceability index to age, traffic, environment, and structural and materials properties. First cost and annual maintenance cost figures were established to be used in the life-cycle cost analysis. The annual maintenance costs proved to be the most difficult figures to estimate due to the extremely large variability. After several attempts, the fixed period cumulative cost approach was identified as the best alternative to reducing the variability of the annual maintenance costs. In order to accomplish true network optimization, the entire NDOT highway system was subdivided into project segments (total number of segments is in excess of 800). Several databases were formed to house the data corresponding to each project segment. The NOS will use the performance models in conjunction with the supporting databases to conduct a fully automated optimization analysis.

Field and Laboratory Performance of Superpave Mixtures in Nevada

The Nevada Department of Transportation (NDOT) has strongly supported Superpaver-related research and implementation efforts since the completion of the Strategic Highway Research Program. However, it is one of the state highway agencies that has not implemented use of the Superpave mix design system. Instead of rapidly implementing Super-pave in the mid-1990s, NDOT chose to build field test sections composed of Superpave and the mixtures conventionally used at the time (Hveem) and placed side-by-side to evaluate the Superpave system relative to the system in use. Extensive laboratory evaluations were conducted on the materials and mixtures placed on each section, and field performance was monitored over time. This study presents a synopsis of the field and laboratory performance information gathered over the past 6 years. Planned NDOT evaluations of Superpave mixtures based on the findings of this effort to date are outlined.

Development of Performance-Related Specifications for Hot-Mix Asphalt Pavements Through WesTrack

The primary objective of the FHWA-sponsored WesTrack project is to further the development of performance-related specifications for hotmix asphalt construction. This objective is being achieved, in part, through the accelerated loading of a full-scale test track facility in northern Nevada. Twenty-six hot-mix asphalt test sections constructed to meet the criteria set forth in a statistically based experiment design are providing performance data that will be used to improve existing (or develop new) pavement performance prediction relationships that better account for the effects that “off-target” values of asphalt content, air-void content, and aggregate gradation have on such distress factors as fatigue cracking, permanent deformation, roughness, raveling, and tirepavement friction. The concept of the planned new performance-related specification and how it will incorporate the modified pavement performance prediction models are described. The current plan for assessing contractor pay adjustments (i.e., penalties and bonuses) based on data collected from the as-constructed pavement is also discussed.

MINIMUM PAVEMENT TEMPERATURE MODELING AND MAPPING FOR ALASKAN CONDITIONS

According to the Superpave design system, the selection of asphalt binder using performance-based binder specifications depends in part on the expected high and low pavement temperatures. For low-temperature cracking considerations, the binder should satisfy minimum pavement temperature requirements selected for design. In this case, knowledge of minimum pavement temperature during the design period is essential for proper binder selection. In this study, field temperature data for a number of sites were collected and analyzed. Correlations between minimum air and pavement temperature were established for different Alaskan climatic zones. Results indicate that the minimum pavement temperature could be lower or higher than minimum air temperature. However, for low temperature ranges the pavement temperature is generally warmer than air temperature by 2 to 7° C, depending on climate zone. Comparisons of minimum pavement temperature for Alaskan conditions with Superpave and the Asphalt Institute criteria for selecting minimum pavement design temperature were made. Results show that the criteria are not appropriate for Alaskan pavements. The Superpave prediction is conservative whereas the Asphalt Institute’s prediction model is unconservative. In addition, contour maps corresponding to 50 percent and 98 percent reliability were developed for minimum air and pavement temperatures. The temperature correlations and contour maps provide a simple tool for design engineers to estimate minimum design pavement temperature for selecting appropriate binder specifications.

Selecting Most Desirable Hot-Mix Asphalt Mixtures

Aggregate properties and gradations have been shown to have a significant impact on the strength and performance of HMA mixtures. Strength properties, such as the resilient modulus and tensile strength, and performance properties, such as moisture sensitivity, permanent deformation, and low-temperature cracking, play a major role in the field performance of HMA mixtures. However, it is believed that by changing the aggregate gradation and the asphalt binder, a desirable hot-mix asphalt mixture can be achieved for any source of aggregate. The data and analysis of a laboratory research study that evaluated four gradations and four asphalt binders in conjunction with five sources of Nevada aggregates are summarized. The measured material properties include the Superpave binder and aggregate properties, the strength properties, and the permanent deformation and low-temperature cracking of the mixtures. The data analysis indicated that the resilient modulus and tensile strength tests can be used in lieu of the more complicated triaxial repeated-load test to ensure against rutting and that the binder property can be used to ensure against low-temperature cracking.