The Loma Prieta earthquake near San Francisco in 1989 and the Northridge earthquake near Los Angeles in 1994 led to a sense of urgency in seismic strengthening of bridges and buildings throughout California. A technique that was just beginning to be used at the time, carbon fiber fabric epoxy bonded to the exterior of structural concrete members, proved to be a valuable tool in this effort. For the past 20 years, the materials and techniques of this approach have been perfected and adapted for use in other strengthening projects and for blast resistance.
Carbon fiber has very high tensile strength and is also very lightweight. When bonded to the exterior of a concrete column, beam, or slab, it can add significant strength without adding weight that would increase the load on foundations and other structural members. The composite material is called fiber-reinforced plastic (FRP). FRP wraps are easy to apply and can be used on any size or shape of structural member. Traditional techniques for strengthening, such as adding concrete and reinforcing steel around the outside of a structural member (often with shotcrete), external post-tensioning, or adding structural steel supports (shoring) often are more expensive due to the extra work to get everything into place.
The primary reason to use this technique is to add strength to an existing structure. In some cases, it might be used on new construction, although at this time that is usually only in response to some sort of design or construction error. In appropriate applications, FRP strengthening can be 30% to 50% less expensive than traditional strengthening due to the ease of installation.
There are a number of proprietary FRP strengthening systems on the market and nearly all of this work is done using one of those systems. The system manufacturers match the fibers with the resins to get the required strength upgrade. Carbon fiber is the preferred fiber in these FRP composites, although other fibers are sometimes used—mostly glass, basalt, or aramid (Kevlar). “The only reason we would provide glass or basalt is simply cost,” says Jim Butler, president of manufacturer HJ3 Composite Technologies, Tucson, Ariz. “Carbon is expensive and sometimes you don't need quite as much strength and then we would use glass or basalt. We are more favorable toward basalt as our low-cost fiber because it has greater abrasion resistance and it doesn't wick. Glass can wick and absorb moisture and chemicals leading to failure.”
“Carbon fiber is used in strengthening work due to a change in the structure's use or due to construction defects,” says Jay Thomas, vice president, strengthening division of Structural Preservation Systems, Hanover, Md. “Most engineers aren't familiar with the strengthening of an existing structure, so they will put out more of a performance spec where they define the existing capacity and the new requirements and ask us to provide not just construction but also design backup and materials.”
“I like to refer to them as construction anomalies,” says Scott Arnold, technical director for FRP manufacturer Fyfe, San Diego. “We are a product supplier who mostly interfaces with structural engineers who have a problem. We will look at the conditions and figure out if it's feasible to get there. Then we will look at whether the carbon brings value compared to other techniques—will it save time and money compared to more traditional alternatives. About 30% of the time, I tell them they really need new concrete rather than using a carbon fiber system. It's important to understand the benefits and the limitations of any product. Carbon fiber is expensive—of the thousands of projects I've been involved with no one ever said it was cheap. But they did it because it saved them money over the alternatives.”
One application for FRP strengthening gaining in popularity is strengthening of concrete tanks, culverts, and prestressed concrete water pipes. The FRP layer not only strengthens the structure but provides an impermeable layer. “Corrosion protection is important,” says Mo Ehsani, president of QuakeWrap, Tucson, Ariz. “If corrosion has gone on for some time, you have lost a lot of the strength of the rebar. Not only can an FRP wrap be designed to make up for that but it also can provide a layer of impervious materials to protect the structure from moisture intrusion and further corrosion. With one application of these fabrics or laminates, we can provide both strength and watertightness.”
Applying FRP wraps to structural concrete isn't difficult, but does require experience. “One hundred percent of the quality is due to workmanship,” says Thomas. “There are distinct cookbook steps and you have to do them all. When we see problems it's usually from someone who hasn't done it before and they are missing steps.”
All of the FRP strengthening system manufacturers consulted require some level of expertise in an installer. “We offer a two-day class to certify contractors to install our products” says Ehsani. “We only sell to certified installers or install it ourselves or require the job be done under our supervision. We do not sell the material to someone who just places an order because if you don't know what you're doing, you could really give a bad name to the product and the industry.”
Step one is surface preparation, starting with simply cleaning the concrete to remove any chemicals or dirt. For most applications, this is followed by shotblasting or water blasting to achieve a roughened surface profile. Arnold recommends a Concrete Surface Profile (CSP) 2 or 3, “like a light sandblasting.” He notes, however, there are two types of applications to consider: bond critical and contact critical. Bond-critical applications rely completely on the bond of the material to the surface of the concrete to transfer the stresses. Contact-critical applications are where the FRP is bonded to itself and creates confinement of the structural member. An example of a contact-critical application is a column where the FRP wraps completely around onto itself. In these situations, says Arnold, “We can even wrap right over paint although it must be in intimate contact with the concrete.”