Civil Engineering Reference
In-Depth Information
appropriate exposure classification. Because of the risk of
surface imperfections that might occur on exterior concrete
placed in late fall and winter, many concrete contractors
choose to delay concrete placement until spring. By waiting
until spring, temperatures will be more favorable for ce-
ment hydration; this will help generate adequate strengths
along with sufficient drying so the concrete can resist
freeze-thaw damage.
Before placing concrete, the heaters under a formed
deck should be turned on to preheat the forms and melt
any snow or ice remaining on top. Temperature require-
ments for surfaces in contact with fresh concrete are the
same as those outlined in the previous section “
Concreting
on Ground
.” Metallic embedments at temperatures below
the freezing point may result in local freezing that
decreases the bond between concrete and steel reinforce-
ment.
ACI Committee 306
suggests that a reinforcing bar
having a cross-sectional area of about 650 mm
2
(1 in.
2
)
should have a temperature of at least -12°C (10°F) immedi-
ately before being surrounded by fresh concrete at a tem-
perature of at least 13°C (55°F). Caution and additional
study are required before definitive recommendations can
be formulated. See
ACI 306
for additional information.
When slab finishing is completed, insulating blankets
or other insulation must be placed on top of the slab to
ensure that proper curing temperatures are maintained.
The insulation value (
R
) necessary to maintain the
concrete surface temperature of walls and slabs above-
ground at 10°C (50°F) or above for 7 days may be esti-
mated from Fig. 14-16. To maintain a temperature for
longer periods, more insulation is required.
ACI 306
has
additional graphs and tables for slabs placed on ground at
CONCRETING ABOVEGROUND
Working aboveground in cold weather usually involves
several different approaches compared to working at
ground level:
1. The concrete mixture need not be changed to generate
more heat because portable heaters can be used to
heat the undersides of floor and roof slabs.
Nevertheless, there are advantages to having a mix
that will produce a high strength at an early age; for
example, artificial heat can be cut off sooner (see Table
14-3), and forms can be recycled faster.
2. Enclosures must be constructed to retain the heat
under floor and roof slabs.
3. Portable heaters used to warm the underside of
formed concrete can be direct-fired heating units
(without venting).
Table 14-3.
A. Recommended Duration of Concrete Temperature in Cold Weather-Air-Entrained Concrete*
Protection from early-age freezing
For safe stripping strength
Conventional
High-early strength
Conventional
High-early-strength
Service category
concrete,** days
concrete,
†
days
concrete,** days
concrete,
†
days
No load, not exposed
‡
favorable moist-curing
2
1
2
1
No load, exposed, but later
has favorable moist-curing
3
2
3
2
Partial load, exposed
6
4
Fully stressed, exposed
See Table B below
B. Recommended Duration of Concrete Temperature for Fully Stressed, Exposed, Air-Entrained Concrete
Days at 10°C (50°F)
Days at 21°C (70°F)
Required percentage of
Type of portland cement
Type of portland cement
standard-cured 28-day
strength
I or GU
II or MS
III or HE
I or GU
II or MS
III or HE
50
6
9
3
4
6
3
65
11
14
5
8
10
4
85
21
28
16
16
18
12
95
29
35
26
23
24
20
* Adapted from Tables 5.1 and 5.3 of
ACI 306
. Cold weather is defined as that in which average daily temperature is less than 4°C (40°F) for 3
successive days except that if temperatures above 10°C (50°F) occur during at least 12 hours in any day, the concrete should no longer be
regarded as winter concrete and normal curing practice should apply. For recommended concrete temperatures, see Table 14-1. For concrete
that is
not
air entrained,
ACI Committee 306
states that protection for durability should be at least twice the number of days listed in Table A.
Part B was adapted from Table 6.8 of
ACI 306R-88
. The values shown are approximations and will vary according to the thickness of con-
crete, mix proportions, etc. They are intended to represent the ages at which supporting forms can be removed. For recommended concrete
temperatures, see Table 14-1.
** Made with ASTM Type I, II, GU, or MS portland cement.
† Made with ASTM Type III or HE cement, or an accelerator, or an extra 60 kg/m
3
(100 lb/yd
3
) of cement.
‡ “Exposed” means subject to freezing and thawing.
