Page 175 - Masonry and Concrete

Masonry Construction Techniques
174 CHAPTER FIVE

particularly if the cooler winter months lengthen the drying process.
Extended wetting will also prolong cement hydration, producing large
amounts of calcium hydroxide, which may also be taken into solution
and leached to the surface to cause calcium carbonate stains.
Covers such as water-repellent tarps or heavy plastic sheets should
extend a minimum of two feet down each side of uncompleted walls
and be held securely in place. During construction, scaffold planks
should also be turned on edge at the end of each day so that rain will
not splash mortar droppings or dirt onto the face of the masonry.

5.7.3 Cold Weather
Cold weather causes special problems in masonry construction. Even
with sufficient mixing water, cement hydration and strength develop-
ment in mortar and grout will stop at temperatures below 40°F. Con-
struction may continue during cold weather if the masonry mortar
and materials are kept warm during placement, preventing the
masonry from freezing during the initial hours after placement before
cement hydration and mortar cure are complete. Frozen mortar looks
like it is hardened, but it is not actually cured and will not develop
full design strength or complete bond until it is thawed and liquid
water is again available for hydration. Frozen mortar is easily
scratched from joints, has a “crows feet” pattern on the surface of
tooled joints, and may flake at the surface. Cement hydration will
resume if the temperature of the masonry is raised above 40°F and its
liquid moisture content exceeds 75%. When these conditions are
maintained throughout the curing period, ultimate strength develop-
ment and bond will be the same as those attained under moderate
conditions.
The rate at which masonry freezes is influenced by the severity of
temperature and wind conditions, the temperature and absorption
characteristics of the units, the temperature of reinforcing steel and
metal accessories, and the temperature of the mortar at the time of
placement. Wet mortar mixes expand more when frozen than drier
ones, and expansion increases as the water content increases. During
freezing weather, low-moisture-content mixes and high-suction units
are desirable, but regardless of the conditions, mortar and grout con-
sistency must provide good workability and flow so that bond is max-
imized. During cold-weather construction, it may be desirable to use a

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