## 3 Meso-Scale Simulation of Moisture Transfer Characteristics
### Mesoscopic Moisture Migration Characteristics in Wetting and Drying
#### 3.2.2 Difference in Moisture Diffusion Characteristics Between Wetting and Drying  
In order to further explain the difference in the moisture migration characteristics between the concrete wetting and drying process, the average humidity distribution inside concrete is obtained according to the calculation results, as shown in Figure 7. The results show that there is an obvious \"inflection point\" in the average humidity distribution inside the concrete whether in the wet or dry process. According to Equation (1), the solution of concrete humidity distribution should be a smooth quadratic curve in general. However, due to the \"S\" growth relationship between the moisture diffusion coefficient and RH, the slope of the quadratic curve changes, which leads to the occurrence of the \"inflection point\". Since the moisture diffusion coefficient reaches the maximum when RH \(\geq\) 0.9, the \"inflection point\" will appear at a higher relative humidity. At the same time, the difference in the humidity distribution under different aggregate volume fractions gradually increases with the increase in the humidity migration distance. This is mainly due to the influence of the increase in the aggregate volume fraction on the overall moisture diffusion coefficient of concrete, which makes the humidity evolution of the same part have  
Figure 5: RH distribution of concrete with different aggregate volume fractions at 72 h in wetting process: (**a**) aggregate volume fraction of 0.3; (**b**) aggregate volume fraction of 0.45; and (**c**) aggregate volume fraction of 0.6. Figure 6: RH distribution of concrete with different aggregate volume fractions at 168 h in drying process: (**a**) aggregate volume fraction of 0.3; (**b**) aggregate volume fraction of 0.45; and (**c**) aggregate volume fraction of 0.6.  
a time difference. At the same time, the continuity of wet migration in space makes the time difference accumulate with the wet migration process, which eventually leads to the increasing difference in the spatial distribution of humidity. Although this difference is small, the difference in wetting is greater than that in drying. Considering the time difference between the wetting and drying process and the phenomenon of the \"inflection point\", the average relative humidity distribution at different moments was obtained by taking the model with an aggregate volume fraction of 0.3 as an example, as shown in Figure 8. The results show that the distance from the boundary to the \"inflection point\" in the wetting and drying process increases with the increase in time, and the change in RH at the \"inflection point\" is small, but it is always greater than 0.9. This is due to the fact that in the wetting process, the boundary \(x=0\) is the first part where the humidity starts to rise. This is due to the wetting process. The boundary \(x=0\) is the first part where the humidity began to rise. According to Equation (1), it can be seen that the \"point of inflection\" on the left side of the humidity gradient is small, and the right side of the humidity gradient is larger, so in the process of wetting, the \"point of inflection\" on the left side of the humidity changes slower than the right side, while the drying process is the opposite. In addition, the wetting process of concrete is faster than the drying process, because the RH of the wet boundary is 1, the moisture diffusion coefficient at the boundary is the largest, and the wet boundary transmits water to the concrete with a large moisture diffusion coefficient. The RH of the drying boundary is 0.5, and the moisture diffusion coefficient at the drying boundary is small, so the boundary transports moisture outward with a small moisture diffusion coefficient.