What is the FRP System?
FRP System is a complete range of composites made from very high strength and extremely high mechanical strength fibres and polymeric resins specially formulated for the strengthening and static and seismic upgrading of structures made from normal, pre-stressed and reinforced concrete, steel, masonry or wood.
The term FRP stands for Fibre Reinforced Polymer. FRP’s are part of the more vast family of “structural composites” and are made from strengthening fibres set in a polymer matrix. In fibre reinforced composites, the fibres act as loadbearing members to offer strength and stiffness, while the matrix, apart from protecting the fibres, acts as an element that transfers the stresses between the fibres and matrix and the structural member to which the composite has been applied. The fibres may be disposed in any direction, depending on design specifications, in order to optimise the mechanical properties of the composite in the directions required. The particular characteristic of structural composites is that they provide better, or at least more “complete”, mechanical properties than those that would otherwise be provided by the single components. In polymeric matrix composites the matrix is generally made from epoxy resins and, by mixing them with an appropriate reactant, they polymerise (cure) to form a solid, glassy material. Strengtheners are made from:
Carbon fibres: carbon fibres are either high strength with a high modulus of elasticity or high strength with a very high modulus of elasticity (HM).
Glass fibres: glass fibres are either type E or type A.R. (alkali resistant).
Basalt fibres: their properties lie somewhere between those of carbon and glass fibres, with mechanical strength comparable with that of carbon fibres and a modulus of elasticity similar to that of glass fibres.
Metallic fibres: steel fibres with very high mechanical strength.
FRP’s have already been used for many years in the naval, aeronautic and military sectors where they have been exploited for their unparalleled specific strength (which means their level of tensile strength per unit of weight). Thanks to their increasingly widespread use and optimisation of the production processes adopted to manufacture them, in particular carbon fibres, costs are now much lower and has led to FRP’s being introduced also into the construction sector.
FRP in the building industry
The use of FRP’s in the construction industry applies mainly to the renovation of weak or damaged structures and the static and seismic upgrading of structures. In this context, repair work based on the use of high performance composites is more cost effective than traditional methods if the overall economic valuation takes into consideration the time required and the tools and equipment employed for the intervention, the costs involved in putting a structure out of service and the estimated working life of the structure itself once the intervention has been completed.
In fact, thanks to their low weight, FRP’s do not require special equipment or lifting gear to put them in place, only a small workforce is required to place the materials in a very short space of time and, in many cases, it is not even necessary to interrupt the normal activities of the structure itself.
Types of FRP used in building work
Fibre reinforced polymer matrix materials are heterogeneous, anisotropic, composite materials that have a linear elastic behaviour up to their failure point. Structural composites are used to strengthen structures in the form of the following types of fabric:
uniaxial, where all the fibres run in a longitudinal direction along the length of the fabric and are held together in a non-structural, lightly woven pattern;
biaxial, made up of an orthogonal weft-warp weave that is normally in a well balanced pattern (the same percentage of fibres in both directions);
quadriaxial, in which the fibres run in various directions along the plane of the fabric.
The fabrics are supplied dry in the form of rolls and then impregnated either before laying (“wet system”) or after laying (“dry system”).
As well as fabrics, there are also rigid elements available that are pre-impregnated with resin using an industrial extrusion process where the element is pulled, called pultrusion. These elements are supplied in the form of plates and bars, and are bonded to the structure to be strengthened using epoxy resins with a thixotropic consistency.
The main parameter that defines the characteristics of FRP strengthening is not its tensile strength, which is always far greater than the workloads to which FRP strengthening is subjected, but its modulus of elasticity. The higher the modulus of elasticity of the fibres, the higher the amount of stiffness they supply.
Types of intervention using FRP
There is a very wide range of applications where FRP may be employed:
- repairing and static and seismic upgrading of unstable or weak structures where the shear strength needs to be supplemented;
- confinement of compressed or compressed/flexed members (pillars, bridge piles, chimneys) to improve their load-bearing properties or their ductility where longitudinal reinforcement also needs to be supplemented.
- strengthening flexed members by creating an external sleeve to the areas subjected to tensile loads;
- repairing structures with localised impact damage, such as bridge beams hit by trucks carrying tall or wide loads;
- seismic upgrading and restoration of domed structures without increasing their seismic mass and without the risk of liquids percolating towards the internal surface;
- creating sleeves around beam-pillar hinge zones for seismic upgrading;
- strengthening load-bearing members in buildings whose structural system has been modified due to new architectural requirements or change in use;
- repairing structures damaged by fire;
- seismic upgrading of reinforced concrete industrial buildings.