## 1 Introduction
Thus, such a retrofit is able to easily upgrade the flexural capacity of slabs as the demands are mainly flexural. The same also applies to the flexural upgrade of RC beams for gravitational loads. The shear upgrade  
Figure 2: **(a)** Shear failure of an RC column at the ground floor of a four-story RC building. Athens, Greece 1999 earthquake sequence. **(b)** Detail of the same shear damage [1]. presents certain difficulties that are discussed within the rest of the sections of this study. Moreover, the flexural or shear upgrade of beams or columns for seismic loading is more difficult as the regions to be retrofitted are near and include the RC column-to-beam joints. For the RC columns, the flexural upgrade is again quite difficult as the regions to be retrofitted are the ones where the columns are joined with the foundation or with the beam-to-column joints. Alternatively, the shear and the compression capacity of RC columns can be effectively upgraded by externally applied CFRP close hoop strips. A large volume of research has been published utilizing strips made by FRP systems based on epoxy resins as well as high-strength steel fibers (known as SRG or SRP) with either cementitious grouts (SRG) or with organic resins (SFRP) by several researchers [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43]. A typical mode of failure is reported to be the delamination of these FRP strips [35; 36; 37]. Using FRP strips based on steel wires is relatively limited when compared to the strips based on carbon fibers (CFRP). This study focuses on the specific problem of RC T-beams which are under-designed in shear. This is a relatively common problem that is due to outdated seismic code provisions with relatively low seismic force levels used in the past, which resulted in relatively low shear force demands for such RC beams in multi-story RC buildings. According to such an outdated seismic design, the shear demands for T-beams were resulting from the combinations including dead and live load and they were met by relatively light transverse reinforcing with open hoop steel stirrups. Instead, current seismic codes result in much larger shear force demands than before at regions where such beams join the columns. In addition, they require that the transverse reinforcement consists of closed hoop steel stirrups. Therefore, it is necessary to strengthen such under-designed RC T-beams deficient in shear capacity. Figure 1b depicts a retrofitting scheme whereby such shear capacity upgrade is carried out by constructing an RC jacket employing extra closed hoop stirrups narrowly spaced. Such a scheme is relatively difficult because it requires the partial breaking of the slab in order to place the closed hoop stirrups and then cast the RC jacket. A relatively simple alternative is to try to attach externally, on the web of such RC T-beam, strips of fiber-reinforced polymers (FRP) in the form of stirrups bonding them to the concrete surface with special epoxy resins. However, even in this case, the main difficulty is to try to add closed hoop FRP stirrups. A large number of experimental  
Figure 3: **(a)** Shear failure of an RC short column at the ground floor of a two-story RC building. Aigio, Greece 1995 earthquake sequence. **(b)** Detail of shear damage of a shear wall at the ground floor of a four-story RC building. Pyrgos, Greece 1993 earthquake sequence [1]. investigations were performed with rectangular beam specimens which did not include the slab. In this case, the absence of the slab allows the externally attached FRP strips to have the form of closed hoop stirrups. However, in the vast majority of multistory RC buildings, the slabs are cast in unison with the supporting beams, thus forming typical T-beam structural elements; this does not allow easy passing of such FRP strips in order to form the closed hoop stirrups that are required in order to bring the shear capacity to meet the demands posed by the upgraded seismic design.