## 2 Materials and Methods
### Methods  
The mix proportion considered at the beginning of the experimental program is shown in Table 1[6]. The water to cement ratio was kept constant to 0.47 for all mix proportions. The mix proportions were denoted as Ref, for the reference mix without any rubber aggregates, and RuC for rubberized concrete, followed by a number showing the replacement percentage, by volume, of sand by rubber aggregates. The data were converted to mass units when preparing the mix proportions, as presented in Table 1.  
The rubber aggregates were set to replace the natural sand, with a maximum dimension of 4 mm. The replacement percentages, by volume, were 40%, 60% and 80%. The apparent density of the rubber aggregates was experimentally determined, 506 kg/m\({}^{3}\), and was found to be in line with similar reported data in the scientific literature [13]. A total number of 20 cylinders (\(\upphi\)100 \(\times\) 200 mm) were cast, resulting in a total number of 80 specimens. The specimens were demolded at 24 h after casting and cured in water for 28 days. At the age of 28 days, a cylindrical specimen of each mix proportion was cut in 30 mm thick slices to assess the distribution of the rubber aggregates within the concrete mix, as shown in Figure 2. After 28 days, the samples were kept in laboratory conditions (23 \(\pm\) 2 \({}^{\circ}\)C and relative air humidity of 40-50%) until the day of testing, 5 years later. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Mix** \\ **Designation** \\ \end{tabular}  } & \multirow{2}{*}{**Cement (C) CEM 142.5R**} & \multirow{2}{*}{**Water (W)**} & \multirow{2}{*}{**W/C**} & \multicolumn{4}{c}{**Aggregates**} \\ \cline{3-4} \cline{6-7}  & & **[kg/m\({}^{3}\)]** & & & **Sand** & **Rubber** & **Sort 4–8 mm** & **Sort 8–16 mm** \\ \cline{2-7}  & **[kg/m\({}^{3}\)]** & & & **[kg/m\({}^{3}\)]** & **[kg/m\({}^{3}\)]** & **[kg/m\({}^{3}\)]** & **[kg/m\({}^{3}\)]** \\ \hline Ref & & & & 582 & - & \\ \cline{2-7} \cline{6-7} \cl  
The static longitudinal modulus of elasticity was assessed in accordance with specifications of SR EN 12390:13 [14]. Cyclic loading was applied within limits mentioned in the code, and three individual values were obtained for each specimen. One cylinder was loaded in compression until failure in order to set the correct upper and lower limits of the loading cycles, as shown in Figure 3a. Therefore, the static longitudinal modulus of elasticity was assessed for 19 specimens only, with 3 distinct measurements for each specimen. The experimental setup is shown in Figure 3b. A hinge was used between the loading platens of equipment and the concrete cylinder in order to ensure an even distribution of the compressive load. The compressive and splitting tensile strengths were determined in accordance with SR EN 12390:3 [15] and SR EN 12390:6 [16], respectively. The considered loading rate for determining the compressive strength was 0.6 MPa/s (4.71 kN/s), whereas, for the tensile splitting strength, a loading rate of 0.05 MPa/s (0.4 kN/s) was adopted. The dynamic longitudinal modulus of elasticity, \(\mathrm{E_{d}}\), was assessed following the guidelines of ASTM C215:14 [17] and was based on the first resonant frequency (ERF) obtained from the Impact Echo Method. The experimental setup is shown in Figure 4. An accelerometer was placed at one end of the concrete cylinder along the longitudinal axis of the specimen to record the response of the sample when subjected to a small impact load at the opposite end. The signal was recorded on a computer via a data acquisition system.