Civil Engineering Reference
In-Depth Information
Burlap must be free of any substance that is harmful
to concrete or causes discoloration. New burlap should be
thoroughly rinsed in water to remove soluble substances
and to make the burlap more absorbent.
Wet, moisture-retaining fabric coverings should be
placed as soon as the concrete has hardened sufficiently to
prevent surface damage. During the waiting period other
curing methods are used, such as fogging or the use of
membrane forming finishing aids. Care should be taken to
cover the entire surface with wet fabric, including the
edges of slabs. The coverings should be kept continuously
moist so that a film of water remains on the concrete sur-
face throughout the curing period. Use of polyethylene
film over wet burlap is a good practice; it will eliminate
the need for continuous watering of the covering.
Periodically rewetting the fabric under the plastic before it
dries out should be sufficient. Alternate cycles of wetting
and drying during the early curing period may cause
crazing of the surface.
Wet coverings of earth, sand, or sawdust are effective
for curing and are often useful on small jobs. Sawdust from
most woods is acceptable, but oak and other woods that
contain tannic acid should not be used since deterioration
of the concrete may occur. A layer about 50 mm (2 in.) thick
should be evenly distributed over the previously mois-
tened surface of the concrete and kept continuously wet.
Wet hay or straw can be used to cure flat surfaces. If
used, it should be placed in a layer at least 150 mm (6 in.)
thick and held down with wire screen, burlap, or tarpau-
lins to prevent its being blown off by wind.
A major disadvantage of moist earth, sand, sawdust,
hay, or straw coverings is the possibility of discoloring
the concrete.
Fig.12-6. Impervious curing paper is an efficient means of
curing horizontal surfaces. (69994)
curing-paper patches. When the condition of the paper is
questionable, additional use can be obtained by using it in
double thickness.
In addition to curing, impervious paper provides
some protection to the concrete against damage from sub-
sequent construction activity as well as protection from
the direct sun. It should be light in color and nonstaining
to the concrete. Paper with a white upper surface is prefer-
able for curing exterior concrete during hot weather.
Plastic Sheets
Impervious Paper
Plastic sheet materials, such as polyethylene film, can be
used to cure concrete (Fig. 12-7). Polyethylene film is a
lightweight, effective moisture retarder and is easily
applied to complex as well as simple shapes. Its applica-
tion is the same as described for impervious paper.
Curing with polyethylene film (or impervious paper)
can cause patchy discoloration, especially if the concrete
contains calcium chloride and has been finished by hard-
steel troweling. This discoloration is more pronounced
when the film becomes wrinkled, but it is difficult and
time consuming on a large project to place sheet materials
without wrinkles. Flooding the surface under the covering
may prevent discoloration, but other means of curing
should be used when uniform color is important.
Polyethylene film should conform to ASTM C 171
(AASHTO M 171), which specifies a 0.10-mm (4-mil)
thickness for curing concrete, but lists only clear and white
opaque film. However, black film is available and is satis-
factory under some conditions. White film should be used
for curing exterior concrete during hot weather to reflect
the sun's rays. Black film can be used during cool weather
Impervious paper for curing concrete consists of two
sheets of kraft paper cemented together by a bituminous
adhesive with fiber reinforcement. Such paper, con-
forming to ASTM C 171 (AASHTO M 171), is an efficient
means of curing horizontal surfaces and structural con-
crete of relatively simple shapes. An important advantage
of this method is that periodic additions of water are not
required. Curing with impervious paper enhances the
hydration of cement by preventing loss of moisture from
the concrete (Fig. 12-6).
As soon as the concrete has hardened sufficiently to
prevent surface damage, it should be thoroughly wetted
and the widest paper available applied. Edges of adjacent
sheets should be overlapped about 150 mm (6 in.) and
tightly sealed with sand, wood planks, pressure-sensitive
tape, mastic, or glue. The sheets must be weighted to
maintain close contact with the concrete surface during
the entire curing period.
Impervious paper can be reused if it effectively retains
moisture. Tears and holes can easily be repaired with
