## 2 Experimental Database and Methods
### Dataset Characteristics  
The process of concrete confinement with FRP composites has been found to significantly improve compressive strength. Concrete is known to significantly expand when subjected to uniaxial unconfined compression. It has been shown that when a material with high axial strength such as a fiber reinforced polymer is used for confinement, the unconfined concrete compressive strength increases significantly [23; 24]. The increase is related to the confining material properties since it reacts to concrete's lateral expansion and gets axially stressed. Therefore, higher concrete confined strength is obtained when FRPs with higher tensile strength are used for confinement [23; 25]. The most common composites used for confinement are based on aramid, carbon, and glass fibers. Data points used in this study originate from a purely experimental dataset covering various FRP confined concrete cylindrical specimens [15]. The experimental database incorporates all concrete strength categories, along with different types of confining FRP materials. The total number of conducted experiments made for each category is provided in the last column of Table 1. The experiments were all conducted in short, circular, column-like specimens with diameters varying from 50 to 100 mm and lengths from 100 to 316 mm. There are eight possible input parameters that affect the output of confined compressive strength. Table 2 provides a brief description and statistical information on the inputs and the output. These refer to geometrical characteristics of the concrete specimens (diameter, \(D\), and height, \(H\)), FRP layer properties (thickness of the FRP, \(t\), and number of FRP layers, \(L\)), as well as measures of mechanical properties, such as the respective unconfined concrete strength, \(f_{co}\), (without FRP layers), the elastic modulus, \(E_{f}\), and the FRP ultimate tensile stress and strain, \(f_{f}\) and \(\varepsilon_{\mu}\), respectively. The partial dependence plot given in Figure 2 expresses the average marginal effect on the compressive strength when each one of the eight input variables changes, while, in parallel, the remaining inputs are fixed. It is observed that the effect of \(f_{co}\) on \(f_{cc}\) is crucial, since, if all other inputs remain constant, as \(f_{\alpha}\) spans from its minimum to its maximum value, \(f_{cc}\) is significantly altered (Figure 2c). Similar \(f_{cc}\) behavior is acquired for the \(t\) parameter (Figure 2g). These two parameters denote that the unconfined concrete strength and the FRP thickness, i.e., the main structural components of the specimen, mostly characterize the compressive strength. The number of FRP layers, \(L\), seems important only for the first layers, while its increase after a certain value does not provide extra strength (Figure 2h). Geometrical characteristics \(D\) and \(H\), (Figure 2a,b, respectively) are only negatively affecting \(f_{cc}\) when they have small values (i.e., small specimens), while this effect \begin{table}
\begin{tabular}{c c c c} \hline
**Specimen Type** & **Concrete Strength** & **Description** & **N** \\ \hline A: AFRP-H & High & Aramid FRP & 25 \\ B: AFRP-N & Normal & Aramid FRP & 67 \\ C: CFRP-H & High & Carbon FRP & 135 \\ D: CFRP-L & Low & Carbon FRP & 22 \\ E: CFRP-N & Normal & Carbon FRP & 574 \\ F: GFRP-H & High & Glass FRP & 53 \\ G: GFRP-N & Normal & Glass FRP & 234 \\ H: & High & High Modulus & 24 \\ HM\_UHM\_CFRP-H & & Carbon FRP & 24 \\ I: HM\_UHM\_ & & High Modulus & 50 \\ CFRP\_N\_ & Normal & Carbon FRP & 114 \\ J: UB\_TUBE\_H & High & FRP Tubes & 178 \\ K: UB\_TUBE\_N & Normal & FRP Tubes & 178 \\ \hline \end{tabular}
\end{table} 
Table 1: Specim characteristics. is no longer valid for larger specimens. FRP elastic modulus, \(E_{f}\), and tensile strength, \(f_{f}\), are two features that have an important impact on \(f_{cc}\), as can be seen from the respective Figure 2d,e.