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Bond Strength
The single most important property of mortar is bond strength, and it is critical that this bond be complete, strong, and durable. The
mechanical bond between individual bricks, blocks, or stones unifies the wall as a system, provides resistance to tensile stress, and seals against the penetration of moisture. The strength and extent of the bond are affected
by many variables of material and workmanship. Complete and intimate contact between the mortar and the unit is essential, and workability influences the ease with which the mortar spreads and covers the surfaces. Rough units
have a very porous surface that is highly receptive to the wet mortar and increases adhesion . The moisture content and suction of the units, the water retention of the mortar, and curing conditions such as temperature,
relative humidity, and wind combine to influence the completeness and integrity of the mechanical and chemical bond. Voids at the mortar-to-unit interface offer little resistance to water infiltration and facilitate subsequent
disintegration and failure if freezing occurs. ASTM E514 gives a laboratory test method for measuring susceptibility to water penetration. The test subjects a sample wall to a pressure differential and application of water on
the high pressure side. The time, location and rate of leakage is observed and interpreted.
Workmanship is also very important in bonding. Full mortar beds must be laid down by the mason to assure complete coverage of all
contact surfaces. Once a unit has been placed and leveled, additional movement will break or seriously weaken the bond. The high water retention of cement-lime mortars allows more time for placing units on bed joints before
evaporation or the suction of adjacent units alters the plasticity and flow of the mortar. In aligning the masonry, laboratory tests show that tapping the unit to level will increase bond strength 50 to 100% over hand pressure
alone. Because of the many variables involved, it is difficult to develop laboratory tests of bond strength that produce consistent results. Flexural bond strength is presently measured by ASTM E518. This test involves creating
a stack bonded assembly of brick and mortar tested asa simple beam. Presently a bond wrench test is being observed as an aleternative.
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