494 indicate that adequate F/T durability is ex-
Task 5: Critical strength
pected of concrete that provides a durability fac-
Concrete is susceptible to ice damage at early
tor (DF) of 80 or greater.
age because either its pore structure is underde-
veloped or its moisture content is too high. As a
Task 4: Equivalent insulation
concrete matures, its water chemically combines
The American Concrete Institute (ACI 1988)
with cement, with the result that the concrete
specifies that concrete placed during cold weather
increases in strength and decreases in freezable
should be maintained at a certain temperature for
water content. At some strength the quantity of
a given amount of time. For example, ACI pro-
freezable water falls below a critical level, which
vides a series of tables outlining the amount of
creates empty space within the concrete, enabling
insulation that is needed to maintain concrete at
the concrete to accommodate the growth of ice
10C (50F) for up to seven days. The amount of
crystals without being damaged. Concrete that
insulation required is related to the ambient tem-
attains a compressive strength of 3.5 MPa (500
perature, the shape of the structure, and the ce-
psi), the critical strength, is expected to be resis-
ment factor of the concrete. Because Pozzutec 20
tant to one cycle of freezing and thawing (ACI
accelerates the generation of heat from cement
1988). The objective of this test was to determine
during the first few days, concrete made with this
if Pozzutec 20 affected this value.
admixture should require less thermal protection
The test was accomplished by allowing 75-
than admixture-free concrete. The objective of this
150-mm (3 6 in.) cylinders of fresh concrete to
task was to determine the minimum ambient tem-
cure at room temperature until they attained a
perature at which an uninsulated cylinder of con-
compressive strength of 1.7, 3.4, and 5.2 MPa (250,
crete made with Pozzutec 20 can be cured to pro-
500, and 750 psi). They were then transferred to a
duce a compressive strength equal to that of
20C (4F) freezing room overnight, after which
admixture-free concrete cured at 10C (50F). This
they were returned to room temperature and
minimum curing temperature could then be com-
cured until being strength-tested after 3, 7, and 28
pared to the ACI tables to determine the amount
days. The strengths of the once-frozen cylinders
of insulation that would have been necessary to
were compared to control cylinders that were
protect normal concrete if cured at that same low
never frozen to determine if the various freezing
temperature. This insulation value was termed
scenarios caused a loss of strength.
"equivalent insulation," signifying the amount of
insulation that Pozzutec 20 could safely replace.
Results and discussion
The test consisted of making three batches of
concrete, each with a Type I cement and a differ-
Task 1: Strength vs. temperature
ent dosage of Pozzutec 20. The concrete was mixed
Strength gain of concrete is the result of chemi-
and cast into numerous 75- 150-mm (3 6 in.)
cal and physical reactions between cement and
cylinder molds, and then capped and distributed
water. At room temperature, the reaction process
among various curing rooms, each maintained at
is most easily observed as a rise in temperature of
a different temperature. At 7, 14, and 28 days,
curing concrete. The amount of temperature rise
three cylinders were removed from each room
depends on how quickly the cement hydrates and
and compression-tested after the cylinders were
how quickly the generated heat is lost from the
warmed up to 10C (50F). Two additional batches
concrete to the outside environment. Figure 1
of concrete made with Type III cement tested the
shows typical temperature histories for 75- 150-
value of using a high early strength cement. Table
mm (3 6 in.) cylinders of concrete cured at vari-
6 gives the test makeup.
ous temperatures. Results for the 308-kg/m3 (517
lb/yd3) mixes are not provided, as these mixes
Table 6. Equivalent insulation tests.
tended to segregate when Pozzutec 20 was added.
Cure temperature
Because this is considered a low cement content
C (F)
Mixture ID
for winter concreting, work with this cement fac-
2,0
10 (50)
tor was not pursued further.
2,1
4, 2, 0, 2 (40, 35, 32, 28)
Figures 1a, 1b, and 1c show the effect of ce-
2,2
4, 2, 0, 2 (40, 35, 32, 28)
ment type, cement amount, and Pozzutec 20 on
2,3
4, 2, 0, 2 (40, 35, 32, 28)
the temperature of curing concrete. It should be
*2,0
4, 2, 0, 2 (40, 35, 32, 28)
*2,2
4, 2, 0, 2 (40, 35, 32, 28)
noted that these figures do not represent field
conditions, as most field structures are more mas-
*Denotes Type III cement.
5
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