Determination of Equilibrium Moisture Content of Building Materials: Some Practical Difficulties

Abstract

Though the definition of equilibrium moisture content of porous materials is straightforward from the point of view of equilibrium thermodynamics, experimental determination of this quantity may not always be easy. Many factors play roles from an experimentalist's point of view. Hysteresis, the reference dry state, the initial moisture content, the history of the test specimen, the size of the test specimen, the drying technique, and drying temperature—all influence the value that is arrived at by an experimentalist. This paper reports results from several series of measurements on gypsum board, eastern white pine, aerated concrete, and calcium silicate brick using the ASTM Standard C 1498 in the hygroscopic ranges and pressure plate techniques above the hygroscopic range of equilibrium moisture content. These results show the ambiguities that may surround the experimental definition of equilibrium moisture content. The results presented in the paper demand a few refinements in the existing ASTM Standard C 1498 as well as in the procedure that is used in the pressure plate method. The paper proposes a few refinements for both. The need for the development of an ASTM standard for the pressure plate technique is identified.