Internal friction study of the influence of humidity on set plaster

Internal friction, one of the useful techniques for studying material changes at microscopic level, is used to investigate water effect on mechanical behaviour of set plaster. Internal friction was measured as a function of strain amplitude at different humidity conditions. A weak dependence is observed at low-strain amplitude, followed by a steep linear rising above a threshold strain amplitude. Increasing the relative humidity (RH) increases the internal friction and lowers the threshold of the rising part. The results provide an experimental evidence of a relative sliding of gypsum crystals enhanced in the presence of water, and a simple viscoelastoplastic rheological model is proposed.