In order to study the hysteretic freezing characteristics of clay, a test method is proposed to measure the relationship between Unfrozen Water Content (UWC) and temperature in clay. Based on the lump system analysis, the interior temperature of sample remains essentially uniform all the time during a heat transfer process if its volume is sufficiently small. Four small-volume clay samples were frozen and thawed by natural convection in isothermal air, and their temperature-time curves were recorded. Based on the principle of energy conservation, a thermal model was proposed to predict the characteristic curves of UWC through analyzing the relationship between temperature change and heat energy in each stage of freezing and thawing cycle. This method was applied to test clay with moisture content of 35%. Four clay samples were frozen to -27°C in -30°C isothermal air and then thawed by natural convection in 28°C isothermal air. The results showed that the UWC in freezing process is always greater than that in thawing process, and the maximum difference is about 4.3%~4.6% for the temperature in the range of -4~-5°C. Thus, the effect of hysteresis should be considered when measuring and analyzing the thermal and mechanical properties of clay.
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