Introduction
Pavement is a special treatment given to natural soil to make it more stable and pliable. Among the four common means of transportation which are land, air, water, and rail, the road which is the key component of land transportation seems to be one of the most important because it leads us to our final destination. From ancient days, there have been many ways of treating roads with the use of various stable materials such as stones. In the present time treatment of roads has developed to the use of modern materials such as laterite, sand, tar, bitumen, chippings, and asphalt. These materials when arranged properly in the required order in layers over the natural soil to provide a stable and even surface for the movement of vehicles constitute the definition of Pavement.
Pavement design and construction is a complex process that must be handled with extreme care to ensure that the pavement meets its expected design life. There are basically two types of pavement – flexible pavement and rigid pavement. Other types of pavement usually come under composite pavement which incorporates some attributes of the two above. Flexible pavement is usually common especially in developing countries of the world probably because of the low initial cost of construction and riding quality. However, the riding quality of rigid pavement can be improved by overlaying it with flexible pavement which further increases its construction cost. A research work carried out by the author shows that the initial cost of constructing rigid pavement is approximately three (3) times the initial cost of constructing flexible pavement.
Prior to the construction of any civil engineering structure, design is usually carried out to ensure that specific defined functions are met. Four (4) key design aspects are usually found in pavement design. These include:
- Route or geometric design
- Materials or mix design
- Structural or thickness design
- Hydraulic or drainage design
Geometric Design
The main purpose of geometric design are rider’s comfort and safety. In this design, the Engineer works closely with the Surveyor who provides him with survey points and drawings for the design. The safe design speed, vertical and horizontal curves, and stopping and passing sight distances are all incorporated into this design. Previously, geometric design was done manually and it was very tedious. Presently, there are many software such as Autodesk Civil 3D, Civil CAD, Bentley MX Roads, and IIT Pave that can be used to accomplish the design.
Materials or Mix Design
The basic materials for flexible and rigid pavement are asphalt and concrete respectively. Therefore, the design methods are distinct. Rigid pavement requires concrete mix design. The processes for this design can be found in different codes of practice depending on the code common in one’s country. The main purpose of concrete mix design is the determination of different constituents of cement, fine aggregate, coarse aggregate, and water to achieve the desired strength of concrete.
Flexible pavement requires asphalt mix design. Asphalt mix design is a very tedious process that takes a long time to accomplish. The commonest method for this design is the Marshall Mix design method. There are other methods available such as Superpave, Hveem, and Texas gyratory method. The main purpose of the design methods is the determination of the optimum binder content.
Structural or Thickness Design
This design involves the establishment of the thickness of different layers of the pavement to ensure that the axle loads are distributed safely to the foundation of the pavement. The different layers of the pavement include subgrade, subbase, road base, base course, and wearing course. This is basically the British practice. American practice would identify the layers as subgrade, subbase course, base course, binder course, and surface course while Indian practice would identify the layers as subgrade, subbase course, base course, and surface course. There are three broad methods involved in this design:
- Empirical methods,
- Mechanistic-empirical method
- Mechanistic method.
The first method has been in use and is still common while the last two are still under development. Some empirical methods used in the design include:
- Asphalt Institute method
- AASHTO method
- CBR method
- Transport Research Laboratory (TRL) method
The California Bearing Ratio (CBR) method is common in Nigeria. The key elements required for this design using this method are subgrade CBR-value and traffic survey data.
Hydraulic or Drainage Design
This is a very important aspect that is often neglected. The reason is that water is an enemy to pavement especially in tropical countries where laterite that contains some fine clay particles are massively used in road construction. Compaction curves show us that there is an optimum moisture content (OMC) where a soil attains maximum dry density (MDD) and beyond this point, the dry density of the soil decreases drastically. This MDD and OMC are used to compact soils to determine the CBR value. After the determined CBR value has been used in design to determine other material thicknesses, necessary controls are made on-site during construction to ensure that the laboratory MDD is achieved. If the pavement is not adequately protected from water through adequate drainage facilities, excess water would permeate into the subgrade and weaken its strength and resistance under load and the pavement would soon fail.
Drainage structures usually required on highways include culverts, bridges, and drains. The data required for this design are basically hydrological data which include rainfall records for drains and river regimes for culverts and bridges.
