Rising to Pieces

Advances in precast concrete slab construction give method a promising future

Bridges Article February 21, 2003
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Stories originate out of Washington, D.C., on a daily basis. So I really can't think of a better place to be during the second week of January than at the Transportation Research Board's annual meeting. There--at the Washington Hilton, Omni Shoreham and the Marriott Parkman--the newsmakers of the industry are offering up enough tips to fill every available pocket in the brain.
I was able to sit in on countless discussions, including one which looked at how the future of transportation was interpreted back in the 1920s, 30s, 40s and 50s. Let's just say the one prognosticator who thought private helicopters would be the way to go by the year 2000 was off . . . way off. But there was one story which in my mind stood out from the rest, one which just may be a top application years from now.
When the day comes when we are buzzing around in high-tech automobiles, speed of construction will be a far heavier weight than the one contractors are handling now. What better way to address that problem than using precast concrete slabs to create new highways and rehab old ones? The process has been used sparingly across the country, and all the cases fell in the category of concrete repair. Crews would cut out a damage section and replace with a new precast slab.
This practice took a positive turn in the spring of 2002, when the Texas Department of Transportation completed a precast pavement pilot project which is currently fueling a concept developed by the Center for Transportation Research at the University of Texas, Austin. The project, on a piece of I-35 near Georgetown, Texas, called for the use of 2,300 ft of post-tensioned precast concrete panels on either side of a new bridge. Both full-width (11 m) and partial-width (5, 6 m) panels were used. The partial-width panels were tied together through transverse post-tensioning. The pavement thickness of these slabs were 8 in., but the post-tensioning actually gave the pavement the fatigue life of a 14-in. thickness. The panels were cast on a long line casting bed 400 ft in length. Long-line casting allowed for 10 full-width panels and up to 20 partial-width panels to be cast at one time. When casting was complete there were 123 full-width and 216 partial-width panels. According to researchers, the mix design used for the panels was similar to the one used for precast prestressed bridge beams. It called for Type III cement with a water-cement ratio of 0.42 and a superplasticizer for increased workability. Specifications called for the panels to reach a 28-day compressive strength of 5,000 psi.
Once the panels arrived at the site a slow-setting segmental bridge epoxy was applied to the panel edges, which aided in the assembly by acting as a lubricant and helped seal joints between panels to protect the post-tensioning strands and prevent grout leakage.
Installation of the first 25 full-width panels took about eight hours, but by project's end 25 panels could be positioned in six hours. Once the panels were in place post-tensioning strands were fed into the ducts at the central stressing pockets and fed through the panels in both directions to self-locking anchors in the joint panels. All post-tensioning was complete 24-48 hours after panel placement.
The initial benefits of this construction method are quite impressive. Those who worked this pilot project believe the process is a key expediting agent. The panels are cast and reach a desired strength in a controlled environment, and can be put in place overnight or during a weekend. The cost of the Georgetown experiment was rather high, but in time as the practice is perfected the expense could go down. And the money saved in user costs would impress many.
My only concern is how the pavement will perform under constant wear-and-tear. Will the slabs stay tight, or will they loosen over time and create an entrance for water and salt?
But what's important here is the roadbuilding industry is evolving to match our run-and-gun society. This is a good thing, because I really don't see any of us running on air anytime soon.

About the author: 
Bill Wilson is editor of Roads & Bridges.
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