Page 92 - Masonry and Concrete

Understanding Masonry
91
UNDERSTANDING MASONRY

of thermal and acoustical resistance. Three
of the most important properties of brick
are strength, absorption, and freeze-thaw
resistance.

Strength: Brick are much stronger than
they need to be for simple one- and two-
story construction. Compressive strength
can range from 1,500 to 22,500 psi. The
majority of brick produced in the United
States and Canada exceeds 4,500 psi. Mor-
tar is not as strong as brick, so when mortar
and brick are combined, the compressive
strength of the masonry drops to about
1,000-2,000 psi, depending on the mortar
mix and the exact brick strength. Even at a
very modest 1,000 psi, a brick wall could
theoretically support its own weight for a
height of more than 600 feet without crush-
FIGURE 4-5
ing. To resist the bending stress of wind
loads, though, the wall also needs flexural Uneven color distribution. (from Beall, Christine,
Masonry Design and Detailing, 4th edition, McGraw-
strength. Flexural strength requires good
Hill, New York).
bond between the mortar and the units,
and good bond is a function of brick tex-
ture and absorption, mortar quality, and workmanship.

Absorption: When fresh mortar comes in contact with a brick, the
mortar paste is absorbed into the surface pores, contributing to the
strength of the bond between brick and mortar. Brick that is very
moist cannot properly absorb the mortar paste, and the lower bond
strength reduces resistance to wind loads and cracking. Brick that is
very dry absorbs too much water so that the mortar cannot cure prop-
erly and develop adequate bond strength. Moist brick should be
allowed to dry before use so that its absorption is increased, and dry
brick should be hosed down so its absorption is reduced. The mortar
could also be mixed with a little more or a little less water. Too much
or too little water in the mortar, however, decreases its workability so
it is better to adjust the moisture content of the brick instead of the

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