Civil Engineering Reference
In-Depth Information
6. Protect the concrete with temporary coverings, such
as polyethylene sheeting, during any appreciable
delay between placing and finishing.
7. Fog the slab immediately after placing and before fin-
ishing, taking care to prevent the accumulation of
water that may reduce the quality of the cement paste
in the slab surface.
8. Add plastic fibers to the concrete mixture to help
reduce plastic shrinkage crack formation.
Fogging the concrete before and after final finishing is
the most effective way to minimize evaporation and
reduce plastic shrinkage cracking. Use of a fog spray will
raise the relative humidity of the ambient air over the slab,
thus reducing evaporation from the concrete. Fog nozzles
atomize water using air pressure (Figs. 13-10 and 13-11) to
create a fog blanket. They should not be confused with
garden-hose nozzles, which leave an excess amount of
water on the slab. Fogging should be continued until a
suitable curing material such as a curing compound, wet
burlap, or curing paper can be applied.
Other methods to prevent the rapid loss of moisture
from the concrete surface include:
• Spray application of temporary moisture-retaining
films (usually polymers); these compounds can be
applied immediately after screeding to reduce water
evaporation before final finishing operations and
curing commence. These materials are floated and
troweled into the surface during finishing and should
have no adverse effect on the concrete or inhibit the
adhesion of membrane-curing compounds.
• Reduction of time between placing and the start of
curing by eliminating delays during construction.
If plastic shrinkage cracks should appear during fin-
ishing, striking each side of the crack with a float and
refinishing can close the cracks. However, the cracking
may reoccur unless the causes are corrected.
CURING AND PROTECTION
Curing and protection are more critical in hot weather than
in temperate periods. Retaining forms in place cannot be
considered a satisfactory substitute for curing in hot
weather; they should be loosened as soon as practical
without damage to the concrete. Water should then be
applied at the top exposed concrete surfaces—for example,
Fig. 13-10. Fog nozzle. (9853)
Fig. 13-11. Fogging cools the air and raises the relative
humidity above flatwork to lessen rapid evaporation from
the concrete surface, thus reducing cracking and im-
proving surface durability. (69956)
Fig. 13-9. Dampening the subgrade, yet keeping it free of
standing water will lessen drying of the concrete and
reduce problems from hot weather conditions. (69955)
