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Thin asphalt overlays are extremely useful as a routine maintenance/pavement preser- vation tool. One of the appealing factors for thin overlays is that they generally are more economical than thicker dense-graded layers. The thin layers allow pavement managers to overlay more lane-miles with the same tonnage. Thin overlays are often shown to have lower life-cycle costs than do other types of pavement preservation treatments. The use of thin overlays has been standard practice for many years for some agencies. In some states, for example, placement of surface courses less than 1.5 in. (38 mm) thick has been performed routinely for more than 40 years, even on high-traffic-volume and heavy-truck routes, such as interstates. The degree to which thin asphalt overlays are successful depends in large part on the project selection and amount of distress in the existing pavement. Pavements that are failing or have failed cannot be successfully treated with a thin overlay alone; they must be repaired so that a stable foundation is provided before the thin overlay is placed. 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, rehabilita- tion, and preservation. This was accomplished by conducting a literature review of thin overlay use and through a survey distributed to all state transportation agencies; agencies in the District of Columbia, Puerto Rico, and selected Canadian provinces; and selected consultants and contractors. The survey had a 90% response rate from U.S. agencies (47 of 52 agencies) and received eight responses from private industry, for a total of 55 of 60 responses (92%). Agencies surveyed reported the service life of thin overlays is generally between 7 and 11 years. The reasons most often expressed for the variability are differences in traffic, weather, existing pavement condition (and level of distress) at the time of the overlay, and the use of different quality standards when thin overlays are placed on interstate projects versus second- ary and local roads. The variability in service life may also be the result of the wide variety of situations in which thin overlays are used. Twenty-two (40%) survey responses indicated that thin overlays are used on interstate projects; 38 (69%) responses indicated that the overlays are used on primary and secondary routes; and 29 (53%) reported thin overlays are used on low-traffic-volume routes. Agencies reported several practices that have been observed to improve performance. One practice reported by several survey respondents is selecting the right candidate. Sometimes thin overlays have been used in mill-and-fill operations to surface badly cracked pavements simply because funding was not available to add additional structure or perform the rehabili- tation needed. In such cases, the thin overlay is fulfilling a purpose, but it is not expected to have a substantial service life. To give thin overlays a chance for success, agencies reported that such overlays should be applied before structural failure begins. Indeed, much of the success of several agencies is the result of resurfacing with thin overlays early in the dete- rioration cycle, before extensive rehabilitation is needed. Others reported that it is important SUMMARY tHIN ASPHALt CONCREtE OVERLAYS
2 the existing roadway not be rutted or structurally unsound. In addition, the existing surface should not be rough because a thin overlay alone may not be able to correct the distress to the desired smoothness. The proper amount and application of tack coat are needed to create adequate bond with the existing pavement to prevent raveling and slipping, and provide for a long service life. When these practices are followed, thin overlays can be expected to perform well for many years.
