Institute for Transportation

About the research

The current practice for enhancing timber bridge durability is the installation of a waterproof wearing surface (bituminous) layer and is recommended for most climates. The goal is to keep the timber superstructure dry which, in conjunction with preservatives by pressure treatment, permits the bridge to have considerably more longevity of service. In practice, the asphalt wearing surface can indeed keep a large majority of the bridge deck surface dry by sheltering it from precipitation, but fails to drain surface water entirely from the bridge deck. As the water drains laterally towards roadway shoulders and longitudinally towards bridge abutments or piers, it often collects and accumulates into wood components resulting in pockets of wood moisture exceeding 20 percent. These perimeter regions of a bridge deck superstructure, which include railing and curb systems or reflective cracking, are prone to early deterioration when sufficient moisture is present. This accumulation of moisture along the bridge curb and scupper zones may compromise the multiple-bolt connections required with many of the crash-tested bridge railing systems. The over-reliance on preservative treatments for durability, while permitting many bridge components to absorb or accumulate moisture in-service, leads to premature wood decay activity and a shortened service life. More effective drainage details are needed to keep the timber superstructure components protected from moisture accumulation and should have the following attributes: robustness to resist maintenance vehicles (graders and snowplows); ruggedness to resist weather events (ice, hail, and winds), ventilation to permit air-drying should the member(s) become wet, and serviceability to easily replace individual members with maintenance. The development of these innovative moisture control strategies should be guided by the different timber bridge superstructures, crash-tested railings, and (end-grain exposure near) abutments. Cost-to-benefit ratios should be estimated to help justifications for the increased initial costs.