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40 CHAPTER SIX CONCLUSIONS Thin polymer overlays (TPOs) have become a widely used construction method in North America for protect- ing bridge surfaces, restoring skid resistance, and a bridge preservation treatment for extending the lives of decks. The use of TPOs tripled from 1990 to 1999 and more than quadrupled from 1999 to 2008. More than half of the states and several of the Canadian provinces have used TPOs. AASHTO, the American Concrete Institute, and a number of states have developed specifications for the construction of TPOs. Three types of overlays are widely used: multiple layer that consists of two or more layers of resin and aggregate; slurry that consists of a single layer of primer, resin, and aggregate followed by a seal coat; and premixed that uses resin and aggregate mixed in continuous batching machines or concrete mixers and placed on decks using screeds. The most common resins and monomers are epoxies, polyester- styrenes, methacrylates, and epoxy-urethanes. Most agen- cies use contractors to install TPOs, but some use their own forces. TPOs give the best performance when applied to decks that are in good condition. TPOs must be constructed in accordance with established principles to provide good per- formance. The most important factors for the success of TPOs areâ ⢠Sound, dry substrate that requires quality repair procedures; ⢠Adequate preparation to provide a clean, textured dry surface; ⢠Environment including dry and warm weather; ⢠Experienced applicator and good workmanship to ensure proper application of materials; ⢠Involvement of resin supplier or manufacturer to assist contractor in proper handling, mixing, and application of resins; and ⢠Thermal compatibility that requires low-modulus res- ins and compatibility of resins and repair materials. TPOs have an important role in bridge maintenance and protection. They are particularly appropriate in high-traffic areas in which lane closures must be minimized and for structures that cannot accommodate significant increases in dead load. When constructed properly, TPOs can provide a service life of 20 to 25 years. RESEARCH NEEDS Research is needed to improve the performance and to extend the life of TPOs. Suggested areas of research include ⢠Development of resins, including those from recycled plastics, that will perform under extreme conditions, such as very low or very high temperatures, heavy traf- fic volumes, or heavy loads; ⢠Development of polymer concretes that will self-heal in the event of cracks and delamination; ⢠Development and validation of tests that will more accurately predict long-term performance of polymer concrete systems when used in a wide range of tem- peratures, heavy loads, and heavy traffic volumes; ⢠Improved analytical techniques to select material prop- erties and thicknesses of TPOs to perform satisfacto- rily in the intended conditions; and ⢠Development of nondestructive test procedures that will accurately determine whether the condition of bridge decks is suitable for TPO application.
