## 3 Results and Discussions
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Comparison Sample**} & **Maximum Applied** & \multirow{2}{*}{**Deformation (mm)**} & **Tensile Strength** & **Tensile Strength by** \\  & **Load (N)** & & **(MPa)** & **Law (MPa)** \\ \cline{2-5}  & **37,100** & **0.71** & **1.6** & **1.58** \\ \hline
**AR** & **Average Value (N)** & **Average Value (mm)** & **Average Value (MPa)** & **Average Value (MPa)** \\ \hline
30 & 54,525 & 1.3 \(\pm\) 0.01 & 2.2 \(\pm\) 0.005 & 2.3 \(\pm\) 0.01 \\ \hline
50 & 72,700 & 1.6 \(\pm\) 0.013 & 2.6 \(\pm\) 0.01 & 3.1 \(\pm\) 0.01 \\ \hline
70 & 95,800 & 2.4 \(\pm\) 0.006 & 3.4 \(\pm\) 0.007 & 4.1 \(\pm\) 0.005 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Tensile strength of the different cylindrical samples experimentally tested. Figure 7: Elongation/applied load diagram for the tensile test on _Arundo donax_ fiber. Finally, Figure 9 reports a tensile strength/load applied for each AR considered, with the correspondent trend line, and shows the direct proportionality between the two parameters (elastic phase) before the sample breaks. From this figure, it is possible to notice that the higher the AR, the higher the load applied to the sample, and the higher the strength of the sample before reaching the final failure. Thus, the obtained results seem to confirm a direct proportionality between the AR of _Arundo donax_ fiber and the tensile strength of concrete blocks; as also highlighted by many authors [90], the greater this ratio, the greater the fiber length and then--as a direct consequence--the greater the counter-resistance force that the fiber produces within the concrete. The results obtained in this work suggest therefore that natural fibers have a reinforcing function within the cylindrical concrete blocks considered. In fact, the comparison with a block without natural fiber showed an increase in the maximum applied load and in the consequent deformation before sample break. As a consequence, the tensile strength also increased, always in line with the values reported in the reference law. On the other hand, comparing between them the values obtained for concrete blocks with different ARs of natural fiber, the direct proportionality between this parameter and the tensile strength of the concrete blocks themselves was highlighted: an increase in the AR of _Arundo donax_ fibers generates an increase in the tensile strength of concrete blocks, which will be greater as the size of the considered blocks increases. As reported in the relevant scientific literature  
Figure 8: Increase in tensile strength for each AR considered and comparison with sample without any additive fiber. Figure 9: Tensile strength/load applied diagram for each AR considered. that has tackled this scientific issue regarding the mathematical modeling or empirical equation [91], this is probably a result of the variability in the bond-slip law throughout the fiber, which is expected to be higher for longer fiber. The AR directly influences another aspect: the adhesion between the cement matrix and the reinforcing fiber. With the same composition and dosage, the higher the value of the AR, the greater the adhesion between the two materials, since the contact surface between them increases, and subsequently the performance of the fiber reinforcement improves, because the fibers stressed by traction tend to be more difficult to extract. This highlights the importance of this parameter for researchers and practitioners in the determination of the optimum AR of fibers to be used to reinforce concrete blocks, which can provide maximum strength when used in constructions. Future analysis to identify the bond-slip laws would therefore enable a full assessment of the performance of _Arundo donax_ natural fibers dispersed in concrete matrixes.