Civil Engineering Reference
In-Depth Information
recommended; they reduce solar-heat gain, thus reducing
the concrete temperature. Pigmented compounds should
be kept agitated in the container to prevent pigment from
settling out.
Curing compounds should be applied by hand-oper-
ated or power-driven spray equipment immediately after
final finishing of the concrete (Fig. 12-8). The concrete sur-
face should be damp when the coating is applied. On dry,
windy days, or during periods when adverse weather
conditions could result in plastic shrinkage cracking,
application of a curing compound immediately after final
finishing and before all free water on the surface has
evaporated will help prevent the formation of cracks.
Power-driven spray equipment is recommended for uni-
form application of curing compounds on large paving
projects. Spray nozzles and windshields on such equip-
ment should be arranged to prevent wind-blown loss of
curing compound.
Normally only one smooth, even coat is applied at a
typical rate of 3 to 4 m
2
per liter (150 to 200 sq ft per gallon);
but products may vary, so manufacturer's recommended
application rates should be followed. If two coats are nec-
essary to ensure complete coverage, for effective protection
the second coat should be applied at right angles to the
first. Complete coverage of the surface must be attained
because even small pinholes in the membrane will increase
the evaporation of moisture from the concrete.
Curing compounds might prevent bonding between
hardened concrete and a freshly placed concrete overlay.
And, most curing compounds are not compatible with ad-
hesives used with floor covering materials. Consequently,
they should either be tested for compatibility, or not used
when bonding of overlying materials is necessary. For
example, a curing compound should not be applied to the
Fig. 12-7. Polyethylene film is an effective moisture barrier
for curing concrete and easily applied to complex as well as
simple shapes. To minimize discoloration, the film should
be kept as flat as possible on the concrete surface. (70014)
or for interior locations. Clear film has little effect on heat
absorption.
ASTM C 171 (AASHTO M 171) also includes a sheet
material consisting of burlap impregnated on one side
with white opaque polyethylene film. Combinations of
polyethylene film bonded to an absorbent fabric such as
burlap help retain moisture on the concrete surface.
Polyethylene film may also be placed over wet burlap
or other wet covering materials to retain the water in the
wet covering material. This procedure eliminates the
labor-intensive need for continuous watering of wet cov-
ering materials.
Membrane-Forming Curing Compounds
Liquid membrane-forming compounds consisting of
waxes, resins, chlorinated rubber, and other materials can
be used to retard or reduce evaporation of moisture from
concrete. They are the most practical and most widely
used method for curing not only freshly placed concrete
but also for extending curing of concrete after removal of
forms or after initial moist curing. However, the most
effective methods of curing concrete are wet coverings or
water spraying that keeps the concrete continually damp.
Curing compounds should be able to maintain the relative
humidity of the concrete surface above 80% for seven days
to sustain cement hydration.
Membrane-forming curing compounds are of two
general types: clear, or translucent; and white pigmented.
Clear or translucent compounds may contain a fugitive
dye that makes it easier to check visually for complete
coverage of the concrete surface when the compound is
applied. The dye fades away soon after application. On
hot, sunny days, use of white-pigmented compounds are
Fig. 12-8. Liquid membrane-forming curing compounds
should be applied with uniform and adequate coverage
over the entire surface and edges for effective, extended
curing of concrete. (69975)
