Mortar joint behavior

More recently, Crisafulli and Carr (2007) proposed a new macromodel in order to represent, in a rational but simple way, the effect of masonry infill panels. The model is implemented as a four-node panel element which is connected to the frame at the beam-column joints. Internally, the panel element accounts separately for the compression and shear behavior of the masonry panel using two parallel struts and a shear spring in each direction. This configuration allows an adequate consideration of the lateral stiffness of the panel and of the strength of masonry panel, particularly when a shear failure along mortar joints or diagonal tension... 

Shear Behavior Along the Bed Joints The influence of mortar joints acting as a plane of weakness on the composite behavior of masonry is particularly relevant in case of strong unit-weak mortar joint combinations. Two basic failure modes can occur at the level of the unit-mortar interface tensile failure (mode I) associated with stresses acting normal to joints and leading to the separation of the interface and shear failure (mode II) corresponding to a sliding mechanism of the units or shear failure of the mortar joint. The preponderance of one failure mode over another or the combination of various failure modes is essentially related to the orientation of the bed joints with respect to the principal stresses and to the ratio between the principal stresses. 

Simplified micro-modeling - expanded units are represented by continuum elements, whereas the behavior of the mortar joints and unit-mortar interface is lumped in discontinuum elements. 

Figure 25 describes the thermal expansion of the silica KD brick-only and the silica KD brick/mortar composite samples. The mortar-only sample was also tested. The brick-only and the brick/mortar composite samples are similar except for temperatures above about 1100°F. The mortar softens considerably, as reflected in the mortar-only sample. Due to the confinement of the mortar within the mortar joint in the brick/mortar composite sample, the mortar maintains reasonable strength. This confirms that the true compressive stress-strain behavior of mortar must be tested with the mortar contained in the mortar joint (2). Containment of the mortar is an important and necessary parameter in testing the thermomechanical behavior of mortar. 

Figures 30, 31, and 32 show the creep behavior of the brick-only, mortar-only, and brick/mortar composite samples at 816°C (1500°F), 1094°C (2000°F), and 1372°C (2500°F), respectively. At all three temperatures the brick-only and brick/mortar composite samples exhibit insignificant creep. The mortar-only samples (no confinement) exhibit relatively considerable creep response. The creep tests also confirm that unconfined mortar tests do not replicate the true confined behavior of the mortar in the mortar joint.

Figures 33 and 34 show the temperature dependent thermal expansion of the silica KN brick-only, mortar-only, and brick/mortar composite samples. The interpretive results are quite similar to the interpretive results of the silica KD tests. These results also show that the mortar-only (unconfined mortar) tests do not reflect the true confined thermomechanical behavior of mortar in mortar joints. Strength patterns as a function of temperature are also similar to the silica KD brick.

Schacht CA. The effect of mortar joints on the thermomechanical behavior of refractory brick lining systems in cylindrical vessels. AISE Spring Conference, Birmingham, AL, March 1985. 

Masoruy is a composite material composed of masomy units with a regular arrangement that are coimected with mortar commonly at horizontal bed and vertical head joints. The interface between units and mortar represents in general an important role on the mechanical behavior of the composite material submitted to distinct types of loading. 

The shear behavior of mortar masonry joints is characterized experimentally based on direct shear tests by following the typical shear test configuration as shown in Fig. 9a. The typical behavior of mortar masonry joints tmder increasing shear load and constant pre-compression load is presented in Fig. 9b. The general shape of the shear stress-shear displacement is characterized by a sharp initial linear stretch. The peak load is rapidly attained for very small shear displacements. Nonlinear deformations develop in the... 

In case of moderate pre-compression stresses, for which the nonlinear behavior of mortar is negligible and the friction resistance takes the central role, the shear resistance of masoiuy bed joints is linearly dependent on the compressive... 

The stress state is calculated by assuming that masonry is an isotropic and homogeneous material, which does not correspond to its actual behavior, since tensile strength is dependent on the orientation of the principal stress regarding the mortar bed joints. For height-to-width ratios QiH) higher than 1.5, from which walls can be... 

The nonlinear response of the joints, which is then controlled by the unit-mortar interface, is one of the most relevant features of masonry behavior. Two different phenomena occur in the unit-mortar interface, one associated with tensile failure (mode I) and the other associated with shear failure (mode II). 

Different test setups have been used for the characterization of the tensile behavior of the unit-mortar interface. These include (three-point, four-point, bond-wrench) flexural testing, diametral compression (splitting test), and direct tension testing. An important aspect in the determination of the shear response of masonry joints is the ability of the test setup to generate a uniform state of stress in the joints. This objective is difficult because the equilibrium constraints introduce nonuiuform normal stresses in the joint. 

Masonry is a composite building material consisting of tliree discrete phases units, joint mortar, and unit-mortar interface. The last is the weakest phase governing the behavior of masonry to horizontal loads. 

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