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Citation: Li, M.; Fini, E.; Jia, X.; Song,
B.; Wang, Y. Effect of Aging on
Healing Capacity of Bituminous
Composites Containing
Polyphosphoric Acid. Materials 2023,
16, 3333. https://doi.org/10.3390/
ma16093333
Academic Editor: Giovanni Polacco
Received: 3 April 2023
Revised: 19 April 2023
Accepted: 21 April 2023
Published: 24 April 2023
Copyright: © 2023 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 (https://
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4.0/).
materials
Article
Effect of Aging on Healing Capacity of Bituminous Composites
Containing Polyphosphoric Acid
Mingxia Li 1, *, Elham Fini 2, Xiaomin Jia 1, Baiyang Song 1and Yanhong Wang 1
1School of Civil Engineering, Luoyang Institute of Science and Technology, No. 90 Wangcheng Avenue,
Luolong District, Luoyang 471023, China; lymzy1119@163.com (X.J.); baiysong@foxmail.com (B.S.);
yhwanglla@126.com (Y.W.)
2School of Sustainable Engineering and Built Environment, Arizona State University, 660 S. College Avenue,
Tempe, AZ 85287-300, USA; efini@asu.edu
*Correspondence: limingxia925@hotmail.com
Abstract:
This study examines how aging affects the healing capacity of bituminous composites
containing polyphosphoric acid (PPA). PPA is commonly used in bituminous composites to enhance
its elasticity, however, PPA effectiveness highly depends on other constituents on the matrix and
the environmental (internal and external) factors. In terms of internal factors, the interplay between
PPA and various bitumen modifiers have been extensively studied. Here, we study how external
factors such as exposure to ultraviolet radiation affect PPA’s efficacy, measured in terms of change
in bitumen’s healing index. The study results showed that the introduction of PPA to bituminous
composites significantly increases the bitumen healing index, however, the change in the healing
index becomes less pronounced as aging progresses. The presence of additives such as taconite were
found to affect the effect of PPA on bitumen’s healing index. For instance, bitumens containing 30%
taconite showed the highest increase in their healing index in the presence of PPA among studied
scenarios. Overall, bitumen containing PPA had a higher healing index than those without PPA
regardless of the extent of aging and dosage of modifiers. This, in turn, indicates that PPA is highly
effective for enhancing bitumen healing. This can be attributed to the role of PPA in promoting
intermolecular interactions within the bitumen matrix.
Keywords: polyphosphoric acid; healing; modified bitumen; aging; ultraviolet
1. Introduction
Bitumen is significantly used as a binder in mixtures for pavement material. During
the service period, it is exposed to complex environmental conditions, such as ultraviolet
(UV) oxidation. UV aging could trigger pavement destruction and seriously weakens the
lifespan of bitumen pavement. Currently, the healing capacity is attracting more and more
attention as a potentially helpful factor for promoting the lifespan of service [
1
]. As such,
investigating and evaluating the effect of UV aging on the healing performance of bitumen
will be helpful because it directly influences the service life of bitumen pavement [
2
]. In
order to enhance asphalt self-healing, many scholars have explored the healing effects
using different physical methods [
3
–
5
]. Some studies have attempted to use microcapsules,
hollow-fiber tubes, and nanoparticles [6–14].
He et al. have studied the effect of thermos-oxidative aging on the healing capacity of
bitumen. They found that a higher level of aging correlates to a smaller healing index. This
difference heightened after multiple damage-healing cycles [
15
]. Xu and Qu et al. utilized a
molecular dynamic simulation to investigate the effect of oxidative aging on healing capac-
ity. The results showed that aging weakened the nano-aggregation behavior of asphaltene
molecules and reduced the translational mobility of bitumen molecules. Compared to
virgin bitumen, aged bitumen has a higher activation-energy barrier for self-healing [
16
,
17
].
Vallerga’s research indicated that UV radiation could apparently influence the softening
Materials 2023,16, 3333. https://doi.org/10.3390/ma16093333 https://www.mdpi.com/journal/materials
Materials 2023,16, 3333 2 of 10
point and ductility of bitumen [
18
]. Glotova et al. have investigated how UV aging affects
bitumen by testing its chemical properties, structure, and composition contents before
and after aging. They found that UV radiation could significantly degenerate the perfor-
mance indicators above [
19
]. Wu et al. conducted a Fourier infrared test and a dynamic
shear rheometer (DSR) test using a base bitumen and polymer-modified bitumen with a
high-pressure mercury lamp. The results showed that UV radiation could obviously age
bitumen, and different intensities were posed to different aging effects [
20
]. Although the
researchers above indicated UV radiation can change the chemical contents and technical
performance of bitumen material, they focused less on the UV-aging effect on the healing
property of bitumen.
Polyphosphoric acid (PPA), as a modifying agent, has become the most important acid
used in bitumen [
21
–
23
]. It can be produced by deriving H
3
PO
4
from heating P
2
O
5
or the
dehydration of H
3
PO
4
at high temperatures [
24
]. Introducing PPA to bitumen improves
its high-temperature grade [
25
–
27
]. Edward et al. have analyzed the rheological perfor-
mance of bitumen modified by PPA. They pointed out that PPA had a remarkable effect
on the medium- and high-temperature rheological behavior of bitumen. Besides, PPA also
promoted the resistance deformation utilizing a repeated creep test [
28
,
29
]. Zhang et al.
investigated the structural characteristics of bitumen modified by PPA before and after
short- or long-term thermal aging. The study showed that the suitable addition of PPA
improved the major physical and rheological properties of SEBS-modified (SM) bitumen,
and the improved properties became more obvious with further aging [
30
]. Also, they
investigated the effect of PPA on the rheological behaviors of HVM bitumen before and
after aging. The suitable gelation induced by PPA not only improved the high-temperature
rheological performance of HVM bitumen but also increased the low-temperature rhe-
ological performance [
31
]. With the aim of improving the compatibility and high- and
low-temperature properties of SBR-modified bitumen, Liang et al. studied the results of
adding PPA to achieve a further modification. It showed that PPA remarkably enhanced
the adhesion ability, high-temperature elasticity, and anti-rutting ability of modified bitu-
men [
32
]. Bennert and Martin researched the recyclability of PPA-modified bitumen. RAP
(Reclaimed Asphalt Pavement) containing PPA is not detrimental to hot-mix bitumen and
should perform in a similar manner to RAP containing SBS-modified bitumen [
33
]. Some
studies also showed that when PPA-modified bitumen was used alone, the properties of
bitumen and its mixture would be weakened. Thus, the PPA was often used in conjunction
with other modifiers [34–36].
There are also studies that evaluated the effects of PPA and aging on bitumen per-
formance. Zhang et al. found that a moderate gelation by 0.8 wt % PPA in styrene–
ethylene/butylene–styrene-modified bitumen improved the softening point and elasticity
before and after aging without negative effects [30].
Based on the lack of research on the healing capacity of bitumen in UV-aging envi-
ronmental conditions, the current direction of research focuses on the effect of PPA on
the self-healing ability of asphalt binder with taconite, investigated for the first time, and
this paper also evaluates how external factors such as exposure to ultraviolet radiation
affects PPA’s efficacy, measured in terms of change to the bitumen’s healing index. These
are attributes that help us to understand how PPA improves capacities of bitumen holis-
tically. Different UV-aging durations (0 h, 100 h and 200 h), with or without PPA, and
different dosages of taconite were considered during the tests to evaluate effects on healing
performance. Experiments were conducted using DSR equipment. The healing index
was calculated according to the complex shear modulus. Also, this study analyzed the
promoted effect of taconite on the healing performance of bitumen modified by PPA. It was
found that ultraviolet (UV) radiation can weaken the healing capacity of bitumen modified
by polyphosphoric acid and taconite. Inversely, polyphosphoric acid and taconite filler
play a positive role on the healing capacity of modified bitumen.
Materials 2023,16, 3333 3 of 10
2. Materials and Methods
2.1. Material and Sample Preparation
PG 64-22, which can be used in areas where the maximum temperature of the pave-
ment is 64
◦
C and the minimum temperature is
−
22
◦
C, was used in this study. It was ob-
tained from the Holy Frontier Corporation in Arizona, and its specific gravity is
1.041 g/cm3
at 15.6
◦
C and absolute viscosity is 179 Pa
·
s at 60
◦
C. Taconite was acquired from a Min-
nesota mine’s tailings. Based on the standard testing method [
37
–
39
], the values of specific
gravity, abrasion resistance, and the dosage of deleterious materials were 2.803 g/cm
3
,
<40, and 40–45%, respectively. Four types of bitumen specimens with different dosages
of taconite and 1% PPA were tested (0%, 10%, 20%, 30%) (Figure 1). Polyphosphoric acid
(83.3% P
2
O
5
, so-called 115% H
3
PO
4
) was acquired from Sigma-Aldrich. One percent wt %
PPA was mixed in samples.
Materials 2023, 16, x FOR PEER REVIEW 3 of 10
2. Materials and Methods
2.1. Material and Sample Preparation
PG 64-22, which can be used in areas where the maximum temperature of the pave-
ment is 64 °C and the minimum temperature is −22 °C, was used in this study. It was
obtained from the Holy Frontier Corporation in Arizona, and its specific gravity is 1.041
g/cm3 at 15.6 °C and absolute viscosity is 179 Pa·s at 60 °C. Taconite was acquired from a
Minnesota mine’s tailings. Based on the standard testing method [37–39], the values of
specific gravity, abrasion resistance, and the dosage of deleterious materials were 2.803
g/cm3, <40, and 40–45%, respectively. Four types of bitumen specimens with different dos-
ages of taconite and 1% PPA were tested (0%, 10%, 20%, 30%) (Figure 1). Polyphosphoric
acid (83.3% P2O5, so-called 115% H3PO4) was acquired from Sigma-Aldrich. One percent
wt % PPA was mixed in samples.
Figure 1. Bitumen specimens with taconite and PPA.
The PPA–taconite modified-binder samples were prepared using the high shear
mixer. The mixing temperature, time, and mixing speed were 135 °C, 30 min, and 2000
rpm, respectively. Seing up the PPA-modified bitumen with taconite began with the low
shear blend of 500 rpm for first 10 min, and following, the speed of blender was raised to
2000 rpm for a constant 20 min.
2.2. Ultraviolet (UV) Testing
In order to make sure the thick film was consistent, about 6 g of unaged sample was
evenly poured onto a rubber mold. Then, the mold with the sample was placed into a UV
chamber, which was a closed box with a UV-light source 10 cm away from the boom.
The testing temperature was 65 °C, the UV-radiation intensity was 0.71 W/m2, and the
wavelength was 340 nm. The UV-aging time continued for 100 h and 200 h, respectively.
Two replicates were conducted for each sample.
2.3. Healing Testing (Dynamic Shear Rheometer)
To evaluate the healing capacity of modified bitumen using PPA and taconite, a rhe-
ometry test was conducted utilizing a time-temperature sweep (loading-rest-loading) fol-
lowing ASTM-D7552-09 [40] by Anton Paar MCR302, whose maximum torque is about
230 mNm. A parallel plate set-up with 8 mm diameter and 2 mm thick samples were used
at 25 °C. The test was performed for two replicates at a strain of 5% and a frequency of 10
Hz. All samples were loaded until its complex shear modulus (G*) was reduced to 50% of
the initial value (G0), which was named Ga. The loading was then stopped, and a rest pe-
riod of 900 s was implemented. After the rest period, the loading was repeated until the
complex shear modulus was reduced to 50% of Gb, where Gb was the initial modulus after
the rest period. Healing index was then calculated using Equation (1) following prior
works [41–43].
𝐻𝐼 =
𝐺−𝐺
𝐺−𝐺
(1)
G0—Initial dynamic shear modulus before rest period;
Ga—Ending dynamic shear modulus before rest period;
Figure 1. Bitumen specimens with taconite and PPA.
The PPA–taconite modified-binder samples were prepared using the high shear mixer.
The mixing temperature, time, and mixing speed were 135
◦
C, 30 min, and 2000 rpm,
respectively. Setting up the PPA-modified bitumen with taconite began with the low
shear blend of 500 rpm for first 10 min, and following, the speed of blender was raised to
2000 rpm for a constant 20 min.
2.2. Ultraviolet (UV) Testing
In order to make sure the thick film was consistent, about 6 g of unaged sample was
evenly poured onto a rubber mold. Then, the mold with the sample was placed into a
UV chamber, which was a closed box with a UV-light source 10 cm away from the bottom.
The testing temperature was 65
◦
C, the UV-radiation intensity was 0.71 W/m
2
, and the
wavelength was 340 nm. The UV-aging time continued for 100 h and 200 h, respectively.
Two replicates were conducted for each sample.
2.3. Healing Testing (Dynamic Shear Rheometer)
To evaluate the healing capacity of modified bitumen using PPA and taconite, a
rheometry test was conducted utilizing a time-temperature sweep (loading-rest-loading)
following ASTM-D7552-09 [
40
] by Anton Paar MCR302, whose maximum torque is about
230 mNm. A parallel plate set-up with 8 mm diameter and 2 mm thick samples were used
at 25
◦
C. The test was performed for two replicates at a strain of 5% and a frequency of
10 Hz. All samples were loaded until its complex shear modulus (G*) was reduced to 50%
of the initial value (G
0
), which was named G
a
. The loading was then stopped, and a rest
period of 900 s was implemented. After the rest period, the loading was repeated until
the complex shear modulus was reduced to 50% of G
b
, where G
b
was the initial modulus
after the rest period. Healing index was then calculated using Equation (1) following prior
works [41–43].
HI =
Gb−Ga
G0−Ga(1)
G0—Initial dynamic shear modulus before rest period;
Ga—Ending dynamic shear modulus before rest period;
Gb—Initial dynamic shear modulus after rest period.
Materials 2023,16, 3333 4 of 10
3. Results and Discussion
3.1. Effect of UV Aging on the Healing Capacity of Modified Bitumen
Figure 2presents the healing index of bitumen modified by PPA and taconite samples
before and after different UV-aging times. As shown in Figure 2, there were obviously
differences among the healing indexes of unaged, 100 h UV-aged, and 200 h UV-aged
samples. The healing indexes were weakened when the UV-aging period increased. It is
suggested that the effect of UV radiation on the healing capacity has a remarkable influence.
This may due to a dramatic increase in the stiffness and the loss of viscoelasticity after 20 h
of UV exposure [
44
]. With a longer aging time, there is a larger amount of asphaltene in
the asphalt. This makes the modified binders harder and, therefore, less able to flow at the
same temperature, impairing its healing ability.
Materials 2023, 16, x FOR PEER REVIEW 4 of 10
G
b
—Initial dynamic shear modulus after rest period.
3. Results and Discussion
3.1. Effect of UV Aging on the Healing Capacity of Modified Bitumen
Figure 2 presents the healing index of bitumen modified by PPA and taconite samples
before and after different UV-aging times. As shown in Figure 2, there were obviously
differences among the healing indexes of unaged, 100 h UV-aged, and 200 h UV-aged
samples. The healing indexes were weakened when the UV-aging period increased. It is
suggested that the effect of UV radiation on the healing capacity has a remarkable influ-
ence. This may due to a dramatic increase in the stiffness and the loss of viscoelasticity
after 20 h of UV exposure [44]. With a longer aging time, there is a larger amount of as-
phaltene in the asphalt. This makes the modified binders harder and, therefore, less able
to flow at the same temperature, impairing its healing ability.
Figure 2. Healing indexes of bitumen modified by PPA at 0, 100 h, and 200 h UV-aging times.
For unaged and 100 h UV-aged samples, the healing index was improved gradually
with the increase of the dosage of taconite included. When the aging period was 200 h, the
healing index was reduced. To further understand the effect of UV radiation on the heal-
ing performance of modified bitumen, the delta healing index of 0–100 h and 100–200 h
UV-aging periods were calculated (Figure 3). Regardless of whether PPA was included, it
showed that the healing index of the samples exposed to 0–100 h UV radiation were weak-
ened more than that of the samples exposed to 100–200 h, besides the samples with 30%
taconite content during the 100–200 h aging period. This indicates that the healing index
decreased slowly with an increase in the aging time. During the 0–100 h aging period,
those differences, which were reduced by 18.31% and 7.68% for without and with PPA,
were the highest while the dosage of taconite was 10%, respectively. However, during the
100–200 h aging period, the highest difference, which was reduced by 4.08% and 20.16%,
respectively, occurred while the dosage of taconite was 30%. It can be concluded that the
healing capacity would be greatly destroyed when the UV-aging time was 200 h, even
though it had 30% taconite and 1% PPA.
0%
10%
20%
30%
40%
50%
60%
0 100 200
Healing Index
UV-aging Time (hrs)
0% 10% 20% 30%
Figure 2. Healing indexes of bitumen modified by PPA at 0, 100 h, and 200 h UV-aging times.
For unaged and 100 h UV-aged samples, the healing index was improved gradually
with the increase of the dosage of taconite included. When the aging period was 200 h,
the healing index was reduced. To further understand the effect of UV radiation on the
healing performance of modified bitumen, the delta healing index of 0–100 h and
100–200 h
UV-aging periods were calculated (Figure 3). Regardless of whether PPA was included,
it showed that the healing index of the samples exposed to 0–100 h UV radiation were
weakened more than that of the samples exposed to 100–200 h, besides the samples with
30% taconite content during the 100–200 h aging period. This indicates that the healing
index decreased slowly with an increase in the aging time. During the 0–100 h aging period,
those differences, which were reduced by 18.31% and 7.68% for without and with PPA,
were the highest while the dosage of taconite was 10%, respectively. However, during the
100–200 h aging period, the highest difference, which was reduced by 4.08% and 20.16%,
respectively, occurred while the dosage of taconite was 30%. It can be concluded that the
healing capacity would be greatly destroyed when the UV-aging time was 200 h, even
though it had 30% taconite and 1% PPA.
3.2. Effect of PPA on the Healing Capacity of Bitumen with Taconite
Figure 4presents the healing indexes of unaged (a), 100 h aged (b), and 200 h aged
(c) samples modified with or without PPA. As shown in Figure 4, the healing index was
promoted after adding PPA to bitumen with taconite regardless of the UV-aging time.
Due to a moderate dosage of PPA gelation, the rheological behavior of bitumen was
improved [
45
]. The addition of PPA can increase the viscosity and adhesion ability of
bitumen [
31
,
32
]. It is also helpful in enhancing the resistance of deformation, improving
the self-healing property.
Materials 2023,16, 3333 5 of 10
Materials 2023, 16, x FOR PEER REVIEW 5 of 10
Figure 3. Delta healing indexes of samples during the 0–100 h and 100–200 h UV-aging periods
without (a) and with (b) PPA.
3.2. Effect of PPA on the Healing Capacity of Bitumen with Taconite
Figure 4 presents the healing indexes of unaged (a), 100 h aged (b), and 200 h aged
(c) samples modified with or without PPA. As shown in Figure 4, the healing index was
promoted after adding PPA to bitumen with taconite regardless of the UV-aging time.
Due to a moderate dosage of PPA gelation, the rheological behavior of bitumen was im-
proved [45]. The addition of PPA can increase the viscosity and adhesion ability of bitu-
men [31-32]. It is also helpful in enhancing the resistance of deformation, improving the
self-healing property.
After 100 h of UV aging, the healing index of samples with PPA was much higher
than that of samples without PPA, which were 34.87%, 35.16%, 38.95%, and 44.84%, while
the dosage of taconite was 0%, 10%, 20%, and 30%, respectively. This indicates that PPA
has a very powerful promotion effect on the healing property of bitumen. When exposed
to 200 h of UV aging, it was found that the healing index of samples with PPA decreased
with an increase in the taconite dosage. However, they were still a lile higher than that
of samples without PPA. To evaluate the effect of PPA on the healing performance of aged
bitumen with taconite, the delta healing index, which represents the difference value of
the healing indexes between taconite-modified binders without and with PPA, were com-
puted (Figure 5). Regardless the type of UV aging, the delta healing indexes were almost
all positive. While the UV-aging time was 100 h, the delta healing indexes were 18.47%,
16.47%, 16.45%, and 14.68%, respectively. With the increase in the taconite dosage, thee
delta healing indexes decreased. When the UV-aging time was 200 h, the promotion of
PPA on the healing property was gradually weakened. This was due to the healing prop-
erty of bitumen being destroyed when the UV-aging time was 200 h. This result is con-
sistent to that concluded in Section 3.1.
Figure 3.
Delta healing indexes of samples during the 0–100 h and 100–200 h UV-aging periods
without (a) and with (b) PPA.
Materials 2023, 16, x FOR PEER REVIEW 6 of 10
Figure 4. Healing indexes of samples without and with PPA at 0 (a), 100 h (b), and 200 h (c) of UV
aging.
Figure 5. Delta healing indexes between samples without and with PPA.
-5%
0%
5%
10%
15%
20%
0% 10% 20% 30%
Delta Healing Index
Dosage of Taconite
Unaged 100hrs UV-aged 200hrs UV-aged
Figure 4.
Healing indexes of samples without and with PPA at 0 (
a
), 100 h (
b
), and 200 h (
c
) of
UV aging.
Materials 2023,16, 3333 6 of 10
After 100 h of UV aging, the healing index of samples with PPA was much higher
than that of samples without PPA, which were 34.87%, 35.16%, 38.95%, and 44.84%, while
the dosage of taconite was 0%, 10%, 20%, and 30%, respectively. This indicates that PPA
has a very powerful promotion effect on the healing property of bitumen. When exposed
to 200 h of UV aging, it was found that the healing index of samples with PPA decreased
with an increase in the taconite dosage. However, they were still a little higher than that
of samples without PPA. To evaluate the effect of PPA on the healing performance of
aged bitumen with taconite, the delta healing index, which represents the difference value
of the healing indexes between taconite-modified binders without and with PPA, were
computed (Figure 5). Regardless the type of UV aging, the delta healing indexes were
almost all positive. While the UV-aging time was 100 h, the delta healing indexes were
18.47%, 16.47%, 16.45%, and 14.68%, respectively. With the increase in the taconite dosage,
thee delta healing indexes decreased. When the UV-aging time was 200 h, the promotion
of PPA on the healing property was gradually weakened. This was due to the healing
property of bitumen being destroyed when the UV-aging time was 200 h. This result is
consistent to that concluded in Section 3.1.
Materials 2023, 16, x FOR PEER REVIEW 6 of 10
Figure 4. Healing indexes of samples without and with PPA at 0 (a), 100 h (b), and 200 h (c) of UV
aging.
Figure 5. Delta healing indexes between samples without and with PPA.
-5%
0%
5%
10%
15%
20%
0% 10% 20% 30%
Delta Healing Index
Dosage of Taconite
Unaged 100hrs UV-aged 200hrs UV-aged
Figure 5. Delta healing indexes between samples without and with PPA.
3.3. Effect of Taconite on the Healing Capacity of Bitumen Modified by PPA
Figure 6shows bitumen modified by PPA with different dosages (0, 10%, 20%, and
30%) of taconite samples before and after UV aging. It can be observed that there are
remarkable differences among healing indexes of the four kinds of samples. For unaged
samples, the healing index was improved by increasing the taconite dosage. When included,
0%, 10%, 20%, and 30% dosages of taconite yielded healing indices of 36.88, 42.84, 45.32,
and 46.81%, respectively. After conducting 100 h of UV aging, the healing capacity still
showed a promoting trend; the values were 34.87, 35.61, 38.95, and 44.84%, respectively.
This indicates that taconite can enhance the healing performance of bitumen due to the role
of taconite in promoting its thermal conductivity, which is consistent with the results of our
published paper [
46
]. Higher thermal conductivity can make the temperature of binders
increase more quickly and reach a critical point that drives the asphalt binder to start the
healing behavior.
However, when handled with 200 h of UV aging, the healing property was reduced,
and with the addition of taconite, it increased. The delta healing indexes of different
taconite dosages were also calculated (Figure 7). It can be seen that the delta healing index
was increased with the dosage of taconite. When concentrations of 10%, 20%, and 30%
taconite were added, they were increased by 5.96, 8.44, and 9.93% for unaged samples,
respectively. After 100 h of aging, the delta healing indexes deceased. However, after 200 h
of UV aging, the indexes were reduced significantly, and the values were 0.03,
−
2.71, and
Materials 2023,16, 3333 7 of 10
−
9.9%, respectively. Hence, introducing taconite can enhance the healing performance of
modified bitumen with PPA. However, when it was aged significantly longer, taconite will
be harmful for the healing performance.
Materials 2023, 16, x FOR PEER REVIEW 7 of 10
3.3. Effect of Taconite on the Healing Capacity of Bitumen Modified by PPA
Figure 6 shows bitumen modified by PPA with different dosages (0, 10%, 20%, and
30%) of taconite samples before and after UV aging. It can be observed that there are re-
markable differences among healing indexes of the four kinds of samples. For unaged
samples, the healing index was improved by increasing the taconite dosage. When in-
cluded, 0%, 10%, 20%, and 30% dosages of taconite yielded healing indices of 36.88, 42.84,
45.32, and 46.81%, respectively. After conducting 100 h of UV aging, the healing capacity
still showed a promoting trend; the values were 34.87, 35.61, 38.95, and 44.84%, respec-
tively. This indicates that taconite can enhance the healing performance of bitumen due
to the role of taconite in promoting its thermal conductivity, which is consistent with the
results of our published paper [46]. Higher thermal conductivity can make the tempera-
ture of binders increase more quickly and reach a critical point that drives the asphalt
binder to start the healing behavior.
Figure 6. Healing indexes of PPA-modified samples with different dosages of taconite.
However, when handled with 200 h of UV aging, the healing property was reduced,
and with the addition of taconite, it increased. The delta healing indexes of different taco-
nite dosages were also calculated (Figure 7). It can be seen that the delta healing index was
increased with the dosage of taconite. When concentrations of 10%, 20%, and 30% taconite
were added, they were increased by 5.96, 8.44, and 9.93% for unaged samples, respec-
tively. After 100 h of aging, the delta healing indexes deceased. However, after 200 h of
UV aging, the indexes were reduced significantly, and the values were 0.03, −2.71, and
−9.9%, respectively. Hence, introducing taconite can enhance the healing performance of
modified bitumen with PPA. However, when it was aged significantly longer, taconite
will be harmful for the healing performance.
0%
10%
20%
30%
40%
50%
60%
0% 10% 20% 30%
Healing Index
Taconite Dosage
unaged 100hrs 200hrs
Figure 6. Healing indexes of PPA-modified samples with different dosages of taconite.
Materials 2023, 16, x FOR PEER REVIEW 8 of 10
Figure 7. Delta healing indexes between samples without and with taconite.
4. Conclusions
Polyphosphoric acid (PPA), a modifying agent, is popularly used in bitumen to en-
hance its elastic property. The effect of it on the healing performance was evaluated in this
study. Since UV aging has a significant effect on bitumen, test samples were also handled
with UV aging. Finally, we compared the healing ability of samples without or with PPA
or taconite at different aging times. The results indicated that ultraviolet (UV) radiation
can weaken the healing capacity of bitumen modified by polyphosphoric acid and taco-
nite. Inversely, polyphosphoric acid and taconite fillers play a positive role on the healing
capacity of modified bitumen. Based on the above findings, the dosage of 30% taconite
introduced into bitumen with 1% PPA are recommended for producing a mixture with a
longer service lifespan. The following are the detailed conclusions drawn from our eval-
uation of the resulting modified bitumen.
(1) Ultraviolet (UV) radiation can weaken the healing capacity of bitumen modified by
polyphosphoric acid and taconite. The longer the aging time, the slower the decay
rate of the healing ability. When adding 30% taconite content, the healing index of
bitumen modified by 1% PPA were 46.81, 44.84, and 24.68% at unaged, 100 h aged,
and 200 h aged conditions, respectively.
(2) Polyphosphoric acid can slow down the loss of healing capacity between unaged and
100 h aged bitumen with taconite. Comparing the delta healing indexes of samples
without and with PPA, the values decreased from −6.95, −18.31, 16.64, and −12.24%
to −2.01, 7.68, 3.96, and −4.08% for taconite dosages of 0, 10, 20, and 30%, respectively.
(3) The addition of taconite filler plays a positive role on the healing capacity of modified
bitumen. When subjected to 100 h of UV aging and modified by 1% PPA, the healing
index of 10, 20, and 30% taconite content introduced were increased by 5.96, 8.44, and
9.98%, respectively, compared with that without taconite.
(4) Polyphosphoric acid can improve the enhancing effect of taconite on the healing ca-
pacity of bitumen. As for 100 h aged samples with 30% content taconite, the healing
index with 1% PPA was increased by 4.41% compared to that without PPA.
(5) After 200 h of aging, the healing property of bitumen decays fast even though the
sample is modified by polyphosphoric acid and taconite.
Author Contributions: Investigation, X.J. and B.S.; Resources, Y.W.; Writing—original draft, M.L.;
Writing—review & editing, E.F. All authors have read and agreed to the published version of the
manuscript.
-10%
-5%
0%
5%
10%
15%
0 100 200
Delta Healing Index
UV-aging Time (hrs)
10% 20% 30%
Figure 7. Delta healing indexes between samples without and with taconite.
4. Conclusions
Polyphosphoric acid (PPA), a modifying agent, is popularly used in bitumen to en-
hance its elastic property. The effect of it on the healing performance was evaluated in
this study. Since UV aging has a significant effect on bitumen, test samples were also
handled with UV aging. Finally, we compared the healing ability of samples without or
with PPA or taconite at different aging times. The results indicated that ultraviolet (UV)
radiation can weaken the healing capacity of bitumen modified by polyphosphoric acid
and taconite. Inversely, polyphosphoric acid and taconite fillers play a positive role on
the healing capacity of modified bitumen. Based on the above findings, the dosage of 30%
taconite introduced into bitumen with 1% PPA are recommended for producing a mixture
Materials 2023,16, 3333 8 of 10
with a longer service lifespan. The following are the detailed conclusions drawn from our
evaluation of the resulting modified bitumen.
(1)
Ultraviolet (UV) radiation can weaken the healing capacity of bitumen modified by
polyphosphoric acid and taconite. The longer the aging time, the slower the decay
rate of the healing ability. When adding 30% taconite content, the healing index of
bitumen modified by 1% PPA were 46.81, 44.84, and 24.68% at unaged, 100 h aged,
and 200 h aged conditions, respectively.
(2)
Polyphosphoric acid can slow down the loss of healing capacity between unaged and
100 h aged bitumen with taconite. Comparing the delta healing indexes of samples
without and with PPA, the values decreased from
−
6.95,
−
18.31, 16.64, and
−
12.24%
to
−
2.01, 7.68, 3.96, and
−
4.08% for taconite dosages of 0, 10, 20, and 30%, respectively.
(3) The addition of taconite filler plays a positive role on the healing capacity of modified
bitumen. When subjected to 100 h of UV aging and modified by 1% PPA, the healing
index of 10, 20, and 30% taconite content introduced were increased by 5.96, 8.44, and
9.98%, respectively, compared with that without taconite.
(4)
Polyphosphoric acid can improve the enhancing effect of taconite on the healing
capacity of bitumen. As for 100 h aged samples with 30% content taconite, the healing
index with 1% PPA was increased by 4.41% compared to that without PPA.
(5)
After 200 h of aging, the healing property of bitumen decays fast even though the
sample is modified by polyphosphoric acid and taconite.
Author Contributions:
Investigation, X.J. and B.S.; Resources, Y.W.; Writing—original draft, M.L.;
Writing—review & editing, E.F. All authors have read and agreed to the published version of
the manuscript.
Funding:
The authors acknowledge the financial supported by Luoyang Institute of Science and
Technology PhD Start-up Program (No. 21010768), the China Scholarship Council (Certificate number:
201908500149), and the National Science Foundation (No. 1928807). The authors greatly appreciate
Jeff Long and Peter Goguen with Arizona State University for their assistance and guidance with
conducting laboratory experiments.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data that support the findings of this study are available on request
from the corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
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