Stiffness plasticity degradation of masonry mortar under compression: preliminary results: Perda de rigidez da argamassa de assentamento da alvenaria comprimida: resultados preliminares

The main goal of this research is to determine the mechanical properties of bedding mortar by assessing the mortar damage onset, the stiff ness
plasticity degradation and the apparent Poisson´s ratio under compression. Two mortar types, 1:0.5:4 and 1:1:6 (cement:lime:sand ratio), were used
and tested at 28 days; specimens had diameter-to-height (d/h) ratios of 0.3 and 1.0. These diameter-to-height (d/h) ratios were chosen to evaluate
the effect of confinement caused by the friction between the steel plates of the testing machine and the sample. Numerical models were developed,
and their response compared with the experimental results. From the experimental results, it was concluded that there are meaningful differences in
their responses with weak and strong mortar types and different d/h ratios. The d/h ratio influences the relationship between the stress and strength
and the apparent Poisson´s ratio of the specimen, which is defined herein as the ratio of the horizontal to vertical strain, regardless of cracking of the
specimen. The mortar damage onset and stiff ness plasticity degradation for both mortar types and d/h ratio are different and depend on the stress/
strength ratio level. All samples with a d/h ratio of 0.3 show a constant decrease in the volumetric strain until failure but with negligible expansion
on the horizontal deformation. In contrast, samples with a d/h ratio of 1.0 present an increase of stiff ness after development of the first crack, which
causes the increase of the sample volume. Numerical simulation and experimental results for mortar 1:0.5:4 with a d/h ratio of 0.3 are similar until
approximately 10 MPa, after which the numerical results diverge from the experimental results. For the d/h ratio of 1.0, the vertical strain results are
also similar, but the horizontal strains results near failure are very different. The model can not represent the nonlinear increase of the horizontal strain
near failure probably because the crack propagation and the stiff ness plasticity degradation could not be controlled. For mortar 1:1:6, vertical strains
from numerical and experimental results are similar, but again the model can not reproduce the nonlinear increase of horizontal strain near failure.