Civil Engineering Reference
In-Depth Information
base slab of a two-course floor. Similarly, some curing
compounds may affect the adhesion of paint to concrete
floors. Curing compound manufacturers should be con-
sulted to determine if their product is suitable for the
intended application.
Curing compounds should be uniform and easy to
maintain in a thoroughly mixed solution. They should not
sag, run off peaks, or collect in grooves. They should form
a tough film to withstand early construction traffic
without damage, be nonyellowing, and have good mois-
ture-retention properties.
Caution is necessary when using curing compounds
containing solvents of high volatility in confined spaces or
near sensitive occupied spaces such as hospitals because
evaporating volatiles may cause respiratory problems.
Applicable local environmental laws concerning volatile
organic compound (VOC) emissions should be followed.
Curing compounds should conform to ASTM C 309
(AASHTO M 148). A method for determining the efficiency
of curing compounds, waterproof paper, and plastic sheets
is described in ASTM C 156 (AASHTO T 155). ASTM C
1151, discontinued in 2000, also evaluates the effectiveness
of curing compounds. Curing compounds with sealing
properties are specified under ASTM C 1315.
Steam Curing
Steam curing is advantageous where early strength gain
in concrete is important or where additional heat is re-
quired to accomplish hydration, as in cold weather.
Two methods of steam curing are used: live steam at
atmospheric pressure (for enclosed cast-in-place structures
and large precast concrete units) and high-pressure steam
in autoclaves (for small manufactured units). Only live
steam at atmospheric pressure will be discussed here.
A typical steam-curing cycle consists of (1) an initial
delay prior to steaming, (2) a period for increasing the
temperature, (3) a period for holding the maximum tem-
perature constant, and (4) a period for decreasing the
temperature. A typical atmospheric steam-curing cycle is
shown in Fig. 12-9.
Steam curing at atmospheric pressure is generally
done in an enclosure to minimize moisture and heat losses.
Tarpaulins are frequently used to form the enclosure.
Application of steam to the enclosure should be delayed
until initial set occurs or delayed at least 3 hours after final
placement of concrete to allow for some hardening of the
concrete. However, a 3 - to 5 - hour delay period prior to
steaming will achieve maximum early strength, as shown
in Fig. 12-10.
Steam temperature in the enclosure should be kept at
about 60°C (140°F) until the desired concrete strength has
developed. Strength will not increase significantly if the
maximum steam temperature is raised from 60°C to 70°C
(140°F to 160°F). Steam-curing temperatures above 70°C
(160°F) should be avoided; they are uneconomical and
Internal Moist Curing
Internal moist curing refers to methods of providing mois-
ture from within the concrete as opposed to outside the
concrete. This water should not effect the initial water to
cement ratio of the fresh concrete. Lightweight (low-den-
sity) fine aggregate or absorbent polymer particles with an
ability to retain a significant amount of water may provide
additional moisture for concretes prone to self desiccation.
When more complete hydration is needed for concretes
with low water to cement ratios (around 0.30 or less),
60 kg/m
3
to 180 kg/m
3
(100 lb/yd
3
to 300 lb/yd
3
) of satu-
rated lightweight fine aggregate can provide additional
moisture to extend hydration, resulting in increased
strength and durability. All of the fine aggregate in a mix-
ture can be replaced with saturated lightweight fine
aggregate to maximize internal moist curing. Internal
moist curing must be accompanied by external curing
methods.
Initial concrete temp. = 21
°
C (70
°
F)
160
Steam temp. in enclosure held to 60
°
C (140
°
F),
until desired concrete strength is developed
140
60
Temp. reduced at
20
°
C (40
°
F)/hr until
120
within 10
°
C (20
°
F)
40
of outside air
100
Outside air
at 10
°
C
Steam applied to
enclosure at rate of
10 to 20
(50
°
F)
80
20
°
C (20 to 40
°
F)/hr
Forms Left in Place
1
2
60
3
4
Forms provide satisfactory protection against loss of mois-
ture if the top exposed concrete surfaces are kept wet. A
soaker hose is excellent for this. The forms should be left
on the concrete as long as practical.
Wood forms left in place should be kept moist by
sprinkling, especially during hot, dry weather. If this
cannot be done, they should be removed as soon as prac-
tical and another curing method started without delay.
Color variations may occur from formwork and uneven
water curing of walls.
40
0
0
5
10
15
20
24
Time after placing, hours
1
2
3
4
Initial delay prior to steaming 3 to 5 hours
Temperature increase period 2
1
/
2
hours
Constant temperature period 6 to 12 hours*
Temperature decrease period 2 hours
*Type III or high-early-strength cement,
longer for other types
Fig. 12-9. A typical atmospheric steam-curing cycle.
