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Sizes of Stone

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SIZES OF STONE. The size of stone used for road metal depends upon the hardness and toughness of the stone and upon the weight of the traffic. The harder and tougher the material, the smaller it may be broken without danger of its crushing or shat tering under the load of wheels and the impact of hoofs; and the harder and tougher a stone, the smaller it must be broken in order that it may compact well in the road. The stones in the top course should be larger for heavy traffic than for light traffic, to prevent their being ground to powder. Larger stones can be used in the bottom layers of a road than at the top.

One of MacAdam's rules was to exclude any fragment weighing more than 6 ounces. Telford's limit was 8 ounces. A 1§-inch cube of compact limestone weighs about 6 ounces. Another of MacAdam's rules was to exclude any stone that could not readily be put into a man's mouth. These rules are frequently quoted, even now, although improvements in road machinery have made them inappropriate with present methods. When these rules were established, the road was not rolled but was compacted by traffic; and as the stone was broken by hand little or no fine material was produced, and hence the road was bound chiefly by manure and dirt brought on by the traffic. Rolling and the use of stone dust for a binder make a material difference in the sizes of the stone per missible.

The bottom course of a macadam road built of soft stones is often composed of fragments 3 to 4 inches in greatest dimensions ; but if it is built of hard tough stone, the sizes are 2 to 2§- inches. The size of rock in the lower courses is not so important as that for the surface course (see § 332). The top course of hard tough stones is usually 1 to 2 inches for heavy traffic, and to 1 inch for light traffic. It is often claimed that a smooth road can not be built

with stones 2 inches in diameter; but with sufficient rolling and a good binding material, a comparatively smooth road may be se cured with such stones, and the road will last much longer than one built of finer material.

The custom is to lay the stone in courses of substantially one size, although some road builders prefer to have the sizes mixed when thrown into the road. The only advantage of the latter practice is that with a skilful proportioning of the sizes less rolling is required; lm t, it is objectionable owing to the difficulty of getting the several ,Izes properly proportioned and keeping them thor oughly mixe I. There is generally too much fine material in the mixed sizes, which makes the road wear rapidly and unevenly.

Connected with the crusher and run with the same power is generally a rotary screen having meshes of three sizes—usually about I, lf, and 21 inches. Fig. 61, page 216, shows a common arrangement of crusher, elevator, screen, and bins.

For economic reasons the size of stone in the several courses and their thickness should be adjusted so as to use, if possi ble, all of the output of the crusher. The output of the various sizes varies considerably with the character of the stone. With a hard stone, half or more of the product of the crusher will not pass through the 1--inch screen; while with field stones one half may pass through such a screen. The last gives more "fines" or "screen ings" than (• tn. be used profitably during construction, but the surplus is very useful in maintaining the surface. With some rocks it is difficult to get enough fine material for use in the original con struction.