dominant factors that determine the rate of
mixtures for concrete or masonry may be-
mortar moisture loss. The air temperature
come commercially available in the future.
and the temperatures of the masonry units
9. The laboratory tests showed that the anti-
and the mortar had only a minor effect on the
freeze admixture KC1 had a negligible effect
rate of moisture loss from mortar.
on the freezethaw durability of mortar. This
2. Mortar becomes immune to a single cycle of
admixture decreased the setting times at low
freezing at a moisture content between 8 and
temperature and substantially increased the
10%.
bond between mortar and the units.
3. The time for mortar to reach a moisture con-
10. Although parameters such as spacing factor,
tent of 8% is typically about 4 hours.
specific surface, number of air voids per 25
mm (1 in.), total air content, and pore size
4. Mortar cured at or above 5C (41F) reaches
distribution are used frequently to predict
critical maturity within 6 hours. Critical
freezethaw durability of slumpable con-
maturity is the minimum maturity needed
crete, these parameters are less reliable when
for mortar to withstand one event of freezing
it comes to dry-cast concrete performance as
without damage.
measured by ASTM C 666 (procedure B) or
5. Freezing does not always harm early-age
ASTM C 1262.
mortar. The 7-day strength of mortar can be
11. Although C 666 uses change in relative
increased by around 10% when it is frozen
after about 10 to 16 hours of curing at or
measure freezethaw durability perfor-
above 5C, provided that the time in freezing
mance, cumulative percent weight loss per
temperatures is discounted from the compu-
surface area as used in C 1262 may be a more
tation of the 7 days.
appropriate measure of performance for
6. Current guidance allows mortar to be heated
dry-cast concrete specimens. More study is
up to 50C (120F). This study showed that
needed.
40C (104F) mortar placed in the cold does
12. No physical property measured in this
not stay above freezing appreciably longer
study, including compressive strength, den-
than 5C (41F) mortar. Therefore, there is
sity, or absorption, consistently predicted
minimal benefit from heating mortar above
the same freezethaw resistance of dry-cast
20C (68F). Thin mortar joints will not
concrete products as that measured using C
remain above freezing significantly longer
666 or C 1262.
when higher temperatures are used.
13. The cost comparison and productivity of the
7. Current guidance requires that mortar be
two walls built as a demonstration in this
thermally protected for a minimum of 16
project did not yield significant cost differ-
hours. Based on critical moisture contents
ences when the outdoor air temperature
and critical maturities, thermal protection
averaged 3C (37F). Colder weather will
could be realistically reduced to 4 to 6 hours.
increase heating demand. However, in all cas-
A conservative change to current practice
es where some heating is required, use of anti-
would be to relax the time of thermal protec-
freeze admixtures will reduce the amount of
tion from the current 16 hours to 8 hours.
fuel burned for thermal protection.
8. Antifreeze admixtures are a viable alternative
to thermal protection. A major drawback is
that, at the present time, no antifreeze admix-
RECOMMENDATIONS
ture commercially available is labeled for use
1. The laboratory experiments in this project
with concrete or with masonry. However, the
showed that the time of thermal protection
ingredients of the U.S.-Army-patented anti-
for masonry may safely be reduced from the
freeze admixture KC1 are available as generic
current 16 hours to 8 hours if the following
chemicals from various sources. The recom-
conditions are met:
mended dosage is 4.5% of sodium nitrate and
a) The masonry units are not extremely
1.5% of sodium sulfate by weight of portland
damp. If this is suspected, the units must be
cement. These chemical compounds are usu-
allowed to dry at room temperature until all
ally supplied in powder form and dissolve
visible signs of moisture disappear.
easily into the mix water. Other antifreeze ad-
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