Measurement of crystalline swelling in expansive clay

An experimental system for measuring relationships among relative humidity, moisture content, and the corresponding volume changes associated with hydration-related, or "crystalline," swelling in expansive clay is described. The system is capable of actively controlling relative humidity between approximately 1 and 90% along either wetting or drying paths in a small environmental chamber. Axial strain and moisture content of compacted expansive clay specimens are monitored as the specimen swells or shrinks in response to imposed changes in humidity. Application of the system is demonstrated for a specimen of Wyoming Na+-smectite. Hysteresis is observed between the wetting and drying loops of the humidity-water content relationship as well as the humidity-strain relationship. Characteristic wavy patterns are noted in both relationships that are interpreted to indicate the successive adsorption and desorption of discrete molecular layers of water associated with crystalline swelling. Humidity values at which the transitions in the stable hydrate states occur compare well with previous results obtained from humidity-controlled X-ray diffraction tests. The maximum axial strain attributed to the crystalline swelling process is 16.3%.