The quality of the roadways is determined by its behaviour. For
example it must resist deformation and cracking, be durable over
time, resist water damage, provide a good tractive surface, and yet
be inexpensive, readily made and easily placed. In order to meet
these demands, the mix designer can manipulate all of three
variables:
- Aggregate. Items such as type (source), gradation and size,
toughness and abrasion resistance, durability and soundness, shape
and texture as well as cleanliness can be measured, judged and
altered to some degree.
- Asphalt binder. Items such as type, durability, rheology,
purity as well as additional modifying agents can be measured,
judged and altered to some degree.
- The ratio of asphalt binder to aggregate. Usually expressed in
terms of percent asphalt binder by total weight of HMA, this ratio
has a profound effect on HMA pavement performance. Because of the
wide differences in aggregate specific gravity, the proportion of
asphalt binder expressed as a percentage of total weight can vary
widely even though the volume of asphalt binder as a percentage of
total volume remains quite constant
By manipulating the variables of aggregate, asphalt binder and
the ratio between the two, mix design seeks to achieve the
following qualities:
- Deformation resistance (stability). HMA should not distort
(rut) or deform (shove) under traffic loading. HMA deformation is
related to one or more of the following:
- Aggregate surface and abrasion characteristics.
Rounded particles tend to slip by one another causing HMA
distortion under load while angular particles interlock with one
another providing a good deformation resistant structure. Brittle
particles cause mix distortion because they tend to break apart
under agitation or load. Tests for particle shape and texture as
well as durability and soundness can identify problem aggregate
sources. These sources can be avoided, or at a minimum, aggregate
with good surface and abrasion characteristics can be blended in to
provide better overall characteristics.
- Aggregate gradation. Gradations with excessive fines
(either naturally occurring or caused by excessive abrasion) cause
distortion because the large amount of fine particles tend to push
the larger particles apart and act as lubricating ball-bearings
between these larger particles. A gradation resulting in low VMA or
excessive asphalt binder content can have the same
effect. Gradation specifications are used to ensure
acceptable aggregate gradation.
- Asphalt binder content. Excess asphalt binder content
tends to lubricate and push aggregate particles apart making their
rearrangement under load easier. The optimum asphalt binder content
as determined by mix design should prevent this.
- Asphalt binder viscosity at high temperatures. In the
hot summer months, asphalt binder viscosity is at its lowest and
the pavement will deform more easily under load. Specifying an
asphalt binder with a minimum high temperature viscosity (as can be
done in the Superpave asphalt binder selection process) ensures
adequate high temperature viscosity.
- Fatigue resistance. HMA should not crack when subjected to
repeated loads over time. HMA fatigue cracking is related to
asphalt binder content and stiffness. Higher asphalt binder
contents will result in a mix that has a greater tendency to deform
elastically (or at least deform) rather than fracture under
repeated load. The optimum asphalt binder content as determined by
mix design should be high enough to prevent excessive fatigue
cracking. The use of an asphalt binder with a lower stiffness will
increase a mixture’s fatigue life by providing greater flexibility.
However, the potential for rutting must also be considered in the
selection of an asphalt binder. Note that fatigue resistance is
also highly dependent upon the relationship between structural
layer thickness and loading. However, this section only addresses
mix design issues.
- Low temperature cracking resistance. HMA should not crack when
subjected to low ambient temperatures. Low temperature cracking is
primarily a function of the asphalt binder low temperature
stiffness. Specifying asphalt binder with adequate low temperature
properties (as can be done in the Superpave asphalt binder
selection process) should prevent, or at least limit, low
temperature cracking.
- Durability. HMA should not suffer excessive aging during
production and service life. HMA durability is related to one or
more of the following:
- The asphalt binder film thickness around each aggregate
particle. If the film thickness surrounding the aggregate
particles is insufficient, it is possible that the aggregate may
become accessible to water through holes in the film. If the
aggregate is hydrophilic, water will displace the asphalt film and
asphalt-aggregate cohesion will be lost. This process is typically
referred to as stripping. The optimum asphalt binder content as
determined by mix design should provide adequate film
thickness.
- Air voids. Excessive air voids (on the order of 8
percent or more) increase HMA permeability and allow oxygen easier
access to more asphalt binder thus accelerating oxidation and
volatilization. To address this, HMA mix design seeks to adjust
items such as asphalt content and aggregate gradation to produce
design air voids of about 4 percent. Excessive air voids can be
either a mix design or a construction problem and this section only
addresses the mix design problem.
- Moisture damage resistance. HMA should not degrade
substantially from moisture penetration into the mix. Moisture
damage resistance is related to one or more of the following:
- Aggregate mineral and chemical properties. Some
aggregates attract moisture to their surfaces, which can cause
stripping. To address this, either stripping-susceptible aggregates
can be avoided or an anti-stripping asphalt binder modifier can be
used.
- Air voids. When HMA air voids exceed about 8 percent
by volume, they may become interconnected and allow water to easily
penetrate the HMA and cause moisture damage through pore pressure
or ice expansion. To address this, HMA mix design adjusts asphalt
binder content and aggregate gradation to produce design air voids
of about 4 percent. Excessive air voids can be either a mix design
or a construction problem and this section only addresses the mix
design problem.
- Skid resistance. HMA placed as a surface course should provide
sufficient friction when in contact with a vehicle’s tire. Low skid
resistance is generally related to one or more of the following:
- Aggregate characteristics such as texture, shape, size and
resistance to polish. Smooth, rounded or polish-susceptible
aggregates are less skid resistant. Tests for particle shape and
texture can identify problem aggregate sources. These sources can
be avoided, or at a minimum, aggregate with good surface and
abrasion characteristics can be blended in to provide better
overall characteristics.
- Asphalt binder content. Excessive asphalt binder can
cause HMA bleeding. Using the optimum asphalt binder content as
determined by mix design should prevent bleeding.
- Workability. HMA must be capable of being placed and compacted
with reasonable effort. Workability is generally related to one or
both of the following:
- Aggregate texture, shape and size. Flat, elongated or
angular particles tend to interlock rather than slip by one another
making placement and compaction more difficult (notice that this is
almost in direct contrast with the desirable aggregate properties
for deformation resistance). Although no specific mix design tests
are available to quantify workability, tests for particle shape and
texture can identify possible workability problems.
- Aggregate gradation. Gradations with excess fines
(especially in the 0.60 to 0.30 mm (No. 30 to 50) size range when
using natural, rounded sand) can cause a tender mix. A gradation
resulting in low VMA or excess asphalt binder content can have the
same effect. Gradation specifications are used to ensure acceptable
aggregate gradation.
- Asphalt binder content. At laydown temperatures (above
about 120 °C (250 °F)) asphalt binder works as a lubricant between
aggregate particles as they are compacted. Therefore, low asphalt
binder content reduces this lubrication resulting in a less
workable mix. Note that a higher asphalt binder content is
generally good for workability but generally bad for deformation
resistance.
- Asphalt binder viscosity at mixing/laydown
temperatures. If the asphalt binder viscosity is too high at
mixing and laydown temperatures, the HMA becomes difficult to dump,
spread and compact. The Superpave rotational viscometer
specifically tests for mixing/laydown temperature asphalt binder
viscosity.