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LONG-TERM PERFORMANCE OF POLYMER CONCRETE FOR BRIDGE DECKS SUMMARY Thin polymer overlays (TPOs) consist of a polymer binder and aggregates with a thick- ness not exceeding 25 mm (1 in.). They have provided long-lasting wearing surfaces with many advantages, including very little dead load, very fast cure times, shallow depths that eliminate the need for raising approach slabs, ability to transition from overlaid lane to non-overlaid lane during construction, low permeability, and good frictional resistance. TPOs were first used as single layers of coal tar epoxy broomed onto the concrete deck with fine aggregate broadcast onto the surface in the 1950s. In the 1960s, an oil-extended epoxy was used to improve the performance. Polyester-styrene resins and methyl methac- rylate monomer systems were being installed in the mid-1970s using the broom-and-seed method. Premixed polymer concrete (PC) began to be used as overlays using screeds for placing and finishing. Some of the thicker, brittle layers delaminated owing to thermal incompatibility. As the interest in TPOs increased in the 1980s, material suppliers began to develop resins specifically for these applications. Through a better understanding of the causes of delamination, construction techniques and materials improved, with a corresponding increase in TPO performance. One of the major causes of delamination, thermal incompat- ibility of polymers and concrete, was reduced significantly by the use of higher elongation and lower modulus resins. The Transportation Research Board issued a problem statement, NCHRP Synthesis 25/ Topic 39-11, Performance of Thin Polymer Overlays for Bridge Decks. The scope included collecting information on a variety of topics, including previous research and construction methods, performance based on field applications, primary factors that influence the per- formance, current construction guidelines, repair procedures, factors that influence per- formance of overlays, and successes and failures of TPOs. Information was to be gathered from state departments of transportation and Canadian provinces, a literature review, a survey of vendors, and selected interviews. The literature contains many references to overlays, particularly in the late 1980s, 1990s, and early 2000s. It provides useful information on use of TPOs, pre-overlay evalu- ation, materials, installation, test methods, TPO field sections, failures, service life, cost, warranties, and special applications. Construction procedures, candidates for TPOs, and construction issues (substrate tests, repair, surface preparation, material handling and mix- ing, placement, finishing and curing, and quality control tests) were identified from the literature. The survey responses, from 40 states and seven provinces, revealed that at least 2,400 TPOs have been constructed in the United States and Canada, a fourfold increase over the number installed through 1999. Seven states and three provinces that responded have not used TPOs. Nearly all states use epoxy resins, and only California indicated that it uses pre- dominantly polyester-styrene in premixed overlays. Causes of failures were given. Several states provided specifications for TPOs.
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2 Contractors interviewed were well experienced with TPO construction; they work on bridges in varying conditions of soundness. They discussed repair procedures, installation procedures, and problems encountered and provided recommendations for construction. The material suppliers had considerable experience with TPOs and provided their opinions on problems encountered and recommendations for improvement of materials applications. From the literature and surveys, the factors that influence performance were identified as the (1) soundness of the substrate, (2) surface preparation, (3) compatibility of the overlay and substrate, (4) aggregates, (5) overlay thickness, (6) bridge girder flexibility, (7) environ- ment, and (8) constructability and workmanship. Little information was found on overlay maintenance; some agencies replace delaminated TPOs with similar materials or even with hydraulic cement concrete. The chapter on proven practices covers best candidates for TPOs, overlay types, materials, qualification of substrate, installation methods, construction test methods, special applications, and warranties. TPOs have become an accepted construction method for deck preservation, restoring sur- face friction and extending the lives of decks. The three overlay types, multilayer, slurry, and premixed, are used widely throughout the United States and Canada. These overlays give good performance when placed on decks that are in good condition and when constructed in accordance with established principles such as those specified in Guide Specification for Polymer Concrete Bridge Deck Overlays. When constructed properly on sound decks, TPOs could provide a service life of 20 or 25 years.