# Effect of Nylon Fiber Addition on the Performance of Recycled Aggregate Concrete
## 4 Conclusions  
Experimental works were carried out with the additions of 0, 0.6 and 1.2 kg/m\({}^{3}\) of NF in both CAC and RAC mixes in order to highlight the effect of NF on the mechanical properties of CAC and RAC mixes. The RAC was prepared with a 100% replacement of RA with CA. The main conclusions obtained from the present study are as follows.  
1. Due to the adhered mortar in RA, the compressive strength values of the RAC mixes were significantly lower than those of the CAC mixes. However, we found that the addition of NF led to an increase in compressive strength of both the CAC and RAC mixes. In particular, this trend  
Figure 14: FE-SEM image of RAC3 sample.  
Figure 12: FE-SEM image of RAC1 sample.  
Figure 13: FE-SEM image of RAC2 sample.  
was more remarkable in the RAC mixes with a high content of NF. The compressive strength ratio results revealed that there was a beneficial effect of NF on the increase in compressive strength.
2. As observed in the case of the compressive strength, a similar trend was also examined with the NF content variation for the split tensile strength. Specifically, the test results revealed that there was a significant increase in the split tensile strength, especially with the addition of 1.2 kg/m\({}^{3}\) NF, regardless of concrete types. In the case of the RAC3 mix, we examined an increase of 80.6% in the split tensile strength over the RAC1 mix without NF.
3. From the test results of the UPV, we observed that in the case of the RAC mixes, there was an increase in the UPV with the further incorporation of NF. This may be due to the increase of the materials density and the bridge effect of NF. Additionally, it seems that the UPV results were closely related to those of both the compressive and split tensile strengths observed, with high correlation coefficient values.
4. Based on the results of RCPT, we observed that the addition of NF in the CAC mixes reduced the total charge, compared to the control (CAC1) mix without the addition of NF, and the reduction of the total charge was more remarkable in the concrete mixes with RA. This implies that the usage of NF in the RAC mixes can mitigate the possibility of steel corrosion oriented from external chlorides due to the reduction of micro-cracks in the cement matrix.
5. The microstructural observation of concrete revealed that the micro-cracks propagated along the ITZ between old mortar and aggregate, especially in the RAC mixes. However, for RAC mix with an addition of NF, the NF plays an important role in crack bridging, resulting in a higher strength and lower permeability in concrete.  
Within the scope of this study, we can conclude that the addition of NF enhanced the permeability as well as the mechanical properties, especially in concrete incorporating RA. The enhancement is primarily attributed to the bridge effect of NF, which allowed for a higher development of strength and concrete density.  
This research was funded by the Ministry of Land, Infrastructure and Transport (MOLIT) and the Korea Agency for Infrastructure Technology Advancement (KAIA) for a project on \"Development of manufacture technology for high-quality recycled aggregate and application of concrete (No. 16C16020280)\".  
The author declares no conflicts of interest.