EFFECT OF AIR PRESSURE ON MOISTURE TRANSFER INSIDE POROUS BUILDING MATERIALS

 The effect of air pressure on moisture transfer inside porous building materials cannot be ignored in cases where air cannot escape through the surfaces of the materials; in such cases, air is compressed by the movement of the moisture. Such a situation can be brought about by the sealing or surface-protecting materials that prevent water from penetrating into a surface, which is often seen in a typical water absorption test. However, in the field of architectural engineering, the effect of air pressure under wetting processes has not been sufficiently examined, and the validity of calculation models for the transfer of both air and moisture inside materials has not yet been verified. Therefore, this study investigates the behavior of water under the influence of air-pressure changes and develops a proper calculation model for it.<br> First, we conducted two types of water absorption tests on bricks. In both of the tests, water infiltrated the specimens through their top surfaces, and the time profiles of the water content were measured using a gamma-ray attenuation method. While only the side surfaces of the specimen were impermeable to moisture and air in Case 1, the side and bottom surfaces were impermeable in Case 2. In Case 2, we expected the air pressure inside the specimen to increase, as the air inside it would not be able to escape through the bottom; this would result in the infiltration of the water being prevented. The results for Case 2 show that the moisture resistance of the specimen was seemingly greater than those obtained in Case 1. We also discovered that the amount of water that was absorbed during Case 2 was not significantly different from the amount absorbed during Case 1. Because air bubbles were observed to have escaped through the water on the top surface in Case 2, we determined that the air pressure inside the specimen returned to atmospheric pressure over time. Therefore, we expected the steady states of Cases 1 and 2 to be similar. It should be noted that the measured water content in Case 2 varied depending on the horizontal position of the measurements.<br> Second, we developed a calculation model corresponding to the experiment that was based on the equation for the transfer of both air and liquid water. The calculation results show that the increase in the water content slowed significantly when the changes in the air pressure inside a material were considered. In addition, the calculated and measured water contents agreed well with one another and allowed us to verify the validity of our model. The analysis also showed that when air escapes from the material, the amount that escapes should be adequately considered by the calculation model. According to the results of both the experiments and the simulation, the infiltration process under the influence of the air pressure was as follows: At an early stage in the infiltration process, because the air pressure inside the material was not much higher than atmospheric pressure, water infiltration was not prevented significantly. However, this caused a rapid compression of the air inside the specimen, which resulted in high air pressure within it. This trend reduced the rate of the increase of the air pressure because further water infiltration was reduced by the high pressure. During the final stage of the infiltration, the air pressure began to decrease to atmospheric pressure and water content reached almost the same values as those in Case 1.