## 1 Introduction
[25] discussed the non-uniform distribution of hoop strains in the FRP wraps of elliptical section columns and numerically modeled the experimental results to propose a stress-strain model for the structural response of the columns. The effects of specimen slenderness, which represents a relevant parameter of the problem, have been known for a long time [30; 31; 32; 33; 34; 35] and were at the basis of selecting a height to diameter H/D ratio of 2 for the standard cylinder used for testing unconfined compressive strength. Finite element models have been used over the last twenty years to model the structural response of FRP-confined concrete sections of various geometries and types of reinforcement and confinement with two objectives: (i) modeling the apparent stress-strain relationship or structural response of the confined concrete columns, as well as (ii) predicting the failure load and mode of failure [36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50]. The most popular FEM software package used was ABAQUS, for its choice of standard material models and the flexibility offered by its materials' subroutine [38; 42; 43; 44; 45; 46; 47; 48; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60]. Other FEM codes were also used, such as ANSYS [39; 57], LS-DYNA [45], DYNA3D [37], PRECON3D [41] and MASA [46]. The key, however, to modeling the complex behavior of concrete is the choice of material model, and many models have been used, with the Drucker-Prager [40; 47; 48; 57] and Concrete Plasticity Damage models [54; 55; 56; 58; 59; 60; 42; 51; 54; 55; 54; 56; 57; 58; 61; 62; 63; 64; 65; 66; 67; 68; 69; 60] being the most popular; in addition, Mohr-Coulomb has been used [41], a microplane material model has been used [46] as well as an adapted CAM-CLAY model [53]. Most of the proposed models achieved acceptable to good agreement with experimental data and contributed to the understanding of the field; however, many of these models are highly elaborate or were developed for particular geometries and conditions. It was noted that a limited number of publications have addressed the localization of failure in confined concrete, its effect on the failure of the FRP wrap and, consequently, the confinement efficiency [16; 17; 18; 19; 20; 21; 22; 23; 24; 29]. Given the nature of the problem, the need for a simple FEM approach with a simple material model that allows the study of localization and failure, without the complex refinements particular to plasticity and damage models, was identified and considered in this work. In order to investigate the relationship between localization, failure mode and the observed structural response, a parametric numerical investigation into the behavior under compression of concrete cylinders wrapped with CFRP was carried out and is presented in the remainder of this paper. The number of wraps as well as the slenderness of the cylinders wereas varied and the following results are presented and discussed: (i) the stress-strain or structural response curve; (ii) the CFRP hoop strain profiles; and (iii) the principal strains inside the concrete showing localization patterns. A simple mechanistic model is also proposed to model localization and the second branch of the bi-linear structural stress-strain response. A modification to the design equation for Equation 12.1 in ACI 440.2R17 [1] is proposed, taking into account the slenderness effects and the level of confinement.