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30 Texas Repair Materials Multiple-Layer Polymer Concrete Overlay, ftp://ftp.dot. Repair materials included polymer concrete, latex modified state.tx.us/publ/txdot-info/cmd/cserve/specs/2004/spec/ concrete, magnesium phosphate, proprietary fast-setting ss4429.pdf; ftp://ftp.dot.state.tx.us/pub/txdot-info/cmd/ concretes, and high alumina cement concrete. One indicated cserve/specs/1993/spec/es7675.pdf; ftp://ftp.dot.state.tx.us/ that the California Department of Transportation specifica- pub/txdot-info/cmd/cserve/specs/.../spec/ss4398.pdf. tion requires a 72-hour cure for magnesium phosphate and 1 hour for high alumina cement. In another case, 3 days were Utah required for fast-setting materials to cure before overlaying the repair. www.dot.state.uut.us/main/uconower.gf ?n=504434814 209249743. Installation Virginia Shot blasting was the cleaning method used on flat work; in some cases, grit blasting was used on curbs and gutters. http://www.aot.state.vt.us /conadmin /Document/Sec- It was indicated that if the shot clogs in the shot blaster, the tion%20900%20Items/THINPOLYMEROVERLAY.pdf. deck is too damp and provides a good control. They repair just before installing the overlay. All of them installed epoxy Contact the California, New York, and Virginia DOTs for multiple-layer TPOs; one also installed MMA slurry and specifications through their websites. premix, and the other installed deicing overlays. One con- tractor permits traffic on the first layer after it cures and then shot blasts the first layer the next day and immediately places MATERIALS the second layer. He uses 550 shot for the first application and 460 for the second. One performed mixing as specified, Resin Systems including a completely automated system using a mobile batching machine and modified asphalt paver. Another used Epoxies are preferred by states, with some states using concrete mixers, wheelbarrows, and screeds on rails. epoxy-urethanes; specifications requiring a minimum tensile elongation of 30%. One company, however, sup- Problems Encountered plies the majority of the polyester premix used in North America. One state specifies 20% tensile elongation, and Contractors encountered the following primary problems: one state specifies brand names of low-modulus epoxies. California uses polyesters and requires a minimum elonga- Weather: rain or high or low temperatures; tion of 35%. Cure times related to traffic openings: resin cured too slowly to open as required by contract; Quantities of materials: plans always indicate less than CONTRACTOR INTERVIEWS required; Day or night temperatures affect the work time and Nine contractors having considerable experience with volume of materials that can be placed; TPOs were interviewed by telephone. The contractor ques- Deck condition: decks are often in badly deteriorated tionnaires are in Appendix A. These nine contractors had condition; some DOTs assume that a polyester pre- installed hundreds of TPOs in many states. mixed TPO can restore a deteriorated deck with 18- to 150-mm (0.75- to 6-in.) premixed polyester TPO as Condition of Bridges needed to restore ride quality; the inevitable deteriora- tion of the TPO is usually blamed on the contractor; Most of the bridges on which they had installed TPOs Moisture in deck: excessive moisture reduces bond ranged from sound to seriously deteriorated, but generally strength. they were moderately distressed. Most bridges had been previously repaired, and at least four had repairs of the sub- Recommendations strate performed as part of the TPO installation contract. One contractor said that 80% of his jobs have been on older Contractor recommendations include that-- bridges that had been repaired, and virtually all contracts involved some repair work before the TPO was installed. Bidders be prequalified for TPO experience; Another contractor said that 90% of the bridges he works on Resin manufacturer's representative always be present, are deteriorated. especially if warranty is required;