Performance of a Dynamic Dew Point Method for Moisture Isotherms of Clays

Laboratory tests were conducted to assess the performance of a dynamic equilibrium method for rapidly measuring water vapor sorption isotherms of clays and clayey soils. Emphasis was placed on quantifying temperature stability, measurement reproducibility, establishing a recommended testing protocol, and comparing dynamically generated isotherms with those obtained using a conventional equilibrium method. Adsorption-desorption loops for water activity (a(w)) or relative humidity ranging from 0.03 to 0.95 and containing as many as 140 discrete measurements were obtained using the dynamic method in 12-24 h. Far less intensive sorption isotherms using the equilibrium method required similar to 600 h (25 days). Measurement reproducibility was within 6% of the full-scale range. Water contents measured using the dynamic method at a(w) less than 0.60 compared excellently with values measured using the equilibrium system and are within 10 to 15% of the full-scale range at higher activity. Comparisons between total suction characteristic curves obtained using the dynamic system and the equilibrium system suggest a practical total suction measurement range from about 100 000 to 400 000 kPa for a wide range of clay types.