Roads provide transportation facilities from one place to another. Roads are considered the most important infrastructure required for the development of a region.
Different types of Roads: –
The types of roads constructed are:
- Low-cost roads (earth roads, kankar roads, gravel roads, mourum roads, water-bound Macadam roads)
- Bituminous roads (flexible pavements)
- Cement-concrete roads (rigid pavements).
Low-cost roads
The term low-cost road is used to mean the road whose initial as well as maintenance costs are low. India is a country of villages and it is necessary to provide the facility of a linking road to every village. The low-cost roads are meant for low intensity of traffic.
Bituminous roads
The bitumen, or bituminous materials in the form of tar or asphalt, is one of the major highway construction materials. The useful qualities of bitumen are:
(i) It is an excellent binding material
(ii) It gives an impermeable surface
(iii) It softens when heated.
The consistency of bituminous materials ranges from semi-solid to liquid and these materials are made commercially available to suit the requirements of specifications for various types of bituminous roads. A bituminous road is defined as a road in which bitumen is used in one from or the other as a binder to keep together the coarse aggregates and road metal. Such a road is also sometimes referred to black-top road because it exhibits a black appearance due to the presence of bitumen.
The advantages of bituminous roads are:
(i) Bituminous materials increase the strength of pavement.
(ii) Development of the cracks on the surface of the bitumen is very less.
(iii) Maintenance costs of bituminous roads are less.
(iv) Bituminous materials provide waterproof surface.
(v) The surface of bituminous roads is non-slippery.
(vi) Bituminous materials can effectively resist the adverse effect of rain, changes in temperature and wind.
(vii) When the bituminous layer is provided on the top of an existing low-cost road, it eliminates the dust nuisance.
(viii) Bituminous materials provide smooth, durable and comfortable surface for traffic.
The disadvantages of bituminous roads are:
(i) The construction cost is very high.
(ii) The viscosity of the bitumen aggregate mixtures plays an important role in determining the performance of bituminous roads. So, it is necessary to have control on the proper viscosity of the bitumen aggregate mixtures during mixing and compaction operations while constructing the road.
Cement-concrete roads
The cement-concrete roads are in the form of monolithic slabs of cement concrete, which serve two functions simultaneously, namely;
(i) As the load carrying base
(ii) As the wearing surface.
The cement-concrete roads are becoming popular because of the fact that concrete of desired quality can be prepared by modern techniques of cement-concrete construction.
The advantages of a cement-concrete road are:
(i) It does not develop corrugations and hence provides a noiseless surface.
(ii) It can be designed for more accurate load distribution.
The disadvantages are:
(i) It is difficult to repair and needs expert supervision.
(ii) It is liable to crack, warp and twist.
Classification of Roads by Nagpur Road Plan
The classification based on location and function is a more acceptable classification. According to Nagpur plan, five different categories of roads are as follows:
- National Highways (NHs): National highways are the main highways running through the length and breadth of the country, connecting major ports, foreign highways, capitals of large states and large industrial and tourist centres including roads required for strategic movements of troops.
- State Highways (SHs): State highways are the arterial roads of a state, connecting with the national highways, capitals of adjacent states, district headquarters and important cities within the state. A state highway serves as a connecting link for traffic to and from district roads.
- Major District Roads (MDRs): Major district roads are the important roads within a district, serving areas of production and market and connecting them with each other or with main highways of a district.
- Other District Roads (ODRs): Other district roads are the roads serving the rural areas of production and providing them with an outlet to market centres, Taluka headquarters.
- Village Roads (VRs): Village roads are the roads connecting villages or the groups of villages with each other to the nearest road of higher category.
Components of roads and their functions
Just like other structures, the highway or the road structure is also required to be designed carefully for the traffic load to be carried, physical and geological features, and climatic conditions of the locality and various other factors which would affect the stability and utility of the highway. For the purpose of design, the road structure may be considered to consist essentially of the following four component parts as shown in Figure
- Subsoil
- Subgrade
- Base
- Surfacing.
Subsoil: This is the natural or prepared soil on which the road has to be formed. It should be stable and strong to carry safely the traffic load and weight of roadway construction.
Subgrade: The subgrade or the formation functions as a support to the road surface and its foundation. The life of the road primarily depends on stable and dry subgrade. Its level may be same or above or below the natural ground level. The support given to the road structure by the subgrade is an important factor and hence, considerable attention should be paid to the proper preparation of subgrade before the road structure is laid on it.
Base: The base or foundation may consist of two layers, the bottom layer being known as sub-base or soling or bottoming. The sub-base should be stable and it should be capable of resisting distortion under traffic loads to a great extent. The function of a road base is to transmit the load from the surfacing to the subgrade. It should possess structural stability and should be of sufficient thickness to develop a good bond with the surfacing.
Surfacing: The topmost layer on which the traffic directly travels is known as road surfacing or wearing layer or wearing course. The main function of road surfacing is to provide a smooth and stable running surfacing which is suitable for the type and intensity of traffic anticipated on the road. The surfacing should be impervious and should protect the base and the subgrade from the action of weather and rain water. The desirable qualities of surfacing are durability, stability, on-slipperiness, economical and dust lessness. It should also be able to resist displacement by traffic and should therefore must be well keyed on to the base.
The governing factors which would affect the design of the the above components of road structure are initial cost, availability of local materials, volume and class of traffic, climatic conditions of the locality, etc.
