VMA: ONE KEY TO MIXTURE PERFORMANCE
by
Donald W. Christensen, Jr., Ph.D., P.E.
Senior Engineer
Advanced Asphalt Technologies, LLC
444 E. College Ave., Suite 510
State College, PA 16801
Ramon F. Bonaquist, Ph.D., P.E.
Chief Operating Officer
Advanced Asphalt Technologies, LLC
108 Powers Court, Suite 100
Sterling, VA 20166
Submitted to the South Central Superpave Center for Publication in the National
Superpave Newsletter
February 2005
INTRODUCTION
Over the past 5 to 10 years, highway engineers have become increasingly concerned that
the composition of hot mix asphalt concrete (HMAC) being placed on our roads may not be
providing adequate durability under all conditions. A recent increase in surface distress in many
asphalt concrete pavements has lead the National Cooperative Highway Research Program
(NCHRP) to sponsor research specifically aimed at top-down cracking. Because of the perceived
increase in surface cracking and raveling, many pavement engineers feel that asphalt concrete
mixtures have become too lean—that is, the voids in mineral aggregate (VMA) and effective
binder content by volume (VBE) are too low for adequate fatigue resistance and durability.
Unfortunately, there is substantial evidence that increasing VMA and VBE can decrease the rut
resistance of asphalt concrete pavements. The purpose of this article is to describe recent
research providing guidelines for establishing design VMA values high enough to ensure good
durability while still achieving superior rut resistance.
The information presented in this article has largely grown out of research performed
under NCHRP Project 9-25: Requirements for Voids in Mineral Aggregate for Mixtures
Designed according to the Superpave System, and Project 9-31: Air Void Requirements for
Superpave Mix Design (1). However, the findings, conclusions and recommendations presented
in this article are those of the authors and may or may not reflect the position of the NCHRP or
the Federal Highway Administration.
NCHRP Projects 9-25 and 9-31 are nearing completion, and will probably be concluded
by the time this article is published. However, some refinements in the findings and
recommendations presented in this article are possible as the results of this research are
implemented over the next few years. As is always good practice, highway engineers should use
2
their judgment and experience with local materials and conditions when evaluating and applying
the information presented in this article.
HISTORICAL PERSPECTIVE ON VMA
Using VMA to evaluate and design asphalt concrete mixtures first gained popularity in the
1950’s, largely due to the efforts of Norman McLeod, a Canadian engineer and researcher who
has made many other contributions to hot-mix asphalt technology. He was the first to propose the
use of minimum VMA values for mixtures and was also the first to suggest that minimum VMA
should increase with decreasing aggregate size (2, 3). Prior to McLeod, Marshall mix design
specifications had only two limitations—air voids should be between 3 and 5 % and voids filled
with asphalt (VFA) should be between 75 and 85 %. McLeod felt that VMA provided better
control over many aspects of mixture composition and believed that establishing minimum VMA
values would help ensure good durability.
At about this same time, a controversy began concerning the best way to ensure good
mixture durability. Like McLeod, many pavement engineers believed that VMA and effective
binder content—meaning binder not absorbed into the aggregate—were the critical factors in
achieving durable asphalt concrete. Campen, Goode and Lufsey, among others, argued instead
that the amount of asphalt binder relative to aggregate surface area was critical to achieving good
mixture durability (4, 5, 6). Some engineers chose to express the ratio of asphalt binder to
aggregate surface area as “film thickness,” while others spoke in terms of “bitumen loading
factor (6).”
Pavement engineers and researchers still argue over this terminology today. Some would
like to see mix design specifications address the issue of film thickness, while others vehemently
oppose this idea on the grounds that asphalt cement binder simply does not exist as thin films
3
within asphalt concrete. These engineers instead believe—correctly—that asphalt concrete is a
particulate composite material made up of an intimate mixture of asphalt and aggregate. It is
probably best to go beyond the argument of whether or not asphalt films actually exist in asphalt
concrete mixtures and instead agree that the amount of binder relative to aggregate surface
area—perhaps best referred to as apparent film thickness—plays an important role in mixture
performance.
RUT RESISTANCE AND MIXTURE COMPOSITION
Recent research suggests that engineers on both sides of the VMA vs. film thickness
controversy have been partly right and partly wrong. The ratio of asphalt binder to aggregate
surface area does in fact significantly affect mixture performance, but not in the way that many
early researchers believed. Instead of affecting durability, this factor is probably more closely
linked to rut resistance. All else being equal, as the amount of binder relative to aggregate
surface area increases, the rut resistance of a mixture will decrease.
An even better indicator of this characteristic is the ratio of VMA to aggregate surface
area. In practical terms, this means that technicians and engineers responsible for mixture design
should be wary of mixtures that have high VMA relative to the amount of fines in the aggregate
gradation. Such mixtures will tend to exhibit poor rut resistance compared to mixtures with more
aggregate fines and lower VMA. A good example of this was the extreme early rutting exhibited
by many of the mixtures placed at WesTrack several years ago (7). A team of investigating
engineers determined that this failure was in part due to the excessive film thickness of many of
the mixtures.
To ensure that asphalt concrete mixtures have adequate levels of rut resistance, it is
necessary to establish minimum values for aggregate fineness at different levels of VMA.
4
Furthermore, it must be understood that rut resistance is a function not only of VMA and
aggregate fineness, but also of design compaction level, field compaction level, and binder grade
relative to both climate and design traffic level and speed. An effective model relating these
factors to field rutting rate for data from WesTrack, MN/Road and the NCAT test track project
was developed as part of NCHRP Projects 9-25 and 9-31 (1, 7, 8, 9, 10):
12,500 VMA3.24
RR =
G*
1.08
0.65
S a2.16 Gb2.16 N design
[(100 − VF ) (100 − VD )]
18.6
(1)
Where
RR
= rutting rate (mm/m/ESALs1/3)
= total rut depth in mm, divided by thickness of asphalt concrete in meters (to 0.10
m maximum) divided by the cube-root of traffic in ESALs
|G*|
= shear complex modulus of the binder (Pa), measured at 10 rad/s and at the yearly,
7-day average maximum pavement temperature at a depth of 50 mm
Sa
= specific surface of the aggregate in m2/kg
Gb
= bulk specific gravity of the aggregate
Ndesign = design compaction level (gyrations)
VF
= air void content of the pavement as constructed
VD
= air void content of the mix as designed
It should be pointed out that Equation 1 is based not on the concept of film thickness, but upon
the concept of resistivity. When an asphalt concrete mixture is deformed under loading, the
asphalt binder must flow through the aggregate structure. Conceptually, resistivity represents
how difficult it is to force the asphalt binder to flow through the aggregate structure of a mixture.
As resistivity increases, rut resistance also increases since energy consumed in forcing binder to
flow through the aggregate not available to damage the aggregate structure permanently.
5
Resistivity increases with increasing aggregate surface area, increasing binder viscosity and
decreasing VMA. Although closely related to apparent film thickness, resistivity is related to the
ratio of VMA3 S z2 , and not to the ratio of VBE S a . Furthermore, the concept of resistivity does
not depend in any way upon the existence of binder films within a mixture.
In Figure 1 below, observed rutting rates and those predicted using Equation 1 are
compared. The 90 % prediction interval for new observations for this model is equivalent to a
factor of 2.0. Therefore, if using Equation 1 for design purposes, a factor of safety of 2.0 will
provide 95 % confidence that the selected rut rate will not be exceeded (the 95 % confidence in
this case is for a one-sided limit, while the 90 % confidence interval in Figure 1 is two-sided).
For example, if the maximum allowable rut rate for a given project is 12 mm, designing for an
ultimate rut depth of 6 mm will result in only 1 chance in 20 that the ultimate rut depth will
exceed 12 mm.
It should be pointed out that in analyzing these data, the amount of age hardening that
occurred in the three different projects was accounted for through application of the MirzaWitczak global aging system (11). This may not seem to be an important factor, but can be
highly significant, particularly when comparing rutting data from relatively short test track
projects—where there is little opportunity for age hardening—to data from long term test roads
where there can be significant hardening of the binder and asphalt concrete over the length of the
project. This phenomenon in fact probably contributed to the rapid rutting at the WesTrack
facility. High temperature binder grades for test tracks should be increased by one-half to one
full grade, depending upon climate, to provide similar rut resistance to long term pavements. In
general, the hotter the climate, the greater the needed correction to account for lack of age
hardening in test track pavements.
6
Equation 1 suggests that to maintain similar levels of rut resistance, the ratio of
VMA3 S a2 should be kept more or less constant. By applying this principle and examining a
range of asphalt concrete mixtures exhibiting reasonable levels of rut resistance, it is possible to
establish guidelines for aggregate fineness as a function of VMA. However, aggregate fines not
only affect the rut resistance of asphalt concrete mixtures, but also improve their durability by
limiting permeability.
During NCHRP Projects 9-25 and 9-31, a model was developed for estimating in-place
permeability of asphalt concrete mixtures from aggregate specific surface and air void content
(1). However, in order to use this model to establish reasonable minimum values for aggregate
fines, maximum permeability values are needed for asphalt concrete pavements. Recently,
pavement engineers in Florida have suggested such a limit after studying the permeability of
mixtures designed according to the Superpave system (12). They concluded that wearing course
mixtures should exhibit an average in-place permeability of 100 × 10-5 cm/s. By applying the
model developed during NCHRP Projects 9-25 and 9-31 to this suggested permeability limit
(and by assuming a gradual increase in permeability with increasing NMAS and VMA), it is
possible to calculate a second set of aggregate fineness requirements, in addition to those based
upon rut resistance. Fortunately, the two sets of requirements are quite close, so that for practical
purposes a single set of aggregate fineness values as a function of VMA can be established.
These values are given in Table 1, both in terms of aggregate specific surface and FM300, which
is the sum of the percent passing the 75-, 150- and 300-micron sieves (1). FM300 is easy to
calculate, and also relates very closely to specific surface; for aggregate gradations used in the
NCAT test track, NCHRP Project 9-9 and National Pooled Fund Study 176, the correlation
between FM300 and specific surface was r2 = 89 %. Furthermore, based upon these data aggregate
7
specific surface in m2/kg can be estimated simply by dividing FM300 by 5 (1). Table 1 also
includes recommended maximum values for FM300, which were calculated by multiplying the
minimum values by 1.7; these maximums are consistent with the range in FM300 values for
typical dense-graded asphalt concrete mixtures, and will help ensure that mixtures will not
become too difficult to mix or compact because of excessive fines.
As mentioned earlier, in order to apply the recommendations in Table 1 with confidence,
the proper high temperature binder grade must be used for a given climate, design traffic speed
and volume. The LTPPBind Software has been developed for this purpose, though many
highway engineers believe that this software does not provide realistic estimates of required
binder grades. Analysis of the high temperature binder grades (including adjustments for traffic
speed and volume) using Equation 1 suggest that care is needed in using LTPPBind, or a binder
with inadequate stiffness at high temperatures might be selected (1, 8). When using the
LTPPBind software, a good approach is to select high temperature binder grades using 98 %
reliability and applying the “KMC” traffic adjustments. In combination with the recommended
aggregate fineness and VMA values given in Table 1, this should provide a minimum effective
reliability against excessive rutting of about 95 %, and because binder grades are usually
rounded up, the actual reliability will usually be greater than this. Many agencies already have
established protocols for binder grade selection; if the established binder grading system is
working well, there is no need to change it—it should work fine at higher design VMA values
provided the aggregate fineness guidelines of Table 1 are followed.
FATIGUE RESISTANCE AND VMA
Durability of asphalt concrete mixtures is not just a function of permeability, but is also a
function of fatigue resistance. Some researchers have proposed that fatigue resistance increases
8
with increasing VFA, while others have proposed that it improves with increasing binder
content. Research performed during NCHRP Projects 9-25 and 9-31 strongly suggests that it is
binder content by volume that best relates to fatigue resistance (1, 13).
Because design air void content in mixtures designed according to the Superpave system
is fixed at 4 %, it can also be stated that for these materials fatigue resistance increases with
increasing VMA. Many paving engineers have suspected this and have attributed the surface
distress observed in some recently constructed pavements to insufficient VMA and binder
content.
In fact, VMA values and binder contents have decreased significantly over the past 30
years, as illustrated in Figure 2. This plot summarizes VMA and binder contents from several
different research projects and specifications covering asphalt concrete mixtures designed since
the 1970s. In the National Rutting Study by Brown and Cross, the average VMA value for
Marshall mixtures designed and placed in the 1970’s and early 1980’s was found to be 17.0 %
(14). Marshall mix designs placed on the MN/Road mainline test section had slightly less VMA
at 15.3 % (9). Some wearing course mixtures are now being placed with VMA values in the 14
to 15 % range, as seen in the recent Florida permeability study and in the mixtures placed at the
NCAT Test Track (10, 12). As a comparison, SMA mixtures, which in general have been
exhibiting significantly better durability than coarse-graded mixtures designed according to the
Superpave system, have design VMA values in the 17 to 18 % range (15).
Clearly a significant portion of the recent decrease in mixture durability can be attributed
to the reduction in VMA and binder content over the past several decades. Given the success of
SMA and its widespread adoption, there probably is not a need to increase VMA values for
dense-graded HMAC mixtures to the 17 to 18 % range, but there is a need for modest increases
9
in design VMA and binder contents. Based largely upon the historical changes in VMA and
binder contents, the superior durability of SMA mixtures, and the relationship between fatigue
resistance and effective binder content, it is recommended that minimum VMA values (and
implied effective binder contents) be increased 1 % for mixtures designed according to the
Superpave system. These recommendations are summarized in Table 2.
Although the design air void content in the Superpave system is currently set at 4 %,
some agencies allow design air voids to vary up to 1 % from this value. In such cases, VMA and
binder content are no longer directly related, and it becomes important to check minimum
effective binder contents by volume, in addition to evaluating VMA. For this reason,
recommended minimum VBE values are included in Table 2. Because wearing course mixtures
are subject directly to tire loading, and also to air, water and sunlight, it is important that they be
extremely durable. For this reason, Superpave wearing course mixtures should have a minimum
VMA of 16.0 % and a minimum VBE of 12 %, regardless of nominal maximum aggregate size.
Rich base course mixtures should meet similar requirements.
Some engineers may be concerned that returning to richer wearing course mixtures might
result in an increase in rutting. However, this will not be a problem if the changes are done
carefully. As discussed at the beginning of this article maintaining proper levels of aggregate
fineness while increasing VMA will help maintain rut resistance. It must also be recognized that
the general quality of the materials used in producing asphalt concrete pavements has improved
significantly with the implementation of the Superpave system. Many of the rut-prone Marshall
mixtures placed in the 1970’s and 1980’s were designed at low levels of compaction and
included substantial amounts of natural sand. Some also contained poorly crushed gravel as
coarse aggregate. The relationships among binder flow properties, rut resistance, climate and
10
traffic were not well understood, and as a result binders used in heavily trafficked wearing
courses sometimes had insufficient stiffness for such severe applications. With the strict
requirements for aggregate quality, compaction and binder grading now in use within the
Superpave system, VMA and binder contents can and should be increased without fear of
excessive rutting, as long as aggregate fineness is also increased slightly and care is taken in
selecting the appropriate binder PG grade.
VMA AND THE MIX DESIGN PROCESS
Durable, rut resistant HMAC must be designed with a high level of compaction,
reasonably high VMA and binder contents, and adequate fines. Although achieving proper VMA
is in general an inexact, trial-and-error process, a few basic rules can help laboratory engineers
and technicians increase VMA in overly lean mixtures. In general, VMA will increase as the
aggregate blend moves further away from a maximum density gradation. This means that for a
coarse blend—one that falls below the maximum density gradation—VMA can be increased by
adding coarse aggregate. Similarly the VMA of fine blends can be increased by adding fine
aggregate. Adding mineral filler to a mixture will sometimes increase VMA, and sometimes
decrease VMA. Usually, mixing an additional aggregate into an existing blend will decrease
VMA rather than increase it, because the new aggregate will tend to fill voids among the existing
particles. Therefore, the number of aggregate sources in a blend should be decreased and not
increased when higher VMA in a mixture is needed. Weak, friable aggregates with poor abrasion
resistance will break down under moderate to high compaction, making it difficult to achieve
high VMA levels. If this is the case, alternate aggregate sources should be evaluated. Sometimes
aggregate particle shape or angularity can affect VMA, either increasing it or decreasing it
compared to other aggregates of similar gradation. Highway agencies should consider
11
broadening aggregate gradation requirements when possible, to make achieving proper levels of
VMA easier, since beyond selection of the proper aggregate size and fineness, there is little
research to suggest that specific characteristics of aggregate gradation affect pavement
performance in any significant way, while VMA, air voids and binder content are critical to
proper durability and rut resistance.
CONTROL OF VMA DURING FIELD PRODUCTION
Many states allow VMA of asphalt concrete mixtures to fall below the minimum design
value during field production. This is believed necessary because of the variability inherent in
the production of asphalt concrete, and also because of the relatively poor precision of the
specific gravity measurements underlying calculation of air voids, VMA and related parameters.
These are certainly valid concerns, but engineers should understand the strong link between
proper levels of VMA and aggregate fines, mixture durability and rut resistance. Although
occasionally allowing VMA to drop up to 1 % below suggested minimum values during
production is probably not a major problem, plant personnel should strive to achieve an overall
average VMA close to the design value. Highway agencies should consider paying for asphalt
binder as a separate item to remove what can otherwise be a strong incentive to produce mixtures
with the lowest VMA and minimum binder content possible while still earning full contract
price.
SUMMARY
Obtaining proper VMA is essential to producing durable and rut resistant asphalt concrete
mixtures. To ensure adequate fatigue resistance, the minimum VMA values for mixtures
designed according to the Superpave system should be increased by 1 % over current
recommended minimums. In cases where design air void content is allowed to vary from 4 %,
12
the implied minimum effective binder content should also be increased 1 % and enforced in
addition to minimum VMA values. Because of severe exposure to the effects of traffic loading
and the elements, wearing course mixtures and rich base course mixtures should have a
minimum VMA of 16 % and a minimum VBE of 12 %, regardless of nominal maximum
aggregate size. Adequate rut resistance can be achieved regardless of VMA by making certain
that the proper binder grade is selected for a given application and that the aggregate blend
contains sufficient fines relative to the design VMA. Adequate fineness will also help keep the
permeability of in-place mixtures low. Although some variability in VMA is inevitable during
field production, plant personnel should strive to keep overall VMA levels close to design
values.
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REFERENCES
1. D. W. Christensen and R. F. Bonaquist, NCHRP Projects 9-25 and 9-31 Draft Final Report,
Sterling, VA: Advanced Asphalt Technologies, LLC, November 2004.
2. N. W. McLeod, “Relationships Between Density, Bitumen Content, and Voids Properties of
Compacted Bituminous Paving Mixtures,” Proceedings, Highway Research Board, Volume
35, 1956.
3. N. W. McLeod, “Voids Requirements for Dense-Graded Bituminous Paving Mixtures,” in
ASTM STP-252, American Society for Testing and Materials, 1959.
4. W. H. Campen, J. R. Smith, L. G. Erickson and L. R. Mertz, “The Control of Voids in
Aggregates for Bituminous Paving Mixtures,” Proceedings of the Association of Asphalt
Paving Technologists, Vol. 26, 1957.
5. W. H. Campen, J. R. Smith, L. G. Erickson and L. R. Mertz, “The Relationship Between
Voids, Surface Area, Film Thickness, and Stability in Bituminous Paving Mixtures,”
Proceedings of the Association of Asphalt Paving Technologists, Vol. 28, 1959.
6. J. F. Goode and L. A. Lufsey, “Voids, Permeability, Film Thickness Vs. Asphalt Hardening,”
Proceedings of the Association of Asphalt Paving Technologists, Vol. 34, 1965.
7. E. R. Brown, L. Michael, E. Dukatz, J. Scherocman, G. Huber, R. Sines, J. D’Angelo and C.
Williams, Performance of Coarse-Graded Mixes at WesTrack—Premature Rutting, Final
Report, FHWA-RD-99-134, June 1998, 20 pp.
8. D. W. Christensen and R. F. Bonaquist, “Rut Resistance and Volumetric Composition of
Asphalt Concrete Mixtures,” Preprints, Journal of the Association of Asphalt Paving
Technologists, 2005.
9. R. Mulvaney and B. Worel, MnROAD Mainline Rutting Forensic Investigation, St. Paul,
MN: Minnesota Department of Transportation, October 2002, 91 pp.
10. E. R. Brown, B. Prowell, A. Cooley, J. Zhang and R. B. Powell, “Evaluation of Rutting
Performance on the 2000 NCAT Test Track,” Preprints, Journal of the Association of
Asphalt Paving Technologists, March 2004.
11. M. W. Mirza and M. W. Witczak, “Development of a Global Aging System for Short and
Long Term Aging of Asphalt Cements,” Journal of the Association of Asphalt Paving
Technologists, Vol. 64, 1995, pp. 393-424.
12. B. Choubane, G. Page and J. Musselman, “Investigation of Water Permeability of Coarse
Graded Superpave Pavements,” Journal of the Association of Asphalt Paving Technologists,
Vol. 67, 1998, p. 254.
13. D. W. Christensen and R. F. Bonaquist, “Practical Application of Continuum Damage
Theory to Fatigue Phenomena in Asphalt Concrete Mixtures,” Preprints, Journal of the
Association of Asphalt Paving Technologists, 2005.
14. E. R. Brown and S. A. Cross, “A National Study of Rutting in Hot Mix Asphalt (HMA)
Pavements, Journal of the Association of Asphalt Paving Technologists, Vol. 61, 1992, pp.
535-573.
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15. E. R. Brown, J. E. Haddock, C. Crawford, C. S. Hughes and T. A. Lynn, Designing Stone
Matrix Asphalt Mixtures, Volume II (a)⎯Research Results for Part 1 of Phase I, NCHRP 9-8
Final Report, Prepared for the National Cooperative Highway Research Program, Auburn
University, July 1998.
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TABLES
Table 1. Minimum Aggregate Fineness Requirements for Rut
Resistance and Permeability.
VMA
Vol. %
20.0
19.0
18.0
17.0
16.0
15.0
14.0
13.0
12.0
Minimum
Sa
m2/kg
7.4
6.8
6.4
5.8
5.4
4.8
4.3
3.9
3.4
Minimum
FM300
Maximum
FM300
38
35
32
30
27
25
22
20
18
64
59
55
50
46
42
38
34
30
Table 2. Suggested Minimum Values for VMA and
VBE for Dense Graded Asphalt Concrete
Mixtures.
Aggregate
NMAS
mm
9.5
12.5
19.0
25.0
37.5
Minimum
VMA
Vol. %
16
15
14
13
12
16
Minimum
VBE
Vol. %
12
11
10
9
8
FIGURES
Observed Rut Rate
1.E+01
1.E+00
NCAT
MN/Road
WesTrack
Equality
90 % PI
1.E-01
1.E-02
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
Predicted Rut Rate
Figure 1. Observed Rut Rates for Three Test Roads and Values Predicted Using Equation
1 (includes line of equality and 90 % prediction interval for new observations).
VMA, Volume %
0
5
10
15
20
25
Marshall Designs 1980's (1)
Marshall Designs ca. 1992 (2)
Superpave Designs 1996 (3)
Binder
Air Voids
Superpave Designs 2000 (4)
SMA Mixtures (5)
Figure 2. VMA, Effective Binder and Air Void Contents (by volume) for Different Mix
Types over the Past 20 to 30 years (1, 2, 3, 4, 5).
17