## 3 Results
### Effect of Amount of PET Fiber and Its Geometrical Properties  
In order to find out the effect of various amounts of PET fibers, along with the impact of geometrical characteristic of PET fibers on the compressive strength of PFRC, the network's capability in predicting unseen data is utilized. To do so, the other input features are kept constant and the only parameters that vary are fiber content, fiber length, and fiber shape in the PFRC. The assumed mix design for generalization analysis is summarized in Table 3. The results hereafter are reported for the assumed mix design with the water-to-binder ratio of 0.3.  
Figure 5: Relative importance of the input parameters in the proposed model.  
The effect of variation in fiber length and width is shown as the aspect ratio of the fiber, which is the ratio of length over width of the PET fibers. Figure 6 demonstrates the variation of the compressive strength due to changes in the PET content, PET shape, and PET fiber aspect ratios. As can be seen, the effect of an increase in the aspect ratios for all shapes is negative. In other words, increasing the length or reducing the width of the PET fibers results in a reduction of the 28-day compressive strength of PFRC. Furthermore, this negative impact is more dominant in higher amounts of fiber content. This can be attributed to the fact that using PET fiber, especially with a higher content and/or higher aspect ratios, causes flaws between the PET fibers and the concrete mass. This defect in PFRC will make a weak region inside the dense mass of concrete matrix and can be the origin of crack formations during loading. This is in accordance with the experimental results [12; 44; 45; 46; 47; 48].  
By comparing the results of Figure 6, the effect of PET fiber shapes can be determined. String-shape PET has an almost constant compressive strength for a wide range of various aspect ratios. This means that the effect of this PET shape type on increasing the compressive strength of PFRC can be neglected. This may be due to a lower bond strength between the PET fibers and concrete matrix. In this context, [PERSON] and [PERSON] have presented the comparable results [49]. However, the other fiber shapes that have a better interaction with the concrete mass have a wider range of higher compressive strength, resulting in a more stable response in the PFRC. In PET fiber shapes, except the string one, the optimum amount of PET fiber usage is lower than 10 to 15 kg/m\({}^{3}\), which roughly equals 0.7 to 1% fiber replacement. The same results were obtained in the experimental studies of [PERSON] et al. [50], [PERSON] and [PERSON] [51], [PERSON] and [PERSON] [52], [PERSON] et al. [44], and [PERSON] et al. [42].  
The effect of PET fiber content is also obvious in Figure 6. As can be seen, an increase in the fiber content results in an adverse effect on the compressive strength of PFRC. This is in accordance with the experimental studies of [PERSON] and [PERSON] [53], [PERSON] and [PERSON] [54], and [PERSON] et al. [55]. This may be due to clogging effect of PET fiber in higher dosages. Moreover, it can be argued that the compressive strength of PFRC is under the influence of both fiber content and fiber aspect ratio for any fiber shapes. It means that these two parameters simultaneously affect the 28-day compressive strength of concrete containing PET fibers.