Pict. 4.6. Road filling eith crushed gravel

4.1.3. 4.1.3. Wearing surfaces

The office of the wearing surface is to protect the foundation from the wear of the traffic and the effects of surface water, and to support the weight of the traffic and transmit it to the foundation. It must furnish a comparatively smooth unyielding surface that affords good foothold for draft animals and good adhesion for motor vehicles, and on which the resistance to traction will be a minimum. The material of which it is composed must possess strength to resist crushing and abrasion, and its fabric must be practically impervious.

The wearing surfaces most commonly employed for roads and streets are composed of:

1) gravel, broken stone, furnace slag, and similar granular materials bound with colloidal cement formed by the action of water on the plastic elements of rock and clay;

2) broken stone, gravel, and sand bound with:

a) bituminous cement;

b) hydraulic cement;

3) stone blocks;

4) brick;

5) wood blocks.

In type (1), a certain amount of moisture is essential to successful binding. When this is lacking, as in the summer season, the binding material becomes dry and brittle, and the fragments at the surface are displaced by the action of the traffic; an excess of moisture destroys the binding power; and the surface is quickly broken up by the traffic. Wearing surfaces of type (2a) are usually limited in life not merely by the wear of traffic, but by the fact that all bitumens slowly alter in chemical composition when exposed to atmospheric action, and in time become brittle. Type (2b) is subject to cracking under expansion and contracting, due to changes of temperature, and is liable to wrear unevenly owing to irregularity in mixing and the segregation of the ingredients while the concrete is being put in place.

When a defective spot begins to wear, it extends very rapidly under the abrasive action of the traffic. A bituminous wearing surface differs from the previously described water-bound broken-stone surface only in the kind of binder and the quality of the stone. The bituminous binder is prepared from asphalt, asphaltic oils, refined water-gas tars, refined coal tars, and combinations of refined tars and asphalts. The essentials necessary to the successful construction of a bituminous covering are:

1) the exclusion of both subsoil and surface water from the foundation;

2) a solid unyielding foundation;

3) a stone of suitable quality and size;

4) that the stone shall be entirely free from dust, otherwise the dust will interpose a thin film between the stone and the bituminous binder and prevent the latter from adhering to the stone;

5) if the stone is to be used hot, that it shall not be overheated; and if is to be used cold, that it shall be dry, for if wet or damp, the bituminous material will not adhere to it;

6) that the bituminous cement shall be of suitable quality.

Two general methods with various modifications in the minor details are employed for applying the bituminous binder to form the wearing surface, the penetration method, and the mixing method.

Penetration method: the stone is spread and packed slightly by rolling. The bituminous binder is then applied by one of the following ways: by hand from pouring pots; by a nozzle leading from a tank cart.

Mixing method: the stone to be used for the wearing surface, varying in size is cleaned and dried, then mixed with a sufficient quantity of the binder to coat all the stones thoroughly (Pict. 4.7.).