Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.39
In practice, most concrete is cast at about 70 � 20�F. Research on the effects
of casting temperature shows highest strengths for concrete cast at 40�F and significant
but practically unimportant increasing loss of strength from 40�F to90�F.
For higher temperatures, the loss of strength becomes important. So does increased
shrinkage. The increased shrinkage is attributable not only to the high temperature,
but also to the increased water content required for a desired slump as temperature
increases. See Fig. 9.5.
For ordinary building applications, concrete suppliers control temperatures of
concrete by cooling the aggregates and, when necessary, by supplying part of the
mixing water as crushed ice. In very hot weather, these precautions plus sectional
casting, to permit escape of the heat of hydration, may be required for massive
foundation mats. Retarding admixtures are also used with good effect to reduce
slump loss during placing and finishing.
(‘‘Hot Weather Concreting,’’ ACI 305R; and ‘‘Standard Specifications for Structural
Concrete,’’ ACI 301.)
9.38 CURING CONCRETE
Curing of concrete consists of the processes, natural and artificially created, that
affect the extent and rate of hydration of the cement.
Many concrete structures are cured without artificial protection of any kind. They
are allowed to harden while exposed to sun, wind, and rain. This type of curing is
unreliable, because water may evaporate from the surface.
Various means are used to cure concrete by controlling its moisture content or
its temperature. In practice, curing consists of conserving the moisture within newly
placed concrete by furnishing additional moisture to replenish water lost by evaporation.
Usually, little attention is paid to temperature, except in winter curing and
steam curing.
Most effective curing is beneficial in that it makes the concrete more watertight
and increases the strength.
Methods for curing may be classified as:
1. Those that supply water throughout the early hydration process and tend to
maintain a uniform temperature. These methods include ponding, sprinkling, and
application of wet burlap or cotton mats, wet earth, sawdust, hay, or straw.
2. Those designed to prevent loss of water but having little influence on maintaining
a uniform temperature. These methods include waterproof paper and impermeable
membranes. The latter is usually a clear or bituminous compound
sprayed on the concrete to fill the pores and thus prevent evaporation. A fugitive
dye in the colorless compound aids the spraying and inspection.
A white pigment that gives infrared reflectance can be used in a curing compound
to keep concrete surfaces cooler when exposed to the sun.
The criterion for judging the adequacy of field curing provided in the ACI 318
Building Code is that the field-cured test cylinders produce 85% of the strengths
developed by companion laboratory-cured cylinders at the age for which strength
is specified.
(‘‘Standard Practice for Curing Concrete,’’ ACI 308; ‘‘Standard Specification for
Curing Concrete,’’ ACI 308.1; and ‘‘Standard Specifications for Structural Concrete,’’
ACI 301.)