Concrete Aggregates

Gravel should also meet these same require ments, except as to angular shape, its particles being usually rounded by erosion. It must, in addition, be clean, because clay or loam adhering to the particles weakens or destroys the adhesive power of the cement. Tests made by the Boston Transit Commission showed a tensile strength of 605 lbs. per square inch for concrete made with clean gravel, as against 446 lbs. when made with dirty gravel.

If a gravel and a crushed stone are of equal hardness, the stone, on account of its angularity, will probably give a better binding quality to concrete than will the gravel, which is smooth and rounded. Gravel, however; is used quite as extensively as crushed stone, and with good results; and it has frequently the advantage of greater cheapness and greater accessibility of supply.

Even in the same bank of gravel or the same pile of crushed stone, the proportions of fine and coarse material will vary at different points. As a result, the only way to secure absolute uni formity in the concrete, is to separate the fine material from the coarse by screening, and then, remix in the proportions required by the speci fications. This adds to the cost, but insures the best results.

Sizes to Use. In the case of both stone and gravel, the coarse material should predominate, and, as already noted, the size of the coarsest particles of stone should be as large as can be handled in the work. Not only is the strength of the concrete increased thereby, but the 'use of leaner and cheaper mixtures is rendered feasible with equally good results. In mass concrete, however, the stones, if too large, are liable to separate from the mortar unless placed by hand or derrick as in rubble concrete, and a practical maximum size is or 3 inches. In thin walls, floors, columns, tanks, conduits, and other reinforced constructions, a 1-inch maxi mum size is generally as large as can be easily worked between the steel, though sometimes 11/2-inch is used. In some cases where the walls are very thin—say 3 or 4 inches—a maxi mum size is more convenient to handle.

Rubble Stone and Its Use.

Sometimes a saving of expense may be secured in the con struction of massive work such as walls, piers, abutments, dams, breakwaters, etc., by the intro duction of large stones into the mass. When this is done, the stones should be used only in the middle or center of the wall, and each one should be so placed that there will be ample room between it and its neighbor. Thorough tamping about the stone is necessary to insure the cement adhering to its surface, and also to insure a per fect filling of the voids. Stones used in this way are better if their surface is irregular than if they are smooth or rounded, as the voids of the irregular stone afford better holding power for the cement.

These large, irregular pieces of stone are called rubble, and the concrete made by using them is known as rubble concrete. The saving of expense in using this form of concrete is due to the fact that not only is the expense of crush ing the stone done away with, but also there is less cement required than with finer aggregate, since a large percentage of the space is filled with solid stone.

In some classes of work, great weight in con crete is desirable—as, for example, in break waters, dams, etc., where there will be great

shock or pressure; and to such cases rubble concrete is very well adapted, as it is much heavier per cubic foot or cubic yard than ordi nary concrete. Compared with rubble masonry, it will usually be cheaper than the latter. This will always be the case if the rubble concrete can be placed without the construction of forms; but in the case of walls of medium thickness, say 3 to 4 feet, the saving in the concreting material will ordinarily be more than offset by the cost of the forms.

In proportioning rubble for concrete, the amount of stone is fixed at a certain percentage of the space to be filled. This percentage will vary with the size of the stone that is used, being larger with the larger sizes of stone, and smaller with the smaller sizes. In other words, a greater percentage of a given volume will be filled if the stone is large size than if it is small size. If the rubble is of such size that it can be handled regu larly by one man or two men, about 20 to 25 per cent of the space filled by the concrete will con sist of the stone, leaving 80 to 75 per cent to be filled with mixed concrete. In the case of stones that can be handled only by a derrick, the per centage may run from about 33 per cent for the smaller sizes to from 55 to 65 per cent for large size stones averaging 1 to 2 cubic yards each or larger, the amount of mixed concrete necessary thus ranging from 66 to 35 per cent or less, according as the stone is increased in size.

The stones should be completely enveloped in the concrete, and the outermost pieces should be buried at least 5 to 12 inches beneath the sur face. A very wet mixture is generally used in placing rubble concrete; and in this case the stones may be placed much closer together than when a medium or dry mixture is used. If the concrete is not mushy enough to flow into and fill all the spaces, it must be thoroughly tamped to insure that none of the crevices are left empty.

Fireproof Aggregates.

Aggregates differ greatly in the resistance they offer to high tem peratures. But even an aggregate naturally weak in this respect—such as limestone—will give a concrete available for fireproof construc tion, if, in depositing the concrete, the precau tion is taken to insure the burial of the coarser aggregate behind a facing very rich in cement.

Slag and pure quartz sand are two of the best fireproof aggregates. The former is itself a product of enormously high temperatures. Its angularity, density, and cheapness are all in its favor. In the selection of slag, however, care should be taken to avoid any that contains sul phur, since that chemical is detrimental to Port land cement.

Other substances that are considered fire proof are firebrick, broken pottery, hard clink ers, cinders, and pumice-stone or lava, all of which have been through the fire. If cinders are used, they should be thoroughly screened through a mason's screen, to remove the dust.

Limestone, granite, and flint crack and even decompose under the action of great heat. Con crete made from "coke breeze," as the refuse from coke furnaces is called, will gradually crack and ,crumble when subjected to high tempera tures.