Design Principle Based

This section discusses about the design principle based innovative applications of road materials. Discussion has been divided into two parts viz.,

• Structural design considerations, and
• Mix design considerations

Optimum pavement design thickness

In Mechanistic-Empirical pavement design, generally sustainability of a pavement structure against fatigue and rutting failures is considered, for which the critical responses are: (a) the tensile strain at the bottom fibre of bituminous layer and (b) the vertical strain at the top of the subgrade. A number of design thickness combinations of bituminous and granular layers are possible which satisfies the above mentioned requirement.

Standard design charts developed by various organizations (Shell 1978; Austroads 1992; Asphalt Institute 1981; IRC:37-2001) are available; these design charts generally provide thickness composition of bituminous and granular layers, depending upon other input parameters viz. temperature, traffic, design life, subgrade strength, material type etc. A designer can choose any suitable granular layer thickness, and, corresponding thickness of bituminous layer to safeguard against structural failures can be read from these charts.

Fig 5. Typical pavement design chart

POINT A - Safe from rutting but oversafe from fatigue considerations.

POINT B- Safe from rutting but unsafe from fatigue considerations.

POINT C- Safe from fatigue but insafe from rutting considerations.

POINT D- Safe from fatigue but oversafe from rutting considerations.

POINT E - Unsafe from both rutting and fatigue considerations.

Point F- Oversafe from both rutting and fatigue considerations

POINT O- Just safe from both rutting and fatigue considerations.

Figure 5 illustrates a typical design chart. The design chart consists of two curves: fatigue curve and rutting curve. The fatigue curve shown as COD in Figure 5 represents the points, which are just safe from fatigue consideration. Similarly, the rutting curve shown as AOB in Figure 5 represents those points which are just safe from rutting consideration. Figure 5 shows various points like A, B, C, D, etc. They are safe, oversafe or unsafe from fatigue or rutting considerations. The reader can point the cursor on the respective points to know about their status. In the design chart the fatigue curve and the rutting curve intersects at a point (point O in this case) that may be called as structurally balanced design point (Narasimham, Misra and Das, 2001). Thickness of pavement layers chosen according to this point will result in a pavement deign which would fail due to fatigue and rutting simultaneously and can be assumed that the pavement materials are fully utilized. Similarly, there could be cost optimal point, where bituminous and granular layer thicknesses are selected such that the total cost of materials used is minimized, without compromising with the structural adequacy of the pavement. The cost optimal point may or may not coincide with the structurally optimal point (Narasimham et al. 2001).