Civil Engineering Reference
In-Depth Information
period may be 3 weeks or longer for lean concrete mix-
tures used in massive structures such as dams; conversely,
it may be only a few days for rich mixes, especially if Type
III or HE cement is used. Steam-curing periods are nor-
mally much shorter, ranging from a few hours to 3 days;
but generally 24-hour cycles are used. Since all the desir-
able properties of concrete are improved by curing, the
curing period should be as long as necessary.
For concrete slabs on ground (floors, pavements, canal
linings, parking lots, driveways, sidewalks) and for struc-
tural concrete (cast-in-place walls, columns, slabs, beams,
small footings, piers, retaining walls, bridge decks), the
length of the curing period for ambient temperatures
above 5°C (40°F) should be a minimum of 7 days; addi-
tional time may be needed to attain 70% of the specified
compressive or flexural strength. When the daily mean
ambient temperature is 5°C (40°F) or lower,
ACI
Committee 306
recommendations for curing should be
followed to prevent damage by freezing.
A higher curing temperature provides earlier strength
gain in concrete than a lower temperature but it may
decrease 28-day strength as shown in Fig. 12-11. If strength
tests are made to establish the time when curing can cease
or forms can be removed, representative concrete test
cylinders or beams should be fabricated in the field, kept
adjacent to the structure or pavement they represent, and
cured by the same methods. Equipment is available that
can monitor internal concrete temperatures and match
that temperature in the concrete cylinder curing box; this
is the most accurate means of representing in-place con-
crete strengths. Cores, cast-in-place removable cylinders,
and nondestructive testing methods may also be used to
determine the strength of a concrete member.
Since the rate of hydration is influenced by cement
type and the presence of supplementary cementing mate-
rials, the curing period should be prolonged for concretes
made with cementing materials possessing slow-strength-
gain characteristics. For mass concrete (large piers, locks,
abutments, dams, heavy footings, and massive columns
and transfer girders) in which no pozzolan is used as part
of the cementitious material, curing of unreinforced sec-
tions should continue for at least 2 weeks. If the mass con-
crete contains a pozzolan, minimum curing time for
unreinforced sections should be extended to 3 weeks.
Heavily-reinforced mass concrete sections should be
cured for a minimum of 7 days.
During cold weather, additional heat is often required
to maintain favorable curing temperatures of 10°C to 20°C
(50°F to 70°F). Vented gas or oil-fired heaters, heating
coils, portable hydronic heaters, or live steam can be used
to supply the required heat. In all cases, care must be taken
to avoid loss of moisture from the concrete. Exposure of
fresh concrete to heater or engine exhaust gases must be
avoided as this can result in surface deterioration and
dusting (rapid carbonation).
High-early-strength concrete can be used in cold
weather to speed-up setting time and strength develop-
ment. This can reduce the curing period, but a minimum
temperature of 10°C (50°F) must be maintained.
For adequate deicer scale resistance of concrete, the
minimum curing period generally corresponds to the time
required to develop the design strength of the concrete at
the surface. A period of air-drying after curing will
enhance resistance to scaling. This drying period should
be at least 1 month of relatively dry weather before the
application of deicing salts.
Curing temperature,
F
°
40
60
80
100
120
6
40
At 28 days
5
30
4
3
20
SEALING COMPOUNDS
At 1 day
2
Sealing compounds (sealers) are liquids applied to the
surface of hardened concrete to reduce the penetration of
liquids or gases such as water, deicing solutions, and
carbon dioxide that cause freeze-thaw damage, corrosion
of reinforcing steel, and acid attack. In addition, sealing
compounds used on interior floor slabs reduce dusting
and the absorption of spills while making the surface
easier to clean.
Sealing compounds differ in purpose from curing
compounds; they should not be confused as being the
same. The primary purpose of a curing compound is to re-
10
1
0
10
20
30
40
50
Curing temperature,
°
C
Fig. 12-11. One-day strength increases with increasing
curing temperature, but 28-day strength decreases with
increasing curing temperature (
Verbeck and Helmuth 1968
).
