Mix design considerations
Non-standard gradation
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.
However one can 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.
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).
|
