Open Geomechanics

The role of disjoining pressure on the drying shrinkage of cementitious materials

Syeda Rahman, Zachary Grasley — Thu, 03 Aug 2023 09:02:00 +0000

Drying induced shrinkage is often attributed to two major mechanisms- capillary pressure in the bulk pore solution and disjoining pressure in the liquid film separating the vapor phase from the pore wall or separating solid surfaces in nanometric pores. There is sufficient ambiguity in literature regarding the relative contribution of these two mechanisms, as well as the means to quantify their contributions. The objective of this manuscript is to evaluate the contribution of disjoining pressure in the drying shrinkage of cementitious materials. An unconventional approach to determining disjoining pressure within the framework of continuum mechanics is presented. This approach utilizes the conservation of linear momentum to derive a generalized expression of the disjoining pressure from the Lorentz force vector. The expression suggests that disjoining pressure is essentially an osmotic pressure at the contact surfaces that counters the electrostatic contribution to linear momentum. The proposed theory accurately predicts measurements of osmotic pressure found in the literature for the swelling of charged bilayers in a dilute salt solution. Applied to the shrinkage problem, the theory suggests that shrinkage stress is induced by the reduction in the potential gradient between the liquid film and bulk solution from the reference (fully saturated) state. The reduction in the potential gradient is caused by an increase in the concentration of the solutes in the pore solution when liquid water is removed as the relative humidity decreases.

Shear strength of angular granular materials with size and shape polydispersity

Shear strength characterization of coarse granular materials often requires modifying the original material in order to fit samples in standard testing devices. This is done, however, at the expense of changing the particle size distribution (psd), employing scaling-down techniques such as parallel grading or scalping methods. Such procedures hide, nevertheless, another challenge. As a given particle size can present a characteristic grain shape, altering the grain size distribution can strongly modify the distribution of grain shapes. While the effects of grain shape on shear strength have been vastly covered in the literature, the effect of having different shapes along grain sizes has yet to be systematically assessed and understood. This article explores the critical shear strength of samples composed of particles with size-shape correlations using 2D discrete element simulations. Two cases of particle shape variability across grain sizes are studied: (1) the sharpness of grains' corners - modeled via the number of sides of regular polygons - and (2) the geometric irregularity of grains - where the corners of a polygon are not necessarily evenly spaced. The effects of these geometrical properties on the shear strength are assessed through a series of numerical simple shearing tests up to large levels of deformation. We find that granular materials presenting different number of sides across grain sizes can strongly modify their mechanical response depending on the grain-size correlation. On the contrary, grain shape irregularity turns out not to have a major effect on the critical shear strength. Microstructural analyses allow us to identify how each correlation affects load transmission mechanisms between grains, and the contribution of each grain shape class to the macroscopic shear strength. This work shows that particle sizes are not the only sample descriptor to consider when applying scaling-down techniques. It is equally key to characterize particle shapes across grain sizes to capture the material's mechanical response adequately.

Introductory consideration supporting the idea of the release of unloading elastic waves in the steady state response of hysteretic soil

Piotr Kowalczyk — Mon, 13 Nov 2023 14:12:55 +0000

Unintended and unwanted high frequency oscillation motion
is sometimes observed in small-scale experimental works and in numerical
simulations when soil is subjected to simple harmonic input
motions. This high frequency motion has been often attributed to the
drawbacks of the actuating systems in experimental setups and to numerical
noise in computational analyses. This work presents introductory
consideration supporting the hypothetical idea that the recorded
and the computed high frequency oscillation motion can possibly be the
consequence of an unrecognized before physical phenomenon of soil
elastic waves released upon unloading due to soil inherent hysteretic
stress-strain behaviour and affecting the steady state response of soil
to harmonic excitation. To this aim, simplified numerical studies representative
of the most basic soil mechanical properties are carried out.
The results reveal potential importance of soil-released unloading elastic
waves and their reflections inside a soil column when understanding
free field response in numerical simulations representative of small scale
experimental setups. Chosen numerical cases are compared with
available examples of experimental works from the literature. In addition,
two further cases are analyzed, including a case showing the potential
importance of soil-released elastic waves in the response of soil
to real earthquakes, and a case showing the response of structural elements.

The role of disjoining pressure on the drying shrinkage of cementitious materials

Syeda Rahman — Mon, 08 Jan 2024 14:51:47 +0000

Drying induced shrinkage is often attributed to two major mechanisms- capillary pressure in the bulk pore solution and disjoining pressure in the liquid film separating the vapor phase from the pore wall or separating solid surfaces in nanometric pores. There is sufficient ambiguity in literature regarding the relative contribution of these two mechanisms, as well as the proper means to properly quantify their contributions. The objective of this manuscript is to evaluate the proper contribution of disjoining pressure in the drying shrinkage of cementitious materials. An unconventional approach to determining disjoining pressure within the framework of continuum mechanics is presented. This approach utilizes the conservation of linear momentum to derive a generalized expression of the disjoining pressure from the Lorentz force vector. The expression suggests that disjoining pressure is essentially an osmotic pressure at the contact surfaces that counters the electrostatic contribution to linear momentum. The proposed theory accurately predicts measurements of osmotic pressure found in the literature for the swelling of charged bilayers in a dilute salt solution. Applied to the shrinkage problem, the theory suggests that shrinkage stress is induced by the reduction in the potential gradient between the liquid film and bulk solution from the reference (fully saturated) state. The reduction in the potential gradient is caused by an increase in the concentration of the solutes in the pore solution when liquid water is removed as the relative humidity decreases.