Concrete Aggregates

A convenient measure for the mortar is a glass gradu ate, about one and one-half inches in diameter, gradu ated to 250 cubic centimeters. A convenient weight of cement plus sand for a test is 350 grams. The sand is dried and mixed with cement in the calculated propor tions, in a shallow pan about ten inches in diameter and one inch deep. The mixing is done conveniently with a 4-inch pointing trowel. The dry, mixed material is formed into a circle as in mixing cement for briquettes, and sufficient water added to make mortar of plastic con sistency, similar to that used in laying brick masonry. After mixing about five minutes, the mortar is introduced, about 20 cubic centimeters at a time, into the graduate ; and, to expel any air bubbles, it is lightly tamped with a stick having a blunt end. The mortar is allowed to settle in the graduate for one or two hours, until the level becomes constant, when the surplus water is poured off, and the volume of the mortar in cubic centimeters is read.

The other sands that are to be compared with this are then prepared in the same way. After testing all the mortars, the sand which produces the strongest mortar is immediately located as that in the mortar of lowest volume. In other words, the best sand is that which pro duces the smallest volume of mortar with the given pro portions by weight. If the sands that are being compared differ appreciably in specific gravity, allowance for this, of course, must be made in weighing them out to be tested, so that the sum of the volumes of the sand particles will be the same in each case. In general, however, the sand which, in any given proportion, produces a mixture of smallest volume, will also produce a mortar of greatest strength.

In a number of tests made with the same proportions of cement and sand by weight, the mortar of medium sand was found to occupy a volume per cent in excess of coarse sand mortar; while fine sand mortar exceeded coarse sand mortar by 17 per cent in volume—the results here again favoring the coarser material.

By similar systematic trials, the best mix ture of two or more sands, or of various sands with various other aggregates, for making con crete, may also be found. Larger quantities of materials must be used than in testing sand for mortar as above described; and, instead of a glass graduate—which would be too weak to stand the necessary light ramming—a short length of cast-iron pipe, closed at one end, may be used. Those aggregates which, mixed with cement in the required proportions, produce the smallest volume of concrete, are ordinarily the best, though, in this case, the shape of the particles and the hardness of the coarser aggre gates must also be considered, as explained below.

Standard Sand.

As a standard sand for test ing purposes, the Committee on Uniform Tests of Cement, of the American Society of Civil Engineers, recommends the natural sand from Ottawa, I11., screened to pass a sieve having 20

meshes per linear inch, and retained on a sieve having 30 meshes per linear inch. The commit tee says that sand having passed the No. 20 sieve shall be considered standard when not more than one per cent passes a No. 30 sieve after one minute of continuous sifting of a 500-gram sample.

Selection of Stone and Gravel.

So wide a variation in texture and strength is found in stones, even of the same class, that no hard and fast rule can be laid down as to their compara tive values for concrete. The actual, intrinsic strength of the stone itself is an factor, the hardest stone—other things being equal—producing the strongest concrete.

In order

of their value for concreting pur poses, different stones may be listed approxi mately as follows: (1) Trap; (2) Granite; (3) Gravel; (4) Marble; (5) Limestone; (6) Slag; (7) Sandstone; (8) Slate; (9) Shale; (10)Cinders. Sandstone is ordinarily not worth more than three-fourths the value of trap, and the value of slate is less than half that of trap.

The difficulty of making anything like a final, general comparison is further increased by the fact that the relative value of specimens of any one kind of stone is affected by the proportions used, and will also vary with the age of the con crete. Gravel concrete, for example, because of the rounded surfaces, may, at the age of one month, be weaker than concrete made from comparatively soft broken stone; but at the end of one year may far surpass the latter in strength. Gravel makes a dense mixture, and, if much cheaper than crushed stone, may ordi narily be substituted for it.

Where stone is used as aggregate, the rela tive size of the particles affects the quality of the concrete, just as in the case of sand; but the strength and density of the concrete depend also on the hardness of the stone itself, on the shape of the particles, and on their being kept within certain limits as to maximum size.

Porous stones, like sandstone, should be avoided if possible. They are inherently weak, and, unless thoroughly soaked before mixing, will give trouble by absorbing excessive amounts of water, thus preventing plasticity and a thor ough mixture. The best crushed stone is that which is hard; with angular fragments, prefer ably of cubical shape and ranging up to the maximum size that can be handled in the work; with all particles smaller than, say, one-fourth inch screened out to be used as sand; and with the sizes of the remaining coarse stone varying from small to large, the coarsest predominating. A flat-grained material packs less closely than, and is always inferior to, stone of angular or cubical fracture.