# Mechanical Behavior of Carbon Fiber-Reinforced Concrete Structures After Structural Failure
## 3 Results y Analysis
In summary, the confined specimens, both with and without the addition of fibers, show improvements in maximum stress and ultimate strain compared to the unconfined specimens [36]. This demonstrates that the jointly confined specimen possesses good mechanical properties, with the U 600 fiber yielding the best results in this regard. This suggests that combining multiple confinement methods can be beneficial for improving the mechanical properties of structures subjected to loading. Next, the existing theoretical models will be validated using the results obtained from the experimental tests, comparing the values with the formulations of ACI 440-17 (ACI PRC-440.2-17: _Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures_) [48] and FIB (_Externally bonded FRP reinforcement for \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimens** & **Stresses** & \(\begin{array}{c}\sigma_{\text{max}},\sigma_{\text{utt}}\\ \text{(N/mm^{2})}\end{array}\) & **(\%)** & \(\begin{array}{c}\varepsilon_{\text{max}},\varepsilon_{\text{utt}}\end{array}\) & **(\%)** \\ \hline SA-2-U 300 & Type I & \(40.81\pm 1.17\) & \(14\) & \(0.007\pm 0.04\) & \(33\) \\ PP-3-U 300 & Type I & \(46.45\pm 0.26\) & \(14\) & \(0.009\pm 0.014\) & \(33\) \\ \hline SA-5-U 300 & Type II & \(52.39\pm 0.89\) & \(-14\) & \(0.009\pm 0.009\) & \(4\) \\ PP-4-U 300 & Type II & \(45.15\pm 0.68\) & \(-14\) & \(0.010\pm 0.027\) & \(4\) \\ \hline SA-2-U 600 & Type I & \(62.21\pm 0.27\) & \(-15\) & \(0.015\pm 0.03\) & \(4\) \\ PP-1-U 600 & Type I & \(52.86\pm 1.57\) & \(-15\) & \(0.016\pm 0.028\) & \(4\) \\ \hline SA-5-U 600 & Type II & \(64.41\pm 0.89\) & \(-0.3\) & \(0.016\pm 0.02\) & \(10\) \\ PP-4-U 600 & Type II & \(64.24\pm 0.66\) & \(-0.3\) & \(0.018\pm 0.035\) & \(10\) \\ \hline SA-1-BI 230 & Type I & \(30.05\pm 0.41\) & \(-0.2\) & \(0.007\pm 0.01\) & \(-14\) \\ PP-1-BI 230 & Type I & \(29.98\pm 0.59\) & \(-0.2\) & \(0.006\pm 0.017\) & \(-14\) \\ \hline SA-4-BI 230 & Type II & \(30.21\pm 0.61\) & \(-1\) & \(0.005\pm 0.02\) & \(15\) \\ PP-6-BI 230 & Type II & \(29.95\pm 0.22\) & \(-1\) & \(0.006\pm 0.001\) & \(15\) \\ \hline SA-2-BI 360 & Type I & \(31.91\pm 0.85\) & \(-11\) & \(0.006\pm 0.01\) & \(-6\) \\ PP-1-BI 360 & Type I & \(28.28\pm 0.70\) & \(-11\) & \(0.005\pm 0.018\) & \(-6\) \\ \hline SA-6-BI 360 & Type II & \(38.35\pm 0.23\) & \(-6\) & \(0.008\pm 0.007\) & \(5\) \\ PP-4-BI 360 & Type II & \(35.88\pm 0.61\) & \(-6\) & \(0.008\pm 0.017\) & \(5\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison between Type I and Type II stress specimens for specimens with and without polypropylene fibers. RC structures. International Federation for Structural Concrete, Laussanne) [49]. Figure 3 illustrates the two stages of the stress-strain curve of a jointly confined specimen during a uniaxial compression test. As can be observed, the stress-strain curve in this study has two well-differentiated zones:  
**Elastic zone**: In this stage, the stress-strain curve is a straight line. As the load increases, the line gradually and slightly curves. This curve is similar to the constitutive curve of unconfined concrete. That is, the elastic behavior of confined concrete is similar to that of unconfined concrete, although there may be some differences due to the confinement provided by the CFRP. **Plastic stress stage**: In this stage, the presence of CFRP prevents the rapid expansion caused by the internal cracks in the concrete. This results in a triaxial stress state in the core area of the concrete. During this stage, the stress-strain curve is practically a straight line, allowing the confinement to enable the concrete to reach higher stress levels before failure compared to unconfined concrete. Table 6 shows the models to be validated. Using the ACI model [48]. the calculation of the maximum strength of confined concrete can be determined using Equations (3) and (4) proposed by [PERSON] and [PERSON].