Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centres, the need of seismic retrofitting is well acknowledged. In India after many earthquakes, the Codes were revisited as more than 50 % of the Indian land has become seismically active which was not taken care of in the previous codes.
The new code developed – IS 1893 (Part 1) 2002, assigns four levels of seismicity – it divides active zones into 4 Zones namely Zone 2, 3, 4 and 5 with higher zones denoting high activity. The specifications given in the design code (IS 1893: 2002) are not based on detailed assessment of maximum ground acceleration in each zone using a deterministic or probabilistic approach. Instead, each zone factor represents the effective period peak ground accelerations that may be generated during the maximum considered earthquake ground motion in that zone. Many buildings constructed before this code are not adequate and hence fall in the danger zone.
With this the need of Seismic retrofitting has risen as many buildings need to be retrofitted in accordance with the new code of practice.
Externally bonded fibre reinforced polymers (FRPs) in the form of continuous carbon (C), glass (G) or aramid (A) fibres bonded together in a matrix made of epoxy, vinylester or polyester, are being employed extensively throughout the world in retrofitting reinforced concrete structures. Their high strength-to-
weight ratio, immunity to corrosion and easy handling and installation are making FRP jackets the material of choice in an increasingly large number of seismic retrofitting projects, despite the relatively high material costs.
FRP as a technology
A Fiber Reinforced Polymer (FRP) composite is defined as a polymer (plastic) matrix, either thermo set or thermoplastic, that is reinforced (combined) with a fibre or other reinforcing material with a sufficient aspect ratio(length to thickness) to provide a discernable reinforcing function in one or more directions. FRP composites are different from traditional construction materials such as steel or aluminium. FRP composites are anisotropic (properties apparent in the direction of the applied load) whereas steel or aluminium is isotropic (uniform properties in all directions, independent of applied load). Therefore, FRP composite properties are directional, meaning that the best mechanical properties are in the direction of the fibre placement.
Composites are composed of:
- Epoxy – The primary functions of the resin are to transfer stress between the reinforcing fibres, act as a glue to hold the fibres together, and protect the fibres from mechanical and environmental damage. The most common resins used in the production of FRP grating are polyesters (including orthophthalic-“ortho” and isophthalic-“iso”), vinyl esters and phenolics.
- Reinforcements – The primary function of fibres or reinforcements is to carry load along the length of the fibre to provide strength and stiffness in one direction. Reinforcements can be oriented to provide tailored properties in the direction of the loads imparted on the end product. The largest volume reinforcement is glass fibre.
- Fillers – Fillers are used to improve performance and reduce the cost of a composite by lowering compound cost of the significantly more expensive resin and imparting benefits as shrinkage control, surface smoothness, and crack resistance.
- Additives – Additives and modifier ingredients expand the usefulness of polymers, enhance their processability or extend product durability.