# 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  
#### 3.3.1 Combined Compressive Strength  
The measured mechanical properties of the specimens in Groups 2, 3, and 4 are summarized in Table 4. In that table, the subscripts CC and FC identify the specimens made with compound concrete and fresh concrete, respectively. So \(f_{\mathrm{\star,300,CC}}\), \(\widetilde{\epsilon}_{\mathrm{\star,200,CC}}\), and \(E_{\mathrm{\star,300,CC}}\) are, respectively, the combined compressive strength, combined peak strain, and combined elastic modulus of the 300 mm compound concrete specimens; and \(f_{\mathrm{\star,300,FC}}\), \(\widetilde{\epsilon}_{\mathrm{\star,300,FC}}\), and \(E_{\mathrm{\star,300,FC}}\) are, respectively, the compressive strength, peak strain, and elastic modulus of the 300 mm control fresh concrete specimens. The table shows that the  
Figure 7: Variation of \(f_{\mathrm{\star,150}}\)/\(f_{\mathrm{\star,300}}\) with maximum aggregate size \(s_{\mathrm{max}}\). cubic, prismatic, or cylindrical compressive strength of the compound concrete made of DCLs and FC decreased gradually with increased replacement ratio. This is presumably due to the 150 mm cubic compressive strength of the demolished concrete (40.8 MPa) being lower than that of the fresh concrete (53.0 MPa on the test day). The strength ratios of \(f_{\text{cu,300,CC}}/f_{\text{cy,300,CC}}\) and \(f_{\text{pr,300,CC}}/f_{\text{cy,300,CC}}\) listed in Table 4 are also presented in Figure 8. It is evident from this figure that:  
1. With increased replacement ratio of DCLs, the \(f_{\text{cu,300,CC}}/f_{\text{cy,300,CC}}\) strength ratio generally increased. The strength ratio for the specimens with a replacement ratio of 20% was almost the same as that for the specimens with a ratio of 30%, and about 4.7% higher than that for the control specimens made of FC alone. But when the replacement ratio was 40%, the strength ratio was 14.7% higher than that for the control specimens. This implies that although both the cubic and cylindrical compressive strengths of the compound concrete decreased with increasing replacement ratio, the cubic strength declined more slowly than the cylindrical strength, probably due to the end restraint of the machine's platens on the lateral expansion of the cubic specimens being much more significant than for the cylindrical specimens. The negative effect of the DCLs is thus more prominent with cylinders. 2. The \(f_{\text{pr,300,CC}}/f_{\text{cy,300,CC}}\) strength ratio did not change very much with increasing replacement ratio. This is because for cylinders and prisms with the same height, the same lateral dimension,  
\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**} & \multirow{2}{*}{\(f_{\text{-300,CC}}\) (\(f_{\text{-300,CC}}\)) (MPa)} & \multicolumn{3}{c}{\(\bar{x}_{\text{-300,CC}}\) (\(\bar{x}_{\text{-300,CC}}\)) (\(\times 10^{-6}\))} & \multicolumn{3}{  
and the constraint caused by the loading platens, the rate of decrease in \(f_{\text{pr,300,CC}}\) with increasing replacement ratio should be similar to that of \(f_{\text{cy,300,CC}}\). For the prismatic and cylindrical specimens with replacement ratios of 20%, 30%, and 40%, the average value of that strength ratio was about 1.13, which is 13% higher than the \(f_{\text{pr,300,FC}}/f_{\text{cy,300,FC}}\) strength ratio of the control specimens made of fresh concrete alone. The prisms' compressive strength apparently decreased more slowly than that of the cylinders. It should be noted that the cylindrical and prismatic specimens were cast in different placement directions: the cylinders were cast parallel to loading direction, while the prisms were cast perpendicular.