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This paper re-evaluates the effect of sunflower oil capsules on the mechanical and self-healing properties of dense-graded asphalt mixtures. Different percentages of capsules (0.50 wt%, 0.75 wt% and 1.00 wt%) were mixed into dense asphalt. The influence of capsules on the properties of asphalt such as density, indirect tensile strength, particle loss, fatigue life, and self-healing, has been investigated. The distribution and integrity of the capsules has been also evaluated by means of CT Scans. It has been proven that capsules can survive the mixing and compaction process of asphalt mixture, do not decrease its mechanical properties and they rupture and release the oil under a high compression loading. Higher capsule content in the mixture resulted in higher oil release ratios. Furthermore, the oil released from the capsules significantly increased the self-healing capability of mixtures. Results from previous research were validated, where it had been found that 0.5% of capsules is the optimal content to obtain good mechanical performance, without affecting the rheological properties of dense-graded asphalt mixtures.
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ORIGINAL ARTICLE
Influence of encapsulated sunflower oil on the mechanical
and self-healing properties of dense-graded asphalt mixtures
Jose Norambuena-Contreras .Quantao Liu .Lei Zhang .Shaopeng Wu .
Erkut Yalcin .Alvaro Garcia
Received: 30 November 2018 / Accepted: 23 June 2019
The Author(s) 2019
Abstract This paper re-evaluates the effect of
sunflower oil capsules on the mechanical and self-
healing properties of dense-graded asphalt mixtures.
Different percentages of capsules (0.50 wt%,
0.75 wt% and 1.00 wt%) were mixed into dense
asphalt. The influence of capsules on the properties
of asphalt such as density, indirect tensile strength,
particle loss, fatigue life, and self-healing, has been
investigated. The distribution and integrity of the
capsules has been also evaluated by means of CT
Scans. It has been proven that capsules can survive the
mixing and compaction process of asphalt mixture, do
not decrease its mechanical properties and they
rupture and release the oil under a high compression
loading. Higher capsule content in the mixture resulted
in higher oil release ratios. Furthermore, the oil
released from the capsules significantly increased the
self-healing capability of mixtures. Results from
previous research were validated, where it had been
found that 0.5% of capsules is the optimal content to
obtain good mechanical performance, without affect-
ing the rheological properties of dense-graded asphalt
mixtures.
Keywords Dense asphalt mixture Encapsulated
rejuvenators Mechanical properties Self-healing
capability
1 Introduction
In the most parts of the world, asphalt mixture is the
preferred surface material to build highway and urban
roads due to its low roughness and economy. Never-
theless, cracking due to changes of temperature,
moisture, and repeated loads may severely reduce
the service life of asphalt pavements [1]. Asphalt is the
mixture of aggregates, which give structural strength
to the material, and bitumen, a liquid that holds the
aggregates together and which viscosity is tempera-
ture-dependant. Asphalt mixture is a self-healing
material, because bitumen has the capacity to flow
and drain into the cracks [2]. Asphalt pavements can
autonomously repair their internal cracks if the
J. Norambuena-Contreras E. Yalcin A. Garcia (&)
Nottingham Transportation Engineering Centre, Faculty
of Engineering, School of Civil Engineering, University
of Nottingham, University Park, Nottingham, UK
e-mail: alvaro.garcia@nottingham.ac.uk
J. Norambuena-Contreras
LabMAT, Department of Civil and Environmental
Engineering, University of Bı
´o-Bı
´o, Concepcio
´n, Chile
Q. Liu L. Zhang S. Wu
State Key Laboratory of Silicate Materials for
Architectures, Wuhan University of Technology, Luoshi
Road 122, Wuhan 430070, China
E. Yalcin
Department of Civil Engineering, Faculty of Engineering,
Firat University, Elazig, Turkey
Materials and Structures (2019) 52:78
https://doi.org/10.1617/s11527-019-1376-3(0123456789().,-volV)(0123456789().,-volV)
conditions are appropriate, such as aggregate grada-
tion, and bitumen’s or mastic’s viscosity, [3,4].
Nevertheless, the bitumen tends to age and its
viscosity to increase during the mixing, compaction
and service life, which reduces the self-healing
capability of asphalt mixture and hence, reduces its
lifespan [5].
To improve the self-healing properties of asphalt
roads and extend their lifespan, researchers have
developed two technologies that improve the drainage
of bitumen into the cracks. The first technology
involved mixing metallic additives into the asphalt
mixture and heating them by means of an electromag-
netic field such as induction or microwave radiation
[6], to make bitumen expand and push it into the
cracks [7]. The second technology was the addition of
encapsulated oil into the asphalt mixture [8]. When the
capsules were ruptured due to traffic loading, the oil
was released and diffused into the mixture. Therefore,
the viscosity of bitumen around the capsule reduced
while the concentration of oil was still high, and
bitumen could drain more easily into the cracks [9].
This paper will focus on the use of capsules to improve
the healing properties of asphalt roads.
There are different types of capsules that have been
previously developed, representative examples, albeit
not the only ones, are those developed by Su et al. [10],
which are micron-sized with core–shell structure, and
those developed by Garcia et al. [11], which are
millimetre-sized, with a porous internal structure. It
has been proven that encapsulated oil can survive
mixing and compaction of asphalt mixture, and
improve its self-healing properties [12]. In addition,
Tabakovic
´et al. [13], have developed hollow fibres
that contain healing agents agent rather than capsules,
although these fibres are still in a premature phase of
their development.
In this paper, we will focus on the capsules
developed by Micaelo et al. [14] and Al-Mansoori
et al. [1517] for asphalt self-healing (ca-alginate
capsules). They are composed of a membrane of
calcium-alginate capsules that contains sunflower oil
and protects it from the environment. Similar capsules
were also manufactured by Norambuena-Contreras
et al. [18] and Xu et al. [19] for self-healing of asphalt
mastic, dense asphalt mixture and Stone Mastic
Asphalt [20]. In these works it was proven that the
capsules had a positive effect on the compatibility,
particle loss and self-healing properties of the mixture,
whereas an adverse effect on stiffness modulus when
the content of capsules substantially increased [1619]
and, the reason for this is probably the low strength of
the capsules; they should have been made stronger. If
the content of capsules is too low, their effect on the
self-healing properties of asphalt will be limited. If an
excess of capsules is added to the asphalt pavement,
the stiffness and rutting resistance may be reduced
because if strength has not been correctly designed
[15].
This paper aims to re-evaluate and summarise the
effect of the addition of the capsules developed in Ref.
[15] on the durability, including fatigue performance
and self-healing, of dense-graded asphalt mixtures. To
achieve this goal, three percentages of capsules,
0.50%, 0.75% and 1.00%, have been added to dense
asphalt mixture. Properties such as indirect tensile
strength, particle loss and fatigue life of the mixtures
with capsules were investigated. Moreover, the influ-
ence of different percentages of capsules on the self-
healing of the asphalt mixture was examined. Finally,
X-ray computed tomography tests were carried out to
evaluate the distribution and integrity of the capsules
in the mixture. The experimental design can be seen in
Fig. 1.
2 Materials and test methods
2.1 Raw materials
The asphalt mixture used in this research was a
standard dense asphalt mixture AC-13, whose grada-
tion is shown in Table 1. Basalt gravel with density
2.976 g/cm
3
, limestone filler with density 2.669 g/
cm
3
, and unmodified bitumen with penetration 77.5
(0.1 mm) and softening point 49.1 C were used to
prepare the mixture. Moreover, Ca-alginate capsules
with a density of 1.116 g/cm
3
were prepared. The
rejuvenating agent used was sunflower oil with a
density of 0.92 g/cm
3
, smoke point 227 C and flash
point 315 C[14]. The polymeric structure of the
capsules was made of sodium alginate (C
6
H
7
O
6
Na)
and calcium chloride (CaCl
2
) provided in granular
pellets with 93% purity. The main thermal and
mechanical properties of the used calcium-alginate
capsules can be consulted in [18].
78 Page 2 of 13 Materials and Structures (2019) 52:78
2.2 Preparation of the Ca-alginate capsules
The encapsulation procedure is shown in Fig. 2and
step-by-step described in Al-Mansoori et al. [16]. Ca-
alginate capsules were prepared at 20 C by ionotropic
gelation of sodium alginate in the presence of calcium
ions [1416]. The capsule preparation method con-
sisted of the following three steps: (1) preparation of
the sodium-alginate emulsion; (2) preparation of the
calcium-chloride solution and capsule synthesis; and
(3) drying and storing of the capsules. Capsules were
prepared in a relationship 1:3 at vol of rejuvenator and
Ca-alginate polymer, respectively. Figure 3a shows an
optical image of the capsules with a size 2.5 mm.
Figure 3b shows a SEM image of the cross-section of
an individual capsule composed of a multi-cavity
structure. A total of 2.5 kg of capsules were manu-
factured in this work.
2.3 Manufacturing of asphalt mixture samples
Asphalt mixture test specimens with, and without, Ca-
alginate capsules were manufactured using 4.7% of
binder content. In the mixtures with capsules, three
different capsule contents as a percentage of the total
mixture mass were used: 0.5%, 0.75% and 1.0%, thus
providing an oil-to-bitumen content by mass in
bitumen of approximately 6.97%, 10.46% and
13.95%, respectively. Due to the low content (no
more than 1.00%), capsules were added directly to the
asphalt mixture without changing the aggregate
gradation.
The cylindrical samples used for mechanical tests
were manufactured according to the following proce-
dure: (1) the aggregates and bitumen were pre-heated
at 160 C for 12 h and 4 h, respectively; (2) the
asphalt mixture components were mixed with the
laboratory mixer at 160 C; (3) the capsules, at 20 C,
were added to the mixture and mixed for 15 s; (4) the
asphalt mixtures containing capsules were poured into
a mould and compacted using the rutting plate
moulding machine; (5) asphalt test cylinders
(U100 mm 950 mm) were drilled from each plate
Manufacture of asphalt that contains capsules
Bulk density and air void content
Indirect tensile strength
Particle loss characterisation
Fatigue characterisation
Self-healing capability test
Distribution of capsules in the asphalt (CT-Scan)
Without capsules (WO/C)
0.50% capsules
0.75% capsules
1.00 capsules
Cylindrical test samples,
Gyratory compacted
Prismatic test samples,
Roller compa cted Quantification of oil released from the capsules
Properties of the asphalt Additional tests
+
+
+
+
Fig. 1 Experimental design of the study
Table 1 Gradation of the AC-13 asphalt mixture
Sieve size (mm) Passing (%)
16 100.0
13.2 96.2
9.5 75.2
4.75 47.4
2.36 30.8
1.18 23.9
0.6 16.6
0.3 12.3
0.15 9.1
0.075 6.9
Water +
Calcium chloride
Cross-linked
alginate
Ca-alginate
Capsules
5000 ml
Drop
Sodium-alginate
emulsion
Calcium-chloride
solution
Dropping-funnel
2000 ml
Fig. 2 Scheme of the encapsulation procedure by ionotropic
gelation
Materials and Structures (2019) 52:78 Page 3 of 13 78
using a pavement coring machine. Additionally,
cylindrical cores (U35 mm 950 mm) were drilled
from the cylinders for CT-Scan characterisation.
Furthermore, asphalt mixture slabs were manufac-
tured using the rutting plate moulding machine, and
prismatic samples were cut for the crack-healing
measurements. Asphalt mixtures were manufactured
in batches of 14 kg using a laboratory mixer with a
total capacity of 25 kg. After mixing, mixtures were
transferred to the steel moulds for their compaction
and sawing as follows: asphalt slabs of
306 9306 960 mm
3
were compacted using a roller
compactor to reach air voids content 5%. Then, prisms
with dimensions 150 9100 960 mm
3
were sawed
from a selection of slabs, and a transverse notch with
dimensions 5 95mm
2
was made at the mid-point of
the beams to facilitate the start of a located single
crack surface during crack-healing tests.
2.4 Bulk density and air void content
The bulk density and air void content play important
roles in the mechanical properties of asphalt mixtures,
including their fatigue life [18]. The maximum density
of test specimens was determined through the BS EN
12697-5 [21] by the mathematical method. In addition,
the bulk densities of the specimens with, and without,
capsules were determined through the BS EN 12697-6
[22] by measuring the bulk density-saturated surface
dry. Besides, the air void content of each test specimen
was calculated based on the previous calculation of the
maximum and bulk densities, as follows:
AVi%ðÞ¼ qmax qi
qmax

100 ð1Þ
where AV
i
is the air void content of each asphalt test
specimen in %, q
max
is the theoretical maximum
density without voids in g/cm
3
, and q
i
is the bulk
density of each test specimen in g/cm
3
.
2.5 Indirect tensile strength
Indirect tensile strength (ITS) is an indicator of the
cohesion between binder and aggregates or Ca-algi-
nate capsules. Indirect tensile tests were used to
characterise the effect of the capsule content on the
mechanical properties of mixtures, compared with
mixtures without capsules. Cylindrical test specimens
were incubated at 25 C for 4 h before the experiment.
The load was applied at a rate of 50 mm/min until the
peak load was reached. ITS of the specimens was
calculated as follows:
ITS ¼2F
pDH ð2Þ
where ITS is the indirect tensile strength in MPa; Fis
the peak load in N;Dis the diameter of the cylinder in
mm; His the thickness of the cylindrical specimen in
mm.
2.6 Particle loss tests
The authors understand that the Ca-alginate capsules
may reduce the adhesion between the aggregates and
the mixture, causing increased roughness. For this
reason, the particle loss (ravelling) resistance of
5mm
50 μm
(a) (b)
Multi-cavity
structure
Fig. 3 a Optical image of Ca-alginate capsules, and bSEM image of the cross-section of an individual capsule
78 Page 4 of 13 Materials and Structures (2019) 52:78
asphalt mixtures with, and without, capsules was
evaluated through particle loss tests. Test specimens
were placed into a water bath at 20 C for 20 h and
then put into a Los Angeles abrasion machine without
steel balls. The drum was rotated at a speed of 30 rpm
for 300 revolutions. The result was calculated by
comparing the mass of the specimens before, and after,
being tested.
PL %ðÞ¼ Wi1Wi2
Wi1

100 ð3Þ
where PL is particle loss of each test specimen in %;
W
i1
is the initial mass of the sample in g; W
i2
is the
residual mass of the test specimen in g.
2.7 Fatigue characterisation
Fatigue characterisation of asphalt mixtures with, and
without, different contents of capsules was studied
using indirect tensile fatigue tests at a wide range of
forces, adapted from Ref. [23]. A Universal Testing
Machine UTM-25 was used to apply half-sine
repeated loads until the cumulative amount of dis-
placement led to the test specimen fracture. The test
frequency was 1 Hz. In each loading cycle, the loading
time was 0.1 s and the intermittent time was 0.9 s.
This experiment was performed at a standard test
temperature of 15 C. The fatigue lives obtained under
different stress levels were analysed by the following
regression equation:
Nf¼K1
r0

n
ð4Þ
where N
f
is the number of fatigue cycles when the test
specimen breaks; r
0
is the horizontal tensile stress at
the specimen center in MPa, and Kand nare fatigue
regression constants.
2.8 Self-healing capability tests
The influence of capsule addition on the self-healing
capability of mixtures was evaluated using the
mechanical crack-healing test previously developed
by Al-Mansoori et al. [1517]. The prisms where
frozen at -20 C and tested under three-point
bending. After, the two halves of the beams were
placed together, in a steel mould, with a thin plastic
sheet in the crack, to avoid the contact between both
faces. A confined static compressive load of 75 kN
was applied on the surface of asphalt prisms to
simulate traffic loading and break the capsules. The
pressure value used in the test is the same as that used
by the Mobile Load Simulators (such as MLS30) for
simulating a large-scale accelerated damage on the
asphalt pavements with self-healing purposes [24].
Finally, once the capsules had been damaged, the
plastic sheet was removed, the prism halves placed
together, and the prisms rested at 20 C. The step-by-
step test procedure has been described in Noram-
buena-Contreras et al. [18]. Figure 4shows a sche-
matic description of the test procedure.
Crack-healing performance of asphalt beams with,
and without, capsules was quantified as the healing
level reached from the three-point bending strength
recovery of cracked beams tested under a three-point
bending test after a defined healing time. Ten different
healing times (i.e. 5, 24, 48, 72, 96, 120, 144, 168, 192
and 216 h) were tested. The healing level reached by
each beam after a specific healing time was defined as
follows:
HL %ðÞ¼ Fihealed
Fiinitial

100 ð5Þ
where HL is the healing level defined as the relation-
ship between the maximum force of the beam initially
tested, F
initial
, and the maximum force measured in the
same asphalt beam after the healing process, F
healed
.
2.9 Quantification of oil released level
by the capsules
The level of oil released by the Ca-alginate capsules
into the asphalt was chemically quantified by means of
Fourier-Transform Infrared Spectroscopy (FTIR) tests
according to Micaelo et al. [14] and Norambuena-
Contreras et al. [18].
FTIR tests, were carried out on two sample groups
as follows: (1) As a reference, test samples of virgin
bitumen with the same amount of rejuvenator con-
tained in the different percentages of capsules (i.e.,
simulating 100% of oil released from 0.5%, 0.75% and
1.0% of capsules added to the mixture). (2) Test
samples were randomly extracted, using a hot knife,
from beams that had been made with and without
capsules before, and after, healing tests. The results
Materials and Structures (2019) 52:78 Page 5 of 13 78
were compared to those of the reference, to know the
percentage of oil in the capsules.
FTIR analysis was developed using a spectrum
device, set in the absorption mode in the wavenumber
range of 400 to 4000 cm
-1
at a resolution of 4 cm
-1
.
The absorbance spectrum in FTIR curves was nor-
malised, and the absorbance area under the spectrum
was measured [25]. Figure 5shows an example of the
curves obtained from asphalt samples containing 0.5%
of Ca-alginate capsules, where the oil releasing effect
on the bitumen was evaluated from changes in the
absorbance peak between the wavenumbers 1700 to
1800 cm
-1
[17]. Using the absorbance area results,
the Oil Released Level, ORL, from the capsules into
the asphalt beams before, and after, crack-healing tests
was defined as follows:
ORL %ðÞ¼ A1745Si
A1745Ref

100 ð6Þ
where A
1745-Si
is the absorbance area at the peak
1745 cm
-1
for the specific asphalt sample containing
different percentages of Ca-alginate capsules (i.e.
0.5%, 0.75% and 1.0%), and A
1745-Ref
is the absor-
bance area at the peak 1745 cm
-1
for the asphalt
sample with the 100% of oil released in asphalt,
depending on the percentage of capsules added to the
mixture.
2.10 CT-Scan characterisation of the samples
with capsules
The distribution of the capsules and their structural
integrity inside asphalt cores were evaluated by means
of CT-Scan analysis following the procedure
described in [18] and based on the following steps:
(1) Several CT-Scan tests on cores were developed
using a GE Sensing and Inspection Technologies
GMBH Phoenix VTomeX M operated at 200 kV and
180 lA; (2) The reconstruction and corrections of
scans were performed using the Phoenix Datos X2
Reconstruction software; (3) Once reconstructed into
3D volumes, 2D image slices were exported in TIFF
format along the XY axis.
100 mm
F
-20ºC +20ºC
Compressive load
Temperature chamber
Steel mould
Healing time: 5-216 h
F
+20ºC
Steel plate
With plastic
membrane
Step 1: Crack generation
Steel mould
Without plastic
membrane
Capsules
Step 2: Capsules activation Step 3: Healing process
Flexural load
Asphalt beam
Repeat Step 1
Fig. 4 Schematic diagram of the crack self-healing test on the asphalt slabs (modified from [20])
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
1700 1710 1720 1730 1740 1750 1760 1770
Absorbance (-)
Wavenumber (cm-1)
Asphalt with oil
Asphalt without oil
Normalised
area under the
FTIR curve
Fig. 5 Example of FTIR curves of asphalt samples without,
and with, oil (0.5% of capsules)
78 Page 6 of 13 Materials and Structures (2019) 52:78
3 Results and discussion
3.1 Effect of capsule addition on the density
and air void content of dense asphalt
Table 2shows the average bulk density and air void
content of asphalt mixtures with, and without, Ca-
alginate capsules. Average values were calculated
from 5 tested specimens for each capsule content in the
mixture. In Table 2it can be seen that the addition of
capsules slightly decreased the density of the asphalt
mixture. The reason for this reduction is that the
density of the capsules (1.116 g/cm
3
at 20 C[18]) is
lower than that of the aggregates and mastic in the
mixture. However, the addition of 0.5% may con-
tribute to reduce the air void content of the mixture, as
it has been reported before by Al-Mansoori et al. [14].
3.2 Distribution of capsules in the asphalt
Figure 6shows the results of X-ray computed tomog-
raphy tests developed on asphalt cores with capsules.
The capsules can resist the manufacturing process and
mainly keep their oil content until they break due to
the effect of external loading. Figure 6a shows that
samples with 0.5% of capsules presented a good
spatial distribution inside the mixture with minor
percentage of broken capsules because of the manu-
facturing process, while mixtures with higher contents
of capsules showed some clusters of capsules, see
Fig. 6b. The reason for the cluster of the image is not
clear and more research could be used in this direction;
although it could have happened because the capsules
were already clustered before mixing due to a poor
curing of the alginate. The dispersion of similar
capsules had been previously quantified in Ref. [14]
and did not show any remarkable results.
Furthermore, CT-Scan images shown in Fig. 6c, d
showed that the capsules used in this study can resist
mixing and compaction, and that they get damaged by
the interaction with the aggregates. In previous studies
[18,20] the authors observed the microstructure of the
capsules by scanning electron microscope (SEM)
images, concluding that the structure of the designed
capsules is a porous microbead, see Fig. 3b. It is not
yet clear how the oil is released from the capsules and
must be part of a more extensive study but, the authors
propose that this happens because the capsules simply
get squeezed by the aggregates around them (see
Fig. 9c). If this is true, capsules in porous asphalt may
release the oil faster than capsules in denser mixtures.
As the capsules present a multi-core structure, when
one of the pockets of oil is broken by the pressure of
the aggregates during the cyclic loading of the asphalt,
the rest of the oil’s pocket may still remain intact,
although this is a topic for future research.
3.3 Effect of capsules on the indirect tensile
strength (ITS) of asphalt mixture
Figure 7shows the ITS results of asphalt samples
without and with capsules. Average values were
calculated from 3 tested specimens for each capsule
content in the mixture. It is shown in Fig. 7a that
asphalt mixtures with 0.5% of capsules are stiffer than
asphalt mixtures without capsules, as well as mixtures
with 0.75% and 1.00% of capsules. Figure 7b shows
the average maximum ITS values registered at a
displacement of 3.05 mm. It shows that asphalt
mixture with 0.5% of capsules has higher average
ITS than asphalt mixture without capsules, or with
0.75% and 1.00% of capsules. The reason can be
attributed to the lower air void content of the asphalt
mixture containing 0.5% of capsules. When more Ca-
alginate capsules were added to the mixture, the air
void content of the mixture increased (see Table 2),
and more oil was released from the capsules, decreas-
ing the stiffness and ITS of the asphalt mixtures.
Table 2 Density and air void content of asphalt mixture with, and without (WO/C), capsules
Type of mixture Bulk density (g/cm
3
) Theoretical maximum density (g/cm
3
) Air void content (%)
WO/C 2.559 2.679 4.5
0.5% caps 2.557 2.669 4.2
0.75% caps 2.529 2.668 5.2
1.00% caps 2.499 2.664 6.2
Materials and Structures (2019) 52:78 Page 7 of 13 78
3.4 Effect of capsules on the particle loss
properties of asphalt mixture
Figure 8shows the particle loss percentage of asphalt
samples with, and without, capsules. Average values
were calculated from 5 tested specimens for each
capsule content in the mixture. In Fig. 8it can be seen
that the average particle loss of the asphalt mixture
without capsules and with 0.5%, 0.75% and 1.00% of
capsules was 4.94%, 5.07%, 5.76% and 6.06%,
respectively. It means that the addition of Ca-alginate
capsules increases the particle loss of asphalt mixtures.
The mixture with 0.5% of capsules has almost the
same average particle loss resistance than the mixture
without capsules. As well, visual inspection of test
specimens containing capsules showed that they can
successfully resist the impacts during the particle loss
test. Similar results have also been reported in 2017 by
Al-Mansoori et al. [15].
5mm 5mm
0.5 mm
0.5 mm
Before
After
(a) (b) (c)
(d)
Group of
capsules
Broken
capsules
Undamaged
Damaged
Fig. 6 CT-Scan reconstruction of the asphalt mixture cores containing a0.5% (based on [18]) and b0.75% of capsules, and two
individual capsules from the mixture with 0.5% of capsules, one undamaged (c) and the other one damaged (d)
Fig. 7 a Average ITS curves and bmaximum average ITS results for asphalt samples with, and without (WO/C), capsules
78 Page 8 of 13 Materials and Structures (2019) 52:78
3.5 Effect of capsules on the durability
to of asphalt mixture
Furthermore, Fig. 9shows the fatigue properties of
asphalt samples with and without capsules. Average
values were calculated from 3 tested specimens for
each capsule content in the mixture. In Fig. 9the
addition of 0.5% capsules improves the fatigue
resistance of asphalt mixture at all stress levels. It
can be attributed to the lower air voids content and
higher strength of the mixture containing 0.5% of
capsules, see ITS results in Fig. 7. The addition of
more capsules (i.e. 0.75% and 1.00% of capsules)
decreases the fatigue resistance of asphalt mixture.
This can be attributed to that an excess of capsules
increases the air voids of the mixture, as seen in
Table 2.
3.6 Influence of the capsule amount on the self-
healing capability of the mixtures
Figure 10a shows the healing levels reached by the
asphalt samples with, and without, capsules, at
different healing times. Each point in the graph
represents one tested beam depending on the capsule
content and healing time. The high number of tests
done and good fitting of the results tell us abou the high
quality of the data obtained. It can be seen that: (1) the
healing levels achieved by the asphalt samples with
capsules were higher than those reached by test beams
without capsules, and (2) the healing level of asphalt
beams with, and without, capsules increased with the
healing time until a maximum value. It can be seen
that the maximum healing level value was reached at a
healing time of 96 h and remained mostly constant
until 216 h, see continuous healing range in Fig. 10a.
Hence, the healing level reached by the beams
between 96 and 216 h can be considered as the
maximum healing level of the mixtures.
Consequently, the average maximum healing levels
for asphalt mixtures without capsules and containing
0.5%, 0.75% and 1.00% of capsules were 20.44%,
49.04%, 54.32% and 54.86%, respectively. These
results proved that higher capsule contents into the
mixture resulted in higher healing levels for all the
healing times studied, see Fig. 10a. This is because
higher capsule contents increase the probability of
breaking more capsules as a result of the external
compression load applied during the test, which also
increase the potential released oil inside the asphalt by
mechanical effects [18].
The level of oil released in asphalt measured by
FTIR tests after tests was 35.45%, 45.02% and 60.93%
for Ca-alginate capsule contents of 0.5%, 0.75% and
1.00%, respectively, see Fig. 10b. Nevertheless, this
level of oil released in asphalt is not only related to the
application of the external load, but also to the
manufacturing process of the mixtures where the
capsules can also be damaged. After the manufactur-
ing process of the mixtures, low levels of oil released
were quantified: 3.22%, 4.09% and 5.54% for capsule
contents of 0.5%, 0.75% and 1.00%, respectively.
Healing levels presented in Fig. 10a also show that
capsule contents higher than 0.5% did not present a
Fig. 8 Results of particle loss of asphalt samples with, and
without (WO/C), capsules
Fig. 9 Results of average fatigue tests of asphalt samples with,
and without (WO/C), capsules
Materials and Structures (2019) 52:78 Page 9 of 13 78
significant increase in the maximum healing levels,
see results of 0.75% and 1.00% in the maximum
healing range. This result can be due to an excess of
rejuvenating agent (i.e. oil-to-bitumen content) inside
the asphalt mixture. In this study, oil-to-bitumen
content by mass in bitumen was 6.97%, 10.46% and
13.95% for capsule contents of 0.5%, 0.75% and
1.00%, respectively. Previous researches [26] devel-
oped on aged asphalt binders rejuvenated with
vegetable oil, proved that contents over 7.0% by mass
of bitumen, can be detrimental to the properties of the
mixture, affecting the rheological properties of bitu-
men. Therefore, based on the results of this study,
0.5% of capsule addition can be considered as the
optimal percentage for asphalt healing without affect-
ing the rheological properties of the dense-graded
asphalt mixture.
3.7 Summary and discussion of results
Table 3provides a general overview of the effect of
the Ca-alginate capsule addition on the density, air
void content, indirect tensile strength, average loss
resistance, fatigue performance, and healing proper-
ties of dense-graded asphalt mixture. In this Table, it
can be seen that depending on the capsule content
added to the mixture (i.e. 0.5%, 0.75% or 1.00%) the
effect of the capsules on the properties evaluated was
different. The Ca-alginate capsules reduced the aver-
age bulk density of the mixture regardless of the
amount of capsules added, which is logical because
they had lower density than the aggregates; although
the effect of adding 0.5% capsules on density was very
low (-0.1%, see Table 3) compared with mixtures
containing a higher capsule content; this makes think
the authors that this is an appropriate amount to mix in
the asphalt. In [15] and [16] it was evidenced that Ca-
alginate capsules adapt their shape to the aggregates
because their composition and spherical shape make
them flow better during manufacturing process, see
also Fig. 6c. Capsule contents of 0.75% and 1.00%
increased the average air void content of the mixture,
while 0.5% of capsules reduced the air void in a 6.7%
compared with the mixture without capsules.
Furthermore, capsule contents of 0.75% and 1.00%
negatively affected the maximum indirect tensile
strength (ITS), while a 0.5% of capsules increased
the ITS in an 11% compared with the mixture without
capsules. Conversely, capsule addition negatively
affected the average particle loss resistance of the
mixture. Nonetheless, the effect of adding 0.5%
capsules on particle loss resistance was low
(-2.5%, see Table 3) compared with the mixture
with a higher capsule content. The reason for the
increase in particle loss could be that capsules adapted
their shape to the surrounding aggregates into the
0
10
20
30
40
50
60
70
80
90
100
04080120160200240
Healing level (%)
Healing time (h)
WO/C
0.5% caps
0.75% caps
1.00% caps
0
10
20
30
40
50
60
70
80
90
100
0.50% 0.75% 1.00%
Amount of 0il released (%)
Capsule content in the mixture
Before healing
After healing
(a) (b)
Max. Healing Range
Fig. 10 Results of the: acrack-healing levels for the asphalt beams with, and without (WO/C), capsules depending on the healing time,
and bpercentage of oil released inside the different mixtures before, and after, healing process
78 Page 10 of 13 Materials and Structures (2019) 52:78
asphalt matrix, modifying the density of the material
and consequently increasing the air void content in the
mixture.
Furthermore, it was observed that capsule contents
of 0.75% and 1.00% negatively affected the average
fatigue life, while a 0.5% of capsules increased the
average fatigue life in a 29.3% compared with the
mixture without capsules. Based on previous studies
of the authors [1518], the negative effect of the
addition of 0.75% and 1.00% of capsules on the
mechanical properties of the asphalt mixtures could
have been magnified by the size of the capsules
because they are part of the solid skeleton and they act
as soft aggregates into the bituminous matrix, see
Fig. 6.
Conversely, it was found that capsules affected
positively the self-healing capability of the asphalt
mixture increasing in all the amounts studied the
maximum healing level value, which happened at
96 h. Once the maximum healing level was reached, it
remained constant. This increase in the healing level
reached by the mixtures with capsule addition of 0.5%,
0.75% and 1.00% was of 100.9%, 121.3% and
126.3%, respectively. Nevertheless, as shown in
Table 3, a higher addition of capsules does not
necessarily mean improved performance, because a
higher number of capsules can also affect their spatial
distribution inside the mixture.
4 Conclusions
This paper shows results about the effect of encapsu-
lated sunflower oil on the density, air void content,
indirect tensile strength, average loss resistance,
fatigue performance, and healing properties of
dense-graded asphalt mixtures. Based on the results,
the following conclusions have been obtained:
It was proven that Ca-alginate capsules, can affect
the bulk density and air void content of asphalt
mixtures. The reason for this may be the oil lost by
the capsules during asphalt mixing and compaction
and the spherical shape of the capsules. Neverthe-
less, it was observed that 0.5% of capsule content
does not noticeably reduce these properties on the
evaluated mixtures.
The Ca-alginate capsule addition has an influence
on the mechanical properties of the dense asphalt
mixture. It was found that 0.5% of capsules is the
optimal content for asphalt mixture to obtain good
mechanical properties. Compared to plain asphalt
mixture without capsules, the mixture containing
0.5% of capsules has enhanced the indirect tensile
strengt, fatigue life and maintained the same
particle loss resistance. Note that the air void
content of the mixture was affected by the addition
of capsules, and that may have had an influence on
the results.
On the contrary, asphalt mixtures with capsule
contents of 0.75% and 1.00% presented a negative
effect on the indirect tensile strength, fatigue, and
particle loss properties of the tested specimens.
The negative effect was attributed to the capsule
size, since they act as soft aggregates into the
bituminous matrix.
CT-Scan results proved that Ca-alginate capsules
can survive the mixing and compaction process of
asphalt mixture and they will release the oil inside
the asphalt by the effect of external compression
loading. Higher capsule amounts added in the
asphalt mixture resulted in higher oil release ratios.
In addition, CT-Scan results also proved that the
Table 3 Comparison of
asphalt mixtures with
capsules to mixtures
without them (values in %)
Property evaluated Capsule content by total weight of the mixture
(Notation: .decrease and mincrease)
0.50% 0.75% 1.00%
Average bulk density .(-0.1) .(-1.2) .(-2.3)
Average air void content .(-6.7) m(15.6) m(37.8)
Maximum indirect tensile strength m(11.0) .(-7.3) .(-25.4)
Average particle loss resistance .(-2.5) .(-16.5) .(-22.7)
Average fatigue life m(29.3) .(-23.0) .(-51.3)
Maximum healing level (at 96 h) m(100.9) m(121.3) m(126.8)
Materials and Structures (2019) 52:78 Page 11 of 13 78
capsules were fixed in the asphalt structure show-
ing a strong adhesion to asphalt mastic by effect of
a good interlocking with aggregates.
The oil released from the Ca-alginate capsules
significantly increases the self-healing capability
of the dense graded asphalt mixtures, increasing
their healing level with all the capsule contents
studied until a maximum healing level value,
reached after 96 h of healing. Once the maximum
healing level was reached, the healing level of the
mixtures remained constant. This was because a
new state of physical equilibrium was attained at
the crack surface.
It was proven that higher capsule amounts added in
the asphalt mixture resulted in higher healing
levels, because a greater amount of rejuvenator is
released in the asphalt. However, 0.5% of capsule
addition can be considered as the optimal percent-
age for asphalt self-healing without affecting the
rheological properties of the mixtures.
Acknowledgements The authors express their gratitude for
the funding given by Highways England UK, through the
Research Project 558065, and to the Royal Society-Newton
Mobility Grant, IE150750. The first author thanks the
Government of Chile, since his Postdoctoral scholarship was
funded by CONICYT/BECAS CHILE 74170030. Additionally,
the fifth author thanks the financial support given by the
Scientific and Technological Research Council of Turkey
(TUBITAK) (application no. 1059B141600780) for the
research scholarship granted.
Compliance with ethical standards
Conflict of interest The authors declare that they have no
conflicts of interest.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://
creativecommons.org/licenses/by/4.0/), which permits unre-
stricted use, distribution, and reproduction in any medium,
provided you give appropriate credit to the original
author(s) and the source, provide a link to the Creative Com-
mons license, and indicate if changes were made.
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... Calcium alginate capsules containing different healing agents were fabricated by orifice-bath method. Based on previous research [33,35,36], the encapsulation procedure of calcium alginate capsules is shown in Figure 1: (1) 8 g sodium alginate was added into 392 g deionized water and the mixture was stirred until the sodium alginate was completely dissolved. (2) An amount of 40 g healing agent and 2 mL Tween 80 were added to the sodium alginate solution and then sheared for 15 min at 5000 rpm to form an oil-in-water emulsion. ...
... The asphalt mixture designed had an air void (VV) of 4.0%, a bulk density of 2.489 g/cm 3 , void in mineral aggregate (VMA) of 15.6% and void filled with asphalt (VFA) of 74.6%, meeting the requirements of "Technical Specification for Construction of Highway Asphalt Pavement (JTG-F40-2004)" [37]. It is reported that the content of calcium alginate capsules shouldn't be higher than 0.5% to avoid the adverse effect on the water sensitivity and fatigue resistance of asphalt mixture [36,38]. In this research, calcium alginate capsules with a content of 0.4% were added to asphalt mixture without changing the gradation to study their effects on the selfhealing properties of the mixture. ...
... Asphalt mixture beams with size 95 mm × 50 mm × 45 mm were obtained from rutting plates and a 10 mm × 4 mm notch was created in the middle of each beam to make sure crack appears at the same position in the following fracture-healing test. It is reported that the content of calcium alginate capsules shouldn't be higher than 0.5% to avoid the adverse effect on the water sensitivity and fatigue resistance of asphalt mixture [36,38]. In this research, calcium alginate capsules with a content of 0.4% were added to asphalt mixture without changing the gradation to study their effects on the selfhealing properties of the mixture. ...
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... Again a notch was cut into the underside of the beam measuring 5×5 mm bisecting the face. This asphalt was featured in experiments conducted in [Norambuena-Contreras et al., 2019a]. ...
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... As mentioned, the use of microcapsules for healing purposes can affect the mechanical function of asphalt pavements (Micaelo et al., 2020;Norambuena-Contreras et al., 2019a). So, Su et al. (2021) investigated the effects of the empty microcapsules remaining after the healing process on the mechanical responses of asphalt pavement. ...
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The inherent self-healing ability of asphalt is insufficient and fails to timely repair the cracks due to the combined effect of temperature variation, air oxidation, ultraviolet exposure and traffic loading. Rejuvenator encapsulation based on self-healing asphalt is a green sustainable preventive maintenance technology for asphalt pavement. During the last decade, rejuvenator encapsulation for asphalt self-healing has been a research hotspot and calcium alginate hydrogels encapsulating rejuvenator is a promising self-healing technology. Hence, this review sheds light on the recent advances of calcium alginate hydrogels encapsulating asphalt rejuvenator including self-healing capsules and fibers. The synthesis methods of calcium alginate capsules and fibers containing rejuvenator were elaborately introduced, and their surface morphology, interior structure, mechanical strength, thermal stability, rejuvenator content, distribution and survival in asphalt materials were systematically analyzed. Besides, the effect of capsules and fiber on the mechanical property and pavement performance of asphalt concrete were explored. Additionally, a comprehensive review about the effect of calcium alginate capsules and fibers on self-healing ability of asphalt materials were presented, and the rejuvenator release mechanism and release ratio of them in asphalt mixtures were expounded. In a nutshell, this review aims at highlighting the current research achievements on alginate capsules and fibers containing rejuvenator in asphalt materials, and inspiring enhanced self-healing methods for smart and sustainable maintenance of asphalt pavement.
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Asphalt pavements and bituminous composites are majorly damaged by bitumen aging and fatigue cracking by traffic load. To add, maintenance and reparation of asphalt pavements is expensive and also releases significant amounts of greenhouse gases. These issues can be mitigated by promoting asphalt self-healing mechanisms with encapsulated rejuvenators. The ability of the required microcapsules to be resilient against high temperatures, oxidation, and mechanical stress is essential to promote such self-healing behavior without compromising the field performance of the asphalt pavement. This work proposes, for the first time, the use of extremely resistant biobased spores for the encapsulation of recycled oil-based rejuvenators to produce more resilient self-healing pavements. Spore encapsulants were obtained from natural spores (Lycopodium clavatum) by applying different chemical treatments, which enabled the selection of the best morphologically intact and clean spore encapsulant. The physical, morphological, and physicochemical changes were examined using fluorescence images, ATR-FTIR, SEM, size distribution, XRD, TGA and DSC analyses. Sunflower oil was used as the encapsulated rejuvenator with an optimal sol colloidal mixture for sporopollenin-oil of 1:5 (gram-to-gram). Vacuum, passive, and centrifugal encapsulation techniques were tested for loading the rejuvenator inside the clean spores and for selecting the best encapsulation technology. The encapsulation efficiency and the profiles of the accelerated release of the rejuvenator from the loaded spores over time were studied, and these processes were visualized with optical and inverted fluorescence microscopy. Vacuum encapsulation was identified as the best loading technique with an encapsulation efficiency of 93.02 ± 3.71%. The rejuvenator was successfully encapsulated into the clean spores, as observed by optical and SEM morphologies. In agreement with the TGA and DSC, the microcapsules were stable up to 204 °C. Finally, a self-healing test was conducted through fluorescence tests to demonstrate how these biobased spore microcapsules completely heal a crack into an aged bitumen sample in 50 min.
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Asphalt is a kind of material with self-healing ability, but the rate of self-healing can not repair the damage under real load in time. Microcapsule technology is considered to be an effective technology to improve the self-healing ability of asphalt. Clarifying the self-healing behavior of microcapsule asphalt and quantitatively evaluating the improvement effect are the premise of further practical application of microcapsule technology. In this study, a fatigue-healing-fatigue test was performed using a dynamic shear rheometer. The ratio of dynamic modulus decay rate before and after asphalt healing (HI) was employed to evaluate the self-healing performance of microcapsule asphalt. In addition, the fatigue-healing-fatigue tests of microcapsule asphalt with different damage degree, microcapsule content and asphalt type were carried out under different rest time and healing temperature conditions; The test results showed that HI increased nonlinearly with the increase of rest time, healing temperature and microcapsule content, and HI decreased nonlinearly with the increase of damage degree. Then, based on the polymer self-healing theory, a macro self-healing behavior model of microcapsule asphalt considering two-stage healing process was established, which can accurately predict the macro healing ability of microcapsule asphalt. Finally, the micro mechanism of repairing asphalt microcracks by microcapsules was obversed via fluorescence microcopy.
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This paper presents an experimental study to evaluate the mechanical and crack-healing properties of stone mastic asphalt (SMA) mixtures with encapsulated rejuvenators. With this goal, calcium alginate capsules with encapsulated sunflower oil as the rejuvenating agent have been manufactured and added into the SMA mixtures. Physical and mechanical properties of SMA with and without capsules have been evaluated following the British standard tests. Healing properties of SMA by the action of capsules have been assessed using three-point bending (3PB) tests applied on test beams conditioned at different healing times, from 5 to 216 h. The spatial distribution of the capsules in the SMA mixtures was evaluated by using X-ray computed microtomography. Results showed that the capsules can resist the manufacturing process without significantly reducing their properties. Additionally, testing of the mechanical properties of SMA mixtures with and without encapsulated rejuvenators presented similar results. Moreover, capsules showed a good spatial distribution inside the SMA samples. It was found that capsules with encapsulated oil increase the crack-healing properties of SMA when compared to mixtures without encapsulated rejuvenators. Overall, the results proved that the capsules with asphalt crack-healing purposes can be safely used in asphalt pavement construction without affecting its properties.
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This paper presents the self-healing results of asphalt mixtures by the action of capsules containing sunflower oil as encapsulated rejuvenator. Three different capsule contents, 0.10, 0.25 and 0.50% by total weight of the mixture, were added to the samples. The mechanical and thermal properties of capsules have been evaluated. In addition, the effect of the capsule addition and the healing temperature on the self-healing properties of asphalt mixtures have been evaluated through three-point bending tests on the cracked asphalt beams with, and without, capsules. The test was implemented by comparing the strength recovery of the broken beams after healing to their original flexural strength. It was proven that the capsules can resist the mixing and compaction processes and break inside the asphalt mixture as a result of applying external mechanical loads, releasing the encapsulated oil. The capsules content in asphalt mixture has a significant influence on the healing level, where a higher capsule content led to obtaining higher healing levels. Likewise, asphalt with, and without, capsules presents an increase of the healing level when the temperature increases. Finally, it was proved that healing temperature has higher influence on the healing levels of the asphalt below 40ºC.
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In this paper, steel slag/steel fiber composite asphalt mixture were prepared. The effects of the addition of steel slag and/or steel fibers on the mechanical, thermal, induction heating and healing properties of asphalt mixture were investigated. The results showed that adding steel slag and/or steel fibers improves the water stability, particle loss resistance and fracture energy of asphalt mixtures. The addition of steel fibers increased the thermal conductivity and thermal diffusion of the asphalt mixture, and steel slag showed a reverse effect. Steel slag asphalt mixture cooled more slowly than steel fiber asphalt mixture, which is beneficial to crack healing of asphalt mixture. The composite of steel fibers and steel slag can enhance the induction heating speed, heating homogeneity and thus enhance the induction healing ratio of asphalt mixture. It is concluded that steel slag/steel fibers composite asphalt mixture achieves good mechanical and induction healing properties.
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The self-healing of cracks in asphalt mixture is mainly due to the drain of the bitumen contained in the space between the aggregates, into the cracks. Until now, it was believed that the physical principles affecting the flow of bitumen are influenced by gravity and surface energy of bitumen and the aggregates. In this study, we show that the thermal expansion of bitumen plays an important role in the self-healing of asphalt mixture. To demonstrate this, asphalt mortar beams were manufactured and broken in two pieces by means of three-point bending tests. Self-healing was induced in asphalt mixture by increasing its temperature using a convection oven, at temperatures that ranged from 40 °C to 120 °C. The self-healing ratio was calculated by comparing the force required to break the test specimens before and after heating. Furthermore, a test was designed that consisted of bitumen raising through a capillary tube from a bitumen container. To account for the effect of thermal expansion, the bitumen container was fully enclosed except for the capillary tube. To account for the effect of surface energy on the bitumen’s capillary flow, the capillary tube was placed in a container that was open to the atmosphere. The rise of bitumen was monitored at temperatures that ranged from 40 °C to 120 °C. Finally, activation energies were derived from the rise of bitumen in the capillaries, viscosity changes and the self-healing progression. It was found that the activation energy of asphalt self-healing is similar to that of bitumen rising due to thermal expansion, which confirms the contribution of thermal expansion on asphalt self-healing by the effect of increasing temperature.
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In recent decades, researchers have revealed the great healing potential of asphalt and proposed various novel methods to inspire and improve the self‐healing capacity of asphalt aimed to prolong the service life of asphalt pavement. In this review, up to date research progresses in induction healing and embedded rejuvenator encapsulation are presented, respectively. Meanwhile, the trial section applications of induction healing and capsule healing are highlighted, which show promising results. Finally, some recommendations for the future development of self‐healing asphalt are proposed. Incorporating self‐healing technologies in asphalt pavement has huge potential to prolong the service life of asphalt pavement. To this aim, induction healing and capsule healing technologies have been tested in laboratory and applied in the trial sections. This review paper states the development of these technologies and illustrates how they revolutionize asphalt pavement design.
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Researchers have demonstrated that the rejuvenator encapsulation method is a promising autonomic self-healing approach for asphalt pavements, where by the self-healing system improves the healing capacity of an asphalt pavement mix. However, potentially high environmental risk via leaching of hazardous chemicals such as melamine formaldehyde renders the technology unsuitable for widespread use in road design. This paper explores the potential for the use of more environmentally friendly and economically viable rejuvenator encapsulation method, where the calcium alginate is used as rejuvenator encapsulation material. The capsule morphology and microstructure were studied using the Microscopy and X-ray tomography. Capsules thermal resistance and mechanical strength were investigated using the Thermogravimetric analysis (TGA) and micro-compressive tests. The results demonstrated that the capsules have sufficient thermal and mechanical strength to survive the asphalt production process. The healing efficiency of the system was evaluated by embedment of calcium-alginate capsules encapsulating rejuvenator in an asphalt mastic beams and subjected to monotonic three-point bend (3PB) loading and healing programme. The results illustrated that the calcium-alginate capsules encapsulating rejuvenator can significantly improve healing performance of the asphalt mastic mix.
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This paper presents the self-healing results of asphalt mastic by the action of calcium-alginate capsules containing sunflower oil. The morphological, physical, thermal and mechanical properties of the capsules have been evaluated. Additionally, the effect of the capsule oil content and the healing temperature on the self-healing properties of asphalt mastic have been evaluated. It was proven that the capsules can resist the mixing and compaction processes and break inside the asphalt mastic due to mechanical loads, releasing the oil. Healing levels in the asphalt mastic samples with capsules were greater than samples without capsules. The healing level depended on the oil content of the capsules and temperature.
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Ten years’ field aged asphalt specimens have collected and being detailed studied. The aging index, viscoelastic behavior of aged binder recovered from the cored specimens, indirect tensile strength and residual fatigue life of the top layer and underneath layer were studied. Saturates, Aromatics, Resins and Asphaltenes (SARA) analysis and Fourier Transform Infrared Analysis (FTIR), viscoelastic analysis, Indirect Tensile (IDT) resilient modulus and fatigue test were performed. Both SARA, FTIR and viscoelastic analysis indicates that traffic loading does have positive degradation influence on asphalt binder. FTIR shows that traffic loading has limited influence on the chemic changes of S[dbnd]O compounds, while has positive contribution on degrading the C[dbnd]C bonds and hence promotes the oxidize aging. Although surface layer can protect underneath asphalt layer from aging, 10-year field aging would introduce significant deterioration onto the underneath layer. The effective aging depth can go as deep as to the second layer. Fatigue tests illustrate that 10-year service has resulted in very weak asphalt pavement surface. Its fatigue life is very sensitive to traffic loading.
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In this research calcium-alginate capsules containing vegetable oil that can release their content due to a mechanical trigger have been made and mixed in asphalt mixture to improve its natural self-healing properties. The physical, mechanical and self-healing properties of asphalt mixture containing these capsules have been evaluated for the first time. Three different capsule contents were used, with oil-to-bitumen ratio 1.1, 2.8 and 5.5, respectively. Capsules were strongly bonded to the asphalt mixture and results showed similar mechanical performance to that of asphalt with and without capsules in the water sensitivity, particle loss and permanent deformation tests. This shows that capsules for asphalt self-healing can be safely used in the road, without affecting its quality. Asphalt containing capsules had slightly lower stiffness, which can be easily solved by reducing the size of the capsules in the future. Furthermore, a new method for testing asphalt containing capsules was designed and tested. It was found that cracked asphalt mixture with capsules recovered 52.9% of initial strength at 20 °C versus 14.0% of asphalt mixture without capsules.