# Effect of Dry-Wet Cycle Periods on Properties of Concrete under Sulfate Attack
## 3 Results and Discussion
### Relative Dynamic Elastic Modulus  
The RDEM of the concrete varied with the number of dry-wet cycles (Figure 8 and Table 5). In general, the relative dynamic elastic moduli were similar between the groups, firstly increasing and then decreasing. However, the numbers of dry-wet cycles required to reach a given stage were different. For example, the longer the dry-wet cycle period, the fewer the cycles required to reach the peak value. In the initial stages, the RDEM in each group decreased slightly. At this point, the dry-wet cycles played a leading role, and the erosion effect of sulfate was weak.  
T3, T7, T14, and T21 peaked in relative dynamic elastic modulus after 28, 12, 6, and 4 dry-wet cycles, respectively. There are two aspects that caused the RDEM of the concrete to rise at this stage. On the one hand, hydration still happened inside the concrete and the soaking process was conducive to an increase in the dynamic elastic modulus; on the other hand, SO4\({}^{-2}\) from the solution entered the interior of the concrete during the dry-wet cycles and reacted with the cement hydration products, producing gypsum and ettringite (among other products), which filled the internal pores and increased the density of the concrete. This inhibited the further entry of SO4\({}^{-2}\), thus reducing the sulfate attack and improving the RDEM [32; 33].  
After peaking, the RDEM of each group began to decline--most rapidly for T7, with an average decrease of 0.7% per cycle. This is because as the attack continued, the combined effect of the concrete damage caused per cycle and the number of cycles sharply increased.  
The shorter the dry-wet cycle period was, the more rapidly capillary adsorption would replenish SO4\({}^{2-}\) in the wetting process, which reduced the concentration difference and humidity gradient inside the concrete during the drying process. This slowed down the SO4\({}^{2-}\) migration to deep concrete and weakened the influence of a single cycle inside the concrete. Too frequent dry-wet cycles were not conducive to the reaction of SO4\({}^{2-}\) with the cement hydration products, as discussed in detail below regarding the damage mechanism. With the further extension of the dry-wet cycle period, although the time available for SO4\({}^{2-}\) to react with cement hydration products was increased, the interval between the wetting process was longer, and the supply of SO4\({}^{2-}\) was limited. Therefore, the deterioration rate for the concrete gradually decreased.  
Figure 8: Variation of relative dynamic elastic modulus among different dry-wet cycle periods.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Cycles**} & \multicolumn{2}{c}{**T3**} & \multicolumn{2}{c}{**Average**} & \multicolumn{1}{c}{**Relative**} \\ \cline{2-5}  & **1** & **2** & **3** & **Value** & **Value** \\ \hline
0 & 43.8 & 43.2 & 43.8 & 43.6 & 0.00 \\
7 & 43.4 & 42.9 & 43.3 & 43.2 & \(-\)0.92 \\
14 & 45.1 & 45.5 & 46.2 & 45.6 & 4.59 \\
21 & 47.6 & 45.9 & 47.7 & 47.1 & 7.95 \\
28 & 47.6 & 47.6 & 47.4 & 47.5 & 9.02 \\
35 & 47.4 & 46.4 & 46.8 & 46.9 & 7.49 \\
42 & 46.5 & 45.6 & 45.9 & 46.0 & 5.50 \\
49 & 46.4 & 46.1 & 46.4 & 46.3 & 6.19 \\
56 & 46.4 & 45.0 & 46.1 & 45.8 & 5.12 \\
63 & 44.8 & 44.5 & 44.9 & 44.7 & 2.60 \\
70 & 43.4 & 43.0 & 42.8 & 43.1 & \(-\)1.22 \\
77 & 40.8 & 41.2 & 40.8 & 40.9 & \(-\)6.12 \\ \hline \multicolumn{5}{c}{T7} \\ \cline{2-5}  & 1 & 2 & 3 & \\ \hline
0 & 44.7 & 44.3 & 44.5 & 44.50 & 0.00 \\
3 & 43.7 & 44.3 & 44.6 & 44.20 & \(-\)0.67 \\
6 & 45.6 & 46.0 & 46.3 & 45.97 & 3.30 \\
9 & 47.6 & 47.6 & 46.7 & 47.30 & 6.29 \\
12 & 47.4 & 46.8 & 48.8 & 4