The different types of road construction materials

The natural earth track cannot withstand modern heavy traffic loads very satisfactorily because it lacks in strength and good riding surface. It is therefore, important to construct some structure in the form of pavements on the top of natural surface to enable it to support wheel load safety and to provide a good riding surface for a longer period.

So properly constructed and maintained roads reduce the wear and tear of vehicles and also increase the speed of transportation. Therefore the construction of roads should be done properly according to the specification.

1. Construction of Earth Roads

First of all sub-grade is prepared all the road surface is brought to the required camber and gradient.

·         Then the surface is rolled properly and well wetted with water.

·         Then a layer of about 10 cm thickness of graded soil is spread evenly.

·         This layer of soil is rolled at optimum moisture content first with suitable roller and then finished with light roller.

·         If a second layer is required to be laid, it should be laid in the same way as the first layer and rolled properly.

·         Finally finished surface is watered for about 5 days for curing and no traffic is allowed in these days.

2. Construction of Gravel Roads

Following steps are followed in the construction of gravel roads:

·         First of all the sub-grade is prepared and the road surface is brought to the required camber and gradient. This prepared surface is then properly compacted.

·         After compaction of the sub-grade, the mixture of gravel and clay is spread with more thickness at the center and less thickness towards the edges so that a required camber is provided. Clay in the mix acts as binder for gravel and sand.

·         The gravel mix so spread is then compacted to the required camber. The size of gravel ranges between 6 mm to 35 mm. This surface is rolled with light roller till the required compaction is achieved.

·         This compacted surface is then watered for 4 to 5 days for curing. During this period of curing traffic is not allowed to pass through the surface.

·         After curing the road is opened to traffic but the water is sprinkled on the surface for further 10 to 15 days.

·         Proper maintenance of the road surface should be done to keep it in working conditions for the traffic.

3. Construction of Water Bound Macadam Roads

The construction of water bound macadam roads we require course aggregates, screening and binding materials if necessary. Screening is used for filling voids in the aggregates. It is the residue of the same coarse aggregates. All these materials are stacked along the road side before its construction starts. The quantity of screening required per 100 sq.m of road surface is 30 cubic meter.

The construction of water bound macadam roads is done in the following stages:

·         Preparation of Sub-grade

After completing the earth work in embankment and cutting, the formation level is prepared. The Sub-grade is generally in the form of a trench having a thickness equal to the thickness of finished pavement. The sub-grade is prepared according to the camber and the grade of the road. It is then thoroughly compacted with road roller of weight not less than 8 tonnes. Before rolling, water is sprinkled on the sub-grade. Any low pot, if developed during rolling should be rectified and the surface brought to the required grade. If the soil is clayey, a layer of granular material like natural sand, moorum, gravel,literate or kankar should be spread with 10 to 15 cm thickness. Earthen Krebs along the road edge of 15 cm depth are also constructed to hold the road materials in proper position.

·         Preparation of the Base Course of Foundation

The consists of 12 to 18 cm size boulder of broken pieces of stones, over burnt bricks, or brick soling. Care should be taken to keep minimum possible voids in hand packing of boulders. Voids are filled with small pieces of stones. The width of this course is kept 60 cm wider than the pavement width of road. The surface is then compacted with 10 tonne roller as the base course is laid with boulders.

4. Construction of Bituminous Roads

Following types of bituminous roads are generally constructed:

·         Surface painting road

Surface painting or surface dressing bituminous roads are used where traffic is small. For the construction of these roads, first water bound macadam 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 charcoal 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, water proof and dustless surface of the road. About 2 kg of binder per sq.m. area of road surface is used and 1.8 to 2 cm of stone chipping are required per 100 sq.m. of the surface.

In case of heavy traffic two layers or dressing are provided to the road surface. In second coat 1.5 kg of bitumen or tar is provided per sq meter of road surface.

·         Bituminous Macadam road

This type of road construction, the aggregates are spread in the prepared base. It is then rolled with roller and then the bituminous binder is sprayed on the surface which is penetrates to full or part depth of the compacted aggregate and thus binds them together.

·         Bituminous Concrete roads

These types of roads are used for heavy and mixed traffic. The base of this road is also water bound macadam. First of all the surface of water bound macadam is cleaned with wire brushes. Side Krebs of soil are 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 mixed with road metal in a mixing plant. This hot mixture is spread on the cleaned surface of water bound macadam evenly and properly between the Kerbs in a thickness of 5 to 10 cm. This surface is then rolled with 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 water bound macadam 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.

5. Construction of Cement Concrete Roads

The cement concrete roads pavements may be constructed with or without sub-base. The concrete road slab perform the function of base course as well as wearing surface. The thickness of road slab varies between 15 to 20 cm depending upon the traffic load. For much heavy traffic loads reinforcement may be provided in the concrete road slab.

Before laying the concrete slab, the sub-grade is prepared as usual. Then the cement concrete mix is laid in alternate panels, which are marked before laying the concrete. The panels may be kept 2.5 m to 3.5 m square. The completed surface of concrete slab is cured for 10 to 15 days before it is opened to traffic.