Measuring permeability, Young's modulus, and stress relaxation by the beam-bending technique.

摘要

Recent interest in the permeability of cement paste, mortars, and concrete lies in the need to gain further understanding of mechanisms affecting the durability of these materials. Conventional techniques for measuring permeability are cumbersome and often take days to complete just one measurement. This thesis proposes a new technique for measuring the permeability. The advantage of this technique is that the results are obtained in a few minutes to a few hours; moreover, there is no problem with leaks or need for high pressures. The method is particularly well suited for examining the changes in permeability and viscoelastic properties of young cement paste samples.; When a saturated rod of a porous material is instantaneously deflected under three-point bending, two types of relaxation processes occur simultaneously: hydrodynamic relaxation, caused by the flow of liquid in the porous body to restore ambient pressure, and viscoelastic relaxation of the solid network. By measuring the decrease in the force required to sustain a constant deflection, it is possible to obtain the permeability and Young's modulus from the hydrodynamic relaxation function, in addition to the stress relaxation function of the sample. The exact viscoelastic solution is developed and the total relaxation is shown to be very closely approximated as the product of the hydrodynamic and stress relaxation functions.; The analytical results are verified on porous Vycor® glass saturated in various solvents, including normal alcohols, water, and glycerol. The results show excellent agreement with the theory. Consistent with observations of previous workers, the permeability is found to be influenced by the size of the solvent molecule; by assuming that the pore surfaces are covered with a monolayer of immobile solvent, the observed variation can be explained.; The evolution of the permeability, Young's modulus, and stress relaxation function are reported for Type III Portland cement paste with water-cement ratios ranging from 0.4 to 0.7 from the age of 1 day to 21 days. The permeability and Young's modulus agree with the range reported by other workers. The stress relaxation function is shown to preserve its shape during aging; that function is numerically transformed into the creep function.