Page 132 - Masonry and Concrete

Understanding Masonry
131
UNDERSTANDING MASONRY

providing full mortar joints with good bond, by controlling cracking,
and by the judicious application of protective coatings to very absorp-
tive units.

Provide Full Mortar Joints: Full mortar joints and good bond
between units and mortar are extremely important, not only to the
strength and stability of masonry walls, but also to their weather resis-
tance. Wind-driven rain of sufficient strength and duration can pene-
trate even well-built masonry walls, but if the joints are only partially
filled, moisture penetration is substantially increased along with the
likelihood of leakage to the interior of the building. Full mortar joints
can easily be achieved with proper technique, but mortar bond is
affected by the mortar materials, the mortar mixture, and the texture
and absorption of the unit surface.
Both portland cement and lime mortars and masonry cement mor-
tars allow water to penetrate through masonry walls. The amount of
water entering the wall is generally higher with masonry cement mor-
tars, but when workmanship is poor, joints are only partially filled,
and flashing and weeps are not functional, either type of mortar can
produce a leaky wall. There are no industry standards or guidelines
identifying varying amounts of water penetration that are either
acceptable or unacceptable. A wall system with well-designed and
properly installed flashing and weeps will tolerate a much greater vol-
ume of water penetration without damage to the wall, the building, or
its contents than one without such safeguards. Ultimately, the work-
manship and the flashing and weep hole drainage system will deter-
mine the success or failure of most masonry installations.

Control Cracking: Masonry walls are very strong in compression and
can support their own weight for a height of several hundred feet or
support the weight of a building for 15 to 20 stories. Masonry walls are
also relatively brittle and can bend very little without cracking. In
unreinforced masonry, the bond between mortar and units is what
holds the walls together and gives them a modest amount of resistance
to pulling (tension) and bending (flexure). The bond between mortar
and units is also the weakest link in masonry, and cracks usually occur
along the lines of the mortar joints. A building may contain thousands
of linear feet of mortar joints along which cracks can open if the

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