Thin Asphalt Concrete Overlays (2014)

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3 PURPOSE AND SCOPE OF SYNTHESIS Thin asphalt concrete overlays have become so common for pavement preservation and rehabilitation that their use has become standard practice by many agencies. These thin over- lays are accepted practice owing to their numerous advantages (Newcomb 2009). The overlays • Provide long service life when placed on structurally sound pavements, • Provide a good riding surface, • Reduce noise at the tire-pavement interface when fine- graded mixtures are used, • Maintain grade and slope geometry with little environ- mental impact, • Are recyclable, and • Are easily maintained. Thin overlays are well established in some states, but other states are just beginning to develop practices and procedures for their use. Little consensus exists on exactly what consti- tutes a thin asphalt concrete overlay. What some agencies may specify as a minimum layer thickness, other agencies refer to as a maximum layer thickness for surface courses. Like- wise, there is disagreement as to when thin overlays should be employed, the materials and mix design to be used, and effective construction practices. With the advent of innovative pavement technologies and greater use of recycled, reclaimed, and alternative materials, it is important to document current experience and practices. The objective of this synthesis is to review the current state of the practice and research efforts on the use of thin asphalt concrete overlays for pavement maintenance, rehabilitation, and preservation. This synthesis includes a literature review, including both U.S. and international technologies, such as the ultrathin bonded wearing course (UTBWC) process devel- oped in France, as well as a survey of state departments of transportation (DOTs) and selected agencies to determine the current use of thin asphalt concrete overlays. In addi- tion, information has been gathered from selected individuals and private industry representatives who have experience with thin asphalt concrete overlay. The information gathered includes the following: • Typical agency definitions, thicknesses, design service life, and selection criteria; • Effect of pavement condition and preparation on performance; • Current mixtures and mix designs used by agencies; • Uses as a function of traffic levels and speed; • Specifications for materials and construction; • Construction techniques; • Quality control/quality assurance procedures; • Performance; • Agency typical annual lane miles paved and cost information; • Innovative technologies and reclaimed and recycled (resource responsible) material use; and • Ongoing research and needs. SURVEY RESPONSE A survey was conducted to determine how thin asphalt over- lays were used as a pavement preservation tool and for gen- eral highway system upgrades. The web-based survey was distributed to all 50 states, the District of Columbia, Puerto Rico, several Canadian provinces, and selected consultants and contractors. Forty-seven of 52 U.S. jurisdictions (90%) responded to the survey, as well as eight companies from private industry, for a total of 55 of 60 (92%). DEFINITION Little consensus exists regarding the point at which the thick- ness of a pavement overlay classifies it as a thin lift overlay because most agencies do not define mixes as “thin” or “non- thin.” For example, in Alaska the minimum thickness for an asphalt overlay is 2 in. (50 mm), and four other responses to the synthesis survey indicated thicknesses to 2 in. (50 mm) would be considered a thin overlay. However, in many states a thin overlay is considered to be no more than 1 in. (25 mm). Twenty-four (38%) of the survey responses (Figure 1) stated that a thin overlay was considered to be no more than 1 in. (25 mm) in thickness, with six agencies (11% of the responses) indicating a thin overlay was considered to be less than 0.75 in. (19 mm) thick. Some agencies consider overlays of 1.0 to 1.5 in. (25–38 mm) to be “normal” instead of thin, and practi- cally all surface courses for the agency are within that thick- ness range. A few agencies responded to multiple categories because the thickness allowed depends on the mix type being used for surfacing. For example, an agency may use an UTBWC at less than 0.75 in. (19 mm) or a 4.75-mm nominal chapter one INTRODUCTION

4 maximum aggregate size (NMAS) mix of less than 1 in. (25 mm) thick. For purposes of this synthesis, thin overlays are defined as surface courses typically placed no more than 1.5 in. (38 mm) thick. Based on the survey, 86% (55 of 64) of the responses will fit this definition; however, some agencies responded to more than one category. BACKGROUND “As road and highway agencies struggle to do more work with less money, thin overlays are increasingly answering the need” (Anderson 2011). Many who are responsible for pavement management at the state and local levels have renewed interest in the successful use of thin asphalt overlays to preserve pave- ment structure and as a form of pavement maintenance. As a result, numerous projects and test sections have been used to experiment with assorted variations in materials and con- struction methods to find economical solutions for pavement preservation. Developments in technologies, such as warm mix asphalt (WMA) and use of spray-paver equipment, have also intro- duced alternative construction methods and materials for eval- uation. Although many reports have been written on these trial projects, there is no single report that contains the results of these test projects. Therefore, this synthesis provides valuable information for those who currently use thin asphalt overlays and those who are exploring for the first time the possibility of using thin overlays to meet their pavement needs. The use of thin asphalt overlays is a relatively new con- cept in some areas but a well-established practice in others. For that reason, typical experiences in the United States are provided and experiences of agencies in other nations are considered. PREVIOUS RESEARCH OR RELATED SYNTHESES The National Asphalt Pavement Association (Newcomb 2009) Information Series 135 is the most recent summary of thin asphalt overlay construction found in the literature search. The report discusses how the character of pavement con- struction has changed over the years from that of building new roads to maintaining and preserving existing infrastruc- ture. Thus, there is much interest in using thin overlays as a method of extending the available funds for maintenance and preservation so that a greater number of lane-miles can be resurfaced annually. During this same transitional period, new technologies and improved materials have helped extend the service life of asphalt pavements. A 2012 survey on pavement preservation treatments in cold regions (Zubeck et al. 2012) found that in conditions of heavy studded-tire usage, crack-sealing, patching, and thin overlays are the most commonly used treatments. Those treatments are used almost exclusively in moist climate conditions. The average service life of thin overlays and UTBWC mixtures in that environment is 6 years or more. A synthesis performed for the Montana DOT by Cuelho et al. (2006) summarized survey responses for expected ser- vice life and cost per lane-mile for several types of preventive maintenance treatments. Table 1 shows that thin overlays are comparable in cost to chip seals and microsurfacing when the additional service life is considered. NCHRP Synthesis 260 (Geoffroy 1998) specifically focused on surface treatments and a single layer of hot mix asphalt (HMA) less than 50 mm (2 in.) over an unbound base. The predominant topics examined in the report were pavement type selection and structural design. The questionnaire was distributed to federal, state, and local agencies. More than half of the 286 respondents used thin-surfaced pavements, and more than half of the user agencies were counties. These FIGURE 1 Responses to definition of thin overlay. (Source: Survey responses.)

5 treatment that most effectively addressed the deficiencies of the existing pavement. Therefore, the benefits of each treat- ment type are used for selecting the treatment. Those ben- efits generally relate to total cost, typical service life of the treatment, and life-cycle cost. Other factors, such as pave- ment condition, functional classification, and type of pave- ment being overlaid, also were used to evaluate the different treatment options. In some cases, local policies and mandates were important factors. ORGANIZATION OF SYNTHESIS A discussion of the various mix types used for thin overlays and their selection criteria is included in chapter two. Chapter three provides information on the design and construction of thin overlays. Chapter four provides findings on how agencies assess performance of thin overlays and how those surfaces are maintained and rehabilitated. Case examples of a few agencies that provided supplemental information are included in chapter five, and conclusions are provided in chapter six. The survey questionnaire is provided in Appendix A, and responding agencies and private industry participants are listed in Appendix B. Appendix C provides questionnaire responses, and Appendix D gives an example of a decision tree matrix provided by the Ohio DOT for the agency’s general system flexible pavements. (The decision tree matrix has been refor- matted to fit the constraints of page size.) local agencies have limited technical resources and rely on their expertise for pavement type selection and design. The primary factors in the selection and design are traffic vol- ume, funds available, route classification, and truck volume. However, the study focused on new construction and did not evaluate the use of asphalt mixtures in a thin overlay. NCHRP Synthesis 222 (Zimmerman and ERES Consultants 1995) presented descriptions of the various methodologies used to determine project selection and pavement preservation treatment recommendations. Some of the processes used were found to be highly automated to remove as much subjectiv- ity as possible. The synthesis survey found three predominant strategies for project and treatment selection: • Pavement Condition Analysis—was based on ratings of current pavement condition; • Priority Assessment Models—used prediction models to forecast pavement condition, thereby making the method useful for “what if” scenarios with alternative treatments; and • Network Optimization Models—used prediction mod- els to evaluate the entire network and establish fund- ing needs so agencies could set priorities and decide on treatment selection. Responses to NCHRP Synthesis 222 indicated that the most important basis for treatment selection was finding the TABLE 1 SUMMARY OF EXPECTED LIVES AND COSTS FOR PREVENTIVE MAINTENANCE TREATMENTS Preventive Maintenance Treatment Average Service Life (Years) Cost per Lane-Mile (12 ft wide) Thin overlay 8.4 $14,600 Double chip seal 7.3 $12,600 Microsurfacing 7.4 $12,600 Slurry seal 4.8 $6,600 Source: Cuelho et al. (2006).