Strengthening concrete members with carbon fiber fabric and epoxy composites
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).
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.
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. 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.
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.
With bond-critical applications, proper surface preparation and application is essential for success. It’s not just a coating; you have to get the bond so that significant stress can be transferred. If you don’t open up the pores of the concrete so your resins mechanically lock into it, you’re putting up an expensive coating that’s doing nothing. Bond-critical applications typically occur with slabs or walls.
Following surface preparation and removal of dust, any defects in the concrete are repaired—holes and cracks filled with epoxy or polyurea. Sharp edges, corners, and other form lines should be smoothed to prevent stress concentrations. Then the concrete is treated with a primer to allow the bond to develop as deep as ¼ inch into the concrete. This is immediately followed by placement of the saturated fiber fabrics.
Fiber sheets come in rolls that are typically 24 or 48 inches wide. Prior to application, it must be saturated with resin or wetted out. Resins are color-coded two-part materials that are premeasured to simplify mixing. For a small job, you can make a temporary table and pour the saturating resin over the fabric and work it in with a squeegee.
For larger jobs, saturating machines pass the fabric through a vat of resin and run it between two steel wringer rollers to force the resin into the fabric and squeeze off the excess—sort of like a giant wallpaper machine. These sheets can be temporarily wound around a piece of 4-inch PVC pipe and unrolled into positioned on the structure. The fabric is easy to cut simply using scissors.
Note that carbon fiber fabrics can be either unidirectional or bidirectional, and the primary reinforcing fibers can be oriented in various directions. The vast majority of FRP fabric used in structural strengthening is unidirectional carbon. The installer needs to be sure to apply the fabric in the correct orientation, in the correct locations, and in the correct number of layers or plies. The fabric can be placed in up to three layers.
A topcoat is applied next. Topcoats come in a variety of formulations up to a higher-end epoxy that can handle pH 2 to 12. Curing takes about 24 hours, which can be accelerated with temperature.
Fire protective coatings often are applied last and four-hour ratings can be achieved. Sometimes the best fire protection may simply be a layer of drywall. Testing after installation often is required with the most common tests being a tap test and a pull-off test to check the bond of the epoxy to the concrete surface. Tap tests are just like hammer sounding for concrete delamination.
Blast resistance of a column or other structural members can be improved with FRP surface applications. Wrapped columns can resist large blast forces.
One application is putting fiber on the top of a slab. An explosion can lift the slabs and reverse curl them, and they can collapse when they come down leading to progressive collapse, so the fiber can be installed on the top side of the slab to prevent that. We’ve also wrapped columns so that even if the concrete is destroyed by a blast, the carbon fiber holds it together—what’s inside the carbon can be complete rubble and it can still be supporting the building.
Repairing prestressed concrete water pipes also is becoming big business. There are miles of pressurized water pipe that have deteriorated sections. Carbon fiber sheets can be laid up to the inside of these pipes or pre-impregnated shells or FRP can be laid up and epoxied to the pipe.
FRP strengthening isn’t right for every situation. The limitation is usually the strengthening limit. There’s a distinct upgrade percentage where you can use carbon—some projects need greater strength upgrade and that leads to more conventional approaches.
It’s a viable tool but one that needs a lot of technical backup to make sure it’s being used right and designed specifically for a structure and that it’s applied properly. It comes with just as much liability as any other structural member. But where there are construction anomalies or a construction situation that’s not buildable, we might be able to work with the engineers to solve the problem.
There are many applications where FRP would offer a more economical alternative but because of the lack of awareness jobs are done in ways that are more difficult and expensive. Contractors need to be able to identify applications where these products would be of benefit.