Table 1. Effect of temperature on strength gain* of
freezing point depression of mix water and the
control (admixture-free) mortar.
strength gain of mortar as a function of time and
temperature.
Strength†
Temp
(C)
1 day
3 days 7 days 14 days
28 days
56 days
Materials
50
17.7
33.6
58.1
69.3
75
77
This study used mortar as a rapid way to eval-
40
16
38.1
69.7
79.2
85
85
uate various chemicals. Mortar, instead of con-
30
13.1
36.6
72.1
87.3
94
93
20
10
30
70
90
100
105
crete, simplified mixing operations, reduced
10
3.7
15
49
75.6
97
110
material handling, and permitted smaller test
5
2
14
30.1
56.7
83
100
specimens, which meant less material had to be
5
1
6.7
14
16.6
20.3
46.8
taken to the landfill. In addition, mortar closely
10
0
0.2
1.7
2.7
4.2
51.1
20
0
0
0
0.2
0.8
37.3
simulates concrete because it has the same paste
to aggregate transition zones, except that the
* The strengths for 20, 5, 10, and 20C are results pro-
aggregate is smaller. The mortar was non-air-
duced at CRREL on 0.39 w/c mortar. The others are expected
strengths according to ACI (1988).
entrained,* used Type I portland cement at a
† Strengths are given as percentages of the 28-day strength of
1:2.75 ratio with sand, and was made with water-
control mortar cured at 20C. The mortars were cured at the
to-cement (w/c) ratios between 0.39 and 0.45 (the
temperature shown in the left most column for 28 days, then
they were cured at 20C for the remaining 28 days.
majority of the testing was at 0.41 w/c). The sand,
sieved for use in concrete, had a bulk specific
gravity (saturated surface-dry) of 2.65 and a
20C. Thus, hot weather might allow the project
moisture absorption of 1.1%. The mixing water
to go faster but it can result in permanent strength
was from the taps at CRREL.
loss.
In the long run, concrete becomes stronger
Mixing
when cured at temperatures that are low but
Mixing of the mortar followed standard labo-
above freezing. Their drawback is that they slow
ratory procedures. The mortars were mixed in a
down strength gain, which can adversely affect
Hobart mixer according to ASTM (1987) Standard
construction schedules. If concrete can be mixed
C305 with minor modifications. The chemical ad-
and protected so that its temperature can be kept
ditives were dissolved in the mixing water before
at 5C, preferably at 10C, strength gain can be
the entire solution was placed into a damp mix-
rapid enough to avoid too much delay. Table 1
ing bowl. The mixer was turned on at low speed
shows that mortar cured at 10C develops 63 and
for 30 seconds while the cement was added. The
50% less strength compared to mortar cured at
mixer was stopped, the sides of the bowl were
20C for 1 and 3 days, respectively. By 56 days,
scraped down, and mixing was resumed for 45
this strength deficit turns into a strength surplus
seconds while the sand was added. The mixer
of nearly 5%. Compared to 50C, it becomes a
was briefly stopped, changed to medium speed,
33% strength surplus. Curing at 5C produces
and run for 30 seconds. Then, the mixer was
similar results. Thus, cold weather can be the best
stopped for 11/2 minutes before finally being run
time to cast concrete.
at medium speed for 1 minute.
However, too much cold leads to problems. If
concrete freezes at an early age, it can be per-
Sample preparation and curing
manently damaged. As Table 1 shows, the 1-day
The mortar was mixed at room temperature.
strength of 5C mortar is only 10% that of mortar
Immediately after mixing it was cast into 50.8-
cured at 20C; no appreciable strength developed
101.6-mm plastic cylindrical molds, vibrated on a
at 10 or 20C. Though hydration occurs at tem-
table to ensure consolidation, capped with plastic
peratures as low as 20C, albeit slowly, less than
lids, and stored at 20, 5, 10, and 20C (some
half of the concrete's strength can be recovered by
mixes were not stored at 20C to simplify test-
thawing. Thus, cold weather protection is
ing). The samples were placed into their respec-
required to achieve strong concrete when the
tive curing rooms within 30 minutes after water
temperature dips below 0C.
TEST PROGRAM
* Non-air-entrained mortar was used intentionally to deter-
mine the effect additives have on strength development with-
The test program defined the effects of various
out having to contend with the variability of entrained air.
chemicals, singly and in combination, on the
Entrained air is recommended for field applications.
2
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