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Asphalt Paving of Treated Timber Bridge Decks
Oilborne preservatives commonly used in bridge construction include creosote, pentachlorophenol (penta), and copper naphthenate (American Wood Preservers' Association 1997). Creosote is a naturally occurring coal tar product. Penta and copper naphthenate are pesticide chemicals that are dissolved in a type A (heavy oil) or a type C (light oil) solvent. The heavy oil solvent is diesel oil. Light oil solvent is as viscous as mineral spirits. The oil carrier, particularly a type A heavy oil, protects the wood from rapid moisture change and minimizes wood shrinkage, checking, and splitting. Excessive checking and splitting allow fungi and insects to penetrate the interior of the wood, causing the wood to deteriorate and eventually leading to the loss of structural integrity.
Waterborne preservatives commonly used in bridge construction include chromated copper arsenate (CCA), ammoniacal copper/zinc arsenate (ACZA), and similar products. Waterborne treatments chemically bond with the wood. Because these treatments do not use an oil medium, timber treated with waterborne preservatives expands and contracts more quickly with moisture change and is susceptible to heavy checking and splitting over time.
The pressure-treatment process for waterborne preservatives significantly increases the moisture content of freshly treated wood. If waterborne-treated wood is not redried after treating, the wood will shrink after installation. The redrying—or curing—process lowers the moisture content gradually, minimizing cracking and splitting. Waterborne preservatives are not recommended for large structural members, particularly glued-laminated timber.
In consultation with the Forest Service in 1996, the Western Wood Preservers Institute (WWPI) and the Canadian Institute of Treated Wood (CITW) published a set of specifications for timber treatment, Best Management Practices for the Use of Treated Wood in Aquatic Environments (BMPs).
In 2002, the Michigan Timber Bridge Initiative published Best Management Practices (BMPs) for the Use of Preservative-Treated Wood in Aquatic Environments in Michigan. This document contains much of the same information as the 1996 WWPI publication, but includes a discussion of the U.S. Department of Labor, Environmental Protection Agency's consumer information sheets and the environmental risks associated with the use of most common wood preservatives.
Both sets of BMPs seek to minimize the amount of treatment chemicals dispersed into the environment by controlling treatment procedures, mandating cleaning procedures after treatment, limiting chemical loading, and requiring visual inspection before installation of structures using preservative-treated wood. These BMPs were prepared to protect water quality and the diversity of life forms found in lakes, streams, estuaries, bays, and wetlands. A secondary result of complying with these specifications has been the improved performance of asphalt pavements on timber bridge decks treated in compliance with the BMPs.
Benefits of Cleaning Procedures After Treatment—In 2000, the Forest Service's Forest Products Laboratory inspected and measured preservative retention levels in six creosote-treated bridges in rural Michigan (Wacker, Crawford, and Eriksson 2002). Two of these bridges were in the same county and had the same type of superstructure. One bridge had undergone cleaning procedures after treatment, as required by the BMPs—the other bridge had not. Core samples revealed similar creosote retentions in both bridges. The bridge that was not cleaned after treatment exhibited excessive underside leakage of creosote, bleeding of asphalt and creosote on the roadway surface, and pavement rutting. The bridge that was cleaned after treatment had none of these problems.
Recommended cleaning procedures after treatment with creosote are:
Expansion bath—Following the pressure period, heat the creosote 10 to 20 °F above press temperatures for a minimum of 1 hour. Pump the creosote back to storage and apply a minimum vacuum of 24 inches of mercury for at least 2 hours.
Steaming—After the pressure period, once the creosote has been pumped back to the storage tank, a vacuum shall be applied for a minimum of 2 hours at a vacuum of not less than 22 inches of mercury to recover excess preservative. Release the vacuum back to atmospheric pressure and steam for a 2-hour period. Maximum temperature during this process shall not exceed 240 °F. Apply a second vacuum for no less than 4 hours at a pressure of 22 inches of mercury.
The long-term benefits of complying with this requirement can be seen in the performance of the asphalt pavement and the reduction of excess creosote on the visible surfaces of treated wood.
Example 7—The Barlow and Cruzen bridges in Alcona County, MI, are two-lane, single-span, stress-laminated, creosote-treated timber deck bridges. The bridges are similar in design. Both were part of the creosote retention study in Michigan. Timber materials of the Barlow Bridge were cleaned after treatment and show almost no bleeding of creosote (figures 13a and 13b) on exposed treated timber surfaces or through the asphalt pavement. The timber materials of the Cruzen Bridge were not cleaned after treatment and show excessive amounts of creosote (figures 14a and 14b) on timber surfaces and through the asphalt pavement. The American Wood Protection Association (AWPA), formerly the American Wood Preserver’s Association, set minimum creosote retention levels for these bridges as 12 pounds per cubic foot. The average measured retention levels were 46.2 pounds per cubic foot for the Barlow Bridge and 52.2 pounds per cubic foot for the Cruzen Bridge. Creosote retention levels were excessive for both bridges. However, the Barlow Bridge shows no significant bleeding. The difference appears to be that the Barlow Bridge received the BMP-recommended cleaning procedures after treatment.
Figure 13a—The Barlow Bridge in Alcona County, MI,
was made from timbers that were cleaned
after being treated with preservative.
Figure 13b—Proper cleaning prevented creosote-
treated timbers from bleeding through the
asphalt pavement on the Barlow Bridge.
Figure 14a—The Cruzen Bridge in Alcona County, MI,
was made from timbers that were cleaned
properly after being treated with preservative.
Figure 14b—Creosote-treated timbers on the
Cruzen Bridge were not cleaned after
Creosote is leaching from the timbers.
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