Time lag in measuring pore humidity in concrete by a gage in finite cavity

Measurements of the relative humidity of water vapor in concrete are important for understanding, predicting and controlling the shrinkage and creep of concrete, as well as other processes such as the ASR. A sufficient time is required for the pore water to diffuse into (or out of) the gage cavity. To determine this time, the nonlinear partial differential equation of diffusion of moisture through concrete is solved numerically. The cavity is simplified as spherical and its volume is considered to be far smaller than the volume of surrounding concrete, which permits assuming spherical symmetry of the pore humidity field. The body of the surrounding concrete is assumed to be so large that the water escape into (or intake from) the cavity does not change the pore humidity in concrete appreciably. For the current measurement practice, the humidity difference between the concrete pores and the cavity is found to drop below the inevitable error of the gage after the lapse of about 24 h, and an acceptable error of about 1% is achieved in about 10 h. It is shown that extrapolation by a decaying power function of time can shorten this time to 2 h. A further shortening could be achieved by reducing the volume of the cavity, although any contact of the gage with the concrete must be avoided.