| Field Note: Tips for Cold Weather Masonry Construction
By Jake Ribar, Principal Masonry Evaluation Engineer,
Construction Technologies Laboratories, Skokie, IL
a masonry project this winter? Below-normal temperatures (40°F
and below) do not necessitate any significant changes of the mortar
mixtures or the masonry units. Rather, your principal concern will
be to keep the newly constructed masonry from freezing. Here are
Mortar – Ideal temperatures
for the placement and curing of masonry mortar is the range of 70°F
+ 10°F. In cold weather (40°F and below) mortar materials
need to be heated, otherwise the mortar is likely to exhibit slower
setting times and lower early strengths.
Water acts as a lubricant in the plastic mortar and is required
for hydration of the portland cement contained in the mortar. While
mortar stiffens as water is absorbed by units and evaporates, the
hardening of mortar is a result of the reaction between the portland
cement and water. This reaction rate is temperature dependent and
is slowed or stopped when the cement paste is below 40°F.
During the critical early curing stages when the free water content
of mortar is high (above 6%) the temperature of the in place masonry
should be maintained above freezing to avoid disruptive expansion
in the mortar due to freezing. While measures to reduce initial
water content of the mortar and increase the dispersion rate of
the free moisture in the masonry limits the time that the masonry
is vulnerable to disruptive freezing expansion, such measures must
be appropriately balanced with other considerations of good masonry
practice. For example: The use of a clean well graded sand will
reduce the water requirement of mortar without the detrimental loss
of workability that may occur as a result of arbitrarily cutting
back on the water content. Heating the mortar materials and providing
heated enclosures for newly constructed masonry will increase both
the dispersion rate of the free moisture and the reaction rate with
the cement compounds.
|Sand can be heated over fire in
a pipe, and water can be heated in metal drums.
Masonry Units – Only dry masonry
units should be used during cold weather masonry construction. Wet
units may become frozen before construction and impair the performance
of the mortar and consequently the wall assembly. Further, dry units
should not be excessively cold because they will cool the mortar
rapidly and could cause freezing. Cold masonry units that are wet
and frozen must be thawed, but carefully, to prevent overheating.
Preheated masonry units exhibit all the usual performance characteristics
of units used during normal construction, except the heated unit
may absorb more water from the mortar.
Absorptive masonry units do have an advantage in cold weather over
units with very low absorption. The absorptive unit can absorb excess
water from the mortar and lessen the possible disruptive expansion
in the mortar on first freezing. However, even with absorptive units,
the temperature of masonry needs to be initially maintained at a
level that assures adequate curing of the fresh mortar. Units with
very low absorption capability (glass block, for example) may require
extended heating of the masonry to avoid disruptive freezing of
the mortar. The slow stiffening of mortar resulting from the low
absorption of the unit will limit productivity during construction
and could contribute to color variations in mortar joints as a result
of tooling wet mortar joints.
Materials – At cold temperatures,
Type I cement can be replaced with Type III cements which hydrates
at a faster rate. You may also consider changing to a higher strength
mortar than you would normally use. For example: If ASTM C270 Type
N mortar is specified for normal temperatures, the typically lower
water retention and higher strength gain of a Type S mortar may
be more appropriate for cold weather, particularly if low absorption
masonry units are used.
Admixtures – Mortar admixtures
are acceptable in cold weather masonry construction, but only if
they are laboratory tested at the temperature extremes at which
they will be used.
For more detail, see: “Recommended Practices & Guide Specifications
for Cold Weather Masonry Construction,” International Masonry
Industry All-Weather Council. Available from International Masonry
Institute, Washington, DC.