A rich mixture, proportions 1:2:4 - that is, 1 barrel (4 bags) packed Portland cement (as it comes from the manufacturer), 2 barrels (7.6 cubic feet) loose sand, and 4 barrels (15.2 cubic feet) loose stone - is used in arches, reinforced-concrete floors, beams, and columns for heavy loads; engine and machine foundations subject to vibration; tanks; and for water-tight work.

A medium mixture, proportions 1: 2 1/2:5 - that is, 1 barrel (4 bags) packed Portland cement, 2 1/2 barrels (9.5 cubic feet) loose sand, and 5 barrels (19 cubic feet) loose gravel or stone - may be used in arches, thin walls, floors, beams, sewers, sidewalks, foundations, and machine foundations.

An ordinary mixture, proportions 1:3:6 - that is, 1 barrel (4 bags) packed Portland cement, 3 barrels (11.4 cubic feet) loose sand, and 6 barrels (22.8 cubic feet) loose gravel or broken stone - may be used for retaining walls, abutments, piers, floor slabs, and beams.

A lean mixture, proportions 1:4: 8 - that is, 1 barrel (4 bags) packed Portland cement, 4 barrels (15.2 cubic feet) loose sand, and 8 barrels (30.4 cubic feet) loose gravel or broken stone - may be used in large foundations supporting stationary loads, backing for stone masonry, or where it is subject to a plain compressive load.

These proportions must not be taken as always being the most economical to use, but they represent average practice. Cement is the most expensive ingredient; therefore a reduction of the quantity of cement, by adjusting the proportions of the aggregate so as to produce a concrete with the same density, strength, and impermeability, is of great importance. By careful proportioning and workmanship, water-tight concrete has been made of a 1:3:6 mixture. In floor construction where the span is very short and it is specified that the slab must be at least 4 inches thick, while with a high-grade concrete a 3-inch slab would carry the load, it is certainly more economical to use a leaner concrete.

An accurate and simple method to determine the proportions of concrete is by trial batches. The apparatus consists of a scale and a cylinder which may be a piece of wrought iron pipe 10 inches to 12 inches in diameter capped at one end. Measure and weigh the cement, sand, stone, and water and mix on a piece of sheet steel, the mixture having a consistency the same as to be used in the work. The mixture is placed in the cylinder, carefully tamped, and the height to which the pipe is filled is noted. The pipe should be weighed before and after being filled so as to check the weight of the material. The cylinder is then emptied and cleaned. Mix up another batch using the same amount of cement and water, slightly varying the ratio of the sand and stone but having the same total weight as before. Note the height in the cylinder, which will be a guide to other batches to be tried. Several trials are made until a mixture is found that gives the least height in the cylinder, and at the same time works well while mixing, all the stones being covered with mortar, and which makes a good appearance. This method gives very good results, but it does not indicate the changes in the physical sizes of the sand and stone so as to secure the most economical composition as would be shown in a thorough mechanical analysis.

There has been much concrete work done where the proportions were selected without any reference to voids, which has given much better results in practice than might be expected. The proportion 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.

Table V. Proportions Of Cement, Sand, And Stone In Actual Structures

Structure

Proportions

Reference

C. B. & Q, R. R.

Reinforced Concrete Culverts

1:3:6

Engr. Cont., Oct. 3, '06

Phila. Rapid Transit Co.

Floor Elevated Roadway.

1:3:6

" " Sept. 26, '06

Subway

Walls..

1:2.5:5

Floors..

1:3:6

C. P. R. R.

Arch Rings..

1:3:5

Piers and Abutments.

1:4:7

Cement Era, Aug. '06

Hudson River Tunnel Caisson

1:2:4

Eng. Record, Sept. 29, '06

Stand Pipe at Attleboro, Mass. Height, 106 feet.

1:2:4

,, ,, ,, 29, '06

C.C.& St.L.R.R.,Danville Arch

Footings..

1:4:8 or 1:9.5

,, March 3,'06

Arch Rings..

1:2:4

Abutments, Piers..

1:3:6 or 1:6.5

N. Y. C. & H. R. R. R.

Ossining Tunnel

Footing..

1:4:7.5

" " " 3, '06

Walls..

1:3:6

Coping..

1:2:4

American Oak Leather Co.

Factory at Cincinnati, Ohio.

1:2:4

,, ,, ,, 3,'03

Harvard University Stadium.

1:3:6

New York Subway

Roofs and Sidewalks.

1:2:4

Tunnel Arches..

1:2.5:5

Wet Foundation 2' th. or less

1:2:4

" " exceeding 2'

1:2.5:5

Boston Subway..

1:2.5:4

P. & R. R. R.

Arches...

1:2:4

" " Oct. 13, '06

Piers and Abutments.

1:3:6

BrooklynNavy Yd. Laboratory

Columns..

1:2:3 Trap rock Eng. News, March 23, '05

Beams and Slabs..

1:3:5 ,, ,,

Roof Slab...

1:3:5 Cinder

Southern Railway

Arches...

1:2:4

Piers and Abutments.

1:2.5:5