Nearly half of U.S. bridges are nearing the ends of their projected life cycles. An estimated quarter of U.S. bridges are deficient in some fashion. That makes construction of new bridges that are safer, more resilient, and less expensive a high priority for transportation departments all over the country.
Three centers at the Institute for Transportation at Iowa State University are helping Iowans make critical progress on all three bridge building fronts. These units partnered with the Iowa Department of Transportation (DOT) and Buchanan County in east central Iowa to carry out a Federal Highway Administration-funded program grant aimed at fostering innovative concepts in bridge construction. ISU’s Bridge Engineering Center (BEC), the National Concrete Pavement Technology (CP Tech) Center, and the Center for Earthworks Engineering Research (CEER) played a part in accomplishing the project goals.
Buchanan bridge features
An aged timber bridge spanning Prairie Creek in Buchanan County was replaced in 2014 with a cast-on-site adjacent box beam bridge, fabricated from a high-performance concrete mixture made with light-weight fine aggregate to improve internal curing. The bridge abutments used a new type of reinforced soil and featured testing of a vertical drain to help during severe rain events.
Brent Phares, director of the Bridge Engineering Center, said, “The construction of this rural bridge allowed us to work on two key bridge design elements. We assisted Buchanan County with design of the geosynthetic-reinforced soil (GRS) abutments and helped create the superstructure design with the Iowa Office of Bridges and Structures.”
The Buchanan bridge project was part of the Innovative Bridge Research and Deployment (IBRD) program sponsored by the Federal Highway Administration that encourages the use of inventive new practices in the bridge industry. A key part of this effort was devising a way to construct the bridge without the use of a traditional overhead crane. This offers advantages to Iowa counties where bridge projects often are carried out by county road staff who may not have ready access to a crane. (The project showed that using backhoes may be the optimum way to lift the cast-on-site beams onto the bridge span.)
Phares pointed out that the construction of beams on-site followed by moving them over the abutments saved considerable construction time. He noted, “Motorists will be inconvenienced for a much shorter period of time when this tactic is used, leading to less traffic disruption in the area.”
New bridge passes tests
The ISU transportation research units were involved in testing and evaluation of the bridge throughout the project. They conducted tests on the concrete mix used for the beams, the elements of abutment construction, and the placement of the prefabricated superstructure on the GRS abutments. When the bridge was completed, they continued to monitor its performance for three years (2014, 2015, and 2016) via live load tests and other data collection mechanisms.
Researchers found that the tests showed the bridge joints were well connected and performed well. The inclusion of a vertical sheet drain in one of the bridge abutments yielded improved drainage conditions over the abutment without a vertical sheet drain. This information could prove valuable in the future if rainfall events worsen.
Phares says they would like to see the cast-on-site beams deployed and tested for a longer span or on a multiple-span bridge to see how widely the technique might be applied with success. Further investigation of the optimum backfill materials, with emphasis on the types regularly used in Iowa, could yield benefits for bridge composition in the future.
A report on the tests performed on the bridge by the ISU research group can be seen here: http://www.intrans.iastate.edu/research/documents/research-reports/Victor_Ave_over_Prairie_Creek_IBRD_bridge_eval_w_cvr.pdf