Taking Precast Concrete to the Limit

A new fire/loading chamber can test precast concrete beams at temperatures of burning jet fuel, similar to the catastrophic conditions in New York’s Twin Towers on Sept. 11, 2001.

Pondering the potential of cutting-edge research for the precast industry poses several tantalizing questions: Can new materials make precast concrete fire and impact proof? Can precast concrete withstand the extreme heat of a jet-fuel fire? Will carbon-fiber reinforcing inside a precast concrete beam sustain 2,300 degrees F (1,260 C)? Will precast structures made with innovative materials remain serviceable in fire or ice? Can bridge service life be doubled?

Researchers hope to find the answers to these and many other civil engineering questions with a new high-heat and impact testing chamber. In 2006, the Center for Innovative Materials Research (CIMR) was built at Lawrence Technological University, Southfield, Mich., utilizing an $11 million cooperative research agreement with the U.S. Army Research Laboratory (ARL) and the U.S. Army Tank-Automotive Research, Development and Engineering Center (TARDEC). The new fire/loading chamber was completed in 2008, and a new environmental/loading test chamber will also be complete later this year.

Fire/loading chamber accommodates full-scale structures

The U.S. military must test vehicle structural components to evaluate vehicle structural integrity under various extreme environmental and loading conditions, including fire, impact and high-heat exposure. Lawrence Tech has been awarded $1.8 million in federal defense appropriations to build a fire/loading chamber for testing vehicle components and bridge elements for various uses.

To a visitor, the extraordinarily large dimensions of CIMR facility and the fire chamber are the first clues that this is not a typical structural testing lab. Its ceiling is 30 feet high (9.1 meters), and the 7,200-square-foot (670-square-meter) center boasts a 50,000-pound-capacity (22,680-kilogram) gantry crane that can accommodate testing of full-scale structural components such as portable battlefield bridges up to 50 feet (15 meters) in length. A full-sized military Humvee (FIGURE 1) could be loaded into the fire chamber.

Thermal-insulating ceramic blocks and insulating blankets were used to build the testing chamber; the interior dimensions of the furnace are 22 feet long, 10.5 feet wide and 8.5 feet high (3.1 by 2.6 by 6.7 meters). The chamber sits on an insulated, reinforced concrete foundation that is 7 feet (2 meters) deep (see Figure 2). The deep concrete foundation supports a massive steel superstructure that holds an impact MTS actuator that can deliver static or dynamic loads with impact force up to 150,000 pounds (68,000 kilograms) using a 160-gallon-per-minute (606-liter-per-minute), closed-loop hydraulic system. This means that a structure can be simultaneously subjected to combined conditions of extreme heat or fire and impact or repeated loading.

With this chamber, researchers can simulate extreme cases of potential fire and loading conditions, including fire events that may occur in residential, commercial or military buildings. They also can simulate the effects of missile and bomb attacks on military vehicles and shields.

Simulating arctic or desert conditions

According to CIMR director and chair of the civil engineering department, University Distinguished Professor Nabil Grace, “The environmental/loading chamber presently under construction will be able to simulate the ability of vehicle components and structural components to handle loads in different conditions, from freezing to dry heat, salt spray, rain, UV light or 100 percent humidity.”

The environmental chamber’s actuator can impose a sustained static load like that of dead loading on a bridge as well as dynamic or repeated impact loading to a structure under extreme conditions (see Grace’s article on carbon fiber reinforcing materials for precast, prestressed concrete structures in the September-October 2008 issue of Precast Solutions magazine). With the soon-to-be-completed environmental chamber, CIMR can reproduce extreme conditions such as those that occur in Antarctica or Iraq.

“We need to have a testing chamber with the potential to create extreme conditions, because military testing standards are much more rigorous than industry standards,” explains Mena Bebawy, a Ph.D. candidate and research fellow at LTU. In fall 2008, he explained current research initiatives. “Right now we are testing a newly patented fire-resistant, carbon-fiber fabric used to increase the load-carrying capacity of precast and reinforced concrete beams.”

Another research initiative at CIMR is the testing of 7,000 psi precast American Association of State and Highway Transportation Officials (AASHTO) I- and T-beams reinforced with a patented ductile-hybrid CFRP fabric. Photos show the results after a one-third scale AASHTO beam with CFRP reinforcing bars that was heated to 2,300 F (1,260 C) in the fire chamber for eight hours of continuous incineration (see Figure 3). This maximum testing temperature is chosen because 2,300 F is the maximum heat exposure that could be generated by any organic fire, from jet fuel to incendiary devices. In other words, the new fire chamber can take precast concrete to the most extreme limit of fire exposure possible.

The U.S. Army’s TARDEC is currently working with the university on research that measures the capabilities of armored structures for mine resistance. “We are collaborating with the ARL and TARDEC to test applications for carbon fiber in a structural-fabric frame,” Grace says. “Using the new test chambers at CIMR, we are part of important research to develop materials that will protect our troops and vehicles, and it is likely that our findings will find applications in commercial and consumer applications.”

Research provides real-world applications for precast industry

While research into innovative materials is of critical importance to the military, civil projects and America’s transportation infrastructure will likely demonstrate the greatest potential of these materials. Here are some anticipated results:

• Ensure building safety. Testing in CIMR’s new environmental chamber will have a direct impact on precast concrete bridge designs. The fire chamber can expose structural components to 2,300 F (using ASTM test standard E119), which approximates the catastrophic conditions at New York City’s World Trade Center on 9/11. Because the organic epoxy matrix in typical CFRP material starts to melt at approximately 150 F (66 C) and ignites at 300 F (149 C), it isn’t long before this material’s matrix is lost under elevated-heat conditions. Once the epoxy matrix is compromised, the carbon fibers lose their structural integrity. The ongoing research in CIMR is dedicated to developing a fire-resistant CFRP material system that can tolerate high temperatures for several hours. This project is currently funded under a contract with the US-DOT and Michigan Economical Development Corporation (MEDC). For more details, visit www.NabilGrace.com.
• Provide permanent corrosion resistance. New reinforcing materials like carbon, glass and aramid fiber-reinforced polymers (FRPs) can replace steel in precast concrete bridges and eliminate corrosion that can lead to structural deterioration. These materials have been tested extensively at Lawrence Tech (see Figure 4) with funding from the U.S. Army, the U.S. Department of Transportation, National Science Foundation and Michigan Department of Transportation. Once these new reinforcing materials have been thoroughly researched and documented to be effective over time, transportation agencies can begin to build more durable bridges with significantly longer service life.
• Double service life. “Corrosion damage caused by deicing salts and other chemicals in concrete bridges can be virtually eliminated by replacing the steel with carbon fiber,” says Grace, “and the use of these innovative materials can double the lifespan of precast bridges in regions that experience harsh environmental conditions.” In CIMR, precast concrete bridge beams 50 feet (15 meters) in length can be tested under both static and dynamic loading up to 1 million pounds of force. Doubling the service life of America’s bridge infrastructure would be an outcome of tremendous value to both state and federal transportation agencies and to U.S. taxpayers.
• Convert existing structures for heavier loads. Bebawy says that one of the practical applications of research on the patented ductile hybrid CFRP fabric materials would be the retrofitting of existing precast structures. “If an office building were to be converted to a library with much higher loading, or if a bridge were redesigned to sustain the load of heavier traffic,” Bebawy says, “this newly developed ductile hybrid fabric could be applied to the underside of the precast beams to increase moment-carrying capacity of the structure.” One ply of this fabric can increase the capacity of a precast beam by 30 percent, two plies can increase the strength by 60 percent, and so on.
• Demonstrate green building potential. The great advantage of this new technology is not only in increasing structural capacity; anything the precast industry can do to help maximize use of material resources and decrease construction waste demonstrates the ability of the industry to contribute to green construction. We already know that a precast concrete structure offers exceptional durability, quality and aesthetic appeal. The precast industry can also boast the flexibility and environmental friendliness of precast buildings to be retrofitted to serve a totally new function without demolition and new construction. This would be of great competitive advantage in a market that is increasingly concerned about saving energy and resources.
• Decrease precast labor costs. Photos show the remarkable light weight of non-steel reinforcing cages. Fewer iron workers are required to fabricate and move reinforcing systems in a precast concrete facility. Plastic ties or common nylon-coated wires can be used to assemble CFRP reinforcing cages (see Figure 5). Plastic ties can be applied manually or with a tie gun.

Cost and special handling considerations for innovative materials

The cost of non-steel reinforcement can be four to five times more expensive than reinforcing steel. With the current steel market crisis hampering precast concrete bidding and production, the tremendous inflation in steel rebar prices and lack of availability are serving to modify price differences between the two materials.

Epoxy adhesive used in applications of CFRP fabric is costly. Currently, researchers pay $1,500 for a 50-gallon (190-liter) drum of epoxy; 5-gallon (19-liter) containers can cost as much as $500.

Technicians familiar with using non-steel reinforcement are quick to note that CFRP reinforcing cannot be handled like common steel rebar. Special precautions must be taken:

• CFRP reinforcing materials cannot be directly exposed to sunlight in the precast yard and must be stored indoors.
• Workers must take special care to ensure the non-steel bars and tendons are not dragged over the ground or otherwise handled in a manner that may scratch or damage the rods.
• Because non-steel reinforcing rods generate strength in the axial direction and have little compressive strength, they cannot be bent or they will easily break (see Figure 6).

Ongoing research using innovative new materials and testing chambers that can simulate extreme conditions for precast concrete elements opens up unlimited market possibilities for the precast concrete industry. New, non-steel materials show the promise of more than doubling the weight-bearing capacity of existing structures. From doubling the life of America’s bridges to providing safety and security for citizens and the military to green building scenarios, new applications for precast concrete designs hold unlimited potential for a product that is already known for its quality and durability.

Sue McCraven, NPCA senior technical consultant, is a civil engineer, technical writer and editor, and environmental scientist who has contributed numerous articles and studies to prominent scientific journals.