damage from early-age freezing. Interestingly,
made from a single mortar batch. The test consist-
the strongest portland cementlime mortars
ed of periodically bringing one set of three cylin-
ders into a 20C (4F) room for at least 24 hr fol-
resulted from a moisture content of 12%, and the
strongest masonry cement mortar came from a
lowed by room temperature curing for a total of 7
moisture content of 10%. Moisture contents
days, excluding the time in the cold room. The
above and below these levels weakened the mor-
cylinders were then stripped from their plastic
tar. Despite efforts to mix and consolidate all of
molds and tested in compression. Strengths were
the batches equally regardless of the mixing
compared with those of control samples that
water content, difficulties in working with drier
were never frozen. The first set of cylinders went
mortar batches may account for some of the
into the cold room immediately after the samples
reductions in strength of the mortars with lower
were cast. Additional cylinders were brought to
water-to-cement ratios.
the cold room at 1-hr intervals through the first 12
hr, as well as a final set of cylinders at 48 hr. The
Critical maturity
cylinders were kept in the cold room overnight,
The development of pore structure in mortar is
and returned to a warm room the following after-
another mechanism that produces frost resis-
noon.
tance. As a mortar matures, its water chemically
Figure 12 shows the 7-day compressive
combines with cement, with the result that the
strengths for these cylinders. The dotted line in
mortar increases in strength and decreases in free
the figure represents the 7-day strength of the
water. At some age the quantity of freezable
control mortar that was never frozen. As can be
water falls below a critical level and creates
seen, if a mortar is frozen at too early an age the
empty pore space within the mortar that enables
mortar loses strength. However, it is also clear
the mortar to accommodate waterice expansion
that once a mortar attains a certain maturity it can
without damage. The objective of this experiment
resist frost damage. In this case, keeping the mor-
tar at room temperature for a minimum of 5 3/4 hr
was to determine the age at which mortar first
becomes resistant to a single cycle of freezing and
before freezing enabled the mortar not only to re-
thawing.
sist frost damage but to begin to gain strength in
A type M masonry cement mortar was utilized
excess of the control. In fact, curing the mortar for
for this test series. It was mixed to a 16% moisture
12 hr at room temperature before exposing it to
freezing temperatures produced a 10% increase
content at room temperature according to exist-
in strength. To confirm that there is a benefit to
ing standards and cast into cylindrical samples
measuring 50 100 mm (2 4 in.) using vibration
freezing mortar, similar testing was done with the
other mortar types frozen at times between 8 and
as the method of consolidation. All of the mortar
60 hr and then cured at room temperature for 28
cylinders cast for this phase of the testing were
14
12
7-Day Control (never frozen)
10
8
0
8
16
24
32
40
48
Time (hr)
Figure 12. Critical maturity.
13
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