Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.11
entraining cement preferable to separate admixtures. This practical approach is preferable
also for very small projects to be supplied with ready-mixed concrete. Economy
with excess cement will be achieved whenever volume is so small that the
cost of an additional sack of cement per cubic yard is less than the cost of a single
compression test.
For engineered construction, some measure of quality control is always employed.
In general, all measurements of materials including the cement and water
should be by weight. The ACI 318 Building Code provides a sliding scale of
overdesign for concrete mixes that is inversely proportional to the degree of quality
control provided. In the sense used here, such overdesign is the difference between
the specified and the actual average strength as measured by tests.
ƒ� c
Mixing and delivery of structural concrete may be performed by a wide variety
of equipment and procedures:
Site mixed, for delivery by chute, pump, truck, conveyor, or rail dump cars.
(Mixing procedure for normal-aggregate concretes and lightweight-aggregate
concretes to be pumped are usually different, because the greater absorption of
some lightweight aggregates must be satisfied before pumping.)
Central-plant mixed, for delivery in either open dump trucks or mixer trucks.
Central-plant batching (weighing and measuring), for mixing and delivery by
truck (‘‘dry-batched’’ ready mix).
Complete portable mixing plants are available and are commonly used for large
building or paving projects distant from established sources of supply.
Generally, drum mixers are used. For special purposes, various other types of
mixers are required. These special types include countercurrent mixers, in which
the blades revolve opposite to the turning of the drum, usually about a vertical axis,
for mixing very dry, harsh, nonplastic mixes. Such mixes are required for concrete
masonry or heavy-duty floor toppings. Dry-batch mixers are used for dry shotcrete
(sprayed concrete), where water and the dry-mixed cement and aggregate are
blended between the nozzle of the gun and impact at the point of placing.
(‘‘Guide for Measuring, Mixing, Transporting, and Placing Concrete,’’ ACI
304R.)
9.9 ADMIXTURES
The ACI 318 Building Code requires prior approval by the engineer of admixtures
to be used in concrete.
Air Entrainment. Air-entraining admixtures (ASTM C260) may be interground
as additives with the cement at the mill or added separately at the concrete mixing
plant, or both. Where quality control is provided, it is preferable to add such admixtures
at the concrete plant so that the resulting air content can be controlled for
changes in temperature, sand, or project requirements.
Use of entrained air is recommended for all concrete exposed to weathering or
deterioration from aggressive chemicals. The ACI 318 Building Code requires air
entrainment for all concrete subject to freezing temperatures while wet. Detailed
recommendations for air content are available in ‘‘Standard Practice for Selecting