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Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project
The 'bump at the end of the bridge' caused by bridge approach slab settlement is an ongoing problem for Iowa and many other State highway agencies. The bump not only degrades the ride quality of a roadway, but also presents a safety issue for drivers and increases impact loads on bridges. Approach slab settlement is generally caused by a loss of support due to consolidation or erosion of the underlying embankment fill, and can be accompanied by failure of the paving notch. While problems with the embankment fill material and paving notch must ultimately be resolved through improved construction practices, a separate but equally important issue is how to reconstruct bridge approach slabs that have already failed due to settlement. This is particularly challenging in urban areas where lane closures must be minimized to reduce the impact of reconstruction on the traveling public. The solution must not only be something that will permit the approach slab to be reconstructed quickly (during overnight or weekend closures), but something that will provide good long-term performance even if future loss of support occurs.
The primary objective of this research was to evaluate the viability of using precast prestressed concrete panels for bridge approach slab reconstruction. Ultimately, this construction process will be utilized for bridge approach slabs in urban areas where lane closures for reconstruction must be minimized. A secondary objective was to evaluate and develop standard details for precast prestressed concrete bridge approach slabs that can be adapted to virtually any approach slab configuration.
The demonstration project was constructed on the northbound lanes of a new bridge over the Floyd River on the realignment of Highway 60 just east of Sheldon, Iowa. Precast prestressed concrete approach slabs were constructed at either end of the bridge. The layout for the precast panels was selected such that it could be adapted to various approach slab configurations of different thicknesses, widths, lengths, and skew angles.
The precast panels were post-tensioned in both directions after installation on site. Post-tensioning not only improves the performance of the approach slab by keeping it in compression to minimize or even eliminate cracking, but also gives the approach slab the ability to act as a 'slab bridge,' spanning over voids that may form due to consolidation or erosion beneath it over time. Post-tensioning also allows for a longer section of continuous pavement between joints, permitting the expansion joint to be moved as far away from the abutment as feasible to minimize the risk of water infiltrating the embankment fill at the abutment.
The precast panels were fabricated in summer 2006 and installed over a 2-day period (for each approach slab) in August and September 2006. In general, each panel was installed in approximately 15–30 minutes. This installation rate will likely improve as contractors become more familiar with the construction technique. While this project was constructed on a new bridge and the roadway was completely closed to traffic during construction, the intent was to evaluate the overall process and project details. The viability of this construction technique was clearly demonstrated, and rapid reconstruction of existing approach slabs under stringent time constraints will be the next step.
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