Choice of Concrete Mixtures

In the specifications of the American Railway Engineering and Maintenance of Way Associ ation (1903), the required consistency of con crete is specified as follows: "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 workmen." Richness of Mixture. When classified ac cording to relative richness in cement, concrete mixtures are distinguished under four different headings—namely, Rich, Medium, Ordinary, and Lean.

Rich Mixture. A

1 :2 :4 concrete—that is, a mixture containing ingredients in the propor tion of 1 barrel (4 bags) of packed Portland cement just as it comes from the manufacturer, to 2 barrels (7.6 cubic feet) of loose sand, to 4 barrels (15.2 cubic feet) of loose gravel or broken stone—would be called a rich mixture. This gives a very strong concrete adapted to all kinds of reinforced work that will be subjected to very heavy loads and stresses, such as reinforced floors, arches, beams, girders, columns, founda tions for heavy vibrating machinery, etc. It is also used where water-tightness and air-tight ness are desired, as in the construction of reser voirs, water conduits, silos, tanks, cisterns, troughs, culverts, building walls, etc.

Medium Mixture.

A 1:2.5:5 concrete—that is, in the proportions of 1 barrel (4 bags) Port land cement, to 2.5 barrels (9.5 cubic feet) loose sand, to 5 barrels (19 cubic feet) loose gravel or broken stone—would be called a medium mix ture. This also gives a very strong concrete, and is adaptable to ordinary machine foundations, thin foundation walls, building walls, arches, ordinary ground floors, sidewalks, gutters, sewers, culverts, etc.

Ordinary Mixture. A 1 :3 :6 concrete—that is, in the proportions of 1 barrel (4 bags) Port land cement, to 3 barrels (11.4 cubic feet) loose sand, to 6 barrels (22.8 cubic feet) loose gravel or broken stone—would be called an ordinary mix ture. This gives a strong concrete, but of less strength than either a medium or a rich mixture. It is adaptable to massive work, such as heavy walls, retaining walls, piers, abutments, etc., which will be subjected to considerable strain. It is sometimes used for reinforced concrete in floors and walls; but is more generally used with out reinforcement, in foundations, footings, ground floors, sidewalks, gutters, etc.

Lean Mixture. A 1:4:8 concrete—that is, in the proportions of 1 barrel (4 bags) Portland cement, to 4 barrels (15.2 cubic feet) loose sand, to 8 barrels (30.4 cubic feet) loose gravel or broken stone—would be called a lean mixture. This gives a concrete weak in tensile strength, but of considerable compressive strength, and therefore specially adaptable to comparatively unimportant massive work where the concrete will be subjected only to plain compressive strains, as in large and thick foundations sup porting stationary loads, backing for stone masonry, sub-bases for sidewalks, driveways, etc.

As a rough guide that will enable the concrete worker or contractor to tell at a glance approxi mately the kind of mixture appropriate for various classes of work, Table X has been com piled. The indications of mixtures given in the table must not be taken as exact and final speci fications for any one of the particular kinds of work mentioned, but merely as approximate guides to what is average current practice. No Ilard and fast lines of universal application can be drawn which will set off certain mixtures as the only ones adaptable to certain kinds of work. The allowable mixtures are separated by no well defined boundaries, but merge into one another by insensible degrees; and a considerable vari ation from the indicated proportions may not only be permissible in some cases, but may even be advisable for the sake of strength as well as of economy.

Variations Allowable in Mixtures. If,

for example, the aggregates are well graded, and their proportions carefully determined by one of the methods before described, smaller propor tions of cement may be used for each class of work. If, on the other hand, the sand is very fine, the quantity of cement should be increased 10 to .15 per cent. Instances of construction will occur in which the strength of concrete required for safety may be obtained from almost any mix ture ranging from lean to rich; and in such cases, choice of mixture will probably be dictated by considerations of cost or by empirical rules based on what has been found most advantageous in actual practice.