## 3 Results and Discussion
### Modulus of Elasticity  
The results of the modulus of elasticity tests are shown in Figure 6. The mean values obtained on 730-day-old specimens ranged from 48.64 GPa (C55_SFA15_SF3) to 51.80 GPa (C100_SFA30_SF3). A correlation between compressive strength and elastic modulus values was observed: the HPC with the lowest mean compressive strength (C55_SFA15_SF3) was also the one with the lowest mean elastic modulus. For the mixtures with higher compressive strengths (C100_SFA30_SF3 and C100_SFA15_SF3), the mean values of elastic modulus were approximately 6% higher than those obtained for C55_SFA15_SF3. The obtained results were compared (Table 7) with the values estimated by the three different models recommended for HSC by ACI 363 [30], Eurocode 2 (EN 1992-1-1) [26] and NS 3473 [31] standards, respectively:  
\[E_{c}=3.32\sqrt{f_{c}}+6.9 \tag{7}\]  
\[E_{c}=22(0.1\cdot f_{c})^{\frac{1}{3}} \tag{8}\]  
\[E_{c}=9.5(f_{c})^{\frac{1}{3}} \tag{9}\]  
where \(f_{c}\) is the cylinder mean compressive strength in MPa, and \(E_{c}\) is the modulus of elasticity in GPa. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Mixture**} & \multirow{2}{*}{**Model**} & \multicolumn{6}{c}{**Tested Value/Predicted Value Difference [\%]**} \\ \cline{3-9}  & & **2** & **7** & **14** & **28 * & **56** & **90** & **730** \\ \hline \multirow{3}{*}{C55\_SFA15\_SF3} & ACI Committee 209 & \(+\)271.0 & \(+\)48.2 & \(+\)5.6 & - & \(+\)0.9 & \(-\)3.3 & \(+\)0.3 \\  & JSCE & \(+\)201.7 & \(+\)33.4 & \(+\)0.4 & - & \(+\)2.8 & \(-\)0.4 & \(+\)5.1 \\  & Eurocode 2,MC 2010 & \(+\)511.1 & \(+\)72.5 & \(+\)10.4 & - & \(-\)1.4 & \(-\)5.5 & \(+\)0.8 \\ \hline \multirow{3}{*}{C100\_SFA15\_SF3} & ACI Committee 209 & \(+\)124.7 & \(+\)9.8 & \(+\)7.2 & - & \(+\)1.1 & \(+\)2.7 & \(+\)13.4 \\  & JSCE & \(+\)82.7 & \(-\)1.2 & \(+\)2.0 & - & \(+\)2.9 & \(+\)5.7 & \(+\)18.9 \\  & Eurocode 2,MC 2010 & \(+\)270.1 & \(+\)27.8 & \(+\)12.0 & - & \(-\)1.3 & \(+\)0.3 & \(+\)14.0 \\ \hline \multirow{3}{*}{C100\_SFA30\_SF3} & ACI Committee 209 & \(+\)82.5 & \(-\)0.7 & \(-\)2.6 & - & \(-\)2.0 & 0.0 & \(+\)3.1 \\  & JSCE & \(+\)48.4 & \(-\)10.6 & \(-\)7.4 & - & \(-\)0.2 & +3.0 & \(+\)8.1 \\ \cline{1-1}  & Eurocode 2,MC 2010 & \(+\)200.5 & \(+\)15.5 & \(+\)1.8 & - & \(-\)4.3 & \(-\)2.3 & \(+\)3.6 \\ \hline \hline \end{tabular}  
*—the actual value of the 28-day compressive strength was used for model calculations.  
\end{table} 
Table 6: Differences between tested and model-predicted compressive strength values. The significant differences observed between obtained and estimated values could be explained by the use of coarse basalt aggregate and the fact that modulus of elasticity was tested on 730-day-old samples. The Eurocode 2 (EN 1992-1-1) [26] indicates that the modulus of elasticity estimated from Equation (8) should be increased by 20% when basalt coarse aggregate is used in concrete. In such cases, the recalculated estimated values would be 51.90 GPa (C55_SFA15_SF3), 53.04 GPa (C100_SFA15_SF3) and 53.41 GPa (C100_SFA30_SF3). In the same standard [26], it has been pointed out that the concrete age has a significant influence on its modulus of elasticity. Thus, the secant modulus of elasticity of concrete at the age of \(t\) days should be estimated from the following equation:  
\[E_{cm}(t)=(f_{c}(t)/f_{c})^{0.3}\cdot E_{cm} \tag{10}\]  
where \(E_{cm}(t)\) is the modulus of elasticity at the age of t days in GPa, \(f_{c}(t)\) is the cylinder compressive strength of concrete at the age of \(t\) days in MPa, \(f_{c}\) is the compressive strength of concrete at the age of 28 days, and \(E_{cm}\) is the modulus of elasticity at the age of 28 days in GPa estimated from Equation (8).