Fiber Reinforced Polymer...

The Need

Reinforced Concrete is a very common building material for the construction of facilities and structures. As complement to concrete's very limited tensile strength, steel rebar has been an effective and cost-efficient reinforcement.

However, insufficient concrete cover, poor design or workmanship, and presence of large amounts of aggressive agents including environmental factors all can lead to cracking of the concrete and corrosion of the steel rebar.

For many years, there have been many studies on this corrosion issue, and the interest in FRP (Fiber Reinforced Polymer) has arisen recently as prospective substitute for steel. Careful consideration on potential of FRP rebar to fill the cost and performance needs may suggest appropriate solutions.

The Technology

Composite materials made of fibers embedded in a polymeric resin, also known as fiber-reinforced polymers, can become an alternative to steel reinforcement for concrete structures. Aramid fiber reinforced polymer (AFRP), carbon fiber reinforced polymer (CFRP), and glass fiber reinforced polymer (GFRP) rods are the commercially available products for the construction industry. They have been proposed for use in lieu of steel reinforcement or steel pre-stressing tendons in non pre-stressed or pre-stressed concrete structures. The problems of steel corrosion are avoided with the use of FRPs because these are non-metallic and non-corrosive. In addition, FRP materials exhibit several properties including high tensile strength, which make them suitable for the use as structural reinforcement.

The bond characteristics are responsible to transfer the load from concrete to reinforcement and to develop the required stress in the reinforcement for equilibrium, particularly when concrete is cracked. Service limits in FRP reinforced concrete elements such as deflection, crack width and crack spacing are directly influenced by the bond properties of the reinforcement in concrete.

Fiber reinforced polymer bars are anisotropic materials. Factors such as type and volume of fiber and resin, fiber orientation and quality control during the manufacturing play a major role in the mechanical characteristics.

When comparing a steel bar of 11.3 mm diameter and Carbon FRP rebar of about 9.5 mm diameter, the results shows that the tensile stress-strain curves of the CFRP bar are linear up to failure (All FRP bars are linear elastic to failure). The ultimate tensile strength is at least 1500 MPa, 3 times which of steel rebar. The modulus of elasticity of the CFRP bar is 128 GPa, about 65% that of steel. The CFRP bar exhibited almost the same bond strength to concrete as 11.3 mm diameter steel bar.

As for Glass FRP bar, tensile strength of 9 mm diameter bar is 760 MPa, and the Modulus of Elasticity is 40.8 GPa, much lower than that of steel.

Fiberglass rebar may be a suitable alternative to steel reinforcing in:

• Architectural Concrete: cast stone, architectural cladding, balusters, column facades, window lentils, architectural precast elements, hand railing, statuary and fountains, etc.
• Concrete exposed to de-icing salts in: bridge decks, railroad grade crossings, median barriers, parking garage elements, and salt storage facilities, etc.
• Concrete exposed to marine salts in: seawalls, water breaks, buildings & structures near waterfront, aquaculture operations, and floating marine docks, etc.
• Concrete used near electromagnetic equipment such as: MRI rooms in hospitals, airport radio & compass calibration pads, and concrete near high voltage cables, transformers, substations, etc.

The Benefits

·         Impervious to chloride ion and chemical attack

·         Tensile strength greater than steel

·         1/4th weight of steel reinforcement

·         Transparent to magnetic fields and radio frequencies

·         Electrically and thermally non-conductive

Based on features above, FRP bars appear to be promising alternative to steel reinforcement in concrete structures.