# 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
#### 3.3.2 Combined Elastic Modulus  
For the prisms and cylinders listed in Table 4, the behavior of the relative elastic modulus (i.e., \(E\),\(S_{\text{200,CC}}/E\),\(S_{\text{300,FC}}\)) with increased replacement ratio is depicted in Figure 9. The figure shows that:  
1. The combined elastic modulus of the prisms or cylinders made with DCLs decreased gradually with increasing replacement ratio. This is presumably due to the low compressive strength of demolished concrete (40.8 MPa for a 150 mm cube) compared with fresh concrete (53.0 MPa on the test day). The modulus of elasticity of demolished concrete should also be less than that of fresh concrete.
2. For any given replacement ratio, the relative elastic modulus of the prisms is very close to that of the cylinders. An approximately linear relationship between the relative elastic modulus and the replacement ratio holds for both.  
Figure 10 shows that for a given replacement ratio, the combined elastic modulus of a prism is always larger than that of a cylinder, though the difference is generally small. For the prismatic and cylindrical specimens with replacement ratios of 20%, 30%, and 40%, the average value of \(E_{\text{pr,300,CC}}/E_{\text{cy,300,CC}}\) is only about 1.05.  
Figure 10: Shape effect on combined elastic modulus of prisms and cylinders made of DCLs and FC.  
Figure 9: Variation of relative elastic modulus with replacement ratio of DCLs.  
Equation (2) shows that \(E_{*,300,\text{FC}}\) is in proportion to \(\left(f_{*,300,\text{FC}}\right)^{1/3}\) for the specimens made of FC alone. To examine the relationship between \(E_{*,300,\text{CC}}\) and \(\left(f_{*,300,\text{CC}}\right)^{1/3}\) for DCL-filled specimens, a dimensionless parameter \(R\) was defined:  
\[R=\frac{E_{*,300,\text{CC}}/\left(f_{*,300,\text{CC}}\right)^{1/3}}{E_{*,300, \text{FC}}/\left(f_{*,300,\text{FC}}\right)^{1/3}} \tag{10}\]  
Figure 11 presents a plot of \(R\) versus the replacement ratio. It is evident that:  
1. \(R\) is always less than 1.0. This implies that even when the compressive strength of compound concrete is equal to that of fresh concrete, the elastic modulus will be lower. A similar relationship can also be observed using published test data from previous research [29]. This indicates that the negative effect of DCLs on the combined elastic modulus is more significant than that on the combined compressive strength.
2. The maximum aggregate size of the fresh concrete in study [29] was 25 mm, which is smaller than that of the fresh concrete in this study (35 mm); at the same time, replacement ratio (e.g., 30% in Figure 9), the value of \(R\) calculated from the published test data [29], is almost the same as that based on the test results in this study. So the influence of maximum aggregate size on the value of \(R\) is apparently limited.
3. The value of \(R\) decreases gradually with increasing replacement ratio. This is presumably due to the greater heterogeneity of compound concrete.  
Based on the test data listed in Table 4, an empirical expression can be proposed for the relationship between the combined elastic modulus and the combined compressive strength applying to both horizontally cast prism and vertically cast cylinder.  
\[E_{*,300,\text{CC}}=8108\times\left(1-0.74\times\eta\right)\times\left(f_{*,30 0,\text{CC}}\right)^{1/3}\ \left(0\leq\eta\leq 40\%\right) \tag{11}\]  
The subscript asterisk (*) refers to the shape of the specimen (* = pr for a prism and cy for a cylinder). Figure 12 compares the calculated and measured elastic moduli. It shows that the measured results generally agree well with results calculat  
Figure 11: \(R\) and replacement ratio of DCLs.