Penn State engineers have developed a new computer-controlled, flexible manufacturing process that promises to make carbon fiber concrete reinforcement grids more competitive with the heavier, corrosion prone, labor-intensive steel rods currently used.
The Penn State team, which includes Dr. Renata S. Engel and Dr. Charles E. Bakis, both associate professors of engineering science and mechanics, and Dr. Thomas E. Boothby, associate professor of architectural engineering, has also developed a computer-based analytical method that simulates the grid's behavior. The simulation can be used as a tool, they say, to tailor the grid's strength and stiffness for use in specific new concrete bridge or building construction or as strengthening material that can be applied to the outside of older damaged structures. The group is currently developing grids for use in box culverts, pre-fabricated concrete structures used as water channels under roadways.
Although the new Penn State grids look somewhat like stiff fishnet or the orange plastic netting often used to section off construction sites, the product is far stronger and stiffer. Bakis notes that carbon fiber itself has a strength of 500,000 lbs. per square inch.
An automated, computer-controlled machine at Penn State's Composites Manufacturing Technology Center (CMTC) weaves the new grids from a continuous one-eighth inch wide carbon fiber bundle impregnated with liquid polymer resins which is laid down in a flexible mold. The grid is then pressed and heated to harden it.
The weaving pattern, the Penn State engineers say, is the key to the grid's stiffness and strength. For example, if the grid is woven so that the fibers cross each other perpendicularly at the 90-degree angle junctures, called joints, it has different characteristics than if the fibers follow an L-shaped path at the joints. This L-shaped path, developed by the Penn State researchers, forms a pattern called the staircase.
Bakis notes that, when in use as a re-inforcement, the grid bonds at the joints to the concrete in which it is embedded. The researchers have shown that the grid's bond stiffness with concrete can be increased by 50 percent when it is woven using the staircase pattern. However, the longitudinal stiffness is decreased by about 25 percent.
Bakis, who is CMTC director, notes that the team worked on controlling and modifying stiffness first because it affects the "bounce" that a reinforced concrete structure will have in response to a load. Motorists traveling over a concrete bridge, for example, usually prefer less bounce and more stiffness.
Using the team's new simulation method coupled with their flexible manufacturing process, they can tailor the weave, by combining the staircase and cross patterns, to meet special needs for any particular application, he says.
The manufacturing process can easily be scaled up. "The team has made a special effort to develop a process that can be easily adopted by industry," he says.
While carbon fiber grids are probably more expensive when compared to equivalent steel reinforcing rods, Engel points out that they have several other advantages. For example, steel reinforcing rods must be tied, by hand, into a grid pattern at the site. The carbon fiber grids need no assembly. The lightweight carbon fiber grids can also be handled easily without heavy equipment. In addition, the grids offer the economy and reliability of mass-produced, pre-fabricated materials.
Best of all, says Engel, the grids will not corrode as do steel rods and, for that reason, they promise to lengthen the life of the bridges and other structures they reinforce.
The Penn State team has detailed their latest results in a paper, "Analysis and Design of CFRP Grids for Reinforced Concrete." The authors are Bakis, Suresh Pannala, a master's degree student, Engel and Boothby. The paper was published in the Proceedings of the 12th International Conference on Composite Materials, held July 5-9, 1999 in Paris, France. The research is supported, in part, by the National Science Foundation.
Editors: Dr. Engel is at rse1@psu.edu or Bakis at ceb5@psu.edu or at 814-865-3178.
Contacts:
Barbara Hale
814-865-9481 (o)/ 814-238-0997 (h)
bah@psu.edu