This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Dry concrete is used in foundations which may be subjected to severe compression a few weeks after being placed. It should not be placed in layers of more than 8 inches, and should be thoroughly rammed. In a dry mixture the water will just flush to the surface only when it is thoroughly tamped. A dry mixture sets and will support a load much sooner than if a wetter mixture is used, and generally is used only where the load is to be applied soon after the concrete is placed. This mixture requires the exercise of more than ordinary care in ramming, as pockets are apt to be formed in the concrete; and one argument against it is the difficulty of getting a uniform product.
Medium concrete will quake when rammed, and has the consistency of liver or jelly. It is adapted for construction work suited to the employment of mass concrete, such as retaining walls, piers, foundations, arches, abutments; and is sometimes also employed for reinforced concrete.
A very wet mixture of concrete will run off a shovel unless it is handled very quickly. An ordinary rammer will sink into it of its own weight. It is suitable for reinforced concrete, such as thin walls, floors, columns, tanks, and conduits.
Within the last few years there has been a marked change in the amount of water used in mixing concrete. The dry mixture has been superseded by a medium or very wet mixture, often so wet as to require no ramming whatever. Experiments have shown that dry mixtures give better results in short time tests and wet mixtures in long time tests. In some experiments made on dry, medium, and wet mixtures it was found that the medium mixture was the most dense, wet next, and dry least. This experimenter concluded that the medium mixture is the most desirable, since it will not quake in handling, but will quake under heavy ramming. He found medium 1 per cent denser than wet and 9 per cent denser than dry concrete; he considers thorough ramming important.
Concrete is often used so wet that it will not only quake but flow freely, and after setting it appears to be very dense and hard, but some engineers think that the tendency is to use far too much rather than too little water, but that thorough ramming is desirable. In thin walls very wet concrete can be more easily pushed from the surface so that the mortar can get against the forms and give a smooth surface. It has also been found essential that the concrete should be wet enough so as to flow under and around the steel reinforcement so as to secure a good bond between the steel and concrete.
Following are the specifications (1903) of the American Railway Engineering and Maintenance of Way Association:
"The concrete shall be of such consistency that when dumped in place it will not require tamping; it shall be spaded down and tamped sufficiently to level off and will then quake freely like jelly, and be wet enough on top to require the use of rubber boots by workman."