Some researchers believe that FRP materials could be the future replacement for steel, either for strengthening or reinforcing applications, providing that researchers are able to improve the fire resistance of these materials.
By Nabil F. Grace, Ph.D, P.E. and Mena Bebawy
Precast concrete specifiers know how crucial it is to stay current on structural testing and emerging technologies, and that the source of that information can have a lasting effect on the structures they design. That’s why it’s so important to understand how universities conduct civil engineering research and how new materials analysis will impact the precast industry.
When turning toward university research, here are some sample questions you might ask: What are the country’s best university structural testing labs currently doing, and how will their results impact future precast concrete designs? How do research engineers go about their testing to develop new material technologies for the precast industry? Why is non-steel materials research receiving significant state and federal funding? What practical results are supported by the most current testing data?
What drives research
Deteriorated condition of U.S. bridges One underlying impetus for many civil engineering studies is the seriously poor condition of the nation’s bridges and roadways. Corrosion-induced deterioration can be viewed as “Enemy No. 1” of our nation’s infrastructure. The American Association of State Highway and Transportation Officials (AASHTO) currently estimates the cost of repair and rehabilitation of the nation’s bridges to be about $150 billion. Although reinforced precast and prestressed concrete structures boast long life span and low maintenance cost, corrosion of steel (due to deicing chemicals used on roadways during cold weather) increases maintenance and rehabilitation costs and decreases the expected life span of structures.
Because most of our country’s bridges were built back in the 1950s and 1960s, these structures are now approaching the end of their expected life spans. State and federal highway agencies realize the magnitude of infrastructure deterioration and its potential impact on driver safety. Transportation agencies, therefore, have funded research into potential alternatives to replace steel in concrete to eliminate the problem of corrosion-induced deterioration. In addition, state and federal transportation agency funding supports beam-strengthening, or bridge-rehabilitating, material research.
Growing need for fire-resistant civil and military infrastructure
A second, perhaps more sinister, force fuels another arena of important structural research. The current lack of sufficient understanding of the destructive nature of fire and the fire resistance of existing structures has cost the United States dearly in the loss of thousands of civilian and military lives. Since Sept. 11, 2001, fire-resistant building materials and designs have become a precondition for ensuring greater public safety in occupied buildings.
Building and bridge fires are always a serious safety issue that must be addressed carefully by structural engineers, because whenever a fire occurs, the potential exists for financial loss and, worse, human casualties (see “Why We Need Fire Research”). As an example, the National Fire Protection Agency reported that U.S. fire departments responded to an estimated 1.6 million fires in 2006. In one year alone, these fires resulted in 3,245 civilian fire fatalities, 16,400 civilian fire injuries, and an estimated $13.6 billion in direct property loss. Residential home fires are responsible for 80 percent of all civilian fire deaths.
Looking beyond domestic building occupant security to military safety and preparedness, fire events, including exposure to incendiary devices during wars, are inevitable. Global terrorist threats are a present and growing reality. By funding state-of-the-art research into new fire- and impact-resistant structural materials, the U.S. military is doing everything possible to protect service personnel and civilians on both the domestic and international fronts.
How cutting-edge research is conducted
Non-steel reinforcing test details
To address the critical need for durable transportation infrastructure, the Center for Innovative Material Research (CIMR) at Lawrence Technological University is developing and testing a viable solution for the corrosion problem in existing bridges and roadways. CIMR researchers are testing the use of carbon fiber reinforced polymer (CFRP) bars, tendons and strands in precast, prestressed concrete beams as a replacement for conventional steel reinforcement. CFRP is a non-metallic material, corrosion-free and maintains its structural integrity under severe environmental conditions, providing a service life much longer than that of steel. Non-steel reinforcing material is expected to double the lifespan of highway structures.
Box beams and I-beam tests
Researchers are developing design guidelines for CFRP precast, prestressed box beam and I-beam bridges. These experimental investigations are supported by analytical and numerical studies. Two AASHTO I-beam bridge models were constructed at one-third scale and tested under service and ultimate loading. Both of the bridge models had a span of 41 feet (12.5 meters) and were totally reinforced with non-metallic reinforcement.
The first model was reinforced with CFRP Diversified Composite Inc. (CFRP-DCI) tendons while the second model was reinforced with carbon fiber composite cable (CFCC) strands and stirrups. To construct the bridge model, five I-beams with a stem depth of 19.75 inches (500 millimeters) were constructed individually and then placed together with a center-to-center spacing of 19.75 inches.
Each beam was reinforced with three prestressing non-metallic (CFRP/CFCC) strands in addition to a number of non-prestressing, non-metallic strands. A deck slab with a thickness of 2.5 inches (64 millimeters) was then cast-in-place along with five diaphragms between the beams. The first model was designed to fail in tension by rupture of the reinforcing tendons, while the second model was designed to fail in compression by crushing the concrete.
The test results for the AASHTO I-beam bridge models proved promising. The CFRP-DCI bridge model sustained a maximum load of 146,000 pounds (66 metric tons) with a corresponding maximum deflection of 10.4 inches (264 millimeters). The failure of the bridge model started with the rupture of the prestressing strands followed by delamination and separation of the loaded beam.
The CFCC bridge model sustained a maximum load of 155,000 pounds (70 metric tons) with a corresponding deflection of 9.46 inches (240 millimeters). The failure of the bridge model took place by crushing the concrete. In both models the deflection was large enough to give a visible warning before the failure took place.
Another research team is currently studying the behavior of precast, prestressed box beams reinforced with CFCC strands and CFRP-DCI tendons. A half-scale bridge model with a span of 32 feet (9.75 meters) was constructed using seven side-by-side box beams. Another bridge model with a span of 32 feet was constructed using four side-by-side box beams. From resulting data, researchers were able to draw the main conclusion that, regardless of the configuration of a beam’s cross section, CFRP/CFCC reinforcements can successfully replace steel reinforcement without any reduction in strength or safety.
New beam-strengthening fabric tests
Fiber-reinforced polymer (FRP) strips, sheets, fabric and plates represent the current and future solution for strengthening issues in precast and cast-in-place reinforced concrete structures. The light weight of the FRP materials and the ease of handling and installation give great advantage to the FRP material over the conventional steel plates that have traditionally been used for strengthening deficient structures. However, one of the problems associated with using FRP material is that, unlike steel, FRP material behavior is not characterized by a stress-strain yield plateau (material ductility) and therefore, sudden failure (no visual forewarning) is to be expected in FRP strengthening applications.
To overcome sudden-failure (lack of ductility) behavior, an innovative new ductile hybrid fiber (DHF) fabric was developed recently at the CIMR. With funding from the NSF (National Science Foundation), the DHF fabric is made by breeding yarns of both high-modulus and low-modulus carbon fibers with glass-fiber yarns. Fibers are oriented at three angles: 0 degrees, 45 degrees and minus 45 degrees. The sudden-failure problem has been successfully addressed for the DHF fabric and now the strengthening material exhibits a yield plateau close to that of the steel and gives significant warning and ductility prior to failure.
Fire-resistant material tests
Understanding how materials respond to high-temperature fire events is one of the main purposes for building the CIMR facility. The CIMR is equipped with a state-of-the-art fire/loading chamber that can be used for testing full-scale structural elements under fire/loading conditions (see Precast Inc. January-February 2009 issue, pages 16 to 19 for an article on Lawrence Tech’s new CIMR facility with its high-heat testing chamber). An ongoing fire research project is underway to enhance the fire resistance of FRP materials.
Some researchers believe that FRP materials could be the future replacement for steel, either for strengthening or reinforcing applications, provided that researchers are able to improve the fire resistance of these materials. The matrix of the FRP material is usually composed of an organic epoxy. Organic epoxy is vulnerable to heat and can lose its strength
in temperatures as low as 150 F (66 C), and when subjected to temperatures of 350 F (178 C), the epoxy may eventually ignite. This is why researchers at the CIMR are dedicating a significant effort to develop a fire-resistant FRP material that can be used safely indoors and outdoors.
Using the new fire/loading chamber, researchers are testing innovative fire insulation materials that can be applied over externally applied FRP strengthening materials with the goal of improving the fire rating of the FRP-strengthened concrete structural elements (Figure 8). As an alternative, researchers are testing the use of fire-resistant inorganic epoxy instead of the organic epoxy that typically forms the matrix of the FRP. The outcome of the current research is expected to be a fire-resistant FRP system that can withstand very high temperatures – up to 2300ºF – for several hours without any degradation in strength.
Research potential for the precast industry
FRP materials have the potential to eliminate the concrete industry’s issues with corrosion and the vacillation of steel availability and cost. Although the price of FRP materials is higher than that of steel, the expected increase in the lifespan of the structural elements will greatly minimize the cost of repair and rehabilitation. Establishing a strong production line for FRP materials in this country will eventually reduce manufacturing costs and make these innovative materials more readily available to design engineers and product specifiers.
What to expect in future research
The behavior of FRP materials for civil and structural applications is still under investigation and researchers are making every effort to introduce a deficient-free material. One of the future research projects, however, is to test FRP-reinforced/strengthened concrete elements under a broad range of adverse environmental conditions such as: freezing/thawing cycles; rain; solar radiation; salt spray; and UV light. As investigations into FRP material behavior and durability continue, design guidelines are being developed to enable structural engineers to confidently specify FRP-enhanced precast concrete elements with the same margin of safety as that of steel-reinforced structures.
Sidebar: Why We Need Fire Research
By Sue McCraven
National security mandates to ensure the safety of civilian and military personnel underlie much of the impetus for state-of-the-art fire research. News reports from the Iraq and Afghanistan conflicts continue to relate the horrific destructive power of improvised explosive devices (IEDs). American transportation infrastructure, however, is also susceptible to severe fire damage. For example, an exploding gasoline tanker literally melted steel supports at a highway overpass in Oakland, Calif., in 2007. The early morning blaze collapsed a busy eastbound connector from the Bay Bridge to I-580 onto another connecting roadway beneath. The high heat of the gasoline fire actually melted the connecting bolts and steel beams supporting the overpass and the roadbed was wrenched from its supports.
Civil engineering research to improve the fire-resistance of precast concrete structures is creating new technologies and products that will provide increased security for our military personnel and safer buildings and roadways for the public. Many of these new technologies and materials will find useful applications in other unrelated precast consumer products.
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.
Mena Bebawy earned his B.S. in structural engineering from ASSIUT University in Egypt and received his M.S. in civil engineering from Lawrence Technological University in Southfield, Mich., where he is currently conducting research while completing his Ph.D. in structural engineering.
Nabil F. Grace, Ph.D., P.E., is a University Distinguished Professor and Chairman of the Civil Engineering Department at Lawrence Technological University, where he is director of the Center for Innovative Materials Research (CIMR). Dr. Grace’s work is funded by numerous state and federal agencies and includes $11 million from the U.S. Army Research Lab to test composite materials under fire and loading conditions.