Construction and Maintenance Practices for Permeable Friction Courses (2009)

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3Permeable friction courses (PFC), which include new gen- eration open-graded friction courses (OGFCs), asphalt-rubber friction courses, and porous European mixes (PEM), have gained wide acceptance throughout the southern and western portions of the United States. PFCs are rapidly gaining pop- ularity due to their safety and environmental benefits. Wear- ing layers comprised of PFC mixes have proven a safe driving surface. These mixes reduce hydroplaning and splash and spray while also improving wet weather friction and visibility of pavement markings, especially in wet weather. PFC layers also have been shown to be beneficial to the environment due to the ability to reduce[m1][m2] tire/pavement noise and improving the quality of water runoff during rain events. PFCs also have been identified as a candidate technology for cool pavements. There are numerous differences between PFC and the first generation OGFCs widely used in the United States during the 1970s and 1980s. PFCs typically contain at least 20 percent more asphalt binder (by volume) than conventional OGFC mixes. They are generally designed to have 18 percent air voids or more, whereas conventional OGFC mixture typically con- tained between 10 and 15 percent air voids. The void structure of PFC allows the mix to be more permeable than conven- tional OGFC and less likely to trap water that could freeze. Unlike conventional OGFC, PFCs may contain fibers, polymer- modified asphalt binders, or asphalt-rubber, alone or in com- bination. Permeable friction courses are typically placed in thicker layers than conventional OGFC (1.0 to 2.0 inches as opposed to 1.0 inch or less). The thicker, more open void structure allows PFC to drain larger volumes of water off the roadway surface faster than conventional OGFC and keeps the void structure clean longer through the flushing action of high-speed traffic, and therefore, reducing the potential for loss of permeability over time. These differences have contributed to a longer reported performance life for PFC compared to conventional OGFC. Research on PFC indicates that the mixes typically last between 8 to 12 years, significantly longer than the first generation OGFC mixtures which typically lasted 5 to 7 years. No wide- spread performance problems with PFC such as raveling have been reported, but concerns remain whether PFC mixes will experience the same performance problems that plagued the first generation OGFC mixes used in freeze-thaw environ- ments. In freeze-thaw environments, the associated incon- veniences and increased cost of winter maintenance and the possible related formation of glaze (“black ice”) seem to out- weigh the benefits of PFC. While black ice can form on any pavement under the right environmental conditions, there is information that it is likely to form on PFC earlier and last longer than on other HMA surfaces. These concerns are a likely reason that PFC mixes are used predominately in the warmer climates found in the southern and western regions of the United States and are not used widely in areas that experience frequent freeze-thaw cycles. Pavement maintenance issues and snow and ice removal also are cited as obstacles to further increased use of PFCs in colder climates. Objective The objective of this project was to recommend design, construction, maintenance, and rehabilitation guidelines that will maximize the advantages and minimize the disadvan- tages associated with the use of PFCs. In the context of this project, PFC was generally, but not exclusively, defined as a highly permeable mix containing polymer-modified asphalt binders or asphalt rubber and fibers, alone or in combination. Research Approach In order to accomplish the research objective, a total of five project tasks were required. C H A P T E R 1 Introduction and Research Approach

Task 1 – Conduct a Comprehensive Review of Worldwide Literature on PFC For this task, a critical, in-depth, annotated literature review was conducted. Results from this review were of utmost im- portance to the successful completion of this project and a significant amount of effort was placed on this task. The liter- ature review was conducted to specifically identify methods of designing (including material requirements), constructing, maintaining, and rehabilitating PFCs. Also, emphasis was placed on the safety benefits and performance aspects of con- structed PFC pavements along with the use of PFC in freeze- thaw environments. As the literature review was not limited to the United States, it included international literature. The deliverable of this study was a comprehensive guide- lines document that fully encompassed the life of a PFC, from mix design through rehabilitation. Additionally, the guideline document was to address project specific advantages and dis- advantages for placing PFC mixes. Therefore, since the project statement did not call for laboratory or field work, the literature review was of paramount importance. In addition to the literature review, the researchers also reviewed standards and practices on the design, construction, maintenance and rehabilitation of PFCs; performance criteria were particularly important. These documents were collected from around the world and summarized as part of the Task 1 State of Art. Task 2 – Survey Highway Agencies in the United States and Worldwide on PFC Subtask 1 – Conduct a Survey of Highway Agencies in the US and Worldwide A vast wealth of knowledge and experience on PFC exists in the United States and different parts of the world. As the objective of this task was to extract this information through a survey of specifying agencies, it was important that certain good practices be followed in developing and conducting the survey. The following outlines the proposed approach. The first step was to identify the objectives of the investiga- tion. For NCHRP Project 9-41, the objective of this survey was to obtain as much information as possible regarding methods of design, construction, maintenance, safety, rehabilitation, performance of PFC, and volume of use. The mode of data collection was a web-based submission form. The web-based survey was proposed because of the fol- lowing advantages: 1) there was no need for maintaining hard copies; 2) the survey could be accessed from any computer with an Internet connection; 3) the process of combining all responses for each question (and all other post processing) could be automated, and significant amounts of time could be saved; and 4) respondents from around the world would be able to access the survey. Subtask 2 – Interview Experts The research team also contacted numerous experts in the design, construction, maintenance, rehabilitation, and performance of PFCs. Experts from the U.S. and around the world were interviewed. The interviews were conducted through phone calls and electronically in order to directly discuss specific topics. Subtask 3 – Summarize Information Regarding Design and Construction Obtained from Survey and Interviews The objective of this subtask was to present the survey and interview information in a meaningful and practical manner. The end product of this subtask was a document with two parts: 1) responses to each specific question, from differ- ent respondents; and 2) responses summarized under design, construction, maintenance, rehabilitation, and performance. Subtask 4 – Identify Functional and Performance Criteria The objective of this subtask was to use the information from Subtask 3 to identify functional and performance crite- ria for PFC. The information was critically analyzed to provide answers to specific questions regarding good and poor per- formance. It was very important that any good performance, as well as bad performance, be related to a set of conditions (traffic and climate), materials, and activities (construction and maintenance). Task 3 – State of Practice for Permeable Friction Courses Task 3 involved providing a detailed summary of the re- search efforts conducted during Tasks 1 and 2 in the form of a state-of-art on the use of PFCs. Specific chapters within the state-of-art deal with mix design (including material properties), structural design, construction, maintenance, rehabilitation, performance, and advantages/disadvantages. The chapters include information obtained from the lit- erature review, evaluations of standards and practices, the survey of transportation agencies and interviews of experts. Where needed, our team developed draft standards and/or practices. Task 4 – Develop Guidelines on the Design, Construction, and Maintenance of PFC Based upon the results of Tasks 1 through 3, guidelines were developed on the design (including material requirements), 4

construction, maintenance, and rehabilitation. Two prac- tices were developed, in AASHTO format, for these topics. The first practice provides guidance on mix design and con- struction while the second practice deals with maintenance and rehabilitation. Task 5 – Prepare Final Report The final report was compiled according to guidelines established by NCHRP and to present a clear and concise summary of the findings and conclusions generated during Project 9-41. Report Organization The draft final report for NCHRP Project 9-41 is divided into three volumes. Volume I of this report includes the cur- rent state-of-art for PFCs. This volume provides a synthesis of the annotated literature review, survey results, and areas needing further research. Volume II of this report includes the guidelines for the use of PFC in the form of AASHTO practices. Appendices within Volume II are draft AASHTO standards for use of PFCs. The final volume of the draft final report presents the annotated literature review. Volumes I and II are published as NCHRP Report 640, and Volume III is published as NCHRP WOD 138. 5