Better Bridges

Civil Engineers Test New Concrete for Stronger, More Durable Bridges

 A new kind of concrete called Ductal will allow bridges to hold more weight and last longer. Made of a mixture of sand, cement, water, and small steel fibers, it is 10 times more expensive than traditional materials but also stronger and virtually impermeable, helping bridges become more durable.

AMES, Iowa--Bridges take a beating, and it can really break the bank to repair them. Now, researchers are breaking bridges to learn how to build them better and save you money.

Justin Doornink spends his mornings underneath bridges. He's an engineering student and, as part of his homework, he's installing sensors to measure the impact of traffic on the bridge. He's trying to figure out how to strengthen the structures. One option is ultra-high-performance concrete, which is made from sand, cement, water and small steel fibers.

Brent Phares, a civil engineer and associate director at the Iowa State University Bridge Engineering Center in Ames, says, "It's much, much stronger. It's basically impermeable to water. What those two things mean is you can build a bridge that has a higher capacity and should last a longer period of time."

Brent did a small-scale test with the new concrete, pushing it to its breaking point. It held close to 595,000 pounds -- that's more than seven semi trucks. The material costs 10-times as much as traditional concrete, but you need less of it, and it lasts longer.

"You're never going to advance the state-of-the-art unless you do some research, try some things out, maybe take some risks and see what might ultimately save the taxpayers money," he says.

BACKGROUND: Engineers at Iowa State University have developed a new type of concrete that is much stronger than conventional concrete. It can withstand pressures up to 595,000 pounds -- more than the weight of seven semi trucks.

 LOAD-BEARING BRIDGES: The researchers conducted a load-bearing capacity test using a 71-foot beam made out the new concrete. They applied increasing amounts of hydraulic pressure to the top of the beam to see how much it could withstand before breaking. It finally broke with a loud pop at 595,000 pounds. The ultra-high performance concrete is made from sand, cement, water and small steel fibers. Standard concrete uses coarser materials. The new concrete is specifically engineered to include finer materials and steel fibers, making it denser and stronger.

WHY THE BEAM BROKE: Isaac Newton said it best: for every action there is an equal and opposite reaction. As the hydraulic pressure on the beam increases, the beam responds by exerting an equal but opposite counter-force. But it doesn't do so uniformly: certain areas bear the brunt of the increasing pressure. This produces a strain on the beam, which eventually becomes too great, and the beam cracks.

DIFFERENT DEFORMATIONS: Different materials can withstand different amounts of deformation, a property known as elasticity. Most materials are elastic to some degree: when they are deformed or bent, they will bounce back to their original shape. But elastic materials all have their limits. Metal springs and rubber bands are very elastic. Plaster and glass are not; instead, they are brittle and snap even with a small deformation