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Asphalt Paving of Treated Timber Bridge Decks

Timber Bridge Deck Structural Behavior

Timber plank decks are seldom paved, so they will not be discussed further. Stress-laminated timber decks perform as monolithic slabs and pavement cracking is minimal. Most pavement cracking occurs with the most common type of treated timber bridge deck-the glued-laminated panel deck.

Differential Deflection of Deck Panels

Asphalt pavement seldom cracks because of normal, longitudinal deflection of the bridge's superstructure. The most common cause of pavement cracking on glued-laminated panel deck systems is differential deflection between adjoining deck panels. This is particularly true of transverse glued-laminated deck panel systems. The panels are installed with the laminations perpendicular to traffic flow. Wheel loads moving from panel to panel cause rapid, repetitive, and sometimes significant panel movement at the panel interface. When these wheel load deflections are more than 0.05 inches, the pavement tends to crack. When deflections exceed 0.10 inches, the cracks often ravel (crumble), causing bumps that increase impact to the bridge and lead to moisture problems.

Example 1—The Watchtower Creek Bridge (figures 2 and 3) and West Fork Creek Bridge were constructed on the Bitterroot National Forest in Montana during the summer of 1989. These two-lane, single-span bridges were constructed with transverse glued-laminated deck panels on seven glued-laminated timber beams spaced 48 inches apart. The beams supporting the deck of the Watchtower Creek Bridge are 27 feet long, 8 3/4 inches wide, and 22 1/2 inches deep. The beams supporting the West Fork Creek Bridge are 35 feet long, 8 3/4 inches wide, and 28 1/2 inches deep. Deck panels on both bridges are nailed to the beams and were not mechanically interconnected. The decks were paved shortly after being installed. Within days, the asphalt paving showed reflective cracking (cracking that is reflected up from the deck) directly over all the deck panel joints. The deck panels were treated with pentachlorophenol carried in a heavy oil solvent. The asphalt pavement cracked as soon as the bridges were put in use. The cracking resulted from differential movement of the deck panels, not from panel shrinkage.

photo of cracking over panel joints on bridge
Figure 2—Cracking over panel joints on the Watchtower Creek Bridge
at the Bitterroot National Forest, MT, in 1989.

photo of crackin over panel joints
Figure 3—Cracking over panel joints on the Watchtower Creek Bridge
at the Bitterroot National Forest, MT, in 2000.

The cracks have opened and raveled somewhat over the 14-year life of the bridges. However, the bridges are functional. No timber deterioration was detected in the deck or superstructure members.

Deflection of longitudinal deck panels can also cause asphalt pavement cracking, although the problem is usually less severe because the wheel loads are not crossing the panel joints. Because longitudinal deck panels usually have a long span, the panels are connected to load distribution beams that help distribute wheel loads.

Example 2—The Satsop River Bridge (figures 4 and 5) near Shelton, WA, was constructed in 1996. It is a double-lane, glued-laminated arch bridge with longitudinal glued-laminated deck panels across transverse floor beams. The asphalt pavement cracked within days of installation. The bridge carried a large number of logging trucks. Significant deck movement was observed as loaded trucks crossed the bridge. The longitudinal deck panels were not interconnected with dowels. The deck panels span 10 feet between floor joists and are stiffened with intermediate load distribution beams. The decks are connected to the floor beams and distribution beams with aluminum fasteners.

photo of cracks over deck panel joint on bridge
Figure 4—Longitudinal cracks over the deck panel
on the Satsop River Bridge near Shelton, WA.

photo of distribution beam under bridge
Figure 5—Distribution beam installed
under the Satsop River Bridge.

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