## 2 Materials and Methods
### \"Unit T-Beams\" with Open Hoop CFRP Strips Employing Specific Mechanical Anchoring Devices  
The tested \"unit T-beam\" specimens in this sequence are listed in column (1) of Table 2 with their code name stating the presence (or not) of an anchoring device for the CFRP strips. Moreover, the preparation of the concrete surface where the CFRP is attached is also indicated. Column 2 of Table 2 lists the average maximum value of the axial strain that was measured at locations s.g.1 and s.g.3 [53]. Column (3) of Table 2 lists the maximum total applied load value during each test. Column (4) lists the value of the total axial load based on the measured average maximum axial strain value (column 2) and the measured Young's modulus value of the used CFRP strips equal to 234 GPa. It is assumed that the CFRP strips of the tested specimen develop this maximum strain value for the whole width of its cross-section at both sides of the \"unit T-beam\". Each CFRP layer had a thickness of 0.131 mm and a width of 100 mm. A reasonably good agreement can be seen by comparing the measured total maximum load values (Table 2 column 3) with the corresponding values calculated in the way described earlier (Table 2 column 4). It can be concluded that, for design purposes, it is very important to be able to ascertain with confidence the axial strain level that can be assumed to arise in these FRP strips. Towards this objective, it is important to approximate the effectiveness of the bonding surface or of a specific anchoring device and on this basis to adopt the appropriate axial strain value for the FRP strips. The following summarizes the most important observations of the behavior exhibited by either the used CFRP strips or the specific mechanical anchoring devices used in these tests. (a1) The debonding of the CFRP strip from the concrete surface was observed for strain/stress levels well below the strain limits given by the manufacturers of the FRP materials. The strain/stress levels accompanying this debonding mode of failure continually decrease when one increases the layers of the FRP strip, and consequently its thickness and cross-sectional area, rendering such layer increase totally ineffective unless it is combined with some type of anchoring. This type of failure, which is expected to occur in similar practical applications, is depicted in Figure 12a as observed during the current investigation. (a2) From the preceding discussion, it becomes obvious that the debonding mode of failure prevails in almost all cases where an open hoop FRP strip is simply attached without any anchoring. However, the effective anchoring of such an open hoop FRP strip is not easy. Thus, the second category of modes of failure includes limit states in which the final debonding and failure of the FRP strip is a result of the interaction between the FRP strip and the used anchoring scheme. In many cases, the employed anchoring scheme is insufficient to withstand the level of axial force that the FRP strip can withstand by itself in  
Figure 12: **(a)** Debonding mode of failure. **(b)** Failure of the anchoring scheme accompanied with debonding. **(c)** Tensile failure of the FRP strip. ideal axial tension conditions leading to either local failure of parts of the anchoring scheme or local failure of the FRP strip in areas neighboring the anchor or both. Again, the increase of the layers of the FRP strip, and consequently of its thickness and cross-sectional area, results in a corresponding increase in the demands on the various parts of the anchoring scheme to withstand this increased CFRP strip capacity leading to partial successive failure of the anchoring device. This type of failure is depicted in Figure 12b as was observed during the current investigation for an anchoring scheme that proved ineffective and is not reported further in this paper. (a3) The final mode of failure is in a form of tensile failure of the FRP strip.