Principles Of Proportioning

With suitable materials selected for any concrete work, the next step is to correctly proportion them. Different classes of concrete work require that for best results the mixtures used shall vary somewhat in the proportions of portland cement, sand or stone screenings, and pebbles or broken stone used. Certain proportions may be thoroughly satisfactory for one kind of concrete work or for one portion of a building, yet not be at all suited for other work or for some other portion of the same building. In proportioning concrete mixtures the aim is to use enough cement to fill all the voids or air spaces in the sand and enough sand-cement mortar to fill all the air spaces in the bulk of pebbles or broken stone.

In most buildings the foundations must contain a certain mass of concrete. If there is to be no cellar enclosed by the foundation wall, watertightness of the concrete is not absolutely necessary, so a mixture containing less cement may sometimes answer. But if watertightness is one of the essentials, the concrete should be very carefully proportioned and have sufficient cement in it to fill all the air spaces or voids in the mixed sand and pebbles so as to prevent water from leaking through the foundation. Watertight concrete comes from well graded materials properly proportioned with enough cement, all well mixed to the right consistency, properly placed, and not allowed to dry out after placing. Some classes of concrete work need not be as strong as others. In such cases also a mixture containing less cement may be used.

Arbitrary Mixtures

The proportioning of concrete materials where great exactness is required may involve numerous and somewhat complicated tests of the sand and pebbles or broken stone. In the average run of concrete work such exact methods or determinations are not necessary and it is common to proportion concrete mixtures after somewhat arbitrary methods that have become fixed usually as a result of experience in various classes of work having proved such mixtures satisfactory. These arbitrary methods usually show that a similar ratio is maintained between the quantity of sand and pebbles or broken stone used, while the ratio of cement to the other ingredient is reduced, depending upon the degree of strength, watertightness or density of concrete necessary. Therefore, in most recommended concrete mixtures the bulk of pebbles or broken stone used is approximately twice that of the sand. A table of arbitrary mixtures follows. Suggestions are given under each mixture proposed, that will enable selection of one suited to the class of concrete work contemplated.

Quantities Of Materials Required For Various Mixtures Of Mortar And Concrete

(Based on tables in "Concrete. Plain and Reinforced," by Taylor & Thompson)

(NOTE: Concrete mixtures are generally proportioned by volume. A 1:1:1 mixture means 1 sack of portland cement, 1 cubic foot of sand, and 1 cubic foot of pebbles or broken stone. A 1:2:3 mixture means 1 sack of Portland cement, 2 cubic feet of sand, and 3 cubic feet of pebbles or broken stone. In other words, the first figure stands for the volume of cement, the second for the volume of sand, the third for the volume of pebbles or broken stone. A 1:2 mixture indicates a mortar, that is, a mixture consisting of 1 sack of portland cement and 2 cubic feet of sand. In proportioning concrete by volume I ssck of portland cement is always considered as equal to 1 cubic foot; therefore, if 1-sack batches are being mixed the portland cement need not be measured.)