# Effect of Nylon Fiber Addition on the Performance of Recycled Aggregate Concrete
## 3 Results and Discussion
### Ultrasonic Pulse Velocity  
The ultrasonic pulse velocity (UPV) measurements have been well known as non-destructive methods to assess the quality of concrete [14; 27; 39; 40]. The UPV values for the CAC and RAC mixes measured at 7, 28, and 91 days are shown in Figure 6. According to the classification criterion for concrete based on ultrasonic pulse measurements by [PERSON] and [PERSON] [41], the UPV values for concrete mixes observed in this work can be classified as shown in Table 7. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mixes**} & \multicolumn{3}{c}{**Split Tensile Strength (MPa)**} \\ \cline{2-4}  & **7d.** & **28d.** & **91d.** \\ \hline CAC1 & 2.9 (0.14) & 4.2 (0.10) & 5.1 (0.22) \\ CAC2 & 4.2 (0.20) & 5.2 (0.24) & 6.0 (0.24) \\ CAC3 & 4.4 (0.42) & 5.6 (0.32) & 6.3 (0.33) \\ RAC1 & 1.8 (0.12) & 2.5 (0.14) & 3.1 (0.24) \\ RAC2 & 3.1 (0.22) & 3.9 (0.11) & 4.7 (0.10) \\ RAC3 & 3.7 (0.18) & 4.7 (0.23) & 5.6 (0.19) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Split tensile strength values of the concrete mixes with different NF contents (standard deviation in parenthesis). Figure 5: Split tensile strength ratio of concrete mixes at: (**a**) 7 days, (**b**) 28 days, and (**c**) 91 days. Figure 6: Ultrasonic pulse velocity of concrete mixes. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Pulse Velocity (m/s)**} & **Concrete Classification** \\ \hline V \(>\) 4500 & Excellent \\
3600 \(<\) V \(<\) 4500 & Good \\
3000 \(<\) V \(<\) 3600 & Questionable \\
2100 \(<\) V \(<\) 3000 & Poor \\ V \(<\) 2100 & Very poor \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Concrete classification based on the ultrasonic pulse velocity [41]. As expected, it was observed from Figure 6 that the UPV values increased with curing time. The increase of UPV with time is a natural development due to the increase of the stiffness via the hydration reaction [42]. Among the concrete mixes made with CA (CAC mixes), the UPV values for the CAC2 mix were almost similar to those for the CAC3 mix, which shows that the content of NF in the CAC mixes does not have a significant influence in the variation of the UPV. However, the CAC2 and CAC3 mixes exhibited a better performance with respect to the UPV compared to the CAC1 mix without NF. This may be partially attributed to the bridge effect of NF which leads to the reduction of micro-cracks in the cement matrix by the addition of NF [29; 30; 43]. Generally, with an addition of steel fiber, introducing steel fibers of greater length negatively affected the UPV of the concrete specimens [44]. However, in this study, it can be seen that introducing NF positively affects the UPV, although it was relatively long (19 mm in length). This might be attributed to the increase of the materials density and the bridge effect of the fibers, due to incorporation of NF into the concrete mixes. In the case of the RAC mixes, with the further incorporation of NF, the general increases in the UPV were also examined, showing similar trends with regards to the strength properties (Tables 5 and 6) with the increase of NF. At 7 days, the UPV values of the RAC mixes were comparable with the CAC mixes, ranging from 3240 \(\sim\) 3540 m/s. This can be attributed to the substantially higher water absorption capacity of RA (4.31%) than CA (0.66%), because of the adhered mortar in RA, resulting in the increased porosity in the RAC mixes. However, relatively small increases in the UPV were observed in the RAC mixes at later ages of curing. Similar to the results of the compressive and split tensile strengths, the UPV results indicated that the CAC mixes showed higher values in the UPV than the RAC mixes, regardless of the aggregate types and NF contents.