HMA consists of two basic ingredients:
aggregate and
asphalt binder. HMA mix design is the process of determining what aggregate to use, what asphalt
binder to use and what the optimum combination of these
two ingredients ought to be. There are several different methods used to
go about this process, of which the Hveem, Marshall and Superpave methods are
the most common. The mix design fundamentals discussed here are applicable
to all mix design methods.
Figure 1: Mix Design - adding asphalt, mixing, making a
sample
HMA is a complex material upon which many different, and sometimes
conflicting, performance demands are placed. 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:
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 in the final HMA product (Roberts et al., 1996):
Deformation resistance. HMA should not distort (rut)
or deform (shove) under traffic loading. HMA deformation is related to
aggregate surface and abrasion characteristics, aggregate gradation, asphalt
binder content and asphalt binder viscosity at high temperatures.
Fatigue resistance. HMA should not crack when
subjected to repeated loads over time. HMA
fatigue cracking is related
to asphalt binder content and stiffness.
Durability. HMA should not age excessivly
during production and service life. HMA durability is related to
air
voids as well as the asphalt binder film thickness around each aggregate
particle.
Moisture
damage resistance. HMA should not degrade substantially from
moisture penetration into the mix. Moisture damage resistance is related
to air
voids as well as aggregate mineral and chemical properties.
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
aggregate characteristics or high asphalt binder content.
Workability. HMA must be capable
of being placed and compacted with reasonable effort. Workability is
generally related to aggregate texture/shape/size/gradation, asphalt binder
content and asphalt binder viscosity at mixing and
laydown temperatures.
No matter what specific method is used, the basic mix design procedure
remains basically the same. All mix design processes involve three basic steps:
Aggregate selection. Different agencies/owners specify different
methods of aggregate acceptance. Typically, a battery of
aggregate
physical tests is run periodically on each particular aggregate source.
Then, for each mix design,
gradation and
size requirements are checked.
Normally, aggregate from more than one source is required to meet gradation
requirements.
Asphalt binder selection. Different authorities can and do
specify different methods of asphalt binder evaluation. In Washington
State, most agencies/owners use the
Superpave PG system. Previously,
WSDOT had used aged residue (AR) viscosity grading.
Optimum asphalt binder content determination. Mix design
methods are generally distinguished by the way in which they determine
the optimum asphalt binder content. This process can be subdivided into:
Make several trial mixes with different asphalt binder contents.
Compact these trial mixes in the laboratory. This compaction is
meant to be a rough simulation of actual field
conditions.
Run laboratory tests to determine key sample characteristics.
Pick the asphalt binder content that best satisfies the mix design
objectives.
The end result of a successful mix design is a recommended mixture of
aggregate and asphalt binder. This recommended mixture, which includes
aggregate gradation and asphalt binder type is often referred to as the job mix
formula (JMF). Although the JMF may subsequently be altered based on field
performance, the mix design provides the initial JMF. For HMA manufacturing, target values of gradation and
asphalt binder content are specified based on the JMF along with allowable
specification bands to allow for inherent material and production variability (see Table
1
and Figure 2). These target values and
specification bands are based on the JMF and not any general HMA gradation
requirements. Thus, the mix designer is allowed substantial freedom in
choosing a particular gradation for the JMF and then the manufacturer is
expected to adhere quite closely to this JMF gradation during production.
Table 5.1: An Example Job Mix Formula (JMF) with
Specification Bands
Sieve Size
3/4 inch
1/2 inch
3/8 inch
No. 4
No. 8
No. 30
No. 200
JMF
100
96
70
27
20
14
11.1
Specification Bands
99 - 100
90 - 100
64 - 75
23 - 28
16 - 24
10 - 18
9.1 - 12.0
Figure 2: Job Mix Formula (JMF)
with Specification Bands Example
WAPA Pavement Note on Mix Design
in Washington State
Any particular mix design is specific to the
material with which it was designed. In other words, a mix design is
unique to its asphalt binder grade and aggregate source. Therefore a mix
design produced from a sand and gravel pit in Ellensburg may not be appropriate
for HMA produced using aggregate from a Granite Falls quarry. This does
not, however, imply that either aggregate source is better; they are just
different.
Depending upon the specifying agency/owner, mix
design requirements can vary. The typical specifications and procedures
are listed below by agency/owner:
WSDOT. WSDOT provides an approved mix design
for each mix used on State paving projects. WSDOT is responsible for
determining the optimum asphalt content needed for the contractor-submitted
aggregate and asphalt binder. Thus, the contractor actually sends in his/her proposed
aggregate and asphalt binder type, and WSDOT runs the proposal through the mix
design process. WSDOT approves these mix designs for one calendar year.
Mix designs are almost always intended to produce one of the
standard WSDOT HMA Hveem or
Superpave
mixes.
Cities and Counties. Requirements vary. Typically, a city or county will require a mix design.
Some cities and counties may actually contract out to WSDOT for a mix design
while some may allow a previously approved WSDOT mix design to be used. In
general, it costs between $3,000 and $4,000 to develop a mix design. Given
the option, paving contractors typically use one of
their existing WSDOT mix designs or one that they know works well for the given
material and application.
Private owners. Requirements vary.
Generally, contractors are allowed more freedom to select their own mix design,
however it generally comes from an existing WSDOT mix design or one that they
know works well for the given material and application. It is also
common practice for private paving applications to use a
Modified Class B
(Commercial Mix) to allow for a smoother surface texture than is typically
achieved using WSDOT mix designs.
