## 2 High-Performance Fiber Reinforced Concrete to Repair RC Components
### Concrete Mechanical Characterization
#### 2.2.1 Compression Test  
The compression tests were performed on concrete cubes 100 \(\times\) 100 \(\times\) 100 mm\({}^{3}\) after seven days of standard curing to evaluate the performance of the material after the rapid repair strategy. The material mechanical characteristics were identified for concrete without fibers, with steel (UHS), or with polymer (UHP) fibers considering: (1) Four cubes for each fiber fraction (0, 1, 2, and 3%) for concrete with steel fibers (UHS), and (2) four cubes for each fiber fraction (0, 2, and 4%) for concrete with polymer fibers (UHP). First, the UHS concrete cubes and then the UHP ones were created. The mix design of the concrete matrix was the same for the two concretes. Then, the UHS specimens were tested by an available test apparatus using a force-controlled test method until specimen rupture, applying a vertical load on the specimen with a constant rate of 11.3 kN/s. Next, the UHP specimens were tested by a new test apparatus using a displacement-controlled test method until specimen rupture, by applying constant displacements with a velocity of 1.5 mm/min. The compressive strength was evaluated without differences using the two test methods. The mean compressive strength values are provided for the UHS and the UHP cubes in Table 1.  
The concrete specimens without fiber were built to determine the strengths of the concrete matrixes of the UHS and the UHP, which showed different mean values of compressive strength (Table 1). The results in Table 1 require further investigation because the UHS and the UHP concretes were created at different times. The steel and the polymer fibers had different mechanical properties (Section 2.1). Therefore, the UHS cubes with 2% steel fiber showed a mean compressive strength that is twice that of the UHP cubes with the same fiber volume fraction because the steel and polymer fibers mechanical characteristics, in term of strength, were different (Section 2.1). In terms of the compression strength variation with the fiber volumetric fraction:  
1. The concrete with steel fibers (UHS) showed an increase in strength as the fiber volume fraction increased (Figure 1a). That phenomenon was not constant but it tended to reduce increasing the fiber percentage. 2. The concrete with polymer fibers (UHP) presented a decrease in the strength when the fiber volume fraction increased (Figure 1b). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen** & **Vol (\%)** & **R\({}_{\text{cm}}\) (MPa)** & **SD** & **Fiber** & **Specimen** \\ \hline No fiber & 0 & 93.6 & 2.6 & & \\ Steel & 1 & 100.9 & 3.0 & & \\ Steel & 2 & 110.2 & 2.7 & & \\ Steel & 3 & 116.3 & 3.7 & & \\ \hline No fiber & 0 & 75.7 & 4.0 & & \\ Polymer & 2 & 51.4 & 3.9 & & \\ Polymer & 4 & 56.9 & 4.2 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mean cube compressive strength (R\({}_{\text{cm}}\)) for concrete with steel or polymer fibers considering different fiber volume fractions (Vol) and standard deviation (SD) of the compressive strengths after seven days of curing. The reduction of the polypropylene-based fiber reinforced concrete compressive strength compared to the plain concrete may be due to several different effects. One could be due to the lower Young's modulus of the polymer (E = 4100 MPa), compared to that of the matrix (not measured, but in the order of 40,000-45,000 MPa) [49], which induces stress concentrations, leading to early cracking. That is not the case of using steel fiber characterized by higher modulus (E = 200,000 MPa). As investigated by previous researchers, it can be also related to the interfacial transition zone (ITZ) effect [50] and to the hydrophobic nature of polymer fibers that could produce a void between the fiber and the concrete matrix, reducing the resisting area of concrete specimen. The low bond strength produces breaks in the bond between the aggregate and the cement and creates a large surface area of material breaking in the cement bond that was also observed in [PERSON]'s work [51].