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This summary offers an overview of on a research study that assessed the magnitude of premature asphalt deterioration on timber bridges; identified the primary mechanisms responsible for wear surface deterioration; and suggested methods for improving asphalt pavement performance on timber bridges. The study revealed that approximately 50 percent of counties experience some problems with premature reduced serviceability of the asphalt pavement wear surfaces that cover their timber bridges. The summary looks at possible pavement failure mechanisms and presents the following proposed solutions for controlling timber bridge asphalt pavement cracking: asphalt pavement saw & seal, asphalt pavement fabric or material underlay, removal of extruded oil-type preservative before surfacing, conditioning of bridge timbers to the expected equilibrium moisture content before bridge installation, and tightening of timber decks through maintenance practices.

Timber in nail-laminated and stress-laminated bridges is often installed with moisture contents (MC) near the fiber saturation point. Post-installation moisture loss induces shrinkage in the timber components, which results in loosening of component fasteners. This research project sought to establish the equilibrium moisture content (EMC) of timber bridges in Minnesota. Researchers took seasonal MC measurements on six nail-laminated timber bridges to determine annual MC variations and moisture gradients in individual bridge components: three bridges from northern Minnesota in St. Louis County and three from southern Minnesota in Sibley County. An electrical resistance meter measured moisture content, with oven-dry and toluene distillation methods of MC determination as controls. The study found the average MC of bridge components in St. Louis County was 2 percent-11 percent higher than bridge components in Sibley. The study determined the average MC at a three-inch depth on three of the major bridge components as: • deck laminations 18 percent (Sibley) to 28 percent (St. Louis) • transverse stiffener beams 14 percent (Sibley) to 18 percent (St. Louis) • deck supports 17 percent (Sibley) to 27 percent (SI. Louis) . The results indicate that the regional microclimate may greatly affect MC. Results from this research will allow MC specifications to be determined before bridge installation, helping minimize post-installation moisture-related problems and optimize design calculations. In addition, results will provide necessary data for ongoing research on transverse load-sharing characteristics of longitudinally nail-laminated timber bridges. Finally, this information will provide a basis for inspecting MC in timber bridges.

Asphalt wear surfaces cover 1,378 of Minnesota's timber bridges. This study assessed the magnitude of premature asphalt deterioration on timber bridges; identified the primary mechanisms responsible for wear surface deterioration; and suggested methods for improving asphalt pavement performance on timber bridges. Research methods included surveys, meetings with several county engineers and tours of their timber bridges, interviews with both asphalt and timber bridge industry professionals, and literature reviews. The study revealed that approximately 50 percent of counties experience some problems with premature reduced serviceability of the asphalt pavement wear surfaces that cover their timber bridges. Possible pavement failure mechanisms include low-temperature cracking, reflective cracking from deck fault lines found at deck panel joint lines and deck lamination separations, asphalt fatigue fracturing, and asphalt de-bonding due to oil preservatives interference. The report presents the following proposed solutions for controlling timber bridge asphalt pavement cracking: asphalt pavement saw & seal, asphalt pavement fabric or material underlay, removal of extruded oil-type preservative before surfacing, conditioning of bridge timbers to the expected equilibrium moisture content before bridge installation, and tightening of timber decks through maintenance practices.