Road construction is the process of installing soil stabilisers, asphalt, concrete, and other materials on a defined path to create a smoothed or paved surface that vehicles can move on between two destinations.
This is a complex venture that involves paving, rehabilitation, and/or reclamation of degraded pavements to create a motorable roadway. It can take months or years of planning before work can begin depending on the size and type of the envisioned road.
How Do You Build a Road Step by Step?
While road construction methods vary depending on the type of road under construction, here’s a step-by-step guide outlining how roads are made using asphalt and/or concrete.
5 Basic Steps of Road Construction
Step 1: Planning
Step 2: Setting Out
Step 3: Earthworks
Step 4: Paving
Step 5: Quality Control
This is the initial step in any road construction project. It involves assessing the current and future traffic patterns and performing a cost-benefit study to ensure the road will serve its purpose.
Layout drawings, funding, legal, and environmental issues are all sorted out during this initial stage of road construction to ensure the project runs smoothly without running into financial or legal problems.
2. Setting Out:
Setting out refers to the process of transferring design proposals from drawings into the ground. It demarcates site boundaries, foundations, and other necessary structural parts.
In most road construction projects a series of boards are placed at intervals along the proposed line of the road. A profile board with a fixed height, often referred to as traveller, is deployed to control the excavated levels between the profile boards.
The traveller is placed in the sight-line between two level boards to ensure it can be seen before and after the excavation to adjust levels accordingly.
Like in any other construction project, the level of profile board is measured using a line level – a short spirit level suspended on a nylon string. The string is moved up or down until the bubble is centred.
This is easily the most labour-intensive procedure in road construction.
It entails the deployment of a tractor shovel, grader, or bulldozer on site to remove the topsoil before scraping and grading the site to expose the underlying ground, often known as formation level.
This is the level at which excavation ceases and construction starts.
The soil below the formation level is known as subgrade and it should be tested for strength before embarking on excavation.
If the quality of the subsoil is undesirable, the material may be removed or stabilised. If the cost of excavation of subsoil is deemed uneconomical, sand wicks and sand drains may be applied.
Sand wicks are sand-filled boreholes underneath the road embarkment. They offer greater stability to the soil by reducing the length that water travels in a drainage path to disintegrate water pressure.
On the other hand, sand drains are used to capture ground water.
It is important to provide subsoil drainage to help deal with leakage through pavements and verges from higher ground as well as periodic rise and fall of the water table.
Since the thickness of the pavement depends on the strength of the subgrade, it is important to reinforce the subgrade by removing poor material in cuttings and substituting them with selected fill, providing adequate subsoil drainage, and compacting subgrade to a high dry density.
The subgrade can also be reinforced through soil stabilisation procedures such as the use of chemicals, cement, or bituminous materials.
Since the strength of subgrade diminishes as moisture content increases, it is important to cover the surface in case it will be left exposed for some time. This can be done by covering the subgrade with a medium gauge plastic sheet with 300mm laps or spraying a bituminous binder with a sand topping.
This road construction procedure begins once the subgrade has been prepared and drainage systems fitted. Paving can either be rigid or flexible depending on the precise requirements of a project.
Rigid pavements have a higher flexural strength, longer design life, and lower maintenance costs, while flexible pavements are cheaper to erect and have a higher capacity to expand and contract with temperature variations, hence they do not require expansion joints.
i.) Rigid Paving
This consists of a reinforced or unreinforced insitu concrete slab placed over a thin granular base course. The inflexibility and strength of rigid pavements enable the loads and pressures to be dispersed over a wide area of the subgrade to reduce the potential impact.
From top to bottom, rigid paving is made up of these layers:
- Subgrade (existing soil).
- Subbase course of crushed stone with a thickness of 60 cm.
- Lubricous sheath made of polythene sheeting.
- Insitu concrete paving slab, which can be reinforced using steel fabric or re-bar.
- Asphalt or similar topping as necessary.
Rigid pavements do not have the capacity to expand and contract due to changes in temperature and moisture changes. Traverse and longitudinal joints are fitted between slabs to prevent cracking that happens as a result of restrained deformations caused by temperature and moisture variations.
The spacing of the joints is usually determined by the temperature at which the concrete is laid, the thickness of the slab, expected traffic load, and the presence or absence of slab reinforcement.
ii.) Flexible Paving
Flexible pavements are made of several layers of asphalt or bituminous material overlying the ready subgrade to which all the traffic loads are distributed. They are called “flexible’ because the entire pavement structure bends or deflects when subjected to traffic loads.
The thickness of every individual layer must have the capacity to distribute loads to avert permanent deformation of the road surface.
In flexible paving, the subgrade is compressed with the subbase on top. The subbase layer is usually made of crushed stone or dry lean concrete that is laid and compressed by a road roller machine.
The subbase should not exceed 15 cm and is usually placed once waterproofing is completed.
Above the subbase is the surfacing layer, which consists of the base layer and the wearing course.
The wearing layer is the topmost layer of bituminous material and is often thicker and stronger than the base layer. Its thickness depends on the material requirements and the anticipated traffic loads.
Common materials used for the wearing course include porous asphalt, hot rolled asphalt, dense bitumen tar macadam, and dense bitumen macadam.
On the other hand, the sub layer is usually at least 6 cm thick and is made up of dense bitumen macadam or asphalt. It is applied with the suitable crossfalls and gradients.
5. Quality Control
After a road surface has been put in place, a series of quality tests must be performed before the road construction project can be deemed complete. This procedure involves checking to confirm drainage, grading levels and other aspects of the road are satisfactory.
Once all the checks return positive results, the road can now be opened for use by motorists. Most roads can last for up to 40 years – with major upgrade works due every decade or so.