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The purpose of this synthesis is to summarize available information and to document how departments of transportation and other agencies and owners are currently using thin white- topping (TWT) and ultra-thin whitetopping (UTW) overlays among their various pavement rehabilitation alternatives. Although TWT and UTW overlays have been constructed for decades, their recent popularity is largely the result of a renewed demand for longer-lasting but cost-effective solutions for hot-mix asphalt (HMA) pavement rehabilitation. A whitetopping overlay is constructed when a new portland cement concrete layer is placed on top of an existing HMA pavement system. The concrete thickness for a UTW is equal to or less than 100 mm (4 in.). A TWT is greater than 100 mm (4 in.) but less than 200 mm (8 in.). Conventional whitetopping is an overlay of 200 mm (8 in.) or more. In most cases, a bond between the new concrete and existing HMA layers is not only assumed during design, but specific measures are taken to ensure such a bond during construction. The success of this bond, leading to composite action, has been found to be critical to the successful performance of this pavement-resurfacing alternative. The goal of this synthesis is to collect and report information about the use of TWT and UTW overlays within the highway community. Proper application of whitetopping overlays requires attention at all stages, including project selection, design, materials selection, con- struction, maintenance, and eventual rehabilitation or replacement. This synthesis explores the literature for topics related to each of these stages. A broad range of citations related to white- topping has been gathered, from practical case studies to reports of theoretical modeling. In addition, the results of a survey of the highway community were used to provide firsthand insight of their experience with these overlays. This synthesis provides the practitioner with a comprehensive source for the state of the practice as well as the state of the art in TWT and UWT overlay application. An approach to the proper application of this rehabilitation alternative is also presented, along with accessi- ble citations of third-party information. SUMMARY THIN AND ULTRA-THIN WHITETOPPING
