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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Shear displacement [mm]

Figure 3. Shear stress-shear displacement diagrams in dry joints; (a) dry specimens; (b) saturated specimens.

From the responses in Figure 3, no significant differences were detected between dry and saturated specimens and the peak shear strength was found to increase as the normal stresses increase. By comparing the peak shear stresses, lower values were obtained in case of saturated specimens and higher scatter was found when the maximum pre-compression level (o0 = 1.0 N/mm2) was applied.

Four stages can be identified in the shear stress-shear displacement diagrams. The pre-peak behavior is characterized by a linear extent for low levels of shear stress, associated to the contact of the joint interface, and by a clear non-linear stretch until peak shear is reached. These features can be confirmed from Figure 3, where the pre-peak part is highlighted. A

plateau is found at peak stress, as the shear behavior of dry masonry joints under monotonic loading exhibits considerable plastic deformations associated to the inelastic sliding.

Similarly to what has been reported in literature (Misra 2002, Huang etal. 2002) no shear softening was recorded after peak stress for smooth surfaces, unlike rough rock joints that exhibit remarkable lowering of the shear resistance as the plastic shear displacement increases due to the roughness fracture.

3.2 Monotonic behavior of mortarjoints

The shear stress-shear displacement diagrams of mortar joints with respect to all levels of pre-compression are shown in Figure 4a. The horizontal displacement is considered as the average of the displacements

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