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Civil Engineering Highway & Transportation Engineering Typical Cross-Section of Flexible Pavements (Roads)


Typical Road Structure Cross Section - Road Cross Section Details


Composition of Road Structure:

Road Structure 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 subgrade generally natural gravel. It is usually not provided on subgrade of good quality.

a. Function of Sub base in Road Cross Section

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

b. Characteristics of materials used in Sub Base:

The subgrade material should be clean and free from organic matter and should be able to be compacted by roller, to form stable sub-base. The material should have the following characteristics:

  • Well graded, uniformity coefficient (D60/D10) should not be less than 3.
  • Fraction passing sieve #200 shall not be greater than 2/3rd of the fraction passing sieve #40.
  • Should have a L.L not greater than 25%.
  • P.I not greater than 6
  • CBR should not be less than 25. ArrowSee Also: CBR Test Procedure
  • In coarse grain, aggregate retained by #10 sieve, %age of wear shall not be greater than 5%.
  • The maximum dia of any particle shall not be greater than 2/3rd of the layer thickness of sub-base

Typical particle size distribution for the sub-base (granular) which will the meet strength requirements are:

B.S Sieve Size % By mass of total Aggr passing test sieve
50 100
37.5 80-100
20 60-100
5 30-100
1.15 170-75
0.3 9-50
0075 5-25

* To avoid intrusion of silt and clay material in sub-base from subgrade

D15 (sub base) < 5

D15 (sub grade)

  • Recommended plasticity characteristic for granular Sub Base (Road Note 31) are;


Liquid Limit (L.L)

Plasticity Index (P.I)

Moist or wet tropical

< 35

< 6

Seasonal wet tropical

< 45

< 12

Arid & Semi Arid

< 55

< 20

2. Sub Grade in Road Structure Cross Section:


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;

  • Stability
  • Resistance to abrasion
  • Resistance to penetration of water
  • 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”.

Selections based on the requirements and economy, large volume of heavy vehicles, low traffic volume etc.

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