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Asphalt Paving of Treated Timber Bridge Decks

Asphalt Pavement Behavior

Asphalt pavement is a flexible system, structurally com-posed of layers of asphalt and underlying bases—in this case, treated timber bridge decks. Portland cement concrete pavement is a rigid system where the concrete serves as the structural component. Underlying bases do not contribute to the strength of the system as significantly as in an asphalt system. Products and paving systems appropriate for concrete bridge decks may not be appropriate for timber bridge decks.

Because asphalt pavement is flexible and concrete pavement is rigid, they distribute loads differently. The concrete pavement has greater structural strength and stiffness, distributing loads over a wide area. Minor variations in subgrade strength have little influence on the structural capacity of the concrete pavement.

Asphalt pavement distributes loads over a much smaller area. Asphalt roadway pavement consists of a series of layers, with the highest quality materials at or near the surface. The strength of flexible pavement is a result of built-up layers that distribute loads over the subgrade through all layers, rather than the strength of a slab alone. Because the asphalt pavement overlay on a bridge is quite thin (2 to 4 inches), minor deflections in the bridge deck can crack the pavement.

Asphalt pavement depends on the bridge deck to act as a relatively stiff, monolithic sublayer that provides underlying strength and stability. When individual sections of the deck (such as glued-laminated deck panels) move independently, this movement may cause asphalt pavement to crack and fail at the deck panel joints.

The design of asphalt pavement mixtures is complex. Too little asphalt results in a brittle mix where aggregate doesn't bond together or to the underlying surface. Too much asphalt results in a soft mixture susceptible to rutting and bleeding.

Another difference between asphalt pavement and concrete pavement is that asphalt pavement does not “set” or become totally stable as it cures. The asphalt in an asphalt paving system can dissolve, soften, or leach from the pavement mix. Hydrocarbon solvents used to carry treatment chemicals in treated wood readily dilute and dissolve asphalt. The chemical composition of creosote is similar to asphalt, and the chemicals in penta and copper naphthenate will dissolve asphalt as well as styrene and rubber additives (polymers). Heat also softens asphalt and accelerates the damaging effects of preservative chemicals and solvents.

Asphalt Adhesion to Treated Timber

Very little information is available about the adhesion of asphalt to treated timber. Asphalt should readily adhere to dry, clean wood, but the effects of oilborne preservatives are not well known. Project coordinators also were interested in determining whether adhesion would be less for smooth wood surfaces, such as glued-laminated timber, and the effect temperature has on asphalt adhesion.

Adhesion Testing—Coast Douglas-fir is a common structural wood species in the Western United States. It accepts treatment relatively well. Four 12- by 36- by 2-inch sections of rough, solid coast Douglas-fir were tested. One was treated with creosote, another with penta in heavy oil, a third with copper naphthenate in heavy oil, and a fourth with chromated copper arsenate (a waterborne preservative). Three 12- by 36- by 5 1/8-inch individual sample pieces of glued-laminated coast region Douglas-fir treated with creosote, penta in heavy oil, and copper naphthenate in heavy oil were used for adhesion testing. Chromated copper arsenate is not recommended for use with glued laminates.

Four different asphalt mixes were tested on each of the wood samples. Two of the asphalts were PG 58-28 and PG 64-28. These liquid asphalt samples are used by transportation agencies in the Northern States. The PG 64-28 asphalt is polymer modified; the PG 58-28 asphalt is not. We also selected AC 85/100, a nonpolymer penetration-grade asphalt, and CRS-2P, a cationic, polymer-modified emulsion (an asphalt cement milled into small particles that can be mixed with water and emulsifying agents). These asphalt mixes are similar to those used in some mountainous climates and Northern States.

The adhesion of these asphalts was tested on the two types of wood (solid and glued laminated) and the four types of treatment (creosote, penta, copper naphthenate, and chromated copper arsenate) at room temperature using a tensile-strength testing machine (figure 17). Templates were attached to the wood samples to keep the asphalt a consistent thickness. Fabricated pullout brackets (figure 18) were embedded in the asphalt. The contact area was 1 ½ by 3 ¾ inches. The asphalt samples were preheated to 300 °F before being poured into the templates. After the brackets were embedded in the asphalt, the samples were allowed to cure for 48 hours.

Figure 17—Testing the tensile strength
of asphalt adhesion.

Figure 18—Tension being applied to the
pullout brackets.

When tested in tension, all the samples failed gradually by pulling the asphalt apart. The asphalt did not separate from the treated wood or the metal brackets. There was no measurable difference in adhesion between the solid samples and the glued-laminated samples. Surprisingly, the dry chromated copper arsenate (waterborne treatment) sample didn't differ significantly from the other samples. The adhesion strengths for penta were the lowest for both the solid and glued-laminated samples. The adhesion strengths for copper naphthenate were the highest for both wood types. The factor determining adhesion strength seemed to be how much the treatment type softened the asphalt. The asphalt on the penta wood samples seemed to be softer, and when tested, elongated farther (figure 19). This result is not conclusive because the particular penta wood sample may have had a higher retention of treatment material and oil solvent.

Figure 19—Asphalt adhesion test after the asphalt
failed when it had elongated.

The polymer-modified asphalts performed better on both wood types; PG 64-28 performed slightly better than the polymer-modified emulsion.

To evaluate the asphalts during cold temperatures (tables 1, 2, and 3), the four asphalts were retested on the creosote sample at freezing temperatures. The templates were refilled and the brackets set on the creosote-treated glued-laminate panel. The samples were left outside for 24 hours. The surface temperature of the sample was 28 °F at the time of testing. The polymer-modified asphalts failed at a tensile load 54 percent higher than at room temperature. The nonpolymer-modified asphalts failed at much smaller loads. The nonpolymer-modified samples resulted in a brittle failure as soon as the minimum load was applied.

Table 1—Adhesion test of solid timbers.

Table 2—Adhesion test of glued laminates at 65°F.

Table 3—Adhesion test of glued laminates at 28 °F.

These results do not provide a hard-and-fast measure of asphalt adhesion to treated timber because the tests were conducted quickly and lacked a standardized test methodology. However, they do show asphalt-treated timber adhesion characteristics.

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