Fan Yin

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.

Novel Method for Moisture Susceptibility and Rutting Evaluation Using Hamburg Wheel Tracking Test

The Hamburg wheel tracking test (HWTT) has been widely used as a standard laboratory test to evaluate the moisture susceptibility and rutting resistance of asphalt mixtures. The stripping infection point and the rut depth at a certain number of load cycles are two common parameters obtained from the test. Although these parameters have been widely adopted by several transportation agencies, the accuracy and variability in characterizing mixture properties of these parameters have been questioned. In this study, a novel method to analyze the HWTT results is introduced and three new parameters are proposed to measure the moisture susceptibility and rutting resistance of asphalt mixtures. The new parameters are compared against the current ones to assess their capability to discriminate between three types of asphalt mixtures with different performance results in the HWTT. Significant advantages in characterizing mixture resistance to stripping and rutting are demonstrated by the new parameters. In addition, the effect of antistripping additives and recycled materials on mixture performance in the HWTT is evaluated with mixtures from a field project in Texas. Test results for the new parameters show that the addition of antistripping additives improves the susceptibility of asphalt mixtures to moisture. Specifically, the use of lime is more beneficial for improving mixture performance than a liquid antistripping agent. Conversely, the addition of recycled materials provides mixtures with increased moisture susceptibility but improved rutting resistance.

Properties of Foamed Asphalt for Warm Mix Asphalt Applications

This report presents proposed AASHTO standard test methods for measuring performance-related properties of foamed asphalts and designing foamed asphalt mixes with satisfactory aggregate coating and workability. The objectives of this project were to determine key properties of foamed asphalt binders that significantly influence the performance of asphalt mixtures and develop laboratory protocols for foaming of asphalt binders and laboratory mixing procedures. The production and performance-related properties of foamed asphalt were investigated through a series of laboratory and field experiments. A key finding of the research is that the foaming characteristics of an asphalt binder are primarily affected by its source (i.e., its crude oil slate), the production date for a given refinery and crude oil slate, and polymer modification. A laser-based method was developed to measure parameters associated with the expansion and collapse of foamed asphalt. A digital photographic approach was developed to characterize the size, distribution, and surface area of bubbles formed during production of foamed asphalt. Methods were also identified for determining a coatability index for foamed asphalt and the workability of mixes produced with foamed asphalt. A foamed asphalt mixture design procedure was developed to identify the optimum water content for coating and workability. Finally, the utility and effectiveness of these various methods were verified through their application to foamed asphalt binder and mix produced in full-scale asphalt mix plants. This report fully documents the research. Four appendixes are included: Influence of Binder Properties on Binder Foam Expansion; Draft Commentary on Guidelines Proposed for Revising Appendix to AASHTO R 35; AASHTO Style Standards; and Field Foaming Data Acquisition Form.

Evaluation of Moisture Susceptibility Minimization Strategies for Warm-Mix Asphalt: Case Study

AbstractWarm-mix asphalt (WMA) technologies aid in reducing mixing and compaction temperatures for asphalt concrete mixtures, allowing for savings in fuel consumption and extending haul distances and construction season. The reduced temperatures also provide a greener technology as emissions are lowered at the plant and the construction site. Engineering and environmental benefits promoted the rapid implementation of WMA technologies, but concerns remain regarding the difference in mixture performance of WMA versus hot-mix asphalt (HMA) because of the changes in the production process, specifically in terms of moisture susceptibility. This case study evaluates moisture susceptibility through the use of laboratory tests including the wet indirect tensile (IDT) strength test, the tensile strength ratio (TSR), and the Hamburg wheel tracking test (HWTT) analyzed with a novel methodology. The performance of two WMA technologies (Evotherm DAT and foaming) versus a control HMA is analyzed with and without antist...

Characterising the long-term rejuvenating effectiveness of recycling agents on asphalt blends and mixtures with high RAP and RAS contents

Although the use of high reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) contents in asphalt mixtures is desirable for environmental and economic reasons, these mixtures are prone to cracking, ravelling, and other durability-related pavement distresses mainly due to the heavily aged recycled binders. Highway agencies and the asphalt paving industry have been exploring the use of recycling agents (RA) in order to produce these mixtures with desirable performance. This study focused on characterising the long-term rejuvenating effectiveness of RA on asphalt blends and mixtures with high RAP and RAS contents. Materials from two field projects were used to prepare a number of asphalt blends and mixtures with various combinations of base binder, recycled material, and RA. These blends and mixtures were subject to various aging protocols prior to being characterised for their oxidation kinetics, rheological properties, and cracking resistance. The test results indicated that the RA evaluated in this study were effective in partially restoring the properties of recycled materials, but their rejuvenating effectiveness diminished with aging. Nevertheless, the recycled blends and mixtures with RA achieved equivalent or even better rheological properties and cracking resistance than those with an allowable amount of recycled materials per agency specifications but without RA. In addition, adding RA had no significant effect on the oxidation kinetics of the recycled blends, but increased their susceptibility to physical hardening in response to oxidation. Finally, the correlation between laboratory aging protocols for asphalt blends and mixtures were determined; the laboratory long-term oven aging protocols of 5 days at 85°C on compacted specimens and 1 day at 135°C on loose mix yielded binders with equivalent rheological properties to those subjected to rolling thin film oven (RTFO) plus approximately 10 and 40 h of pressure aging vessel (PAV), respectively.

Short-Term Laboratory Conditioning of Asphalt Mixtures

This report develops procedures and associated criteria for laboratory conditioning of asphalt mixtures to simulate short-term aging. The report presents proposed changes to the American Association of State Highway and Transportation Officials (AASHTO) R 30, Mixture Conditioning of Hot-Mix Asphalt (HMA), and a proposed AASHTO practice for conducting plant aging studies. The report will be of immediate interest to materials engineers in state highway agencies and the construction industry with responsibility for design and production of hot and warm mix asphalt.

Long-term ageing of asphalt mixtures

Ageing of asphalt mixtures occurs during production and construction and continues throughout the service life of the pavement. Although this topic has been studied extensively, recent changes in asphalt mixture components, production parameters, and plant design have raised a need for a comprehensive evaluation that considers the impacts of climate, aggregate type, recycled materials, WMA technology, plant type, and production temperature. In this study, field cores were acquired from seven field projects at construction and several months afterwards, and raw materials were also collected for fabricating laboratory specimens that were long-term oven aged (LTOA) in accordance with selected protocols. The resilient modulus and Hamburg wheel tracking tests were conducted on both specimen types to evaluate the evolution of mixture stiffness and rutting resistance with ageing. The concepts of cumulative degree days and mixture property ratio were proposed to quantify field ageing and its effect on mixture properties. Test results indicated that the LTOA protocols of two weeks at 140°F (60°C) and five days at 185°F (85°C) produced mixtures with equivalent in-service field ageing of 7–12 months and 12–23 months, respectively, depending on climate. Finally, among the factors investigated in the study, WMA technology, recycled materials, and aggregate absorption exhibited a significant effect on the long-term ageing characteristics of asphalt mixtures, while production temperature and plant type had no effect.

Effect of Water Content on Binder Foaming Characteristics and Foamed Mixture Properties

Mechanical foaming has become the most popular method for producing warm-mix asphalt in the United States. The process of mixing cold water with hot binder results in volume expansion and subsequent viscosity reduction of the binder, which is likely to produce a better coating of the aggregates along with improved overall mixture workability. Although mechanical foaming has been widely used in recent years, questions persist regarding the effects that different amounts of water have on binder foaming characteristics and foamed mixture properties. This study developed a novel, noncontact method to measure the expansion and collapse of foamed binder during the foaming process and the evolution of the size and amount of foam bubbles over time. Two parameters were proposed for evaluating the effect of water content on binder foaming characteristics. In addition, the effect of water content on the foamed mixture properties of workability and performance was investigated. The test results indicated that the amount of water used in the foaming process had a significant effect on binder foaming characteristics and foamed mixture properties. The optimum foaming water content could be determined through evaluation of the workability of foamed asphalt mixtures produced at different foaming water contents. Compared with hot-mix asphalt mixtures, equivalent or better performance in laboratory tests was observed for plant-produced and laboratory-produced foamed mixtures at the optimum water content.

Stiffness Characterization of Asphalt Mixtures with High Recycled Material Content and Recycling Agents

Economic and environmental considerations have prompted the use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixtures. However, given the concerns about long-term pavement performance, state departments of transportation (DOTs) tend to limit the quantities of these recycled materials unless certain mixture modifications are made [e.g., use of a softer virgin binder performance grade (PG) and warm-mix asphalt technology, the addition of a recycling agent (RA), or any combination of these modifications]. This study focused on the stiffness characterization of recycled asphalt mixtures with combinations of virgin binder PG, RAP, RAS, and RA. Materials were collected from two field projects in Texas and Indiana, and laboratory specimens were prepared and tested for resilient modulus, and dynamic modulus (|E*|) after short-term oven aging (STOA) and long-term oven aging (LTOA). An RA effectiveness parameter was proposed to quantify the rejuvenating effect of RA, which was defined as the percentage reduction in mixture stiffness for the recycled mixture with RA versus the corresponding control mixture without RA. Furthermore, the |E*| test results were analyzed with a Black Space diagram to discriminate asphalt mixtures with different stiffness and relaxation characteristics. The test results indicated that the incorporation of RA was effective in reducing the stiffness of asphalt mixtures with high recycled material content, but the effectiveness diminished with aging. Moreover, recycled mixtures with a softer and less brittle virgin binder and an RA at a higher dosage showed desirable stiffness and relaxation properties after STOA and LTOA.

Validation of Guidelines for Evaluating the Moisture Susceptibility of WMA Technologies

National Cooperative Highway Research Project (NCHRP) 9-49, which was completed in 2013, developed guidelines for warm mix asphalt (WMA) mix design and quality control to identify and minimize any possibility of moisture susceptibility. The objective of the current project, NCHRP Project 9-49B, was to validate and revise, if necessary, the thresholds in the guidelines developed in NCHRP Project 9-49. The research was based on a survey of the state departments of transportation (DOTs) and paving contractors to identify WMA mixtures with available field performance, mix design, and quality assurance data, including wet indirect tensile (IDT) strengths and tensile strength ratios, wet resilient moduli and ratios, and Hamburg wheel tracking parameters. The survey identified 89 field projects with either IDT or Hamburg wheel tracking results. These results were analyzed to validate the thresholds established for the tests in NCHRP Project 9-49. The key practical outcome of the research is a flowchart presented in this report for conditioning and testing WMA laboratory specimens in the mix design process that incorporates the validated thresholds.