Road construction
Road construction is the act of designing, building and maintaining pavements and upper layers of transportation routes. Road pavements can be made of unbound materials such as soil and gravel, though commonly roads are surfaced with asphalt or concrete.
Design
Road design consists of three technical aspects:
- geometrical road design: design of the line management of the road, including environmental issues.
- structural road design: design of the body of the road
- road safety: design of the safety features of and next to a road
Besides the technical sides of the design, planning and juridical issues must be considered.
Construction
The construction of a road usually starts with the preparation of the construction area in order to build up the pavement. Removal of earth and rock by digging or blasting, construction of embankments, bridges and tunnels, and removal of vegetation are often needed. A variety of road building equipment is employed in road building.
Once these activities are completed, the construction of the road bed begins by compaction of the native soil, known as the subgrade. Weak soils may be stabilized with additives such as portland cement or quicklime, the use of geotextiles, or dug out and replaced with imported soils.
Once the subgrade is prepared, a base course consisting of gravel or crushed stone is usually placed on the subgrade and compacted. High-traffic-volume roads will often have multiple layers of base courses, to reduce expense or increase performance. Depending on the road type, different kinds of layers are build on top of the base course, e.g. asphalt concrete or portland cement concrete. Unbound materials such as soil or gravel are put on low-stressed roads such as farm tracks and country lanes. Roads made of asphalt or concrete provide a smooth and high-friction surface and strengthen the pavement structure by spreading out the vehicle loads applied to the subgrade and are therefore used for high-stressed roads.
Maintenance
Like all structures, roads wear out over time. This effect is primarily due to accumulated damage from vehicles, however environmental effects such as frost heaves, thermal cracking and oxidation often contribute. According to a series of experiments carried out in the late 1950s, called the AASHO Road Test, it was empirically determined that the effective damage done to the road is roughly proportional to the 4th power of axle weight. Therefore, trucks are considered to be the main cause of pavement deterioration, as they have individual axle loading between 5 and 9 times that of most passenger cars.
Maintenance of a road can be divided in several types, from fixing small failures to a complete reconstruction of the road. Usually, road building equipment is used to replace the top material layers and to rebuild them.