Rich bottom bituminous pavement

Increased binder content above the optimum content can appreciably enhance the fatigue life.

Higher bitumen content increases the thickness of the binder film between aggregates resulting in lower stress in the binder film, and thus the fatigue life is improved (Sousa et al., 1998; Harvey et al., 1996).

However, with increased amount of binder content, the bituminous mix tends to be softer and thereby its stiffness modulus value may fall.

A mix designer's objective would be to achieve both high stiffness and high fatigue life.

This mutually contradictory situation can be handled by using a bituminous pavement layer where it is made richer in binder content towards the bottom layer(s).

Since fatigue cracks start from bottom of bituminous layer, higher bitumen content helps to give greater restraint against fatigue cracking.

This concept has been termed as 'rich bottom pavement' (Monismith et al., 2001; Harvey et al., 1997; Harvey and Tsai 1996).

Figures 8 and 9 provide two such options of achieving this condition. In Figure-8, quantity of bitumen is used more towards the bottom of the layer. In Figure-9, two different bituminous mixes are used in two layers, in which use of softer grade of bitumen comes out to be the best alternative (shown as Alternative-II in the figure).

Fig. 8 Rich bottom pavement

Fig. 9 Two grades of bitumen used in two layers