## 3 Results and Discussion
### Evaluation of the Strength Characteristics of Concrete under Cyclic Sulfate Attack  
The results of changes in the compressive strength of vibrated, centrifuged, and vibrocentrifuged concrete samples depending on the number of sulfate attack cycles are presented in Table 9 and Figure 8.  
An analysis of the trend in compressive strength change, shown in Figure 8, showed that the rate of compressive strength drop is the highest for vibrated concrete and the lowest for vibrocentrifuged concrete. Already after 15 cycles, there is a noticeable difference in the drop in strength of vibrated concrete in comparison with centrifuged and vibrocentrifuged concrete. So, in general, after 60 cycles of aggressive action, the loss of compressive strength was 16.4% for vibrated concrete, 11.8% for centrifuged concrete, and 9.5% for vibrocentrifuged concrete.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Cycle Number**} & \multicolumn{3}{c}{**Manufacturing Technology**} \\ \cline{2-4}  & **Vibration** & **Centrifugation** & **Vibrocentrifugation** \\ \cline{2-4}  & \multicolumn{3}{c}{**Compressive Strength (MPa)**} \\ \hline
0 & 43.7 & 47.3 & 50.8 \\ \hline
5 & 43.5 & 47.2 & 50.7 \\ \hline
10 & 43.1 & 46.9 & 50.5 \\ \hline
15 & 42.5 & 46.5 & 50.2 \\ \hline
20 & 41.7 & 46.0 & 49.7 \\ \hline
25 & 41.0 & 45.4 & 49.3 \\ \hline
30 & 40.3 & 44.7 & 48.7 \\ \hline
35 & 39.5 & 44.1 & 48.1 \\ \hline
40 & 38.8 & 43.6 & 47.7 \\ \hline
45 & 38.4 & 43.0 & 47.2 \\ \hline
50 & 37.7 & 42.5 & 46.8 \\ \hline
55 & 37.2 & 42.1 & 46.4 \\ \hline
60 & 36.5 & 41.7 & 46.0 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Change in compressive strength of concrete samples depending on the number of sulfate attack cycles.  
The dependences of the loss of compressive strength on cycle number \(x\) shown in Figure 8 are well approximated by a polynomial of the third degree (8)-(10) with a high coefficient of determination \(R^{2}\).  
\[\Delta R_{Vb.cub}=-0.2137+0.1290x+0.00655x^{2}-6.946\times 10^{-5}x^{3},R^{2}=0.998 \tag{8}\]  
\[\Delta R_{Cb.cub}=-0.0415+0.0353x+0.006860x^{2}-6.969\times 10^{-5}x^{3},R^{2}=0.998 \tag{9}\]  
\[\Delta R_{Vb.cub}=-0.04066+0.0134x+0.00579x^{2}-5.687\times 10^{-5}x^{3},R^{2}=0.998 \tag{10}\]  
The results of changes in the axial compressive strength (prism strength) of vibrated, centrifuged, and vibrocentrifuged concrete samples depending on the number of sulfate attack cycles are presented in Table 10 and Figure 9.  
Figure 8: Dependence of the loss of compressive strength of concrete on the number of sulfate attack cycles (V—vibrated, C—centrifuged, VC—vibrocentrifuged).  
Figure 9: Dependence of the loss of prism compressive strength of concrete on the number of sulfate attack cycles (V—vibrated, C—centrifuged, VC—vibrocentrifuged).  
The dependences of prismatic compressive strength on cycle number \(x\) are approximately similar to the trend in cubic compressive strength (Equations (11)-(13)).  
\[\Delta R_{Vb}=0.160-0.001176x+0.007272x^{2}-3.980\times 10^{-5}x^{3},R^{2}=0.998 \tag{11}\]  
\[\Delta R_{Cb}=0.135-0.004086x+0.005851x^{2}-3.753\times 10^{-5}x^{3},R^{2}=0.999 \tag{12}\]  
\[\Delta R_{Vcb}=0.060-0.01674x+0.005414x^{2}-3.543\times 10^{-5}x^{3},R^{2}=0.999 \tag{13}\]  
Already after 20 cycles, the strength reduction rate of vibrated concrete increases sharply and is the highest among the three types of concrete under consideration. The minimum rate of strength drop was recorded for vibocentrifuged concrete. The maximum drop in prism compressive strength after 60 cycles of sulfate attack was 17.5% for vibrated concrete, 12.7% for centrifuged concrete, and 10.8% for vibocentrifuged concrete.  
It should be noted that in the dependences obtained (5)-(13), the first term of the polynomials is close to zero (within the measurement error). This means that if the number of sulfate attacks \(x\) is zero, then there is no loss of mass and strength.