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ABOUT FIBER REINFORCED POLYMERS





FRP stands for Fiber Reinforced Polymers and it is used to indicate a wide category of composite materials which constitute the frontier of current construction technology. They ... are costitued of polymers matrix where unidirectional or multiaxial fabrics are imbibbed.

Their use in aeronautic construction continues with great success. The revolutionary Centurion aircraft is taking its first experimental flights; it combines lightness of weight with aerodynamic efficiency, and like its predecessor Pathfinder, its structure is entirely made of composite materials (carbon fiber and kevlar) coated in protective film. The Centurion can reach the stratosphere with extremely low-powered engines.

In civilian constructions, FRPs are been used with promising results in three main areas:

1) structures wholly made of composite (beams, panels, outer shells)
2) strengthening of traditional materials such as reinforced concrete, walls and timber
3) reinforcement of existing structural elements of different typologies and materials

The first category is dominated by pultruded profiles, light, resistant and sufficiently stiff; their use is typically envisaged for structural frames.
The main efforts currently aim at solving connection problems. As concerns other plate or shell-type elements, the classical evolution of application in the field of aircraft, ship and automobile constructions can be now found in the first examples of composite civil constructions. The first bridges and housing units entirely made of composite are appeared.

The second category is dominated by bars and meshes obtained with different technologies. These are used to strengthen reinforced concrete beams and masonry walls, or structurally to co-operate with wood. Prestressing is particularly interesting for the high strength/longitudinal modulus of composites cables or bars. Anchorage problems are principal research task fields.
Prestressed and cable stayed bridges are already in use in different parts of the world.

The third category is the fastest growing. Reinforcing deteriorated structures offers the most possibilities for use.
The rapidity of interventions and the great efficiency have convinced many designers to choose this path. The cost is not high compared to traditional technologies both in terms of execution time and construction difficulties.
In seismic areas, the improvement of the structural behaviour of existing structures is often surprisingly effective.
The high "resistance/weight" ratio must be kept in mind, which is "a must" for structures subjected to seismic action. In this context, wrapping technology, with composites prepared on site, represents a real structural engineering revolution. In fact the columns confinement, the bending and shear beams strengthening are achieved easily and effectively by bonding structural elements with resin-impregnated fabrics. Resins constitute the matrix of the composite, while the fabric's fibres vary in nature: carbon, aramide, glass, polivyniialcohol or other mineral fibers.

By dosing layers and morphologies of the composite, it is possible to obtain considerable improvements in resistance and ductility The applications development of this sector is progressing rapidly whereas the methodologies for calculating are not completely organized and collected in codes of recommendations. In particular, reinforced concrete beams shear strengthening is being researched as concerns the schematization of calculations and applications morphologies. In the quasi-totality of these intervention, the reinforcement is external, therefore the transfer of actions from the composite to the original structure takes place on the external film of the reinforced element. In reinforced concrete beams, the external concrete thickness which must be able to transmit the above-mentioned actions is known as "steel cover".
Before each intervention it is necessary to evaluate the consistency of the support, and only if it is considered adequate, can the application be carried out. The methodologies of Non Destructive Testing are a useful tool for acquiring elements of judgement.

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