Andrew Giesler, a graduate student in civil engineering, and his team at New Mexico State University are testing ultra-high performance concrete bridge girders on a large scale to aid the development of bridge design procedures for the state of New Mexico that could lead to a variety of improvements to the state’s infrastructure.
The concrete possesses dramatically increased compressive strengths, a very dense microstructure, and steel fibers that greatly improve post-cracking strength. These properties allow for the design of bridges that can have much longer design lives compared to those constructed with normal strength concrete.
Under the direction of Brad Weldon, assistant professor of civil engineering, Giesler conducted three large-scale flexural tests on 16-foot-long, prestressed UHPC bridge girders to evaluate their flexural strength. Graduate student Mark Manning assisted Giesler with the testing and will be conducting similar tests on a full-scale UHPC girder after Giesler graduates in fall 2014.
UHPC was designed to be stronger and more durable than average concrete. The mixture proportions used to create the UHPC Giesler tested was a modified mixture originally developed by a former NMSU graduate student, under the supervision of Craig Newtson, civil engineering professor. The idea was to use primarily local products to produce UHPC, which would drive down the ultimate cost of the final product. With the new UHPC mixture design, Giesler faced the challenges of integrating UHPC production into an industrial setting.
To familiarize local producers with this material, the prestressed UHPC beams were mixed, cast, and cured at Coreslab Structures in Albuquerque, a company that donated a portion of the materials as well as their equipment and labor. The unique mixture design and consistency of this UHPC required careful observation as it went through the batching process. This particular UHPC, made primarily from New Mexico materials, had never been produced on a large scale prior to the casting of these beams.
“We needed to make sure that Coreslab’s facility would be able to accommodate this new concrete. Some of the procedures were new to them, however the entire process went very smoothly and according to plan,” Giesler said.
UHPC has a longer lifespan than average concrete. Whereas normal strength concrete bridges are designed to last approximately 50 years, UHPC bridges have been estimated to have design lives of up to 150 years.
Alongside Ph.D. candidate Jorge Marquez, Giesler designed and erected a structural testing frame that was needed in order to perform the large-scale testing. The testing frame is anchored to the floor and is designed to withstand the resistance created by the beam testing. Parts of the frame were made from recycled bridge girders donated by the U.S. Department of Transportation. Weldon acquired other materials used to build the frame, as well as the actuators used to load the UHPC girders. In addition, monetary assistance was donated by the Associated Contractors of New Mexico for the fabrication and construction of the frame.
Michael McGinnis, a civil engineering associate professor at the University of Texas at Tyler, traveled to NMSU to assist Giesler with a unique form of structural monitoring. McGinnis specializes in digital image correlation. Using the DIC equipment, pictures were taken of a grid drawn on the surface of the beam used to track the formation and propagation of cracks throughout the testing. DIC is capable of covering a large area and can track small changes in the concrete, aiding the equipment used by Giesler. Giesler’s measurement equipment tracked certain areas of the beams in regions prone to cracking as the beams were loaded. DIC served to validate Giesler’s measurements, as well as capture shear behavior closer to the end of the beams near the supports.
The testing was to study the flexural behavior of the UHPC at a large-scale level to evaluate design procedures that can aid in the future development of standardized design codes. Previously, tests had been done on only small-scale rectangular beams. Large-scale tests provide a more realistic representation of how full-scale UHPC beams will behave in a structure such as a bridge.
“Large-scale testing provides much more accurate data. Hopefully, these tests will help to prove that UHPC can be designed both accurately, and efficiently, using simplified methods,” Giesler said.
UHPC is significantly stronger in compression than normal strength concrete, and has strengths exceeding 22,000 pounds per square inch. The compressive strength of average concrete ranges from 4,000 to 6,000 psi. Currently no bridge design specifications for concrete of this strength exist in the United States. The unique material properties, along with the steel fibers that are included in the mixture, are not accounted for in the common design standards. Giesler hopes that through these large-scale tests, he will be able to present data that will aid in the development of new specifications for the design of UHPC bridges.
Jobe Materials donated a high-strength concrete that Giesler and his team used for a cast-in-place bridge deck on one of his three beams. In a real-life scenario, this deck serves as the surface that vehicles drive on. If the beams are designed correctly, the deck and the bridge beam lock together to make the girder stronger.
“We wanted to investigate how a composite lower-strength concrete deck would influence the flexural behavior of a UHPC girder. We wanted to see if it would serve any practical purpose from a structural performance standpoint,” Giesler said.
Right now, the specific UHPC Giesler tested is not being used in any bridges. Giesler said there are commercially available UHPC’s that are being used in a few bridges in the United States, but the UHPC he tested would need to meet specific specifications before it can be used.
“The uniqueness of this UHPC is that it is local to New Mexico,” Giesler said. “If it does make it to a bridge, it will be the first bridge in New Mexico constructed with UHPC. We need to make sure we are confident in our design procedures before UHPC is implemented into a real bridge.”
Giesler received the Daniel P. Jenny Research Fellowship from the Precast/Prestressed Concrete Institute to help fund his graduate work on UHPC. A variety of equipment and materials were also donated to the UHPC project by the NMDOT, BASF, Dayton Superior, El Paso Machine and Steel, Voss Engineering and Bekaert. The NMSU MTECH Lab also provided assistance and equipment for Giesler to fabricate customized testing equipment. Giesler is currently working on his master’s thesis and hopes to defend it in September. After Giesler completes his master’s degree, he plans on working for an engineering firm in the Southwest, where he can design unique and exciting structures and put his knowledge of structural engineering to use.
Information from NMSU