TransTech Project Spotlight: 1st PPCP Technology for Bridge Approach Slab Construction—Hwy 60, Iowa

The first project demonstrating precast prestressed
concrete pavement (PPCP) technology for
bridge approach slab construction
was installed at both ends of a bridge on
Highway 60 near Sheldon, Iowa

The Iowa Department of Transportation (IOWA-DOT), The Transtech Group, and the Federal Highway Administration (FHWA) published a report titled “Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project” on the reconstruction of bridge approach slabs which have failed due to a loss of support from embankment fill consolidation or erosion, and how it can be particularly challenging in urban areas where lane closures must be minimized. The report documents the successful development, design, and construction of the precast prestressed concrete bridge approach slabs on Highway 60, as well as the challenges and issues that were faced during the project. The report presents recommendations for future implementation of this innovative construction technique.

PROBLEMS:
• 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. 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.
• It degrades the ride quality of a roadway
• Presents a safety issue for drivers
• Increases impact loads on bridges

A separate important issue is how to reconstruct bridge approach slabs that have already failed due to settlement—particularly challenging in urban areas where lane closures must be minimized to reduce the impact of reconstruction on the traveling public.

SOLUTIONS:
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. Precast prestressed concrete pavement (PPCP) is a potential solution for rapid bridge approach slab reconstruction which uses prefabricated pavement panels that can be installed and opened to traffic quickly.

To evaluate this solution, IOWA-DOT constructed: a precast prestressed approach slab demonstration project on Highway 60 near Sheldon, Iowa. 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.
• 2 precast prestressed concrete approach slabs constructed at either ends of a new bridge
• Layout of precast panels selected so it could be adapted to various approach slab configurations of different thicknesses, widths, lengths, and skew angles
• Slabs tied to the bridge abutments as part of an integral approach slab
• Approach slab panels designed to accommodate a 30° bridge skew and a “rooftop” crowned pavement section
• Panels installed over a crushed limestone base using partial-width precast panels in order to simulate lane-by-lane construction for future applications
• Precast panels post-tensioned in both directions after installation on site
• Longitudinal post-tensioning completed from the ends of each approach slab
• Transverse post-tensioning completed from the outside edge of the pavement. Post-tensioning will not only improve 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 beneath it over time due to the mechanisms. 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 in order to minimize the risk of water infiltration into the embankment fill at the abutment.

Home Photo: Temporary post tensioning used to pull panels together