## 5 Conclusions  
A simple axisymmetric finite element model was developed for FRP-wrapped concrete cylinders using the built-in capabilities of a commercial code, PLAXIS, and a Mohr-Coulomb constitutive material law. Without any particular modifications, the model was able to capture the failure localization in the confined concrete showing multiple failure planes and the development of a failure mechanism. The results indicate that the failure mode of the FRP-confined concrete is by localized rupture of the wrap due to extreme strains caused by the localization of failure inside the core. A parametric analysis was carried out for four slenderness ratios, namely H/D = 1, 2, 4 and 6, and for four levels of CFRP confinement, corresponding to 1, 2, 4 and 6 wrap(s). The main observations were as follows:  
* The stress-strain structural response curves capture the experimentally observed typical bi-linear behavior of the FRP-confined concrete. In the first part of the stress-strain curve, the behavior is linear similar to that of the unconfined concrete. The second part shows a pseudo-plastic linear behavior, depending mainly on the FRP wrap mechanical stiffness and the fracture mechanism developed inside the concrete. * For a slenderness ratio H/D equal or greater than 2, the stress-strain responses of the cylinders coincide for the same level of confinement and fan up with increasing stiffness of the confining  
Figure 13: Failure mechanism in a confined concrete cylinder. FRP. The response of the cylinders with H/D = 1 is affected by the additional confinement caused by the friction between the loading platens and the concrete cylinder. * The proposed numerical model captures the strain localization inside the concrete core and reflects that in the development of variable hoop stresses in the confining FRP along the length of the cylinders. Hoop stress profiles allow the prediction of the location of the premature failure of the FRP when its maximum strain is reached as well as the calculation of the average strain or confinement level in the confined system. The results of the localization patterns indicate the existence of a characteristic length, Lc, independent of the level of confinement that is necessary for the localization and development of an unconstrained failure mechanism or solid wedge mechanism. This characteristic length, Lc, was estimated to be equal to 1.5D. These results concur with the findings published in the literature about localization in unconfined and confined concrete. The efficiency factors of the FRP wrap, relating the actual confinement stress to the theoretical one achievable when the whole wrap uniformly reaches its limit strain, were calculated. The calculated values are weakly dependent on the slenderness but show a clear increasing trend with confinement stiffness increases, ranging between 0.692 and 0.874. These values are in agreement with the results reported in the literature. Using a shear-friction analysis approach, a simple rigid wedge mechanistic model is proposed. Equations are proposed that can be used to predict the failure load of the confined concrete as well as the stress-strain structural response of the confined system. The proposed approach can be used to calculate the confinement efficiency factors for various geometries and to refine the design recommendations of ACI 440.2R17, such as in Equation 12.1 [1], by relating its parameters to independently measured material properties. The sensitivity of the results reported in this work to variations in the properties of the FRP and concrete as well as the geometry of the confined cross sections requires further investigation. Further work is needed for the fine tuning of the proposed stress-strain model and its verification against the extensive data available in the literature. Conceptualization, G.K. and M.T. ; methodology, G.K. and M.T. ; software, M.T. ; validation, M.T. ; formal analysis, G.K. and M.T. ; investigation, M.T. ; resources, M.T.