Loam is chiefly clay mixed with sand and a little vegetable matter, lime, etc.; and as a binding material has all the charac teristics of clay.
A very finely divided silica, easily mistaken for clay, is sionally present in gravel, and makes an excellent binding material. Iron oxide is frequently found as a coating on the pebbles in such quantities as to cement them firmly together. These ferru ginous gravels when broken up and put upon a road, will again unite—often more firmly than originally, because of the greater pressure—and form a smooth hard surface, impervious to water. They are much used in road building, gravel from Shark River, N. J.,—much used around New York City—and that from the Ohio river near Paducah, Ky.,—largely used in the neighboring states— being examples.
Comparatively coarse gravel frequently contains some in gredients, as, for example, fragments of limestone or shale, which under the action of traffic and the weather reduce to a powder and form a good binding material. Sometimes gravel contains bits of ironstone (clay cemented with iron oxide) in the form of thin flat chips which break and crush easily under the wheels, and if present in any quantity make a most excellent binding material.
The binding action referred to in the preceding discussion is mechanical; and we come now to the consideration of an action not yet well understood, but which for the present at least will be called chemical action. Experiments seem to prove that if fine powder of certain stones is wetted with water and subjected to compression, a true chemical cementation takes place. Conse quently some stone when broken into small fragments, wetted and traversed by heavy wheels or by a road-roller will be cemented together to a considerable degree. This cementation is due to the fact that the friction of one small piece of stone upon another pro duces a very fine powder at the point of contact, which, when wetted and compressed, forms a weak cement. Owing to the rounded surfaces of water-worn pebbles, this cementing action is much less with gravel than with rough angular fragments of broken stone; but with gravel composed of undecayed rocky fragments this action takes place to a considerable degree. As a rule, pebbles of bluish color will thus cement together, while reddish or brown ones will not, which accounts in part at least for the well known superiority of blue gravel for road purposes. Trap rock possesses the property
of cementation in a high degree, and hence trap gravel is a very excellent road-building material. Limestone possesses a fair de gree of cementation, but is too soft to wear well. Quartz wears well but produces little or no dust for cementation, and besides its surfaces are so smooth and hard that the binder has but little effect; and therefore it rarely happens that a gravel of which more than one half of its bulk is white quartz pebbles proves to be a good road gravel.
The cementation of rocky fragments is much more important in a crushed-stone road than in a gravel one, and therefore the subject will he more fully considered in Chapter V.
The binding elements heretofore discussed exist naturally in the gravel; but gravels are often found that do not contain any binding material, and in such cases it is necessary to add some cementing material.
Clay, shale, hard-pan, marl, loam, etc., are often used for this purpose, chiefly because they are so plentiful and easily applied; but none of them are suitable for the purpose, as they all have the characteristics of a clay binder (see § 234). With any of them, it is difficult to keep the gravel from breaking up—particularly under heavy traffic.
In some localities a poor iron ore is found, which, when mixed with gravel, makes an excellent binder and gives a smooth hard road surface. Bog iron-ore, which occurs in marshes, is usually very good for this purpose.
The fine dust from a stone crusher, when mixed with gravel, will bind it together; but it is seldom feasible to use stone dust on ac count of the expense. When this method of binding gravel is resorted to, the construction partakes of the character of a crushed stone road—a subject foreign to this chapter. The chief difference between a gravel and a crushed-stone road is in the thoroughness of the binding. The binding of a gravel road is due chiefly, and usually solely, to the mechanical action of the binder; while the binding of the broken stone is due to both the mechanical and the chemical action of the binder, and both are stronger with rough angular fragments of broken stone than with water-worn pebbles.