142. Manner of Mixing

The most satisfactory method of mixing concrete by hand is to first prepare a tight floor of plank, or, better still, of sheet iron with the edges turned up about 2 inches, for mixing the materials on.

Upon this platform should first be spread the sand, and upon this the cement. The two should then be thoroughly and immediately mixed by means of shovels or hoes, and the broken stone or aggregates then dumped on top and the whole worked over dry with shovels, and then worked over again while water is added from a sprinkler on the end of a hose. Only as much water should be added as is necessary to cause the cement to completely coat and cause to adhere all the particles of the aggregates. Too much water will lessen the strength of the concrete.

The water used should be clean and at about the temperature of 65°.

There are many machines for mixing mortar, which, for large quantities of concrete, effect a saving in the cost of mixing, and probably do the work more thoroughly and evenly. As soon as the concrete is mixed it should be wheeled to the trenches in barrows and dumped.

Instead of first mixing the cement and sand, very good results may be obtained, with perhaps a little less labor, by depositing the broken stone on the sand after it is spread over the platform, and then the cement on top of the stone, and working the whole over dry with shovels. The first method, however, is to be preferred where an extra quality of concrete is desired.

143. Proportions

The best proportion of cement, sand and aggregates will depend upon the kind and quality of the cement used and the character of the work.

The proportion of sand to aggregates should be such that the sand will just fill the voids in the aggregates. This will, of course, vary with the size of the aggregates and the coarseness of the sand. For stone broken to go through a 2-inch ring, about one-half as much sand as stone is required, on an average, to fill the voids. After one batch of concrete has been deposited and rammed the inspector can generally tell by the appearance whether too much or too little sand has been used.

Natural Cement Concrete. - For concrete foundations under buildings of moderate height, and for foundations for cement pavements, natural cements make as strong concrete as is required.

For the best brands of natural cements 1 part cement, 2 parts sand and 4 parts gravel or broken stone should be used.

[This proportion was used in the foundations of the Brooklyn Bridge.]

Portland Cement Concrete. - For concrete to be used under heavy buildings and under water Portland cement should be used.

For the best brands of cement 2 parts of cement to 5 of sand and 9 of broken stone will answer for almost any building construction. Much larger proportions of sand and aggregates than these are often used, but the author would not recommend a greater proportion than the above unless the quality of the cement is constantly tested and only the best used, and the concrete mixed under rigid inspection.