Available via license: CC BY 4.0
Content may be subject to copyright.
materials
Article
Microencapsulated Bio-Based Rejuvenators for the
Self-Healing of Bituminous Materials
Jose Norambuena-Contreras 1, * , Luis E. Arteaga-Perez 2, Andrea Y. Guadarrama-Lezama 3,
Rodrigo Briones 4, Juan F. Vivanco 5and Irene Gonzalez-Torre 1
1LabMAT, Department of Civil and Environmental Engineering, Universidad del Bío-Bío, Avenida Collao
1202, Concepción, Chile; irenegon@ubiobio.cl
2
LPTC, Laboratory on Thermal and Catalytic Processes, Department of Wood Engineering, Universidad del
Bío-Bío, Avenida Collao 1202, Concepción, Chile; larteaga@ubiobio.cl
3Facultad de Química, Universidad Autónoma del Estado de México, Paseo Colón esq. Paseo Tollocan s/n,
Col. Residencial Colón 50120, Toluca 50000, Estado de México, Mexico; ayguadarramal@uaemex.mx
4CIPA, Centro de Investigación de Polímeros Avanzados, Avenida Collao 1202, Concepción, Chile;
r.briones@cipachile.cl
5Facultad de Ingeniería y Ciencias, Universidad Adolfo Ibáñez, Viña del Mar 2562340, Chile;
juan.vivanco@uai.cl
*Correspondence: jnorambuena@ubiobio.cl; Tel.: +56-41-311-1657
Received: 21 February 2020; Accepted: 20 March 2020; Published: 22 March 2020
Abstract:
Asphalt self-healing by encapsulated rejuvenating agents is considered a revolutionary
technology for the autonomic crack-healing of aged asphalt pavements. This paper aims to explore the
use of Bio-Oil (BO) obtained from liquefied agricultural biomass waste as a bio-based encapsulated
rejuvenating agent for self-healing of bituminous materials. Novel BO capsules were synthesized
using two simple dripping methods through dropping funnel and syringe pump devices, where the
BO agent was microencapsulated by external ionic gelation in a biopolymer matrix of sodium alginate.
Size, surface aspect, and elemental composition of the BO capsules were characterized by optical
and scanning electron microscopy and energy-dispersive X-ray spectroscopy. Thermal stability and
chemical properties of BO capsules and their components were assessed through thermogravimetric
analysis (TGA-DTG) and Fourier-Transform Infrared spectroscopy (FTIR-ATR). The mechanical
behavior of the capsules was evaluated by compressive and low-load micro-indentation tests. The
self-healing efficiency over time of BO as a rejuvenating agent in cracked bitumen samples was
quantified by fluorescence microscopy. Main results showed that the BO capsules presented an
adequate morphology for the asphalt self-healing application, with good thermal stability and
physical-chemical properties. It was also proven that the BO can diffuse in the bitumen reducing the
viscosity and consequently self-healing the open microcracks.
Keywords: asphalt; encapsulated rejuvenators; agricultural waste; bio-oil; self-healing efficiency
1. Introduction
Bituminous materials are viscoelastic composites mainly used for road and airport pavement
construction. These materials include asphalt mixtures, mastics and bituminous binders [
1
]. Amongst
the bituminous materials, asphalt mixtures are the most used for asphalt pavement construction in
the world. When asphalt pavements are exposed to years of mechanical and thermal stresses, as
well as environmental effects (i.e., air oxidation, ultraviolet (UV) radiation, moisture), debonding
at the interface between bitumen and mineral aggregates can occur, resulting in their cracking [
2
].
Cracking of bituminous materials mainly occurs due to the oxidation of the hydrocarbon compounds
of bitumen [
3
], which is a dark thermoplastic material composed of a solid part called asphaltenes
Materials 2020,13, 1446; doi:10.3390/ma13061446 www.mdpi.com/journal/materials
Materials 2020,13, 1446 2 of 16
and a liquid part called maltenes (resins and oils) [
4
]. During the oxidative process, additional
oxygen-containing polar functional groups promote clustering mechanisms, the content of asphaltenes
increases, while maltenes decrease, and all these molecular-scale changes result in an increase of
rigidity of the pavement and, consequently, its damage by cracking [
5
]. Hence, different approaches
and technologies to promote crack closure at an early stage have been recently proposed, based on
the development of self-healing bituminous materials [
6
]. Numerous advances on novel self-healing
bituminous materials for sustainable pavements have been developed over recent years, turning this
topic into an emerging field of study. Currently, two different approaches have mainly been used to
promote asphalt self-healing thereby extending the service life of asphalt pavements: first, an approach
to reduce the viscosity of bitumen by increasing its temperature through externally triggered heating
via induction and microwave radiation [
7
] and, secondly, an autonomic approach by the action of
encapsulated rejuvenating agents [
8
]. Rejuvenating agents consist of lubricating and extender oils with
a high proportion of maltene constituents, which restore the asphaltenes/maltenes ratio in the aged
bitumen (i.e., reduce the stiffness of the aged binder) [
9
]. The concept of self-healing in bituminous
materials over non-linear time is shown in Figure 1. When damage occurs in an asphalt containing
embedded capsules, open microcracks appear and eventually propagate until they reach and break a
capsule releasing the contained rejuvenating agent, which reduces the viscosity of aged bitumen by
diffusion, allowing it to flow through the cracks, sealing them autonomously.
Materials 2020, 13, x FOR PEER REVIEW 2 of 15
bitumen [3], which is a dark thermoplastic material composed of a solid part called asphaltenes and
a liquid part called maltenes (resins and oils) [4]. During the oxidative process, additional oxygen-
containing polar functional groups promote clustering mechanisms, the content of asphaltenes
increases, while maltenes decrease, and all these molecular-scale changes result in an increase of
rigidity of the pavement and, consequently, its damage by cracking [5]. Hence, different approaches
and technologies to promote crack closure at an early stage have been recently proposed, based on
the development of self-healing bituminous materials [6]. Numerous advances on novel self-healing
bituminous materials for sustainable pavements have been developed over recent years, turning this
topic into an emerging field of study. Currently, two different approaches have mainly been used to
promote asphalt self-healing thereby extending the service life of asphalt pavements: first, an
approach to reduce the viscosity of bitumen by increasing its temperature through externally
triggered heating via induction and microwave radiation [7] and, secondly, an autonomic approach
by the action of encapsulated rejuvenating agents [8]. Rejuvenating agents consist of lubricating and
extender oils with a high proportion of maltene constituents, which restore the asphaltenes/maltenes
ratio in the aged bitumen (i.e., reduce the stiffness of the aged binder) [9]. The concept of self-healing
in bituminous materials over non-linear time is shown in Figure 1. When damage occurs in an asphalt
containing embedded capsules, open microcracks appear and eventually propagate until they reach
and break a capsule releasing the contained rejuvenating agent, which reduces the viscosity of aged
bitumen by diffusion, allowing it to flow through the cracks, sealing them autonomously.
Figure 1. Concept of autonomic crack-healing in asphalt over time using encapsulated rejuvenators.
Depending on their structure, capsules can be classified as either spherical polynuclear capsules or
core-shell capsules [10], consisting of a defined core/cargo surrounded by a polymeric shell structure.
Several authors have successfully synthesized and proven the use of numerous encapsulated
rejuvenating agents, such as dense aromatic oil [11], waste cooking oil [12], and sunflower-cooking
oil [13] with asphalt self-healing purposes. Encapsulation procedures, such as in-situ polymerization
and ionic gelation, have mainly been used to produce core-shell and polynuclear (beads)
encapsulated rejuvenators with size ranges of 5–153 µm and 2–7 mm, respectively [11–16]. Recently,
bio-oil (BO) has been introduced by Zhang et al. [17] as a sustainable rejuvenating agent for aged
asphalt binder. Bio-oil is a kind of renewable material which can be obtained from crops, cotton,
straw, wood waste, animal manure and others through pyrolysis [18] or liquefaction processes [19].
In this context, Aguirre and co-workers [20] synthesised double-walled microcapsules with size 153
µm by in-situ polymerisation using polyurethane and urea-formaldehyde, containing a commercial
BO product as the rejuvenating agent. Nonetheless, the petrochemical-based synthetic polymers used
for the encapsulation process can produce a high environmental risk from the leaching of hazardous
chemical compounds, making these capsules unsuitable for their use in asphalt pavements [21].
This paper aims to explore the use of BO obtained from liquefied agricultural biomass waste as
a bio-based encapsulated rejuvenating agent for the self-healing of bituminous materials. To reach
this aim, as a first stage of this research, BO capsules were synthesized using two simple dripping
methods, where the BO agent was encapsulated by ionic gelation in a biopolymeric matrix of sodium
alginate. This bio-based polymer was selected due to its large use in oil encapsulation and high
capacity to form gel at low concentrations [14]. Finally, a comprehensive experimental
characterization of the different BO capsules and their main results are discussed within the paper.
Figure 1.
Concept of autonomic crack-healing in asphalt over time using encapsulated rejuvenators.
Depending on their structure, capsules can be classified as either spherical polynuclear capsules or
core-shell capsules [10], consisting of a defined core/cargo surrounded by a polymeric shell structure.
Several authors have successfully synthesized and proven the use of numerous encapsulated
rejuvenating agents, such as dense aromatic oil [
11
], waste cooking oil [
12
], and sunflower-cooking
oil [
13
] with asphalt self-healing purposes. Encapsulation procedures, such as in-situ polymerization
and ionic gelation, have mainly been used to produce core-shell and polynuclear (beads) encapsulated
rejuvenators with size ranges of 5–153
µ
m and 2–7 mm, respectively [
11
–
16
]. Recently, bio-oil (BO) has
been introduced by Zhang et al. [
17
] as a sustainable rejuvenating agent for aged asphalt binder. Bio-oil
is a kind of renewable material which can be obtained from crops, cotton, straw, wood waste, animal
manure and others through pyrolysis [
18
] or liquefaction processes [
19
]. In this context, Aguirre and
co-workers [
20
] synthesised double-walled microcapsules with size 153
µ
m by in-situ polymerisation
using polyurethane and urea-formaldehyde, containing a commercial BO product as the rejuvenating
agent. Nonetheless, the petrochemical-based synthetic polymers used for the encapsulation process
can produce a high environmental risk from the leaching of hazardous chemical compounds, making
these capsules unsuitable for their use in asphalt pavements [21].
This paper aims to explore the use of BO obtained from liquefied agricultural biomass waste as a
bio-based encapsulated rejuvenating agent for the self-healing of bituminous materials. To reach this
aim, as a first stage of this research, BO capsules were synthesized using two simple dripping methods,
where the BO agent was encapsulated by ionic gelation in a biopolymeric matrix of sodium alginate.
This bio-based polymer was selected due to its large use in oil encapsulation and high capacity to form
Materials 2020,13, 1446 3 of 16
gel at low concentrations [
14
]. Finally, a comprehensive experimental characterization of the different
BO capsules and their main results are discussed within the paper.
2. Materials and Methods
2.1. Materials
Biopolymeric BO capsules for asphalt self-healing were prepared in this study. The polymeric
structure of the capsules was prepared of low-viscosity grade sodium alginate (viscosity at 20
◦
C
200–300 cP for 2% w/v solution) provided as a powder by Buchi (Flawil, Switzerland), and calcium
chloride dihydrate (CaCl
2·
2H
2
O) provided in granular pellets with 70% purity by Winkler (Concepci
ó
n,
Chile). Bio-Oil (BO) obtained from agricultural biomass waste via liquefaction with density 1.25 g/cm
3
,
viscosity at 20
◦
C 750 cP, and pH at 25
◦
C 2.0–2.5 was used as the bio-based rejuvenating agent for
asphalt self-healing. Virgin bitumen with density 1.04 g/cm
3
and penetration grade 80/100 mm at 25
◦
C
was used to quantify the BO healing efficiency.
2.2. Preparation of Bio-Oil Rejuvenating Agent by Liquefaction Process
Bio-oil used as the rejuvenating agent for asphalt self-healing was obtained from agricultural
biomass waste through a solvolysis liquefaction process described by Briones et al. [
19
]. Briefly,
a mixture of PEG#400 and glycerol in proportion 50/50 w/w was used as the solvent in the liquefaction
process. The liquefying solvent and a small amount of H
2
SO
4
(3 wt.% of the solvent) as catalyst were
added into a three-neck flask reactor (250 mL) equipped with a mechanical stirrer (Model OS40-Pro
-LB Pro, Rocky Hill, CO, USA), temperature controller and condenser. When the temperature of the
liquefying media reached 150
◦
C, a specific quantity of oven-dried biomass corn stover residues was
added gradually. The mixture was continuously stirred during the liquefaction process (ca. 45 min)
to obtain a homogeneously liquefied bio-oil product. After ending the reaction, the flask was cooled
down to ambient temperature to stop the reaction and the liquefied products were collected for later
application as encapsulated rejuvenator.
2.3. Preparation of Bio-Based Capsules Containing Liquefied Bio-Oil
Bio-oil capsules were prepared by the cross-linking of sodium alginate (C
6
H
7
O
6
Na) in the presence
of Ca
2+
ions through an external ionic gelation process [
13
], as shown in Figure 2. The preparation
consisted of 5 steps, as follows: (1) sodium alginate was added to deionized water and mechanically
stirred (Model OS40-Pro -LB Pro) at 700 rpm for 30 min, until its complete solution. After that, the
solution was left to rest for about 30 min until all the bubbles disappeared; (2) rejuvenating BO was
then added to the sodium alginate solution while mixing with the mechanical stirrer at 700 rpm until a
stable emulsion was obtained; (3) at the same time, the hardening bath of calcium chloride (CaCl
2
)
in deionized water was prepared, using a magnetic stirrer until complete solution; (4) capsules were
formed by letting the water-alginate-rejuvenator emulsion drop into the calcium chloride hardening
bath. During the capsule formation process, the hardening bath solution was gently agitated using a
magnetic stirrer at 200 rpm. Capsules stayed in the solution for 30 min after the end of the encapsulation
process; and (5) BO capsules were rinsed with deionized water and dried in an electric dryer for 24 h
at 35
◦
C. Finally, the dried BO capsules were stored in a freezer at
−
18
◦
C to avoid the oxidation of
the rejuvenator.
Materials 2020,13, 1446 4 of 16
Materials 2020, 13, x FOR PEER REVIEW 4 of 15
capsules that pumps the emulsion through a metal hollow needle of 1.2 mm diameter at a pressure
flow rate of 2 mL/min. Considering the variables involved in the study (2 methods and 2
concentrations) a total of 4 different types of BO capsules were prepared in this research, named as:
M1–2%; M1–3%; M2–2%; and M2–3%.
2.5. Morphology, Encapsulation Efficiency, and Antioxidant Activity of Bio-Oil Capsules
The morphology of the BO capsules was characterized by their size, surface aspect and internal
microstructure by means of Optical (Leica EZ4, Wetzlar, Germany) and Scanning Electron
Microscopy (Hitachi SU 3500, Chiyoda, Tokyo, Japan), respectively. Additionally, the presence of
elements in the surface of the BO capsules was evaluated by SEM through energy dispersive X-ray
spectroscopy (EDX Bruker Quantax 100, Billerica, MA, USA) for semi-quantitative determinations.
The relative density of the BO capsules was measured following method B of the ASTM D792-13 [22].
The encapsulation efficiency and antioxidant activity of the BO were quantified by chemical tests
described by Guadarrama-Lezama et al. [23]. Thus, encapsulation efficiency of BO was defined as the
ratio between the quantity of BO retained within the capsules and the total BO used to produce them.
Additionally, antioxidant activity of the encapsulated BO was quantified by the 2,2′-azino-bis (3-
ethylbenzothiazoline-6-sulphonic acid (ABTS) technique. Based on this method, the results of
antioxidant activity of the BO were presented as µmol Trolox/mL.
Figure 2. Preparation of the bio-based capsules containing bio-oil (BO). (a) Set-up of the dropping
funnel method (M1); (b) set-up of the microfluidic pressure pump method (M2); (c) detail of BO
encapsulation by simple extrusion through dripping technique using a syringe pressure pump, where
the alginate emulsion is extruded through a metal hollow needle and added dropwise into a
collecting/hardening bath where the biopolymer is cross-linked; (d) BO (BO) encapsulation in calcium
alginate matrix via external gelation where Ca
2+
ions migrate from the aqueous bath (CaCl
2
solution)
to the emulsion drop and, as a result, the alginate chains are progressively cross-linked forming
irregular shaped or polynuclear BO capsules. (c),(d) are based on Martins et al. [10].
2.6. Thermochemical Characterization of Bio-Oil Capsules and Their Components
Chemical structures of the BO capsules and BO were characterized by Attenuated-Total
Reflection Infrared (ATR-IR) spectroscopy by direct transmittance in a single-reflection ATR system.
Infrared spectra were recorded on a PerkinElmer Spectrum Two Fourier Transform Infrared
Spectrometer (Waltham, Massachusetts, USA). Each spectrum was recorded over 20 scans in the
range from 4000 to 400 cm
−1
, with a resolution of 2 cm
−1
. A thermogravimetric analysis (TGA-DGT)
was carried out to evaluate the thermal stability of the BO capsules and their components. The
thermal test was recorded on a TA Instrument TGA Q 50 thermogravimetric analyser (New Castle,
Figure 2.
Preparation of the bio-based capsules containing bio-oil (BO). (
a
) Set-up of the dropping funnel
method (M1); (
b
) set-up of the microfluidic pressure pump method (M2); (
c
) detail of BO encapsulation
by simple extrusion through dripping technique using a syringe pressure pump, where the alginate
emulsion is extruded through a metal hollow needle and added dropwise into a collecting/hardening
bath where the biopolymer is cross-linked; (
d
) BO (BO) encapsulation in calcium alginate matrix via
external gelation where Ca
2+
ions migrate from the aqueous bath (CaCl
2
solution) to the emulsion
drop and, as a result, the alginate chains are progressively cross-linked forming irregular shaped or
polynuclear BO capsules. (c,d) are based on Martins et al. [10].
2.4. Concentrations and Encapsulation Procedures
To evaluate the effect of biopolymer concentrations and encapsulation methods on the properties
of the capsules, two different sodium alginate concentrations (2% and 3% of weight by volume of
water) and encapsulation methods were used for BO capsule preparation. For both concentrations, the
BO/water volume ratio was 0.05. Two simple extrusion-dripping set-up methods by external gelation
were used to prepare the BO capsules: a dropping funnel (M1), and a microfluidic pressure pump
(M2). In both methods, the water-alginate-rejuvenator emulsion was dropped in the calcium chloride
hardening bath. The dropping funnel method (see Figure 2a) constitutes a gravity-drip procedure,
which mainly depends on the viscosity of the emulsion to form the capsules. In contrast, the pressure
pump method (see Figure 2b) constitutes a controlled mechanism to produce capsules that pumps
the emulsion through a metal hollow needle of 1.2 mm diameter at a pressure flow rate of 2 mL/min.
Considering the variables involved in the study (2 methods and 2 concentrations) a total of 4 different
types of BO capsules were prepared in this research, named as: M1–2%; M1–3%; M2–2%; and M2–3%.
2.5. Morphology, Encapsulation Efficiency, and Antioxidant Activity of Bio-Oil Capsules
The morphology of the BO capsules was characterized by their size, surface aspect and internal
microstructure by means of Optical (Leica EZ4, Wetzlar, Germany) and Scanning Electron Microscopy
(Hitachi SU 3500, Chiyoda, Tokyo, Japan), respectively. Additionally, the presence of elements in
the surface of the BO capsules was evaluated by SEM through energy dispersive X-ray spectroscopy
(EDX Bruker Quantax 100, Billerica, MA, USA) for semi-quantitative determinations. The relative
density of the BO capsules was measured following method B of the ASTM D792-13 [
22
]. The
encapsulation efficiency and antioxidant activity of the BO were quantified by chemical tests
described by Guadarrama-Lezama et al. [
23
]. Thus, encapsulation efficiency of BO was defined
as the ratio between the quantity of BO retained within the capsules and the total BO used to produce
them. Additionally, antioxidant activity of the encapsulated BO was quantified by the 2,2
0
-azino-bis
(3-ethylbenzothiazoline-6-sulphonic acid (ABTS) technique. Based on this method, the results of
antioxidant activity of the BO were presented as µmol Trolox/mL.
Materials 2020,13, 1446 5 of 16
2.6. Thermochemical Characterization of Bio-Oil Capsules and Their Components
Chemical structures of the BO capsules and BO were characterized by Attenuated-Total Reflection
Infrared (ATR-IR) spectroscopy by direct transmittance in a single-reflection ATR system. Infrared
spectra were recorded on a PerkinElmer Spectrum Two Fourier Transform Infrared Spectrometer
(Waltham, Massachusetts, USA). Each spectrum was recorded over 20 scans in the range from 4000 to
400 cm
−1
, with a resolution of 2 cm
−1
. A thermogravimetric analysis (TGA-DGT) was carried out to
evaluate the thermal stability of the BO capsules and their components. The thermal test was recorded
on a TA Instrument TGA Q 50 thermogravimetric analyser (New Castle, Delaware, USA). The samples
of ~5 mg were heated between ambient temperature and 600
◦
C at 10
◦
C/min. A constant nitrogen
flow rate of 10 mL/min was used, ensuring an inert atmosphere during the pyrolysis process.
2.7. Mechanical Properties of Bio-Oil Capsules
Compressive strength and micro-indentation hardness of the four different types of BO capsules
were measured at 20
◦
C. Compressive strength of the BO capsules was measured following the
recommendations of the ASTM D695-02 a [
24
]. For the tests, ten capsules of each type were randomly
selected and pre-conditioned for 2 h at room temperature. The compressive strength was measured
by uniaxial parallel plates compression testing on the 10 samples. To do that, individual capsules
were loaded until failure at a loading rate of 0.2 mm/min using an Universal Testing Machine (Test
Resources, Shakopee, Minnesota, USA) with a 5 kN load cell. Besides, a HV-1000A Microhardness
Tester (Russell Fraser Sales Pty Ltd., Kirrawee, New South Wales, Australia) equipped with a Vickers
indenter probe was used to perform the low-load micro-indentation tests on the BO capsules. To
develop the tests, firstly, five BO capsules of each type were randomly selected and embedded in an
epoxy resin mould (3 cm diameter and 2 cm height) and then polished until reaching the indentation
surface (Figure 3a) to ensure a uniform surface to enable the quality of the micro-indentation process.
Materials 2020, 13, x FOR PEER REVIEW 5 of 15
Delaware, USA). The samples of ~5 mg were heated between ambient temperature and 600 °C at 10
°C/min. A constant nitrogen flow rate of 10 mL/min was used, ensuring an inert atmosphere during
the pyrolysis process.
2.7. Mechanical Properties of Bio-Oil Capsules
Compressive strength and micro-indentation hardness of the four different types of BO capsules
were measured at 20 °C. Compressive strength of the BO capsules was measured following the
recommendations of the ASTM D695-02 a [24]. For the tests, ten capsules of each type were randomly
selected and pre-conditioned for 2 h at room temperature. The compressive strength was measured
by uniaxial parallel plates compression testing on the 10 samples. To do that, individual capsules
were loaded until failure at a loading rate of 0.2 mm/min using an Universal Testing Machine (Test
Resources, Shakopee, Minnesota, USA) with a 5 kN load cell. Besides, a HV-1000A Microhardness
Tester (Russell Fraser Sales Pty Ltd, Kirrawee, New South Wales, Australia) equipped with a Vickers
indenter probe was used to perform the low-load micro-indentation tests on the BO capsules. To
develop the tests, firstly, five BO capsules of each type were randomly selected and embedded in an
epoxy resin mould (3 cm diameter and 2 cm height) and then polished until reaching the indentation
surface (Figure 3a) to ensure a uniform surface to enable the quality of the micro-indentation process.
Figure 3. (a) Bio-oil (BO) capsules embedded in epoxy resin; (b) Vickers micro-indentation hardness
testing. The Vickers micro-indenter is a square-based pyramidal-shape indenter made of diamond
with an angle of 136 º between opposite faces. A total of 40 tests were performed on BO capsules.
To carry out the micro-indentation tests, the indentation area was located on the centre of the
BO capsules’ surface using the optical camera of the testing equipment. Micro-indentation tests
consisted of applying a force of 50 gf for 10 s on the surface of each capsule, as shown in Figure 3b.
After removing the force, the Vickers micro-indentation hardness (HV) of each BO capsule was
calculated as follows:
𝐻𝑉 = ∙°
∙
≈.∙
, (1)
where F is the load in kgf, and d is the average diagonal length of the indentation print in mm.
2.8. Self-Healing Efficiency of Bio-Oil as Rejuvenating Agent: a Proof of Concept
The self-healing efficiency of the BO as a rejuvenating agent of asphalt was quantified in cracked
bitumen samples by fluorescence microscopy. For the tests, thin film bitumen samples with
dimensions 20 × 20 × 0.5 mm3 were prepared on a glass petri dish using masking tape and a metallic
spatula. Then, hot bitumen test samples were conditioned for 2 h until they reached room
temperature. After cooling of the samples, a 100 µm-width microcrack was made in the centre of the
sample using a metallic cutting element. Then, using a hollow needle, a drop of BO ~2 mg was
dropped on the cracked bitumen test sample to simulate the rejuvenator release into the open
microcrack. The crack closure by the effect of BO diffusion over time was recorded periodically taking
images with an inverted fluorescence microscope (ICOE IV 5100 FL, Ningbo, China) with phase
d
1
d
2
136°
Indentation print
Vickers indenter
Epoxy resin
Embedded
BO capsules
Micro-indentation
surface
Polished surface
1 cm
1 mm
(a)
BO capsule
(b)
Indentation
depth
Indentation
diagonals
Load
Figure 3.
(
a
) Bio-oil (BO) capsules embedded in epoxy resin; (
b
) Vickers micro-indentation hardness
testing. The Vickers micro-indenter is a square-based pyramidal-shape indenter made of diamond
with an angle of 136◦between opposite faces. A total of 40 tests were performed on BO capsules.
To carry out the micro-indentation tests, the indentation area was located on the centre of the BO
capsules’ surface using the optical camera of the testing equipment. Micro-indentation tests consisted
of applying a force of 50 gf for 10 s on the surface of each capsule, as shown in Figure 3b. After
removing the force, the Vickers micro-indentation hardness (HV) of each BO capsule was calculated
as follows:
HV =2·sin136◦
2·F
d2≈1.8544·F
d2, (1)
where Fis the load in kgf, and dis the average diagonal length of the indentation print in mm.
Materials 2020,13, 1446 6 of 16
2.8. Self-Healing Efficiency of Bio-Oil as Rejuvenating Agent: a Proof of Concept
The self-healing efficiency of the BO as a rejuvenating agent of asphalt was quantified in cracked
bitumen samples by fluorescence microscopy. For the tests, thin film bitumen samples with dimensions
20
×
20
×
0.5 mm
3
were prepared on a glass petri dish using masking tape and a metallic spatula. Then,
hot bitumen test samples were conditioned for 2 h until they reached room temperature. After cooling
of the samples, a 100
µ
m-width microcrack was made in the centre of the sample using a metallic
cutting element. Then, using a hollow needle, a drop of BO ~2 mg was dropped on the cracked bitumen
test sample to simulate the rejuvenator release into the open microcrack. The crack closure by the
effect of BO diffusion over time was recorded periodically taking images with an inverted fluorescence
microscope (ICOE IV 5100 FL, Ningbo, China) with phase contrast and a magnification up to 400
×
.
Crack-width in the images was measured at six positions by image processing software ImageJ
®
(Version 1.52u, NIH Wisconsin, Bethesda, Maryland, USA). The complete crack closing process was
recorded over a maximum time of 120 min. Finally, the self-healing efficiency of the BO rejuvenating
over time was quantified as follows:
SHEti(%)= ICW0−PCWti
ICW0!×100, (2)
where SHE
ti
is the healing efficiency at a time t
i
measured in %, PCW
ti
is the average partial crack-width
at a time timeasured in µm, and ICW0is the average initial crack-width measured in µm.
3. Results and Discussion
3.1. Morphology and Composition of the Bio-Oil Capsules
Figure 4presents the main results of the morphological and composition experimental
characterization developed on the BO capsules. Results showed that BO capsules with different regular
and irregular shaped size were obtained as result of the polymer concentration and encapsulation
method used (Figure 4a). BO capsules with regular spherical size (Figure 4b) were produced majorly
by method 2. SEM images of surface and cross-section of an individual BO capsule are shown in
Figure 4c,d, respectively. From SEM images, it can be observed that the BO capsules showed a dense
membrane in their surface, without polynuclear formation and internal multicavity “egg-box”. This
result is not common considering the external ionic gelation method used [
10
], which suggests that BO
chemically affects the encapsulating process increasing the cross-linking process of the alginate matrix
and, as a result, endowing the capsules with a denser and more rigid membrane and polymer matrix.
Regardless the limited microporous structure shown by BO capsules, they presented an encapsulation
efficiency of BO of 78 ±5%, for both concentrations and methods.
The black color of the BO capsules was given by the color of the BO product (Figure 4e), which
also painted black the hardening bath of calcium chloride (Figure 4f) while the capsules were produced.
In addition, SEM-EDS characterization proved that BO capsules reveal a rough texture on their surface
(Figure 4g) with an element composition of Ca (56.43%) and Na (43.57%), as a result of the materials
used for the synthesis of calcium alginate matrix structure in the presence of Ca2+ions.
Furthermore, the distribution of the different BO capsule sizes is shown by the frequency
histograms in Figure 5a,b, for capsules produced by method 1 and 2, respectively. This histogram
proves that the BO capsule size can be fitted to the log-normal probability distribution (P-values
0.863 (M1–2%); 0.247 (M1–3%); 0.922 (M2–2%); and 0.676 (M2–3%) given by K-S test) used to model
stochastic processes. Statistical size analysis of 100 individual BO capsules by each type registered an
average size of 2.07 mm (SD =0.32 mm), 2.73 mm (SD =0.43 mm), 1.37 mm (SD =0.14 mm) and 1.51
mm (SD =0.10 mm) for capsules M1–2%, M1–3%, M2–2%, and M2–3%, respectively. These results
proved that method 1 produced larger capsules than method 2 (51% and 82%, respectively) and that the
increment of 1% of alginate (2% to 3%) increased the capsule size by 32% and 10% for methods 1 and 2,
Materials 2020,13, 1446 7 of 16
respectively. This result was due to the fact that the BO capsules produced by method 1 (Figure 5c,d)
were more irregular compared with the capsules produced by the method 2 (Figure 5e,f).
Materials 2020, 13, x FOR PEER REVIEW 6 of 15
contrast and a magnification up to 400×. Crack-width in the images was measured at six positions by
image processing software ImageJ
®
(Version 1.52u, NIH Wisconsin, Bethesda, Maryland, USA). The
complete crack closing process was recorded over a maximum time of 120 min. Finally, the self-
healing efficiency of the BO rejuvenating over time was quantified as follows:
𝑆𝐻𝐸%=
100
,
(2)
where SHE
ti
is the healing efficiency at a time t
i
measured in %, PCW
ti
is the average partial crack-
width at a time t
i
measured in µm, and ICW
0
is the average initial crack-width measured in µm.
3. Results and Discussion
3.1. Morphology and Composition of the Bio-Oil Capsules
Figure 4 presents the main results of the morphological and composition experimental
characterization developed on the BO capsules. Results showed that BO capsules with different
regular and irregular shaped size were obtained as result of the polymer concentration and
encapsulation method used (Figure 4a). BO capsules with regular spherical size (Figure 4b) were
produced majorly by method 2. SEM images of surface and cross-section of an individual BO capsule
are shown in Figure 4c,d, respectively. From SEM images, it can be observed that the BO capsules
showed a dense membrane in their surface, without polynuclear formation and internal multicavity
“egg-box”. This result is not common considering the external ionic gelation method used [10], which
suggests that BO chemically affects the encapsulating process increasing the cross-linking process of
the alginate matrix and, as a result, endowing the capsules with a denser and more rigid membrane
and polymer matrix. Regardless the limited microporous structure shown by BO capsules, they
presented an encapsulation efficiency of BO of 78 ± 5%, for both concentrations and methods.
Figure 4. (a) Image of the different BO capsules produced as result of the alginate concentration and
encapsulation method used; (b) individual spherical BO capsule; (c) SEM image of BO capsule; (d)
SEM image of the cross-section of BO capsule; (e) liquid Bio-Oil (BO) obtained from liquefied
agricultural biomass waste; (f) BO capsules in hardening bath of calcium chloride; (g) SEM detail
image of surface structure on BO capsule; (h) SEM-EDS on the BO capsule surface.
The black color of the BO capsules was given by the color of the BO product (Figure 4e), which
also painted black the hardening bath of calcium chloride (Figure 4f) while the capsules were
produced. In addition, SEM-EDS characterization proved that BO capsules reveal a rough texture on
their surface (Figure 4g) with an element composition of Ca (56.43%) and Na (43.57%), as a result of
the materials used for the synthesis of calcium alginate matrix structure in the presence of Ca
2+
ions.
Figure 4.
(
a
) Image of the different BO capsules produced as result of the alginate concentration and
encapsulation method used; (
b
) individual spherical BO capsule; (
c
) SEM image of BO capsule; (
d
) SEM
image of the cross-section of BO capsule; (
e
) liquid Bio-Oil (BO) obtained from liquefied agricultural
biomass waste; (
f
) BO capsules in hardening bath of calcium chloride; (
g
) SEM detail image of surface
structure on BO capsule; (h) SEM-EDS on the BO capsule surface.
Materials 2020, 13, x FOR PEER REVIEW 7 of 15
Furthermore, the distribution of the different BO capsule sizes is shown by the frequency
histograms in Figure 5a,b, for capsules produced by method 1 and 2, respectively. This histogram
proves that the BO capsule size can be fitted to the log-normal probability distribution (P-values 0.863
(M1–2%); 0.247 (M1–3%); 0.922 (M2–2%); and 0.676 (M2–3%) given by K-S test) used to model
stochastic processes. Statistical size analysis of 100 individual BO capsules by each type registered an
average size of 2.07 mm (SD = 0.32 mm), 2.73 mm (SD = 0.43 mm), 1.37 mm (SD = 0.14 mm) and 1.51
mm (SD = 0.10 mm) for capsules M1–2%, M1–3%, M2–2%, and M2–3%, respectively. These results
proved that method 1 produced larger capsules than method 2 (51% and 82%, respectively) and that
the increment of 1% of alginate (2% to 3%) increased the capsule size by 32% and 10% for methods 1
and 2, respectively. This result was due to the fact that the BO capsules produced by method 1 (Figure
5c,d) were more irregular compared with the capsules produced by the method 2 (Figure 5e,f).
Figure 5. (a) Frequency histogram of the size of BO capsules with log-normal fitting produced by the
dropping funnel method (M1); (b) frequency histogram of the size of BO capsules with log-normal
fitting produced by the pressure pump method (M2); (c–f) examples of the produced BO capsules.
Figure 6. (a) Measurement of the relative density of the BO capsules with test method B of the ASTM
D792-13 [22] used in the case of materials with densities that are very close to, or lighter than, water.
In the image, the mass in air of a 100 mg sample BO of capsules was determined, and then it was
immersed in ethanol with density 0.789 g/cm
3
, determining its apparent mass upon immersion and
calculating its relative density; (b) average results of density of BO capsules with St. Dev. error bars.
Figure 5.
(
a
) Frequency histogram of the size of BO capsules with log-normal fitting produced by the
dropping funnel method (M1); (
b
) frequency histogram of the size of BO capsules with log-normal
fitting produced by the pressure pump method (M2); (c–f) examples of the produced BO capsules.
Furthermore, the BO capsules’ size and their morphological micro-structure also affected their
density. To prove it, Figure 6shows the average results of the relative density measured on the different
BO capsules. From these results, it can be concluded that BO capsules produced by method 2 (with
smaller size and volume) presented a higher average relative density compared with those of method
1 and that the alginate concentration increase does not significantly affect their density value. The BO
capsule type M2–3% (average size 1.51 mm) presents the highest value of density (1.505 g/cm
3
; SD
=0.023 g/cm
3
), while the capsule type M1–3% (average size 2.73 mm) registered the lowest density
(1.467 g/cm
3
; SD =0.001 g/cm
3
), as shown in Figure 6. These results suggest that, although the size
Materials 2020,13, 1446 8 of 16
(volume) of the BO capsules affected the density, the average density values for the capsules were very
similar. Overall, the size and composition results of the BO capsules confirm that they can be added
into asphalt matrix as an encapsulated rejuvenator with asphalt self-healing purposes.
Materials 2020, 13, x FOR PEER REVIEW 7 of 15
Furthermore, the distribution of the different BO capsule sizes is shown by the frequency
histograms in Figure 5a,b, for capsules produced by method 1 and 2, respectively. This histogram
proves that the BO capsule size can be fitted to the log-normal probability distribution (P-values 0.863
(M1–2%); 0.247 (M1–3%); 0.922 (M2–2%); and 0.676 (M2–3%) given by K-S test) used to model
stochastic processes. Statistical size analysis of 100 individual BO capsules by each type registered an
average size of 2.07 mm (SD = 0.32 mm), 2.73 mm (SD = 0.43 mm), 1.37 mm (SD = 0.14 mm) and 1.51
mm (SD = 0.10 mm) for capsules M1–2%, M1–3%, M2–2%, and M2–3%, respectively. These results
proved that method 1 produced larger capsules than method 2 (51% and 82%, respectively) and that
the increment of 1% of alginate (2% to 3%) increased the capsule size by 32% and 10% for methods 1
and 2, respectively. This result was due to the fact that the BO capsules produced by method 1 (Figure
5c,d) were more irregular compared with the capsules produced by the method 2 (Figure 5e,f).
Figure 5. (a) Frequency histogram of the size of BO capsules with log-normal fitting produced by the
dropping funnel method (M1); (b) frequency histogram of the size of BO capsules with log-normal
fitting produced by the pressure pump method (M2); (c–f) examples of the produced BO capsules.
Figure 6. (a) Measurement of the relative density of the BO capsules with test method B of the ASTM
D792-13 [22] used in the case of materials with densities that are very close to, or lighter than, water.
In the image, the mass in air of a 100 mg sample BO of capsules was determined, and then it was
immersed in ethanol with density 0.789 g/cm
3
, determining its apparent mass upon immersion and
calculating its relative density; (b) average results of density of BO capsules with St. Dev. error bars.
Figure 6. (a) Measurement of the relative density of the BO capsules with test method B of the ASTM
D792-13 [
22
] used in the case of materials with densities that are very close to, or lighter than, water.
In the image, the mass in air of a 100 mg sample BO of capsules was determined, and then it was
immersed in ethanol with density 0.789 g/cm
3
, determining its apparent mass upon immersion and
calculating its relative density; (b) average results of density of BO capsules with St. Dev. error bars.
3.2. Compressive Strength and Micro-Indentation Hardness of the Bio-Oil Capsules
Figure 7summarizes the results of the compressive tests developed at 20
◦
C on the different BO
capsules. Average curves of the compressive force versus displacement obtained of the capsules are
shown in Figure 7a. It can be observed that the BO capsules presented an elastic-plastic mechanical
behavior with moderate ductility and breakage in plastic deformation. As result, BO capsules M1–2%
and M1–3% recorded the largest average compressive forces and displacements, with values of 93.32
N and 193.93 N at displacements of 0.51 mm and 0.76 mm, respectively. This result proves that the BO
capsules increase their compressive force and displacement with the increase of the sodium alginate
concentration (2 to 3%) and the encapsulation method used (M1 to M2, Figure 2).
Previous research [
8
] proved that the minimum compressive force of capsules required to resist
the asphalt manufacturing processes was 10 N, approximately. Based on this study, the obtained
compression results (Figure 7) suggest that all BO capsules tested at room temperature could: (1)
survive the asphalt manufacturing process, and (2) resist high compressive strength until failure and, as
a result, partially release the encapsulated BO by the effect of an external trigger (such as traffic loads).
Norambuena-Contreras et al. [
13
] proved through compressive tests that the exposure of calcium
alginate capsules to high temperatures (160
◦
C) degrades the polymeric matrix structure of the capsules
making them more brittle, and hence less flexible to mechanical loads. However, the maximum
force resisted by the BO capsules, as shown in Figure 7b, shows that most of the BO capsules (3 of 4
types) registered average compressive forces greater than those obtained by Norambuena-Contreras
et al. [
13
] (50 N at 20
◦
C) for capsules containing sunflower oil as encapsulating rejuvenator. Thus,
considering that the BO capsules have a greater mechanical performance than the calcium alginate
capsules produced in [
13
], it can be concluded that the BO-capsules also can successfully resist the
asphalt manufacturing process. However, this hypothesis should be checked as a second part of this
study. Additionally, results shown in Figure 7b proved that the addition of a higher amount of sodium
alginate (2 to 3%) to the composition of BO capsules helped to increase their compressive strength
because the BO capsules present a denser internal structure, as shown in the SEM image of capsule
cross-section in Figure 4d.
Materials 2020,13, 1446 9 of 16
Materials 2020, 13, x FOR PEER REVIEW 8 of 15
Furthermore, the BO capsules’ size and their morphological micro-structure also affected their
density. To prove it, Figure 6 shows the average results of the relative density measured on the
different BO capsules. From these results, it can be concluded that BO capsules produced by method
2 (with smaller size and volume) presented a higher average relative density compared with those of
method 1 and that the alginate concentration increase does not significantly affect their density value.
The BO capsule type M2–3% (average size 1.51 mm) presents the highest value of density (1.505
g/cm
3
; SD = 0.023 g/cm
3
), while the capsule type M1–3% (average size 2.73 mm) registered the lowest
density (1.467 g/cm
3
; SD = 0.001 g/cm
3
), as shown in Figure 6. These results suggest that, although the
size (volume) of the BO capsules affected the density, the average density values for the capsules
were very similar. Overall, the size and composition results of the BO capsules confirm that they can
be added into asphalt matrix as an encapsulated rejuvenator with asphalt self-healing purposes.
3.2. Compressive Strength and Micro-Indentation Hardness of the Bio-Oil Capsules
Figure 7 summarizes the results of the compressive tests developed at 20 °C on the different BO
capsules. Average curves of the compressive force versus displacement obtained of the capsules are
shown in Figure 7a. It can be observed that the BO capsules presented an elastic-plastic mechanical
behavior with moderate ductility and breakage in plastic deformation. As result, BO capsules M1–
2% and M1–3% recorded the largest average compressive forces and displacements, with values of
93.32 N and 193.93 N at displacements of 0.51 mm and 0.76 mm, respectively. This result proves that
the BO capsules increase their compressive force and displacement with the increase of the sodium
alginate concentration (2 to 3%) and the encapsulation method used (M1 to M2, Figure 2).
Figure 7. (a) Average curves of the compressive force versus displacement of the BO capsules tested
following the recommendations of the ASTM D695-02a [24], including image details of the mechanical
test, broken capsule, and crack type; (b) average results of maximum compressive force of capsules
with St. Dev. error bars; (c) linear correlation between compressive force and capsule size.
Previous research [8] proved that the minimum compressive force of capsules required to resist
the asphalt manufacturing processes was 10 N, approximately. Based on this study, the obtained
compression results (Figure 7) suggest that all BO capsules tested at room temperature could: 1)
survive the asphalt manufacturing process, and 2) resist high compressive strength until failure and,
as a result, partially release the encapsulated BO by the effect of an external trigger (such as traffic
loads). Norambuena-Contreras et al. [13] proved through compressive tests that the exposure of
calcium alginate capsules to high temperatures (160 °C) degrades the polymeric matrix structure of
Figure 7.
(
a
) Average curves of the compressive force versus displacement of the BO capsules tested
following the recommendations of the ASTM D695-02a [
24
], including image details of the mechanical
test, broken capsule, and crack type; (
b
) average results of maximum compressive force of capsules
with St. Dev. error bars; (c) linear correlation between compressive force and capsule size.
Furthermore, Figure 8shows the results of Vickers micro-indentation hardness and depth
registered for BO capsules produced by method 1 (Figure 8a) and method 2 (Figure 8b), respectively.
From these box plots, it can be observed that sodium alginate concentration addition influenced the
hardness and micro-indentation depth of capsules. Thus, BO capsules with 2% of alginate registered
lower average hardness, M1–2% (24.89 MPa; SD =4.67 MPa) and M2–2% (23.90 MPa; SD =7.38 MPa),
while capsules with 3% alginate registered the greatest average hardness, M1–3% (37.24 MPa; SD
=14.63 MPa) and M2–3% (32.35 MPa; SD =9.22 MPa). Additionally, greater hardness results were
obtained in a lower depth value. From Figure 8, it can be concluded that capsule M1–3% was the one
that presented a lower depth (31.14 µm; SD =11.48 µm) and, therefore, greater hardness.
This result coincides with the compressive results, where the BO capsule M1–3% recorded the
greatest average maximum compressive force, as shown in Figure 7b. This result was due to the dense
microstructure of the capsule, which allows it to have a high resistance to hardness and compression.
However, this dense structure reduces its properties to release encapsulated BO. Figure 8c shows an
example of the BO capsules embedded in epoxy resin and Figure 8d shows a cross-section of the
capsule M1–3% after micro-indentation test. In this Figure, an individual micro-indentation print can
be observed, which has been magnified in Figure 8e. The SEM characterization of the BO capsule test
specimens, 24 h after carrying out the micro-indentation hardness test, suggested that the capsules
revealed partial recovery of the deformation. For example, the average of indentation diagonals
registered by the capsules M1–3% after test was close to 200
µ
m, while after 24 h, this value was
reduced to 50
µ
m. This strain recovery phenomenon shown by the BO capsules on the indentation
tests was due to their elastic-plastic mechanical behavior previously proved by the compressive test, as
shown in Figure 7a. In summary, the micro-indentation hardness results obtained (Figure 8) suggest
that all BO capsules tested at room temperature could resist the indentation loads produced by the
aggregates during the asphalt manufacturing process; hence, they could survive to partially release the
encapsulated BO in the asphalt matrix by the effect of an external force trigger.
Materials 2020,13, 1446 10 of 16
Materials 2020, 13, x FOR PEER REVIEW 9 of 15
the capsules making them more brittle, and hence less flexible to mechanical loads. However, the
maximum force resisted by the BO capsules, as shown in Figure 7b, shows that most of the BO
capsules (3 of 4 types) registered average compressive forces greater than those obtained by
Norambuena-Contreras et al. [13] (50 N at 20 °C) for capsules containing sunflower oil as
encapsulating rejuvenator. Thus, considering that the BO capsules have a greater mechanical
performance than the calcium alginate capsules produced in [13], it can be concluded that the BO-
capsules also can successfully resist the asphalt manufacturing process. However, this hypothesis
should be checked as a second part of this study. Additionally, results shown in Figure 7b proved
that the addition of a higher amount of sodium alginate (2 to 3%) to the composition of BO capsules
helped to increase their compressive strength because the BO capsules present a denser internal
structure, as shown in the SEM image of capsule cross-section in Figure 4d.
Furthermore, Figure 8 shows the results of Vickers micro-indentation hardness and depth
registered for BO capsules produced by method 1 (Figure 8a) and method 2 (Figure 8b), respectively.
From these box plots, it can be observed that sodium alginate concentration addition influenced the
hardness and micro-indentation depth of capsules. Thus, BO capsules with 2% of alginate registered
lower average hardness, M1–2% (24.89 MPa; SD = 4.67 MPa) and M2–2% (23.90 MPa; SD = 7.38 MPa),
while capsules with 3% alginate registered the greatest average hardness, M1–3% (37.24 MPa; SD =
14.63 MPa) and M2–3% (32.35 MPa; SD = 9.22 MPa). Additionally, greater hardness results were
obtained in a lower depth value. From Figure 8, it can be concluded that capsule M1–3% was the one
that presented a lower depth (31.14 µm; SD = 11.48 µm) and, therefore, greater hardness.
Figure 8. (a) Box plots of the results of Vickers micro-indentation hardness and depth for BO capsules
produced by method 1; (b) box plots of the results of micro-indentation hardness and depth for BO
capsules produced by method 2; (c) BO capsules embedded in a mould of epoxy resin; (d) SEM image
of the BO capsule M1–3% taken 24 h after test; (e) SEM image detail of the micro-indentation print
(i.e., indentation diagonals) in BO capsule M1–3% taken 24 h after test.
This result coincides with the compressive results, where the BO capsule M1–3% recorded the
greatest average maximum compressive force, as shown in Figure 7b. This result was due to the dense
microstructure of the capsule, which allows it to have a high resistance to hardness and compression.
Figure 8.
(
a
) Box plots of the results of Vickers micro-indentation hardness and depth for BO capsules
produced by method 1; (
b
) box plots of the results of micro-indentation hardness and depth for BO
capsules produced by method 2; (
c
) BO capsules embedded in a mould of epoxy resin; (
d
) SEM image
of the BO capsule M1–3% taken 24 h after test; (
e
) SEM image detail of the micro-indentation print (i.e.,
indentation diagonals) in BO capsule M1–3% taken 24 h after test.
3.3. Chemical and Thermal Properties of the BO Capsules and Their Components
The BO sample was characterized by FTIR-ATR with the aim of identifying specific functional
groups which could lead to an antioxidant activity of the BO and, as result, to a better performance
as rejuvenating agent of aged asphalt binder. Figure 9shows the normalized infrared spectra of BO
obtained from the liquefaction process of corn stover residues. The assignment of spectral bands in the
FTIR spectra was carried out based on that previously reported by Lievens et al. [
25
] and Stuart [
26
],
regarding BO and oxygen-containing compounds. According to the results shown in Figure 9, the
wavenumbers of substantial functional groups in BO are concentrated between 920 and 1720 cm
−1
,
indicating the presence of species containing C–O–H, C–O and C=O bonds. These chemical species,
usually acids, aldehydes, ketones and phenolics, derive from the deconstruction of cellulose and
lignin during the liquefaction process, as has been widely reported [
19
]. Indeed, the absorption bands
recorded between 1000 and 1220 cm
−1
are associated with the C–O stretching in alcohols, glycols and
phenolics. In particular, the lignin-derived phenolics have been identified as species with the potential
to avoid the action of free radicals, thus with antioxidant properties [27].
Moreover, the absorption band at 3295 cm
−1
could be associated with the O–H stretching in water
and carboxylic acids. The latter is also confirmed by both C–O–H in-plane and C–O–H out-of-plane
bending signals at 1420 cm
−1
and 930 cm
−1
, respectively. The carboxylic acids have been identified as
antioxidant compounds and one of the most important functionalities within the BO used [
19
]. Those
signals found between 2700 and 2900 cm
−1
are attributed to the C–H stretching in aliphatic groups (e.g.,
in aldehydes and ketones). The ketones are also responsible for increasing antioxidant activity and, in
some applications, their antioxidant effect can conjugate with that of phenolics. In fact, the antioxidant
activity of the BO measured by the 2,2
0
-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid-ABTS)
technique was 126.30
±
2.7
µ
mol Trolox/mL, which is considerably greater than that reported for the
Materials 2020,13, 1446 11 of 16
antioxidant activity of sunflower oils (23.2
±
1.1
µ
mol Trolox/mL) reported by Guadarrama-Lezama
et al. [
23
]. Sunflower oil was successfully encapsulated in calcium alginate (CaAlg) capsules as a
rejuvenating material with asphalt self-healing purposes.
Materials 2020, 13, x FOR PEER REVIEW 10 of 15
However, this dense structure reduces its properties to release encapsulated BO. Figure 8c shows an
example of the BO capsules embedded in epoxy resin and Figure 8d shows a cross-section of the
capsule M1–3% after micro-indentation test. In this Figure, an individual micro-indentation print can
be observed, which has been magnified in Figure 8e. The SEM characterization of the BO capsule test
specimens, 24 h after carrying out the micro-indentation hardness test, suggested that the capsules
revealed partial recovery of the deformation. For example, the average of indentation diagonals
registered by the capsules M1–3% after test was close to 200 µm, while after 24 h, this value was
reduced to 50 µm. This strain recovery phenomenon shown by the BO capsules on the indentation
tests was due to their elastic-plastic mechanical behavior previously proved by the compressive test,
as shown in Figure 7a. In summary, the micro-indentation hardness results obtained (Figure 8)
suggest that all BO capsules tested at room temperature could resist the indentation loads produced
by the aggregates during the asphalt manufacturing process; hence, they could survive to partially
release the encapsulated BO in the asphalt matrix by the effect of an external force trigger.
3.3. Chemical and Thermal Properties of the BO Capsules and Their Components
The BO sample was characterized by FTIR-ATR with the aim of identifying specific functional
groups which could lead to an antioxidant activity of the BO and, as result, to a better performance
as rejuvenating agent of aged asphalt binder. Figure 9 shows the normalized infrared spectra of BO
obtained from the liquefaction process of corn stover residues. The assignment of spectral bands in
the FTIR spectra was carried out based on that previously reported by Lievens et al. [25] and Stuart
[26], regarding BO and oxygen-containing compounds. According to the results shown in Figure 9,
the wavenumbers of substantial functional groups in BO are concentrated between 920 and 1720 cm
−1
,
indicating the presence of species containing C–O–H, C–O and C=O bonds. These chemical species,
usually acids, aldehydes, ketones and phenolics, derive from the deconstruction of cellulose and
lignin during the liquefaction process, as has been widely reported [19]. Indeed, the absorption bands
recorded between 1000 and 1220 cm
−1
are associated with the C–O stretching in alcohols, glycols and
phenolics. In particular, the lignin-derived phenolics have been identified as species with the
potential to avoid the action of free radicals, thus with antioxidant properties [27].
Figure 9. Normalized infrared spectra of BO obtained from liquefaction of corn stover residues. The
assignment of spectral bands was carried out based on that previously reported by Lievens et al. [25]
and Stuart [26] on BO and oxygen-containing compounds.
Moreover, the absorption band at 3295 cm
−1
could be associated with the O–H stretching in water
and carboxylic acids. The latter is also confirmed by both C–O–H in-plane and C–O–H out-of-plane
5001000150020002500300035004000
Normalized absorbance (a.u.)
Wavelength (cm
-1
)
C-H (2700 - 2900 cm
-1
)
C=O (1715 cm
-1
)
C-O-H (1420 cm
-1
)
C-O (1000 -1220 cm
-1
)
-OH (3295 cm
-1
)
C-O-H (920 - 930 cm
-1
)
Figure 9.
Normalized infrared spectra of BO obtained from liquefaction of corn stover residues. The
assignment of spectral bands was carried out based on that previously reported by Lievens et al. [
25
]
and Stuart [26] on BO and oxygen-containing compounds.
In this context, Norambuena-Contreras et al. [
13
] proved that the sunflower oil released from the
CaAlg capsules significantly increases the self-healing capability of the dense asphalt mixture [
14
]
and Stone Mastic Asphalt [
15
]. However, when the CaAlg capsules are exposed to high temperatures
for long pre-heating periods, they can suffer peroxidation of the encapsulated sunflower oil [
13
].
Considering the FTIR-ATR evidence, it can be concluded that the BO obtained by liquefaction of corn
stover residues has strong antioxidant properties provided by carboxylic acids, phenolics and, to less
extent, by ketone functionalities. So, this liquid has the potential to be used for asphalt self-healing
applications. However, for future encapsulation applications by means of biopolymers, interactions
between the BO and capsule materials should be carefully analyzed. In this study it was proven that
the interaction of carboxylic acids, CaCl
2
and alginate could lead to a cross-linked structure of the BO
capsule and even to the erosion of its surface, as shown in the SEM image in Figure 4g, which may
decrease the efficiency in releasing the BO during the asphalt healing process [28].
Furthermore, thermogravimetry (TGA) and derivative thermogravimetry (DTG) profiles of the
biopolymer (BioPoly) containing 2% and 3% of alginate are shown in Figure 10 along with the capsules
(BO Cap) prepared from those biopolymers and containing the BO. The thermal stability of the
biopolymer (BioPoly) used for the preparation of the capsules was confirmed by the TGA curves in
Figure 10. From these figures, it can be observed that the decomposition at 160
◦
C—temperature
of asphalt mixture preparation—was below 10% for both BioPoly_2% (Figure 10a) and BioPoly_3%
(Figure 10b), respectively. Two major DTG peaks, corresponding to 10–12% and 36–40% wt. loss, were
identified for the BioPoly. The first peak around 185
◦
C is associated with dehydration reactions in the
alginate; while the second one, at 278 ◦C, corresponds to the degradation of the CaCO3.
Materials 2020,13, 1446 12 of 16
Materials 2020, 13, x FOR PEER REVIEW 11 of 15
bending signals at 1420 cm−1 and 930 cm−1, respectively. The carboxylic acids have been identified as
antioxidant compounds and one of the most important functionalities within the BO used [19]. Those
signals found between 2700 and 2900 cm−1 are attributed to the C–H stretching in aliphatic groups
(e.g., in aldehydes and ketones). The ketones are also responsible for increasing antioxidant activity
and, in some applications, their antioxidant effect can conjugate with that of phenolics. In fact, the
antioxidant activity of the BO measured by the 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulphonic
acid-ABTS) technique was 126.30 ± 2.7 µmol Trolox/mL, which is considerably greater than that
reported for the antioxidant activity of sunflower oils (23.2 ± 1.1 µmol Trolox/mL) reported by
Guadarrama-Lezama et al. [23]. Sunflower oil was successfully encapsulated in calcium alginate
(CaAlg) capsules as a rejuvenating material with asphalt self-healing purposes.
In this context, Norambuena-Contreras et al. [13] proved that the sunflower oil released from
the CaAlg capsules significantly increases the self-healing capability of the dense asphalt mixture [14]
and Stone Mastic Asphalt [15]. However, when the CaAlg capsules are exposed to high temperatures
for long pre-heating periods, they can suffer peroxidation of the encapsulated sunflower oil [13].
Considering the FTIR-ATR evidence, it can be concluded that the BO obtained by liquefaction of corn
stover residues has strong antioxidant properties provided by carboxylic acids, phenolics and, to less
extent, by ketone functionalities. So, this liquid has the potential to be used for asphalt self-healing
applications. However, for future encapsulation applications by means of biopolymers, interactions
between the BO and capsule materials should be carefully analyzed. In this study it was proven that
the interaction of carboxylic acids, CaCl2 and alginate could lead to a cross-linked structure of the BO
capsule and even to the erosion of its surface, as shown in the SEM image in Figure 4g, which may
decrease the efficiency in releasing the BO during the asphalt healing process [28].
Furthermore, thermogravimetry (TGA) and derivative thermogravimetry (DTG) profiles of the
biopolymer (BioPoly) containing 2% and 3% of alginate are shown in Figure 10 along with the
capsules (BO Cap) prepared from those biopolymers and containing the BO. The thermal stability of
the biopolymer (BioPoly) used for the preparation of the capsules was confirmed by the TGA curves
in Figure 10. From these figures, it can be observed that the decomposition at 160 °C—temperature
of asphalt mixture preparation—was below 10% for both BioPoly_2% (Figure 10a) and BioPoly_3%
(Figure 10b), respectively. Two major DTG peaks, corresponding to 10–12% and 36–40% wt. loss,
were identified for the BioPoly. The first peak around 185 °C is associated with dehydration reactions
in the alginate; while the second one, at 278 °C, corresponds to the degradation of the CaCO3.
Figure 10. TGA and DTG results of the biopolymer matrix (BioPoly) and BO capsules (BO Cap) for
concentrations of (a) 2% of alginate; and (b) 3% of alginate. Tsh is the DTG shoulder temperature.
Likewise, the weight loss below 300 °C is caused by the loss of hydroxyl groups (–OH) in the
alginate, and above this temperature decarboxylation reactions take place forming CO2 as the main
product [29]. When BO is confined into the CaAlg capsules (BO Cap_2% and BO Cap_3%) the TGA
0 100 200 300 400 500 600
0
20
40
60
80
100
BO Cap_2%
BioPoly_2%
Temperature (°C)
TGA (.wt%)
(a)
-9
-6
-3
0
DTG (wt.%/min)
T
sh
Carbonization/
Crosslinking
Decomposition
0 100 200 300 400 500 600
0
20
40
60
80
100
BO Cap_3%
BioPoly_3%
Temperature (°C)
TGA (.wt%)
(b)
Decomposition Carbonization/
Crosslinking
-9
-6
-3
0
DTG (wt.%/min)
T
sh
Figure 10.
TGA and DTG results of the biopolymer matrix (BioPoly) and BO capsules (BO Cap) for
concentrations of (a) 2% of alginate; and (b) 3% of alginate. Tsh is the DTG shoulder temperature.
Likewise, the weight loss below 300
◦
C is caused by the loss of hydroxyl groups (–OH) in the
alginate, and above this temperature decarboxylation reactions take place forming CO
2
as the main
product [
29
]. When BO is confined into the CaAlg capsules (BO Cap_2% and BO Cap_3%) the TGA
profiles of the BioPoly and that of the capsules containing the BO are very similar until 175
◦
C. Above
175
◦
C, the decomposition of the BO starts showing a DTG shoulder at Tsh =190
◦
C, which could
be attributed to the breakage of C-H and C–O–H in lighter molecules, such as carboxylic acids, and,
to some extent, to the decomposition of the alginate itself. Then, a maximum peak is found around
250
◦
C corresponding to the overlapping of the BO capsule decomposition and to the pyrolysis of
aldehydes, phenolics and ketones contained in the BO (based on Figure 9). Those peaks found above
400 ◦C correspond to the carbonization of the alginate matrix via cross-linking reactions.
Overall, TGA results of BO capsules and their components are in line with those found in the
BO spectrum FTIR and suggest that the encapsulation process of BO as rejuvenating agent leads to a
thermally stable material with the potential to be used for asphalt self-healing applications.
3.4. Self-Healing Efficiency of the Bio-Oil as Encapsulated Rejuvenator
The self-healing capability of the BO as rejuvenating agent for asphalt was quantified in this
study by means of fluorescence microscopy. Figure 11 presents the results of healing efficiency of
BO as a proof of concept measured over time. From this Figure, it can be observed that BO obtained
from the liquefaction of corn stover residues has the capability to close the
µ
-crack in the bitumen by
effect of the diffusion of BO in bitumen. In Figure 11a, it can be seen that BO has a healing efficiency
close to 50% at a time of 120 min of evaluation. Likewise, the
µ
crack-width showed a linear trend
with the healing time. The healing process occurs because the bio-rejuvenator (i.e., BO) increases the
content of viscous components of asphalts and decreases the viscosity of bitumen, allowing it to flow
over time healing its open crack. In short, BO can dissolve the bitumen on both sides of the crack.
Likewise, fluorescence images have allowed the measurement of the crack-closing over time because
the diffusion of bituminous molecules caused by Brownian motion [30] was accelerated by the BO.
The self-healing capability results of BO in virgin bitumen are promising and future application in
aged asphalt samples should be evaluated. Zhang et al. [
17
] reported that the addition of bio-rejuvenator
from biomass waste decreases the activation energy of aged asphalt binder and increases the temperature
susceptibility and fluidity of aged asphalt binder, with the potential to rejuvenate it.
Previous studies by Aguirre et al. [
20
] proved that encapsulated commercial BO can be used for
asphalt self-healing. So, encapsulated BO obtained from liquefaction of corn stover residues can be
potentially used as a sustainable rejuvenator for aged asphalts in order to promote their
µ
crack-healing.
Nevertheless, future research using different amounts of BO capsules should be carried out to prove
Materials 2020,13, 1446 13 of 16
this effect in bituminous materials (i.e., including asphalt mixtures, asphalt mastics and bituminous
binders) along all different length scales.
Materials 2020, 13, x FOR PEER REVIEW 12 of 15
profiles of the BioPoly and that of the capsules containing the BO are very similar until 175 °C. Above
175 °C, the decomposition of the BO starts showing a DTG shoulder at Tsh = 190 °C, which could be
attributed to the breakage of C‒H and C–O–H in lighter molecules, such as carboxylic acids, and, to
some extent, to the decomposition of the alginate itself. Then, a maximum peak is found around 250
°C corresponding to the overlapping of the BO capsule decomposition and to the pyrolysis of
aldehydes, phenolics and ketones contained in the BO (based on Figure 9). Those peaks found above
400 °C correspond to the carbonization of the alginate matrix via cross-linking reactions.
Overall, TGA results of BO capsules and their components are in line with those found in the
BO spectrum FTIR and suggest that the encapsulation process of BO as rejuvenating agent leads to a
thermally stable material with the potential to be used for asphalt self-healing applications.
3.4. Self-Healing Efficiency of the Bio-Oil as Encapsulated Rejuvenator
The self-healing capability of the BO as rejuvenating agent for asphalt was quantified in this
study by means of fluorescence microscopy. Figure 11 presents the results of healing efficiency of BO
as a proof of concept measured over time. From this Figure, it can be observed that BO obtained from
the liquefaction of corn stover residues has the capability to close the µ-crack in the bitumen by effect
of the diffusion of BO in bitumen. In Figure 11a, it can be seen that BO has a healing efficiency close
to 50% at a time of 120 min of evaluation. Likewise, the µcrack-width showed a linear trend with the
healing time. The healing process occurs because the bio-rejuvenator (i.e., BO) increases the content
of viscous components of asphalts and decreases the viscosity of bitumen, allowing it to flow over
time healing its open crack. In short, BO can dissolve the bitumen on both sides of the crack. Likewise,
fluorescence images have allowed the measurement of the crack-closing over time because the
diffusion of bituminous molecules caused by Brownian motion [30] was accelerated by the BO.
Figure 11. (a) Results of healing efficiency and µ-crack with over time; (b–e) Procedure to measure
the healing efficiency of BO on film bitumen test samples of 20 × 20 × 0.5 mm; (f–i) Fluorescence
microscopy images on closed µ-crack by the effect of BO diffused in bitumen over time.
The self-healing capability results of BO in virgin bitumen are promising and future application
in aged asphalt samples should be evaluated. Zhang et al. [17] reported that the addition of bio-
rejuvenator from biomass waste decreases the activation energy of aged asphalt binder and increases
the temperature susceptibility and fluidity of aged asphalt binder, with the potential to rejuvenate it.
Previous studies by Aguirre et al. [20] proved that encapsulated commercial BO can be used for
asphalt self-healing. So, encapsulated BO obtained from liquefaction of corn stover residues can be
potentially used as a sustainable rejuvenator for aged asphalts in order to promote their µcrack-
healing. Nevertheless, future research using different amounts of BO capsules should be carried out
to prove this effect in bituminous materials (i.e., including asphalt mixtures, asphalt mastics and
bituminous binders) along all different length scales.
Figure 11.
(
a
) Results of healing efficiency and
µ
-crack with over time; (
b–e
) Procedure to measure
the healing efficiency of BO on film bitumen test samples of 20
×
20
×
0.5 mm; (
f–i
) Fluorescence
microscopy images on closed µ-crack by the effect of BO diffused in bitumen over time.
4. Conclusions and Future Research
This paper explored the use of BO obtained from liquefied agricultural biomass waste as a
potential bio-based encapsulated rejuvenating agent for asphalt self-healing. Based on the results, the
following conclusions have been obtained.
Bio-Oil (BO) capsules with different size (regular and irregular shaped size) and with a BO
encapsulation efficiency of 78% were obtained as result of two different polymer concentrations (2%
and 3%) and using two different encapsulating dripping methods: a dropping funnel (M1) and a
microfluidic pressure pump (M2). Encapsulating method 1 produced BO capsules with greater size
(average range size 2.07–2.73 mm) than encapsulating method 2 (average range size 1.37–1.51 mm). In
addition, it was proven that the increase of 1% of alginate (2% to 3%) increased the capsule size.
BO capsules produced by method 1 were more irregular compared with the capsules produced by
method 2. SEM characterization on capsule samples showed that the BO capsules presented a dense
membrane on their surface, without polynuclear formation and internal multicavity.
Likewise, it was proven that BO capsule size and their micro-structure affected their density. BO
capsules M2–3% (average size 1.51 mm) presented the highest value of density (1.505 g/cm
3
), while
capsules M1–3% (average size 2.73 mm) registered the lowest density (1.467 g/cm3).
BO capsules presented an elastic-plastic mechanical behavior with moderate ductility and breakage
in plastic deformation. Results of compressive characterization proved that the BO capsules increased
their compressive force and displacement with the increase of sodium alginate concentration (2 to 3%)
and the encapsulation method used (M1 to M2).
Compression results obtained suggest that all BO capsules tested at room temperature could
survive the asphalt manufacturing process and resist high compressive strength until failure and, as a
result, partially release the encapsulated BO by the effect of an external trigger (such as traffic loads).
Additionally, it was proven that sodium alginate concentration addition influenced the hardness
and depth of BO capsules. BO capsules with 2% of alginate registered the lowest average hardness,
while capsules with 3% alginate registered the greatest average hardness.
Moreover, greater hardness results were obtained at a lower depth value. It was concluded that
capsule M1–3% was the one that presented a lower depth and hence a greater hardness. This result
coincides with the compressive results. In short, micro-indentation hardness results obtained suggest
that all BO capsules tested at room temperature could resist the indentation loads produced by the
Materials 2020,13, 1446 14 of 16
aggregates during the asphalt manufacturing process, and therefore, survive for partial release of the
encapsulated BO in the asphalt matrix by effect of an external force trigger.
Based on FTIR-ATR results, it was concluded that the BO obtained by liquefaction of corn stover
residues has strong antioxidant properties (antioxidant activity 126.30
µ
mol Trolox/mL). So, this liquid
has the potential to be used for asphalt self-healing applications.
Overall, TGA results of BO capsules and their components were in accordance with the BO
spectrum FTIR and allow the suggestion that the encapsulation process of BO as a rejuvenating agent
leads to a thermally stable material with the potential to be used for asphalt self-healing applications.
It was proven that BO has the capability to close the
µ
-crack in the bitumen by the effect of
the diffusion of BO in bitumen, achieving a healing efficiency close to 50% at a time of 120 min of
evaluation. Self-healing capability results of BO in virgin bitumen are promising for evaluation of its
future application in aged asphalt sample and, hence, encapsulated BO can be potentially used as a
sustainable rejuvenator for aged asphalts with the aim of promoting their µcrack-healing.
Future research to evaluate in more detail the chemical interactions between the BO and capsule
materials should be developed because these interactions could lead to a cross-linked structure of the
BO capsule and even to the erosion of its surface, which may decrease the efficiency in releasing the BO
rejuvenating agent during the asphalt healing process. To avoid this phenomenon, this study proposes
a future research, albeit with the use of the coextrusion dripping technique to encapsulate the BO agent.
Additionally, based on the limitations of this study, as future research the authors suggest carrying out
several tests to 1) evaluate the rejuvenating effect of the BO on short-term and long-term aged binders
by FTIR-ATR tests; 2) evaluate the spatial distribution of the BO capsules and their integrity inside the
asphalt mixtures by X-ray computed tomography; and 3) quantify by tests the effect of the mixing
order and the aging time on the mechanical stability and self-healing properties of asphalt mixtures
with, and without, BO capsules.
Author Contributions:
Conceptualization, J.N.-C.; methodology, J.N.-C. and I.G.-T.; software, J.N.-C.; validation,
J.N.-C. and I.G.-T.; formal analysis, J.N.-C. and L.E.A.-P.; investigation, J.N.-C.; resources, J.N.-C., I.G.-T., A.Y.G.-L.,
R.B., and J.F.V.; data curation, J.N.-C.; writing—original draft preparation, J.N.-C. and L.E.A.-P.; writing—review
and editing, L.E.A.-P., A.Y.G.-L., R.B., J.F.V. and I.G.-T.; visualization, J.N.-C.; supervision, J.N.-C.; project
administration, J.N.-C.; funding acquisition, J.N.-C. All authors have read and agreed to the published version of
the manuscript.
Funding:
This research was funded by the National Commission for Scientific & Technological Research
(CONICYT) from the Government of Chile, through the Research Project FONDECYT Regular 2019 No. 1190027.
Acknowledgments:
The authors want to thank to the students Matias Fuentes, Felipe Muñoz and Jose L. Concha
from LabMAT at the Universidad del B
í
o-B
í
o, for their technical support during the development of the testing
program. The authors thanks to the CONICYT-FONDEQUIP Program EQM-140088 for the acquisition of the
Hitachi Scanning Electron Microscope (ESEM). Fourth author (R.B) thanks to CONICYT through the Research
Projects FONDECYT Initiation 11160914 and International Cooperation Program/REDES 180165. Fifth author
(J.F-V) thanks to FONDECYT Initiation 11170957 and PIA-CONICYT ANILLO (ACT 17203). Finally, the financial
support for the first author (J.N-C) from the Royal Society (United Kingdom) through the Project International
Exchanges 2019 Round 1 (Ref. IES\R1\191015) is also gratefully acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Partl, M.N.; Di-Benedetto, H.; Bahia, H.U.; Canestrari, F.; De-la-Roche, C.; Piber, H.; Sybilski, D. Advances in
Interlaboratory Testing and Evaluation of Bituminous Materials: State-of-the-Art Report of the RILEM Technical
Committee 206-ATB; Springer: Dordrecht, The Netherlands, 2013.
2.
Airey, G.D.; Choi, Y.-K. State of the art report on moisture sensitivity test methods for bituminous pavement
materials. Int. J. Road Mater. Pavement Des. 2002,3, 355–372. [CrossRef]
3.
Airey, G.D. State of the art report on ageing test methods for bituminous pavement materials. Int. J. Pavement
Eng. 2003,4, 165–176. [CrossRef]
4.
Lu, X.; Isacsson, U. Effect of ageing on bitumen chemistry and rheology. Constr. Build. Mater.
2002
,16, 15–22.
[CrossRef]
Materials 2020,13, 1446 15 of 16
5.
Morian, N.; Hajj, E.; Glover, C.; Sebaaly, P. Oxidative aging of asphalt binders in hot-mix asphalt mixtures.
Trans. Res. Rec. 2011,2207, 107–116. [CrossRef]
6.
Ayar, P.; Moreno-Navarro, F.; Rubio-G
á
mez, M.C. The healing capability of asphalt pavements: A state of the
art review. J. Clean. Prod. 2016,113, 28–40. [CrossRef]
7.
Norambuena-Contreras, J.; Garcia, A. Self-healing of asphalt mixture by microwave and induction heating.
Mater. Des. 2016,106, 404–414. [CrossRef]
8.
Garc
í
a, A.; Schlangen, E.; van de Ven, M.; Sierra-Beltr
á
n, G. Preparation of capsules containing rejuvenators
for their use in asphalt concrete. J. Hazard. Mater. 2010,184, 603–611. [CrossRef]
9.
Karlsson, R.; Isacsson, U. Investigations on bitumen rejuvenator diffusion and structural stability (with
discussion). J. Assoc. Asph. Paving Technol. 2003,72, 463–501.
10.
Martins, E.; Poncelet, D.; Rodrigues, R.C.; Renard, D. Oil encapsulation techniques using alginate as
encapsulating agent: Applications and drawbacks. J. Microencapsul. 2017,34, 754–771. [CrossRef]
11.
Su, J.F.; Schlangen, E. Synthesis and physicochemical properties of high compact microcapsules containing
rejuvenator applied in asphalt. Chem. Eng. J. 2012,198, 289–300. [CrossRef]
12.
Su, J.F.; Qiu, J.; Schlangen, E.; Wang, Y.Y. Investigation the possibility of a new approach of using microcapsules
containing waste cooking oil: In situ rejuvenation for aged bitumen. Constr. Build. Mater.
2015
,74, 83–92.
[CrossRef]
13.
Norambuena-Contreras, J.; Yalcin, E.; Garcia, A.; Al-Mansoori, T.; Yilmaz, M.; Hudson-Griffiths, R. Effect of
mixing and ageing on the mechanical and self-healing properties of asphalt mixtures containing polymeric
capsules. Constr. Build. Mater. 2018,175, 254–266. [CrossRef]
14.
Norambuena-Contreras, J.; Liu, Q.; Zhang, L.; WU, S.; Yalcin, E.; Garcia, A. Influence of encapsulated
sunflower oil on the mechanical and self-healing properties of dense-graded asphalt mixtures. Mater. Struct.
2019,52, 78. [CrossRef]
15.
Norambuena-Contreras, J.; Yalcin, E.; Hudson-Griffiths, R.; Garc
í
a, A. Mechanical and self-healing properties
of stone mastic asphalt containing encapsulated rejuvenators. J. Mater. Civ. Eng.
2019
,31, 04019052.
[CrossRef]
16.
Micaelo, R.; Al-Mansoori, T.; Garcia, A. Study of the mechanical properties and self-healing ability of asphalt
mixture containing calcium-alginate capsules. Constr. Build. Mater. 2016,123, 734–744. [CrossRef]
17.
Zhang, R.; You, Z.P.; Wang, H.N.; Ye, M.; Yap, Y.K.; Si, C. The impact of bio-oil as rejuvenator foraged asphalt
binder. Constr. Build. Mater. 2019,196, 134–143. [CrossRef]
18.
Zhang, R.H.; Zhao, T.S.; Tan, P.; Wu, M.C.; Jiang, H.R. Optimization of bio-asphalt using bio-oil and distilled
water. J. Clean. Prod. 2017,165, 281–289. [CrossRef]
19.
Briones, R.; Serrano, L.; Llano-Ponte, R.; Labidi, J. Polyols obtained from solvolysis liquefaction of biodiesel
production solid residues. Chem. Eng. J. 2011,175, 169. [CrossRef]
20.
Aguirre, M.A.; Hassan, M.M.; Shirzad, S.; Mohammad, L.N.; Cooper, S.; Negulescu, I.I. Laboratory testing of
self-healing microcapsules in asphalt mixtures prepared with recycled asphalt shingles. J. Mater. Civ. Eng.
2017,29, 04017099. [CrossRef]
21.
Xu, S.; Tabakovi´c, A.; Liu, X.; Schlangen, E. Calcium alginate capsules encapsulating rejuvenator as healing
system for asphalt mastic. Constr. Build. Mater. 2018,169, 379–387. [CrossRef]
22.
ASTM D792-13. Standard Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement;
ASTM International: West Conshohocken, PA, USA, 2013.
23.
Guadarrama-Lezama, A.Y.; Dorantes-Alvarez, L.; Jaramillo-Flores, M.E.; P
é
rez-Alonso, C.; Niranjan, K.;
Guti
é
rrez-L
ó
pez, G.F.; Alamilla-Beltr
á
n, L. Preparation and characterization of non-aqueous extracts from
chilli (Capsicum annuum L.) and their microencapsulates obtained by spray-drying. J. Food Eng.
2012
,112,
29–37. [CrossRef]
24.
ASTM D695-02a. Standard Test Method for Compressive Properties of Rigid Plastics; ASTM International: West
Conshohocken, PA, USA, 2002.
25.
Lievens, C.; Mourant, D.; He, D.; Gunawan, R.; Li, X.; Li, C.-Z. An FT-IR spectroscopic study of carbonyl
functionalities in bio-oils. Fuel 2011,90, 3417–3423. [CrossRef]
26.
Stuart, B. Infrared Spectroscopy: Fundamentals and Applications, 1st ed.; John Wiley & Sons: West Sussex, UK,
2004; pp. 71–93.
Materials 2020,13, 1446 16 of 16
27.
Chandrasekaran, S.R.; Murali, D.; Marley, K.A.; Larson, R.A.; Doll, K.M.; Moser, B.R.; Scott, J.; Sharma, B.K.
Antioxidants from slow pyrolysis bio-oil of birch wood: Application for biodiesel and biobased lubricants.
ACS Sustain. Chem. Eng. 2016,4, 1414–1421. [CrossRef]
28.
Pillay, V.; Danckwerts, M.P.; Fassihi, R. A crosslinked calcium-alginate-pectinate-cellulose acetophthalate
gelisphere system for linear drug release. Drug Deliv. 2002,9, 77–86. [CrossRef]
29.
Zhao, W.; Qi, Y.; Wang, Y.; Xue, Y.; Xu, P.; Li, Z.; Li, Q. Morphology and thermal properties of calcium
alginate/reduced graphene oxide composites. Polymers 2018,10, 990. [CrossRef]
30.
Shu, B.; Wu, S.; Dong, L.; Norambuena-Contreras, J.; Yang, X.; Li, C. Microfluidic synthesis of polymeric fibers
containing rejuvenating agent for asphalt self-healing. Constr. Build. Mater. 2019,219, 176–183. [CrossRef]
©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
