## 4 Discussion
### Confinement Efficiency  
The ratio of the average FRP hoop strain to the maximum hoop strain that was calculated in Table 1 can be viewed as the efficiency factor of the FRP wrap relating the actual confinement stress to the theoretical one achievable when the whole wrap reaches its limit strain uniformly. The calculated values ranged between 0.692 and 0.874 and are in agreement with the observations and experimental results reported in the literature. [PERSON] et al. [PERSON] _et al._ (2018) were among the first to introduce a strain efficiency factor accounting for the discrepancy between their experimental results and predictions. Based on their  
Figure 9: Characteristic length, Lc, and failure mechanisms for H/D = 2 and H/D = 4.  
experimental observations, [PERSON] et al. [13] proposed an empirical reduction factor of 0.5. [PERSON] and [PERSON] [5; 7] reported an efficiency factor of 0.586 based on the calibration of their experimental results on standard-size cylinders. [PERSON] and [PERSON] [74] computed 0.58 from a database of 251 test results, the majority of which pertain to standard-size cylinders. [PERSON] and [PERSON] [15] reported efficiency factors between 0.57 and 0.61 from the experimental results on small-, medium- and large-scale columns. [PERSON] and [PERSON] [6] had previously analyzed results from 19 published data sets that contain 192 data points and obtained a mean efficiency factor of 0.598 with a standard deviation of 0.248. [PERSON] and [PERSON] [75] analyzed 832 datasets collected from 99 experimental studies published between 1992 and mid-2013, and reported confinement efficiency factors between 0.675 and 0.709. [PERSON] and [PERSON] [76] carried out an extensive investigation on the effects of various parameters on the efficiency of the FRP-confined concrete, covering their own experimental data [77] and others'. They reported an efficiency factor close to 0.73 for the circular specimens considered in their study.  
The results of the parametric numerical modeling indicate that the confinement efficiency increases with the confining stiffness of the wrap, which agrees with experimental observations. Figure 10 shows the non-linear increase in the confinement efficiency with the number of confining FRP wraps for the various H/D ratios; all calculated values are greater than the ACI recommended 0.55.  
ACI 440.2R17 recommends one single conservative value of 0.55 for the efficiency factor in design Equation 12.1 [1], for all geometries and levels of confinement. The theoretical efficiency factor obtained from a numerical analysis, such as the one presented in this work, and a strength reduction factor can be employed in the formulation of an alternative design equation. Assuming, for example, a strength reduction factor of 0.75, since fracture controls the failure of the FRP-confined concrete, then the recommended efficiency factor for a given confined concrete configuration would be equal to the value of the predicted confinement efficiency obtained from the numerical model for the corresponding geometry and confinement level multiplied by 0.75. For example, assuming a confinement equivalent to six wraps, then the predicted confinement efficiency is 0.85, which gives \(0.75\times 0.85=0.64\), and which is 16% higher than the ACI recommended value of 0.55.