Abstract: In order to investigate whether there is a unique correspondence between the relative humidity of 100% inside dam concrete and the pore water saturation of 100%, dam secondary concrete specimens with a water-binder ratio of 0.5 and different fly ash contents (0, 35%) were formed. The relative humidity inside the concrete specimens was monitored by temperature and humidity sensors and the pore water saturation of the concrete was calculated using a weighing test. The results show that the relative humidity inside the dam concrete with a water-binder ratio of 0.5 was kept at 100% in the insulation humidity state; however, the corresponding pore water saturation was not necessarily 100%; the pore water saturation of concrete without fly ash and with 35% fly ash was 85%~89% and 73%~76%, respectively. This indicates that 100% of the pore water saturation of the dam concrete corresponds to 100% of the internal relative humidity, and the internal relative humidity of 100% is not the only relative humidity corresponding to the pore water saturation of 100%. In the insulation humidity state, for concrete with a water-binder ratio of 0.5, the pore water saturation of concrete with 35% fly ash is less than that of concrete without fly ash.