## 3 Results and Discussions
### Influence of Crumb Rubber on Pervious Concrete Properties
#### 3.1.2 Compressive Strength  
The addition of rubber has a negative effect on the compressive strength of pervious concrete, which can be seen in Figure 6. It can be seen that both rubber particle size and rubber content have a strong effect on compressive strength. The compressive strength of RMPC decreases with the increase of rubber content. Compared to CPC, with a compressive strength of 22.2 MPa, the compressive strengths of CCR- and FCR-modified mixtures are 14.6-20.4 MPa and 19.7-21.9 MPa, respectively. As we all know, the compressive strength of pervious concrete comes from the bonding force and  
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline \multirow{2}{*}{**Mix ID**} & \multicolumn{6}{c}{**Compressive Strength After Freeze-Thaw Cycles (MPa)**} \\ \cline{2-9}  & \multicolumn{2}{c}{**25 Cycles**} & \multicolumn{2}{c}{**50 Cycles**} & \multicolumn{2}{c}{**75 Cycles**} & \multicolumn{2}{c}{**100 Cycles**} \\ \cline{2-9}  & **Mean** & **St. Dev.** & **Mean** & **St. Dev.** & **Mean** & **St. Dev.** & **Mean** & **St. Dev.** \\ \hline Control & 21.2 & 1.00 & 18.1 & 0.49 & 16.7 & 0.54 & 14.2 & 0.90 \\ CCR-2 & 19.5 & 0.49 & 16.9 & 0.65 & 15.7 & 0.29 & 13.8 & 1.08 \\ CCR-4 & 18.0 & 0.33 & 15.5 & 0.62 & 14.6 & 0.57 & 13.6 & 0.46 \\ CCR-6 & 16.6 & 0.25 & 14.3 & 0.57 & 13.6 & 0.62 & 12.8 & 1.01 \\ CCR-8 & 14.3 & 0.62 & 12.8 & 0.34 & 12.3 & 0.48 & 11.7 & 0.56 \\ FCR-2 & 21.0 & 0.22 & 18.4 & 0.34 & 17.4 & 0.42 & 15.3 & 0.86 \\ FCR-4 & 20.5 & 0.48 & 18.2 & 0.52 & 17.2 & 0.70 & 15.6 & 0.62 \\ FCR-6 & 20.0 & 0.34 & 17.6 & 0.57 & 17.2 & 0.61 & 15.8 & 0.66 \\ FCR-8 & 19.3 & 0.56 & 17.5 & 0.33 & 17.0 & 0.41 & 16.5 & 0.41 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Freeze–thaw results of RMPC.  
Figure 5: Permeability coefficient of RMPC versus rubber content.  
strength of coarse aggregate; the addition of rubber weakens the bonding force between the aggregate and cement paste, and results in a reduction of compressive strength.  
The effect of rubber particle size on compressive strength can also be found in Figure 6. The CCR-modified mixture has lower compressive strength than the FCR-modified mixture at the same incorporation level. The maximum difference between CCR- and FCR-modified mixtures is 5.1 MPa at the 8% level, which means that the small rubber particle is more advantageous to the compressive strength of RMPC. This is because the roles of FCR and CCR in pervious concrete are quite different. Generally, pervious concrete consists of a coarse aggregate framework and cement paste. CCR exists as a framework in pervious concrete by replacing the coarse aggregate; FCR plays the role of a filler rather than the framework. The addition of CCR weakens the role of the framework in strength formation. The adverse impact of FCR on compressive strength is lower compared to CCR. Figure 6 also indicates that the difference in compressive strength between CCR- and FCR-modified mixtures increases with the increasing incorporation level. This is because the adverse impact of rubber on compressive strength increases with the increasing rubber content. FCR-modified mixtures have a maximum strength loss of 11% at the 8% incorporation level, which means that FCR is more effective in modifying pervious concrete without sacrificing excessively compressive strength.