Lightweight high-performance concrete (LWHPC) can significantly reduce the cost of constructing bridges while increasing load capacity. According to Benjamin A. Graybeal, Ph.D., P.E., who heads the Federal Highway Administration's (FHWA's) Structural Concrete Research Program, LWHPC can be just as strong as conventional concrete. “You just need to appropriately design the mix to achieve the desired properties,” he says.
Aggregates—such as shale, slate, and clay with particular chemical compositions—are essential elements of LWHPC. When these aggregates are heated, gasses inside them expand as a result of a chemical reaction. This expansion allows voids, or small holes, to form inside the aggregates, which become lighter and are later crushed for use as an aggregate in LWHPC.
“The type of aggregate used in the LWHPC mix impacts the performance of the concrete. Obviously, lighter rocks make the final concrete lighter. Since the aggregates make up a larger proportion of the overall mix, stronger and/or stiffer rocks also can have a significant impact on the performance of the mix.”
For the past 5 years, Graybeal has been working on a project aimed at determining the behavior of LWHPC, particularly in regard to the gap of equilibrium densities ranging from conventional lightweight to normal weight concrete. “There is a very specific issue that we’re addressing through traditional structural testing, where we build the components and break them to look at basic structural behaviors like bending and shear and the development of rebar,” he says. “So it’s somewhat straightforward, but it’s looking at a newer material, a newer version of concrete.”
Recently the Expanded Shale, Clay and Slate Institute (ESCSI) recognized Graybeal and his colleague, Gary Greene, Jr., Ph.D., P.E., a project engineer at Professional Service Industries and research contractor at TFHRC, for leadership and dedication in advancing comprehensive research on LWHPC girders. At its midyear meeting held May 9–13, 2011, in Birmingham, AL, ESCSI presented Graybeal and Greene with its prestigious Frank G. Erskine Award, an honor that celebrates individuals, companies, associations, and partnerships—outside the expanded shale, clay, and slate (ESCS) industry—who recognize the unique properties of ESCS and have demonstrated its use through design, promotion, or implementation.
“The research that Drs. Graybeal and Greene are conducting at TFHRC is significant for our industry because it is providing a tremendous amount of data on bond strength, development length, shear performance and prestress losses for high performance lightweight concrete that we have not had previously,” ESCSI Structural Committee Chairman Ken Harmon, P.E., remarked in a press release. “This should give bridge engineers confidence to design prestressed bridge girders with high strength lightweight concrete.”
LWHPC can reduce the dead load carried by substructures and superstructures, and because the concrete members are lighter, transportation and erection costs will decrease. “It’s not that the concrete itself is less expensive,” points out Graybeal. “It’s that the structural design becomes simpler, so the whole project becomes less expensive. If you can make the bridge lighter, it can save you money; you can make your beams smaller because they don’t have to carry as much load.”
Perhaps more importantly, you can make the substructure smaller. Foundation design is largely dependent on the dead weight of the structure. “So if you can make the structure lighter, it can have a big impact on how many piles you have to drive, or how big your spread footings need to be,” Graybeal adds.
Another incentive for utilizing LWHPC relates to span extension and logistics. Bridge girders are often transported across bridges that lead to the project site, so they can’t be too heavy. “If those bridges can’t carry the load because the pieces are too heavy, then you can’t use them, which means you need to use smaller pieces, potentially leading to more substructure elements and shorter spans,” explains Graybeal.
LWHPC has the potential of making bridge pieces 15 percent lighter and 10 percent longer than similar applications of conventional concrete. “In some cases this could mean the difference between a one-span bridge and a two-span bridge, or a two-span bridge and a three-span bridge, which can be an enormous cost-saver because you’re eliminating a substructure,” Graybeal recognizes.
The American Association of State Highway and Transportation Officials (AASHTO) and numerous concrete specialists are eager to see the results of the study, which could lead to code provision changes in the load and resistance factor design (LRFD) for bridges, specifically the AASHTO LRFD Bridge Design Specifications.
“It’s become a rather high profile project. We’ve been working on it since 2006, and people are really interested in the results,” says Graybeal. “We frequently provide updates, but because there are so many tests involved, it just can’t happen quickly. The upside is that when we’re done we will have completed 96 full-scale structural tests and will have thoroughly demonstrated a wide range of critical structural behaviors.”