Masonry in Tension, Shear and Biaxial Stress

  • Arnold W. Hendry


The resistance of masonry to tensile or shear stresses is dependent on the bond between mortar and masonry units. The mechanism of bond between unit and mortar is incompletely understood but is known to be influenced by a large number of factors. These have been set out by Groot [1] in the matrix form shown in figure 4.1. It will be clear from this that the problem of masonry bond is extremely complex and most investigations so far have been of a phenomenological nature, applying to specific combinations of materials. However, some more fundamental work has been reported in which the physical and chemical nature of the brick/mortar interface has been studied. Experiments by Grandet [2] on the interaction between brick material and cement paste have shown that this is critically affected by the formation of a micro-layer of ettringite (3CaSO4. Al2O3. 3CaO. 31H2O) at the clay-cement interface, and by the respective mean diameters of the pores of the brick and of the micro-crystals of the ettringite. It is necessary for the pore size of the brick material to be greater than 0.05 mm for a mechanical bond to be formed, and also for the cement to be properly hydrated behind the ettringite layer, despite the withdrawal of some of the water by suction from the brick. If the brick is dry, and has a high suction rate, there will be a partially hydrated zone in the cement paste, to a depth of several millimetres, and possessing poor mechanical strength. The movement of water between the brick and the mortar, and the resulting effects on the development of the mechanical bond between these components, was shown to be considerably affected by their specific surfaces and capillary dimensions. Grandet concluded that it is possible on the basis of these observations to obtain some indication of the likely behaviour of cement mortars, which would, however, also be influenced by such other parameters as the compaction of the mortar, its cement content and water retentivity. The effects of water movement on bond between various combinations of mortar and unit have been studied by Groot [3] who concluded that a primary cause of poor bond in clay masonry is the presence of an unduly high proportion of fine material at the mortar/brick interface. In the case of calcium silicate bricks, poor bond results from the presence of too many fine pores in the bricks.


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Copyright information

© Arnold W. Hendry 1998

Authors and Affiliations

  • Arnold W. Hendry
    • 1
    • 2
  1. 1.University of EdinburghUK
  2. 2.The British Masonry SocietyUK

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