A mini project on study of Bituminous Road

layers

By Y7CE220 Y7CE221

On guidance of MR.K.Vijaya Kishore sir

INTODUCTION TO BITUMEN Bitumen is a black or dark colored solid or viscous cementitious substance having an adhesive properties.

It consists chiefly high molecular weight hydrocarbons derived from distillation of petroleum or natural asphalt.

It is a semi-solid hydrocarbon product produced by removing the lighter fractions (such as liquid petroleum gas, petrol and diesel) from heavy crude oil during the refining process.

Bitumen is often confused with tar. Although bitumen and coal tar are similarly black and sticky, they are distinctly different substances in origin, chemical composition and in their properties.

Tars are residues from the destructive distillation of organic substances such as coal, wood, or petroleum.

INTRODUCTION TO BITUMINOUS ROAD

Bituminous road is a road constructed by using bitumen. It is also called flexible pavement because it changes its shape according to nature of load and sub base.

A road or pavement is a structure consisting of superimposed layers of processed materials above the natural soil sub-grade, whose primary function is to distribute the applied vehicle loads to the sub-grade.

The pavement structure should be able to provide a surface of acceptable riding quality, adequate skid resistance, favorable light reflecting characteristics, and low noise pollution.

The ultimate aim is to ensure that the transmitted stresses due to wheel load are sufficiently reduced, so that they will not exceed bearing capacity of the sub-grade.

This type of pavement has four layers namely, sub grade (bottom most), sub base, base and wearing surface (top most).

Thickness of layer decreases normally from bottom to top.

Cross section of bitumen road

Road Layers

1.Sub grade layer The SUBGRADE is the layer of naturally occurring material or the fill

material the road is built upon. 

The strength of the SUBGRADE layer is measured using the CBR test. 

The strength of the SUBGRADE is an important factor influencing the thickness of the road pavement design.  

Where the SUBGRADE is weak, i.e. a low CBR, it will be necessary to have a CAPPING LAYER over the subgrade to increase the strength before the actual road pavement thickness is designed.    

2.Capping layer When the California Bearing Ratio (CBR) of the subgrade is less

than 5%, it is normal to require a suitable capping layer of low cost material. 

This capping layer is usually a granular type material designed to provide a working platform on which sub-base construction can proceed with minimum interruption from wet weather.

capping is also used to minimize the effect of a weak subgrade on

pavement strength. 

3. Sub base layer This particular layer in ROAD PAVEMENT construction, is quite often used to refer, some what

misleadingly, to the material GRANULAR SUB-BASE. 

The primary functions of sub base layer are:- - Improve drainage. - Minimise Frost action damage. - Provide a working platform for construction. The material used in this layer -Granular sub base 

-Cement bound material

GSB Type 1.

This material is typically crushed stone, crushed slag, crushed concrete or non-plastic well-burnt shale.

GSB Type 2 It is a much finer material, has a much wider grading envelope and is of a lesser "engineering"

quality than GSB Type 1. Therefore it does not generate as much interlock as a Type 1 and is consequently not as strong  

Grading range for type 1 GSB Grading range for type 2 GSB

4.Road base (Base) The ROADBASE (base) is the main load-bearing / load spreading layer in the

road structure and is usually 100mm. or more thick depending on the loading of the traffic for which the road is designed. 

The ROADBASE (base) is usually a bituminous material, dense bitumen macadam (asphalt concrete) or hot rolled asphalt.

The cement bound material (CBM) can also be used in this particular layer.

Generally it is made of crushed slag, crushed stones and other stabilized materials

-Aggregates and Hot Mix Asphalt are usually used in this layer.

Gradation for Base layer

5.Binder Course This is the layer of material below the SURFACE COURSE and above the

ROADBASE.

The BASECOURSE (binder course) is a load spreading layer, spreading the load imposed on the WEARING COURSE (surface course) over a wider area of the ROADBASE (base).   

It can also be a specialized layer, e.g. both stiff and impervious under a POROUS ASPHALT wearing course to prevent the ingress of water into the road fabric.

BASECOURSE (binder course) is most commonly a bituminous material, and can be either HOT ROLLED ASPHALT or DENSE BITUMEN MACADAM (asphalt concrete).

This is a strengthening layer of the pavement and should be at least 40mm, preferably 50mm thick.

The material used as a binder course is 'chunkier' than a wearing course, usually comprising 20mm or 28mm aggregate in a bitumen binder, known as Dense Bitumen Macadam (DBM).

For binder course 50-80mm thick, 20mm material (DBM) is used. For binder course 70-150mm thick, 28mm material (DBM) is used.

20 mm binder course Dense Bitumen Macadam

6. Wearing course or Surface course

The WEARING COURSE (surface course) is the top layer of the ROAD  PAVEMENT and is designed :-

to be impervious to the ingress(entering) of water. To have an even RUNNING SURFACE,.

To be durable, and have a high resistance to skidding, and

To be chosen so as not to deform under the weight of traffic appropriate to the road.

Bitumen mastic as a wearing course

Bitumen mastic is an intimate homogeneous mixture of mineral fillers and well graded fine and coarse aggregates with hard grade bitumen cooked and laid hot.

It has been successfully used as a wearing course on bridge decks and in different situations of heavy duty road pavements. It has got the following main advantages:

appreciable capacity to absorb shocks -cracks are self healing

prevents bleeding

long service life

Besides these six layers mentioned above there are also: Seal Coat: It is a thin surface treatment used to water-proof the surface

and to provide skid resistance. Tack Coat: It is a very light application of asphalt, usually asphalt

emulsion diluted with water. It provides proper bonding between two layers of binder course and must be thin, uniformly cover the entire surface, and set very fast.

Prime Coat: It is an application of low viscous cutback bitumen to an

absorbent surface like granular bases on which binder layer is placed. Unlike tack coat, prime coat penetrates into the layer below, plugs the voids, and forms a water tight surface.

Some of the common aggregates used in road paving

MACHINERIES FOR THE PAVEMENT OF THE ROAD 1. Rollers 2. Dozers 3. Graders 4. Plate Compactors 5. Planers 6. Asphalt pavers 7. Tandem Vibratory roller 8. Paver 9. Bitumen Sprayer 10. Millers 11. Asphalt Splitting Machines

References

Introduction to pavement design by Prof. Tom V. Mathew 2009-02-27.

Highway engineering by S.K.Khanna and Justo.

Construction equipments and planning by Mohesh Verma

http://www.pavingexpert.com/subgrade.htm

AASHTO Guide for the Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, DC, 1993

Thank You