Thermodynamics of interacting continua with surfaces and creep analysis of concrete structures

Creep, shrinkage and delayed thermal dilatations of concrete have source mainly in the interaction with water of the internal surfaces of solids in the cement paste microstructure. This interaction is believed to consist in changes of thickness or mass content of mono- and multimolecular adsorbed water layers confined between two solid adsorbent surfaces. These changes are brought by diffusion of water and other molecules along the layers. In the present paper a consistent formulation of the above mechanism based on surface thermodynamics is presented and the isotropic macroscopic stress-strain relations are derived. These relations indicate the form of the dependence of material parameters on pore humidity and temperature, and restrict considerably the number of possible forms of constitutive equation that would have to be assumed on a purely phenomenological basis if the material were regarded as general interacting continua in which strains are general functionals of the history of water content and temperature. A viscoelastic material with time-dependent properties is obtained as the special case for constant humidity and temperature. A suitable, numerically stable algorithm of step-by-step time integration of stress and strain problems is also presented. Analysis of experimental data aimed at numerical determination of the material parameters is presently in progress at Northwestern University.