Aging Infrastructure Requires a Better Understanding of the Long-Term Behavior of Bridge Girders

Long-term behavior of prestressed concrete bridge girders the focus of a research project conducted at The University of Texas at Austin


Newswise — When looking at a typical prestressed concrete bridge girder, there may not be any visually noticeable deflections from traffic or any other loading, but this does not tell the entire story of the girder. What may not be observed by the casual observer is evident to and accounted for by the engineer who designed that girder. One of the reasons for this discrepancy is that most of these deformations occur slowly over time. While the casual observer may not be able to perceive any of these deformations, the long-term durability of a structure is greatly affected by them.

“Long-term durability is a major issue for today’s infrastructure. In order to create concrete bridges with longer service lives and better performance, we must better understand the long-term behavior of these members,” said David Garber, PhD, (researcher on project and now an Assistant Professor at Florida International University).

Prestressed concrete bridge girders rely on a pre-compression force applied by tensioned steel strands for good durability performance and long life. The effectiveness of this pre-compression force will naturally decrease over time (due to the material properties of both steel and concrete) and must be accounted for at the time of initial construction by the design engineer. The loss in effectiveness of the pre-compression force is often called prestress loss. Underestimating the prestress loss can lead to cracked girders with shorter life-spans; overestimating can lead to wasted material and other constructability issues.

A better understanding of the long-term behavior of prestressed concrete bridge girders was the focus of a research project conducted at The University of Texas at Austin under the leadership of Oguzhan Bayrak, PhD (Professor at The University of Texas at Austin) and also assisted by José Gallardo (Adjunt Professor at Technological University of Panama), and Dean Deschenes (Laboratory Manager at Simpson Gumpertz & Heger).

In order to gain a better understanding of this long-term behavior, an experimental study was conducted in which 30 full-scale bridge girders were constructed and instrumented. As the long-term behavior is greatly influenced by the materials and construction techniques used, several different precast fabrication plants were utilized for construction. The constructed girders were then shipped from the plants to several different environmental exposure sites and stored there for up to three years. During this period, the behavior of these girders was measured using vibrating wire strain gauges embedded in the test specimens.

Since bridge girders do not just sit around when they are actually in use, the next focus of the research program was to load and test them at the Ferguson Structural Engineering Laboratory (FSEL) at UT. “Strain monitoring in the field can only get us so far,” said Garber, “In order to truly understand the behavior of an aging girder, structural testing should be conducted in a controlled, laboratory setting.” With this in mind, all 30 of the bridge girders were shipped back to FSEL and were flexural service load tested.

Since bridge girders do not just sit around when they are actually in use, the next focus of the research program was to load and test them at the Ferguson Structural Engineering Laboratory (FSEL) at UT. “Strain monitoring in the field can only get us so far,” said Garber, “In order to truly understand the behavior of an aging girder, structural testing should be conducted in a controlled, laboratory setting.” With this in mind, all 30 of the bridge girders were shipped back to FSEL and were flexural service load tested.

“A designer comes into a project typically knowing where the bridge will be built, but generally not knowing how it will be built,” says Garber. “He or she is then only able to specify the concrete strength to the fabricator, which is not sufficient for adequately estimating the long-term behavior of a structure with a high level of confidence.”

The next step for the research team will be to work on determining ways to improve the methods for designers to better predict and estimate the long-term behavior of prestressed concrete girders. This will hopefully lead to girders with better performance and longer design lives.

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