Perpetual pavement

A perpetual bituminous pavement may be defined as a pavement designed and built to last longer than fifty years without requiring major structural rehabilitation or reconstruction (APA101 2001). This pavement may only require periodic replacement of top wearing surface and recycling of old pavement material (TRL 2001; AA-2 2001).

The concept of full depth bituminous pavement is in vogue from 1960s in USA. Nunn and his associates of Transport Research Laboratory, UK found (Nunn et al., 1997) that thick bituminous pavements tend to show long lasting performance and may require only minor surface repairs. California Department of Transportation in collaboration with University of California, Berkeley (Monismith et al., 2001) first implemented concept of perpetual pavement in a rehabilitation planning project.

In full depth bituminous pavement, the thickness is so designed that the fatigue and rutting strains developed are below the permissible limit (MS-1 1999 ). If the thickness is chosen to be sufficiently large so that the fatigue strain is close to the endurance limit, then the fatigue life becomes very long, and the pavement may be said to have attended 'perpetual life'. A perpetual pavement, in general, is made up of the following layers:

• The top wearing surface is designed in such a way that it is water-tight as well as removable and hence replaceable. Stone Matrix Asphalt (SMA) or Open Graded Friction Course (OGFC) are recommended. They also produce less noise due to tyre-pavement interaction.
• The intermediate layer is constituted with good quality aggregates and designed to be strongly resistive to rutting.
• The bottom part is made resistant to fatigue cracking by making it rich in bitumen and choosing a gradation that has less voids.

Figure-6 schematically represents the layer composition of a typical perpetual pavement.

A perpetual pavement is a full depth bituminous pavement in most of the cases. The principles based on which it is designed (mix design and structural thickness design) are the following:

• The pavement layers are chosen in such a way that they are rut resistive. The pavement is chosen to be adequately thick such that the vertical subgrade strain is low. Since subgrade contributes to the major part of rutting, low vertical subgrade strain would cause low level of rutting.
• The wearing surface should be adequately water-proof. The surface should be so designed that it can be repaired or recycled and the whole pavement will not require any major reconstruction (AA-2 2001).
• The thickness of the bituminous layer is chosen in such a way that the horizontal tensile strain ( e t ) developed is less than the endurance limit (refer Figure-7) of the bituminous mix, hence its laboratory fatigue life (N) becomes infinity (AA2-2001, Nunn et al. 1997). It is justifiable to design the pavement as 'bottom rich' (refer to next section), which shifts the endurance limit to higher level.
• The temperature gradient tends to be steeper towards the surface of the pavement (TRL 2001, Newcomb 2001) as shown schematically in Figure-7. Therefore the bituminous mixes with temperature susceptible binder should be avoided as surface course. Use of modified binder could be helpful in this regard.