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CHAPTER 7
Selection of Asphalt Concrete
Mix Type
The selection of an appropriate HMA mixture for a specific paving application is important
in designing new pavements and for rehabilitation strategies for existing pavements. The type of
mixture selected for the various layers of a pavement has a major effect on the cost, constructability,
and long-term performance of the pavement. Mixtures with lower binder contents and lower
quality aggregates are less expensive. To facilitate placement and compaction, thinner layers
should be made with smaller nominal maximum aggregate size mixtures, while thick base
layers should be made with larger nominal maximum aggregate sizes. Mixtures at the surface
of a pavement should have relatively high binder content to make them more resistant to
the damaging effects of traffic and the environment. Lower binder contents can be used in mix-
tures for intermediate and base courses because they are protected by the layers above them.
Careful consideration of mix type is an important factor when staged construction is used,
because the base or intermediate courses must serve temporarily as the surface during the first
stages of construction.
This chapter provides recommendations for mixture type selection considering traffic, environ-
ment, constructability, and economics. It discusses the appropriate use of the three HMA mix types
that can be designed using the procedures presented in this manual: dense-graded, gap-graded
(GGHMA), and open-graded friction course (OGFC). Although the types of mixtures to be used
in a project are usually selected during the design phase, it is important that mixture designers
understand the rationale behind the selection of mixtures for specific applications. In some cases,
the engineer responsible for a mix design may be asked to suggest a mix type for a given application.
The recommendations presented in this chapter largely follow those contained the National
Asphalt Pavement Association (NAPA) Publication IS 128, HMA Pavement Mix Type Selection
Guide. The interested reader should refer to this publication for additional information concerning
mixture type selection.
Pavement Structure and Construction
As discussed in Chapter 2, asphalt concrete pavements are engineered structures consisting of
multiple layers or courses of hot-mix asphalt (HMA) and other materials. The structural HMA
layers are usually referred to as surface, intermediate, and base courses depending on their location
in the pavement structure. The intermediate course is sometimes called the binder course. Some
pavements with higher traffic volumes may also include a wearing course composed of OGFC
placed over the surface course. Each HMA layer in a pavement is composed of different materials
and is placed in one or more lifts using separate paving operations. Each layer has a specific
function that affects the type of mixture that should be specified and used. Figures 7-1 and 7-2
show typical cross sections for asphalt pavements commonly encountered in new construction
and rehabilitation.
91
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92 A Manual for Design of Hot Mix Asphalt with Commentary
HMA wearing course HMA wearing course
HMA intermediate course HMA intermediate course
crushed HMA base
aggregate course
base
crushed
aggregate
prepared subgrade subbase
prepared subgrade
(a) Conventional HMA Pavement
(b) Deep-Strength HMA Pavement
HMA wearing course
HMA wearing course
HMA leveling course
HMA intermediate course
PCC
HMA base crushed
course aggregate
subbase
prepared subgrade
prepared subgrade
(c) Full-Depth HMA Pavement
(d) Composite Pavement
Figure 7-1. Cross-sections for typical asphalt pavements in new construction.
As shown in Figure 7-1, there are four types of new pavements depending on the type of
base and the overall thickness of the HMA layers. Conventional flexible pavements, shown in
Figure 7-1a, consist of relatively thin layers of HMA constructed over an unbound aggregate
base. In this type of pavement, the unbound aggregate base is thick and is the major load-carrying
element in the pavement. Conventional flexible pavements are primarily used on roadways with
low traffic volumes. Flexible pavements that carry moderate to high traffic volumes are either
deep-strength or full-depth. Deep-strength HMA pavements, shown in Figure 7-1b, have a
relatively thick HMA base constructed on an unbound aggregate subbase, while in full-depth
HMA pavements, shown in Figure 7-1c, all layers above the prepared subgrade are constructed
with HMA. The HMA base is the primary load-carrying element in both of these pavement types.
The unbound aggregate subbase in deep-strength HMA pavements provides a working platform
for paving, and in some areas, additional thickness for frost protection. Composite pavements,
shown in Figure 7-1d, consist of an HMA surface constructed on Portland cement concrete (PCC).
The PCC is the primary load-carrying element in composite pavements. Composite pavements
are constructed by design in some urban areas or during lane widening on PCC rehabilitation
projects that include an HMA overlay where it is desired to maintain the same pavement cross
section in the new lanes and the existing lanes.
Perpetual pavement, a relatively new concept, is intended to provide a pavement with a very
long-lasting underlying structure combined with a durable wearing course. Ideally, the pavement
structure should last 50 years or more without replacement, while the surface course might need
replacement every 20 years. Selection of mixtures for perpetual pavements is discussed at the end
of this chapter.
Pavement rehabilitation with HMA can result in two types of pavements as shown in Figure 7-2.
Rehabilitation of existing asphalt pavements, shown in Figure 7-2a, is almost always accomplished
using an HMA overlay. Prior to constructing the overlay, areas of the pavement that exhibit
alligator or fatigue cracking must be repaired to full depth because the base of the existing
pavement remains the primary load-carrying element in the flexible pavement after construction
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Selection of Asphalt Concrete Mix Type 93
HMA overlay
HMA leveling course
full-depth existing HMA
repair pavement
crushed aggregate
subbase
subgrade
(a) HMA Overlay on Existing HMA
Pavement
HMA overlay
HMA leveling course
joint existing PCC
repair pavement
crushed aggregate
subbase
subgrade
(b) HMA Overlay on Existing PCC
Pavement with Joint Repair
HMA overlay
HMA leveling course
Rubblized PCC
base
crushed aggregate
subbase
subgrade
(c) HMA Overlay on Rubblized PCC
Pavement
Figure 7-2. Cross-sections for typical asphalt pavements
in rehabilitation.
of the overlay. If the existing surface course is in reasonably good condition, there is adequate
vertical clearance, and safety hardware can accommodate an increase in pavement elevation, the
overlay may be placed directly on the existing surface course. If the existing pavement includes
an OGFC; the surface course is rutted, cracked or highly weathered; or it is important to maintain
the existing elevation of the pavement, then the existing pavement is milled to an appropriate
depth prior to placement of the overlay. A thin leveling or scratch course of variable thickness
may be placed on the existing or milled pavement to improve smoothness prior to placing the
HMA overlay. If strengthening is required due to an anticipated change in traffic volume, an
intermediate course may also be added. Rehabilitation of existing PCC pavements with HMA
involves placing one or more layers of HMA over the PCC. The HMA may be placed directly
on the existing PCC, shown in Figure 7-2b, after repair of cracked PCC slabs and joints that
exhibit poor load transfer. When the HMA is placed directly on the intact PCC, the PCC is the
primary load-carrying element of the rehabilitated pavement. The HMA overlay is often saw cut
at the location of the PCC joints to control reflective cracking in the HMA. The saw cuts are sealed
at the time of construction. Alternatively, as shown in Figure 7-2c, the PCC slab may be broken
or rubblized to control reflective cracking. In this case much thicker HMA layers are placed over
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94 A Manual for Design of Hot Mix Asphalt with Commentary
the broken or rubblized PCC. The new HMA base serves as the primary load-carrying element
in the rehabilitated pavement. A thin leveling course of variable thickness may be placed on the
broken or rubblized PCC to improve smoothness prior to placing the HMA layers.
The sections that follow describe in greater detail the function and characteristics of each of
the HMA layers shown in Figure 7-1 and 7-2. These characteristics are important factors in the
selection of appropriate mixture types for each layer.
Surface Course
The surface course is the uppermost structural layer in an asphalt pavement. In most cases it
is the top layer of the pavement and also serves as the wearing course. Since it is directly exposed
to traffic and environmental forces, it must be produced with the highest quality materials. The
surface course provides the following characteristics for an asphalt pavement:
· Adequate wet weather friction for safety
· High resistance to load-induced rutting, shoving, and surface cracking
· High resistance to thermally induced cracking
· Low permeability to minimize surface-water infiltration
· High durability to resist disintegration due to the combined effects of aging, traffic loading,
and freeze-thaw effects
· Appropriate surface texture for noise control, safety, and aesthetics
· Smoothness.
Because the surface course is made with the highest quality materials, economics dictate that
it be the thinnest pavement layer, typically 25 to 75 mm (1.0 to 3.0 in) thick. Surface course
mixtures are typically only one lift thick and made with nominal maximum aggregate sizes of
12.5 mm or less. Smaller nominal maximum aggregate size mixtures can be placed in thinner
layers, have higher binder contents, and, when compacted to the same in-place air void content,
have lower permeability than larger nominal maximum aggregate size mixtures. Surface courses
contain highly angular aggregates and an appropriate performance-graded binder to resist
traffic and environmental forces. If the surface course is also the top layer in the pavement, then
the aggregates must be resistant to polishing under traffic loading to provide appropriate skid
resistance over the service life of the pavement. Dense-graded and GGHMA mixtures are commonly
used as surface courses.
OGFC Wearing Course
Some moderate- to high-traffic pavements may include an OGFC as a wearing course on
top of the surface course to improve skid resistance, reduce splash and spray, and reduce noise.
These characteristics of OGFCs are the result of the open pore structure of these mixtures.
OGFCs are made with durable crushed aggregates and often include modified binders and fibers
to increase the binder content and improve durability. Because OGFCs are very permeable, the
surface course directly beneath them must be impermeable to minimize infiltration of water into
the pavement structure. To avoid trapping water in the pavement structure, OGFCs should be
daylighted at the shoulders and milled from the pavement before placing future overlays.
Intermediate Course
The intermediate or binder course consists of one or more lifts of HMA between the surface and
base courses. Not all pavements have an intermediate course; the need for an intermediate course
depends on the overall thickness of the HMA and the thickness of the base and surface courses.
