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OCR for page 241
CHAPTER 8
Design of Dense-Graded
HMA Mixtures
Chapter 8 is probably the most important chapter in the Manual. It describes in detail the rec-
ommended procedure for designing dense-graded HMA mixtures. Much of the material pre-
sented here also appears in other chapters--it is repeated in Chapter 8 for the convenience of the
reader, and so that Chapter 8 can be used as a stand-alone document for designing dense-graded
HMA mixtures. For example, the tables on aggregate specifications also appear in Chapter 4.
Many of the tables that appear in Chapter 8 are either identical or nearly identical to tables used
in the Superpave system, as described in AASHTO standards M 323 and R 35 and the Asphalt
Institute's SP-2 manual. Many of these tables have been only slightly modified, based upon the
results of various recent research projects. Some of the tables, such as those providing guidelines
for interpreting various performance tests, do not appear in standards and publications deal-
ing with Superpave. As described below, in some cases development of these tables was simply a
matter of presenting typical current practice. However, for some of the performance tests devel-
oping meaningful guidelines for interpreting results was a complex task.
Chapter 8 presents a brief history of HMA mix design methods, in order to provide inexperi-
enced technicians and engineers with some background. Of particular interest is the information
provided on the Superpave system, which is quite similar to the mix design method presented in
the Manual. After the background section, Chapter 8 presents a short summary of the proposed
mix design procedure, followed by a detailed, step-by-step description. This includes numerous
example problems with solutions.
An important feature in Chapter 8 of the Manual is the frequent references to HMA Tools,
which is a Microsoft Excel spreadsheet application designed to accompany the Manual. HMA
Tools is a powerful spreadsheet which can perform virtually all of the calculations needed when
performing an HMA mix design. The examples given in Chapter 8 generally refer to HMA Tools.
As pointed out in the Manual, it is not necessary to use HMA Tools to perform mix designs
according to the recommended method, but if other software is used, it will usually be necessary
to update the various specifications and limits to reflect those given in the Manual.
Table 8-1 in the Manual shows recommended grade adjustments for traffic level and speed. This
table is identical to Table 6-3, presented and discussed in detail previously in the Commentary
as Table 6. Readers should refer to this discussion for the derivation of the grade adjustments
shown in Table 8-1.
Table 8-2 lists compaction effort as a function of design traffic level. The values for design
gyrations--Ndesign--are identical to what appears in AASHTO R 35. However, values for Ninitial and
Nmax have been eliminated from the table. Ninitial and Nmax requirements have been eliminated on
the basis of work done during NCHRP Project 9-9(1)(35); as documented in NCHRP Report 573,
two of the major conclusions of this project were that neither Ninitial nor Nmaximum correlated with
241
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242 A Manual for Design of Hot Mix Asphalt with Commentary
rutting observed in an extensive field experiment and are not required in designing HMA mix-
tures (35). NCHRP Report 573 also suggested new gyration levels, as listed in Table 9 below.
NCHRP Report 573 recommends two sets of compaction levels--one for binders with a high
temperature grade of 76 and greater (or for binders used in mixes placed more than 100 mm
from the pavement surface), and one set for other binders. Furthermore, the overall level of com-
paction is lower than is currently suggested in R 35 (35). The recommended compaction levels
are based on matching densification as it occurs in the gyratory compactor and as it occurs under
traffic loading. It should be pointed out that the correlations reported in NCHRP Report 573
between densification during laboratory compaction and under traffic loading are not strong--
34 and 37% for the two different compactors used in the study (35). Furthermore, this study does
not address the strong effect laboratory compaction has on rut resistance, as noted in NCHRP
Report 567 (4). In the procedure given in the Manual, the current compaction levels are main-
tained for several reasons. First, HMA mixtures made with binders of grade PG 76-XX and higher
are in general polymer modified and usually intended for pavements subjected to very heavy traf-
fic loading--major urban highways which demand the highest levels of reliability against rut-
ting. The significant reduction in Ndesign recommended in NCHRP Report 573 could reduce the
rut resistance of such HMA designs to an unacceptable level. A second factor to consider when
evaluating the use of a different set of compaction levels for what, in effect, are mostly polymer-
modified binders is the probable adoption of the MSCR test to grade asphalt binders at high
temperatures. Use of this test might result in changes in binder grade selection that in combi-
nation with a change in compaction levels could yield inadequate performance for mixtures
that should exhibit outstanding levels of performance. A third consideration is that there has
of yet been little time to validate the findings of NCHRP Report 573. Additional time is needed
for industry input and further independent evaluation of the proposed compaction levels prior
to implementation.
Table 8-3 lists the primary control sieve (PCS) size for different NMAS, along with the PZC
control point, which is the % passing above which an aggregate gradation is considered a
"coarse" gradation and below which it is considered a "fine" gradation. Table 8-3 is nearly iden-
tical to Table 4 from AASHTO M 323-5, with the exception that Table 8-3 includes information
on 4.75 mm NMAS gradations, while Table 4 in M 323-5 does not.
Table 8-4 in the Manual (Table 10 below) lists recommended NMAS for different types of
dense-graded HMA mixtures, along with recommended lift thicknesses. The values for NMAS
follow directly from the recommendations of Chapter 7 on mix type selection--specifically,
Table 7-3, which lists recommended mix types and NMAS values for different traffic levels.
Table 7-3 in turn was based on the recommendations given in NAPA's publication HMA Pavement
Mix Type Selection Guide, IS 128 (33). Lift thickness values are based on recommendations given
Table 9. Compaction effort as a function of design traffic
level as recommended in NCHRP Report 573 and as given in
AASHTO R 35 and the mix design manual (35).
From NCHRP Report 573:
20-Year
Design Ndesign for Binders PG 76-
Traffic, XX or for Binders Used in Ndesign in AASHTO
Million Ndesign for Binders Mixes Placed > 100 mm from R 35 and in the Mix
ESALs < PG 76-XX Pavement Surface Design Manual
< 0.30 50 NA 50
0.30 to < 3.0 65 50 75
3.0 to < 10 80 65 100
10 to < 30 80 65 100
> 30 100 80 125
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Commentary to the Mix Design Manual for Hot Mix Asphalt 243
Table 10. Recommended aggregate nominal maximum aggregate
sizes (NMAS) for dense-graded HMA mixtures--table 8-4 in the mix
design manual.
Application Recommended Recommended Lift Thickness, mm
NMAS, mm Fine-Graded Coarse-Graded
Mixtures Mixtures
4.75 15 to 25 20 to 25
Leveling course mixtures
9.5 30 to 50 40 to 50
4.75 15 to 25 20 to 25
Wearing course mixtures 9.5 30 to 50 40 to 50
12.5 40 to 65 50 to 65
19.0 60 to 100 75 to 100
Intermediate course mixtures
25.0 75 to 125 100 to 125
19.0 60 to 100 75 to 100
Base course mixtures 25.0 75 to 125 100 to 125
37.5 115 to 150 150
9.5 30 to 50 40 to 50
Rich base course mixtures
12.5 40 to 65 50 to 65
in NCHRP Report 531: lift thickness values 3 to 5 times NMAS for fine-graded mixtures, and 4
to 5 times NMAS for coarse-graded mixtures.
Table 8-5 in the Manual lists maximum and minimum VMA values as a function of aggre-
gate NMAS. Minimum VMA values are the same as those specified in AASHTO M 323-04.
However, there is a note to the table allowing agencies to increase minimum VMA values by
up to 1.0%, in order to improve field compaction, fatigue resistance, and durability. The note
also contains a caution that if VMA is increased, care should be taken to ensure that the result-
ing mix maintains adequate rut resistance. This note has been included to address the concern
of many agencies that HMA mixtures designed according to existing Superpave methods often
exhibit durability problems--raveling, surface cracking, and moisture damage. Many agen-
cies have already increased minimum VMA values for Superpave mixes in order to address
these perceived problems. Allowing an increase of up to 1.0% in minimum VMA addresses the
concerns of agencies that have experienced durability problems in Superpave mixes, but allows
those agencies that have not seen such problems to maintain minimum VMA values at the cur-
rent levels specified in M 323-04.
Maximum VMA values in the Manual are held to 2% above the minimum values. VFA is no
longer specified. One of the reasons for eliminating the requirements for minimum and maxi-
mum VFA is that the relationship among VMA, VFA, and design air voids is complex and makes
simultaneous control of all three difficult and confusing. The requirements currently given in
Table 6 of M 323 in fact require some effort to interpret precisely; specification of minimum and
maximum VFA, in combination with the specified design air void content of 4.0%, establish an
alternate set of minimum and maximum VMA values, since VMA = 4.0/(1-VFA/100). The implied
VMA values calculated in this way are either equal to or less than the stated VMA values (allow-
ing for rounding errors), and so the stated minimum VMA values are not ambiguous. However,
specifying a minimum VMA and a design air void content also implies a minimum VFA, since
VFA = (VMA-4.0)/VMA × 100%. In this case, the implied minimum VFA values are often greater
than those specifically listed in Table 6 of M 323-04. The conservative interpretation would be
that the highest of the two alternate sets of minimum VFA values applies, but the standard as writ-
ten is somewhat ambiguous. Table 11 lists the minimum VMA values and minimum and max-
imum VFA values specified in Table 6 of M 323-04, along with the minimum VFA calculated
from the specified minimum VMA, and the calculated maximum VMA values calculated from
the maximum VFA. The approach used in the Manual--simply specifying a minimum and max-
imum VMA--is simpler and avoids the ambiguity inherent in trying to simultaneously control
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244 A Manual for Design of Hot Mix Asphalt with Commentary
Table 11. Specified minimum VMA values and implied
minimum and maximum VMA values calculated from VFA
values specified in table 6 of AASHTO M 323-04.
Design Calculated Calculated
NMAS, Traffic Level, Minimum Minimum Maximum Minimum Maximum
mm Million VMA, VFA, VFA, VFA, VMA,
ESALs % % % % %
4.75 < 3.0 16.0 70 80 75.0 20.0
4.75 3.0 16.0 75 78 75.0 18.2
9.5 < 3.0 15.0 65 78 73.3 18.2
9.5 3.0 15.0 73 76 73.3 16.7
12.5 All 14.0 65 75 71.4 16.0
19 All 13.0 65 75 69.2 16.0
25 0.3 12.0 65 75 66.7 16.0
25 < 0.3 12.0 67 75 66.7 16.0
37.5 All 11.0 64 75 63.6 16.0
air voids, VMA, and VFA. For the smaller NMAS values and higher traffic levels, the approach
in the Manual results in similar ranges for VMA. For the larger NMAS value, the approach in the
Manual is somewhat more restrictive, since M 323-04 in effect specifies maximum VMA values
3 to 4% higher than the minimum for these aggregates. However, in reality, because of the high
cost of asphalt binders most mix designers select VMA values very close to the minimum values
specified in M 323-04. Therefore, the difference in the two approaches is probably negligible in
practice.
Tables 8-6 and 8-7 in the Manual list aggregate control points for aggregate blends of differ-
ent NMAS values. These tables contain values identical to those given in Table 3 of AASHTO
M 323-04. However, in the Manual, the aggregate control points are given as suggested limits,
and not specified limits as is done in M 323-04. This change was made because it provides the
mix designer with much greater flexibility in obtaining specified VMA values, and virtually all
evidence relating HMA performance to composition suggests that it is much more important to
control VMA than to control details of aggregate gradation. For instance, none of the models
discussed in Chapter 6 relating HMA composition to rut resistance and fatigue resistance con-
tain factors related to aggregate gradation as predictor variables (16, 17, 18, 22, 23, 24). Although
NCHRP Report 405 states that aggregate gradation effects rut resistance and fatigue resistance,
these statements are made without support--and in fact made with the admission that eval-
uating the effect of aggregate gradation on HMA performance was outside the scope of the
project (20).
Aggregate specification properties--coarse aggregate fractured faces, flat and elongated par-
ticles, fine aggregate angularity, and clay content--are given in Tables 8-8 through 8-11. These
tables are identical to Tables 4-6 through 4-9 given in Chapter 4. As discussed in the Commen-
tary section dealing with Chapter 4, these tables are very similar to the corresponding tables in
M 323-04. The reader should refer to the Commentary section on Chapter 4 for a discussion of
these tables.
Table 8-12 in the Manual lists requirements for dust/binder ratio; it is reproduced here as
Table 12. The requirements given for 4.75 mm NMAS mixes are identical to those given in
AASHTO M 323-04. The requirements for other mixes--allowable dust/binder ratios in the
range of 0.8 to 1.6, with an option of lowering this range to 0.6 to 1.2--are slightly higher than
those in M 323-04. In M 323-04, the specified range for mixes other than 4.75 mm NMAS is from
0.6 to 1.2, with an option of raising this range to 0.8 to 1.6. The requirements in the Manual are
therefore similar, but encourage slightly higher dust/binder ratios. There are two reasons for this
increase. The first is that research performed during NCHRP Projects 9-25 and 9-31 showed a
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Commentary to the Mix Design Manual for Hot Mix Asphalt 245
Table 12. Requirements for
dust/binder ratio--table 8-12 in
the mix design manual.
Allowable Range for
Mix Aggregate Dust/Binder Ratio, by
NMAS, mm Weight
> 4.75 0.8 to 1.6A
4.75 0.9 to 2.0
A
The specifying agency may lower the allowable
range for dust/binder ratio to 0.6 to 1.2 if
warranted by local conditions and materials. The
dust/binder ratio should however not be lowered if
VMA requirements are increased above the
standard values as listed in Table 8-5.
strong relationship between permeability and aggregate surface area--as aggregate surface area
increases, permeability tends to decrease, all else being equal (13). Therefore, specifying slightly
higher dust/binder ratios should result in mixes with lower permeability to air and water and
improved durability. The second reason for encouraging slightly higher dust/binder ratios in
HMA mixes is that the design method given in the Manual attempts to encourage slightly higher
VMA and asphalt binder contents in order to improve the durability of the resulting mixtures.
One example of how this is done is the VMA requirements described above, which include an
option of increasing the minimum VMA values by up to 1% to improve field compaction, fatigue
resistance, and durability. It was found in NCHRP Projects 9-25 and 9-31 that rut resistance of
HMA mixes tends to increase as aggregate specific surface increases relative to VMA. Since the
Manual encourages higher VMA values, higher dust/binder values are also encouraged in order
to maintain or improve rut resistance compared to mixes designed according to the Superpave
system. The extremely premature rutting of many of the mixtures placed at the WesTrack facil-
ity was attributed in part to high VMA and relatively low dust/binder ratios (36). Promoting an
increase in dust/binder ratio will help to prevent such failures in the future.
Table 8-20 in the Manual (reproduced here as Table 13) lists recommended minimum values
for flow numbers as a function of design traffic level. The values in this table are based on a
relationship between flow number values and maximum allowable traffic level estimated using
the resistivity/rutting model given earlier as Equation 1. Data used in developing this relationship
was collected by the FHWA in one of their field trailers, for nine different projects in New England,
New York State, Nebraska, North Carolina, Minnesota, and Wisconsin. The mix composition,
binder G* /sin values, flow number values, and related data appear in Table 14 of this report.
Data forwarded by the FHWA included tests of both laboratory-prepared mixes having differ-
ent binder contents and field produced mix. A meaningful relationship between flow number
and calculated maximum traffic could only be developed using the laboratory mixes. The rea-
son this relationship did not hold up for the field-produced mixes is not clear, but it is possibly
due to differences in age hardening during production, transport, and sample storage. The
design air void content was known precisely only for mixes which used the design binder content.
For the other two mixes for each project--one above and one below the design binder content--
the design air void content was estimated using the following equation (3):
Vadb = Vad - 2.5 ( Pb - Pbd ) (7)
where
Vadb = Estimated design air voids at some binder content Pb
Vad = Design air voids at design binder content Pbd
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246 A Manual for Design of Hot Mix Asphalt with Commentary
MA0467 ME0359 ME0570
100,000 MN0465 NC0360 NE0569
NY0466 WA0463 WI0357
Fit
10,000
Flow Number
1,000
y = 6.222x1.145
100 R2 = 0.894
10
1.0 10.0 100.0 1000.0
Estimated Maximum MESALs
Figure 2. Relationship between flow number and estimated
maximum traffic from FHWA field data on six projects.
Another complication in calculating the allowable traffic values shown in Figure 2 and Table 14
is that, in most cases, it is not clear whether the binders used in the nine mixes was polymer mod-
ified or not--as seen in Equation 1, there is a factor M that is applied to mixes made using poly-
mer modified binders to account for the superior rut resistance of these materials compared to
non-modified binders of the same high-temperature grade. Recent surveys of asphalt binder pro-
ducers suggests that, at the time these mixes were produced (mostly in 2004) approximately 90%
of PG 64-28 binders and PG 70-22 binders were modified (37, 38). The PG 70-28 binder used
for the MN0465 project was clearly modified, based on the rheological behavior of the binder.
From this information and the observed relationship between binder flow properties and mix-
ture flow number, it was assumed that five of the binders used in the nine projects were polymer
modified. The four projects in which non-modified binders were assumed to be used in the
mixes were NC0360, WI0357, NE0569, and WA0463. Analysis of the data using this assumption
provided good results, but it appeared that the value of M used in Equation 1 (7.13) was some-
what too high. The best correlation between estimated maximum traffic and flow number was
found when the value of M was assumed to be 3.0. The lower value of M might be because the
modified binders in the nine projects contained only relatively small amounts of polymer, or
possibly because much of the testing was performed at intermediate temperatures (mostly 38 or
45 °C), where the effect of polymer modification on rut resistance may not be as large as it is at
higher temperatures.
The values of flow number appearing in Table 13 were calculated from the regression equa-
tion given in Figure 2 using traffic levels at the midpoint of the given design traffic range. For a
Table 13. range of 3 to 10 million ESALs, a value of 6.5 million ESALs was used to estimate a minimum
Recommended flow number of 53. For the 10 to 30 million ESAL range, a value of 20 million ESALs was used
minimum flow to calculate the minimum flow number of 190. For traffic above 30 million ESALs, a traffic level
number require- of 65 million ESALs was used to calculate the required flow number value of 740. Selecting these
ments (Table 8-20 in values, rather than the maximum for each range was done to provide some insurance against
the Mix Design excessive rutting, while avoiding being too restrictive, which might result in having a large num-
Manual). ber of mixes fail the performance test and needing to be redesigned.
Traffic Minimum
Level Flow Number
Table 15 lists recommended minimum flow time values as given in the Manual (Table 8-21 in
Million Cycles the Manual). These flow time values were calculated by developing a regression equation relating
ESALs flow time and flow number, as shown in Figure 3. This plot includes data from five projects included
<3 ---
3 to < 10 53 in the NCHRP Project 9-19 database (39). The data used in the plot are shown in Table 16. The
10 to < 30 190 flow time values in Table 15 and the flow number values given in Table 13 are intended to be,
30 740 for all practical purposes, equivalent.
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Commentary to the Mix Design Manual for Hot Mix Asphalt 247
Table 14. Results of AMPT tests and related properties used in calculation of flow number limits (39).
Binder Voids Agg.
Grade as Design Design Spec. Agg. Allowable Flow
Project ID PG- Pb Tested Voids Ndesign VMA Surface Gs Temp. |G*|/sin M Traffic No.
Wt.% Vol.% Vol.% Gyr. Vol.% M2/kg °C Pa MESALs
MA0467 4.6-1 64-28 4.6 6.9 5.8 100 17.0 3.96 2.681 45.2 38,073 3.00 48.4 346
MA0467 5.1-1 64-28 5.1 7.2 4.5 100 16.9 3.96 2.681 45.1 38,073 3.00 31.4 216
MA0467 5.6-1 64-28 5.6 7.2 3.3 100 16.9 3.96 2.681 45.0 38,073 3.00 19.1 171
ME0570 5.4-1 64-28 5.4 4.9 5.8 75 15.3 5.28 2.560 54.3 10,265 3.00 26.6 541
ME0570 5.9-1 64-28 5.9 5.0 4.5 75 15.3 5.28 2.560 54.1 10,265 3.00 17.6 351
ME0570 6.4-1 64-28 6.4 5.0 3.3 75 15.6 5.28 2.560 54.0 10,265 3.00 10.2 148
NC0360 4.5-1 70-22 4.5 8.0 3.9 100 13.9 6.32 2.599 45.0 73,343 1.00 131.1 1121
NC0360 5.0-1 70-22 5.0 7.9 2.6 100 14.0 6.32 2.599 45.1 73,343 1.00 70.5 801
NC0360 5.5-1 70-22 5.5 8.2 1.4 100 14.3 6.32 2.599 45.2 73,343 1.00 22.4 411
NY0466 4.5-1 64-28 4.5 7.1 5.5 100 13.5 4.24 2.618 44.9 44,891 3.00 153.2 1911
NY0466 5.0-1 64-28 5.0 7.0 4.2 100 13.5 4.24 2.618 44.9 44,891 3.00 106.4 841
NY0466 5.5-1 64-28 5.5 7.6 3.0 100 13.2 4.24 2.618 44.9 44,891 3.00 61.1 666
WI0357 4.4-1 64-22 4.4 7.1 6.7 100 15.2 3.64 2.725 31.3 326,035 1.00 478.5 9416
WI0357 4.9-1 64-22 4.9 7.0 5.4 100 15.1 3.64 2.725 31.8 326,035 1.00 364.2 7896
WI0357 5.4-2 64-22 5.4 6.9 4.2 100 15.0 3.64 2.725 31.6 326,035 1.00 255.8 5653
ME0359 5.3 - 1 64-28 5.3 4.7 5.8 75 14.5 3.78 2.599 37.3 96,540 3.00 322.1 5472
ME0359 5.8-2 64-28 5.8 4.8 4.5 75 14.3 3.78 2.599 37.6 96,540 3.00 224.7 4177
ME0359 6.3 - 2 64-28 6.3 4.8 3.3 75 14.4 3.78 2.599 37.5 96,540 3.00 135.1 2356
MN0465 4.8-1 70-28 4.8 7.9 6.0 100 15.5 3.78 2.686 44.7 38,036 3.00 53.7 283
MN0465 5.3-1 70-28 5.3 8.0 4.7 100 15.4 3.78 2.686 44.8 38,036 3.00 37.4 251
MN0465 5.8-1 70-28 5.8 8.0 3.5 100 15.4 3.78 2.686 44.8 38,036 3.00 23.1 186
NE0569 5.0-1 64-28 5.0 7.5 5.0 96 15.8 6.06 2.596 37.8 92,875 1.00 145.0 1083
NE0569 5.5-4 64-28 5.5 7.9 3.7 96 15.6 6.06 2.596 38.0 92,875 1.00 91.3 1294
NE0569 6.0-5 64-28 6.0 7.1 2.5 96 15.8 6.06 2.596 37.7 92,875 1.00 53.7 374
WA0463 5.5-1 64-22 5.5 7.9 4.9 100 14.8 5.26 2.717 44.8 41,895 1.00 45.7 378
WA0463 6.0-2 64-22 6.0 8.2 3.6 100 14.7 5.26 2.717 44.9 41,895 1.00 28.8 239
WA0463 6.5-1 64-22 6.5 7.9 2.4 100 14.5 5.26 2.717 45.0 41,895 1.00 16.5 156
MN/Road NCAT ALF
1.0E+07
Indiana Nevada
Fit
1.0E+06
1.0E+05
Flow Time, s
1.0E+04 Table 15.
Recommended
1.0E+03 minimum flow time
requirements--
1.0E+02 table 8-21 in the mix
design manual.
1.0E+01 y = 0.362x1.009
R2 = 0.707 Traffic Minimum
Level Flow Time
1.0E+00
Million s
1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 ESALs
Flow Number <3 ---
3 to < 10 20
Figure 3. Plot of flow time as a function of flow number for five 10 to < 30 72
projects from the NCHRP project 9-19 database (39). 30 280
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248 A Manual for Design of Hot Mix Asphalt with Commentary
Table 16. Data from NCHRP project 9-19 used to correlate flow number
and flow time (39).
Con. Dev. Flow Flow
Project Phase Section Stress Stress Temp. Voids No. Time
lb/in2 lb/in2 °C Vol.% s
MN/Road 1 Cell 16 0 207 37.8 7.7 2,041 730
MN/Road 1 Cell 17 0 207 37.8 8.0 2,482 360
MN/Road 1 Cell 18 0 207 37.8 5.9 2,991 935
MN/Road 1 Cell 20 0 207 37.8 6.1 659 236
MN/Road 1 Cell 22 0 207 37.8 6.9 1,511 770
MN/Road 2 Cell 01 0 173 37.8 6.5 683 226
MN/Road 2 Cell 01 0 173 54.4 6.8 27 6
MN/Road 2 Cell 03 0 173 37.8 6.3 416 267
MN/Road 2 Cell 03 0 173 54.4 6.6 49 6
MN/Road 2 Cell 04 0 173 37.8 6.5 753 371
MN/Road 2 Cell 04 0 173 54.4 6.8 24 21
NCAT 2 E06 0 173 37.8 6.9 12,289 1,807
NCAT 2 E06 0 173 54.4 7.1 1,926 455
NCAT 2 N02 0 173 37.8 5.3 23,181 31,953
NCAT 2 N02 0 173 54.4 5.5 6,860 3,024
NCAT 2 N03 0 173 37.8 5.5 22,563 324,161
NCAT 2 N03 0 173 54.4 6.6 1,211 102
NCAT 2 N05 0 173 54.4 6.1 3,682 17,009
NCAT 2 N07 0 173 54.4 5.8 22,203 26,677
NCAT 2 N11 0 173 54.4 6.2 39,000 2,576
NCAT 2 N12 0 173 37.8 5.2 29,605 43,763
NCAT 2 N12 0 173 54.4 5.6 39,000 18,230
ALF 2 Cell 05 0 138 54.4 9.2 241 104
ALF 2 Cell 07 0 69 54.4 10.6 7,761 13,208
ALF 2 Cell 07 0 138 54.4 10.4 6,028 2,611
ALF 2 Cell 08 0 138 54.4 9.3 6,651 3,542
ALF 2 Cell 09 0 138 54.4 7.3 376 202
Indiana 2 4-A 64-28 0 173 54.4 7.7 165,931 2,251
Indiana 2 4-B 64-28 0 173 54.4 3.7 15,060 25,151
Indiana 2 6-B 64-16 I 0 173 54.4 6.4 3,159 589
Nevada 2 HV 64-22b 0 173 37.8 5.8 29,497 47,851
Nevada 2 HV 64-22b 0 173 `54.4 5.9 1,494 1,676
Nevada 2 HV 64-22T 0 173 54.4 6.8 541 205
Nevada 2 HV AC 20-P B 0 173 54.4 7.7 1,009 61
Nevada 2 HV AC 20-P T 0 173 37.8 6.2 3,071 5,603
Nevada 2 HV AC 20-P T 0 173 54.4 6.2 1,858 144
Nevada 2 SP 64-22 B 0 173 54.4 1.8 4,531 5,649
Nevada 2 SP 64-22 T 0 173 37.8 5.8 22,050 1,210
Nevada 2 SP 64-22 T 0 173 54.4 5.8 149 47
Nevada 2 SP AC-20P B 0 173 54.4 1.8 67,000 3,228
Nevada 2 SP AC-20P T 0 173 54.4 5.7 1,087 159
Table 17 (Table 8-22 in the Manual) lists recommended minimum rut depths for the Accel-
Table 17. Recom-
erated Pavement Analyzer (APA) test. At the time of the writing of the Manual and Commen-
mended maximum
rut depths for
tary, there was not a large amount of data on the use of the APA as a performance test. In NCHRP
the APA test-- Report 508, the results of research evaluating the APA were reported (40). It was found that
table 8-22 in the although there were reasonable correlations between rut depths determined with the APA and
mix design manual. those observed in the field on a project-by-project basis, an overall relationship for multiple proj-
ects could not be developed. As a result, NCHRP Report 508 does not provide specific guidelines
Traffic Maximum
Level Rut Depth
for interpreting the results of the APA test (40). For the purposes of providing such guidelines
Million Mm in the Manual, test procedures and requirements for several states were reviewed (41, 42, 43).
ESALs The most common conditions, as reported by Mr. Chad Hawkins at the 2006 APA User Group
<3 ---
3 to < 10 5 Meeting, for running the APA have been reported as a 100 lbf load applied through the hose inflated
10 to < 30 4 to a pressure of 100 lb/in2; the rut depth is measured after 8,000 loading cycles (43). The most
30 3 common test temperature is 64°C. These are the conditions used in Oklahoma--the values in
OCR for page 249
Commentary to the Mix Design Manual for Hot Mix Asphalt 249
Table 16 are, in fact, the same as those used in Oklahoma's APA specification, although Okla-
homa includes requirements at lower traffic levels (41). North Carolina's specification uses
somewhat higher maximum rut depth values, but its standard specifies a load of 120 lbf and a
hose pressure of 120 lb/in2 (42). According to LTTPBind Version 3.1, the typical high-temperature
PG binder grade in Oklahoma is very close to 64.0, so the test temperature in their APA standard
corresponds to the high temperature binder grade at 98% reliability. This is the basis for the sug-
gested test temperature for the APA equal to the high-temperature binder performance grade
specified by the local state highway agency for traffic levels of 3 million ESALs or less.
Establishing guidelines for interpreting the results of the Hamburg wheel tracking test is dif-
ficult because the test is not widely used (43). The Manual gives test conditions and minimum
passes to a half-inch rut depth as specified by the State of Texas (44). Test conditions for the
Hamburg test as specified by the Texas Department of Transportation are as follows (44):
· Specimen dimensions: 150 mm (6 in.) in diameter, 62 ± 2 mm (2.4 in.) thick
· Wheel load: 705 ± 2 N (158 ± 0.5 lb)
· Air void content: 7 ± 1%
· Test temperature: 50 ± 1°C
Test requirements used in Texas--in terms of minimum passes to a 0.5-inch rut depth--are
given in Table 18 (Table 8-23 in the Manual). The Manual states that agencies wishing to use the
Hamburg device as a performance test should consider doing an engineering study to develop
appropriate requirements for their local conditions and materials.
Suggested test values for both maximum permanent shear strain (MPSS) determined using the
repeated shear at constant height (RSCH) test and strength measured using the high-temperature
indirect tension (HT/IDT) test were calculated in a manner similar to that used to develop suggested
requirements for the flow number and flow time test. Data from NCHRP Projects 9-25/9-31 were
used in combination with the rutting-resistivity model (Equation 1) to estimate maximum allow-
able traffic, which was then compared to test values for MPSS and IDT strength (4, 27). For both
sets of test data, the air void content varied from about 2 to about 6%, with an average of 4.0%. The
protocol for the HT/IDT test is to compact specimens using the design gyrations (Ndesign) which is
what was done for the 9-25/9-31 tests. However, the protocol for RSCH/MPSS (AASHTO T 320) is
to prepare specimens at an air void content of 3.0 ± 0.5%. This requires an adjustment to the MPSS
values reported in 9-25/9-31, which was made by developing a relationship between MPSS and al-
lowable traffic at the design voids, calculating the allowable traffic at 3% voids, and then adjusting
the MPSS value according to the difference between the estimated allowable traffic values at the de-
sign voids and at 3% air voids. As discussed in the analysis of flow number values, in order to avoid
rejection of a large number of mixes by performance testing, the traffic level used in estimating the
values for both HT/IDT strength and MPSS was the midpoint of the traffic levels for the 3 to 10 mil-
lion ESALs and 10 to 30 million ESAL ranges; for traffic levels above 30 million ESALs, a value of
65 million ESALs was used to estimate the minimum IDT and maximum MPSS.
Table 18. Texas requirements
for Hamburg wheel tracking
test--table 8-23 in the mix
design manual (44).
High-Temperature Minimum Passes to
Binder Grade 0.5-inch Rut Depth
PG 64 or lower 10,000
PG 70 15,000
PG 76 or higher 20,000
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250 A Manual for Design of Hot Mix Asphalt with Commentary
A final adjustment to HT/IDT strength is needed because the suggested protocol--testing at a
loading rate of 50 mm/min at 10°C below the average 7-day maximum pavement temperature,
rather than at 3.75 mm/min at 20°C below the critical high pavement temperature--gives IDT
strength values about 10% higher than the original protocol, as used in 9-25/9-31. The relationship
between IDT strength using the two protocols is shown in Figure 4, reproduced from a letter report
prepared for PennDOT as part of a small research project investigating the IDT strength test (45).
Figures 5 and 6, respectively, show the relationships between MPSS and IDT strength and allowable
ESALs determined from the 9-25/9-31 data. Tables 19 and 20 show recommended values for max-
imum MPSS and minimum IDT strength as determined from the relationships shown in the two
figures. The data on which this analysis is based are summarized in Table 21 (4).
IDT Str. at 40 C and 50 mm/min
120
100
y = 1.10x
equality
80 R2 = 0.99
60
40
20
0
0 20 40 60 80 100 120
IDT Str. at 30 C and 3.75 mm/min
Table 19.
Recommended Figure 4. Comparison of IDT strengths from the
maximum values for two procedures (45).
MPSS determined
using the SST/RSCH 8.0
MPSS, % at 3 % Voids
test--table 8-24 in the
mix design manual. 6.0
y = -1.13Ln(x) + 5.5
R2 = 76 %
Traffic Maximum Value 4.0
Level for MPSS
Million %
ESALs 2.0
<3 ---
3 to < 10 3.4
10 to < 30 2.1 0.0
30 0.8 0 20 40 60 80 100 120 140
Design Traffic, Million ESALs
Table 20. Figure 5. Plot of MPSS vs. allowable million ESALs
Recommended for NCHRP 9-25/9-31 data.
minimum high-
temperature 800
indirect tensile
strength require-
IDT Strength, kPa
600
ments--table 8-25
in the mix design 400
manual.
y = 92.6Ln(x) + 68
Minimum 200 R2 = 89 %
Traffic HT/IDT
Level Strength 0
Million kPa
ESALs 0 50 100 150
<3 --- Design Traffic, Million ESALs
3 to < 10 270
10 to < 30 380 Figure 6. Plot of IDT strength vs. estimated allow-
30 500 able million ESALs for NCHRP 9-25/9-31 data.
OCR for page 251
Commentary to the Mix Design Manual for Hot Mix Asphalt 251
Table 21. Data used in developing correlations between maximum permanent shear strain,
high-temperature IDT strength and estimated allowable traffic (4).
Agg. Traffic
Aggregate, NMAS, Binder Agg. Spec. Binder MPSS @ HT/IDT @ 7%
Gradation Grade Temp. Ndesign Gs Surface |G*|/sin VTM VMA 3% Voids Strength Voids
°C Gyrations m2/kg Pa Vol.% Vol.% % lb/in2 MESALs
KY limestone, 19
PG 64-22 54 100 2660 4.32 14,500 4.40 15.10 149.6 4.75 5.59
mm, coarse
KY limestone, 19
PG 64-22 54 50 2660 4.32 14,500 3.30 17.40 114.1 7.97 0.78
mm, coarse
KY limestone, 19
PG 64-22 54 100 2648 4.75 14,500 3.60 12.10 226.8 3.92 13.14
mm, dense
KY limestone, 19
PG 64-22 54 50 2648 4.75 14,500 4.00 13.60 182.0 4.83 3.68
mm, dense
Crushed gravel, 19
PG 64-22 54 100 2566 3.76 14,500 3.20 14.00 202.0 3.62 2.91
mm, coarse
Crushed gravel, 19
PG 64-22 54 75 2566 3.76 14,500 4.20 14.90 192.1 3.71 2.29
mm, coarse
Crushed gravel, 19
PG 64-22 54 100 2575 4.32 14,500 4.40 12.80 316.9 1.83 10.09
mm, dense
Crushed gravel, 19
PG 64-22 54 75 2575 4.32 14,500 3.40 13.00 273.5 2.64 4.31
mm, dense
VA limestone, 9.5
PG 64-22 54 100 2671 4.40 14,500 4.30 15.80 160.0 4.71 4.76
mm, coarse
VA limestone, 9.5
PG 64-22 54 50 2671 4.40 14,500 3.50 18.20 6.46 0.75
mm, coarse
VA limestone, 9.5
PG 76-16 54 100 2671 4.40 66,200 4.30 15.80 314.8 1.61 38.29
mm, coarse
VA limestone, 9.5
PG 58-28 54 100 2671 4.40 4,880 4.30 15.80 108.6 6.21 1.07
mm, coarse
VA limestone, 9.5
PG 64-22 54 100 2659 6.21 14,500 4.50 18.70 199.6 4.19 6.48
mm, fine
VA limestone, 9.5
PG 64-22 54 50 2659 6.21 14,500 3.90 20.30 7.39 1.44
mm, fine
VA limestone, 9.5
PG 76-16 54 100 2659 6.21 66,200 4.50 18.70 416.8 2.12 52.14
mm, fine
Granite, 12.5 mm,
PG 64-22 54 125 2632 4.79 14,500 4.20 13.10 2.33 16.37
dense
Granite, 12.5 mm,
PG 76-16 54 125 2632 4.79 66,200 4.20 13.10 589.6 0.24 131.71
dense
Granite, 12.5 mm,
PG 58-28 54 125 2632 4.79 4,880 4.20 13.10 164.8 2.54 3.67
dense
Granite, 12.5 mm,
PG 64-22 54 125 2631 6.15 14,500 3.50 14.90 2.20 14.49
fine
Granite, 12.5 mm,
PG 76-16 54 125 2631 6.15 66,200 3.50 14.90 550.6 0.33 116.59
fine
Granite, 12.5 mm, fine PG 58-28 54 125 2631 6.15 4,880 3.50 14.90 171.4 3.00 3.25
VA limestone, 9.5
PG 64-22 60 100 2671 4.40 6,920 4.30 15.80 116.2 4.90 1.72
mm, coarse
VA limestone, 9.5
PG 76-16 60 100 2671 4.40 33,100 4.30 15.80 235.5 2.39 14.78
mm, coarse
VA limestone, 9.5
PG 64-22 60 100 2659 6.21 6,920 4.50 18.70 133.1 5.42 2.35
mm, fine
VA limestone, 9.5
PG 76-16 60 100 2659 6.21 33,100 4.50 18.70 300.6 2.73 20.13
mm, fine
Granite, 12.5 mm,
PG 64-22 60 125 2632 4.79 6,920 4.20 13.10 198.2 2.36 5.93
dense
Granite, 12.5 mm,
PG 76-16 60 125 2632 4.79 33,100 4.20 13.10 436.1 0.65 50.85
dense
Granite, 12.5 mm,
PG 64-22 60 125 2631 6.15 6,920 3.50 14.90 247.2 2.51 5.25
fine
Granite, 12.5 mm,
PG 76-16 60 125 2631 6.15 33,100 3.50 14.90 460.8 1.30 45.01
fine
OCR for page 252
252 A Manual for Design of Hot Mix Asphalt with Commentary
The final critical part of the mix design procedure as described in the Manual is adjustment of
the performance test temperature to account for slow traffic speeds. This is necessary because as
traffic speed decreases, permanent deformation can significantly increase. Three approaches are
possible to account for this effect: (1) increase or decrease required performance test value as
traffic speed decreases; (2) decrease test loading rate as traffic speed decreases; or (3) increase test
temperature as traffic speed decreases. There is not enough data at this time to use the first
approach and it would also be a relatively complicated approach. The second approach is not
feasible, since the loading rate on some of the proposed test methods is fixed and is not normally
varied. Therefore, the third approach is suggested. The Manual suggests increasing the test tem-
perature 6°C for slow traffic and 12°C for very slow traffic. These adjustments were calculated
directly from Equation 6 given above, using a traffic speed of 70 kph for fast traffic, 25 kph for
slow traffic and 10 kph for very slow traffic.
