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Asphalt Paving of Treated Timber Bridge Decks
Creosote for bridge timbers should be derived entirely from coal tar, as required in AWPA P1/P13. Penta and copper naphthenate treatment chemicals can be carried in a heavy oil solvent (AWPA type A) or a light oil solvent (AWPA type C). The type A solvent provides more protection against moisture intrusion and usually is preferred by bridge engineers. However, type C solvent is often used in more sensitive environments. It provides a cleaner surface with less potential for solvent dripping.
When timber is improperly treated, or if the cleaning procedures are not followed after treatment, chemicals and solvents (treatment residues) will be present at, or may migrate to, timber surfaces. This can occur as a natural process in a treated timber or may be accelerated by heat or compressive stressing forces, as is the case for stress-laminated bridges. Excessive creosote, penta, copper naphthenate, or oil solvents reduce pavement-to-deck adhesion, soften the asphalt in the pavement mix, cause bleeding and pavement rutting, and dissolve paving membranes. In extreme cases, the mixture of asphalt, treatment solvent, and treatment chemicals can leach into the environment.
To minimize problems associated with preservatives:
AWPA treatment standards provide minimum requirements for preservative penetration and retention. The BMPs strive to meet these standards without using more chemicals than necessary. The average preservative retention of wood treated to BMP standards should not exceed 150 percent of the AWPA required minimum retention.
Setting precise maximum chemical loading levels is difficult because of the inherent variability found in wood, including cell structure and the proportion of sapwood (newly formed outer wood) to heartwood (inactive wood).
Glued-laminated and solid members are sometimes treated in the same batch. Glued-laminated timber has a higher sapwood-to-heartwood ratio than solid material. Because sapwood is more permeable than heartwood, the glued-laminated timber usually retains significantly more preservative than solid timbers from the same batch. Glued laminates are dried to 16-percent moisture content to increase the penetration and retention of treatment chemicals and solvents. Solid materials are dried to 19-percent moisture content.
Example 8—The Cameron Bridge across the Manistee River in Crawford County, MI, was constructed in 1995 as a two-lane, two-span, stress-laminated timber box-beam bridge. The deck is comprised of a laminated box section with southern pine glued-laminated webs spaced every 24 inches. Solid pin oak dimensional wood stressed between the glued-laminated webs forms the box section. This bridge was also part of the Michigan creosote-treated bridge study. The glued-laminated webs had an average creosote retention of about 12.5 pounds per cubic foot. The solid wood had an average creosote retention of about 5.1 pounds per cubic foot. Although these are not exceptionally high retention levels, excess creosote dissolved the asphalt-impregnated paving membrane between the deck and the asphalt paving over the tops of the glued-laminated members (figure 15). The long stripes of creosote bleeding through the asphalt paving show the locations of the glued-laminated beams.
Figure 15—The Cameron Bridge in Crawford County, MI,
had asphalt bleeding over glued-laminated webs.
Several publications discuss the environmental effects of treated wood on aquatic environments. The first, Assessment of the Environmental Effects Associated with Wooden Bridges Preserved with Creosote, Pentachlorophenol, or Chromated Copper Arsenate (Brooks 2000), involves studies of actual preservative concentrations in the soil and water adjacent to bridges treated with those chemicals.
The second, Environmental Impact of Preservative-Treated Wood in a Wetland Boardwalk (Weyers and others 2001), is a study of the effects of four different wood treatment products in sensitive wetland environments.
Both reports indicate minimal risk to the environment from preservatives lost from timber bridges. Any risk can be minimized or eliminated through better treatment, cleaning after treatment, and construction and maintenance practices.
Example 9—The previously discussed Cruzen Bridge in Alcona County, MI, was part of the State's creosote-retention study. The two-lane, single-span, stress-laminated timber-deck bridge was constructed in 1995. The asphalt paving was placed shortly after the deck was stressed. The following summer, bridge users began complaining of excessive bleeding of the asphalt (figure 16). The bridge has creosote bleeding on the underside and sides of the deck. Creosote retention levels in this bridge deck average about 52 pounds per cubic foot. The deck laminations are 6 4/inch-wide glued-laminated sections of southern yellow pine and red pine. When laminated decks are stressed, preservatives are squeezed out. A higher concentration of preservatives occurs at the junctions of the deck laminations. At the Cruzen Bridge, these concentrations of creosote bled up through the asphalt pavement. The spacing of the asphalt lines on the surface of the pavement match the width of the glued-laminated beams in the stressed deck. This bridge did not have a paving membrane, so the visible bleeding is pavement asphalt and excess creosote.
Figure 16—Asphalt bleeding directly over creosote
concentrations on the Cruzen Bridge
in Alcona County, MI.
As specified in the Western Wood Preservers Institute's Best Management Practices for the Use of Treated Wood in Aquatic and Wetland Environments (2011), cleaning procedures should be required after treatment. Treatment retentions should be limited to 150 percent of the AWPA retention minimums.
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