The use of FRP materials in strengthening structures such as beams, floors, bridges, columns, silos, cooling towers and chimneys has rapidly gained acceptance worldwide since the development of the technique in the 1980’s. For beams and bridges, the technique involves bonding either unstressed or pre-stressed carbon fibre reinforced polymer (CFRP) plates to the underside (soffit) or bottom flange of the beam. This has the effect of increasing the capacity of the lower part of the beam which is under tension. For materials such as reinforced concrete and cast iron, which are strong in compression but weak in tension, this is usually the most critical area. Pre-stressing the CFRP plates has the effect of further reducing the tensile stress on the bottom flange, hence the load capacity is increased. CFRP plates can also be bonded to the sides of beams near to the supports to increase the shear capacity. For structures such as columns, silos and cooling towers, the reinforcement is applied cross-wrapped or in bands, with the tensile capacity of the carbon fibres acting as confinement.
The most obvious advantage of the use of CFRP plates over the more traditional use of bonded steel plates is the reduction in dead load on the structure. Carbon fibres have a much higher tensile strength than steel but with much less weight. They are also non-corroding. Glass fibre reinforcement is a lower cost alternative to carbon fibres. The bonding between the CFRP plates and the structure is of great importance for the technique to be successful. To prevent failures, non-destructive test methods for the detection of voids indicating bond failure such as the use of ultrasonic and transient pulse thermal imaging have been developed.
Reasons for choosing FRP materials for refurbishment:
- Highly cost effective method of maintaining or upgrading existing structures.
- Quick application results in lower disruption and shorter contract periods.
- The technique may allow continued usage of structure or facility during strengthening works.
- Increases the capacity with minimal addition of dead load to the structure.
- Materials are easy to transport and handle – no lifting gear required. Easy to use at height.
- Ability to work in confined areas and in situations with difficult access (e.g. tunnels, basements)
- Minimal plate preparation required – by use of peel ply plates
- Plates may be any length with no lap joints needed.
- For wall strengthening overlaps are simple.
- Plates may be thinner than alternative steel – less reduction in headroom.
- Durable, corrosion resistant.
- Good fire performance
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