Methods of measuring the quantities and mixing the ingredients for concrete, and the equipment available, vary greatly. For very small projects where mixing is performed on the site, the materials are usually batched by volume. Under these conditions, accurate proportioning is very difficult. To achieve a reasonable minimum quality of concrete, it is usually less expensive to prescribe an excess of cement than to employ quality control. The same conditions make use of air 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. Æ’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.)