## 3 Results and Discussions
### Compressive Strength  
The compressive strength was measured on 10 cylinders in accordance with SR EN 12390-3 [15]. The load was applied at a constant rate of 0.6 MPa/s (4.71 kN/s). The obtained values are shown in Figure 12.  
Figure 11: SineDamp function for fitting the steady-state response of the specimen.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mix**} & \multicolumn{2}{c}{**Vibration Decay, \(\Lambda\)**} & **Pulsation** & \(\xi\) \\ \cline{2-4}  & **[Hz]** & **[Hz]** & **[\%]** \\ \hline Ref & 471 & 10,019 & 0.75 \\ \hline RuC40 & 1561 & 8278 & 3.00 \\ \hline RuC60 & 1260 & 7339 & 2.73 \\ \hline RuC80 & 1072 & 6343 & 2.69 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Material damping.  
Compared to the data from [6], the compressive strength of the reference mix increased by 22.64% from 35.56 MPa to 43.61 MPa. However, all other concrete mixes, the ones involving rubber aggregates, showed a decrease in the values of the compressive strength with an almost 35% decrease for RuC80. Only a 3.65% decrease was observed for RuC40. The obtained results are in line with the decreased values of both the static and the dynamic moduli of elasticity and the decrease in the values of the material damping with the increase in the replacement percentage.  
A possible explanation may reside in the higher number of voids present at the ITZ level for concrete mixes with large volumes with rubber particles.  
The conversion equations from compressive strength to static modulus of elasticity were checked for accuracy and suitability of being applied to rubberized concrete, although they were not specifically developed and proposed for this purpose. Hence, the equation given by Eurocode 2 [27] (Equation (6)) and the equation given in ACI 318-14 [28] (Equation (7)) were investigated. The obtained results are summarized in Table 4.  
\[\mathrm{E_{cm}}=22\times\left(\frac{\mathrm{f_{cm}}}{10}\right)^{0.3},\text{ where }\mathrm{f_{cm}}=\mathrm{f_{ck}}+8[\mathrm{MPa}] \tag{7}\]  
\[\mathrm{E_{c}}=4.7\sqrt{\mathrm{f_{c}}^{\prime}} \tag{8}\]  
where \(\mathrm{f_{c}}^{\prime}\) and \(\mathrm{f_{ck}}\) are the compressive strength of concrete obtained from laboratory investigations [MPa], \(\mathrm{E_{c}}\) and \(\mathrm{E_{cm}}\) are the static moduli of elasticity [MPa].  
It can be observed that the best prediction for the reference mix was given by Equation (7). Equation (6) overestimated the experimental results by as much as 22%. All equations significantly overestimated the experimental results with the increase in the rubber content. The data are in line with the results reported in previous studies [6] and underline  
\begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Mix**} & **Ec (Eurocode 2, Equation (6))** & **Exp/Ec,6** & **Ec (ACI 318-14, Equation (7))** & **Exp/Ec,7** \\ \cline{2-5}  & **[MPa]** & **[\%]** & **[MPa]** & **[\%]** \\ \hline Ref & 35,995 & 78.37 & 28,825 & 90.89 \\ \hline RuC40 & 30,795 & 56.38 & 22,711 & 77.56 \\ \hline RuC60 & 29,296 & 45.72 & 20,978 & 67.21 \\ \hline RuC80 & 27,749 & 35.35 & 19,180 & 56.43 \\ \hline \end{tabular}
\end{table}
Table 4: Predicted values for the static modulus of elasticity as function of the compressive strength.  
Figure 12: Compressive strength of Ref and RuC mixes.  
the limitation of currently available equations in predicting the modulus of elasticity of rubberized concrete with large volumes of rubber aggregates. Therefore, further studies should be conducted with the aim of proposing accurate prediction equations in view of the increasing use of rubberized concrete in the construction industry.