How to make longer, stronger concrete girders

Engineering researchers at the University of Alabama, Tuscaloosa, Alabama, are testing massive concrete girders in a campus laboratory to find ways for bridges to span longer distances with fewer supports underneath.

Longer, more durable concrete girders would mean fewer support structures underneath the bridge, and that could lead to lower construction costs.

Besides cost saving, longer spans would mean fewer disturbances over water, wetlands or other natural habitats. In urban areas, such as interstates that sit over city streets, fewer supports would mean less disruption of traffic and business below.

The two-year project with the Alabama Department of Transportation should end with recommendations on how to design and build concrete girders that can span 180 feet, about 15 feet longer than the longest concrete girders currently used in the state.

Bridges presently needing longer spans use different designs and more expensive materials than concrete girders.

The project is focused on small cracks that appear at the end of concrete girders soon after fabrication. Concrete often cracks without safety issues, but, as girders get longer, the cracks become more problematic for the long-term durability of the girder, said Dr Wei Song, UA assistant professor of civil, construction and environmental engineering.

Song works on the project with Dr Sriram Aaleti, assistant professor, and Dr Jim Richardson, associate professor, both in UA’s civil, construction and environmental engineering department. Also involved are graduate students David Burkhalter and Vidya Sagar Ronanki.

“We want to make sure these cracks don’t happen,” Aaleti said. “We have different designs near the end, and we hope to find the optimal design.”

The research team designed three different versions of the girder end zone after performing computer simulations. They then worked with Hanson Pipe and Precast, an international building products company and one of the largest manufacturers of concrete products in North America, to fabricate the concrete girders at a facility near Birmingham.

The UA team positioned sensors inside the girder before the concrete was placed. Data from the sensors will help the research team determine which girder end zone detail performs best. Four 54-foot long concrete girders – one is a control girder with a traditional design – will be brought to the Large Scale Structures Lab on campus and tested with a load of close to 1 million pounds.

The first girder was delivered earlier this semester, and it is already attached to testing equipment. A load jack will bear down on the girder until the concrete begins to crack. From there, the sensors on the inside and outside of the girder will give the team data on how the girder handled the load.

The tests should help ALDOT and its contractors design and fabricate longer girders that do not crack.

“You want to improve your design from day one and reduce the amount of cracks so they can last longer, and, as a result, spend less money maintaining them,” Song said.

Pictured left, graduate students David Burkhalter, left, and Vidya Sagah Ronanki, both in civil, construction and environmental engineering, work on a 54-foot concrete girder in the Large Structures Lab on UA’s campus.

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