## 1 Introduction  
The confinement of reinforced concrete columns in aging infrastructure with multiple layers of fiber reinforced plastics (FRP) has gained wide acceptance as a repair and strengthening method and considerable experimental and theoretical research has been devoted to the topic during the last 30 years. International and national organizations have issued standards and technical recommendations for the use of FRP wraps to reinforce concrete columns, such as in ACI 440.2R17 [1] and CS806 [2], reflecting the state of the art. The strengthening effect is achieved by the passive resistance that the elastic FRP confining wrap offers to the lateral expansion of concrete under compression. In spite of multiple research efforts, no unified model has been established to predict the stress-strain response of FRP-confined concrete or to predict its failure strength. Several models, mostly empirical in nature, were proposed in the literature to simulate the apparent stress-strain or structural response of the FRP-confined compression sections, such as [PERSON] et al. [3], [PERSON] and [PERSON] [4] and [PERSON] and [PERSON] [5], to cite some of the earliest. These models have been calibrated against the authors' own sets of experimental data showing limited predictive capabilities for other data sets [4; 6]. The bi-linear stress-strain model by [PERSON] and [PERSON] [5; 7] for FRP-confined concrete was adopted in ACI 440.2R17 [1] and the design formulae were also recommended based on a modified form of the same model. One of the main shortcomings encountered by the proposed models and the published design recommendations is the difficulty in determining the actual failure strain of the FRP wrap and hence the effective confinement pressure. In closely monitored experiments, the FRP confining device is observed to fail at lower average strains than those measured for FRP tensile coupons, of the order of 50% to 80%, therefore providing a proportionally reduced confining stress [5; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. ACI 440.2R17 [1] has adopted a bi-linear stress-strain relationship for FRP-confined concrete and recommended the use of a conservative strain efficiency factor of 0.55 for the FRP in order to estimate the confinement stress. A minimum confinement ratio of 0.08 of the unconfined compressive strength was recommended by ACI 440.2R17 to ensure a non-descending second branch in the stress-strain response diagram. This suppression of softening was discussed in detail in [PERSON] [26] and [PERSON] et al. [27]. Different explanations have been proposed for the low average failure strain of the FRP wraps, the most plausible being that stress concentration regions caused by cracking and dilation of the concrete lead to the development of local stress concentrations in the FRP wraps [5; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. Using numerical modeling, the authors showed that the hoop strain distribution along the height of the cylindrical concrete specimens was not uniform. Their results suggested that localization and the development of a failure mechanism in concrete caused the development of stress concentration zones where the ultimate FRP strain was reached, thus resulting in premature failures, as was noted in experimental observations [28]. Using a photogrammetric digital image correlation technique (DICT) that allows capturing strains from the surface of the FRP-confined concrete, [PERSON] and co-workers [16; 17; 20], [PERSON] and co-workers [19; 21; 23] and [PERSON] and [PERSON] [29] measured the non-uniformity of the hoop strains across the height of the tested concrete cylinders and the sharp rise of these strains points to localized failure. Extensive arrays of electrical resistance strain gauges were also utilized by other researchers [18; 22; 24] to confirm the non-uniform distribution of the hoop strains in the FRP along the height of the cylindrical concrete samples, and the localization of failure. [PERSON] et al.