BITUMINOUS ROADS

Bituminous roads are those which are painted with bituminous materials at its surface. These are high-cost roads.
The main functions of providing a seal coat on a bituminous surface are as under:
1) To make the surface watertight.
2) To provide a more desirable surface texture.
3) To reduce the slipperiness of the surface.
4) To obtain an existing dry or weather surface.

Following types of bituminous roads are generally constructed:
Surface painting road.
Bituminous Macadam road
Bituminous Concrete road.
Sheet Asphalt road.

Surface painting road.

Surface painted or surface dressing bituminous roads are used where traffic is small. For the construction of these roads, first W.B.M road is constructed as described early. The surface is cleaned with wire brushes and brooms to make grooves in the joints of the surface. Then a layer of hot colter or asphalt is sprinkled over the surface. Over this layer, a layer of stone chippings is laid and rolled properly. This sets the aggregate into the binder. The thickness of this layer is between 2 to 3 cm. This surface dressing provides a thin, waterproof and dustless surface to the road. About 2 kg of binder per sq. m area of the road surface is used and 1.8 to 2 cup m of stone chippings are required per 100 sq. m of the surface.
In case of heavy traffic, two layers of dressing are provided to the road surface. In second coat 1.5 kg of bitumen or tar is provided per sqm of the road surface.

Bituminous Macadam Road

In this type of road construction, the aggregates are spread on the prepared base. It is then rolled with the roller and then the bituminous binder is sprayed on the surface which penetrates to full or partial depth of the compacted aggregate and thus binds them together.
There are two types of Bituminous Macadam surface road.

Grouted Macadam
This is the method of bituminous road construction in which the aggregates are bound together by grouting or penetrating bitumen into the voids in full or part depth of compacted aggregates. It is called grouted or penetrated method. This method is also called built-up spray grout macadam.
The grouted method may be full grouted macadam or semi- grout macadam road. When the bitumen penetrates to the full depth it is called full grouted method and when the bitumen penetrates into the aggregates to part depth, it is called semi- grouted macadam.
Full grout macadam construction is adopted in regions of heavy rainfall and the semi- grout macadam is constructed in the regions of average rainfall. The recommended thickness for full grout is 5 to 8 cm and that for semi- grout macadam construction, it is usually kept 5 cm.

Premix Type Construction
This is the type of road in which aggregates and binder are mixed together in a mixing plant before these are laid on the surface of the road. The W.B.M surface is cleaned with wire brushes. Refined tar and road metal are heated to a temperature of 120° and mixed in mixing plant. 56 kg of bitumen is required for 1 cu m of road metal. This mixture is laid on the W.B.M surface in a thickness of 5 cm. The surface is then rolled with a roller. Over this, a premixed sealing coat is applied and the road surface is rolled finally with proper care.

Bituminous Concrete Road

These types of roads are used for heavy and mixed traffic. The base of this road is also W.B.M. First of all the surface of W.B.M is cleaned with wire brushes. Side Krebs of soil is formed to support the bituminous surfacing.
The road metal of size of 6.3 mm is heated to a temperature of 180°. Then coarse sand of 1.3 mm size is also mixed in the road metal in the ratio of 1:2. Then bitumen or tar is heated at the same temperature as road metal and mixed with road metal in a mixing plant. This hot mixture is spread on the cleaned surface is then rolled with the roller to proper camber and grade. Over this a sealing coat of coarse sand and bitumen is applied on this surface and surface is again rolled. This type of road surface provides a smooth surface and pleasant looking finish.

Sheet Asphalt Road

Sheet asphalt provides a better type of surface. This type of road surface is applied to W.B.M base or concrete base. It consists of 60 mm thick layer of asphaltic concrete over which 20 to 40 mm thick carpet of sand- bitumen mix is laid.