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A Comparison between Static and Repeated Load

Test to Predict Asphalt Concrete Rut Depth

Farhan Alzaidy

Department of Civil Engineering
College of Engineering
University of Baghdad

Baghdad, Iraq
f.alzaidy1901m@coeng.uobaghdad.edu.iq

Amjad Hamad Khalil Albayati

Department of Civil Engineering
College of Engineering
University of Baghdad

Baghdad, Iraq
a.khalil@uobaghdad.edu.iq

Abstract-Rutting has a significant impact on the pavements'

performance. Rutting depth is often used as a parameter to assess

the quality of pavements. The Asphalt Institute (AI) design

method prescribes a maximum allowable rutting depth of 13mm,

whereas the AASHTO design method stipulates a critical

serviceability index of 2.5 which is equivalent to an average

rutting depth of 15mm. In this research, static and repeated

compression tests were performed to evaluate the permanent

strain based on (1) the relationship between mix properties

(asphalt content and type), and (2) testing temperature. The
results indicated that the accumulated plastic strain was higher

during the repeated load test than that during the static load

tests. Notably, temperature played a major role. The power-law

model was used to describe the relationship between the

accumulated permanent strain and the number of load

repetitions. Furthermore, graphical analysis was performed

using VESYS 5W to predict the rut depth for the asphalt
concrete layer. The α and µ parameters affected the predicted rut

depth significantly. The results show a substantial difference

between the two tests, indicating that the repeated load test is

more adequate, useful, and accurate when compared with the

static load test for the evaluation of the rut depth.

Keywords-asphalt concrete; rut depth; VESYS

I. INTRODUCTION

Rutting is a distress mechanism in flexible pavements.
Recently, due to the increased truck tire pressures and the lack
of maintenance that result in roadway deterioration [1], rutting
has become the predominant mode of flexible pavement
failures. Rutting is primarily caused by the accumulation of
permanent deformations in the pavement or its layers.
Furthermore, a major portion of rutting in the surface layer of
flexible pavements is subjected to high tire pressures and heavy
axle loads. High tire pressures decrease the contact area
between the tire and the pavement, producing high stress,
which aggravates deformation in flexible pavements. In
addition, environmental conditions affect significantly the
surface layer of the pavements [2].

The permanent deformation of the Asphalt Concrete (AC)
mixture has been studied by several researchers for various
materials using different testing procedures. Furthermore,
various parameters and prediction models have been developed

to measure rutting resistance. In this study, creep and uniaxial
cyclic compression results of AC mixtures were estimated and
compared based on various asphalt contents, asphalt types, and
temperatures. Many studies have been performed to study the
mechanism of rutting, and many methods have been proposed
for predicting the Rut Depth (RD). Some of these methods are
based on limiting the values of subgrade strain to levels that
prevent rutting at the pavement surface. These methods were
based on the assumption that when the maximum vertical
compressive strain at the top of the subgrade is less than a
critical value, rutting will be limited to an acceptable level for a
specified number of load applications. However, this
methodology does not essentially preclude the occurrence of
rutting that might occur in the asphalt layer. Therefore, a layer-
strain method was proposed to improve the above process by
including additional analysis to evaluate the amount of rutting
occurring in the asphalt layer [3]. It is reported [4] that
excessive permanent deformation can lead to longitudinal
depressions in wheel paths. This can be followed by
longitudinal cracking. Subsequently, water can penetrate these
cracks and deteriorate the roadway, causing it to lose its load-
carrying capacity. Furthermore, excessive permanent
deformation can reduce the driving comfort and can lead to
icing or hydroplaning because of water getting collected in
wheel paths.

AASHTO classified traffic control and environmental
factors as the major causes of AC rutting. However, highway
authorities have little control over these two factors.
Furthermore, there are some limitations that prevent the
redressing of problems that are within the control of highway
agencies [5]. Several highway engineers focused on
ascertaining the factors that can help address the problem of
rutting to propose better mechanisms to reduce the RD.
Authors in [6] performed several creep tests to study the
permanent deformation of AC and developed a rutting
prediction model based on a statistical method which
considered factors such as stress, temperature, asphalt
viscosity, effective asphalt content, air voids, and load
repetition. To evaluate the parameters that affect the AC mix,
authors in [7] evaluated the influencing factors and suggested
variables such as the asphalt content, binder viscosity, air
voids, and test temperature that affect the AC mix.

Corresponding author: Farhan Alzaidy

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Furthermore, additives were also used to improve the strength
of asphalt materials, e.g. using rubber modified asphalt binder
[8]. Most of the existing permanent deformation models for
asphalt mixes have been empirically determined based on
laboratory test results. As a simple performance test [9], a
uniaxial repeated load test is generally used to characterize the
permanent deformation response of asphalt mixes. Permanent
deformation measured using repeated load tests, as depicted in
Figure 1, is generally composed of three stages [10], primary,
secondary, and tertiary. In the primary stage, the rate of
permanent deformation accumulates quickly and tends to
decrease, reaching a constant value in the secondary stage.
Finally, the rate starts to increase rapidly and accumulates
again in the tertiary stage [11]. The objective of these tests is to
simulate the repeated load conditions that occur on the road.
Authors in [12] compared the responses of 3 mixes consisting
of conventional and modified binders under both creep and
repeated loading. For creep loading at 37°C and confining
pressure of 207KPa, the difference among the mixes was not
discernible. The results of repeated load testing suggest that the
repeated loading test may be more applicable than the creep
test to calculate the permanent deformation characteristics of
asphalt mixes. The results of Strategic Highway Research
Program (SHRP) [13] also suggest that more deformation
occurs in repeated loading than in creep loading for the same
materials and other test conditions. Apart from the two
conventional procedures used for repeated load tests, i.e.
termination of the test at 10,000 loading cycles or 5%
accumulated strain, regression coefficients a (intercept) and b
(slope) were also used to determine the rutting susceptibility of
AC mixtures [14]. Authors in [15] proposed a method based on
repeated axial load tests to determine the flow number of
bituminous mixtures. Static and repeated load tests were used
to evaluate the rutting potential of asphalt mixtures, and the
results indicated that the specimens of mixtures experienced a
tertiary flow state in the repeated load test. However, no
specimen achieved a tertiary flow state in the static load test.
This can be attributed to the static pattern of loading that
induces less strain when compared with that of repeated
loading [16].

Fig. 1. Typical permanent deformation behavior of asphalt mixture.

II. MATERIALS CHARACTERIZATION

A. Asphalt Cement

To characterize the properties of the base bitumen,
conventional tests such as the penetration test, softening point
test, flash point test, and penetration and ductility after the thin-
film oven test were performed. These tests were performed in
conformity with the relevant test parameters listed in Table I.

TABLE I. PROPERTIES OF ASPHALT CEMENT

Property

ASTM

design

ation

Penetration grade

40–50

Penetration grade

60-70

Test

results

SCRB

specification

Test

results

SCRB

specific

Penetration at

25°C, 100gm, 5s

(0.1mm)

D-5 42 40-50 67 60-70

Softening point

(°C)
D-36 51 …… 45 …..

Ductility at 25°C,

5cm/min (cm)
D-113 >100 >100 …. >100

Flash point (°C) D-92 289 Min. 232 292 Min. 232

Specific gravity D-70 1.041 …… 1.028 ……

Residue from

thin-film oven

test:

D-1754

-Retained

penetration (% of

original)

D-5 59.5 55+ 64.1 52+

-Ductility at 25°C,

5cm/min (cm)
D-113 80 25+ 100+ 50+

B. Aggregates

Coarse and fine aggregates were chosen and mixed in
appropriate proportions to meet the wearing course gradation
prescribed by the SCRB specification (SCRB, R/9 2003). The
gradation curve of the aggregates is depicted in Figure 2. To
obtain the properties of the coarse and fine aggregates used in
the mixture, conventional tests, such as the specific gravity test,
Los Angeles abrasion resistance test, sodium sulfate soundness
test, and sand equivalent test were performed. The results,
along with the specification limits set by the SCRB, are listed
in Table II.

Fig. 2. Aggregate gradation curve.

C. Mineral Filler

The filler is a non-plastic material that must pass through
sieve No. 200 (0.075mm). The physical properties of the filler
used in this study are listed in Table III.

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TABLE II. PHYSICAL PROPERTIES OF AGGREGATES

Property
ASTM

designation

Test

results

SCRB

specification

Coarse aggregates:

1. Bulk specific gravity

2. Apparent specific gravity

3. Water absorption (%)

4. Wear by Los Angeles

abrasion (%)

5. Soundness loss by sodium

sulfate solution (%)

6. Fractured pieces (%)

C-127

C-131

C-88

2.611

2.689

0.443

18.7

3.1

…..

30 max.

10 max.

95 min.

Fine aggregates:

1. Bulk specific gravity

2. Apparent specific gravity

3. Water absorption (%)

4. Sand equivalent (%)

C-127

2419

2.663

2.697

0.727

45 min.

TABLE III. PHYSICAL PROPERTIES OF THE MINERAL FILLER

(LIMESTONE DUST)

Property Test results

Specific gravity 2.794

Passing sieve No. 200 (0.075mm) 94

D. Specimen Preparation

The cylindrical specimens used in this study were 101.6mm
(4in) in diameter and 203.2mm (8in) in height. The fractions of
the aggregates were separated into groups and the aggregates
retained on the pan were discarded and replaced by the mineral
filler (limestone dust). Based on the graduation specifications
depicted in Figure 2, the aggregates were mixed into a batch of
3800g on the mixing bowl and heated to 150°C in a
temperature-controlled oven. Asphalt was also heated in a
container to a temperature in the range of 135–140°C for 2min,
and the contents of the bowel were thoroughly mixed manually
on a hot plate. To ensure a uniform compaction temperature,
the bowel with its contents was transferred to an oven and
stored for 10min at 140°C. A preheated to 100°C compaction
mold was prepared and a 4-in paper disk was inserted to cover
the mold base plate. The interior edge of the mold was
lubricated using a brush to facilitate the specimen extraction.
The specimen was compacted by the double plunger method
with a load of 65000lb (29491kg) applied using a hydraulic
compression machine at the Material Laboratory of the Civil
Engineering Department of the University of Baghdad. The
load was applied to each end of the specimen for 1min. Finally,
36 specimens were carefully transferred to a smooth, flat
surface and allowed to cool overnight at 25ºC and then were
removed from the mold using a hydraulic extractor. The
specimens were then numbered and stored in a bag.

III. EVALUATION OF PERMANENT DEFORMATION AND

TESTING RESULTS

Permanent strain values were determined using a Pneumatic
Repeated Load System (PRLS) apparatus manufactured under
the auspices of the Civil Engineering Department of the
University of Baghdad [17] as depicted in Figure 3. Axial
repeated and static load tests were performed. The cylindrical
specimens, 101.6mm (4in) in diameter and 203.2mm (8in) in
height, were subjected to conditioning in the PRLS test
chamber at various testing temperatures (20°C, 40°C, and

60°C) for 2h. A linear variable differential transformer was
used to monitor the deformation of the specimens under each
load cycle. Compressive loading was applied at 138kPa in the
form of a rectangular wave with a constant loading frequency
of 60cycles/min and loading sequence including 0.1s load
duration and 0.9s rest duration.

Fig. 3. A photograph of the PRLS.

A. Permanent Deformation Characteristic Properties for

Repeated Load Test

The values of the permanent strain were obtained at the
following load repetitions: 1, 2, 10, 100, 500, 1000, 2000,
3000, 4000, 5000, 6000, 7000, 8000, 9000, and 10000.
Permanent strain (��) was calculated using (1):

�� �
�� � ��

(1)

where �� is the axial permanent microstrain, �� is the axial
permanent deformation, and h is the specimen's height. The
accumulative axial permanent strain versus the number of
loading repetitions (N) is depicted in Figure 4.

Fig. 4. Cumulative permanent strain vs loading cycles from the repeated

load test (log-log scale).

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B. Permanent Deformation Characteristic Properties for

Static (Creep) Load Test

Data for the following loadings were recorded at 1, 10, 30,
60, 100, 260, 360, 500, 1000, 2000, and 3000s. The values of
the permanent strain were calculated at these loadings using the
following expression:

�� �

� � ��

(2)

where �� is the axial creep deformation. The cumulative axial
permanent strain versus the number of loading times is
depicted in Figure 5.

Fig. 5. Cumulative permanent strain vs loading times from the creep load
test (log-log scale).

C. Regression Analysis

Several formulas have been proposed to describe the

relationship between �� and N. Authors in [18, 19] suggested a
log-log relationship, as described in the following equation:

�� � ��
� (3)

where N is the number of load repetitions, and a and b are
positive regression constants, as depicted in Figure 4. The
parameter a is the intercept with the permanent strain axis, and
b is the slope of the linear portion of the logarithmic
relationship. The double-logarithmic-scaled relationship
between �� and N can be approximated using a linear function.

IV. VESYS THEORY

Authors in [20] developed the VESYS software using
multilayer visco-elastic theory to predict the RD in flexible
pavement layers. The basic assumption of VESYS rutting
models is that permanent strain is related to resilient strain as
follows:

����� � ����
�� (4)

where ����� is the vertical permanent strain at the N
th
load

repetition, and � and � are material properties that are
functions of the stress state and temperature respectively. The
material parameters are defined as � � 1� � and � � �� ��⁄ .
These two parameters govern the permanent deformation
behavior of the model. In particular, � is the rate of decrease or
increase in permanent deformation as the number of load
applications increases, and � is the constant of proportionality
between permanent and elastic strains (� 0). Values greater
than 1 indicate premature rutting. The most critical task in

using this model is the prediction of � and � accurately for
each pavement layer within the pavement system. The
permanent deformation parameters could be predicted using
laboratory or field data [21].

The comparative evaluation of the impact of repeated and
creep stresses on permanent deformation parameters is listed in
Table IV. Intercept a represents the deformation of the
specimen after the first load repetition, whereas slope b refers
to the rate of deformation throughout the fatigue life of asphalt
concrete. Furthermore, this comparison also includes the
average values of permanent strain and parameters α and β.

TABLE IV. PERMANENT DEFORMATION PARAMETERS

Temperature (°C) Asphalt content (%) Asphalt grade

20 40 60 4 4.6 5.2 40-50 60-70

Repeated Load Test (RLT)

εp 266 5840 30000 9458 16160 16700 11160 17040

a 55 150 430 230 210 190 160 260

b 0.184 0.40 0.58 0.348 0.4 0.43 0.394 0.398

α 0.816 0.60 0.42 0.652 0.6 0.57 0.606 0.602

µ 0.105 0.270 0.50 0.340 0.285 0.250 0.23 0.36

Creep Load Test (CLT)

εp 532 1730 9295 3850 3480 4219 3680 4020

a 210 555 1430 798 660 730 690 770

b 0.119 0.144 0.264 0.165 0.176 0.187 0.175 0.177

α 0.881 0.856 0.736 0.835 0.824 0.813 0.825 0.823

µ 0.120 0.145 0.260 0.165 0.175 0.189 0.176 0.178

V. DISCUSSION

The accumulated strain at the flow number increased for the
Repeated Load Test (RLT) and exhibited the opposite behavior
for the Static Load Test (SLT). The significant difference in the
values of �� at the failure load for both tests is illustrated in
Figure 6. Table V lists the ANOVA analysis that explains the
impacts of the testing variables on the plastic strain. The Table
lists the degrees of freedom (the number of variables used
minus one), the F-test results, and the p-value (significance) for
each test. The results particularly indicate that temperature
plays a major role affecting plastic deformation (F value >>
Fcrit.). The variables (intercept and slope) used in this model
when plotted on a log scale can be useful indicators of rutting
resistance. The minimal values of each provide a good
indicator of rutting resistance. Furthermore, the slope
coefficient b is also used as a term of creep rate (fc) to an
assessment of the asphalt performance in resistance to
permanent deformation. The intercept values of RLT increased
by 2.7 and 7.8 factors with an increase in temperature from
20°C to 40°C and 60°C respectively, whereas SLT increased
by 2.6 and 6.8 respectively. The content and grade of the
asphalt had only a minor impact. The slope or fc values for
RLT increased by 2.17 and 3.15 with an increase in
temperature from 20°C to 40°C and 60°C respectively, whereas
that of SLT increased by 1.21 and 2.21 respectively. The
content and grade of the asphalt did not affect the slope of the
SLT. The sensitivity analysis of the VESYS 5W rutting model
clearly indicated that α and µ had a considerable effect on the
predicted RD. The values of both parameters were measured
using RLT and SLT. Figures 7–9 illustrate the impact of
temperature and mixture properties (asphalt content and grade)

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on the RD of the AC layer. As mentioned above, temperature is
considered to be the main factor that affects rutting in the AC
layer. The results indicated that RD increased with an increase
in temperature for both tests, as depicted in Figure 7. At 20°C,
the RD increased slightly. However, at 40°C and 60°C, the RD
value was larger for RLT than for SLT by a factor of 6.8 and
4.02 respectively. The asphalt content and grade had no effect
on the RD for the creep test, as depicted in Figures 8 and 9. The
higher value of α and lower value of µ coefficients indicate
pavements with lower rutting. At a lower value of α, rutting
will develop over the entire pavement life and the mainstream
of the rutting will occur at the initial stage and taper off with
the remaining life of the pavement when the AC layer is soft
(higher initial strain) or the climatic region is hot (higher
temperatures).

(a)

(b)

(c)

Fig. 6. Impact of testing temperature and asphalt mixture on the plastic

strain.

TABLE V. IMPACT OF VARIABLES AND TEST METHODS ON PLASTIC

STRAIN ( WO-WAY ANOVA)

Source of Variation df F P-value F crit.

Repeated Load Test (RLT)

Temperature (°C) 2.000 38.474 6.03E-06 3.885

Asphalt grade 1.000 2.666 0.128 4.747

Asphalt content (%) 2.000 1.381 0.310 3.326

Static Load Test (SLT)

Temperature (°C) 2.000 291.438 6.74E-11 3.885

Asphalt grade 1.000 1.096 0.316 4.747

Asphalt content (%) 2.000 1.149 0.396 3.326

Fig. 7. Temperature vs predicted rut depth on AC layer (VESYS 5W

analysis).

Fig. 8. Asphalt content vs predicted rut depth on AC layer at 40°C

(VESYS 5W analysis).

Fig. 9. Asphalt grade vs predicted rut depth on AC layer at 40°C (VESYS

5W analysis).

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Fig. 10. Impact of ESAL on the prediction of rut depth in AC layer at 40°C

(VESYS 5W analysis).

To compare the trend of variation in RD with equivalent
single-axle load (ESAL), Figure 10 depicts the application of
various ESALs on the AC layer. The total RD for RLT is
higher than that for SLT by a factor of 8.4. Therefore, it is
significant that the RD at the first million repetitions is
approximately the 44% of the total RD. Moreover, we can
compare the predicted RD for the AC layer evaluated using
RLT with the measured RD estimated using (5):

"� � �� �# (5)

where "� is the rutting depth (mm), �� is the permanent strain

(mm/mm), and # is the total thickness (mm) of the asphalt
concrete layer. �� is evaluated based on the model developed in
[17] as follows:

$%& � �34.463+0.983log�
��.��1234 �.��5678� +

1.961log9 +1.812log; +0.656log= � 37.277log? �
4.951log@ �1.455logA� 1.93logB (6)

where $%& is the log of accumulated permanent micro strain at
the N

th
load repetition, T is the test temperature (°C), S is the

stress level (psi), D is the applied stress duration (s), B is the
percentage absorbed asphalt (by weight of aggregates), A is the
percentage of air voids, M is the percentage of voids in mineral
aggregates, and F is the voids filled with asphalt. Similarly,
using the trend of variation in RD with ESAL, as depicted in
Figure 11, the values of RD for both the predicted and the
measured data are closed. Therefore, the use of repeated load
tests to predict RD is significantly accurate and reliable.

Fig. 11. Comparison between the predicted and the measured rut depths for

various ESAL repetitions.

In general, to evaluate the rutting resistance properly the
parameters from unconfined RLTs in [9,14] can be used.

VI. CONCLUSION

In this study, the power-law model was used to obtain
parameters for simulating the permanent deformation of the
asphalt layer under uniaxial repetitive and static compression
loads. The results indicated that the accumulated plastic strain
for the repeated load test was higher than that of the static load
test for various mixture properties and climatic conditions
(temperature). VESYS 5W was used to predict the rut depth at
the asphalt concrete layer subjected to equivalent single-axle
loads. The α and µ parameters exhibited a considerable impact
on the predicted rut depths. Finally, the results of this study
indicated a high difference between the above two tests. The
predicted rut depth based on repeated load tests was in
reasonable agreement with the actual rut depth.

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