The use of fiber-reinforced composite materials has registered, during the last years, a particularly signficant increase to their use for the restoration and strengthening of both masonry and R.C. structures thanks to their excellent mechanical properties and to their capability to offer undoubted advantages in terms of reversibility and not invasive application.
On the other hand, together with the several advantages already mentioned, the use of FRP for civil engineering applications has posed also several problems such as the need of using always epoxy resins for bonding and impregnating the fibers, the need of leveling the substrate by a thin layer of putty to assure no defects, the need of a chemical bond (never mechanical) to transfer shear stresses from the substrate to the lamina. Finally all the problems related to fire resistance and to the mistrust of public offices in using such innovative and modern materials on ancient ones.

In this scenery a new generation of composite materials is nowadays available also for the Italian market, the SRP (Steel Reinforced Polymer) materials and the SRG (Steel Reinforced Grout) materials consisting of steel fibers having a high tensile strength (Ultra High Tensile Steel, Hardwire™) and braided in order to create chords immersed in either epoxy resin (SRP) or in a mortar matrix, that allow to obviate to all the above-mentioned problems, however maintaining the most important advantages of the FRP technology.

The main properties of this innovative composite materials can be summarized as follows:

• steel ( rising from the evolution in pearlitic or ipereutettoidic steels during wire drawing) increases the ductility of the strengthened element thanks to a pseudo-ductility generated by the stretch of filaments that are rolled up in cords, after they have been pulled, without undergoing essential yielding, maintaining an almost perfectly elastic behaviour till rupture;
• thanks to its shear strength, steel can simplify problems concerning connections and anchorages;
• the use of cords does not need solely epoxy resins for matrix since it shows a high compatibility with every kind of thermoplastic or thermoset polymeric and also with mortar matrixes;
• the impregnation with a mortar matrixes can overcome problems concerning fire-resistance of the composite material and considerably reduce the installation costs since there is no more the need of having perfectly smoothed surfaces, since the mortar previously used for leveling can now be used also for impregnating the fibers;
• thanks to the copper or zinc coating of steel wires, the material shows its scouring resistance, by guaranteeing durability in the long run and an optimal adherence through the main resins/mortars available on market.

It is possible to specify the physical-geometrical properties which influence the mechanical response of the composite:

• the strength class of the filament, included between 2400 and 4000 MPa;
• the diameter of the wire, included between 0.20 and 0.48 mm;
• the cord type according to the way filaments are twisted (ST2, 3SX, 3X2);
• the density of the type, that is the number of cords present per unit length ( low, medium, high). This parameter, besides affecting the mechanical response of the composite, allows the use of a wider scale of matrixes with different viscosities.

The above-mentioned materials are produced by Hardwire LLC in USA and are nowadays available also for the Italian/European market thanks to TEC.INN. S.r.l.