Four (4) key factors are usually considered in the design of rigid pavement. These include:
1. Amount, type and weight of present and anticipated traffic
The volume and character of the present and anticipated traffic would determine the number and width of the traffic lane. The weight and frequency of wheel loads would guide the slab structural design.
2. Supporting power and character of the subgrade
In the design, the supporting power of subgrade is evaluated based on the modulus of subgrade reaction, k which is usually gotten from plate load/bearing tests. The k-value represents the load (kN) per square metre (m) on a loaded area divided by the deflection (m) of the subgrade under the load. The unit of the k-value is kN/m2/m.
3. Climate
The climatic region to which the project is located determines the type and degree of exposure the pavement would receive. With this knowledge, established procedures would be followed to design the concrete mix that will have the required durability as well as strength. The climate region also tends to indicate the type of soil that may be encountered in the project. Soils that may be satisfactory subgrade in one climate may be unstable or troublesome under another climate because of differences in temperature, humidity and rainfall. The knowledge of climate and soil type is very important when determining the need for the type and thickness of base or treatment to be used for the subgrade.
4. Concrete
Flexural strength (often known as modulus of rupture) of the concrete is used in the design of road (rigid) pavement. This is unlike the compressive strength we are often acquainted with and which can be found in many sources. The reason for the use of flexural strength is because when a concrete pavement slab is loaded to the point that a break point develops, the concrete fails in flexure. It is designed in such a way that the flexural strength capacity would not be exceeded under load. Generally, concrete have small flexural strength when compared to its compressive strength. Table 1 shows three grades of flexural strength of concrete based on BS 5328 (Table 2). The minimum grade of concrete for rigid pavement is grade 40 (1:1/2:1)
Table 1; Flexural strength grades of concrete
Based on HD 26/06 (DMRB 7.2.3) rigid pavement can be classified as
- Continuously Reinforced Concrete Pavement (CRCP), (normally with an asphalt overlay of minimum thickness 30mm); or
- Continuously Reinforced Concrete Base (CRCB) with an asphalt overlay of 100mm.
Other forms of rigid construction, permitted for limited use and in special circumstances but only with approval from the Overseeing Organisation, include:
- Unreinforced Jointed Concrete (URC);
- Jointed Reinforced Concrete (JRC).
