To get to clarify what Fibre-reinforced plastics are it is necessary to start from the origins. They are organic-based materials with a high molecular weight, consisting of pure polymers or mixed with additives or various charges.
Let’s take a step back.
Fibre-reinforced plastics are fibre Reinforced Polymers (or FRPs or fibre-reinforced polymer matrix materials or fiber-reinforced materials) constitute a wide range of composite materials, consisting of an organic polymer matrix with which is impregnated a continuous fiber reinforcement with high mechanical properties.
The first FRP composite material was made in the 20th century: only in the early 1940s when the first fiberglass artifact, a boat, was produced. Then in the 1960s, high-strength carbon and boron fibers appeared; while in the early 1970s the aramid fiber, under the commercial name of kevlar, was unearthed.
Back to the Plastic FRPs.
Loaded plastics are composite materials in which the matrix is precisely the chosen plastic material, inside which are drowned: carbon fibers, glass, kevlar, or even wood.
Fibre-reinforced plastics are a category of loaded plastics that specifically uses fiber materials to mechanically enhance the strength and elasticity of plastics (matrix or binding agent). The amount of mechanical improvement depends on different factors: the properties of the matrix and the fiber, their relative volume, and the length and orientation of the fiber within the matrix.
Like all composite materials, FRPs have anisotropic and heterogeneous behavior but exhibit predominantly linear elastic behavior until collapse.
These materials have different peculiarities, which vary according to the type of individual FRP and determine its scope. However, all fiber-reinforced products have common characteristics such as:
- high lightness
- high mechanical strength
- high corrosion resistance
- high thermal resistance
- high dielectric and amagnetic properties
The fibers used for the production of FRP must have either high mechanical strength and/or high elastic modulus. The most common are:
- Carbon: the fiber-reinforced material is known as CFRP (Carbon Fiber Reinforced Polymer).
- Glass: the fiber-reinforced material is called GFRP (Glass Fiber Reinforced Polymer).
- Aramid: the fiber-reinforced material is known as AFRP (Aramidic Fiber Reinforced Polymer).
Less used are boron fibers and ceramic fibers.
The fibers consist of very thin continuous filaments (diameter about 10 μm) which are commercially available in various forms, among which the most common are:
- single strand (monofilament)
- filament beam or spinning cable (tow): made up of thousands of strands parallel to each other, assembled without twisting;
- spun yarn: obtained from thousands of strands parallel to each other, assembled by twisting;
- roving: wire obtained by assembling without twisting a number of spun yarns, arranged parallel to each other.
The fibers before their use are transformed into tissues or pultruded.
The fibers used to reinforce the polymer matrix can be transformed into fabrics.
From the geometric point of view, a distinction can be made in:
- monoaxial fabrics: consisting of fibers or bundles of fibers all arranged in parallel lines and held together by a texture of filaments which may be of the same material as the warp fibers or, more often, of a different material (e.g. nylon or polyester).
- biassial fabrics: they are obtained by weaving bundles of fibers in two orthogonal directions. They can be made using both fibers of the same type in both directions and fibers of different nature (e.g. carbon in one direction and aramid in the other).
- multiaxial fabrics: they are obtained by a arranged the fibers in several directions that are variously inclined between them. On the market, there are triaxial fabrics, with bundles of fiber woven in three inclined directions of 120° to each other, and quadriaxial fabrics characterized by the presence of four different orders of fibers tilted 135° to each other.
The tissues thus obtained are used impregnated with polymer resins.
Pultrusion is the production process by which some composite artifacts are made.
This technology consists of an extrusion process similar to that used for the production of bricks during which the fibers are pulled to ensure their perfect alignment before being impregnated with the polymer matrix.
A pultrusion system normally includes:
- a continuous fiber coil unfolding station;
- a tub for impregnation of fibers with polymer resin;
- a hot or microwave accelerated forming and curing station to allow rapid curing;
- a system of tracks or jaws used to exert traction force and to allow the product to advance;
- a cutting station of the finished product in the desired length.
With pultrusion, only constant section products are made and with fibers all oriented in a single direction.
Therefore, with this technology, only foils, profiles, and bars of various shapes and sections are produced.
Pultruded profiles are, for example, used in construction for the realization of fixtures.
On the market, there is a wide range of FRP products in the form of foils, ribbons, fabrics, bars, profiles, used in various sectors such as:
- electrical and energy;
- transport (automotive, rail, naval and airport, aerospace, etc.);
- especially in the field of construction.
Focusing on the construction field FRP fibers are used, in the civil construction sector, for the structural reinforcement of masonry, reinforced concrete buildings, and in the structural recovery of buildings affected by seismic disasters.
Specifically, FRP carbon fibers are appreciated for their high mechanical properties, they are used for the reinforcement of individual elements such as nodes and columns: in these cases, they are applied with confinement and “circle” techniques. The same fibers are also used for the reinforcement and consolidation of bending and cutting beams and floors.
FRP REINFORCED CONCRETE
FRP-reinforced concretes are obtained by associating fabrics, bars, foils, and tapes in fiber-reinforced composite material with normal or prestressed concrete structures.
The combination of the two materials in construction is increasingly used for the recovery of existing structures, thus avoiding the demolition of the same. However, FRP materials are also used in the construction of new constructions.
The most frequent cases are:
- restoring the durability and carrying capacity (consolidation) of bewitched structures;
- structures for which, in the course of their lives, accidental project loads have varied;
- statically unsuitable structures due to design or realization errors;
- adaptation of structures following changes in current regulations (e.g. seismic regulations);
- repairs of structures following a seismic event.
In the seismic field, FRPs allows to increase the bearing capacity and/or ductility of a structure without the introduction of new seismic masses.
The most used technologies for structural recovery are:
- manual lay up: it is currently the most widespread technology, it consists of impregnating unidirectional or multidirectional dry fabrics directly in place using epoxy resins that have the function of both matrix and adhesive to the structural substrate. This technology can be according to two methodologies:
- dry lay-up – suitable for reduced work, essentially consists of a first phase in which a very fluid epoxy primer is laid out with a brush or roller on the concrete support that creates the best conditions for the adhesion of the fabric, then the first layer of fabric is applied with the fibers aligned along the desired direction and finally, the fabric is impregnated by a fluid epoxy resin also applied with roller or brush. With the same sequence the next layers apply;
- wet lay-up – more suitable for carrying out work on large surfaces., than the previous method, immediately after laying the primer, the fabric previously cut to the desired size and immersed directly in a tray containing the fluid epoxy resin is applied.
- plate bonding or plating method: consists of structural bonding with epoxy resins of pultruded foils (generally rectangular) directly on the substrate, suitably leveled, of normal or prestressed reinforced concrete limbs to be consolidated. This method is based on the most classic beton plaque in which the foils used are made of steel (glued and/or bolted).
- Near Surface Mounted Bars or NSM: Consists of structural bonding with epoxy resins of pultruded cylindrical or rectangular bars. In this case, it is created in the thickness of concrete that constitutes the iron cover, special grooves where the FRP bars are placed, and then glued. The epoxy resins used as an adhesive guarantee both an excellent adhesion between the concrete and the FRP material, and an adequate transmission of the cutting efforts between the two materials without dangerous vicious phenomena occurring which result in relative sliding between the two materials.
In the field of new construction, FRP materials have not had the development they had hoped for. These materials, while having a high lightness and mechanical strength in comparison to the classic building materials, have different limits of application:
- fragility of the connection systems between various elements; to overcome this problem, increasingly performing adhesives are being developed, to replace joints in metallic material, which until a few years ago seemed to be the only possible alternative;
- impossibility of processing to obtain special pieces (realization of brackets and shaped irons in the c.a. structures);
- high costs.
An operation has been made to use FRP sections to replace steel sections for the construction of metal carpentry and FRP bars to replace traditional steel reinforcement (only in the form of straight bars or load-sharing nets) for the construction of reinforced concrete structures.