CRFP FOR STRUCTURAL STRENGTHENING

Carbon fiber reinforced polymer is an extremely strong and light fiber-reinforced material which contains carbon fibers. The binding polymer is often a thermoset resin such as epoxy, but other thermoset or thermoplastic polymers, such as polyester, vinyl ester or nylon, are sometimes used. The fibers provide the main reinforcing elements while the polymer matrix (epoxy resins) acts as a binder, protects the fibers, and transfers loads to and between the fibers.

Fiber-reinforced polymer (FRP) composites have been used for structural strengthening for almost 30 years. During that period, the use of FRP for retrofit projects has been gaining more popularity among design professionals over conventional strengthening techniques, such as installation of supplemental structural steel frames and elements. FRP strengthening of existing structures can involve complex evaluation, design, and detailing processes, requiring a good understanding of the existing structural conditions along with the materials used to repair the structure prior to FRP installation.

The most common FRP systems for concrete strengthening applications are carbon fiber based (CFRP). Carbon has superior mechanical properties and higher tensile strength, stiffness, and durability compared with glass fiber based systems. The use of prefabricated CFRP bars and plates is typically limited to straight or slightly curved surfaces; for example, the top side or underside of slabs and beams. Prefabricated FRP elements are typically stiff and cannot be bent on site to wrap around columns or beams.

FRP fabric, on the other hand, is available in continuous unidirectional sheets supplied on rolls that can be easily tailored to fit any geometry and can be wrapped around almost any profile. FRP fabrics may be adhered to the tension side of structural members (e.g. slabs or beams) to provide additional tension reinforcement to increase flexural strength, wrapped around the webs of joists and beams to increase their shear strength, and wrapped around columns to increase their shear and axial strength and improve ductility and energy dissipation behavior.

The adhesive systems used to bond FRP to the concrete substrate may include a primer that is used penetrate the concrete substrate and improve bond of the system; epoxy putty to fill small surface voids in the substrate and provide a smooth surface to which the FRP system is bonded; saturating resin used to impregnate the fabric and bond it to the prepared substrate; and protective coating to safeguard the bonded FRP system from potentially damaging environmental and mechanical effects. Most epoxies for FRP strengthening systems are adversely affected by exposure to ultraviolet light, but can be protected using acrylic coatings, cementitious coatings, and other types of coatings. The resins and fiber for a FRP system are usually developed as one system, based on materials and structural testing. Mixing or replacing a component of one FRP system with a component from another system is not acceptable and can adversely affect the properties of the cured system.

The bond between FRP system and the existing concrete is critical, and surface preparation is essential to most applications. Any existing deterioration or corrosion of internal reinforcement must be resolved prior to installation of the FRP system. Failure to do so can result in damage to the FRP system due to delamination of the concrete substrate.

FRP systems provide a very practical tool for strengthening and retrofit of concrete structures, and are appropriate for:

• Flexural strengthening

• Shear strengthening

• Column confinement and ductility improvement

FRP systems have also been successfully used for seismic upgrading of concrete structures. These applications include mitigating brittle failure mechanisms such as shear failure of unconfined beam-column joints, shear failure of beams and/or columns, and lap splice failure. FRP systems have also been to confine columns to resist buckling of longitudinal steel bars. These FRP schemes increase the global displacement and energy dissipation capacities of the concrete structure, and improve its overall behavior. Because of the resistance to corrosion, FRP composites can be utilized on interior and exterior structural members in all almost all types of environments

Reference: Strengthening of Concrete Structures Using FRP Composites By Tarek Alkhrdaji, Ph.D., P.E. In Articles, Building Blocks Comments 19

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