Institute for Transportation

An asphalt paving system protects the structural elements of timber bridge decks from tire wear; reduces the penetration of moisture to other superstructure members, such as beams, stringers, diaphragms, and their associated hardware; and provides a skid-resistant roadway surface.

This report, which was prepared in response to concerns expressed to the U.S. Department of Agriculture (USDA) Forest Service, Forest Products Laboratory, Wood In Transportation program, and the San Dimas and Missoula Technology Development Centers, discusses problems with recently constructed timber bridges that were paved with asphalt. Numerous publications and articles were reviewed; agency and industry professionals were consulted, and asphalt adhesion and paving membrane solubility were tested. Information was collected at treated timber bridges in Alaska, Montana, Oregon, Washington, Michigan, and Wisconsin. Many of the bridges were performing very well, others exhibited one or more problems.

Ensuring long-term pavement performance and minimizing environmental problems for bridges with treated timber decks is the goal of this project. Some effects of waterborne preservatives are covered, but the focus is primarily on timber treated with oilborne preservatives.

Asphalt paving failures on the decks of treated timber bridges are caused by one or more of the following deficiencies:

*Bridge deck design and construction
*Type and quantity of the wood’s preservative treatment
*Design and installation of the asphalt paving system.
*Deck deflection and movement (the primary causes of pavement cracking)

Preservative treatment and asphalt paving system problems are often related. The treatment’s interaction with asphalt cement (asphalt) is the main cause of pavement delamination and asphalt bleeding and leakage. Improper treatment practices compound improper paving system design, and vice versa.