Fiber Reinforced Plastics (FRP):Introduction-Properties
Concrete structures regardless of their purpose are disintegrating. A common link for this breakdown is that steel reinforcement is being used to strengthen the flexural capacity of the structures. Reinforcing steel will corrode when contact is made with humid or salty environments. When steel corrodes, it is expanding which creates tensile forces in the concrete. As the concrete reaches its limit in tension it begins to crack and spall. This spalling creates an even better environment for the corrosion to propagate even further.
Example: The deterioration of concrete bridges due primarily to corrosion of the reinforcing steel in the concrete is a major concern today consuming billions of dollars on their repair around the world. The primary cause of the deterioration is the corrosive action of steel on concrete cause by deicing chemicals and salt water harsh environment.
Methods used to extend the longevity or protection of structures against corrosion are the use of sealers, increase cover depth, increase concrete density, and additives to retard the chemical process. A promising solution to the problem is the use of Fiber Reinforced Plastics (FRP) as a replacement for reinforcing steel. The use of FRP as reinforcement has the following advantages of:
- High tensile strength,
- Corrosion resistance,
- Flexibility, and
- Electromagnetic resistance.
FRP is comprised of high strength fibers bonded in a polymer matrix. It has been used by the aerospace and automotive industry for quite some time. FRP reinforcement can be used for marine and water exposed structures, piers, docks, suspension and cable-stayed bridges. FRP reinforcing rods can be use to combat deicing salts in bridge decks, parapets, retaining walls, foundations, and curbs. FRP reinforcing can be use to combat saltwater for the same type of structures or components. Other areas where FRP can be used are wastewater and chemical corrosion areas and low electrical conductivity areas. Projects that have FRP used in them are bridges in Germany, Japan, China, and the United States.
The ultimate strength of FRP is stronger than steel for same diameter, however unlike steel, the compression strength is less than the tensile strength. The strength is close to twice that of steel. The stress-strain diagrams for FRP show it is linearly elastic to rupture. The stresses in Glass FRP (GFRP) bars are well below ultimate when failure occurs.
The modulus of elasticity is twenty to twenty-five percent of that of steel. The low modulus of elasticity of FRP may lead to the deflection limit state controlling designs due to it being one quarter that of steel. Lower stiffness produces load deflection that is almost linear. At ultimate load, deflection in GFRP is double that of steel, but due to the higher strength and ductility at failure, deflection of the steel reinforcing is greater.
The specific gravity of FRP material is a quarter of that of steel which makes it easier to handle.
FRP and concrete have similar coefficients of thermal expansion which will aid in its use.
The bond strength of FRP reinforcing is not as high as steel reinforcing bars, but then again epoxy coated bars reduce bond strength.
The bond strength has been determine to be two-thirds that of steel reinforcement.
The fatigue of GFRP reinforcing is good up to half their strength.
The use of FRP as reinforcement has the following advantages of light weight, high tensile strength, corrosion resistance, flexibility, and electromagnetic resistance.
The disadvantages of FRP are low modulus of elasticity, higher cost, low failure strain, anchorage methods, bond to concrete, and ultraviolet light sensitivity.
FRP can be made of carbon, aramid, or glass.
Fiber reinforced plastics are often made by the pultrusion process. The fibers are impregnated with a resin and pulled through a dye that forms the geometry of the section. The sections can be hollow tubes, I shape or round rods. The round rods are wrapped with additional fiber to form ribs which aids in the bonding to concrete. Hooks and bends are difficult to make. It may be necessary to make connections like plumbing or use grids where straight sections will not work.
Carbon and aramid FRP can have high fatigue characteristics, three times more than steel, glass fatigue characteristics are generally less than steel.
Of the three types of FRP, mentioned here, CFRP (Carbon Fiber Reinforced Concrete) has the highest tensile properties. Aramid FRP (AFRP) has the higher strain at failure, but also is most effected by water.
Glass FRP (GFRP) is the least expensive and is sand finished for better bonding properties.
FRP material resists temperatures as high as 105°C. At temperatures in excess of 205°C, FRP reinforcement loses some of its flexural capacity.