Bitumen Road Construction.
In various layers, such as the base course, binder course, and surface course, bituminous pavements are constructed.
These layers are constructed of various materials and the bituminous pavements have different functions.
Layers in a Bituminous Pavement.
Bituminous road has three layers. Which is as follows.
- Bituminous Base Course.
- Bituminous Binder Course.
- Bituminous Concrete Layer.
#1. Bituminous Base Course
- In a bituminous pavement, the base course layer consists of mineral aggregates mixed along with bituminous materials including such asphalt, stones as well as sands. This coating is used as the base for the positioning of the surface course or binder.
#2. Bituminous Binder Course
- An intermediate layer between the base course and the surface layer is the Binder course layer. That is the first layer in the case of a bituminous two-layer resurfacing.
- The bituminous binder course, also called the levelling course, is composed of a bituminous-aggregate mixture.
#3. Bituminous Concrete Layer
- A mixture of aggregates continually graded from maximum size to minimum size is a bituminous concrete layer (typically less than 25 mm to 0.075 mm aggregates).
- Enough bitumen is applied to the blend such that it is essentially impervious to the compacted concrete mix and has reasonable transverse and elastic properties.
Also, Read: What is Bitumen And Bitumens Types
What Is Bituminous Macadam?
Bituminous Macadam (BM) consists of crushed aggregates and bituminous binder heated and mixed in a hot mix plant at specified temperature, transported to the construction site, laid with a mechanical paver and compacted by a roller.
The BM is laid in a compact thickness of 50 to 100 mm. two different gradations of aggregates have been suggested by the IRC to provide open-graded mixture suitable for base course of highway pavements.
Therefore the BM layer should be covered by a suitable surfacing course before exposing to weather or traffic.
In case there is a delay in laying appropriate bituminous layers ( such as binder course and/or surfacing course) above the BM layers as per the design, at least a seal coat should be applied to prevent Ingress of water during unexpected rains and possible damaged to the BM layer.
With regard to load dispersion properties and toughness, the BM base course is known to be far superior to other forms of base course materials such as penetration macadam, WMM or WBM, providing the overall structural design satisfies the exact specifications at the location.
In terms of the granular base course layer, the equivalence ratios of the BM base course are assumed to be 1.5 m; in other words, one unit thickness of the BM base course layer is considered equal to 1.5 unit thickness of the granular base course layer and therefore of the construction methods of penetration.
Bitumen Road Construction Process:
The initial surface is prepared by fixing the potholes or rust if any. At least a week before the surface course is laid, Premix chippings fill out the defects.
If the current pavement is extremely high, a sufficient bituminous levelling course of thickness is offered to lay a bituminous concrete surface course on a binder course instead of directly laying it on a WBM.
It is preferable to lay AC layers over a bituminous base or binder route. A bitumen tuck coat is applied at 6.0 to 7.5 kg per 10 sq.m area, which can be increased to 7.5 to 10 kg for a non-bituminous base.
In a hot mixing plant with the necessary power, the premix is prepared with the desired quality control. It is possible to heat the bitumen up to 150-177 deg C and the aggregate temperature does not vary from the binder temperature by more than 14 deg C.
The hot mixed material is obtained by the transporters from the mixture and delivered to the site at a temperature of 121 to 163 degrees C by a mechanical paver. The camber and the layer thickness are checked correctly.
In the strength of the resultant pavement structure, the regulation of the temperatures during the mixing and the compaction is of considerable importance.
A mixture is thoroughly compacted by rolling at a pace of not more than 5 km per hour after it is put on the base course.
The initial or breakdown rolling is done with a roller of 8 to 12 tons and the intermediate rolling is done with a pneumatic fixed wheel roller of 15 to 30 tons with a tire pressure of 7 kg per sq.cm. With water, the wheels of the roller are kept moist.
The amount of passes required depends on the layer’s thickness. On the following day, rolling in warm weather tends to increase the density if the original rolling was not sufficient. Final rolling or finishing is carried out by a tandem roller of 8 to 10 tonnes.
Quality control of bituminous concrete construction.
In order to ensure the consistency of the resultant paving mixture and the pavement surface, regular inspections are carried out at the site.
- Grading Aggregate.
- Bitumen Grade.
- Aggregate Temperature.
- Temperature During Blending and Compaction of the Paving Mix.
At least one sample of a blend discharged by a hot mixing plant every 100 tonnes is obtained and tested for the above criteria. There are also Marshall tests performed.
A 3.0 m straight edge should be tested against the AC wall. The longitudinal undulations should not exceed 8.0 mm and the number of undulations of more than 6.0 mm should not exceed 10 undulations of 300 m in length. There should be no undulations in the cross-traffic profile exceeding 4.0 mm.
Advantage of Bitumen Road:
- It does not use any joints; it thus offers a flat surface for riding. As compared to concrete pavements, it also has less sound pollution. In the bituminous pavement, wear and tear become less, thereby preserving the smoothness.
- A gradual process is a deformation and the failure of the bituminous pavement. The asphalt paving exposes delicate defects.
- They have the choice of being easily restored. In reversing the route for traffic, they do not consume time; as they set fast.
- This helps to carry out staged construction in a situation where challenges are met with fund limitation or traffic prediction problems.
- Compared to concrete paving, the initial expense and total maintenance cost of bituminous pavement are smaller.
- They are resistant to high melting temperatures and are not impacted by de-icing materials.
Disadvantage of Bitumen Road:
- Bituminous surfaces are less durable.
- Compared with concrete paving, poor tensile strength.
- Extreme weather and inappropriate weather conditions tend to render slippery and porous bituminous pavement.
- Bitumen with impurities can cause soil contamination, hence the melting of ground water. In small quantities, it may have hydrocarbons.
- During building and service life, blocking of pores and drainage routes.
- Further salting-to stop snow in the winter season.
- High building costs during extreme temperature conditions.
Application of Bitumen Road:
- Bitumen is a by-product of the method of distilling crude oil. Crude oil itself is a hydrocarbon composition. Petrol, diesel, high octane oils, and gasoline are the main items available.
- Bitumen is left behind as these oils are refined from crude oil. Further treatment of by-products produces pure bitumen to make it clean from impurities.
- Although the primary demand for goods is of paramount significance to society, the survival of bitumen as a commodity by product is long. This is used as a modern construction material by product, without looking after any other fresh materials.
- Bitumen’s physical and chemical properties are observed to be a function of the level of load, temperature, and loading length. That is a substance that is thermoplastic and viscoelastic.
- These dependencies allow one to genuinely enter road traffic so that the properties of a bitumen mix can be varied depending on the measured stress levels. This bitumen flexibility, depending on the path use, results in a wide range of bitumen mixes.
- The fact that bitumen has a desirable melting point, which effectively assists in both surface dressing and wearing resistance, is highly appreciable.
- The bitumen melting point should not be too high so that it can quickly be melted during the pavement laying process. Around the same time, bitumen has a melting point, which under high temperatures will not cause the already cast path to melt and distort.
- Although the bitumen melting point is favorable, it can be melted back to its original condition. This is called the method of asphalt recycling.
- Instead of taking them to landfill sites, the torn-up asphalt parts are brought back to the recycling facility. It is possible to reuse this recycled blend. If required, to keep the mix live again, the old bitumen is blended with new bitumen and new aggregates.
- It is free from hydrocarbons and thus not harmful, as explained in the processing of bitumen. To get free of organic materials as well as impurities, the substance is refined to the maximum.