# Effect of Nylon Fiber Addition on the Performance of Recycled Aggregate Concrete
## 3 Results and Discussion  
### Compressive Strength  
Table 5 presents the compressive strength values of both CAC and RAC mixes with different NF content. Due to the adhered mortars in recycled aggregates, the compressive strength values of RAC mixes is much lower than CAC mixes. For example, it can be seen that the compressive strength value of the RAC1 mix without NF decreased by 27% at the age of 28 days, compared with that of the CAC1 mix. At 91 days, the CAC1 mix also exhibited a higher compressive strength value, showing 41.2 MPa, than the RAC1 mix (30.1 MPa). It is presumed that the higher absorption and the larger pores, due to the adhered mortar in the RAC1 mix, contributed to a decreased strength in the RAC1 mix.  
The compressive strength generally increased when NF was added to the mixes. Among the concrete mixes with RA, the highest compressive strength was obtained from the RAC3 mix with 52.6 MPa at 91 days of curing. For the CAC mixes, the CAC3 mixes exhibited a good development in compressive strength. It was therefore obvious that the addition of NF led to the increase of compressive strength in both the CAC and RAC mixes. It is worth noting that despite the use of RA, the RAC3 mix with 1.2 kg/m\({}^{3}\) of NF showed much higher compressive strength values, ranging from 27-41%, compared with the CAC1 mix with CA, for all stages of the curing.  
In order to highlight the effect of RA and NF on the compressive strength of concrete mixtures, the compressive strength ratio (CSR) of concrete mixes at 7, 28 and 91 days was calculated, and the correspondent results were shown in Figure 4a-c, respectively. The CSR results shown in Figure 4 exhibited almost the same trend, regardless of the curing ages. However, it must be noted that when NF was added in the RAC mixes, there was a beneficial effect on the increase in compressive strength, as a similar result was reported by [PERSON] et al. [37]. They found an approximately 11.5% increase in the compressive strength of concrete with 0.6 kg/m\({}^{3}\) of NF content, compared to control concrete without NF.  
From Figure 4, it was clearly observed that the CSR increased when the NF contents increased. Increases of CSR in the high content NF containing mixes (CAC3 and RAC3) were more remarkable  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Charge Passed (Coulombs)** & **Chloride Ion Penetrability** \\ \hline \(>\)4000 & High \\
2000–4000 & Moderate \\
1000–2000 & Low \\
100–1000 & Very low \\ \(<\)100 & Negligible \\ \hline \hline \end{tabular}
\end{table}
Table 4: Chloride ion penetrability based on the charge passed [36].  
Figure 3: Schematic of the rapid chloride ion penetration test based on ASTM C1202-18 [36].  
than for the low content NF mixes such as CAC2 and RAC2. At all curing periods, the higher CSR values were obtained from the RAC3 mix with an NF content of 1.2 kg/m\({}^{3}\), which means that, as the NF content increases, the compressive strength development of recycled aggregate concrete becomes more effective. For example, the CSR of the RAC3 concrete was 194% at 7 days, while the value for the CAC3 mix was only 136%. More importantly, the improved behavior of concrete with NF may be the result of the high content of fibers which form a network that acts as a bridge in the cement matrix, resulting in a reduction of micro-cracks [16; 27; 38].