Guide for Pavement Friction (2009)

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xi • Mineralogical and petrographic properties. ¾ Aggregate composition/structure and mineral hardness. • Physical and geometrical properties. ¾ Angularity, shape, and texture. • Mechanical properties. ¾ Abrasion/wear resistance. ¾ Polish characteristics. • Durability properties. ¾ Soundness. Several test methods are available for characterizing aggregate frictional properties. The extent of aggregate testing and characterization required as part of the friction design process will vary from agency to agency, based on the types of aggregates available, the variability of aggregate properties, the quality and historical performance of available aggregates, and the anticipated applications (e.g., mix types, roadway functional class). Since laboratory material testing does not guarantee friction performance in the field, it is essential that testing be used in conjunction with field performance history to identify acceptable aggregate types. Macro-texture is defined by the type of surface paving mixtures and/or surface texturing techniques applied. Several different surface mix types and finishing/texturing techniques are available for use in constructing new pavements and overlays, or for restoring friction on existing pavements. The more commonly used mix types and texturing techniques are presented in this report along with the typical macro-texture levels achieved. Pavement– tire considerations, such as noise, splash/spray, and hydroplaning, and general considerations, such as constructability, cost, and structural performance, are not directly discussed in this report, however, they must be an integral part of any policies developed for the application of these mixes and texturing techniques.

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1 CHAPTER 1. INTRODUCTION BACKGROUND Although the mission statements and goals of state departments of transportation (DOTs) vary from agency to agency, the terms “safe” and “safety” are prominently included in most pronouncements about highways. While the provision of highway facilities for mobility and economic purposes is usually the main priority of a highway agency, the safe operation of all vehicles using those facilities is often an equal or complementary priority. As there are many facets and features in a highway system, there are many areas in which improvements to highway safety can take place. In one such area—pavement–tire friction (or, simply, pavement friction)—the campaign for improvements has been waged for decades, with many studies taking place at the state, national, and international levels. The results of these studies have been used to develop new or updated policies and standards for measuring and controlling friction. Various forms of friction guidance have also been developed and made available over the years by transportation agencies and organizations. Most notable among these are: • Federal Highway Administration (FHWA)—Technical Advisories on skid crash reduction, pavement friction courses, and pavement texturing. • National Cooperative Highway Research Program (NCHRP)—Syntheses and Reports covering, among other things, safety, friction testing, and surface drainage. • American Association of State Highway and Transportation Officials (AASHTO)— Guides, Manuals, and Guide Specifications for highway geometric design, construction, maintenance, and pavement management. • Pavement industry groups—Bulletins and Manuals on surface mixture selection and texturing. • International Agencies (e.g., United Kingdom, Australia, Japan)—Various guides, manuals, and reports on friction testing, design, and safety management. Comprehensive guidance covering both the policy and technical aspects of designing for and managing pavement friction has been limited to Guidelines for Skid-Resistant Pavement Design, published by AASHTO in 1976. This document recommended pavement specifications that would yield the desired frictional properties upon completion of construction and that would maintain adequate long-term friction. It also discussed the importance of aggregate selection and mixture design for both asphalt- and concrete- surfaced pavements, and the role of micro-texture and macro-texture in pavement surface friction. DESCRIPTION OF THE PROBLEM Many changes have taken place since the development the 1976 AASHTO Guide. In addition to the continuous growth in the amount and type of highway traffic and the

2 increased focus on the needs of the highway user (i.e., safer and more comfortable roads), substantial technological changes have occurred in the following areas: • Pavement materials and mixture design properties. • Friction-testing methods and equipment. • Construction procedures and standards. • Vehicle and tire characteristics. • Highway geometric design practices and standards. Although much information and guidance related to pavement friction is available, it is quite fragmented and has not been integrated into a comprehensive administrative policy and design tool for addressing friction issues. Therefore, a new Guide for Pavement Friction is needed to assist highway engineers in (a) understanding the complex subject of pavement friction and its importance to highway safety and (b) instituting pavement management and design practices and processes that optimize friction safety, while recognizing and considering the effects on economics and other pavement–tire interaction issues (e.g., noise, splash/spray, visibility/glare). PROJECT OBJECTIVES AND SCOPE The primary objective of this research project was to develop a new Guide for Pavement Friction, for consideration and adoption by AASHTO and for subsequent use by highway engineers involved in designing, constructing, and managing pavement surfaces. The new Guide would address both asphalt (i.e., flexible and semi-rigid) and concrete (i.e., rigid) pavements and would serve as a supplement to existing structural and/or mix design practices. The new Guide would not address winter maintenance issues (i.e., snow and ice removal/treatment). The scope of this research project consisted of the following tasks and subtasks: Phase I • Task 1—Collect and Review Information Related to Pavement Friction. ¾ Subtask 1a—Perform Literature Search and Review. ¾ Subtask 1b—Conduct State and Industry Surveys and Interviews. • Task 2—Prepare Updated Detailed Work Plan for Developing the Guide for Pavement Friction. • Task 3—Prepare Interim Report. Phase II • Task 4—Execute Approved Work Plan ¾ Subtask 4a—Refine and Further Develop Basics of Pavement Friction and Texture. ¾ Subtask 4b—Develop Recommendations for Pavement Friction Management. ¾ Subtask 4c—Develop Recommendations for Asphalt Pavement Friction Design, Construction, and Restoration. ¾ Subtask 4d—Develop Recommendations for Concrete Pavement Friction Design, Construction, and Restoration.

3 ¾ Subtask 4e—Develop Methods for Evaluating Impacts of Other Factors on Pavement Friction Design. ¾ Subtask 4f—Prepare Draft Guide. ¾ Subtask 4g—Prepare Revised Guide. ¾ Subtask 4h—Develop Training Materials. • Task 5—Prepare Final Report. The key deliverables for this study—the Guide and this final project report—were developed as individual, stand-alone documents. OVERVIEW OF REPORT This report is presented in seven chapters. Chapter 1 is this introduction. Chapter 2 discusses the effort of gathering information through an immense literature search, state surveys, and state and industry interviews. Chapter 3 describes the nature of the relationship between pavement friction and safety. Chapter 4 provides important background information on pavement friction and texture, including their definitions, the factors that affect them, and how each parameter is measured and reported. Chapters 5 and 6 focus on the two most important aspects of the new Guide for Pavement Friction—friction management and friction design, respectively. Descriptions of the processes, activities, and technical matters associated with each function are provided in these chapters, along with the information, data, and ideas used to support the guidelines featured in the Guide. The final chapter summarizes the key findings of this research study and presents the study recommendations. Five appendices are also included in this report. Appendix A is a bibliography that lists all of the documents obtained and reviewed in Task 1. Appendix B contains the six-page questionnaire survey administered to state highway agencies (SHAs). Appendix C presents a summary of the state survey responses. Appendix D provides a summary of the results of interviews with selected states and industry organizations. And, appendix E is a primer on the fundamental concepts of pavement friction.

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