A road is a wide way route on land in between two places. It is a specially prepared surface which allows travel by foot or some form of conveyance, including (but not limited to) a motor vehicle, cart, bicycle, or horse. Roads are an integral part of infrastructural development when it comes to development and progression of communities as well as nations. A road provides access to employment, social needs, health and education services etc. hence, road networking can easily be categorized as one of the top priorities for infrastructural development.
Bitumen road construction consists of various steps. Following is a brief explanation of those;
Step1: Preparation of base course layer
The base course in pavements is a layer of material in a road way. It is located under the surface layer consisting of the wearing course and sometimes an extra binder course. Its thickness ranges from 100 to 150 millimeters. A surface is prepared by removing the pot holes or rust if any. Then on clean surface base course layer laid down.
Step 2: Application of Tuck Coat
It is desirable to lay asphalt concrete layer over a bituminous base. Asphalt concrete is a composite material commonly used to surface roads, parking lots, airports, as well as the core of embankment dams. It consists of mineral aggregate bound together with asphalt, laid in layers. While construction of road, a tack coat of bitumen is applied at 6.0 to 7.5 kg per 10 sq.m area, this quantity may be increased to 7.5 to 10 kg for non-bituminous base.
Step 3: Preparation and placing of Premix
The premix is prepared in a hot mix plant of a required capacity with the desired quality control. The bitumen may be heated up to 150 – 177 deg C and the aggregate temperature should not differ by over 14 deg C from the binder temperature. The hot mixed material is collected from the mixture by the transporters, carried to the location and is then spread by a mechanical paver at a temperature of 121 to 163 degrees Celsius. The camber and the thickness of the layer are accurately verified.
Control of the temperatures during the mixing and the compaction are of great significance in the strength of the resulting pavement structure.
Step 4: Rolling
A mix, after it is placed on the base course, is thoroughly compacted by rolling at a speed not more than 5 km per hour. The initial or break down rolling is done by 8 to 12 tones roller and the intermediate rolling is done with a fixed wheel pneumatic roller of 15 to 30 tones having a tire pressure of 7kg per sq.cm. The wheels of the roller are kept damp with water. The number of passes required depends on the thickness of the layer. In warm weather, an additional step of rolling on the very next day, helps to increase the density if the initial rolling if it was not adequate. The final rolling or finishing is done by 8 to 10 tons tandem roller.
Step 5: Quality control of bituminous concrete construction
Routine checks are carried out at site to ensure the quality of the resulting pavement mixture and the pavement surface. Periodical checks are made for
- Aggregate grading
- Grade of bitumen
- Temperature of aggregate
- Temperature of paving mix during mixing and compaction
At least one sample for every 100 tons of the mix discharged by the hot mix plant is collected and tested for above requirements. Marshall Tests are also conducted. For every 100 sq.m of the compacted surface, one test of the field density is conducted to check whether it is at-least 95% of the density obtained in the laboratory. The variation in the thickness allowed is 6mm per 4.5m length of construction.
Step 6: Finished surface
The AC surface should be checked by a 3.0 m straight edge. The longitudinal undulations should not exceed 8.0 mm and the number of undulations higher than 6.0 mm should not exceed 10 in a length of 300 m. The cross-traffic profile should not have undulations exceeding 4.0 mm.
If all these steps are followed in letter and spirit under supervision of a specialist, then one has a perfect road structure which stands the test of time.