Mix design considerations
Non-standard gradation
Though fatigue performance is considered in structural design of bituminous pavements, as per Indian practice, it is not taken into account while designing the mix.
The fatigue life of the mix can be increased by increasing the bitumen content.
But, Voids in Mineral Aggregates (VMA), being fixed for a given gradation and compaction level, increase in bitumen content will result in less Air Voids (VA), which is undesirable for a mix.
Thus, there is a need to deviate from the specified gradation in order to come up with a new gradation, which possibly can give rise to better fatigue performance, yet without compromising with the VMA and Marshall-stability requirements.
Laboratory study on non-standard mixes (as per Indian specifications) showed better performance in terms of fatigue life without compromising with the Marshall and stiffness requirements (Das et al. 2004).
Stone matrix asphalt
Stone matrix asphalt (SMA) is a gap-graded bituminous mix with high percentage of coarse aggregates with high bitumen content. Gap gradation aims at maximizing stone-to-stone contact. This gives a structurally strong mix due to efficient load distribution through the stone-matrix skeleton. The drawback of this method is the absence of medium sized aggregates due to gap
gradation. This may arise possibility of drain-down of low-grade penetration bitumen through the stone matrix . To check this possibility, modifiers, such as cellulose fibers, are used to hold the bitumen in place (Better Roads 2003; GDOT 1995; Decoene et al., 1990).
Research efforts of Georgia Department of Transportation (GDOT) and European experience, has proven that SMA has superior performance and gives the following benefits:
• 30% - 40% less rutting than standard mixes.
• 3 to 5 times greater fatigue life in laboratory experiments.
• lower life cycle cost. |
