Table of Contents for Road Construction Procedure-
- Requirements of a good pavement
- Road Construction Materials
- Road Construction Step by Step
- Earthwork including grading and sloping of subgrade
- Sub Base Course
- Base Course
- Surface Course
- Need of Drainage in Roads
- Final Words on Roads Construction Procedure
India proudly has World’s 2nd-largest road network. The total length of Indian roads is 5.89 million kilometres (as of August 2021). The road network has increased at a CAGR of 17% from 2016-21. For the growth momentum continuation, the Government of India to spend 18% of the capital (in FY 2019-26) on highway development.
Consequently, the increasing roads network has given rise to employment as well as the economy. Further, the demand for highway construction professionals is the highest ever. Because a qualified and experienced professional maintains the balance between scope, time and cost without compromising the quality and safety. Moreover, they are also well versed with road construction materials and road construction procedures.
Road Construction Step by Step
This section explains the process of road construction step by step. The process is for a greenfield project. A new project is a greenfield project. In contrast, the rehabilitation, expansion and reconstruction projects are brownfield projects.
The layers of material laid over each other are the basis of highway construction. Each layer has a designed thickness, material specification and properties. The California Bearing Ratio (CBR) tests the material’s resistance to determine the material layer thickness for a given load.
The below heads explain the road construction step by step.
Earthwork including grading and sloping of subgrade
A subgrade is the lowest layer of a road. It is the foundation for the road. The soil is a subgrade material.
The process of preparing the subgrade is as follows.
- Clean the ground of plants, bushes, organic matter etc.
- Mark the road dimensions like alignment, pavement width and the footpath on the ground.
- Excavate the ground if the road level is lower than the ground. For the road level higher than the ground level, the embankment is prepared for the road. The subgrade level equals road elevation minus the total thickness of the overlying layers.
- If the soil is loose at the desired elevation, compact it with a mechanical roller. If the soil has swelling property on water absorption, replace it with non-expansive soil.
- Grade the subgrade to replicate the top profiles of the road to the subgrade. The elevation and camber are maintained at the subgrade level to drain off the water.
A sub-base layer lays over the subgrade. The material quality and the traffic load over it determines the sub-base layer thickness. The lower the sub-base material quality, the thicker is the layer. A sub-base layer distributes the load evenly to the subgrade. It prevents the capillary action from subgrade to the base course by providing free drainage.
The sub-base material is a mixture of coarse and fine aggregates. The traffic load on the layer and the materials’ properties determine the ratio of coarse to fine aggregate. Several iterations determine the most economical and feasible sub-base material mix.
The sub-base provides stable support to the road. The method of sub-base preparation is detailed below.
- Lay the sub-base material over subgrade, maintaining the thickness of the layer. The maximum allowable variation for material thickness is 6mm for a 4.5 m stretch of road.
- Water the layer before compaction, if needed.
- Compact the layer with a tandem roller or a vibratory roller. The thickness of the sub-base layer and the roller weight determines the required roller repetitions. The number of repetitions is inversely proportional to the roller weight.
- If anywhere the sub-base settles more than the average settlement of the layer, dig down the spot. Remove the failed subgrade portion and replace it with the sub-base material and compact it.
- Check the field dry density (FDD) of the sub-base layer for every 100 sqm area. It should be at least 95% of the maximum dry density (MDD).
A base course lies over the subgrade and acts as a foundation for the surface course. The base course thickness enables it to take the traffic load. Therefore, it has stiffness and strength.
The base course is thinner than the sub base. However, the material quality of the base course is higher than the sub-base material. The base course has good quality crushed aggregates only.
- Apply a low viscous cutback bitumen over the sub base course. It is called a prime coat. A prime coat penetrates the sub-base layer and clogs the pores.
- Spread the base course material over the prime coat. Maintain the base course material thickness across the lateral and longitudinal directions. A road paver machine lays the base course.
- Compact the base course with a tandem roller or a pneumatic roller. The roller speed is not more than 5-10 km per hour.
A surface course is the top layer of the road. It bears all the wear and tear due to traffic on the road. Therefore, it is also known as wearing course. A surface course is made of a rich bituminous mix.
Here is the road construction procedure step by step for the surface course.
- Apply a tack coat over the base course. A tack coat is a thin layer of bitumen emulsion with water. It provides bonding between the base course and surface course.
- A road paver machine lays the surface course over tack coat. The machine is equipped with a material thickness control mechanism. The dump trucks dump the bituminous mix in the paver machine’s hopper.
- A tandem roller compacts the surface course by multiple passes.
- Check the surface with a straight edge. The undulations in longitudinal direction should not exceed 8mm in 300 m length of road. Also, for the lateral direction, the undulations should not exceed 4mm.
Need of Drainage in Roads
A drainage system in highways ensures road survival throughout the service life. Water causes pavement failures. Running water may cause erosion of the subgrade. Therefore, a road should always be dry, i.e., water should not accumulate on or below the pavement.
A highway should have a proper cross profile to drain off the water. The pavement has drains on either side. A drain carries the water and disposes of it suitably. It also prevents subgrade erosion.
Road Construction Materials
The road construction materials are soil, aggregates (coarse and fine), bitumen, binders, and admixtures. The use of each material is exclusive for each layer.
The application of road construction materials is in the following forms-
- Soil as naturally occurring or processed material finds its’ use as subgrade material.
- The coarse and fine aggregate mixture forms the sub-base coarse.
- The aggregate of size 0-10 mm mixed with bitumen forms the base coarse and wearing coarse.
- Bitumen as cutback bitumen, emulsion, asphalt finds application in bitumen mix, tack coat, and prime coat.
- Cement concrete is the core material for rigid pavement.
Conclusion on Roads Construction Procedure
Road construction involves multiple considerations. The finance, public convenience, technical aspects, social impact, environmental impact etc. However, the core construction procedure includes the survey and layout, geometric design, material selection, drainage design, and construction execution.
To continue the core construction activities, a road construction procedure is more or less similar for both rigid and flexible pavement. The article describes the flexible road construction procedure step by step. Furthermore, road construction materials play an important role in road stability and serviceability throughout the service life.
Nowadays, specialised additives and geotextiles find their extensive use in highway construction. Such materials improve the load-carrying capability of the road. Retaining walls also find extensive usage in road construction.
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