Fiber Reinforced Polymers (FRP)


The composite materials consist of high-mechanical reinforcement fibers and a resin matrix that ensures adhesion to the reinforced structural surface and transfers the loads to the fibers. They are used effectively to strengthen and restore structural elements throughout the construction sector. This application is fully described in the relevant bibliography and is widely used in Japan, the USA, Canada and the countries of Northern and Southern Europe, mainly for the reinforcement of concrete structures.

In Italy, the application of composite materials began in 1996 and brought a rapid technological development in the method of restoration and reinforcement of concrete structures and masonry. The composite materials are suitable for solving problems related to the restoration of existing and historic structures, caused by aging and wear over the years or by various factors such as landslides, earthquakes, strong vibrations from vehicle traffic, incorrect construction work as well as subjecting to excessive loads. The problem of restoring listed buildings / monuments exists to a large extent in Italy, but also in other European countries, such as Greece, Spain and Portugal. The purpose of using composite materials is to improve the mechanical behavior of structural elements, that is to improve strength, failure deformation (and therefore plasticity) and the ability to absorb energy.

Composite reinforcement (fibers in a resin matrix) is applied directly to the surface of the structural element to be upgraded. The resulting composite material consists of reinforced polymers (carbon, glass, aramid) of high strength and elasticity and the matrix of epoxy or fireproof resin. The matrix takes the shape of the element and achieves the cooperation of the reinforcement with the carrier which it is called to strengthen by transferring the carrier stresses due to external loads to the reinforcement fibers. Some representative results of application of composite materials are:

  1. Increasing the flexural strength of structural members (pillars, walls, slabs, beams)
  2. Increasing the flexibility of structural members (pillars, walls, slabs, beams)
  3. Increasing shear strength of structural members (pillars, wall, beams)
  4. Increasing Axial strength of structural members (pillars, walls, beams)
  5. Increasing the plasticity of structural elements (pillars, walls, beams)
  6. Reduction of anchor length requirements (increase in floor height)
  7. Increase of joint strength (beams-columns)


There are many applications where tried-and-tested traditional reinforcement methods using injected or sprayed concrete or metal sheets cannot meet their needs due to:

  • Challenging the application space
  • Long application completion time
  • Unavoidable alterations in the architectural style of the building
  • “Destructive” intervention in listed buildings
  • High cost of operation (direct or indirect that makes it prohibitive)

The main advantages of using FRP systems, from composite materials, for the repair, reinforcement and restoration of structures in relation to traditional methods, using conventional materials (such as steel or concrete), are the following:

  1. High mechanical properties – Composite materials are non-conductive materials and have properties such as: increased resistance to aging and corrosion, high resistance to fatigue, shock and creep deformations and resistance to moisture as well as alkaline and acidic environments.
  2. Flexibility in design – The composites are lightweight with extremely high tensile strength, multiples of the strength of steel, and behave linearly. They increase the strength of the structural elements in bending and shear as well as the plasticity of the joints. The active mass of the building does not increase significantly so that the dynamic characteristics of the building are not adversely affected (stiffness, idioperiods, idiomorphs). They can be applied in combination with traditional methods. Their use results in reduced deformations due to beneficial loads (increased rigidity), increased bearing capacity, increased fatigue resistance and reduction or elimination of cracks (increased strength). Also, composite materials have a high ratio of resistance to mass and stiffness to mass, which is a critical factor in seismic reinforcements, as an increase in mass causes an increase in seismic loads.
  3. Small volume of reinforcement – With the use of composite materials the architectural features of the structures remain practically unchanged without changing the geometry. The very small thickness of reinforcement (few mm), with composite materials, does not increase the size of the reinforced elements, in contrast to the traditional methods where a significant useful space of the building is removed (eg concrete mantles).
  4. Ease and speed of application – Their application is simple and requires short preparation at the construction site. The weight of the composite materials is small, they are easy to transport (available in rolls) and their installation does not require heavy or special equipment but simple and small tools. They are cut on the spot to the desired length and width without wasting material resulting in the operation being very fast (in most cases the application time is a few weeks).
  5. Flexibility of application – The application of composite materials is possible even in cases where there is a limitation of the working space. It can be formed on the spot in the project around any surface shape (after its proper preparation).
  6. Minor nuisance in the operation of the building – The evacuation of the space is not necessary and the nuisance to the users is minimal. Reinforcements with composite materials do not require a large workspace and the functions of the building are largely not interrupted during the works.
  7. Reversibility of the reinforcement – The use of composite materials allows the removal of the original reinforcement from the carrier, in order to apply another reinforcement method in the future. This important feature acquires special interest in matters concerning the restoration or reinforcement of buildings of artistic and historical value, since the application of equipped polymers is a non-destructive method of reinforcement.
  8. Architectural result – The applications of composite materials can be coated and painted according to the aesthetic requirements of the project.
  9. More economical solution – Based on the above advantages and given that today the use of composite materials is widespread, it is natural that the solution they provide is more economical and effective than conventional reinforcement methods. Applications of composite materials in addition to the above advantages, have the ability to deal with problems point by point and where required without the need for overall intervention, as in conventional reinforcement methods.