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Asphalt Paving of Treated Timber Bridge Decks
Freshly placed asphalt pavement is not impervious to moisture, but if the roadway surface is crowned or sloped a minimum of 1 percent, very little moisture will penetrate through the pavement to the deck. Standing water damages pavement by interacting with the asphalt cement. The water tends to strip the asphalt from the aggregate particles, weakens the pavement, causes crumbling, and compromises the bond between the asphalt and the deck. Such pavement becomes increasingly susceptible to destruction by weather and the pounding of traffic loads. Water and deicing salts can also damage timber bridges, particularly if the beams that carry the bridge load are steel or if the bridge deck is stressed with high-strength steel rods.
Bridge decks should be crowned, super elevated, or constructed on a grade-preferably 2 percent, but at least 1 percent. Removing surface water before it can percolate through the asphalt surfacing is a simple, effective alternative to using waterproofing paving membranes.
When bridges with glued-laminated deck panels are paved, the deck panel deflection should be limited to 0.05 inch or the deck panels should be mechanically interconnected. To minimize shrinkage, deck panels should be treated with an oilborne preservative and be protected from the weather before installation. If deck panels must be treated with waterborne treatments, the panels should be redried to a maximum 19-percent moisture before being shipped to the jobsite. When stored at the jobsite, the deck panels should be protected from moisture.
Treated wood should meet the requirements specified in Best Management Practices for the Use of Treated Wood in Aquatic and Wetland Environments (Western Wood Preservers Institute 2011) for treatment, posttreatment procedures, and visual inspection before installation. Complying with these BMPs will minimize preservative residue on the timber surfaces as well as future chemical and solvent leaching, reducing environmental risks, and improving the performance of asphalt pavement on treated timber bridge decks.
Design also affects the performance of asphalt pavement. The use of PG-rated asphalt is recommended for sites that are subject to heavy loads, high traffic volumes, and a harsh environment. The Superpave mix design method and PG-rated asphalt are important results of the Strategic Highway Research program and Long-Term Pavement Performance program. State departments of transportation are a good source of information about these asphalts and pavement designs.
Timber decks treated with oilborne preservatives are very resistant to moisture penetration and damage. The deck acts as a water-resistant cover over the beams and hardware. Constructing the bridge on a minimum road grade, crown, or superelevation of 1 percent also will help keep the bridge deck dry. However, if road salts are present, a waterproof paving system may more fully protect critical steel components such, as stressing bars, beams, and connecting hardware.
Paving membranes are designed to leave a continuous layer of flexible asphalt as a barrier to prevent water from penetrating the structure. However, treatment chemicals and oil solvents dissolve asphalt. Paving membranes should be used only on treated timber bridge decks that are free of preservative residue and are expected to remain that way.
When a paving membrane is to be placed on a timber deck treated with creosote or heavy oil solvent treatment, the wood must be treated in compliance with the BMPs by the Western Wood Preservers' Institute. These practices will ensure that the wood was properly prepared before treatment, was subjected to appropriate procedures after treatment, and was properly inspected at the job site. The amount of preservative treatment chemical retained in the wood should be less than 150 percent of the AWPA-specified minimum retention. The pavement system also will perform best if the bridge deck has been allowed to cure before paving to ensure that treatment residue evaporates. The length of time varies, depending on many factors, including climate.
The same requirements should be used for timber decks treated with light oil solvents. Proper curing may be more critical with these treatments. If the light oil solvent has not evaporated from the wood before a paving membrane is placed, the solvent vapors will be trapped. Eventually, they will attack the paving membrane and asphalt in the pavement. The time required to cure timber decks that have been treated with light oil solvents requires further research.
Meet all specifications regarding mix temperature and thickness. Ensure that air temperatures and weather conditions are within recommended limits. If a tack coat is applied, use it sparingly. Minimizing free asphalt is essential for long-term pavement performance.
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