United States Department of Agriculture	Asphalt Paving of Treated Timber Bridge Decks
Forest Service
Technology & Development Program
7100 Engineering November 2003 0371-2809-MTDC

Introduction

An asphalt paving system protects the structural elements of timber bridge decks from tire wear; reduces the penetration of moisture to other superstructure members, such as beams, stringers, diaphragms, and their associated hardware; and provides a skid-resistant roadway surface.

This report, which was prepared in response to concerns expressed to the U.S. Department of Agriculture (USDA) Forest Service, Forest Products Laboratory, Wood In Transportation program, and the San Dimas and Missoula Technology Development Centers, discusses problems with recently constructed timber bridges that were paved with asphalt. Numerous publications and articles were reviewed; agency and industry professionals were consulted, and asphalt adhesion and paving membrane solubility were tested. Information was collected at treated timber bridges in Alaska, Montana, Oregon, Washington, Michigan, and Wisconsin. Many of the bridges were performing very well—others exhibited one or more problems.

Ensuring long-term pavement performance and minimizing environmental problems for bridges with treated timber decks (figure 1) is the goal of this project. Some effects of waterborne preservatives are covered, but the focus is primarily on timber treated with oilborne preservatives.

Figure 1—Paved timber bridge in Oregon

Asphalt paving failures on the decks of treated timber bridges are caused by one or more of the following deficiencies:

• Bridge deck design and construction
• Type and quantity of the wood's preservative treatment
• Design and installation of the asphalt paving system.
• Deck deflection and movement (the primary causes of pavement cracking)

Preservative treatment and asphalt paving system problems are often related. The treatment's interaction with asphalt cement (asphalt) is the main cause of pavement delamination and asphalt bleeding and leakage. Improper treatment practices compound improper paving system design, and vice versa.

Timber Bridge Deck Design

The four most common timber bridge decks in the United States (Wacker and Smith 2001) are:

• Timber plank
• Glued-laminated timber panel
• Stress-laminated timber
• Nail-laminated timber

Timber Plank Decks—Timber plank decks generally are used on low-volume, rural, unpaved roads. Because timber plank decks are too flexible to provide a durable sublayer for asphalt pavement, they are rarely paved.

Glued-Laminated Timber Panel Decks—Glued-laminated panels usually are placed across (transverse to) beams or stringers to create the bridge deck and connected to timber beams mechanically with lag bolts or deformed shank nails. The panels usually are bolted onto steel beams.

Glued-laminated timber panel decks also can be placed longitudinally over transverse floor beams of an arch or suspension bridge. However, short-span bridges may span from abutment to abutment.

Mot asphalt pavement cracking problems with glued-laminated timber panel decks result from a bridge's structural design. Unless deck panels are mechanically interconnected, wheel loads cause the glued-laminated panels to move independently of each other, especially with the more common transverse glued-laminated timber panel decks. This relative displacement causes reflective cracking (cracking that migrates up from the timber deck) of the asphalt pavement above the panel joints. The extent and size of the reflective cracking is proportionate to the deck panel displacement.

Asphalt pavement cracks when panels move independently, either vertically or horizontally. Horizontal movement occurs when glued-laminated panels shrink as they dry. This shrinkage opens gaps at the deck panel interfaces, cracking the pavement if the movement is large.

Stress-Laminated Timber Decks—Stress-laminated timber decks are multiple timber laminations (usually longitudinal) that are stressed into monolithic slabs by high-strength steel reinforcing rods. These laminations can be timber planks, or glued-laminated beams for longer spans.

A stress-laminated deck often is self supporting. Beams or stringers are not needed. Because the deck behaves as a single unit, differential deflection does not occur. Volume change, caused by moisture, is spread across the width of the entire deck, minimizing pavement cracking as the deck shrinks.

Nail-Laminated Timber Decks—Nail-laminated timber decks are timber planks, usually placed across (transverse to) longitudinal beams or stringers. The laminations are nailed to the longitudinal beams. When new, these decks perform much like stress-laminated timber decks. With age, nail-laminated decks become more flexible and frequently show random reflective asphalt pavement cracking. Glued-laminated timber decks often replace nail-laminated in new construction.

Wood Preservatives

Wood preservatives protect wood by inhibiting decay fungi and insects that feed on wood fiber. Preservatives in properly treated wood are stable. Only minimal amounts leave the wood. Preservatives do not penetrate the entire cross section of large structural members—they usually penetrate less than 1 inch. To be effective, the treatment must penetrate deeply enough and supply enough preservatives to create a preservative “envelope” that prevents decay fungi or insects from reaching untreated wood. Applying the appropriate amount of preservatives is critical. Too little will leave the wood vulnerable to decay. Too much will result in preservatives and solvents leaching to the surface of the wood and into the environment. When treated timber decks are paved, leached preservatives and solvents interact with paving membranes and asphalt, potentially causing pavement failure.

Wood preservatives are broadly classified as oilborne or waterborne. Oilborne preservatives generally consist of a pesticide chemical carried in an oil solvent. They are the most commonly used treatment for bridge construction. Because oilborne preservatives leave an oil solvent film on the surface of the wood, they generally are not recommended for applications that allow repeated human contact.

Waterborne wood-treatment chemicals that fixate with the wood tend to be more appropriate for human contact. However, because such chemicals do not produce a water-resistant, oily surface, the wood member can lose or gain moisture rapidly. The change in water volume can split and crack large structural members, exposing untreated wood. Wood treated with waterborne preservatives is rarely recommended for highway bridge construction.

In 1995, the San Dimas Technology and Development Center published Selection and Use of Preservative Treated Wood in Forest Service Recreational Structures (9523-1203-SDTDC). This document provides background on the various preservatives, with recommendations for appropriate use.

Asphalt Pavement

Asphalt pavement can be constructed with hot materials (hot mix) or cold materials (cold mix). In this report, asphalt pavement refers to hot-mix pavement. Asphalt pavement is about 95-percent aggregate and 5-percent asphalt cement.

Asphalt cement is referred to as asphalt. The aggregate provides structural carrying capacity through point-to-point contact, while asphalt holds the aggregate in place under traffic loads and prevents dust. Adding asphalt to aggregate reduces porosity, but asphalt pavement is still permeable.

Modern asphalt includes numerous additives to best fit the local environment and improve the performance of asphalt pavement. Asphalt itself is the end result of the oil refining process. John Norton, Jr., described asphalt as “the bottom of the refinery barrel” (Norton 2002).

Since the 1990s, asphalt specifications have used the performance graded (PG) system under the national Superpave asphalt pavement program. The PG grading system uses two numbers, such as PG 64-22, to reference a grade. In this example, 64 and-22 represent the temperature extremes in degrees Celsius that the pavement is designed to withstand.

Additives are used to create polymer-modified asphalt. Polymers are the most common asphalt additive and have the greatest effect on the performance of asphalt pavement, particularly in the Northern States. Elastomers made from styrene-butadiene-styrene (SBS) or styrene-butadiene-rubber (SBR) are the most common polymer additives. Elastomer polymers add considerable elasticity, ductility, and cracking resistance. In cold weather, polymers significantly increase the asphalt pavement's adhesion to the treated timber deck (see the Asphalt Adhesion to Treated Timber section). Industry testing also has shown that SBS significantly reduces cold weather cracking. Polymers are often added to high asphalt content mixes to stiffen the mix and reduce rutting.

Asphalt Pavement Systems

Paving systems are composed primarily of the asphalt pavement, but can include primers and paving membranes, tack coats, and paving fabrics. Primers should not be confused with prime coats. Prime coats are low-viscosity asphalts that are applied to prepare an aggregate base. They penetrate the base, seal the aggregate, and harden the surface. A prime coat would not be used on bridge decks. Primers are specialty products designed to improve adhesion of paving membranes to a surface—usually concrete bridge decks.

A paving membrane is a fabric, often a nonwoven paving cloth, with polymer-modified (or rubberized) asphalt on one or both sides. This asphalt melts when hot asphalt is applied over it. The melted paving membrane provides a waterproof layer between the pavement overlay and the underlying structure.

A tack coat is a thin layer of liquid asphalt sprayed over the prime coat or base course, or directly onto a bridge deck. A tack coat helps bond the asphalt course to the underlying surface. Paving fabric is usually a nonwoven geotextile placed beneath or between paving layers. Paving fabrics are always placed over a light application of asphalt cement to provide a moisture resistant barrier in the pavement structure (American Association of State Highway and Transportation Officials 2001).

Response to Concerns

Potholes or cracks may form when asphalt pavement fails on timber bridge decks. Asphalt pavement may dissolve and decompose, or the asphalt and preservative chemicals or solvents may bleed to the pavement surface and to the underside of the bridge. When asphalt pavement contains too much asphalt, the asphalt will migrate to the top pavement surface and bleed, or to the bottom of the pavement and drip. Asphalt from the paving membrane can also bleed and drip if it is dissolved by excess preservative chemicals and solvents. Asphalt bleeding can lead to rutting and stripping, which can be accelerated by heavy loads, hot weather, or improper pavement design. Asphalt and preservatives can drip from the underside of a bridge and be released into the environment.

The quality and durability of asphalt pavement on treated timber bridge decks is determined by four main factors:

Structural (serviceability) characteristics—The design of the bridge superstructure affects deck movement and deflection. Deck deflections and shrinkage of timber deck members can cause severe pavement cracking.

Type and amount of preservative treatment chemicals and solvents—Residual treatment chemicals and solvents can be found on the surface of improperly treated wood. These chemicals and solvents will dissolve asphalt from the paving membranes and the asphalt pavement. This dissolved asphalt, along with the preservative, will soften the pavement and bleed to the pavement surface, or leak around or through the deck. Having these products drip into streams and rivers is unsightly and environmentally unacceptable.

Asphalt paving systems—Paving membranes or excessive primers or tack coats, combined with treatment chemicals, often cause improper bonding and excessive concentrations of asphalt in the pavement mix.

Construction and application methods—Variations in construction can cause excessively thick tack coats, affecting asphalt pavement's adhesion to the deck. Inappropriate use of paving membranes can cause a membrane to slip, allowing pavements to bunch and fold. Weather also affects the curing and adhesion of asphalt.

E-mail: wo_mtdc_webmaster@fs.fed.us	Technology & Development Center

Visitor since July 14, 2004