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Hidden Revolution: Composite Rebar Gains Strength

Aug 03, 2020

Concrete has long been used as a building material for its high compressive strength, good durability and low cost. However, its well-known Achilles’ heel is its brittleness and limited tensile strength. This was solved quite handily about a century ago by using reinforcing bars (rebar) of steel in the tension side of concrete structures. Steel rebar is functionally efficient and relatively inexpensive, so it does a good job in most cases. However, steel rebar has its own weakness: susceptibility to corrosion (oxidation) when exposed to salts, aggressive chemicals and moisture. As it corrodes, steel rebar swells and increases the tensile load on the concrete, which begins to crack and spall, creating openings that lead to further and faster deterioration of the steel and concrete. This necessitates costly repair and maintenance and, if allowed to progress far enough, it can compromise the structure’s integrity. Numerous coatings and penetrants have been  introduced over the decades to help seal out moisture from concrete, and rebar itself has been upgraded with epoxy coatings or the use of stainless steel. But it isn’t always possible to prevent corrosion in the long term. Further, steel rebar’s penchant to conduct electrical and magnetic fields makes it undesirable in concrete specified for certain power-generation, medical/scientific-imaging, nuclear and electrical/electronic applications.

FRP value proposition

There are many reasons why fiber-reinforced polymer (FRP) rebar makes sense in some concrete structures. For starters, composite rebar won’t rust or corrode, so it’s ideal for periodic or long-term immersion in fresh water or brine in applications such as retaining walls, piers, jetties, quays, caissons, decks, pilings, bulkheads, canals, offshore platforms, swimming pools and aquariums. It’s also immune to road salt and other deicing chemicals, making it a more durable and less maintenance-intensive choice for roadways and bridges, parking structures, airport runways, Jersey barriers, retaining walls and foundations, curbs, parapets, and slabs on grade. Further, it offers broad resistance to a host of other chemicals found at wastewater treatment plants, solid waste sites, petrochemical plants, pulp and paper mills, pipelines, tanks, cooling towers and chimneys, as well as the alkaline environment of concrete itself.

Another advantage is that the tensile strength of FRP rebar is typically 1.5 to 2 times higher than steel, so it’s a good counterbalance to concrete’s high compressive strength. It also provides excellent fatigue resistance, making it suitable for cyclic loading situations (such those on roads and bridges). Moreover, composite rebar is one-quarter the weight of comparably performing steel. Here there are a number of practical benefits. There is less wear and tear on construction workers who must carry and install it and less need for cranes and other heavy-lifting equipment. It is easily cut with common cutting tools, without damaging saw blades. More rebar can be hauled per truckload without exceeding legal loading limits. For bridges and like structures, the higher strength-to-weight ratio provides either greater carrying capacity for a given structure or possible opportunities to reduce the size and weight of the entire structure. Composite rebar also is useful in weight-sensitive applications where soils have poor load-bearing properties, in seismically active locations or in environmentally sensitive areas where it is undesirable to move heavy equipment.

For electromagnetically sensitive applications, both glass (the most common composite rebar reinforcement) and polymer are inherently nonconductive, so they won’t transmit current, attract lightning strikes or interfere with the operation of nearby electrical devices. That makes it a safer choice in aluminum and copper smelting plants, nuclear power plants, specialized military structures, airport towers, electrical and phone transmission towers, manholes containing electrical or phone equipment, hospitals with magnetic resonance imaging (MRI) equipment and toll-road sensing arrays and collection booths. Because glass-reinforced composite is equally poor at thermal transmission, it can be helpful for maintaining climate control in buildings, patio decks and basements.

Although the initial cost of composite rebar is generally higher than standard steel rebar and is roughly comparable to epoxy-coated steel rebar, when considered on a lifecycle cost (LCC) basis, it can be quite economical — particularly for non-prestressed concrete applications subject to flexure, shear and compressive loadings that typically require frequent repair and maintenance or where there are other issues with metal. For all these reasons and more, composite rebar has slowly begun to gain share in the civil engineering market.

No regs, no progress

Composite rebar got its start in Japan in the 1980s, with carbon and aramid fiber reinforcements in thermoset matrices, and slowly spread to projects in Canada during the early 1990s, says John Busel of the American Composites Manufacturers Assn. (ACMA, Arlington, Va.). But it didn’t really take off, he recalls, until specifications were developed and published for composite rebar in the late 1990s. Busel, director of ACMA’s Composites Growth Initiative, was for 12 years the secretary and then the chair of the American Concrete Institute’s (ACI, Farmington Hills, Mich.) Committee 440 - FRP Reinforcement, during the time that group developed its groundbreaking specifications and design guide for FRP rebar.

“Coming up with products that aren’t supported by testing and research just doesn’t work with civil engineers,” explains Busel. “It takes a lot of data to convince them, and obtaining that takes time.” In view of that reality, Committee 440 was established in the early 1990s and took close to a decade to develop the first edition, published in 1999, updated in 2006, with another update due in 2012. “Now you have standards that architects, engineers, and contractors can put into their plans globally,” says Busel, noting that “the ACI 440.1R has proven to be one of the best-known and best-used specification guides in the world and was definitely worth all the work.”

“The ACI 440 has been an extremely dynamic and active association,” notes Busel’s long-time colleague Doug Gremel. “We haven’t discriminated against any research anywhere in the world. If we can take it and incorporate it into our codes, we do it.” Gremel — the director of Non-Metallic Reinforcing at Hughes Brothers Inc., a director of Hughes subsidiary Aslan Pacific Ltd. (both of Seward, Neb.) and management committee chair at Omaha, Neb.-based Composite Insulated Concrete Systems LLC — adds, “We have no pride of ownership when it comes to this knowledge.”

Despite this growing body of science and experience in the mid- to late 1990s, growth in the use of FRP rebar was slow. The first U.S. installation didn’t appear until 1996 in the McKinleyville/Buffalo Creek Bridge in Brooks County, W. Va. FRP rebar finally gained traction in North America after its inclusion in the Canadian highway bridge code, where it became the default solution for dealing with corrosion caused by Canada’s harsh weather. That, in turn, led to work by the American Association of State Highway and Transportation Officials (AASHTO) to develop specs for the use of glass FRP (GFRP) concrete decks and traffic railings. From that point, U.S. Department of Transportation (DoT) engineers and specifiers had their own design guide to go along with ACI 440. As a result, says Busel, Canada and the U.S. together now have almost 400 bridges with FRP rebar in some aspect of their construction. European installations are growing, but at a slower pace.

Gremel — whose employer, Hughes Brothers, is a global supplier of FRP rebar — says the standards form the objective framework for quality assurance. “We have to provide production-lot certs as evidence that any given ‘mill run’ of rebar meets or exceeds the properties called out in ASTM standards,” he says. “We’re doing tensile modulus and strain tests on every lot we run, just like the steel guys.”

The civil engineering community’s progress toward comfort with FRP rebar might be slow, but it has not discouraged the pursuit of new approaches to its manufacture that could, as the following examples indicate, make the next generation of composite rebar a much more attractive alternative to steel.