Stone is prepared for use in road work by crushing and screening. In the early days of broken-stone roads, all stone was broken by hand, and the roads were care fully constructed of stone broken to approximately uni form sizes without the addition of a binding material. The development of stone crushing machinery has, however, modified practice in this regard and stone crushed by machinery is now almost exclusively used. It gives satisfaction both as to binding properties and durability, and has the advantage of greatly lessening the cost.
The size to which stone should be broken for road material depends to some extent upon the nature of the material. The harder and tougher it is the smaller the pieces may be without danger of crushing or shat tering under the loads and shocks received in the road surface, and the smaller also they will need to be in order to be thoroughly compacted in the road.
It is a general custom to use larger stones in the bottom courses of a road than at the top. A rule very commonly given is that the stones for the lower layers should be at least 2 inches in their greatest diameter, and not more than 3 inches, and that for the surface layer the stones shall not be greater than 2 inches in greatest dimension.
If of very hard rock the surface layer may have t inch to ii inches as an upper limit of size.
The size of the rock in the lower layers does not seem of so great importance as that for the surface layers, as it is not directly subject to the weight or the abrading action of the concentrated wheel-loads, and it is probable that in some cases unnecessary expense is incurred in following the refinements of rigid specifica tions in this particular.
There is a• difference of opinion also among road builders as to the of using stone of uniform size. Some insist quite strenuously upon this point and carefully screen their stone with the object of get ting it as uniform as possible; while others declare that the variation of size is an advantage, and even that the stone should not be screened after coming from the crusher, except to remove the stone above the limiting size and when necessary to get rid of foreign matter in case it should contain clay or earth.
Uniformity of size probably makes the wear more even, but the presence of smaller fragments facilitates the binding together of the material. The best prac tice seems to favor the exclusion of the fine material from the stone, without insisting on too great uniform ity in size (stones being allowed probably from ' inch to I or 2 inches in dimension), and then adding small material after the placing of the stone upon the road to assist binding. If the varying sizes be well dis tributed through the mass of stone, the variation of size has the advantage of lessening the amount of voids, and makes possible to compact the stone in the road with a less quantity of binder. Screening out the fine parts and dust eliminates the danger of having portions of the road made up entirely of fine material, and secures a proper distribution of the binder through the mass of stone.
Gravel is frequently used for roads constructed in the same manner as with broken stone, both with and without the telford foundation. The requirements of a good gravel for this purpose are the same as for a good stone. The stones of the gravel should be sharp and angular, and must possess the qualities of hardness and toughness. Water-worn material is therefore ob jectionable, as it will not compact without the use of large amounts of binding material. In many places a hard flint gravel occurs which is not inferior to the best broken stone. This frequently occurs when the available rock is soft limestone and may be used to advantage as a surface upon a base of the soft rock.
Gravel should be screened to remove the larger stones and the fine material, and then treated in the same manner as broken stone.
Gravel not fit for surface material may often be used to advantage as a base under a surface of hard rock; in many instances, economy would result from the substitution of gravel for broken stone in such work. Slag and cinders may also sometimes be used in the same manner.