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Recent studies on long-life flexible pavements indicate that it may be advantageous to design and construct asphalt mixtures comprising the underlying layers in such a manner that very dense mixtures are produced. This will improve not only the fatigue characteristics but also durability through a decrease in air voids. A 19.0 mm mixture was designed and tested at asphalt contents (ACs) higher than the optimum design level. Stiffer binder and recycled asphalt pavement (RAP) were employed to help maintain stiffness in order to prevent instability. The field voids were predicted to decrease approximately 1.0 to 1.5 percent for each 0.4 percent increase in AC, which would improve durability. Flexural stiffness peaked for an 0.5 percent increase in AC, and fatigue life trended upward but needed approximately 1.0 percent additional asphalt for a major beneficial effect. Permeability improved slightly as AC was increased. The researchers think that the Hamburg test would have been more appropriate for this study than the tensile strength ratio test, which indicated no improvement in stripping susceptibility with an increased AC, because it might simulate field conditions better. In addition, the Mechanistic-Empirical Pavement Design Guide Software (Version 0.900) was used to evaluate trial pavement designs with several design alternatives, including varying the binder performance grade, effective binder volume, and air void content to determine the resultant changes in predicted fatigue cracking and rutting of hot-mix asphalt (HMA) layers. This theoretical pavement analysis indicated that increasing the binder content of the HMA intermediate layer beyond the design optimum and increasing the stiffness of the intermediate layer by increasing the high-temperature binder performance grade slightly decreased the predicted fatigue cracking and reduced the rutting of the HMA layers. The analysis also showed that more significant reductions in the predicted fatigue cracking could be realized by increasing the binder content of the HMA base layer slightly beyond the optimum and by reducing the in-place air void content of the HMA base layer. It was recommended that VTRC should further investigate the effects of higher binder contents and lower air voids on the performance of base mixes. Further study of current void criteria to verify optimum pavement performance is also recommended. This project provides a stepping stone to achieve long-lasting perpetual-type flexible pavement. Designs with a high binder content offer the potential to reduce fatigue cracking 20 to 60 percent by incorporating additional asphalt binder and reducing the void content of asphalt base. The use of RAP to maintain the necessary stiffness for high binder contents should provide comparable stiffness to an increasingly expensive PG 70-22 binder for base material. Some effort is taking place in 2007 for reducing voids in base mixes with high RAP content; however, quantification of the economic benefits from that endeavor will be a future goal.
TRB's National Cooperative Highway Research Program (NCHRP) Report 752: Improved Mix Design, Evaluation, and Materials Management Practices for Hot Mix Asphalt with High Reclaimed Asphalt Pavement Content describes proposed revisions to the American Association of State Highway and Transportation Officials (AASHTO) R 35, Superpave Volumetric Design for Hot Mix Asphalt, and AASHTO M 323, Superpave Volumetric Mix Design, to accommodate the design of asphalt mixtures with high reclaimed asphalt pavement contents.
The purpose of this study was to document the field experience of the Virginia Department of Transportation (VDOT) in the use of high-modulus high-binder-content (HMHB) base layer hot-mix asphalt (HMA) mixtures. Information was gathered with regard to the construction of HMHB base mixtures at three field trial sites in Virginia, and laboratory tests were conducted on samples that were gathered before and during construction. The three locations were the sites of deep rehabilitation or new construction, and HMA base mixtures were used at designed asphalt content, designed asphalt content plus 0.4% additional asphalt, and/or designed asphalt content plus 0.8% additional asphalt. Two of the field trial locations had no construction-related issues; difficulties during compaction occurred at the third. Laboratory testing of materials collected from this location showed the mixture to have a low air void content, a high percentage of voids filled with asphalt, and a binder performance grade that was lower than expected. This was also the only location from which materials were collected for fatigue testing, the results of which did not show a clear relationship between binder content and fatigue life. It is thought that the mixture production and construction issues at this location were site specific and not generally indicative of a larger trend when HMHB mixtures are used. The research showed that HMHB mixtures incorporating 0.4% additional asphalt binder could be successfully constructed but was unable to determine if the same was true of HMHB mixtures incorporating 0.8% additional asphalt binder. Further study may be needed to determine the maximum additional asphalt binder that can be successfully incorporated. Additional studies using repeated-load permanent deformation should be conducted to determine if a cutoff value (or a range) of the flow number can be established to determine optimum performance. This study documented the field and laboratory knowledge gained by VDOT when producing and placing HMHB mixture test sections in an effort to achieve a long-lasting perpetual-type flexible pavement. These designs offer the potential to reduce fatigue cracking by incorporating additional asphalt binder and reducing the void content of the mixture. The use of an adjusted binder grade or RAP to maintain the necessary stiffness for high binder contents should provide the necessary stiffness to minimize the susceptibility for rutting during service. Quantification of the economic benefits of using HMHB mixtures is a future goal that can be realized after longer term study of field performance
One of the emerging solutions to enhance the durability of asphalt pavements is the use of a French asphalt mix known as "High-Modulus Asphalt Concrete (HMAC)." This mix uses a hard asphalt binder, high binder content (about 6%), and low air voids content as compared to Superpave mixtures. The key objective of this study was to develop a cost-effective HMAC mixture using crumb rubber and local materials in Louisiana. To achieve this objective, four HMAC mixtures were prepared using two asphalt binders (PG 82-22 and PG 76-22 plus 10% crumb rubber) and two Reclaimed Asphalt Pavement (RAP) contents (20% and 40%); additionally, a conventional Superpave mixture in Louisiana was prepared as a control mixture. The laboratory performance of these five mixtures was evaluated in terms of workability, dynamic modulus, rutting resistance, and cracking resistance. The AASHTOWare Pavement ME Design software was also used to estimate the long-term field performance of these mixtures. Results indicated that the HMAC mixture prepared with 10% crumb rubber and 20% RAP successfully met the French mix design specifications for HMAC and LaDOTD specifications. This HMAC mix outperformed the control Superpave mix in terms of dynamic modulus, rutting resistance, and cracking resistance. Additionally, this HMAC mixture can reduce the required asphalt thickness by 1.5 or 2 inches based on the traffic level. The cost-effectiveness analysis indicated that this HMAC mixture was more cost-effective than conventional Superpave mixtures in Louisiana. In addition, this mixture is environmentally-friendly since it can reduce the disposal of scrap tires in landfills.
TRB's National Cooperative Highway Research Program (NCHRP) Report 691: Mix Design Practices for Warm-Mix Asphalt explores a mix design method tailored to the unique material properties of warm mix asphalt technologies. Warm mix asphalt (WMA) refers to asphalt concrete mixtures that are produced at temperatures approximately 50°F (28°C) or more cooler than typically used in the production of hot mix asphalt (HMA). The goal of WMA is to produce mixtures with similar strength, durability, and performance characteristics as HMA using substantially reduced production temperatures. There are important environmental and health benefits associated with reduced production temperatures including lower greenhouse gas emissions, lower fuel consumption, and reduced exposure of workers to asphalt fumes. Lower production temperatures can also potentially improve pavement performance by reducing binder aging, providing added time for mixture compaction, and allowing improved compaction during cold weather paving. Appendices to NCHRP Report 691 include the following. Appendices A, B, and D are included in the printed and PDF version of the report. Appendices C and E are available only online.
TRB's National Cooperative Highway Research Program (NCHRP) Report 714: Special Mixture Design Considerations and Methods for Warm-Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary presents special mixture design considerations and methods used with warm-mix asphalt. NCHRP Report 714 is a supplement to NCHRP Report 673: A Manual for Design of Hot-Mix Asphalt. All references to chapters in NCHRP Report 714 refer to the corresponding chapters in NCHRP Report 673.
"ASTM Publication Code Number (PCN) 04-011080-08. - "Sponsored by ASTM Committee D-4 on Road and Paving Materials."-- Foreword. - Includes bibliographical references and indexes. - Electronic reproduction; W. Conshohocken, Pa; ASTM International; 2011; Mode of access: World Wide Web; System requirements: Web browser; Access may be restricted to users at subscribing institutions.
Functional Pavement Design is a collections of 186 papers from 27 different countries, which were presented at the 4th Chinese-European Workshops (CEW) on Functional Pavement Design (Delft, the Netherlands, 29 June-1 July 2016). The focus of the CEW series is on field tests, laboratory test methods and advanced analysis techniques, and cover analysis, material development and production, experimental characterization, design and construction of pavements. The main areas covered by the book include: - Flexible pavements - Pavement and bitumen - Pavement performance and LCCA - Pavement structures - Pavements and environment - Pavements and innovation - Rigid pavements - Safety - Traffic engineering Functional Pavement Design is for contributing to the establishment of a new generation of pavement design methodologies in which rational mechanics principles, advanced constitutive models and advanced material characterization techniques shall constitute the backbone of the design process. The book will be much of interest to professionals and academics in pavement engineering and related disciplines.