Many of the points mentioned concerning the selection of a good sand are also applicable to broken stone. The latter may be produced from almost any moderately hard rock, provided it is not subject to decay. The best material for broken stone is a rather hard and tough rock, which breaks into angular fragments with surfaces that are not too smooth.
Gravel makes a good aggregate, although its surfaces are too smooth and rounding to give the best results. Coarse gravel may be improved by running it through a rock crusher to render some of the fragments angular and rough. A mixture of gravel and broken stone gives excellent results (see § 454). The gravel assists the compacting of the mass, and the fragments of broken stone furnish a good bond. A mixture of this kind also leaves but a small percentage of voids in the mass, and this decreases the amount of mortar required.
293. Sandstones are sometimes said to be better than limestones, but this will depend on their relative hardness and structure, and the use to which the concrete is to be put; no general rule will apply. Some limestones seem to be particularly adapted to concrete-making, as the cement adheres to the surface so firmly. Granite, syanite and trap are excellent for the purpose. Fragments of brick and of other burnt clay products give good results up to the limit of the strength of the pieces, but this limit is not high. Table 155 gives the results of transverse tests of concrete bars made under the author's direction, to show the comparative value of different kinds of stone. The results of these tests are discussed in § 454.
1 Engineering Record, Nov. 17, 1894.
Mr. E. L. Ransome1 has pointed out that "for fireproof work, care should be taken to avoid such aggregates as contain feldspar," and that limestone should not be used if the concrete is likely to be subjected to a long continued red heat. The same writer mentions the fact that finely crushed granite may be inferior to finely crushed limestone for use in concrete; one reason for this being that, "owing to the brittle quality of granite, in crushing it is not only broken into small pieces, but many of these pieces are so bruised or contused that upon a little pressure being exerted upon them, such, for instance, as can be applied by the finger or thumb, they will crumble".
294. The care required in the selection of a proper quality of broken stone or gravel will depend upon the required strength of the concrete. If a strong concrete is required, rich mortar will not be able to make up a deficiency in the strength of the stone; but if a low strength is sufficient, and consequently a poor mortar is to be used, but little will be gained by having a very strong rock from which to obtain broken stone. In this case a rock which presents a good surface to which mortar may adhere is the principal requirement, and a very hard rock need not be insisted upon.
It is frequently required that the broken stone shall be freed from all fine material, resulting from the crushing of the stone, before the mortar is added to form concrete. The wisdom of this requirement is not always clear and depends upon the kind of stone. It has already been stated that some forms of crusher dust or screenings give, if not too fine, most excellent results in mortar; this is especially true of limestone screenings. Again, to retain in the broken stone all of the screenings, will result in diminishing the percentage of voids in the aggregate, and thus decrease the amount of mortar necessary.
On the other hand, if a stone is covered with a layer of moistened, floury dust, it cannot be so readily brought in direct contact with the mortar, and if the mortar does reach the stone it is made less rich by the dust, which acts as so much fine sand. It must be said, however, that so far as our experiments go, they do not confirm this latter theory when a moderate amount of fine material is in question, especially with crushed limestone. There is a reason, however, in some cases why the very fine material which acts as sand should be screened out of broken stone, even if it is again used in the mortar for the concrete; the fine material collects in certain parts of the bin or pile, making the proportions irregular, so that one batch of concrete may have a rich mortar with a comparatively large amount of stone, while another may have a poor mortar with but little stone. If, therefore, all of that portion of the broken stone finer than, say, one-eighth of an inch, be screened out and used as so much sand in making the mortar, the resulting concrete will be better and more nearly uniform in quality.
Material that is really foreign, such as vegetable mold or loam, will be detrimental to the strength of the concrete. Even clay is not permissible here if it adheres to the stone, because if the surface of a piece of stone is smeared with clay, the mortar will not be able to adhere as well to that surface. Clay in a granulated form and not adhering to the stone may be permitted, however, in small amounts, possibly as much as ten per cent., without seriously injuring the concrete for many uses.
When old masonry is torn down, the stones are sometimes crushed for use in concrete, but such stones, having particles of mortar adhering to the surfaces, will not be of first quality for the purpose; their cheapness, however, will frequently outweigh such objections.