# 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
Considering that the compressive strength of a vertically cast specimen is normally less than that of a horizontally cast one [50; 51], it may be surmised that if prisms and cylinders made of DCLs and FC were cast in the same placement direction, the \(f_{\text{pr,300,CC}}/f_{\text{cy,300,CC}}\) strength ratio should be significantly smaller than the observed 1.13. Based on the experimental results published in recent years, a linear formula Equation (3) has been suggested for predicting the cubic compressive strength of compound concrete made of normal-strength DCLs and FC [27; 30]. Taking the observed shape effect into account, Equation (3) can be extended to Equations (4) and (5), which may approximately predict the combined compressive strength of prisms and cylinders:  
\[f_{\text{cu,300,CC}}=f_{\text{cu,300,FC}}\times(1-\eta)+f_{\text{cu,300,DC}}\times\eta \tag{3}\]  
\[f_{\text{*,300,CC}}=S_{*}\times[f_{\text{*,300,FC}}\times(1-\eta)+f_{\text{*,3 00,DC}}\times\eta] \tag{4}\]  
\[S_{*}=1.0+a_{*}\times\eta-b_{*}\times\eta^{2} \tag{5}\]  
Here the subscript asterisk (*) = cu for a cube, cy for a cylinder, and pr for a prism;\(f_{\text{*,300,CC}}\) is the predicted compressive strength of compound concrete with a lateral dimension of 300 mm;\(f_{\text{*,300,DC}}\) and \(f_{\text{*,300,FC}}\) are, respectively, the compressive strength of demolished concrete and fresh concrete with a lateral dimension of 300 mm; \(\eta\) represents the replacement ratio; and a- and b- are coefficients related to the shape effect. The values of \(a_{\text{cu}}\), \(a_{\text{cy}}\), and \(a_{\text{pr}}\) can be taken, respectively, as 0, 0.04, and 1.12, and \(b_{\text{cu}}\), \(b_{\text{cy}}\), and \(b_{\text{pr}}\) can be taken, respectively, as 0, 0.85, and 2.87, according to the test results listed in Table 4. It should be noted that during the calculation, the subscript asterisk (*) on the left- and right-hand sides of Equations (4) and (5) must be assigned with the same meaning. The combined compressive strengths of the cubic, prismatic, and cylindrical specimens in Groups 2, 3, and 4 predicted using Equation (4) were compared with the measured results presented in Table 5.  
Figure 8: Variation of strength ratio with replacement ratio of DCLs. During the calculations, in order to determine the value of \(f_{*,300,\text{DC}}\), it was assumed that the size and shape effects on demolished concrete's strength are approximately the same as those on fresh concrete's strength (i.e., \(f_{*,150,\text{DC}}/f_{\text{cu},150,\text{DC}}=f_{*,150,\text{FC}}/f_{\text{cu},1 50,\text{FC}}\), and \(f_{*,300,\text{DC}}/f_{*,150,\text{DC}}=f_{*,300,\text{FC}}/f_{*,150,\text{FC}}\)). If so, \(f_{*,300,\text{DC}}\) can be determined as:  
\[f_{*,150,\text{DC}}=\frac{f_{*,150,\text{FC}}}{f_{\text{cu},150,\text{FC}}} \times f_{\text{cu},150,\text{DC}} \tag{6}\]  
Figure 7:0  
Except for \(f_{\text{cu},150,\text{DC}}\), all the other strengths on the right-hand side of Equations (6) and (7) can be obtained from Figure 7:1 The measured 150 mm cubic compressive strength of demolished concrete, \(f_{\text{cu},150,\text{DC}}\), was 40.8 MPa, as described in Section 2.1. Table 5 shows that:  
1. The combined compressive strength (\(f_{*,300,\text{CC}}\)) predicted using Equation (4) is generally higher than the measured one. This is mainly because Equation (4) is based on data from previous experiments in which the maximum aggregate size of fresh concrete did not exceed 25 mm. Spaces between the mold and the DCLs and gaps between adjacent DCLs may have been better filled with fresh concrete in that work. The fresh concrete's aggregate size in this study ranged up to 35 mm. This may have allowed some spaces between the mold and the DCLs and some gaps between the adjacent lumps to be filled only with mortar without coarse aggregates. If the homogeneity of the compound concrete was poorer, that could explain the lower measured compressive strength. 2.