Characterization of saturated porous bodies

Measurement of the response of a saturated body to mechanical and thermal strains can be used to determine the permeability and viscoelastic properties of the body. For example, bending a saturated beam creates a pressure gradient in the pores, and as the liquid flows to equilibrate the pressure, the force required to sustain a fixed deflection decreases. Analysis of the kinetics of force relaxation yields the permeability, in addition to the elastic modulus of the body; if viscoelastic relaxation of the solid phase occurs, it can also be measured. This method permits measurement of very low permeabilities in minutes or hours, but it is useful only for structurally homogeneous materials (such as cement paste) that can be formed into slender beams. For concrete, it is more practical to find the permeability by analysis of thermal expansion kinetics. When a saturated body is heated, the liquid expands more than the solid, and the expansion of the liquid stretches the solid network like a spring; consequently, the apparent thermal expansion coefficient is high. During an isothermal hold, the solid phase squeezes the liquid out of the pores and the body contracts. Analysis of the kinetics of thermal dilatation yields the permeability of the body. Recent experiments reveal an anomalously high thermal expansion coefficient for the water confined in the small pores of cement paste.