EFFECT OF MOISTURE CONDITIONING ON CREEP COMPLIANCE AND RESILIENT
MODULUS OF ASPHALT CONCRETE
Jean-Luc Lambert
M.Sc. Student, Department of Civil Engineering, University of Manitoba, Winnipeg, Canada
Ahmed Shalaby
Professor and Head, Department of Civil Engineering, University of Manitoba, Winnipeg, Canada
Mahmoud Enieb *
Assistant Professor, Civil Engineering Department, Faculty of Engineering, Assiut University, Assiut, Egypt
* Civil Engineering Department, Assiut University, Assiut 71516, Egypt
m.enieb@aun.edu.eg
ABSTRACT: Many principal distresses that affect asphalt concrete (AC) relate to a phenomenon called moisture
damage which can lead to longitudinal cracking, spalling, rutting, shoving, stripping and ravelling. The level of
susceptibility of an AC to moisture damage depends on the aggregate and binder properties, volumetric mix
properties, environmental factor, construction method, and pavement design. The research presented in this
paper investigates the effects that moisture and freeze-thaw conditioning has on the volumetric properties of job
mix and core samples for a Type A and Type B AC. The time dependent phenomenological mechanical response
of the job mix and cores before and after conditioning was measured by resilient modulus (MR) and creep
compliance (D(t)) testing at temperatures of -10°C, 5°C, 25°C and 40°C. As a result of the testing program, it
was observed that: (1) the presence of moisture increases the stiffness of AC in a frozen state, (2) in the presence
of moisture, an AC with a higher air void content increases the susceptibility of AC to moisture damage, and (3)
the aggregate gradation has a direct influence on the susceptibility of AC to moisture damage.
KEY WORDS: Asphalt concrete, moisture damage, resilient modulus, creep compliance
1. INTRODUCTION
The development of distresses and failures in asphalt concrete (AC) pavements are caused by mechanisms that
instigate the loss of durability or the ability of the material to resist damage over time. One such mechanism is
moisture damage which causes distress and failure in AC pavements resulting from the presence of moisture, in
the form of a vapour or liquid, originating internally or externally [1]. This reduces the pavements performance
by promoting distresses such as: longitudinal cracking, spalling, rutting, shoving, stripping and ravelling. When
moisture originates or is introduced in the AC a weakening of adhesion and cohesion of the material occurs, due
in part to: binder properties, aggregate properties, volumetric mix properties, environmental conditions, traffic
volume and loads, pavement design and construction practices [2].
Several studies have been conducted to evaluate the mechanisms and causes of moisture damage, and also to
find a test method to measure and predict the resistance of Hot Asphalt Mix (HMA) to moisture damage
[3,4,5,6,7,8,9,10,11,12,13]. The effort that went into these studies reveals that no one test method can accurately
quantify the degree of susceptibility of AC to moisture damage. Instead, a practice to identify the degree of
susceptibility to moisture damage should be standardized and implemented.
In this research, two different hot mix asphalt concrete mixes were tested for resilient modulus and creep
compliance and then subjected to moisture and freeze-thaw conditioning. Once conditioned the samples were retested for resilient modulus and creep compliance. The research provided insight into the volumetric properties
of AC that contribute to the susceptibility to moisture damage and contributes to the body of knowledge required
to establish a standard practice.
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2. THEORETICAL AND ANALYTICAL BACKGROUND
Resilient modulus (MR) and creep compliance (D(t)) testing according to the Superpave Indirect Tensile test
quantifies the time dependent phenomenological mechanical response of AC when subjected to a dynamic and
static load respectively [15,22]. The MR and D(t) respectively quantifies the elastic modulus and stiffness of the
AC.
Static or dynamic
load
Load head
Vertical diametric
axis
Horizontal
diametric axis
Sample
Support
Figure 1. Load application on cylindrical sample [15]
The Long Term Pavement Performance (LTPP) Protocol P07 [15], used in this research project, provides
procedures to determine the MR and D(t) of AC. Both the MR and D(t) tests are carried out on a cylindrical
sample that has a diameter of 150 mm and a thickness that ranges between 25 mm and 50 mm. The MR test is
performed by applying a repetitive haversine waveform load with a 0.1 s load period followed by a 0.9 s rest
period to the sample’s vertical diameter axis, as shown in Figure 1, at temperatures of 5°C, 25°C and 40°C. The
magnitude of the peak dynamic load is chosen such that the horizontal deformation falls within the range of 38
μm and 89μm. Once an appropriate load is selected, the sample is subjected to five cyclic loads in order to
produce deformation and load data. The resulting instantaneous and total resilient deformations measured, as
shown in Figure 2, are averaged over three load cycles. Based on the measured values the MR is calculated
according to Equation 1.
(1)
Where, MR = Resilient modulus [GPa],
= applied repeated stress, = the resilient strain.
The D(t) test is performed by applying a static compressive load with constant magnitude to the sample’s
vertical diametric axis, as shown in Figure 1, for a duration of 100±2 s at temperatures of -10°C, 5°C and 25°C.
The magnitude of the fixed compressive load is chosen such that the horizontal deformation falls within the
range of 38 μm and 89 μm. The resulting deformations measurements at time intervals 1 s, 2 s, 5 s, 10 s, 20 s,
50 s and 100 s, as shown in Figure 3, are used to calculate the creep compliance of the AC based on Equation 2.
t
t
(2)
Where, D(t) = Creep compliance at time t [1/GPa],
time t.
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= the resilient strain at time t and
= applied stress at
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35
34
33
Instantaneous
resilient
deformation
Displacement [μm]
32
Total resilient
deformation
31
30
29
28
27
26
25
0,0
0,1
0,2
0,3
0,4
0,5
0,6
Time [s]
0,7
0,8
0,9
1,0
1,1
Figure 2. Example of resilient modulus deformation measurements
0,20
40
35
0,15
25
0,10
20
0,05
15
10
0,00
5
-0,05
0
0
10
20
30
40
50
60
70
80
90 100
Time [Sec]
Static Load
Deformation
110
120
130
140
Figure 3. Example of creep compliance measurements [15]
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Deformation [µm]
Static Load [kN]
30
3. EXPERIMENTAL METHODOLOGY AND PROCEDURE
3.1. Materials
Fifteen Type A and Type B job mix and cored samples were collected from the field. A summary of the type and
number of samples is presented in Table 1. The term job mix refers to loose AC samples collected during
construction that were compacted using a Superpave Gyratory Compactor (SGC), while the term core refers to
samples that were cored from the AC pavement in the field. Both types of samples had a diameter of 150 mm
and a thickness of between 25 mm and 50 mm. The Type A and Type B samples had a maximum aggregate size
of 19 mm and a maximum nominal size of 12.5 mm with the later AC mix having a finer grading. A type 150200 and 200-300 binder was used in the manufacturing of the Type A and Type B respectively. A summary of
the physical properties of the binders and aggregate gradation are presented in Table 2 and Figure 4 respectively.
Table 1. Type and number of samples
Job Mix Samples
Type A
Type B
3
3
Core Samples
Type A
Type B
4
5
Table 2. Asphalt binder properties
Property
Average penetration [mm] 100g. 5s @ 25°C
Specific gravity @ 150°C
Mixing temperature [°C]
Compaction temperature [°C]
AC Mix Design
Type A
Type B
164.9
236.0
1.0251
1.0219
145
142
135
131
100
90
Percent Passing [%]
80
70
60
50
40
30
20
10
0
0.425
0.075
0.180
2.00
Type A
4.75
Sieve [mm]
Type B
9.5
12.5
16
0.45 power max. density line
Figure 4. Superpave gradation for Type A and Type B mix design
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19
3.2. Volumetric Properties
A volumetric mix design was conducted for both the Type A and Type B in order to determine the bulk specific
density (Gmd), air void content, voids in mineral aggregate (VMA), and maximum theoretical specific gravity
(MTSG) as per ASTM D2726, D3203, D6995, and D2041 respectively [17,18,19,20]. A summary of the
volumetric mix design properties of the Type A and Type B are presented in Table 3. Based on the mix design,
Type A has a lower air void content, higher binder content and higher bulk specific density than Type B.
Table 3. Asphalt concrete mix design
AC Mix Design
Property
Type A
Type B
5.2
5.0
2364
2254
Air voids [%]
4.9
7.9
VMA [%]
13.9
16.6
VFA [%]
64.9
52.2
MTSG
2.485
2.449
Asphalt content [%]
3
Bulk specific density [kN/m ]
3.3. Testing Program
Initially, the bulk specific gravity of each sample was determined by comparing the ratio between the weight of
sample in air to that in a saturated state. The calculated Gmb was used to determine the air void content of each
sample by taking the ratio between the Gmd of the sample and the MTSG of the mix in question. The voids filled
with asphalt (VFA) was calculated by dividing the difference between the VMA and air voids by the VMA.
Once the air void content was determined, the unconditioned samples were subjected to MR and D(t) testing at
temperatures of -10˚C, 5 ˚C, 25 ˚C and 40˚C based on the LTPP Protocol P07 [15]. Using the individual results,
an average MR and D(t) was calculated for the Type A and Type B job mix and cores.
After testing, the samples were subjected to moisture and freeze-thaw (F-T) conditioning according to ASTM
D4867 [21]. This consisted of saturating a sample in water under vacuum until 55% to 70% of air voids were
filled with water. Following the water saturation, the sample was stored in an environmental chamber at -18˚C
for 15 h. Afterwards, the sample was removed from the chamber and placed in a 60˚C water bath for 24 h then
removed and left to cool to room temperature in a sealed bag. Once cooled the conditioned samples were
retested for MR and D(t) according to LTPP Protocol P07 [15].
4. TEST RESULTS AND ANALYSIS
4.1. Volumetric Properties
A difference in air voids between the job mix and core samples were observed for both the Type A and Type B.
This difference in air void content, as shown in Table 4, is attributed to the method by which the samples were
prepared, the binder content and aggregate grading. When considering how the samples were prepared, the job
mix samples were compacted by a SGC machine until the required number of gyrations were applied to achieve
the designed air void content while the core samples were compacted by heavy compaction machinery. From the
air void content test results it appears that the job mix samples were compacted to a higher density than the core
samples. The difference in air void content can also be attributed to the difference in binder content of 5.2% and
5.0% between the Type A and Type B respectively. This results in a VFA of 64.9% for the Type A and 52.2%
for the Type B.
By its self the binder content does not account for the difference in air void content. It is also attributed to the
aggregate gradation. According to the aggregate mix designs, presented in Figure 4, the Type A contains
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approximately 47% coarse aggregates while the Type B contains 33% coarse aggregate. This results in a VMA
of 13.9% in the Type A and 16.6% the Type B due to a denser gradation in the former.
Table 4. Average bulk density and air voids content of ac samples
Average Bulk Density
[kg/m3]
Average Air Voids Content
[%]
Type A Job Mix
2329
5.4
Type A Cores
2267
8.0
Type B Mix
2222
9.7
Type B Cores
2113
14.1
Sample
4.2. Results of Creep Compliance Testing
Creep compliance testing of the conditioned and unconditioned Type A and Type B samples were conducted
according to LTPP Protocol P07. The resulting D(t) values are presented in Figures 5, 6 and 7. Based on the
results, the job mix samples contained lower air void content and had lower creep compliance than the core
samples. As an example, the Type A job mix (5.4% air voids) had a D(t) of 0.68 GPa-1 compared to the cores
(8.0% air voids) that had a D(t) of 1.48 GPa-1.
In addition, the unconditioned job mix samples had higher creep compliance than the conditioned samples at
-10°C. When comparing the creep compliance of the conditioned and unconditioned Type A job mix, as shown
in Figure 5 and 6, the conditioned sample had a D(t) of 0.29 GPa-1 compared to the unconditioned D(t) of
0.68 GPa-1. This observation, however, reverses when the sample temperature is above freezing. At 5°C the
unconditioned and conditioned Type A job mix had a creep compliance of 1.74 GPa-1 and 1.68 GPa-1,
respectively, while at 25°C the unconditioned and conditioned Type A job mix had a creep compliance of
29.01 GPa-1 and 37.07 GPa-1, respectively.
This reversal of stiffness between the conditioned and unconditioned material between -10°C and 25°C can be
accounted for by the presence of moisture in the void structure of the material. When the material contains
frozen moisture at -10°C the stiffness increases in magnitude due to the solidified moisture. Alternatively, when
the material contains moisture at 25°C the stiffness decreases in magnitude due to the loss of adhesion between
the aggregate and asphalt binder.
The creep compliance reversal observed in the job mix is not observed in the core. It is possible that the density
of the core, which is less than the job mix, allows for a greater interconnectivity of the air voids, as reported by
Arambula et al. [16]. As a result, the ability of moisture to disperse and cause a loss of adhesion between
aggregate and asphalt binder increases with air void content in the AC.
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18
Creep compliance [1/GPa]
16
14
12
10
8
6
4
2
0
-10
5
Temperature [˚C]
Type A job mix
Type B job mix
Type A core
Type B core
Figure 5. Unconditioned creep compliance at 50 s
18
Creep compliance [1/GPa]
16
14
12
10
8
6
4
2
0
-10
5
Temperature [°C]
Type A job mix
Type B job mix
Type A core
Type B core
Figure 6. Conditioned creep compliance at 50 s
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275
Creep compliance [1/GPa]
250
225
200
175
150
125
100
75
50
25
0
Unconditioned
Type A job mix
Conditioned
Type B job mix
Type A core
Type B core
Figure 7. Creep compliance at 25oC and 50 s
4.3. Results of Resilience Modulus Testing
Resilient modulus testing of unconditioned and conditioned Type A and Type B job mix and core samples
indicated that the MR reduces with increase in air void content. From Figures 8 and 9, it can be see that the
Type B cores were not tested at 40˚C. This was due to the lack of stability experienced during testing.
Akin to the creep compliance testing, both the Type A and Type B job mix had higher MR values than the cores,
as seen in Figure 8 and 9. Based on the Type A results at 5˚C, the unconditioned job mix (5.4% air voids) had a
MR of 8.8 GPa while the cores (8.0% air voids) had a MR of 5.4 GPa and the conditioned job mix had an MR of
8.7 GPa while the cores had a MR of 2.6 GPa. This reduction of resilient modulus between the job mix and cores
is related to the air void content. Both the job mix and cores have the same constitutive aggregate and binder
content. The only difference in volumetric properties is air void content which is based on the compaction
temperature and compaction energy.
When comparing the Type A and Type B unconditioned and conditioned resilient modulus, the former is higher
than the later. As an example, the unconditioned Type B core has a MR of 0.98 GPa at 25°C and a MR of
0.52 GPa after conditioning. This reduction in resilient modulus is directly attributed to the effect that moisture
and freeze-thaw conditioning has on the AC samples.
In addition, the damage caused by moisture and freeze-thaw cycling on AC is augmented with an increase in air
void content. When comparing the MR of the unconditioned and conditioned Type A at 25°C, there is a 23%
drop in the job mix and a 54% drop in the cores. The higher drop in MR observed in the core material is due to a
higher air void content that allows for a greater degree on moisture damage to transpire.
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9
Total resilient modulus [GPa]
8
7
6
5
4
3
2
1
0
5
25
40
Temperature [°C]
Type A job mix
Type B job mix
Type A core
Type B core
Figure 8. Unconditioned total resilient modulus
9
Total resilient modulus [GPa]
8
7
6
5
4
3
2
1
0
5
Type A job mix
25
Temperature [°C]
Type B job mix
40
Type A core
Type B core
Figure 9. Conditioned total resilient modulus
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5. CONCLUSION
Two different asphalt concretes with asphalt binder content of 5.2% and 5%, air void content of 4.9% and 7.9%,
and VFA of 64.9% and 52.2% respectively where tested for creep compliance and resilient modulus before and
after moisture and freeze-thaw conditioning. Based on the test results and analysis of the study, the following
conclusions were drawn:


The presence of moisture reduces the creep compliance of AC at -10°C. As such, moisture increases the
stiffness of AC in a frozen state.
In the presence of moisture, a lower air void content increases the stiffness and elastic modulus of AC.
This indicates that a lower air void content reduces the susceptibility of AC to moisture damage.
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