Civil Engineers Develop Bridge Systems of Tomorrow
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Civil Engineering Professor and project investigator Richard Gutkowski is turning the discards of progress into the bridge systems of tomorrow. Utilizing salvaged utility poles removed as part of road expansion projects, Gutkowski and graduate student Matthew LeBorgne are recycling reusable wood into short (20-30 ft) to medium span, (40-50 ft) bridges.
Layering wood and concrete to create a composite bridge, the groundbreaking idea is an economical solution for low tax base communities dependent on agricultural economies and the related freight and shipping industry. Research and construction is taking place at the Structures Laboratory, part of Colorado State University's Engineering Research Center (ERC).
Gutkowski aims to replace conventional reinforced concrete slabs, a costly system, with a composite design. Integrating an innovative cambering, or arching technique, the researchers are using tapered utility poles that will be set in alternating directions, beneath a thinner concrete slab, to attain a concave design.
The concrete layer will provide compression strength and a hardwearing surface for vehicle wheels, keeping snow and rain off the wood. The wood layer provides tensile strength by replacing the usual lower non-structural half of the concrete, its steel reinforcement and external temporary shoring with a structural wood layer that self-shores the concrete during the curing process.
The project is the result of cooperation on a worldwide scale. Researchers in New Zealand, Germany, Italy and Sweden have joined forces studying long-term creep, hygrothermal effects such as the flow of moisture between materials, and the effect of humidity changes on deflection over time in composite bridge designs. In Colorado's arid climate, Gutkowski and team have been working to overcome the special properties of dry wood, which draws water from adjacent concrete shrinking it, negatively affecting the curing process.
Although the team is utilizing a notched shear key system - grooves cut in the wood filled with concrete when cast - interconnecting the two layers has proved to be a challenge. Since the mechanics of shrinking concrete have not been studied at relative humidity of less than 40%, Gutkowski has plans to examine this issue using environmental test chamber equipment at the ERC in a later phase of the project.
"Our challenge has been the need to prevent very dry wood from extracting water from the concrete in our notch connection," said Gutkowski. "We appear to have overcome that important piece of the mechanics puzzle by using additives in the concrete and sealing the wood. With that working we can better manage the immediate and long term deflection and stress behavior."
The project is funded by the U.S. Department of Transportation via the Mountain-Plains Consortium within the University Transportation Centers program. In addition, Xcel Energy provided, at little to no cost, uprooted utility poles displaced as part of the widening of LaPorte Avenue from Overland Trail to the earth dam at Horsetooth Reservoir. Xcel Energy is also providing poles from an ongoing roundabout intersection construction site at Vine Drive and Taft Road.
"The idea came to me when I saw them removing the poles as I passed by each day on the way to the ERC," Gutkowski recalled. "It was like a light bulb coming on."