June 29, 2005

Full-Scale Testing at NC State’s Constructed Facilities Laboratory Demonstrates Advantages of Advanced Bridge Materials

Matt Boylan (far left background) watches fellow graduate students Greg Lucier (foreground) and Hatem Seliem deliver concrete to the bridge form with a one-yard bucket. (Photo: Engin Reis)

The American Society of Civil Engineers (ASCE) in their Infrastructure Report Card 2005 (www.asce.org/reportcard/2005/) reported that one in three urban bridges was classified as structurally deficient or functionally obsolete. Across the nation deteriorating older structures need to be upgraded or replaced, which is costly.

The Large Structural Systems Laboratory, which is part of the Constructed Facilities Laboratory (CFL) at North Carolina State University, is engaged in a number of full-scale tests of new materials that will help reduce the cost and duration of bridge repairs. Two projects, in particular, illustrate the exciting new technology. One project involves the testing of a new steel called Microcomposite Multistructural Formable (MMFX) steel, and the other involves the strengthening of old bridge girders with fiber reinforced polymers (FRPs).

In many ways MMFX steel is similar to black steel that is used to reinforce traditional concrete structures. Unlike black steel, the microstructure of the MMFX steel has been altered so that it is highly resistant to corrosion. In addition, MMFX steel is roughly twice as strong as conventional steel in many applications. According to Dr. Sami Rizkalla, Distinguished Professor of Civil Engineering and Construction and Director of the Constructed Facilities Laboratory in the Department of Civil, Construction, and Environmental Engineering at North Carolina State University, concrete buildings and bridges reinforced with traditional steel absorb water, which corrodes the steel over time. “Concrete is like a sponge,” he said. “When the steel in concrete corrodes, it expands. When it expands, it cracks the concrete, and we end up with problems that translate into billions of dollars of deteriorated bridges and structures.”

Because the CFL is one of only four public labs in the United States accredited by International Accreditation Service, Inc. (IAS) to perform compliance testing for the International Code Council (ICC), the MMFX Steel Corporation asked the CFL to help them develop provisions and design guidelines for using MMFX steel as a reinforcement in concrete structures. At approximately the same time, the North Carolina Department of Transportation (NCDOT) commissioned the CFL to determine the safety factor of a concrete bridge deck reinforced with MMFX steel.

The NCDOT recently used MMFX steel to build a bridge deck in Johnston County. Because the material was new, they had designed the bridge in a conservative way: they had selected MMFX steel purely for its corrosion resistance, not for its strength. However, because they were interested in taking advantage of MMFX steel’s increased strength in future projects, the NCDOT commissioned the CFL to build a full-scale section of the same bridge and test it to failure so that the strength and, consequently, the safety of an MMFX-steel-reinforced bridge deck could be determined.

The CFL is in the process of testing three of these full-scale bridge sections to evaluate the structural performance of MMFX-steel-reinforced concrete bridge decks, study the effects of using different reinforcement ratios for MMFX-steel-reinforced concrete bridge decks, compare the behavior to conventionally reinforced concrete bridge decks and develop standards for MMFX-steel-reinforced concrete bridge decks.

The first test has been completed. Rizkalla reported, “We found out that the safety factor in NCDOT’s conservative design is a range of 10 times.” The MMFX-steel-reinforced bridge deck designed and built by NCDOT is capable of carrying a load 10 times greater than what was expected. According to Rizkalla, in future designs the NCDOT will be able to use less MMFX steel to obtain required strength, which will be more economical.

The second test has just been completed, and the results are not available yet. In that test the CFL built to scale and tested to failure a full-scale section of a typical bridge deck — one that had not been reinforced with MMFX steel. The CFL will compare the performance of this second test deck to the performance of the first test deck that had been reinforced with MMFX steel.

In the final test that remains, the CFL will build to scale and test to failure a full-scale section of a deck reinforced with only 60 percent of the MMFX steel used in the first test deck. This test and others will be available for public viewing by way of webcam at www.ce.ncsu.edu/centers/cfl/Webcam/.

The second project involves carbon fiber reinforced polymer (CFRP). The NCDOT has been looking at methods to strengthen and repair existing bridges quickly and effectively. According to Rizkalla, “Some of the existing bridges built 40 to 50 years ago had been designed for truckloads weighing less than what we have now and need to be strengthened to carry the new loads. In other cases, bridges damaged by impact or corrosion need to be repaired.”

For the past 15 years, Rizkalla’s research has been in the field of advanced composite materials with focus on the use of FRP materials for new construction and for the rehabilitation, strengthening and repair of bridges and structures. He is recognized as an expert in the field and was recently named Fellow by the International Institute for FRP in Construction. According to Rizkalla, advanced composite materials are “glass or carbon fibers that can be used as construction material. They are very strong at approximately 6 times the strength of steel and very light at 20 percent the weight of steel.” “They also can be installed quickly,” he added.

The NCDOT commissioned the CFL to evaluate the cost effectiveness and value engineering aspects of using CFRP systems to strengthen or repair bridge girders to extend their service lives by increasing their load-carrying capacity. As part of the project, the NCDOT provided 43-year-old bridge girders for testing. The CFL’s plan called for the testing of sixteen of the 30-ft-long and three of the 54-ft-long prestressed bridge girders strengthened with various CFRP systems. A special adhesive is being used to bond the CFRP to the concrete.

To date the CFL has completed the testing of 17 bridge girders reinforced with CFRP. “We have succeeded in increasing their capacity by 60 percent through the special attachment of this material to existing girders,” Rizkalla reported. “To repair a bridge with this method takes no more than a week. While in Canada, I was involved in repairing several bridges with this technique.”

The MMFX steel and CFRP projects are just two of the many bridge and other large structure projects being performed in the CFL’s Large Structural Systems Laboratory. The lab’s state-of-the-art testing apparati allow professors, staff and graduate students to test advanced materials and large-scale structural systems. The applied research performed in this lab — in this instance, the testing of new materials to strengthen and extend the life of bridges — puts the researchers in a desirable position of directly affecting the lives of Americans.

— mcblief —

Please also see story at www.ncsu.edu/news/press_releases/05_08/157.htm.

The graphic provides an overview of the test setup for evaluating MMFX steel in NCDOT bridges. (Illustration: Hatem Seliem)		Concrete slab is ready for testing under two hydraulic actuators. (Photo: Greg Lucier)		An hydraulic actuator is used to test a 43-year-old bridge girder strengthened with CFRP. (Photo: Owen Rosenboom)

Media contact:
Kathi McBlief, (919) 515-2283, mcblief@eos.ncsu.edu

Technical contact:
Dr. Sami Rizkalla, (919) 513-1733, sami_rizkalla@ncsu.edu
Lee Nelson, (919) 513-4793, lee_nelson@ncsu.edu

/ News Index / News Archives Index /