Amount Of Water

For most classes of concrete work the amount of water required is that which will produce a mixture that when shoveled into a pile will gradually tend to spread of its own weight; at least, it will do so just as soon as disturbed. This is usually called a "jelly" or "quaky-like" consistency. If any more water than required to produce such a consistency is used, the concrete will tend to flow, and except in very few instances, such a mixture is too wet for best results. An exception is when the concrete is to be used in making some object where the space between inner and outer forms is very narrow, then it might be necessary to use a little wetter mixture to make the concrete settle to all parts of the mold or form. Never must enough water be used so that when handling the mixture on a shovel the pebbles or broken stone will separate from the sand-cement mortar. It is not possible to state exactly what quantity of water shall be used for any volume of dry materials owing to the variation in such materials themselves; the sand may be quite dry-at other times it may contain a considerable quantity of moisture. In the first case, more water would be required to make a mixture of the consistency described than in the latter case. Again, when certain kinds of crushed stone are used in place of pebbles, they may absorb more water than other kinds of stone, thus making it necessary to vary the quantity of water required as compared with some other mixture to produce the right consistency.

Machine Mixing

If concrete is to be mixed by machine, a mixer of the batch type is preferable because the several ingredients for every batch of concrete must be separately measured. This insures correct proportions. There are mixers of what are known as the continuous type. These receive the several ingredients of concrete-cement, sand, pebbles or broken stone-from hoppers. Frequently, however, a clogging of the hopper spout due to dampness of the sand or packing of the cement or some other defect of operation will prevent desired proportions being secured, hence the concrete will not be uniform.

Size Of Machine Batches

Batches should not be larger than suited to the capacity of the mixer drum, nor larger than can be placed within thirty minutes after mixing. Care should be taken not to place more materials in the drum than recommended by the manufacturer. There must be enough unoccupied space in the mixer drum so that the concreting materials can tumble about freely while the mixer drum is being revolved, thus making it possible to thoroughly mix the concrete. As a rule, the materials for a batch of concrete should not occupy a volume in the mixer drum exceeding one-third of its entire volume when mixed.

Speed And Time Of Mixing

The mixer drum must not be revolved too rapidly for that will cause the materials to cling to the inner surface of the drum rather than be tumbled about and thoroughly mixed. Mixing should continue for not less than 1 1/2 minutes after all materials have been placed in the drum.