Seavey, Robert T.

This summary offers an overview of on a research study that assessed the magnitude of premature asphalt deterioration on timber bridges; identified the primary mechanisms responsible for wear surface deterioration; and suggested methods for improving asphalt pavement performance on timber bridges. The study revealed that approximately 50 percent of counties experience some problems with premature reduced serviceability of the asphalt pavement wear surfaces that cover their timber bridges. The summary looks at possible pavement failure mechanisms and presents the following proposed solutions for controlling timber bridge asphalt pavement cracking: asphalt pavement saw & seal, asphalt pavement fabric or material underlay, removal of extruded oil-type preservative before surfacing, conditioning of bridge timbers to the expected equilibrium moisture content before bridge installation, and tightening of timber decks through maintenance practices.

Asphalt wear surfaces cover 1,378 of Minnesota's timber bridges. This study assessed the magnitude of premature asphalt deterioration on timber bridges; identified the primary mechanisms responsible for wear surface deterioration; and suggested methods for improving asphalt pavement performance on timber bridges. Research methods included surveys, meetings with several county engineers and tours of their timber bridges, interviews with both asphalt and timber bridge industry professionals, and literature reviews. The study revealed that approximately 50 percent of counties experience some problems with premature reduced serviceability of the asphalt pavement wear surfaces that cover their timber bridges. Possible pavement failure mechanisms include low-temperature cracking, reflective cracking from deck fault lines found at deck panel joint lines and deck lamination separations, asphalt fatigue fracturing, and asphalt de-bonding due to oil preservatives interference. The report presents the following proposed solutions for controlling timber bridge asphalt pavement cracking: asphalt pavement saw & seal, asphalt pavement fabric or material underlay, removal of extruded oil-type preservative before surfacing, conditioning of bridge timbers to the expected equilibrium moisture content before bridge installation, and tightening of timber decks through maintenance practices.

This study evaluated the thermodynamics of stress-laminated bridges under laboratory conditions. After assembling three timber laminated bridge deck panels of 120" x 43" x 12", high tension rods were used to form "stress-laminated" panels. Researchers placed the panels in a laboratory freezer, with cold temperature settings of 10*, 0*, -10*, -20°, and -300 Fahrenheit and repeated the process three times, each with the wood at a different moisture content--a "green" moisture content greater than 30 percent, a 17 percent moisture content, and a 7 percent moisture content (mc). The results showed that the bar force reduction in the green moisture content sample was significantly greater than in either the 17 percent or 7 percent me tests. The study concluded that the moisture content levels and temperature fluctuations cause variations in rod stressing levels; that the tensioning losses occur within a few hours of the temperature drop; that the green moisture content levels have a severe adverse effect on the stressing levels; and that tensioning levels somewhat stabilize with moisture contents below 17 percent. Based on this study, it would appear that any existing stressed bridge decks should be closely monitored until the moisture content of the members is less than 19 percent. Further study may be needed to determine the behavior of bridge decks with a moisture content above 17 percent and below the fiber saturation point.

A retrofit scheme to widen and strengthen nail-laminated timber bridges was evaluated in this project. The scheme consists basically of laying a second, transverse layer of timbers above the existing deck, and casting a grout layer between the two wood ones to insure good force transfer. An old wood bridge was evaluated before and after it was retrofitted in order to investigate the effectiveness of the retrofit technique. In addition, three laboratory specimens, representing portions of the retrofitted bridge deck (ungrouted and grouted), were tested to investigate the strength and the effects of fatigue on the retrofitted bridge deck, and to evaluate the transverse load distribution of the original and retrofitted bridge deck. An analytical model of the retrofitted bridge deck was also developed utilizing the finite element method, the deflection and transverse distribution results from the model studies were compared favorably with the laboratory results.

Approximately 35% of Minnesota's 1,373 bridges are reaching the end of their expected service life. Many of these timber bridges are located in rural, low traffic settings that do not justify the expense of replacement. Extending the life of these bridges will not only save communities money, but will ensure continuous bridge service. This study reviewed both new and traditional techniques for inspecting decaying wood on timber bridges. Newer, nondestructive technologies include stress wave timber or Resistograph drill and infrared thermography. These methods offer many advantages, but they are still expensive and time consuming. Traditional inspection methods might provide the best alternative. The researchers recommend inspecting older bridges every three years and newer bridges every five years, with more careful evaluation of areas that show wood decay. The researchers conclude that the best way to pass on knowledge about timber bridge inspection is through hands-on seminars.

Many of the 1,400 timber bridges in Minnesota need to be improved to meet present day standards. When the desired service level can be attained by widening a bridge six feet or less, a retrofit can be done by placing a second, wider, transverse deck onto the existing deck and substructure. Bridge components must be carefully inspected prior to a retrofit project. Bridge #6641 in Sibley County was retrofitted. First the bituminous surface was removed. A longitudinal beam supported the extended deck. Grout was poured and leveled and then nail-laminated panels were laid transversely. A bituminous surface was laid over the full width of the new deck. The cost of the project was $51,632. (Replacing the bridge was estimated to take 2-3 years and cost $215,000). The county quantified the strength change and load distribution characteristics of the retrofitted bridge deck. Static and dynamic tests were performed both before and after the retrofit. The tests show that adding a second deck effectively decreased the static deflections and improved the transverse load distribution. Nail-laminated timber bridge #2642, also in Sibley County, was retrofitted in 1992 and was load tested again in 1995. All dynamic deflections were lower than those of the post-retrofit tests in 1992. This improvement can be explained in part by the drying of the moisture that was introduced into the bridge deck during grouting. Subsequent drying would add stability to the bridge deck. A retrofitted timber bridge is expected to last an additional 20-40 years.