## 4 Results and Discussion
### Compressive Strength  
Figure 8 depicts the results of the compressive strength of all the tested mixtures incorporating various rubber sizes (R1, R5 and R10) and their dosages (10%, 20% and 30%) at 28 days. Figure 9 shows the compressive strength results at 14, 28 and 56 days for various tested mixtures. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Properties**} & \multicolumn{4}{c}{**Rubber Contents**} \\ \cline{2-5}  & **0\%** & **10\%** & **20\%** & **30\%** \\ \hline Slump (mm) & 136 & 88 & 81 & 76 \\ Compressive strength (MPa) & 55.1 & 40.5 & 33.7 & 22.4 \\ Splitting tensile strength (MPa) & 9.6 & 7.5 & 6.9 & 6.3 \\ Flexural strength (MPa) & 7.4 & 5.9 & 4.4 & 3.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of the concrete mixture incorporating R1 rubber. Figure 7: Slump flow for the mixture incorporating various dosages of rubber. Figure 8: Compressive strength for the mixture incorporating various dosages of rubber at 28 days. Figure 9: _Cont_. All the results reported in Figures 8 and 9 showed a coefficient of variation (CV) of less than 1.2%. The results showed that the compressive strength increased for the mixture incorporating the R5 rubber size compared to that of the R1 for all the tested dosages of rubber (10%, 20% and 30%). For instance, for the mixture incorporating 10% of rubber, an approximately 7% increase in compressive strength was observed for the mixture incorporating R5 rubber compared to that of the similar mixture with the R1 rubber size. Furthermore, a relatively higher increase in compressive strength (i.e., approximately 11%) was observed in between the R5 and R1 rubber sizes for the 30% replacement level. This increase in compressive strength from the R1 to R5 rubber size mixtures was mainly due to the similarity of the gradation curve of the R5 rubber size with the sand particles. Moreover, due to the excessive grinding of rubber to obtain the desired size, (R1) resulted in irregular and flakier shapes of rubber, which might generate flexibility, spring a like effect in the mixture and allow the concrete to deform more under an applied load. The low stiffness and large pore volume attributed to the small rubber size (R1) also decreased the mechanical strengths of the rubberized concrete [42]. On the other hand, compressive strength reduction was observed for the mixture incorporating the R10 rubber size compared to that of the R1 and R5 rubber sizes. For example, for the mixture incorporating 20% of rubber, a decrease in compressive strength of approximately 13% was observed for the R10 rubber size compared to that of the R1 rubber size. A similar decreasing trend was observed between the R5 and R10 rubber sizes. This reduction in compressive strength for R10 compared to the R1 rubber size was mainly attributed to the fact that the aggregates are considered as the strongest load-carrying material in concrete. Rubber is a softer material and is not as strong as aggregates; consequently, it reduces the strength. Moreover, the bonding between the rubber and the other concrete ingredients is weak; therefore, rubberized concrete faces more of a stress concentration at rubber aggregates, causing a reduction in the strength properties [43]. It was observed that the 28-day compressive strength decreased with the higher dosages of rubber (Figure 8). For example, the mixture incorporating the R1 rubber showed approximately 27% and 39% decreases in the 28-day compressive strength for the 10% and 20% dosages, respectively, compared to that of the similar mixtures without the addition of rubber. Similarly, a higher reduction in compressive strength was observed for the mixture incorporating 30% of rubber. The weaker bond between the crumb rubber and the cement promotes the strength reduction properties. The distortion of rubber aggregates compared to the neighboring cement paste initiated the crack immediately. Furthermore, a decreased density of rubber particles also contributed towards the decreased strength [40].