The use of advanced fiber-reinforced composite materials has registered, during the last years, a particularly signficant impulse for the restoration and the strengthening of both masonry and R.C. structures thanks to their excellent mechanical properties (very good capacity/durability/self weight/costs) and to their capability to offer undoubted advantages in terms of reversibility and not invasive intervention, that represent two remarkable aspects for the Supervising Office of Historical Patrimony.
Fiber-reinforced composite materials, well-known through the English acronym Fiber Reinforced Polymers, are materials consisting of long high capacity fibers that are immersed in a polymeric matrix.
The fibers represent the resistant components of the material and show very high axial strengths when subjected to tensile stresses. On the contrary, the polymeric matrix, commonly named resin, aims to protect fibers from the wear and tear and from possible external damages, to assure a good fibers alignment and to guarantee a good strain distribution among fibers, in order to stress them uniformly. Mechanical properties of composites generally depend on the arrangement of the above-mentioned fibers and resin. Generally the percentage of volumetrical fraction of the fibers with respect to the total volume is approximately 70%. Among the various possibilities of fibers available on market, in the construction industry the fibers mostly in use, in performance in terms of strength and in stiffness increasing order: glass, aramid e carbon.
The most common fiber used for strengthening applications of R.C. and P.C. structures is carbon, since there is need to provide high strengths in front of limited strains. It is possible to choose between fibers having a high strength or high modulus according to the problem to deal with. Concerning masonry structures, both carbon and glass fibers are mostly commonly used. Only during the last years aramid fibers started to be applied in the construction industry for the creation of mechanical anchorages of strengthening solutions that implementd carbon and glass fiber sheets, thanks to their higher shear and impact strength.

Among the most commonly used strengthening systems in the Civil Engineering industry we can distinguish:

• Precured Systems: they consist of elements characterized of different shapes, that are prepared in factories through the pultrusion process and that are bonded, using epoxy resins, to the element in need of strengthening. Two different solutions can be adopted:
  1.Bonding of laminates having different sections (generally rectangular) directly on the substrate that is appropriately prepared (usually through hand-brushing or more commonly via sand blasting). This technology is particurarly suggested in case of regular and flat profiles, that do not have either convexity-concavity, and of substrates which are in good conditions (more or little degradation);
  2.Bonding of bars having a circular or rectangular section on grooves previously cut and filled up with resin in order to guarantee adhesion between the bar and the substrate. This technology is known as NSM (Near Surface Mounted Bars).In particular, it is suggested in case of flexural strengthening in negative moment region of slabs, by not keeping the strengthening exposed to mechanical degradation. It does not need any superficial treatments except cutting the groove that is particularly costly and timely conserving. This technology is particurarly used on masonry structures thanks to the possibility to insert the bar into the joint between two clay tiles without compromising the masonry aesthetics.
• " Wet lay-up Systems: they consist of unidirectional and multidirectional fiber sheets and grids that are impregnated in situ through resins representing both the matrix and the adhesive for the interested substrate. The system is the most commonly used in our industry, thanks to their versatility and their possiblity to be applied on any kind of geometrical section and in the most different and difficult working conditions without involving onerous costs for the surface preparation.

According to the circumstances of the project, TEC.INN. S.r.l. chooses the best technology, by guaranteeing always an A typology application as it is defined by the CNR-DT 200/2004 design guidelines, that allows to take advantage of the optimal characteristics of the FRP systems. The most commonly used systems, as testified by field applications, are the in-situ impregnated ones.
An important aspect in strengthening through FRP materials of both procured and wet lay-up systems, concerns the preparation of the substrate on which the strengthening is bonded since the effectiveness of the system relies on the capacity of the FRP lamina to transfer the tensile stresses to the substrate through bond. As a consequence, in case the substrate is not able to absorb the above-mentioned stresses, the retrofit will be ineffective. For this reason, both R.C. and masonry substrates are always carefully prepared in order to eliminate every trace of incoherent materials and to make the substrate ready for the installation of the FRP systems.

The main fields of FRP application can be summarized as follows:

• retrofitting of damaged structures in order to restore their durability;
• variation of live loads due to change in use of change of design codes;
• mistakes of engineering or design construction;
• changes of design codes;
• strengthening and/or retrofitting in seismic areas or after an earthquake shock.

In case FRP are applied on R.C. or P.C. structures, generally there can be an increase around 40-50% and such improvement can either interest flexural, shear or axial strength of either beams, slabs or columns. When dealing with masonry structures, strengthening through FRP aims to prevent/retard the formation of hinges that are responsible for the collapse of the structure.