## 1 Introduction
It requires specialized in situ labor and equipment; it also requires perforating the floor RC slab, by partially breaking structural elements in order to place the needed additional steel reinforcement, either longitudinal or transverse steel reinforcement. Moreover, employing such RC jacketing techniques increases the dimensions of the upgraded structural members, resulting in an increase in their stiffness and their dead load. This may not be always desirable. At the same time, such RC jackets result in a decrease in the available internal free space or applying RC jackets may face prohibitions in the external space of the structural system from adjacent buildings. The increase in mass and stiffness, resulting from RC jacketing, may change the dynamic characteristics of the whole structural system and may also cause, in some cases, undesirably increased demands at specific structural elements. Figure 1: (**a**) RC jacket of columns employing shotcrete (gunite). (**b**) RC jacket of column and beam at the region of their joints employing shotcrete (gunite) [13]. Such RC jacketing of critical structural elements can be combined, when possible, with additions to a structural system of extra shear walls along its height in proper locations. This combination can reduce excessive displacement response in a way that the demands to the critical elements are met with the corresponding capacities [13]. This retrofit concept is quite effective; however, the construction of such jackets represents a degree of practical difficulties and response issues, as mentioned before. During the last decades, alternative retrofitting techniques were developed aiming to provide practical solutions to the in situ RC jacketing difficulties for the various RC structural elements [14; 15; 16; 17; 18; 19; 20; 21]. Obviously, apart from the structural elements themselves, the more difficult regions for effectively retrofitting an existing RC structure are the critical areas of the structural member connections (joints) as well as the connections with the foundation. Both, the structural connections and the foundation are very critical areas that require special consideration in both identifying the nature of the probable structural damage as well as proposing countermeasures. The main flexural structural damage in slabs and beams develops in the areas of maximum bending moments. For the beams, this usually develops near the joints with the columns and shear walls where large bending moments are expected to arise due to the seismic loads. Similarly, at the ends of the beams are areas of large shear forces from the combination of earthquake forces with the dead and live loads; these will cause the appearance of shear damage in the form of diagonal cracks. The presence of large bending moments mainly from the seismic loads together with large axial forces will cause the formation of flexural damage at the top and the toe of columns, whereas the presence of shear forces from the seismic loads together with axial forces will lead to the formation of shear damage at the columns, a very dangerous form of damage (see Figure 2a,b). The presence of large shear forces from seismic loads, together with relatively low-level axial forces, will lead to the development of another dangerous form of shear damage in the shear walls (see Figure 3b), whereas the presence of short columns will lead to the development of large shear forces from seismic loads and the development again of shear damage, as shown in Figure 3a. Alternative retrofitting techniques to RC jacketing have been developed, which employ fiber-reinforced polymers (FRP) attached externally with special organic resins or inorganic matrices to weak structural elements. This external application has in some cases specific practical advantages when the resulting structural performance is upgraded in the desired way [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. It can be applied to various structural elements of an existing RC structure with a varying degree of effectiveness and difficulty.