Building Design and Construction Handbook - Merritt - Ventech!

9.38 SECTION NINE
The second method requires surface preparation of the base slab, by stiff brooming
before final set to roughen the surface and thorough washing before the separate
heavy-duty topping is cast. For the second method, the topping is a very dry (zeroslump)
concrete, made with 3 ⁄8-in maximum-size special aggregate. This topping
should be designed for a minimum strength, � 6000 psi. It must be tamped into
place with powered tampers or rotary floats. (Note: If test cylinders are to be made
from this topping, standard methods of consolidation will not produce a proper test;
tamping similar in effect to that applied to the floor itself is necessary.) One precaution
vital to the separate topping method is that the temperatures of topping and
base slab must be kept compatible.
(‘‘Guide for Concrete Floor and Slab Construction,’’ ACI 302.1R.)
9.36 CONCRETING IN COLD WEATHER
Frozen materials should never be used. Concrete should not be cast on a frozen
subgrade, and ice must be removed from forms before concreting. Concrete allowed
to freeze wet, before or during early curing, may be seriously damaged. Furthermore,
temperatures should be kept above 40�F for any appreciable curing (strength
gain).
Concrete suppliers are equipped to heat materials and to deliver concrete at
controlled temperatures in cold weather. These services should be utilized.
In very cold weather, for thin sections used in buildings, the freshly cast concrete
must be enclosed and provided with temporary heat. For more massive sections or
in moderately cold weather, it is usually less expensive to provide insulated forms
or insulated coverings to retain the initial heat and subsequent heat of hydration
generated in the concrete during initial curing.
The curing time required depends on the temperature maintained and whether
regular or high-early-strength concrete is used. High-early-strength concrete may
be achieved with accelerating admixtures (Art. 9.9) or with high-early-strength cement
(Types III or IIIA) or by a lower water-cementitious materials ratio, to produce
the required 28-day strength in about 7 days.
An important precaution in using heated enclosures is to supply heat without
drying the concrete or releasing carbon dioxide fumes. Exposure of fresh concrete
to drying or fumes results in chalky surfaces. Another precaution is to avoid rapid
temperature changes of the concrete surfaces when heating is discontinued. The
heat supply should be reduced gradually, and the enclosure left in place to permit
cooling to ambient temperatures gradually, usually over a period of at least 24 h.
(‘‘Cold Weather Concreting,’’ ACI 306R; ‘‘Standard Specification for Cold
Weather Concreting,’’ ACI 306.1; and ‘‘Standard Specifications for Structural Concrete,’’
ACI 301.)
9.37 CONCRETING IN HOT WEATHER
Mixing and placing concrete at a high temperature may cause flash set in the mixer,
during placing, or before finishing can be completed. Also, loss of strength can
result from casting hot concrete.
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