# Testing Concrete for the Construction of Winemaking Tanks
## 3 Results and Discussion
### Mechanical and Physical Properties  
As shown in Table 4, a small increase in open porosity was observed in CP10 composition, which is a common feature in concretes/mortars that contain a quantity of pozzolan. This is connected to the lower specific gravity of the pozzolan compared to cement powder [27]. Regarding compressive strength, it seems in principle that there was no significant difference between the two compositions. In C3 composition, it seems that the development of strength was faster in the first 7 days compared to CP10, which is also expected, as the presence of pozzolan leads to the development of strength at a slower rate. In addition, the aim of pozzolan addition was the capturing of portlandite into the structure, rather than contributing to excessive compression. Overall, the strength level achieved was considerably higher than the required for C20/25 concrete, which means that there was room for cement content reduction in the concrete mixture. However, the authors decided to keep the cement content high and have a higher safety margin for the actual production of the concrete. After some real scale trial mixtures, cement content could be reduced to improve the environmental profile of the concrete tanks. Additionally, the static modulus of elasticity for both mixtures was in the expected range of values for concrete, with a small decrease observed for CP10, in agreement to other studies in which the dynamic modulus is slightly reduced due to pozzolan addition [28]. For tensile strength (flexural and splitting) there was no significant difference after the addition of pozzolan, on the contrary, a small increase in splitting tensile strength was observed. Table 4 shows the mechanical properties, as well as the open porosity of the two studied mixtures.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Property** & **C3** & **CP10** \\ \hline Open porosity (\%) & 6.79 & 10.55 \\
7-day compressive strength (MPa) & 40.3 & 35.3 \\
28-day compressive strength (MPa) & 45.8 & 42.7 \\ Static modulus of elasticity (GPa) & 37.2 & 28.0 \\ Flexural strength (MPa) & 7.97 & 7.81 \\ Splitting tensile strength (MPa) & 1.94 & 2.33 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Results of physical and mechanical properties.  
Figure 2: Visualization of the results of the finite element analysis.  
Table 5 gives a comparative analysis of the results for compositions C3 and CP10, before and after the freeze-thaw cycles. The results from the thermal conductivity test on two tiles of each composition are given in Table 6. Table 7 presents the results of the water vapor permeability resistance coefficient for the two compositions C3 and CP 10.  
Regarding the durability test (Table 5), no significant modification is observed in the weight change, structure and strength of the concrete after freeze-thaw cycles.  
The same similar behaviour between the two compositions is also observed in the thermal conductivity/resistance test (Table 6). For both compositions, the thermal conductivity of the concrete is relatively low, compared for example to the standard values of the Greek Regulation on the Energy Performance of Buildings, for lightly reinforced concrete (1.65-2.0 W/(m \(\times\) K)) [29], therefore the composition is considered successful in terms of thermal behaviour, for the construction of the tanks. The water vapor permeability test showed an increased value of the resistance coefficient for the composition with pozzolan (55.8 m) compared to the reference composition (31.8 m). Despite the greater open porosity of the CP10 composition, the lower permeability could be attributed to the pore distribution modifications, as the addition of pozzolan materials refines the pore structure [30]. Both compositions had relatively low water vapor permeability compared to the literature [15].