The conclusion is that dry concrete if thoroughly tamped is the strongest, but also the most expensive to mix and lay; that quaking concrete is nearly as strong as dry, and is more easily mixed, and does not require as much tamping; and that mushy or fluid concrete is considerably weaker, and requires tighter forms, but does not require any tamping. Dry concrete must be employed where great strength is required at an early date; but it must be thoroughly tamped,—a thing difficult to secure with ordinary laborers. Plastic or quaking concrete is suitable for plain concrete in large masses, but care is required to secure a solid surface (see 4 353). A wet or mushy mixture must be used for reinforced concrete, and will usually give a solid surface next to the form without any special care.
for the wet 22.5, the sand being reasonably dry; while other experi menters obtain nominally the same degree of plasticity with less than half as much water.
As a rule, with well-graded ingredients in the usual proportions, plastic or quaking concrete will require about 8 to 10 pounds, or about 1 to 1 gallons, of water per cubic foot. The amount of water required increases slightly as the proportions of sand and stone increase. The water required to produce a plastic or quaking con sistency, in terms of the weight of the cement, is about as follows: for neat cement 20 per cent; for rich mortar 25 to 30 per cent; for rich concrete 30 to 33 per cent; for lean concrete 33 to 50 per cent; and for very lean concrete 50 to 100 per cent. The weight of water in a wet mixture in terms of the weight of the total dry materials is about as follows: for mortar, 10 per cent; and for concrete, 7 or 8 per cent.
If the aggregate is porous, it should be drenched before beginning to mix the concrete, as otherwise the aggregate by absorbing the water employed in mixing may rob the cement of the water required in setting and thus ruin the concrete. Obviously this precaution is most important with dry mixtures.