# Industrial Low-Clinker Precast Elements Using Recycled Aggregates
## 3 Results and Discussion
### Compressive Strength and Modulus of Elasticity  
Figure 6a shows the compressive strength-strain curves for each concrete at 160 days. Several studies [25; 46; 47] show that the concrete's compressive strength decreases with the degree of substitution of RA for NA, but in strain terms, concretes show similar values around 2500 \(\upmu\)m/m for the failure. The exception is the HRR50 mix, which exceeds the values of the rest by almost 1000 \(\upmu\)m/m. Figure 6b shows the same mixtures but at an age of 365 days. The decrease in strength may also be due to  
Figure 4: Density vs. RA content. Figure 5: Porosity (**a**) and absorption coefficient (**b**) vs. RA content. the randomness of the type of RA and its distribution into the mortar matrix, which causes greater uncertainty than conventional mixtures. Table 3 shows the different values of compressive strength obtained at different ages. Table 4 displays the modulus of elasticity, and shows that when using RC, the decrease in the elastic modulus is around 4%. The substitution of 25% by RA implies a decrease in elastic modulus of 5.6%, while the substitution of OPC in this case does not seem to have an influence. In the case of replacing 50% of aggregate by RA, the influence of the substitution of OPC by RC is meaningful, decreasing the elastic modulus by 15%. As for the loss of elastic modulus over time, a greater influence of the cement is observed than the type of aggregate, with a limit that tends to an asymptotic value of around 27 GPa. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Concrete:** & **Substitution [\%]** & \begin{tabular}{c} **Modulus of** \\ **Elasticity [GPa]** \\ \end{tabular} & \
\begin{tabular}{c} **Modulus of Elasticity at** \\ **365 days [GPa]** \\ \end{tabular} &
\begin{tabular}{c} **\% of the Initial** \\ **Elastic Modulus** \\ \end{tabular} \\ \hline HP & 0 & 35.5 & 31.7 & 89.3 \\ HPR & 0 & 34.1 & 29.5 & 86.5 \\ HR25 & 25 & 33.9 & 30.8 & 90.8 \\ HR50 & 50 & 31.9 & 29.3 & 91.8 \\ HRR25 & 25 & 34.2 & 27.9 & 81.6 \\ HRR50 & 50 & 27.8 & 27.4 & 98.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Modulus of elasticity. Figure 6: Compressive strength-strain at 160 (**a**) and 365 (**b**) days. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Concrete:**} & \multicolumn{4}{c}{**Compressive Strength [MPa]**} \\ \cline{2-5}  & **28 days** & **160 days** & **365 days** & \(\Delta_{\text{36-28}}\) [\%]** \\ \hline HP & 51.2 & 53.5 & 56.8 & +10.9 \\ HPR & 46.1 & 50.2 & 46.6 & +1.1 \\ HR25 & 51.7 & 47.0 & 45.1 & \(-\)12.8 \\ HR50 & 51.2 & 48.8 & 42.1 & \(-\)17.7 \\ HRR25 & 45.0 & 47.5 & 43.7 & \(-\)2.9 \\ HRR50 & 41.2 & 47.5 & 42.4 & +2.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Compressive strength at different ages. Some organizations such as EHE-08, ACI, and Eurocode present their expressions to predict elastic modulus at 28 days from the compressive strength. In Expressions (1)-(3): \(E\) is elastic modulus at 28 days [GPa] and \(f_{28}\) is the compressive strength at 28 days [MPa]. EHE-08 [48]  
\[E=8.5\sqrt[3]{f_{28}} \tag{1}\]  
ACI [49]  
\[E=4.7\sqrt{f_{28}} \tag{2}\]  
Eurocode 2 [50]  
\[E=22(f_{28}/10)^{0.3} \tag{3}\]  
These expressions can be used to obtain the predictions and comparisons, with the experimental results shown in Table 5. The ACI method fits quite well in most cases but predicts higher values when the percentage of substitution is 50%. The EHE-08 method is safer, although when the substitution is 50% and the OPC is replaced by RC, higher values are produced due to the heterogeneity of the RA affecting the compressive strength. These types of expressions only satisfactorily fit ordinary concrete models. Figure 7 shows that from approximately 48 MPa, concrete with RA achieved the same compressive strength as concrete with OPC. RA concrete increases its elastic modulus significantly. This might be due to the addition of a new variable, such as RA compared with OPC, which is much more standardized throughout its production process.