Page 133 - Masonry and Concrete

Understanding Masonry
132 CHAPTER FOUR

masonry is not properly designed to accommodate compressive loads,
wind loads, expansion and contraction, or dissimilar movement
between the masonry and adjacent materials. Cracks can also occur as
a result of foundation or soil movements, impact, and other cata-
strophic events. Large cracks may indicate serious structural prob-
lems, but even small cracks are unsightly and provide the potential for
excessive moisture penetration, so understanding the cause and pre-
vention of cracking is an important element in overall performance.
Cracking in masonry is most often related to the expansion and
contraction caused by changes in moisture content. Some shrinking
and swelling occurs alternately through normal wetting and drying
cycles, but more important are the permanent moisture expansion of
clay masonry and the permanent moisture shrinkage of concrete
masonry. Clay masonry begins to reabsorb moisture from the atmos-
phere as soon as it leaves the firing kiln, and as the moisture content
increases, the units expand permanently. Concrete masonry products
are moist-cured to hydrate the portland cement in the mix. Once the
curing is complete, residual moisture evaporates, causing the units to
shrink permanently. Both clay and concrete masonry also expand and
contract very slightly with changes in temperature, but these move-
ments are always less than the initial moisture shrinkage or expansion.
Much of the initial moisture shrinkage or expansion will take place
before the masonry is used, but some almost inevitably occurs after the
units are laid and can cause masonry walls to crack or bow out of place
if the movement is not properly accommodated by flexible anchorage
or fully restrained by reinforcing steel.
Normal shrinkage cracking in concrete masonry can be minimized
by incorporating wire joint reinforcement in the mortar beds of single-
wythe walls or steel bar reinforcement in the grouted cavity of double-
wythe walls. This helps restrain the shrinkage and evenly distributes
the stresses. Special joints can also be installed to force the cracking to
occur at predetermined locations. Concrete masonry control joints are
continuous, weakened joints designed to accommodate the shrinkage
in such a way that cracking will occur in straight lines at predeter-
mined locations rather than zig-zagging across the wall at random
locations. The more joint reinforcement there is in the wall, the farther
apart the control joints can be. The less joint reinforcement, the closer
together the control joints must be.

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