Crossection of a Road

It is important to understand the cross-section of the road.

Road Cross Section is composed of the following components

1.    Sub Base
2.    Base Course
3.    Sub Grade
4.    Surface/Wearing Course

1. Sub Bases:

    It is layer of granular material provided above sub-grade generally natural gravel. It is

usually not provided on sub-grade of good quality.

    a. Function of Sub base in Road Cross Section

        It enables traffic stresses to be reduced to acceptable levels in sub-grade in the Road
        It acts as a working plate form for the construction of upper pavement layers.
        Acts as a drainage layer, by protecting the sub-grade from wetting up.
        It intercept upward movement of water by capillary action.
        It acts as a separating layer b/w sub-grade and road base. By this it prevent the two layers
from mixing up.

3. Base Courses in Road Structure Cross Section

    It is the layer immediately under the wearing surface (Applied whether the wearing surface is

bituminous or cement concrete and whether its a thick or thin bituminous layer).
    As base course lies close under the pavement surface it is subjected to severe loading. The

material in a base course must be of extremely high quality and its construction must be done

carefully. The LA Abrasion test can determine the quality of the aggregate for this purpose.

    a. Types of Base Course

        1. Granular Base Course:

It is a mixture of soil particles ranging in size from coarse to fine. Processing involves crushing

oversized particles and screening where it is necessary to secure the desired grading. The

requirements of a satisfactory soil aggregate surface are;

    a. Stability
    b. Resistance to abrasion
    c. Resistance to penetration of water
    d. Capillary properties to replace moisture lost by surface evaporation upon the addition of

wearing course requirement change.

        2. Macadam Base:

Successive layers of crushed rock mechanically locked by rolling and bonded by stone screening

(rock duct, stone chips etc).

        3. In-water bound Macadam:

The crushed stones are laid, shaped and compacted and then finer materials are added and washed

into surface to provide a dense material.

        4. Treated Bases:

Compose of mineral aggregate and additive to make them strong or more resistant to moisture. Among

the treating agents is bitumen.


4. Surface/Wearing Course in pavement cross section:

The top layers of pavement which is in direct contact with the wheel of the vehicle. Usually

constructed of material in which bitumen is used as binder materials.

    a. Bituminous Pavement:

Consists of combination of mineral aggregate with bituminous binder ranging from inexpensive

surface treatment ¼ in or less thick to asphaltic concrete. For good service throughout the full

life, bituminous pavement must retain the following qualities:

    -Freedom from cracking or raveling.
    -Resistance to weather including the effect of surface water heat and cold.
    -Resistance to internal moisture, particularly to water vapors.
    -Tight impermeable surface or porous surface (if either is needed for contained stability of

underlying base or subgrade).
    -Smooth riding and non skidding surface.

The design should be done so that to meet the above requirements for considerable number of years

(need proper design and construction supervision). Pavements meeting all the requirements above can

be produced if these construction processes are followed:

    -Heat a viscous bituminous binder to make it fluid. Then in a plant, mix it with heated

aggregate. Place and compact the mixture while it is hot.
    -Use fluid bituminous binder. Mix it with aggregate at normal temperature. Mixing may be done

at a plant (plant mix) or on the prepared roadway base (road mix). Spread and compact the mixture

at normal temperature.
    -Add solvent such as naphtha or kerosene to a viscose bituminous binder to make it fluid with

aggregate at normal temperature by either plant or road mix methods. Spread and compact at normal

temperature before solvent evaporates.
    -Use fluid emulsion of viscous bituminous binder in watee. Mix it with aggregate at normal

temperature by either plant or road mix method. Spread and compact at normal temperature before the

emulsion breaks down with its components.
    -Spread and compact clean crushed aggregate as for water bound macadam. Over it, spray heated

dissolved or emulsified bituminous binder which penetrates open areas of the rock and binds the

aggregate together. Thus is commonly called “Penetration Method”.
    -Spread bituminous binder over the roadway surface then cover it with properly selected

aggregate. This is commonly called the “Inverted Penetration Method”.

For better Reference and details go to www.aboutcivil.org my favorite website for civil engineers.