There has been much concrete work done where the propor tions were selected without any reference to voids, which has given much better results in practice than might be expected. The pro portion of cement to the aggregate depends upon the nature of the construction and the required degree of strength, or water-tightness, as well as upon the character of the inert materials. Both strength and imperviousness increase with the proportion of cement to the aggregate. Richer mixtures are necessary for loaded columns, beams in building construction and arches, for thin walls subject to water pressure, and for foundations laid under water. The actual measurements of materials as actually mixed and used usually show leaner mixtures than the nominal proportions specified. This is largely due to the heaping of the measuring boxes.
109. Methods of Mixing Concrete. The method of mixing concrete is immaterial, if a homogeneous mass is secured of a uniform consistency, containing the cement, sand, and stone in the correct proportions. The value of the concrete depends greatly upon the thoroughness of the mixing. The color of the mass must be uni form, every grain of sand and piece of the stone should have cement adhering to every point of As surface.
Ingredients in 1 Cubic Yard of Concrete (Sand Voids, 40 per cent; Stone Voids, 45 per cent; Portland Cement Barrel Yielding 3.65 cu. ft. of Paste. Barrel specified to be 4.4 cu. ft.) This table is to be used when the cement is measured loose, after dumping it into a box, for under such conditions a barrel of cement yields 4.4 cu. ft. of loose cement.
[Tables V to IX have been taken from Gillette's "Handbook of Cost Data."] 110. Wetness of Concrete. In regard to plasticity, or facility of working and moulding, concrete may be divided into three classes: dry, medium, and very wet.
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 S 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 con sistency 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 con siders 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 Rail way 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." 111. Transporting and Depositing Concrete. Concrete is usually deposited in layers of 6 inches to 12 inches in thickness. In handling and transporting concrete, care must be taken to prevent the separation of the stone from the mortar. The usual method of trans porting concrete is by wheel-barrows, although it is often handled by cars and carts, and on small jobs it is sometimes carried in buckets. A very common practice is to dump it from a height of several feet ?nto a trench. Many engineers object to this process as they claim that the heavy and light portions separate while falling and the con crete is therefore not uniform through its mass, and they insist that it must be gently slid into place. A wet mixture is much easier to handle than a dry mixture, as the stone will not so readily separate from the mass. A very wet mixture has been deposited from the top of forms 43 feet high and the structure was found to be waterproof. On the other hand, the stones in a dry mixture will separate from the mortar on the slightest provoca tion. Where it is necessary to drop a dry mixture several feet, it should be done by means of a chute or pipe.