Researchers Use One-of-a-Kind Lab in Quest for Longer, More Durable Bridges
By Adam Jones
Photos by Matthew Wood and Jeff Hanson
A bridge is a bridge is a bridge. Pass under one bridge on the interstate, and you might as well have seen them all. Some are higher or longer, but not different. Right?
States have different design standards, different needs, different soil and geography and different materials to draw from to make concrete. A bridge in Alabama is different from one in Colorado or Georgia.
And, in Alabama, concrete girder bridges – the run-of-the-mill overpasses that dot the landscape – have a maximum span length between supports of 165 feet. That’s not normally a problem, but for some projects a longer span could come in handy, especially since concrete is cheaper and requires less long-term maintenance than the steel used to make longer bridges.
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.
Other states, including Florida, have design standards that allow for longer concrete girders than those in Alabama. But, it’s not as easy as switching to Florida standards.
For one, it would mean the private companies in the state that make concrete girders would have to invest much to change equipment. And, it’s not as simple as ordering bridge girders from Florida manufacturers since transporting long bridge sections is costly.
Engineering researchers at The University of Alabama had an idea. What if girder design was changed, just a bit, to allow the state’s companies to make longer girders using existing equipment?
It sounds simple enough, but there’s a trick. The longer a girder gets, the more likely it is to form cracks at the ends. Concrete often cracks without safety issues, but, as girders get longer, the cracks become more problematic for the long-term durability of the girder. Years down the road, deterioration and corrosion from the cracks can cause costly maintenance issues or even wholesale replacement of the bridge.
The team at UA wanted to see if they could design the girder in such a way that cracking is minimized, even at 185 feet in length – the target they estimated as feasible in Alabama.
“Other states that have longer concrete girders haven’t necessarily solved the cracking issue,” says Dr. Wei Song, a researcher on the team. “Our method aimed to tackle the issue at the root, from the design standpoint.”
The Alabama Department of Transportation funded research to test the idea at UA, which is home to the one-of-a-kind Large Scale Structures Laboratory that can physically test the models run in computer simulations.
Song worked on the project with his UA colleagues, Drs. Sriram Aaleti and Jim Richardson. Also involved were graduate students David Burkhalter and Vidya Sagar Ronanki.
“We wanted to make sure these cracks don’t happen,” Aaleti says. “We have different designs near the end, and we hoped to find the optimal design.”
The two-year project validated the researchers’ idea on design changes, and the team submitted to ALDOT the design plan for the longest concrete span in Alabama. In the process, the UA team created a wealth of data that can be used by civil engineers across the world to improve girder design and fabrication.
A concrete girder is more than just placed and cured concrete. While concrete is strong in compression, it is weak in tension. Steel tendons are placed inside the concrete forms for the girder and are “pre-tensioned” prior to concrete placement. After the new concrete has cured sufficiently, the tendons are cut loose from their external supports, causing the concrete girder to be “pre-stressed” in compression.
Cracks can develop at the ends of girders where stresses are transferred from the pre-stressing strands to the concrete. Since longer girders typically require more pre-stressing strands, longer pre-stressed girders have a higher risk of cracking at the ends.
The research team designed three different versions of the girder end zone after performing computer simulations. They wanted to see if combining two common methods of mitigating cracks would result in longer girders for Alabama.
One method is debonding, placing plastic sheathing around some of the strands at the girder ends. Another method is decreased harping, or lessening the curve of the strands inside the girder.
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 40 strain gauges inside the girder before the concrete was placed, using data from the sensors to detect crack formation once the concrete hardened and the strands were cut from their support beds, transferring the force from the strand to the concrete.
Four 54-foot long concrete girders were created. One was developed with debonding, another with decreased harping, one with both and one without either. They were each brought to the Large Scale Structures Lab and tested with a load of close to 1 million pounds.
It’s testing only the unique capabilities of the Large Scale Structures Lab could provide, Richardson said.
The result of the testing confirmed debonding and decreased harping means contractors in Alabama could fabricate girders at about 185 feet, 20 feet longer than current methods, without changing or upgrading girder fabrication equipment.
“We are using existing forms and technology to increase the maximum length and the durability of Alabama’s pre-stressed concrete girders,” Song said. “We’re talking about thousands of feet of bridges, so, if we can spend less money maintaining them, we’re talking about saving a lot of money.”
Drs. Song and Aaleti are assistant professors and Dr. Richardson is an associate professor, all in UA’s civil, construction and environmental engineering department.
The University of Alabama, the state’s oldest and largest public institution of higher education, is a student-centered research university that draws the best and brightest to an academic community committed to providing a premier undergraduate and graduate education. UA is dedicated to achieving excellence in scholarship, collaboration and intellectual engagement; providing public outreach and service to the state of Alabama and the nation; and nurturing a campus environment that fosters collegiality, respect and inclusivity.