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Bridge ’Speaks’ Through Fiber Optic Technology

Sat December 30, 2000 - West Edition
Mark A. Horner



New Mexicans won’t hesitate to let you know that Bill Gates actually founded Microsoft in Albuquerque in the 1970s. Gates soon took his company to the state of Washington, but the pioneering spirit for innovative technology continues to thrive in New Mexico. Take, for example, a one-of-a-kind bridge being built about 15-mi. (24 km) west of Albuquerque over an often-dusty riverbed called the Rio Puerco. Fiber optic sensors are embedded inside four of the structure’s 12 precast girders. The sensors allow the bridge to essentially “talk” to researchers and reveal specific secrets concerning the impact of stress on its concrete.

“It’s going to be fascinating,” said State Concrete Engineer Bryce Simons of the New Mexico State Highway and Transportation Department.

“I think the monitoring system is the way to the future,” added the project’s principal investigator, Rola Idriss. Idriss is a professor in the civil engineering department at New Mexico State University in Las Cruces. Already, members of her team regularly visit the new bridge. The visiting researchers’ interest is in a computer located just beneath the deck. A quick download provides the history of almost every stress and strain that has impacted this structure.

Idriss has been using fiber optics to monitor the effects of stress on bridges for years. But this is the first time that fiber optics have actually been placed inside a bridge during its construction.

“Absolutely,” stated Simons. “Because the data we wanted to collect is data from absolute ’time-zero,’ when the concrete was still wet and fluid.”

Last summer, workers at CSR’s Albuquerque precast concrete plant labored under a high desert sun. Not unusual. But this concrete — and the project itself—was anything but typical.

“It’s tackier. Sticky,” responded Bill Lujan, quality control worker of CSR, when asked to recall what it was like to work with high strength concrete for two-and-a-half weeks last summer.

“It’s like oatmeal,” added CSR plant engineer Steve Ruiz.

To make matters even more challenging, fiber optic sensors were embedded in the mix as it was steam-cured. CSR production manager Paul Jensen was a bit more direct than his colleagues. “I had never seen anything like it before. My impression? It was a pain in the butt,” Jensen laughed. “To do the whole job, to do the 12 pieces, it took us two weeks. Normally, we’d do 12 pieces in a week.”

However, everyone at CSR sees the potential for profit. “The high-strength concrete gives us the potential to do larger, longer beams and to compete with the steel industry. In some areas, that wasn’t possible prior to this,” stated CSR general manager Loren Later. Along with Later’s sense of optimism, investors expect results and that’s why people in various parts of the world are keeping tabs on the digital data streaming on the Rio Puerco bridge project.

Simons remarked that if one were to walk across this new bridge while bouncing a basketball, the sensors detect it. He added that the computer would even recognize and store two types of data: The relatively slow transference of energy that occurs as footsteps land on the concrete, as well as the lift, and sudden impact of the bouncing ball.

Scientists, however, will be more interested in measuring the strain caused by a big-rig hauling a large load over the bridge than a bouncing ball. What we’re going to be able to measure is any kind of a permanent bending,” explained Simons. “For instance, if a truck pushes down and bends [the prestressed bands within the girders] and it doesn’t bounce back 100 percent, if they bend a little bit, we’re going to be able to see that … we’re going to be able to see what kind of fatigue considerations are active on these bridges.”

Simons said the Rio Puerco bridge project also will advance a standard scientific principal. “We never really had a handle on how much concrete moves as the temperature changes … how much the bridge moves as the temperature changes … . Now, all of a sudden with this technology, we’re going to be able to measure very, very, accurately, not only for a single beam, but on an overall system approach. We’re going to be able to measure what the thermal coefficient for this bridge is. So, now we can begin to determine from wintertime to summertime, from the day’s high temperature to the day’s low, what kind of movement we can expect. And with that kind of information we can improve our design requirements and tolerances to accommodate that.”

Expectations are that the bridge will fatigue later, rather than sooner. “Absolutely,” affirmed Idriss. “And you have significant savings because you have a more durable concrete. And the fact that it is high strength allows you to use fewer girders and choose a longer span for the same girder. You’re going to economize on the actual construction cost.”

“I’ve been around bridges a lot of years and I’ve never seen a bridge deck that hasn’t cracked. And this one has not cracked. It doesn’t have a single hairline crack on it,” revealed a pleasantly surprised Manuel Gamboa, job superintendent, A.S. Horner Construction.

This quiet bridge on a frontage road along Interstate-40 has at least one other state asking, “what if…?” A representative expressed concern that bridges in the San Francisco area might be developing cracks and classic hinges underwater. “I told them that would be an excellent application for these sensors,” said Idriss.