Batching

When bagged cement is used, the field mix proportions are usually given in terms of designated amounts of fine and coarse aggregate per bag (or per 94 pounds) of cement. The amount of material mixed at a time is called a batch. The size of a batch is usually designated by the number of bags of cement it contains, such as a four-bag batch, a six-bag batch, and so on.

The process of weighing out or measuring out the ingredients for a batch of concrete is called batching. When mixing is done by hand, the size of the batch depends on the number of people available to turn it with hand tools. When a machine does the mixing, the size of the batch depends on the rated capacity of the mixer. The rated capacity of a mixer is given in terms of cubic feet of mixed concrete, not of dry ingredients.

On large jobs, the aggregate is weighed out in an aggregate batching plant, usually shortened to batch plant, like the one shown in Figure 11. Whenever possible, a batch plant is located near and used in conjunction with a crushing and screening plant. A crushing and screening plant crushes stone into various particle sizes, then screens them into separate piles. In a screening plant, the aggregate in its natural state is screened by size into separate piles.

The batch plant, which is usually portable and can be taken apart and moved from site to site, is generally set up adjacent to the pile of screened aggregate. The plant may include separate hoppers for several sizes of fine and coarse aggregates, or only one hopper for fine aggregate and another for coarse aggregate. It may have one or more divided hoppers, each containing two or more separate compartments for different sizes of aggregates.

Each storage hopper or storage hopper compartment can be discharged into a weigh box, which can, in turn, be discharged into a mixer or a batch truck. When a specific weight of aggregate is called for, the operator sets the weight on a beam scale. The operator then opens the discharge chute on the storage hopper. When the desired weight is reached in the weigh box, the scale beam rises and the operator closes the storage hopper discharge chute then opens the weigh box discharge chute, and the aggregate discharges into the mixer or batch truck. Batch plant aggregate storage hoppers are usually loaded by clamshell equipped cranes.