## 4 Results and Discussion
### Results of Quasi-Static Tests
#### 4.1.1 Uniaxial Tension Tests on Plate-Like Composite Specimens
The lower diameter, higher stiffness, and strength of the PBO-AS fibers, as well as their adequate bonding to cementitious matrix ensures efficient confinement of the matrix already, prior to cracking [23]. Furthermore, PBO-AS fibers enable better control of micro-cracks, hence enhancing the first crack stress of the composite. In addition to their weak hydrophilicity, the high Young's modulus of PBO-AS fiber ensures narrow crack widths in TRC-PBO in comparison to those in TRC-PE. This influences both the strain at peak stress and the work-to-fracture of the corresponding composites. Work-to-fracture is the area under the stress-strain curves up to the peak load. The potential of the material to develop multiple cracks and exhibit strain-hardening behavior can be characterized by the strain-hardening modulus; i.e., the ratio of tensile strength to first crack stress [17]. It can be observed that while TRC-PBO prevails in both first crack stress (4.6 MPa) and tensile strength (9.4 MPa), TRC-PE exhibits a higher strain-hardening modulus, with a first crack stress of 2.5 MPa and tensile strength of 8.1 MPa. Taking into consideration the strain capacity of 2.8% in the case of TRC-PE and 0.9% for TRC-PBO, it is not surprising that the TRC-PE yields a considerably higher work-to-fracture of 185.7 kJ/m\({}^{3}\) when compared to 67.4 kJ/m\({}^{3}\) in the case of TRC-PBO. The ultimate strain capacity of the investigated composites is decided by both the average crack width and the average crack spacing, which is defined as the ratio of gauge length to the average number of cracks within the gauge length of the specimen. Figure 8 shows tensile stress-strain curves and corresponding average crack widths for representative specimens of TRC-PE and TRC-PBO. Due \begin{table}
\begin{tabular}{l l l l l} \hline \hline  & **First Crack Stress** & **Peak Tensile** & **Ultimate Strain [\%]** & **Work-to-fracture** \\  & **[MPa]** & **Stress [MPa]** & & **[kJ/m\({}^{3}\)]** \\ \hline TRC-M & 3.4 (0.3) & 3.4 (0.3) & – & – \\ TRC-PE & 2.5 (0.5) & 8.1 (0.2) & 2.8 (0.4) & 185.7 (31.3) \\ TRC-PBO & 4.6 (1.1) & 9.4 (0.2) & 0.9 (0.2) & 67.4 (12.1) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Results of the uniaxial quasi-static tests on the investigated composites (average values; standard deviations are given in parentheses). to the 5 second interval between individual image recordings, only a limited number of photos were taken during the tests. This explains the relatively low number of crack width measurement points in the case of TRC-PBO, as shown in Figure 8b. Figure 8: Tensile stress–strain curve and corresponding average crack widths of a representative specimen of both (**a**) TRC-PE and (**b**) TRC-PBO with corresponding composite stresses and crack patterns under quasi-static loading. Despite both materials showing multiple cracking, the crack patterns are very different. At the same strain level, TRC-PBO exhibits finer and denser cracks along the specimen. The average number of cracks of TRC-PE and TRC-PBO at peak stress are 12 and 16, resulting in the average crack spacings of 4 mm and 3 mm, respectively; see Table 5. Though TRC-PBO exhibits more cracks, the low average crack width of 29 \(\upmu\)m at peak load leads to an overall smaller strain capacity when compared to TRC-PE, for which the average crack width is 88 \(\upmu\)m. The relationships between crack density, crack width, and applied stress have been investigated in order to achieve a better understanding of the crack pattern [13; 14; 16; 29]. It can be observed that, at a similar stress level, TRC-PBO shows better crack control. Note that SHCC specimens were also investigated without textile reinforcement. The reference SHCC matrices show inferior mechanical properties compared to the hybrid fiber-reinforced composites presented in this section. Thus, despite the undesirable failure mode of the textile yarns, their contribution is still significant.