Thin and Ultra-Thin Whitetopping (2004)

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3BACKGROUND Portland cement concrete (PCC) overlays over existing hot- mix asphalt (HMA) pavements have been used as a rehabil- itation option for more than 80 years. Coined “whitetopping” by the industry, these overlays have been used on airports; Interstate, primary, and secondary highways; local roads and streets; and parking lots to improve the performance, dura- bility, and riding quality of deteriorated HMA surfaces. Modern whitetopping overlays are commonly classified by thickness and by bond with the HMA. Three distinct cat- egories are found in the literature (1,2): 1. Conventional whitetopping—a concrete overlay of 200 mm (8 in.) or more, designed and constructed without consideration of a bond between the concrete and underlying HMA. 2. TWT—an overlay of greater than 100 mm (4 in.) and less than 200 mm (8 in.) in thickness. In most but not all cases, this overlay is designed and constructed with an intentional bond to the HMA. 3. UTW—with a thickness equal to or less than 100 mm (4 in.), this overlay requires a bond to the underlying HMA to perform well. NCHRP Syntheses of Highway Practice 99 and 204, pub- lished in 1982 and 1994, respectively, document the histori- cal use of whitetopping (3,4). These documents identify most whitetopping projects being constructed as using “jointed plain concrete pavement.” Other types include “jointed rein- forced concrete pavement” and “continuously reinforced con- crete pavement.” In the current synthesis, consideration is given only to whitetopping overlays constructed as jointed plain concrete pavement. Fiber-reinforced concrete (FRC) is commonly used with almost all UTW and some TWT over- lays, and although considered a separate classification in previous NCHRP syntheses (3,4), in this document, it will instead be considered a characteristic of the overlay. Although whitetopping overlays have been used in the United States since 1918, there has been renewed interest in them over the last 10 years as a result of several successful high-profile projects (4–6). In addition, responding to demand caused by rapidly deteriorating highways with only modest increases in funding, the concrete paving industry has adopted whitetopping as a key marketing strategy. The survey conducted as part of this synthesis revealed that more than 50% of the states have constructed a TWT or UTW overlay within the last 5 years and more than 40% have built a project within the last year. In the 10-year period from 1992 to 2001, the American Concrete Pavement Association (ACPA) recorded 282 UTW projects in 35 states, totaling 765,000 m2 (916,000 yd2) (7). TWT overlays are used on highway and secondary roads carrying a wide range of traffic, from light to heavy (1). Gen- eral aviation airports have also used this class of whitetopping for runway, taxiway, and apron pavements (8). Because they are thinner, UTW overlays are best suited for more lightly loaded pavements. These pavements include some intersec- tions, ramps, and light aircraft aprons (9). Overall, it appears from the information gathered that the use of TWT and UTW overlays is on the increase. SYNTHESIS FOCUS AND OBJECTIVE Although conventional whitetopping closely resembles most PCC pavements with respect to design, construction, and main- tenance techniques, TWT and UTW possess some unique characteristics, most notably their thickness, joint spacing, and bond to the underlying HMA (10). The goal of this synthesis is to collect and report infor- mation about the use of TWT and UTW overlays within the highway community. Proper application of these overlays requires attention at all stages, including project selection, design, materials selection, construction, maintenance, and eventual rehabilitation or replacement. During the develop- ment of this synthesis, the literature was reviewed for topics related to each of these stages. A broad range of citations related to TWT and UTW overlays have been identified, from practical case studies to reports of theoretical modeling. In addition, this synthesis presents the results of a survey of the highway community in regard to their firsthand experience with these overlays. This survey includes responses from both the public and private sectors. The objective of this synthesis is to convey the informa- tion compiled in a manner that is easily accessible and under- standable by the highway practitioner. Although a particular emphasis is given toward providing qualitative guidelines on TWT and UTW overlays, numerous references are cited that CHAPTER ONE INTRODUCTION

4Supporting information follows, including the survey ques- tionnaire and a summary of responses (Appendix A) and a select number of case studies on TWT and UTW projects (Appendix B). The user is encouraged to explore these case studies, because many of the concepts included in this syn- thesis were used on those projects. The synthesis has been developed with two uses in mind. The first is to serve as a quick reference guide, each chapter having been developed in a hierarchical fashion, quickly allowing readers to identify their topics of interest. The sec- ond use is as a training aid, with a flow inherent in the mate- rial, allowing interested readers to gain a much better under- standing of this topic as they read the entire report. more fully document the analytical approaches used in under- standing the nature of this rehabilitation technique. SYNTHESIS ORGANIZATION This synthesis report is organized into seven chapters: this introductory chapter (chapter one); an overview chapter (chap- ter two); a chapter on project selection (chapter three); one on whitetopping design (chapter four); one on construction prac- tices (chapter five); a chapter on performance, repair, and reha- bilitation (chapter six); and a summary chapter with conclu- sions and suggestions for further research (chapter seven).