- Why is Bitumen Used in Road Construction?
- Use of Bitumen in Flexible Road Construction
- Requirements of Bitumen Mixes for Road Construction
- Advantages of Bituminous Road Construction Over Concrete Pavements
- Disadvantages of Bituminous Pavement
Why is Bitumen Used in Road Construction?
Bitumen is used in road construction due to various properties and advantages it has over other pavement construction materials. Bitumen gain certain unique properties that are inbuilt in it during its manufacture. The bitumen as a raw material in flexible road construction and bitumen as a mix (composing other materials i.e. aggregates/ pozzolans) serves certain advantages, that prompt to use bitumen widely in road construction.
Use of Bitumen in Flexible Road Construction
The reason behind the significant application of bitumen in flexible pavements are explained below:
1. Production of Bitumen is economical
Bitumen is a by-product of crude oil distillation process. Crude oil itself is a composition of hydrocarbons. The primary products that are available are the petrol, diesel, high octane fuels and gasoline.
When these fuels are refined from the crude oil, the bitumen is left behind. Further treatment of by-product, to make it free from impurities give pure bitumen.
As the primary product demand is of utmost importance to the society, the bitumen as a by product has survival for long. This by product is utilized as a new construction material, without going for any other new resource.
2. Physical and Rheological Properties of Bitumen bring Versatility
The physical and the chemical properties of Bitumen are found to be a function of load level, temperature and the duration of loading. It is a thermoplastic and viscoelastic material.
These dependencies make us to truly access the traffic on the road so that a bitumen mix properties can be varied based on the stress levels calculated. This versatility of bitumen results in a large variety of bitumen mix, based on the road application.
3. The Melting Point of Bitumen is low
It is highly appreciable about the fact that bitumen has a favorable melting point, that helps in both surface dressing and wearing resistance with ease.
The melting point of the bitumen should not be too high, that it can be melted easily during laying the pavement. At the same time, bitumen has a melting point, which would not let the already casted road pave to melt and deform under high temperatures.
In areas of high temperatures, along with this quality of bitumen, the aggregate composition helps to cover up the effect of large temperature.
4. Bitumen can undergo Recycling
As the melting point of bitumen is favorable, it can be melted back to its original state. This is called as asphalt recycling process.
The torn-up asphalt pieces are taken up to the recycling plant, instead of sending them to landfills. This recycled mix can be reused. If necessary, the old bitumen is mixed with new bitumen and new aggregates to make the mix live again.
5. Bitumen gain Adhesive Nature
As explained in the production of bitumen, it is free from hydrocarbon and hence not toxic. The by product is refined to maximum to get rid of organic materials and impurities.
The bitumen has a highly adhesive nature, which keeps the materials in the road mix bind together under strong bonds. These become stronger when the mix is set i.e. ready for vehicle movement.
6. Bitumen has Color Variety
The traditional bitumen is black in color. This is because the dense organic material within bitumen is black in color. Now, when certain pigments are added to bitumen, the color of our choice can be obtained. These are colored bitumen.
It is costly than the normal colored bitumen. The disadvantage of colored bitumen is that it requires more chemical additives and materials.
Requirements of Bitumen Mixes for Road Construction
An overall bitumen mix is used in the construction of flexible pavement to serve the following needs.
- Structural Strength
- Surface Drainage
- Surface Friction
1. Structural Strength of Bituminous Pavements
The figure below shows a typical cross section of flexible pavement, that was developed in the USA. The structural bitumen layer composes of:
- Bituminous surface or wearing course
- Bituminous binder course
- Bituminous base course
The primary purpose of these bitumen mixes is structural strength provision. This involves even load dispersion throughout the layers of the pavement. The loads involved are dynamic or static loads, which is transferred to the base subgrade through the aggregate course.
A granular base with a bituminous surface course is only provided for roads of low traffic. It is just sufficient and economical.
The rebounding effect of bitumen upper layers helps in having resistance against high dynamic effect due to the heavy traffic. Rebounding property is reflected by the stiffness and the flexibility characteristics of the bitumen top layers. When looking from bottom to top, the flexibility characteristics should increase.
Studies have shown that the above mentioned characteristics of aggregates are attained using densely graded bitumen mixes. This mix should make use of nominal maximum size aggregate (NMAS), that must decrease from the base course- binder course – surface course.
The nominal maximum size aggregate (NMAS) = One sieve larger than first sieve-to retain more than 10% of combined aggregate.
There is a higher amount of bitumen content in the wearing course, that make the layer more flexible. This would help in increasing the durability.
2. Surface Drainage of Bituminous Pavements
Subsurface drainage can be facilitated using granular sub base in the construction of flexible pavement. Permeable asphalt treated base (PATB) can be used to provided positive surface drainage in major highways. This would behave as a separate course for facilitating subsurface drainage.
3. Surface Friction of Bituminous Roads
It is essential for the pavement layer to provide enough skid resistance and friction, during vehicle passage, especially in wet condition. This would ensure the safety of the passengers. The macro and the micro surface texture of the asphalt mix contributes towards the surface friction.
The mix gradation i.e. open graded or dense graded will contribute to macro surface texture. The open graded mix have higher macro surface than dense graded. The water is squeezed out from the bottom of vehicle tire when the high macro surface texture is implemented.
The micro surface texture is contributed by the aggregate surface, that is exposed when the above bitumen layer is torn.
Advantages of Bituminous Road Construction Over Concrete Pavements
1. A smooth Ride Surface
It does not make use of any joints; Hence provide a smooth surface to ride. It also gives less sound emission when compared with concrete pavements. The wear and tear are less in the bituminous pavement, thus maintaining the smoothness.
2. Gradual Failure
The deformation and the failure in the bituminous pavement is a gradual process. The concrete pavement shows brittle failures.
3. Quick Repair
They have an option to be repaired to be quick. They don’t consume time in reverting the path for traffic; as they set fast.
4. Staged Construction
This helps in carrying out staged construction in a situation when problems of fund constraint or traffic estimation problems are faced.
5. Life Cost is Less
The initial cost and overall maintenance cost of bituminous pavement are less compared to concrete pavement.
6. Temperature Resistant
They act resistant against high temperature from melting and are not affected by de-icing materials.
Disadvantages of Bituminous Pavement
- Bituminous pavements are less durable
- Low tensile strength compared to concrete pavement
- Extreme weather and improper weather conditions tend to make bituminous pavement slick and soft.
- Bitumen with impurities can cause pollution to soil, hence ground water by their melting. These may have hydrocarbons in small amounts.
- Clogging of pores and drainage path during construction and service life
- More salting- to prevent snow during winter season
- Cost of construction high during extreme conditions of temperature