nucleated while the higher temperature, and the one
would set faster than the control at room temperature
that defines the freezing point, is where ice grows. As
and that would resist freezing and gain significant
strength at 5C (23F) or below. Since the successful
heat is removed from the water, just enough of the water
turns into ice to replace it as heat of fusion and, thus,
admixture combination would have to accelerate
the temperature remains constant until all of the water
cement hydration at low temperature and protect the
solidifies. At that instant the temperature begins to drop
water from freezing, the approach was to combine
again until it reaches ambient.
accelerators with water reducers while maintaining
Strictly speaking, solutions, such as the mixing wa-
workability (flow). Table 4 shows how the admixtures
ters in each mortar in Figure 1, should not freeze at a
were combined and how they affected the mortar com-
single temperature. The solid that freezes from a solu-
pared to the control (mix 1).
tion is pure H2O or ice. As ice grows, the concentra-
The first step evaluated the freezing point depres-
sion and accelerated cure properties of the Type E
tion of alkalis and other additives in the remaining liq-
accelerating admixture combined singly with the Type
uid increases. Progressively lower temperatures are
A and F water reducers to produce mixes 2 through 4.
required if more ice is to grow. However, the constancy
Mix 2 used the Type E accelerating admixture at 67
of the temperature of the control and the mortar con-
percent of its recommended dose with the Type A
taining the low dose of accelerator suggests that, for
water-reducing admixture at 100 percent of its recom-
practical purposes, these two mixes have only one
mended dose. As Table 4 shows, the water-reducing
freezing temperature. The mortar made with the higher
capability of these two admixtures permitted the w/c
concentration of admixture (high dose curve, Fig. 1b)
ratio to be lowered from 0.48 of the control to 0.41
exhibits the expected progression of freezing points
with the result that the freezing point dropped from
whereby the temperature does not hold constant but
1.9C (28.6F) of the control to 3.4C (25.9F). This
continues to fall over time. The main finding from these
combination of admixtures, however, did not satisfy
tests, however, was that none of the admixtures when
the need for a 5C (23F) freeze protection or for an
used alone and when added to mortar in permis-
initial set time of less than 3.2 hours--mix 2's set time
sible amounts lowered freezing points significantly
was 7.25 hours. To speed up hydration and add more
(Table 3).
solutes into the mix water, the Type E admixture was
Combining the admixtures
increased to full dose with the Type A admixture, also
at full dose. This change dropped the freezing point to
Standard practice places no limit on the number of
3.9C (25F) but the initial set increased to 8.25 hours.
admixtures that can be used in concrete, just on indi-
Mix 4 replaced the Type A admixture with Type F and
vidual amounts. The objective, therefore, was to use as
combined it with the Type E admixture, both at full
many admixtures as needed to develop a mortar that
Table 4. Properties of fresh mortar made with admixture combinations. Set times and flow
were conducted at 19C while freezing points were obtained at 20C. The letters in column
2 correspond to admixtures of Table 1. Water contents were changed in all but mixes 8 and
9 to keep flow constant.
Initial
Final
Freezing
Mix
Mortar
set
set
point
Flow
w/c
no.
designation
(hr)
(hr)
(C)
(%)
ratio
1
Control
3.2
4.4
1.9
104
0.48
2
E(67*) + A(100*)
7.25
na
3.4
103
0.41
3
E(100) + A(100)
8.25
10.3
3.9
103
0.39
4
E(100) + F(100)
6.1
na
4.0
102
0.37
5
C(100) + D(625)
8.25
9.5
3.1
101
0.39
6
C(200) + D(625)
5.1
7
4.0
103
0.38
7
C(100) + E(100) +
4.9
6.5
4.9
107
0.39
F(100)
8
C(100) + E(100) +
na
na
5.7
too dry
0.30
F(100)
9
C(100) + E(100) +
na
na
9.2
set too fast
0.34
F(100)+ 2% CaCl2
*Percent of maximum dose recommended by the manufacturer.
5
Previous Page
