# Compressive Behaviors of Prisms Made of Demolished Concrete Lumps and Fresh Concrete
## 3 Results and Discussion
### Mechanical Properties of Specimens Made of DCLs and FC
#### 3.3.4 The Effect of the Characteristic Ratio for Prisms  
The measured mechanical properties of the specimens in Group 5 are summarized in Table 6.  
The variation of the combined compressive strength with the ratio of the characteristic size of the lumps to the specimen's lateral dimension (designated as \(\beta\)) is shown in Figure 15. The ratio ranges from 0.22 to 0.44, and its relationship with the prismatic compressive strength is weak. But when this ratio reaches 0.55, about a 10% decrease in compressive strength is observed. A similar reduction \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multirow{2}{*}{**Notation**} & \multirow{2}{*}{**Serial**} & \multicolumn{2}{c}{\(f_{\text{pro},\text{acon},\text{CC (MPa)}}\)} & \multicolumn{3}{c}{\({}^{\pi}_{\text{PRO/CC (\times 10}\text{-}\text{-}\text{)}}\)} & \multicolumn{3}{c}{\(E_{\text{proj,300,CC (GPa)}}\)} \\ \cline{5-13}  & & & _Mea._ & _Avg._ & _Std._ & _Mea._ & _Avg._ & _Std._ & _Mea._ & _Avg._ & _Std._ \\ \hline \multirow{5}{*}{Group 5} & 1 & 34.0 & & 2483 & & 19.9 & & & & & \\  & PR300-67-0.3 & 2 & 33.6 & 32.8 & 1.4 & 2765 & 2756 & 220 & 19.5 & 18.6 & 1.5 \\  & 3 & 30.8 & & & 3021 & & & 16.5 & & & \\  & 1 & 31.9 & & & 2291 & & & & & 19.8 & \\  & PR300-100-0.3 & 2 & 35.0 & 33.4 & 1.3 & 2762 & 2538 & 193 & 20.6 & 20.5 & 0.5 \\  & 3 & 33.2 & & & 2562 & & & 21.0 & & \\  & 1 & 31.8 & & & 2533 & & & 18.9 & & \\  & PR300-133-0.3 & 2 & 33.9 & 32.5 & 1.0 & 2660 & 2624 & 65 & 18.1 & 18.2 & 0.6 \\  & 3 & 31.9 & & & & 2678 & & & & 17.5 & \\  & 1 & 31.7 & & & 2362 & & & & 20.2 & \\  & PR300-167-0.3 & 2 & 28.4 & 30.3 & 1.4 & 2881 & 2495 & 277 & 20.6 & 20.6 & 0.3 \\  & 3 & 30.8 & & & 2242 & & & 20.9 & & \\ \hline \hline \end{tabular} Note.: _Mea._, measured value; _Avg._, average value; _Std._, standard deviation.  
\end{table} 
Table 6: Experimental results of specimens in Group 5. Figure 14: Calculated and measured \(\widetilde{\varepsilon}_{*,300,\text{CC}}\). has been reported in previous studies [27; 29] using cubic and cylindrical specimens with different characteristic ratios. The observed relationship probably arises because larger lumps have less bonded surfaces with the FC, which is where micro-cracks tend to occur and develop during the loading process. So the occurrence and development of internal cracks and then the energy dissipation of a loaded prism with a larger characteristic ratio are less dispersive, leading to less energy being dissipated before failure and lower compressive strength. Based on the test results listed in Table 6, a bilinear empirical relationship between the combined compressive strength and the characteristic ratio can be established for prisms as:  
\[f_{\text{pr,300,CC}}(\beta)/f_{\text{pr,300,CC}}(\beta=0.33)=\left\{ \begin{array} []{cc}1.00&0.22\leq\beta\leq 0.33\\ 1.14-0.41\times\beta&0.33\leq\beta\leq 0.55\end{array} \right. \tag{14}\]  
Similar relationships have been proposed for cylindrical and cubic specimens [27; 29]:  
\[f_{\text{cy,300,CC}}(\beta)/f_{\text{cy,300,CC}}(\beta=0.33)=1.11-0.28\times \beta\ (0.1\leq\beta\leq 0.6) \tag{15}\]  
\[f_{\text{cu,300,CC}}(\beta)/f_{\text{cu,300,CC}}(\beta=0.33)=1.08-0.21\times \beta\ (0.1\leq\beta\leq 0.6) \tag{16}\]  
Comparing Equation (14) with Equations (15) and (16), it can be seen that with increasing \(\beta\), the rate of decreasing compressive strength is in the order prism > cylinder > cube. The effect of the characteristic size of the lumps on compressive strength is greatest for prisms and least for cubes. This is probably an effect of the end restraints caused by the machine platens on lateral expansion. This would be more significant for cubes than for the lateral expansion of cylinders and prisms. For prisms, the non-uniform stress distribution caused by the presence of sharp edges would be expected to amplify the impact of characteristic lump size on compressive strength.