## 6 Results  
### Compressive Strength  
As concrete is the most versatile construction material, it is for the most part used to take the load in compression. Concrete mix standard deviation and coefficient of variation and compressive strength and was evaluated at 7, 28, and 90 days of curing as appeared in Figure 1 and Table 6 respectively. RCA's highest compressive strength was obtained with the mix having nylon fibers and 20% of silica fume, followed by the mixture which had 0% silica fume and nylon fibers. Increasing the dosage of RCA reduces the compressive strength of concrete. Compressive strength of concrete at 28 days decreases by 7% at 50% RCA and 15% at 100% RCA replacement. Recycled aggregate concrete reduced compressive strength as a comparison to normal aggregate concrete could be attributed to the existence of a greater number of interfacial transition zones ITZs in recycled aggregate concrete as compared to normal aggregate concrete. In recycled aggregate concrete ITZs has four types: (1) between the old and new mortar; (2) ITZ between old mortar and natural stone recycled concrete aggregate; (3) between new mortar and natural concrete aggregate; and (4) between natural stone recycled concrete aggregate and new mortar [50]. In the natural aggregate concrete, there is only one ITZ that is present between fresh mortar and natural aggregate. The existence of multiple ITZs can cause the failure of early bonding in the case of recycled aggregate concrete under compression; hence, recycled aggregate concrete RAC compressive strength is significantly lower as compared to normal aggregate concrete NAC. Furthermore, the water requirement to fulfill the water absorption of aggregates in the case of RAC mixes is very high as compared to NAC, which could also be the significant factor that leads to reduced compressive strength. Though adding more water resolves concrete workability issues, it could cause in concrete with lower compressive strength [11]. Adding 20% silica fume does not have a major effect on the compressive strength of concrete at 7 days of curing. Concrete samples cured at 28 and 90 days, which had silica fume performed considerably better as compared to sample with no silica fume. For example, RCA0-SF20 displays 3% and 7% more compressive strength as compared to RCA0-SF0 28 and 90 days of curing. This could be attributed to the pozzolanic reaction amid free C-H and silica fume particles. Silica fume does not only decrease the C-H amount of cement matrix by changing it to C-S-H gel, but it also enhances the dispersal of particle size in the cement matrix by filling the voids amid fine aggregates and particles of cement. This causes enhancement of compressive performance of concrete samples having 20% silica fume [51]. RCA mixes that had silica fume gained more compressive strength as compared to NCA concrete blends. Figure 1: (**a**) Compressive Strength (**b**) Compressive Strength of Each NFRC Mix w.r.t Plain Concrete (PC). The compressive strength of nylon fiber reinforced concrete is more as compared to a reference sample. Strengthening to cement matrix is provided by nylon fibers and aids in controlling crack propagation. Because of the RCA utilization, nylon fibers recuperate some loss of 4% concerning the reference sample. Net increase in compressive strength because of the nylon fibers addition is more in concrete mixes having silica fume as compared to mixes with no silica fume. For example, concrete samples with silica fume exhibit a net increase of (3-10%) over the reference sample in compressive strength while, concrete samples with no silica fume exhibit (3-5%) net increase only, it can be seen in Figure 1. This might be attributable to enhancement in the bond of cement paste matrix and fibers because of adding silica fume. Silica fume particles are very finer as compared to cement, it could increase binder surface area over nylon fibers.