The thermodynamics of volume change and creep

Abstract

1 With some exceptions, early observers of creep of concrete assumed that time-dependent strain is due to plastic flow or viscous flow. It is now becoming understood that creep, like shrinkage, is a phenomenon depending on adsorption and desorption of water and related phenomena. Understanding shrinking and swelling is prerequisite to understanding creep. An analysis of these phenomena based on thermodynamics is especially instructive and reliable because in general it does not depend on the correctness of the model of structure adopted for the discussion. 2 Spontaneous volume change is due to a change in the balance of forces and counterforces within the specimen. It cannot be due to the removal or addition, per se, of water molecules. 3 The volume of a porous body will vary with the density of the material of which it is made. Material density will vary with hydrostatic pressure, with temperature, or with change in surface tension of the material. 4 Change of surface tension presumably affects both the inner part and the outer tension shell of particulate material, and oppositely. 5 Surface tension, which is the same as surface free energy, varies with the amount of water adsorbed, and that is a function of ambient vapor pressure at constant temperature. The change in solid surface tension can be calculated from the concomitant change in ambient humidity. 6 The volume change of the body corresponding to a given change in surface tension is the resultant of the change of the inner and outer parts, is at present indeterminate, and is in any case a small fraction of the amount of volume change to be accounted for. 7 Data on paste structure show that adsorbed water must produce disjoining pressure in the narrowest places between solid bodies, where adsorption is hindered.