Evaluation of Rutting Resistance
in Warm-Mix Asphalts Containing
Moist Aggregate
Feipeng Xiao, Serji N. Amirkhanian, and Bradley J. Putman
As the asphalt industry becomes more aware of warm-mix
technologies, the need increases to perform research to determine
the feasibility of these technologies. Some European countries are
already using warm-mix technologies to produce asphalt mixes at
lower temperatures without significantly affecting the quality of
the mixtures. While energy savings and air quality improvements
by using warm asphalt are appealing, the performance of warm
asphalt in the United States is not well known. However, a large
number of projects were completed recently. The mix designs,
binder sources, equipment, climate conditions, work practices, and
many other factors are quite different in the United States than in
Europe. Thus, warm-mix asphalt (WMA) requires more investigation and research before being commonly incorporated in the
United States (3– 6).
WMA is an asphalt mixture that is mixed and compacted at
temperatures lower than is conventional HMA. Typically, the mixing
temperatures of WMA range from 100°C to 140°C (212°F to 280°F)
compared with the mixing temperatures of 150°C to 180°C (300°F
to 350°F) for conventional HMA (3–6). Thus, warm asphalt has been
gaining increasing popularity recently, due to rising energy prices,
potential effects on the climate, and more stringent environmental
regulations (3–11).
Studies conducted at National Center for Asphalt Technology
indicated that as the mixing temperatures are reduced for WMA,
the mixtures show increased tendencies toward rutting and moisture
susceptibility (3–5). This is a result of the aggregates used in the
mixture not drying completely. Thus, WMA producers should find
the right balance between lowering the mixing temperatures, using
sufficient amounts of anti-stripping agents and sufficiently drying
the aggregates used in the mixtures. In addition, Xiao et al. found
that the hydrated lime plays an important role in improving the
indirect tensile strength (ITS) and tensile strength ratio values of
WMA mixtures whether the aggregate contained moisture or not (7).
Generally, at a mixing temperature of 100°C to 140°C (212°F to 280°F),
the aggregate may not be completely dried during the mixing process.
Even though some states in the United States and other countries
have specifications that require a completely dry aggregate in WMA
mixtures (12), not much research has been conducted in determining the effects of the moist aggregates with WMA additives that may
result in the rutting failure of the pavement.
The objective of this study was to investigate the influence of
WMA additive, hydrated lime, and moisture content of aggregate
on the rutting resistance of the mixtures using conventional Asphalt
Pavement Analyzer (APA) testing procedures in regard to dry and
conditioned (saturated wet) specimens and to perform statistical
significant analysis for their effects on rutting characteristics.
In recent years, rising energy prices and more stringent environmental
regulations have resulted in an interest in warm-mix asphalt (WMA)
technologies to decrease the energy consumption and emissions associated with conventional hot-mix asphalt production. In this study, the
objective was to conduct a laboratory investigation of rutting resistance
in WMA mixtures containing moist aggregates. Rut depth, weight
loss, and gyration number of dry and conditioned specimens were
measured for all of the mixtures. The experimental design included two
aggregate moisture contents (0% and ∼0.5% by weight of the dry mass
of the aggregate), two lime contents (1% and 2% lime by weight of dry
aggregate), three WMA additives (Aspha-min, Sasobit, and Evotherm),
and three aggregate sources. Thirty-six mixtures were prepared, and
216 specimens were tested in this study. Test results indicated that the
aggregate source significantly affects the rutting resistance regardless
of the WMA additive, lime content, and moisture content. In addition,
rut depth of the mixture containing moist aggregate generally satisfies
the demand of pavement performance without additional treatment. The
mixture with Sasobit additive exhibited the best rutting resistance.
The mixtures containing Aspha-min and Evotherm additives generally
showed a rut resistance similar to that of the control mixture.
Rutting distress, a type of permanent deformation, usually caused
by consolidation or lateral movement of pavement materials due
to traffic loading in any of a pavement’s layers or subgrade, can
result in a vehicle hydroplaning and tend to pull a vehicle toward
the rut path as it is steered across the rut. Rutting damage in flexible
pavements generally depends on three constituents of hot-mix asphalt
(HMA): aggregate, binder, and air void. Aggregate shape and texture
play a key role in determining the interlock level of the aggregate and
thus influence the lateral movement of pavement (1, 2). In addition,
the binder used in various geographic areas and related to the performance temperature is an important factor in rutting. Moreover,
the air void content of the pavement is also a vital part; generally,
high air content can be more prone to rutting. Additionally, some
researchers indicate that the truck speed, contact pressure, layer
thickness, and truck wheel wander also affect the rutting behavior
of asphalt pavement (1).
F. Xiao, Asphalt Rubber Technology Service, and S. N. Amirkhanian and B. J. Putman,
Department of Civil Engineering, 2002 Hugo Drive, Clemson University, Clemson,
SC 29634-0911. Corresponding author: F. Xiao, feipenx@clemson.edu.
Transportation Research Record: Journal of the Transportation Research Board,
No. 2180, Transportation Research Board of the National Academies, Washington,
D.C., 2010, pp. 75–84.
DOI: 10.3141/2180-09
75
76
Transportation Research Record 2180
TABLE 1
Physical and Chemical Properties of WMA Additive
Property
Aspha-min
Sasobit H8
Evotherm H5
Ingredients
Sodium aluminosilicate
Na2O • Al2O3 • 2SiO2
Solid saturated
hydrocarbons
Physical state
Color
Odor
Molecular weight
Specific gravity
Vapor density
Bulk density
pH value
Boiling point
Flashpoint
Solubility in water
Granular powder
White
Odorless
365
2 (20°C)
—
500–600 kg/m3
11–12
—
—
Insoluble
Pastilles, flakes
Off-white to pale brown
Practically odorless
Approx. 1,000 g/mol
0.9 (25°C)
—
—
Neutral
—
285°C (ASTM D92)
Insoluble
Modified tall oil fatty acid
polyamine condensate
water
Viscous liquid
Amber (dark)
Fishy, amine-like
values of Al2O3 and SiO2, while Aggregate II has the lowest value.
Although both Aggregates I and III are granite, their engineering
properties are very different (2, 5). For example, Aggregate I has
the larger Los Angeles abrasion loss, absorption, and specific gravity
values. These properties may affect the performance of mixtures.
EXPERIMENTAL PROGRAM AND PROCEDURES
Materials
The experimental design in this study included the use of three WMA
additives: Aspha-min, Sasobit, and Evotherm, shown in Table 1; two
moisture percentages, 0%, and ∼0.5% by weight of dry aggregate; and
two hydrated lime contents, 1% and 2% by weight of dry aggregate.
Three aggregate sources (designated as I, II, and III) and one binder
grade (PG 64-22) were used for this project (Figure 1).
The engineering properties of the coarse and fine aggregates used
in this study are shown in Tables 2 and 3. Aggregate sources I and III
(granite) are composed predominantly of quartz and potassium
feldspar, while Aggregate II (schist) is a metamorphic rock. Among
the three aggregate sources, Aggregate I has the largest percentage
Aggregate
III
1%
Lime
Control
Same as
Agg. II
~0.5%
Moisture
Sasobit
APA
6 Samples
(3 wet and 3 dry)
FIGURE 1
The mix design procedure included the aggregates used for a
mixture (12.5 mm) that satisfied the specifications set forth by the
South Carolina Department of Transportation (SCDOT) for a surface
Type B mixture. Gradations for each aggregate source (I, II, and III)
are shown in Table 4. The design aggregate gradations for each aggregate source were the same for various WMA additives (Aspha-min,
0%
Moisture
1%
Lime
Aspha-min Evotherm
Mix Design, Sample Fabrication, and Testing
Aggregate
II
Aggregate
I
Same as
Agg. II
1.03–1.08
<1
1.03 g/cm3
9–11
>100°C
—
Water soluble
Flowchart of experimental design.
Same as
2% Lime
2%
Lime
Aspha-min Evotherm
APA
6 Samples
(3 wet and 3 dry)
Control
Sasobit
Xiao, Amirkhanian, and Putman
TABLE 2
77
Physical Properties of Coarse Aggregate
Specific Gravity
Coarse
Aggregate
Soundness, % Loss at
Five Cycles
Los Angeles
Abrasion
Loss (%)
Absorption
(%)
Dry
(BLK)
SSD
(BLK)
Apparent
46
32
30
1.10
0.70
0.50
2.690
2.770
2.630
2.720
2.780
2.640
2.770
2.820
2.660
I
II
III
3
⁄4 to 3⁄8 in.
⁄8 in. to Νο. 4a
Sand
Equivalent
Hardness
0.1
0.9
4.1
—
38
53
5
5
6
3
0.1
0.6
1.7
NOTE: BLK = bulk; SSD = saturated surface dry.
a
Sieve size.
Sasobit, and Evotherm); aggregate moistures; and lime contents.
The rheological properties of the asphalt binder with each WMA
additive were measured by another researcher in the same laboratory,
and results were reported elsewhere (13). A total of 36 mixtures
were used in this study.
To simulate the possibility of using moist aggregate sources in
the field, a preliminary study was conducted (7 ). Initially, several
moisture contents (0.5%, 1%, and 1.5%) were tested. However, after
a series of trials, it was found that an approximately 0.5% moisture
content can be achieved after the aggregate was heated to a warm
mixing temperature.
The temperatures, shown in Table 5, were determined in accordance
with a previous research project (7 ). The mixing temperatures of
materials were employed after a series of trial processes to achieve a
mixing temperature of 121°C to 127°C. The compaction temperature
of 115°C to 121°C was used in this study regardless of WMA and
aggregate types. For a mixture containing damp (moist) aggregate,
initially, 3% hot water (by weight of aggregate) (60°C to 70°C) was
added to the completely dried aggregate (160°C to 165°C) and then
blended by hand for 30 s before mixing (7). After that, the aggregate
contained approximately 0.5% moisture and had a temperature of
121°C to 127°C. Most of the water was evaporated during this process.
Finally, the asphalt binder was poured and mixed with the damp
aggregate. It took approximately 11⁄2 min for a mechanical blender
to completely coat the aggregate particles based on the workability of
the mixture. The purpose of using hot water and a higher temperature
aggregate in this study was to achieve the target moisture content.
The preliminary work indicated that 0.5% moisture content was easy
to obtain. In addition, it was found that the wet aggregate might not
be able to be completely dried when it was heated to 110°C to 127°C
in the oven. The hypothesis was that in the field an aggregate might
still contain some moisture at a relatively high temperature.
For this study, the optimum binder content was defined as the
amount of binder required to achieve 4.0% air voids in accordance with
SCDOT volumetric specifications. After the mix designs were
completed, six Superpave® gyratory compacted specimens for each
mixture (150 mm in diameter and 75 mm in height) were prepared
with 4% ± 0.5% air voids. During specimen preparation, the weight
TABLE 3
Physical Properties of Fine Aggregate
loss value of the mixture was measured due to the evaporation of
moisture during a short-term aging process. These evaporation
values from various mixtures would be helpful in determining the
effects of moisture and hydrated lime content. Moreover, the gyration number of each specimen was also investigated to identify the
influences of mixture type.
The samples were then conditioned in the APA chamber at 64°C
(147°F) for 6 h and tested at the same temperature to determine the rut
depth (8,050 cycles). Three rut samples were tested in dry condition,
and the other three were tested in wet condition. Similar to what
is done for a moisture susceptibility test, the wet rut samples were
saturated to 70% to 80% and submerged in a water bath at 60°C
(140°F) for 24 h, followed by conditioning at 64°C (147°F) for
6 h before testing. This test was conducted in accordance with
AASHTO TP63-03. A total of 216 specimens were evaluated for
rut depth in this study.
ANALYSIS OF TEST RESULTS
Statistical Considerations
The rut depth values of dry and wet specimens were statistically analyzed at the 5% level of significance (0.05 probability of a Type I
error) with respect to the effects of the moisture with hydrated lime
TABLE 4
Aggregate Gradations and Combinations
Specification
Type of Aggregate
Sieve
Limits
Agg. I
Agg. II
Agg. III
19.0 mm
12.5 mm
9.5 mm
4.75 mm
2.36 mm
0.60 mm
0.15 mm
0.075 mm
98–100
90–100
74–90
46–62
25–41
9–21
4–12
2–8
99.0
93.9
88.6
48.8
29.6
18.3
6.6
3.3
99.6
93.7
83.8
49.3
39.1
17.8
8.5
5.1
99.7
94.3
84.7
50.9
32.5
17.3
8.2
5.0
9
61
10
19
1
11
46
16
26
1
30
32
18
19
1
Aggregate Blend
Fine
Aggregate
I
II
III
Fineness
Modulus
Absorption
(%)
SSD
(BLK)
Soundness,
% Loss
2.82
2.81
3.20
0.40
0.20
0.60
2.590
2.650
2.640
4.5
2.8
0.1
Stone Νο. 57
Stone Νο. 789
R.S.
M.S.
Lime
78
Transportation Research Record 2180
TABLE 5
Mixing and Compaction Temperatures of Mixtures
Material Temperatures for Mixing (°C)
Aggregate (A, B, C)
Mix Type
Aggregate
Water
Binder
a
0.5%M +1% lime
a
0.5%M +2% lime
a
0.0%M +1% lime
160–165
160–165
121–127
60–70
60–70
—
127–132b
127–132b
121–127b
Compaction
Temperature (°C)
115–121
115–121
115–121
a
Moisture.
Binder temperature is 148°C when using Evotherm additive.
b
and WMA additive types. Relationship analysis of rut depth and
gyration number and rut depth distribution of various mixtures were
also used to explore the influences of hydrated lime, moisture content,
WMA additive, and aggregate source.
rate during the mixing process, while some might be dried during
the short-term aging process and even contained by the coated
aggregate. Figure 2 shows the total weight loss value of mixtures
after a short-term aging process. This weight loss might include
the combined evaporation of moisture and light oil. For example, in
Figure 2, the mixture without moisture still has a loss of approximately
4 g regardless of aggregate source and WMA additive. As shown
in Figure 2a, the mixture containing moist aggregate has a higher
weight loss than the mixture without moisture. Except for the mixture
with Evotherm, other mixtures show slight increases in weight loss
as the lime percentage increases from 1% to 2%. In Figure 2b and 2c,
though the weight loss increases as the mixture contains moisture,
Weight Loss Analysis
0.35
0.35
0.30
0.30
Loss of mixture (%)
Loss of mixture (%)
The moisture of aggregate was completely evaporated after being
dried in the oven overnight in the laboratory, and this dried aggregate
mixed with asphalt binder produces the mixture. However, as the
mixture contains the moist aggregate, some moisture might evapo-
0.25
0.20
0.15
0.10
0.05
C
A
E
S
0.25
0.20
0.15
0.10
0.05
C
A
E
S
0.00
0.00
1% lime
1% lime
1% lime
2% lime
~0.5% Moisture
Mix type
(a)
1% lime
2% lime
~0.5% Moisture
Mix type
(b)
Loss of mixture (%)
0.35
0.30
0.25
0.20
0.15
0.10
0.05
C
A
E
S
0.00
1% lime
1% lime
2% lime
~0.5% Moisture
Mix type
(c)
FIGURE 2 Weight loss values of samples during curing process: (a) Aggregate I, (b) Aggregate II,
and (c) Aggregate III.
Xiao, Amirkhanian, and Putman
79
the mixture with 1% lime generally shows the highest weight loss
value. With respect to the effects of WMA additive and aggregate
source, Figure 2 illustrates that, in most cases, the mixture from
Aggregate I and containing Evotherm or Sasobit has a relatively
higher weight loss in general. However, the weight loss value of the
mixture with various WMA additives from Aggregates II and III
is not significant.
as the moisture content of the mixture increases (Figure 3b). In
addition, the gyration number values of these mixtures were not more
than 40; it seems that the mixture from Aggregate II was more
easily compacted. The gyration numbers shown in Figure 3c do not
exhibit a clear trend, and these numbers are remarkably different
when 2% lime is used.
Dry Rut Depth Analysis
Gyration Number Analysis
The dry rut results shown in Figure 4a indicate that the rut depths of
both the control mixture and mixture containing Sasobit additive
decrease as the additional moisture and 1% lime were added. Generally, rut depth of the mixture with moisture and 1% lime was the
lowest regardless of WMA type. In most cases, the mixture with
Aspha-min or Evotherm has a higher rut depth, while the mixture
containing Sasobit exhibited the lowest value. In Figure 4b, the
mixture with moisture generally had the lower rut depth regardless
of lime percentage and WMA type. However, the mixture with
moisture and containing Evotherm shows a noticeable higher rut
depth than do other mixtures with Aggregate II. The rut values of
mixtures made from Aggregate III are shown in Figure 4c. It can be
For gyratory asphalt samples, an increase of gyration number typically
reduces its air void content. AASHTO TP63-03 indicates that the
APA sample should be compacted to 4% ± 0.5% air voids before
testing. The gyration number required to reach the target air voids of
each mixture is shown in Figure 3. In Figure 3a, the gyration number
decreases with an increase of moisture or lime contents, or both,
regardless of the WMA additive. In other words, the mixture from
Aggregate I is easier to compact as lime or moisture contents, or both,
increase. The gyration number values of these mixtures with various
WMA additives are generally close. However, in most cases, the
gyration number of the mixtures with Aggregate II slightly increases
70
70
C
A
E
C
S
A
50
40
30
20
10
S
50
40
30
20
10
0
0
1% lime
2% lime
1% lime
1% lime
~0.5% Moisture
Mix type
(a)
1% lime
Gyration number
60
2% lime
~0.5% Moisture
Mix type
(b)
70
C
A
E
S
50
40
30
20
10
0
1% lime
1% lime
2% lime
~0.5% Moisture
Mix type
(c)
FIGURE 3
E
60
Gyration number
Gyration number
60
Gyration number of mixtures: (a) Aggregate I, (b) Aggregate II, and (c) Aggregate III.
80
Transportation Research Record 2180
7.0
7.0
C
E
S
C
6.0
Rut depth (mm)
Rut depth (mm)
6.0
A
5.0
4.0
3.0
2.0
1.0
A
E
S
5.0
4.0
3.0
2.0
1.0
0.0
0.0
1% lime
2% lime
1% lime
1% lime
~0.5% Moisture
Mix type
(a)
1% lime
2% lime
~0.5% Moisture
Mix type
(b)
7.0
C
A
E
S
Rut depth (mm)
6.0
5.0
4.0
3.0
2.0
1.0
0.0
1% lime
1% lime
2% lime
~0.5% Moisture
Mix type
(c)
FIGURE 4
Rut depth values of dry samples: (a) Aggregate I, (b) Aggregate II, and (c) Aggregate III.
seen that the mixture containing Sasobit had the lowest value, and
this value was lower as the mixture contains moisture. However, the
other mixtures generally have similar rut values. Statistical analysis
shown in Tables 6 and 7 indicates that the rut depth of the mixtures
made from Aggregate I was significantly different for the various
lime and moisture contents. However, the mixtures made from
Aggregate III did not have significantly different rut values. With
respect to the aggregate effect on the dry rut values, Figure 4 illustrates
that the rut values from various aggregate sources were generally
different, though they were prepared with the same WMA additive
and moisture. With respect to the effect of WMA additive, Figure 4
illustrates that the mixture containing Evotherm showed a higher
rut depth while the use of Saboit reduced the rut depth. Statistical
analysis in Tables 6 and 7 illustrates that the rut values were not significantly different between control specimens and those containing
the Aspha-min additive. However, the dry specimens containing
TABLE 7 Statistical Analysis of Rut Depth Shown
by Control Group and Additives
Aggregate
TABLE 6 Statistical Analysis of Rut Depth Shown
by Percentages of Lime and Moisture
Dry Sample
Aggregate
I
II
III
C–A
C–S
C–E
A–S
A–E
S–E
N
N
N
Y
Y
Y
N
N
Y
Y
N
Y
N
N
Y
Y
Y
Y
N
N
N
Y
Y
Y
N
Y
N
Y
Y
Y
N
N
N
N
N
Y
Dry sample
I
II
III
Wet Sample
0–1
0–2
1–2
0–1
0–2
1–2
Y
Y
N
Y
Y
N
Y
N
N
Y
Y
Y
Y
Y
Y
N
N
Y
NOTE: 0%–1% lime; 1 = moisture + 1% lime; 2 = moisture + 2% lime;
Y = significant difference; N = no significant difference (α = 0.05).
Wet sample
I
II
III
NOTE: C = control; A = Aspha-min; S = Sasobit; E = Evotherm; Y = significant
difference; N = no significant difference (α = 0.05).
Xiao, Amirkhanian, and Putman
81
Sasobit generally had significantly different rut depths in comparison
with other specimens.
mixture made from Aggregate III had a lower rut resistance when
the aggregate contained moisture. Obviously, the mixture from
various aggregate sources exhibited significantly different rutting
resistance. The rut depth of the mixture from Aggregate II has the
lowest rut value, while the mixture made from Aggregate I showed
the highest one. Generally, after treatment, the mixture containing
Sasobit additive exhibits the greatest rutting resistance in comparison with other WMA additives regardless of lime content, moisture
percentage, and aggregate source. Statistical analysis of wet specimens
was summarized in Tables 6 and 7. No significant difference was
noted in rut depth for the mixtures with Aspha-min and control or
Aspha-min and Evotherm. However, the wet specimens containing
Sasobit generally had significantly different rut depths in comparison
with others.
Wet Rut Depth Analysis
Moisture effect on ITS of the mixture containing moist aggregate had
been investigated by Xiao et al. (7 ). Their research results indicated
that hydrated lime is beneficial in achieving the moisture resistance
of a mixture offset by the moist aggregate. However, the rutting resistance of a mixture containing moist aggregate under a wet condition
(mentioned earlier) is not clearly understood from the standpoint of
the influence of moisture. In this study, Figure 5 shows all rut depths
of various mixtures after warm water bath treatment. In Figure 5a,
it can be seen that the mixtures made from Aggregate I had a slightly
lower rut value when moist aggregate was used regardless of WMA
additive and lime content. Moreover, Figure 5a shows that the mixture with 2% lime had better rut resistance. With respect to the effect
of WMA additive, the mixture containing Sasobit additive had
the best rut resistance. Similarly, as shown in Figure 5b, the mixture
made from Aggregate II generally had a slightly higher rut resistance (lower rut depth) when the aggregate contained moisture. In
addition, the mixture with Sasobit additive had a lower rut depth.
However, unlike Figure 5a and 5b, Figure 5c indicates that the
Difference Analysis Comparing
Dry and Wet Rut Depth
To compare the differences in rut depth for dry and wet conditions,
content shown in Figure 6 was used to state the values (i.e., dry rut
depth minus wet rut depth). The negative values shown in Figure 6
indicate that the dry rut depth is less than the wet one after the treatment. However, the positive values illustrate the dry rut depth is
7.0
7.0
C
E
S
C
6.0
Rut depth (mm)
Rut depth (mm)
6.0
A
5.0
4.0
3.0
2.0
1.0
E
5.0
4.0
3.0
2.0
1.0
0.0
0.0
1% lime
2% lime
1% lime
1% lime
~0.5% Moisture
Mix type
(a)
1% lime
C
(b)
A
E
S
6.0
Rut depth (mm)
2% lime
~0.5% Moisture
Mix type
7.0
5.0
4.0
3.0
2.0
1.0
0.0
1% lime
1% lime
2% lime
~0.5% Moisture
Mix type
(c)
FIGURE 5
A
Rut depth values of wet samples: (a) Aggregate I, (b) Aggregate II, and (c) Aggregate III.
S
82
Transportation Research Record 2180
3.0
C
A
E
Difference in rut depth (mm)
Difference in rut depth (mm)
3.0
S
2.0
1.0
0.0
-1.0
-2.0
-3.0
C
A
E
S
2.0
1.0
0.0
-1.0
-2.0
-3.0
1% lime
2% lime
1% lime
1% lime
~0.5% Moisture
Mix type (Dry-Wet)
1% lime
2% lime
~0.5% Moisture
Mix type (Dry-Wet)
(a)
(b)
Difference in rut depth (mm)
3.0
C
A
E
S
2.0
1.0
0.0
-1.0
-2.0
1% lime
1% lime
2% lime
~0.5% Moisture
Mix type (Dry-Wet)
(c)
FIGURE 6 Differences in rut depth values between dry–wet samples: (a) Aggregate I, (b) Aggregate II,
and (c) Aggregate III.
higher, and the effect of the treatment on rutting resistance can be
neglected.
Figure 6a shows that the dry specimens generally had a lower rut
depth than wet ones, thus exhibiting a better rutting resistance. This
result also indicates that the mixture made from Aggregate I had
higher potential rut depth after treatment in warm water. In Figure 6b,
it can be seen that in most cases, the wet rut depths of the mixtures
with moisture were higher and thus more sensitive to rutting when
exposed to moisture. Results shown in Figure 6c illustrate that the
wet specimen from Aggregate III had a higher rutting resistance
(e.g., dry rut depth is higher). The treated specimens made from
Aggregate III were generally less susceptible to rutting in comparison
with those from Aggregates I and II. In addition, the influence of
WMA additive and lime content on rutting resistance in regard to
the dry or wet condition is generally not significant.
Relationship of Rut Depth and Gyration Number
NCHRP report 478 indicated that an appropriate compaction parameter, gyration number, is related to the stiffness and rutting resistance
of an asphalt mixture (14). In this study, the gyration numbers of
various mixtures were obtained in accordance with compaction tem-
peratures designated in Superpave mix design procedures (Table 5).
Although the compaction temperatures were the same for all types
of warm asphalt mixtures, the gyration number was significantly
different for each mixture. As shown in Figure 7, in most cases, the
gyration numbers of overall mixtures were from 20 to 60, and the
related rut depth values ranged from 1 mm to 6 mm. The general trends
for dry and wet specimens are shown in Figure 7a and 7b. These two
linear trends indicate that, as expected, the increase of gyration
number in the mixture reduces its potential rut depth, though the
coefficient of determination (R2) values of regression models are low.
Distribution Analysis of Rut Depth
To study further the effects of aggregate source, WMA additive,
and moisture and lime content on the rut resistance, the rut depth
distribution was performed in regard to varying mixtures. This distribution analysis of the mixtures was categorized into dry and wet
groups. Figure 8a and 8b shows the influence of aggregate source.
In this figure, the rut depths in the peak distributions of the mixtures
containing Aggregate II are close to 1 to 2 mm and 2 to 3 mm for
dry and wet specimens, respectively. However, Figure 8a illustrates
that the mixtures containing Aggregate III have the rut peak values
8
1% lime
1% lime+0.5% M
8
2% lime+0.5% M
R2 = 0.10
5
4
R2 = 0.001
3
2
Rut depth (mm)
Rut depth (mm)
6
R2 = 0.30
0
20
FIGURE 7
40
60
Gyration number
(a)
80
6
5
4
R2 = 0.151
3
R2 = 0.015
2
R2 = 0.376
0
20
40
60
Gyration number
(b)
80
Relationships between rut depth and gyration number: (a) dry sample and (b) wet sample.
40
40
I
II
I
III
Frequency (%)
30
20
10
0
II
III
30
20
10
0
0~1 1~2 2~3 3~4 4~5 5~6 6~7 7~8
Rut range of dry sample (mm)
(a)
0~1 1~2 2~3 3~4 4~5 5~6 6~7 7~8
Rut range of wet sample (mm)
(b)
30
30
A
E
S
C
Frequency (%)
C
Frequency (%)
2% lime+0.5% M
0
0
20
10
0
A
E
S
20
10
0
0~1 1~2 2~3 3~4 4~5 5~6 6~7 7~8
Rut range of dry sample (mm)
0~1 1~2 2~3 3~4 4~5 5~6 6~7 7~8
Rut range of wet sample (mm)
(c)
(d)
40
+Moisture
+Lime
30
1% L
1% L+M
2% L+M
20
10
0
Frequency (%)
40
Frequency (%)
1% lime+0.5% M
1
1
Frequency (%)
1% lime
7
7
30
1% L
1% L+M
2% L+M
20
10
0
0~1 1~2 2~3 3~4 4~5 5~6 6~7 7~8
Rut range of dry sample (mm)
(e)
0~1 1~2 2~3 3~4 4~5 5~6 6~7 7~8
Rut range of wet sample (mm)
(f)
FIGURE 8 Distributions of rut depth for mixtures: (a) aggregate effect, dry sample; (b) aggregate effect,
wet sample; (c) WMA effect, dry sample; (d) WMA effect, wet sample; (e) moisture effect, dry sample; and
(f ) moisture effect, wet sample.
84
of 4 to 5 mm (highest distribution for dry specimens). In addition, the
rut distribution of the wet specimen containing Aggregate II shows
that this curve is moving toward the right side of the x-axis. In
other words, these rut values are generally higher. With respect to
the influence of WMA additives, Figure 8c and 8d indicates that the
mixtures containing Sasobit generally exhibit a peak rut depth of
1 to 2 mm, while other mixtures do not seem noticeably different
regardless of the dry or wet specimens. The lime and moisture effects
were presented in Figure 8e and 8f. For dry specimens (Figure 8e),
it can be seen that the increase of lime and moisture content results
in a shift of the curve to the left, which means that these mixtures
produce lower rut values, generally. In Figure 8f, the distribution
curves of three mixtures do not exhibit any trends; their rut values
are generally in the range of 1 to 6 mm.
FINDINGS AND CONCLUSIONS
The following conclusions were drawn based upon the experimental
results obtained from this laboratory investigation of WMA mixtures
that contain moist aggregate:
• The experimental results found that the weight loss of the
mixtures containing moisture generally is greater than that of the
control mixture during a short-term aging process due to additional
moisture.
• In general, the rutting resistance of a mixture is related to the
aggregate type (source) regardless of moisture or lime contents, or both.
Aggregate II has a better rut resistance than Aggregates I and III do.
The rut depths of these mixtures were generally less than 7 mm.
• The influence of moisture on rut depth can be neglected, and it
even results in a better rut resistance in some cases. The increase
of lime content does not significantly affect the rut resistance of the
mixture. The warm water bath treatment methodology illustrates
that the rut depth of the mixture containing moist aggregate was
generally low and thus satisfied the requirement of rut resistance of
the asphalt pavement.
• Test results indicate that the mixtures containing Sasobit
additive have the lowest rut depth. In addition, the mixtures containing Aspha-min and Evotherm additives generally show a similar rut
resistance with the control mixture. Moreover, analysis of the experimental results from the dry and wet specimens also shows that the
rut depth value of warm asphalt mixtures can satisfy the demand of
field performance.
• Relationship analysis of rut depth and gyration number illustrates that the gyration numbers of these mixtures are generally from
20 to 60, and the rut values of these mixtures are from 1 to 6 mm, in
general. Additionally, as expected, the test results show that the
increase of gyration number reduces the rut depth of the mixture.
• The rut depth distribution analysis illustrated the rut depth range
of various mixtures. It also indicates that rutting resistance is related
to the aggregate sources. In addition, the distribution curve of the
mixture with Sasobit exhibits the lowest rut depth regardless of dry
or wet condition. Furthermore, one distribution curve shows that the
mixture with moist aggregate has a slight improvement in the rutting
resistance under the dry test condition.
Transportation Research Record 2180
ACKNOWLEDGMENTS
Financial support was made possible through a grant from South
Carolina’s Department of Health and Environment Control and the
Asphalt Rubber Technology Service of Clemson University.
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The Characteristics of Nonbituminous Components of Bituminous Paving Mixtures
Committee peer-reviewed this paper.