Construction of roads is quite a vast subject. Here, some salient areas or aspects of construction of roads with flexible pavement has been discussed briefly. Purpose is to enable one to quickly have an overall grasp on practical road construction based primarily on British and American specifications.
Unlike a rigid pavement where the load bearing layer is made up of concrete, a flexible pavement has load bearing layer made up of compacted aggregates only. Since, there are no binding materials like cement or bitumen in these layers these are sometimes termed as unbound layers as well.
A typical road formation consists of a lane or several lanes plus two shoulders on both sides plus two verges or slopes on either sides that eventually meet the ground level. While the shoulders too are sometimes sealed with bitumen, quite often they are left unsealed. They are slightly sloped outwards as per requirements.
When a section is considered through this formation we get a road section that is made up of a sub-grade plus the overlying layers. The sub-grade is earthen which is formed by either cutting existing natural ground to required levels or by filling selected earth-fill in compacted layers to the required levels.
All the overlying layers as a whole is called the pavement of a road in technical parlance. In common parlance however, a path with a hard surface, meant for the pedestrians, at the side of a road is widely called as a pavement. The discussion here is about the former one.
A common pavement usually consists of the following: a) A granular sub-base b) Aggregate base course c) Bituminous seal. The bituminous seal consists of a) Tack coat & Prime coat on top of aggregate base course b) Bituminous base course c) Bituminous wearing course.
To put it in a nutshell,
Cross-section of a flexible pavement road = sub-grade plus the overlying layers (i.e., the pavement)
Pavement = granular sub-base plus aggregate base course plus bituminous seal.
Bituminous seal = tack and prime coats plus bituminous base course plus bituminous wearing course.
The usual construction practice is to complete the road section upto the bituminous base course during construction phase. This is mainly to protect the underlying unbound layers as well as to minimise dust generation. The wearing course, if laid during the construction phase, is most likely to be damaged due to movement of heavy machinery and hence, is laid at the end of the construction phase. Bituminous seal often conforms to local specifications as well in order to suit local environmental conditions.
The pavement structure may vary to some extent, in terms of layer thickness or number of layers etc., from place to place or project to project but the basic concept remains almost the same as discussed above. Construction of flexible pavement will be discussed in the next post (Part 2).