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CHAPTER 5
Mixture Volumetric Composition
Understanding the terminology used to describe the volumetric composition of asphalt
concrete and the ability to perform a volumetric analysis are two of the most important skills that
engineers and technicians must master in order to develop effective asphalt concrete mix designs.
Most of the specifications for HMA and related materials used throughout the United States and
Canada are expressed, at least in part, in terms of volumetric composition. This chapter describes
the basic terminology used in volumetric analysis, including such important concepts as air void
content, voids in the mineral aggregate, and apparent film thickness. Besides basic definitions of
such terms, an effort has been made to briefly describe how each of the main mix composition
factors affect pavement performance. Because accurate bulk and maximum theoretical specific
gravity data are essential to performing volumetric analysis, these tests are described in some
detail in this chapter. Equations are presented for performing a complete volumetric analysis,
and a detailed example problem is given at the end of the chapter. Because most engineers and
technicians use spreadsheets and similar tools to perform the calculations involved in volumetric
analysis of HMA mixtures, detailed reading of the equations and example problems will not be
necessary for many readers of this manual; however, the equations and example problem are
presented to (1) make the chapter complete and (2) for engineers and advanced technicians who
wish to have a thorough understanding of volumetric analysis or who are interested in developing
their own customized spreadsheets for performing volumetric analysis.
Composition Factors
Asphalt concrete primarily consists of three different components or phases: aggregate, asphalt
binder, and air. Materials like concrete, which consist of particles held together by a cement of
any type, are called composites. Some asphalt concrete mixtures contain small amounts of other
additives, such as cellulose fibers, mineral fibers, ground rubber, and polymers. Although such
additives may affect workability and performance significantly, these additives almost always
represent a very small percentage of the overall volume and mass of the asphalt concrete. Engineers
and technicians should remember the three major components of asphalt concrete--aggregate,
asphalt, and air. These three components are the key to understanding volumetric analysis.
The composition of asphalt concrete can be described in terms of either weight or volume.
The asphalt binder content of a mixture, for example, is often given in terms of percent of total
mix weight, whereas air void content is always given as a percent of total volume--it must be given
this way, since the mass of air voids in an asphalt concrete specimen is essentially zero. Although
composition of asphalt concrete mixtures can be given in terms of weight, traditionally the most
common and most important method of describing and analyzing asphalt concrete composition
is by volume. This is what is meant by the term "volumetrics" or "volumetric analysis" of asphalt
concrete--characterizing the composition of an asphalt concrete mixture by relative proportions
46
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Mixture Volumetric Composition 47
100% Voids
Asphalt
80%
Aggregate
60%
40%
20%
0%
Lean Surface Stone
Base Course Matrix
Course Asphalt
Figure 5-1. Typical composition by
volume of different HMA mix types.
by volume of aggregate, asphalt, and air voids. Although this may sound like a simple task, it can
become quite complicated when absorption of asphalt by the aggregate must be accounted for
or when only incomplete information on a mixture is available. It is essential that engineers and
technicians responsible for developing asphalt concrete mix designs or performing quality
control operations have a thorough understanding of volumetric analysis.
Typical asphalt concrete mixtures, as designed in the laboratory, contain about 84 to 90%
aggregate, 6 to 12% asphalt binder, and about 4% air voids by volume. Figure 5-1 illustrates
the typical composition of three different types of HMA--in all cases, almost all of the mixture
volume is made up of aggregate. Asphalt concrete is mostly composed of aggregate. If the volume
percentage of two of these components is known, the other can be determined by subtraction.
For example, if we know that a mixture is to be designed with 4% air voids and 10% asphalt binder
by volume, we can calculate the amount of aggregate required as 100 - (4 + 10) = 86% by volume.
It is important to remember this when specifying the composition of asphalt concrete. It would be
impossible, in this example, to specify a mixture with 4% air voids, 10% minimum binder content
and 87% minimum aggregate content by volume--a mixture with a minimum of 87% aggregate by
volume could have no more that 13% total volume of air voids and asphalt binder. This is only one
example of the complex interrelationships involved in characterizing and specifying the volumetric
composition of asphalt concrete. More examples are given throughout this chapter. But first, we will
discuss in more detail different volumetric factors used in volumetric analysis.
Air Voids
"Air voids," when applied to asphalt concrete, means small pockets of air that exist within the
asphalt binder and between aggregate particles. Air void content does not include pockets of air
within individual aggregate particles, or air contained in microscopic surface voids or capillaries
on the surface of the aggregate. Figure 5-2 shows the different ways in which air exists in asphalt
concrete mixtures. Designing and maintaining the proper air void content in HMA and other
mix types is important for several reasons. When air void contents are too high, the pavement
may be too permeable to air and water, resulting in significant moisture damage and rapid age
hardening. When air void contents are too low, the asphalt binder content may be too high,
resulting in a mixture prone to rutting and shoving.
When discussing the air void content of asphalt concrete mixtures, it is first necessary to specify
what type of specimen or sample we are testing or analyzing. We can measure the air void content
of asphalt concrete in the following types of specimens:
· Specimens compacted in the laboratory when developing a mix design
· Specimens compacted in the laboratory from material produced at the plant as part of quality
assurance testing
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48 A Manual for Design of Hot Mix Asphalt with Commentary
Air voids
Pores on
aggregate
surface
Air pockets
within
aggregate
particle
Asphalt Binder
Figure 5-2. Air in asphalt concrete. Air can exist in pores on the
aggregate surface, pockets within aggregate particles, or voids within
the asphalt binder or between the binder and aggregate particles.
Only the last type of air is included in the air void content of asphalt
concrete mixtures.
· Specimens taken from the roadway as cores immediately after construction as part of quality
assurance testing
· Specimens taken from a wheel path of a roadway as cores after several years or more of traffic
loading
· Specimens taken from between the wheel paths of a roadway as cores after several years or
more of traffic loading
The typical air void content of these different types of specimens will usually vary substantially,
so it is important when discussing or specifying the air void content of asphalt concrete to make
sure it is clear what type of specimen has been tested or is required to be tested.
In the Superpave method of designing HMA, the air void content in laboratory mix designs
is held constant at 4.0%. Some agencies have, however, expanded the allowable range for air void
content to as wide as 3.0 to 5.0%, which was the design range used in the Marshall mix design
method. The in-place air void content of HMA pavement is often assumed to be about 7%, but
recent research (as reported in NCHRP Report 573) suggests that immediately after construction
the air void content of HMA pavements typically ranges from about 6 to 11%, with a median
value between 8 and 9%. Cores taken from a newly constructed pavement will generally have air
void contents in this range. However, once the pavement is opened to traffic, the repeated loading
as trucks pass over the pavement will tend to further compact the material in the wheel paths of
the pavement. Many engineers assume that the air void content in the wheel paths of an asphalt
concrete pavement should, within a few years, reach about the same value as was used in the
laboratory mix design, but, as documented in NCHRP Report 573, this is not always the case.
Cores taken outside the wheel path will undergo little or no change in air void content with
time, since their location is not subjected to traffic loading. Therefore, if there is a question about
how well an asphalt concrete pavement has been constructed several years after construction, this
can only be determined from cores taken from between the wheel paths. Samples of HMA loose
mix produced at the plant and compacted in the laboratory (commonly termed plant-produced/
laboratory-compacted) should have air void contents close to the design value. However, plant-
produced HMA often contains more mineral dust than was used in the mix design, which tends
to reduce the air void content of laboratory-compacted specimens. This reduction can be avoided
by making sure that aggregate gradations used in preparing mix designs accurately reflect what
is typically produced at the plant.
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Mixture Volumetric Composition 49
Determining air void content is one of the main purposes of volumetric analysis. Unfortunately,
there is no simple direct way to determine the air void content of an asphalt concrete specimen.
Air void content is determined by comparing the specific gravity (or density) of a compacted
specimen with the maximum theoretical density of the mixture used to make that specimen. For
example, if the compacted density of an asphalt concrete specimen is 95.3% of the theoretical
maximum specific gravity, the air void content is 100 - 95.3 = 4.7%. Mixture-specific gravity and
the volumetric analysis of asphalt concrete are discussed in more detail later in this chapter.
Binder Content
Binder content is one of the most important characteristics of asphalt concrete. Use of the proper
amount of binder is essential to good performance in asphalt concrete mixtures. Too little binder
will result in a dry stiff mix that is difficult to place and compact and will be prone to fatigue
cracking and other durability problems. Too much binder will be uneconomical, since asphalt
binder is, by far, the most expensive component of the mixture and will make the mixture prone
to rutting and shoving. Typical asphalt binder contents range from 3.0% or less (for lean base
course mixtures) to over 6.0% (for surface course mixtures and rich bottom layers), which are
designed for exceptional durability and fatigue resistance.
As mentioned earlier in this chapter, asphalt binder content is most often stated and specified
as a percentage of total mix weight. A ton of hot mix that is 5.2% asphalt binder will contain
2,000 × 0.052 = 104 pounds of binder. However, there are two problems with this way of stating
asphalt content. First and most important, it is the asphalt content by volume and not by weight
that dictates performance, and asphalt content by total mix weight is a function of both asphalt
content by volume and aggregate specific gravity. Consider two asphalt concrete mixtures,
both with an air void content of 4.0% and an asphalt binder content of 12.0% by volume.
One mixture is made with a limestone aggregate with a specific gravity of 2.50, and the other with
a dense diabase having a specific gravity of 3.20. If no asphalt binder is absorbed by the aggregate
(as we will discuss below, not a very good assumption), the asphalt content of the limestone mix
will be 5.35% by total mix weight, while the asphalt content of the diabase mixture will be 4.23%
by total mix weight. The difference in asphalt binder content by weight is over 1.0%, even though
these mixtures contain identical asphalt contents by volume! To avoid this problem, many agencies
now specify minimum binder content by weight as a function of aggregate specific gravity.
The second problem with stating asphalt binder content by total mix weight is that most
aggregates tend to absorb asphalt binder. Asphalt binder absorbed by an aggregate is tightly
held in microscopic pores on the aggregate surface and does not significantly contribute to the
durability of a mixture. The amount of absorption varies widely, depending on aggregate type.
Dense igneous rocks, such as diabase and basalt, might only absorb a few tenths of a percent of
asphalt binder from a mixture, while porous sandstones and slags might absorb from 1 to as much
as 4% of the asphalt binder from a mixture. The term "effective binder content," abbreviated as
Vbe, is used to describe the amount of asphalt binder in a mixture not including that absorbed
by the aggregate (see Figure 5-3). For example, if the total asphalt content of a mixture is 5.3%
by weight, and the aggregate absorbs 0.4% binder by total mix weight, the effective binder
content of this mixture is 5.3 - 0.4 = 4.9%. If a mixture is to be designed to have 11.0% asphalt
content by volume, not including the 1.0% of the binder absorbed by the aggregate, then the total
asphalt binder content must be 11.0 + 1.0 = 12.0% by volume.
Theoretically, the most effective way of characterizing and specifying asphalt binder content
is effective binder content by volume, since this avoids the two problems described above. However,
effective asphalt content by volume can only be determined through volumetric analysis and
cannot be determined with a high degree of precision. Asphalt concrete plants are almost always
designed to control asphalt binder content as a percentage of total mix weight. For these reasons,
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50 A Manual for Design of Hot Mix Asphalt with Commentary
Figure 5-3. Effective asphalt content. Effective asphalt
includes asphalt binder not absorbed by the aggregate--
in this sketch, the cross-hatched gray area represents
effective binder.
many agencies specify minimum total binder content by weight and give tables, graphs, or formulas
adjusting this minimum according to the aggregate specific gravity. Unfortunately, there is no
simple way to account for aggregate absorption when specifying asphalt binder content, since
absorption varies so widely among aggregate types, and even substantially within aggregate from
a given quarry. As discussed below, one way to specify effective binder content by volume is to
control both air void content and voids in the mineral aggregate at the same time.
Voids in the Mineral Aggregate
Voids in the mineral aggregate (VMA) refers to the space between aggregate particles in an
asphalt concrete mixture (see Figure 5-4). VMA is also often used to characterize loose aggregate,
Figure 5-4. Voids in mineral aggregate. Dark and light
gray areas represent aggregate particles, black area asphalt
binder and white areas air voids; voids in mineral aggregate
(VMA) is composed of asphalt binder and air voids--black
and white areas.
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Mixture Volumetric Composition 51
but its meaning is exactly the same--the volume percentage of space between aggregate particles.
VMA is numerically equal to the air void content plus the effective binder content by volume.
Therefore, establishing a single design air void content (such as the 4.0% used in Superpave mixtures)
and then controlling VMA is the same as controlling effective binder content. For example,
a Superpave 12.5-mm mixture designed at 4.0% air voids with 14.0% minimum VMA has a
minimum effective binder content of 14.0 - 4.0 = 10.0% by volume.
Some engineers and agencies have proposed that VMA should be defined as total binder
content plus air void content, both by volume. The only advantage to using this definition is that
it makes aggregates with high absorption appear to be more economical than they are, since
defining VMA in this way includes the large volume of binder absorbed by such aggregate.
Defining VMA in this way is a non-standard practice and can result in mixtures that are deficient
in asphalt binder, difficult to place and compact, and prone to fatigue cracking.
Voids Filled with Asphalt
Voids filled with asphalt (VFA) is the percentage of VMA filled with asphalt binder--the balance
is air voids. An asphalt concrete mixture with a VMA of 16% and an effective asphalt content
of 12% has a VFA value of (12/16) × 100% = 75%. In this case, 25% of the VMA is air voids.
Consider a second mixture, with 15% VMA and 5% air voids. The effective asphalt content
is then 15 - 5 = 10%, and the VFA is (10/15) × 100% = 67%. VFA is calculated by dividing the
effective binder content by the VMA and multiplying by 100%.
In designing asphalt concrete mixtures, VFA is closely related to both VMA and Vbe. This is
because with the design air void content constant at about 4.0%, as VMA increases, Vbe increases
and VFA also increases. Therefore, in most cases VFA should be thought of as simply an indicator
of mix richness, like VMA or Vbe. If design voids are fixed or allowed to vary only over a narrow
range, there is little point in simultaneously controlling VMA, Vbe, and/or VFA. In fact, simul-
taneous control of strongly interrelated volumetric factors can lead to confusion and conflict
during the mix design process and during construction. Figure 5-5 shows the relationships between
air void content, VFA, and VMA (Figure 5-5a) and Vbe (Figure 5-5b).
It is not entirely clear what aspects of performance are related to VFA that are not also
strongly related to other volumetric factors, especially Vbe. Some engineers have proposed that
fatigue resistance increases with increasing VFA. However, VFA and Vbe are strongly related.
Recent research strongly suggests that Vbe is a somewhat better overall indicator of fatigue resist-
ance in asphalt concrete mixtures. Therefore, in order to control or evaluate fatigue resistance,
engineers and technicians should either use Vbe, or VMA at a constant design air void content.
There is then little need to independently specify VFA. Relationships between mixture compo-
sition and performance are discussed in more detail in Chapter 6 of this manual.
Apparent Film Thickness
"Film thickness," when applied to asphalt concrete mixtures, refers to the average thickness of
binder coating aggregate particles in the mixture. Some engineers and researchers have proposed
that this is an important characteristic related to several aspects of pavement performance--
mixtures with low film thickness will be brittle and prone to durability problems, while mixtures
with high film thickness will have too much asphalt and may be prone to rutting and shoving.
Film thickness is, however, a controversial concept among paving engineers. Many engineers
strongly oppose the use of this term, since there are, in fact, no real films of asphalt binder within
an asphalt concrete mixture; the asphalt binder exists as a single homogenous phase binding the
aggregate particles together. Critics of film thickness point out that there is no way to physically
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52 A Manual for Design of Hot Mix Asphalt with Commentary
100
90
VFA, %
80
70 3%
60 4%
50 5%
40
5 10 15 20 25
(a) VMA, Volume %
90
80
VFA, %
70 3%
60 4%
5%
50
40
0 5 10 15 20
(b) Vbe, Volume %
Figure 5-5. Relationship between air void
content (labels to the left of curves), VFA and
(a) VMA, and (b) Vbe.
separate an aggregate particle with an intact asphalt binder film from a compacted asphalt con-
crete mixture. However, this criticism does not address the issue of whether or not calculated
film thickness values are correlated to pavement performance. For this reason, it is suggested that
engineers and technicians use the term "apparent film thickness" rather than "film thickness,"
thereby avoiding the main objection of many critics that such films do not physically exist.
Figure 5-6 illustrates the concept of apparent film thickness.
Recent research (documented in NCHRP Report 567) strongly suggests that there are reasons why
apparent film thickness should relate to performance, especially rut resistance. Rut resistance of
asphalt concrete mixtures increases as VMA decreases and aggregate fineness increases. Because
binder content decreases with decreasing VMA, this means that rut resistance should increase
aggregate
id
vo
ss
air
kn e
t
al
film
ph
hic
asphalt
as
t
absorbed
rbed
as
ph aggregate
ab so
alt
lt
a sp h a
aggregate
asphalt
Figure 5-6. Concept of apparent film thickness.
Calculation of apparent film thickness should not
include asphalt binder absorbed by the aggregate.
