## 3 Materials and Methods  
### Confined Test Pieces  
Nine cylindrical concrete test pieces were produced, measuring 150 mm in diameter and 300 mm in height in accordance with regulation NCh 170 [69] (Figure 1).  
The concrete dosage we used can be seen in Table 2.  
The test pieces were removed from their molds after 24 h and were placed in a humidity chamber, immersed in limewater for 28 days, after which the faces of the test piece were flattened and given a parallel structure using a Triple Hi-Kenna TSURU-TSURU concrete specimen end grinding machine, in accordance with NCh 1172 [70].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**ACI 440-17** & **FIB** \\ \hline \(f_{cc}=f_{c}+\varphi_{f}\cdot 3.3\cdot f_{l}\) & \(f_{cc}=f_{c}\left(2.254\sqrt{1+7.94\frac{f_{c}}{f_{c}}}-2\frac{f_{c}}{f_{c}} -1.254\right)\) \\ \(\frac{2E_{f}\cdot n+t_{f}\cdot 0.55+\varepsilon_{flu}}{D}\) & \(f_{l}=\frac{2E_{f}\cdot n+t_{f}\cdot k+\varepsilon_{flu}}{D}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: ACI and FIB models for confined concrete.  
The six remaining test pieces were reinforced by gluing the carbon fiber fabrics with epoxy resins. Both the carbon fiber fabrics and the epoxy resins were supplied by MAPEI. The gluing was carried out by applying a first layer of resin and gluing the fabric to the specimen, leaving an overlap of 150 mm (indicated by the company) to avoid detachment of the fabric when it entered the load. Subsequently, a roller was applied to the outer surface to eliminate air pockets and imperfections in the execution. The tissue was cut with lesser height than the specimen to prevent it from being loaded in the compression direction, which would cause it to buckle. Two kinds of unidirectional carbon fiber fabric were used: a carbon fiber fabric with a normal modulus of elasticity and high strength (HR) with the trade name C-UNI-AX, and a fabric with a high modulus of elasticity (HM) with the trade name C-UNI-AX-HM. The geometrical and mechanical characteristics of both fabrics are set out in Table 3. Here, \(e\) is the thickness equivalent of the fabric, \(\sigma_{r}\) is the ultimate tensile strength, \(E\) is the longitudinal modulus of elasticity, \(\varepsilon_{ft}\) is the ultimate tensile strain, \(F_{r}\) is the mechanical capacity of the fiber under tensile stress per unit of length, and \(K_{r}\) is the rigidity of the fiber under tensile stress per unit of length. All the pieces were subsequently compression tested until breaking point. The tests were performed in the Materials Laboratory of the Central University of Santiago de Chile (LEMUC), using a Druck-und Biegeprufmaschine press at a speed of 1 MPa/s and following regulation NCh 1037 [71].  
\begin{table}
\begin{tabular}{c c} \hline
**Material** & **Dosage (kg)** \\ \hline Cement & 346 \\ Gravel & 879 \\ Sand & 950 \\ Water & 123 \\ Plasticizer & 2.1 \\ \hline \end{tabular}
\end{table}
Table 2: Dosage of the concrete used.  
Figure 1: Image of the nine test pieces produced for compression testing.  
The breaking strain and modulus of elasticity values of both fabrics made it possible to classify them as being of high strength and high modulus [11].  
The choice of these types of fiber made it possible to compare the influence of the rigidity of the fabric on the compressive behavior of the confined test pieces, due to the fact that both fabrics provide practically the same mechanical capacity (801.8 \(\mathrm{N/mm}\) compared to 727.6 \(\mathrm{N/mm}\)) but have very different rigidities (38.2 \(\mathrm{kN/mm}\) compared to 64.3 \(\mathrm{kN/m}\)).