Relationship between timber bridge characteristics and asphalt pavement wear surface performance

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 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.