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3 CHAPTER ONE INTRODUCTION BACKGROUND SCOPE Polymer concrete (PC) overlays were first used in the 1950s In 2007, TRB issued a problem statement, NCHRP Topic as single layers of coal tar epoxy broomed onto the concrete 39-11, Performance of Thin Polymer Overlays for Bridge substrate and seeded with fine aggregate. These overlays Decks. The scope of the topic included the following statement: were not very impermeable, nor were they durable under traf- Thin polymer overlays, which consist of a polymer fic. In the 1960s, an oil-extended epoxy was used to improve binder; e.g., epoxies, polyesters, or methacrylates, and the performance. By the mid-1970s, polyester-styrene res- aggregates are constructed with a thickness of no more ins and methyl methacrylate monomer systems were being than 25 mm (1.0 in.). They have the advantages of (1) placed using the broom-and-seed method (1). Premixed PC adding very little dead load; (2) very fast cure times; (3) shallow depths that eliminate need for raising approach that was screeded in place began to be used. In many cases, slabs; (4) transition from overlaid lane to non-overlaid the thicker, more brittle layers delaminated because of ther- lane during construction; (5) low permeability; (6) long- mal incompatibility of the overlay and substrate. lasting wearing surface; and (7) frictional resistance. Many thin polymer overlays have been installed and it is critically important to summarize their performance Through increased attention to monomer and resin in one document. formulations and a better understanding of the causes of delamination and other distresses, the performance of thin polymer overlays (TPOs) has increased significantly. Con- Considerable performance history over the past 20 years is siderable efforts have gone into improving the resistance to now available and the synthesis study collected information on chemical and mechanical attack and into understanding the a variety of topics: (1) previous research, specifications, and requirements for surface preparation, mixing and placing procedures on TPOs; (2) performance based on field applica- the PC, and in curing (2). tions; (3) the primary factors that influence the performance, including traffic, chemical contamination, alkalisilica reac- Initially, the reviews of TPOs were not favorable. For tion, corrosion, concrete strength, air content, moisture, envi- example, in 1984 Furr (3) stated concerning sand-filled ronment including temperature and climate, use of tire chains epoxy TPOs, "these overlays generally have proved to and studs, methods of removing existing concrete, aggre- be a poor solution to the surfacing and waterproofing gate, surface preparation, material compatibility including problem." He went on to say that of 12 states, only one substrate, treatments, and patching; (4) current construction had found one epoxy that had performed well. However, guidelines related to surface preparation, mixing and place- since that time, the improvement in performance has been ment, consolidation, finishing, and curing; (5) repair proce- substantial, although some problems still exist. It is now dures; (6) factors that influence the performance of overlays, understood that flexible resins used in thin layers with including life-cycle cost, benefits and costs, bridge deck con- wear-resistant aggregates are essential in producing TPOs dition, service life extension, and performance; and (7) suc- that are thermally compatible with the concrete decks and cesses and failures of TPOs, including reasons for both. are long wearing (3). It was learned in this survey of states and provinces that most problems occur because of errors Information was gathered from state departments of in workmanship. transportation (DOTs) and Canadian provinces, a literature review, a survey of vendors, and selected interviews. The use of TPOs has increased significantly in recent years. Sprinkel (4) reports that before 1990, 139 TPOs had been placed. There was a threefold increase between MAJOR DEFINITIONS 1990 and 1999, with 416 additional overlays having been placed. Considerable experience and data now permit more The focus of this report is TPOs. There are several impor- informed conclusions to be drawn relating to best practices tant distinctions to be made. The overlays are noncementi- for constructing TPOs. tious, that is, there is no hydraulic cement such as portland