Viscoelasticity of PPA/SBS/SBR Composite Modified Asphalt and Asphalt Mixtures Under Pressure Aging Conditions

Abstract

The viscoelastic behavior of asphalt mixtures is a crucial consideration in the analysis of pavement mechanical responses and structural design. This study aims to elucidate the molecular structure and component evolution trends of polyphosphoric acid (PPA)/styrene butadiene styrene block copolymer (SBS)/styrene butadiene rubber copolymer (SBR) composite modified asphalt (CMA) under rolling thin film oven test (RTFOT) and pressure aging (PAV) conditions, as well as to analyze the viscoelastic evolution of CMA mixtures. First, accelerated aging was conducted in the laboratory through RTFOT, along with PAV tests for 20 h and 40 h. Next, the microscopic characteristics of the binder at different aging stages were explored using Fourier-transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC) tests. Additionally, fundamental rheological properties and temperature sweep tests were performed to reveal the viscoelastic evolution characteristics of CMA. Ultimately, the viscoelastic properties of CMA mixtures under dynamic loading at different aging stages were clarified. The results indicate that the incorporation of SBS and SBR increased the levels of carbonyl and sulfoxide factors while decreasing the level of long-chain factors, which slowed down the rate of change of large molecule content and reduced the rate of change of LMS by more than 6%, with the rate of change of overall molecular weight distribution narrowing to below 50%. The simultaneous incorporation of SBS and SBR into CMA mixtures enhanced the dynamic modulus in the 25 Hz and −10 °C range by 24.3% (AC-13), 15.4% (AC-16), and reduced the φ by 55.8% (AC-13), 40% (AC-16). This research provides a reference for the application of CMA mixtures in the repair of pavement pothole damage.

Full text

Academic Editor: Andrew N. Hrymak
Received: 6 February 2025
Revised: 1 March 2025
Accepted: 4 March 2025
Published: 6 March 2025
Citation: Yu, Z.; Ling, X.; Fan, Z.;
Zhou, Y.; Ma, Z. Viscoelasticity of
PPA/SBS/SBR Composite Modified
Asphalt and Asphalt Mixtures Under
Pressure Aging Conditions. Polymers
2025,17, 698. https://doi.org/
10.3390/polym17050698
Copyright: © 2025 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://creativecommons.org/
licenses/by/4.0/).
Article
Viscoelasticity of PPA/SBS/SBR Composite Modified Asphalt
and Asphalt Mixtures Under Pressure Aging Conditions
Zongjie Yu 1,2,* , Xinpeng Ling 2,3, Ze Fan 2,3, Yueming Zhou 2,3 and Zhu Ma 4
1College of Transportation Engineering, Changsha University of Science and Technology,
Changsha 410114, China
2
Xinjiang Institute of Transportation Sciences Co., Ltd., Urumqi 830000, China; xinpenglingxjk@163.com (X.L.);
zefanxjk@163.com (Z.F.); yuemingzhou2024@163.com (Y.Z.)
3Key Laboratory of Highway Engineering Technology and Transportation Industry in Arid Desert Regions,
Urumqi 830000, China
4Xinjiang Highway and Bridge Testing and Inspection Center Co., Ltd., Urumqi 830000, China;
zhumajczx@163.com
*Correspondence: 13511844380@163.com
Abstract: The viscoelastic behavior of asphalt mixtures is a crucial consideration in the
analysis of pavement mechanical responses and structural design. This study aims to
elucidate the molecular structure and component evolution trends of polyphosphoric acid
(PPA)/styrene butadiene styrene block copolymer (SBS)/styrene butadiene rubber copoly-
mer (SBR) composite modified asphalt (CMA) under rolling thin film oven test (RTFOT)
and pressure aging (PAV) conditions, as well as to analyze the viscoelastic evolution of
CMA mixtures. First, accelerated aging was conducted in the laboratory through RTFOT,
along with PAV tests for 20 h and 40 h. Next, the microscopic characteristics of the binder at
different aging stages were explored using Fourier-transform infrared spectroscopy (FTIR)
and gel permeation chromatography (GPC) tests. Additionally, fundamental rheological
properties and temperature sweep tests were performed to reveal the viscoelastic evo-
lution characteristics of CMA. Ultimately, the viscoelastic properties of CMA mixtures
under dynamic loading at different aging stages were clarified. The results indicate that
the incorporation of SBS and SBR increased the levels of carbonyl and sulfoxide factors
while decreasing the level of long-chain factors, which slowed down the rate of change
of large molecule content and reduced the rate of change of LMS by more than 6%, with
the rate of change of overall molecular weight distribution narrowing to below 50%. The
simultaneous incorporation of SBS and SBR into CMA mixtures enhanced the dynamic
modulus in the 25 Hz and
−
10
◦
C range by 24.3% (AC-13), 15.4% (AC-16), and reduced the
φ
by 55.8% (AC-13), 40% (AC-16). This research provides a reference for the application of
CMA mixtures in the repair of pavement pothole damage.
Keywords: road engineering; PPA/SBS/SBR composite modified asphalt mixture; FTIR;
GPC; viscoelasticity; dynamic modulus
1. Introduction
Asphalt aging is one of the factors leading to the deterioration of pavement perfor-
mance [
1
]. Currently, researchers have begun to explore the effects of asphalt aging on its
performance through numerous laboratory experiments [
2
,
3
]. Studies have demonstrated
that the complex modulus of SBS modified asphalt increases with aging time while the
phase angle decreases [
4
,
5
]. This trend results in reduced adhesion and the onset of stress
relaxation in SBS modified asphalt. The study found that the addition of polymers can
Polymers 2025,17, 698 https://doi.org/10.3390/polym17050698

Polymers 2025,17, 698 2 of 20
achieve three effects: first, it reduces the problem of surface raveling; second, it slows down
the aging rate of the matrix asphalt; and third, it enhances the aging performance of the
polymer-modified asphalt [6–9].
During the aging process of asphalt, short-chain compounds participate in addition
and polymerization reactions, resulting in the formation of longer-chain compounds. The
transformation of aromatic fractions and resins into asphalt contributes to the leading to
asphalt hardening and embrittlement of the material [
10
,
11
]. Modified asphalt obtained by
incorporating SBS and SBR into the base asphalt is used due to its excellent performance [
12
].
The presence of carbonyl compounds and sulfur oxides in aged asphalt contributes to
a significant reduction in the effectiveness of SBS and SBR modifiers. Aged modified
asphalt exhibits increased hardness and a notable decrease in low-temperature ductility,
which reflects the combined effects of oxidation of asphalt molecules and degradation of
compounds [11,13–15].
The inclusion of suitable components can enhance both high-temperature performance
and cohesiveness [
16
]. Polystyrene can improve the low-temperature performance and
adhesion of the CMA [
17
], while polyurethane enhances water stability [
18
], PPA improved
the SBS modified asphalt’s resistance to permanent deformation and enhanced their elastic
recovery properties [
19
,
20
]. After adding PPA, the penetration and ductility of composite
modified asphalt were reduced, while the softening point, rutting factor and viscosity
increased [
21
]. PPA prompted the clustering of SBS particles and confined the swelling
of SBS [
22
]. The SBS/SBR modified asphalts with the addition of PPA show significantly
better high-temperature properties, the ability of asphalt to resist rutting is improved, and
the elastic recovery is increased [
23
]. Although these materials have improved various
properties to different extent, their workability and stability are poor in cold weather, partic-
ularly during the winter snowfall season. This suggests that the performance enhancement
of cold mix asphalt materials through PPA/SBS or PPA/SBR additives is limited. While the
use of a single, PPA/SBS or PPA/SBR additives can partially address the problems of low
strength and poor water stability, there is insufficient mention of whether the durability of
cold mix asphalt materials is improved under aging conditions. Therefore, it is necessary
to investigate the durability of cold patch materials under aging conditions.
In existing research, there are numerous evaluation methods for assessing the impact of
multi-field coupling effects on the performance of asphalt mixtures. Guo analyzed the adhe-
sion failure patterns of asphalt mixtures under temperature–humidity coupling conditions
using digital image processing technology [
24
]. Yang investigated the aging characteristics
of polymer-modified asphalt under temperature–irradiation coupling conditions through
multi-scale morphological characterization techniques [
25
,
26
]. However, the diverse eval-
uation methods face limitations in their promotion and application processes [
27
–
31
].
Therefore, focusing on the stress–strain response of asphalt mixtures under multi-field
coupling environments and exploring the viscoelastic evolution of these mixtures can
provide precise quantitative measurements of the effects of multi-field coupling [32].
Viscoelasticity is one of the essential properties that reflect the stress–strain response
of materials. In the study of asphalt mixtures, dynamic and static load tests are commonly
employed to evaluate the linear viscoelastic behavior of the mixtures [
1
]. Under dynamic
loading, specimens are typically cylindrical with dimensions of
Φ
100 mm
×
h150 mm, and
their performance is assessed through dynamic modulus testing. Under static loading, creep
or relaxation tests are usually conducted to maintain a constant stress or strain in the asphalt
mixture, characterizing its creep and stress relaxation behavior under prolonged loading or
relaxation conditions. Proposed predictive methods for the viscoelastic behavior of asphalt
mixtures under aging conditions operate through statistical analysis and the development of
empirical equations [
33
–
36
]. In the conversion and analysis of viscoelasticity under dynamic

Polymers 2025,17, 698 3 of 20
and static loads, extensively explored numerical methods are available for studying the
conversion of viscoelastic properties under dynamic and static loading conditions, along
with their accuracy in transformation [37–43].
GPC and FTIR are commonly used to analyze the composition and functional group
changes in asphalt. Studies have shown that polymer-modified asphalt undergoes an
increase in high molecular weight molecules and a decrease in low and medium molecular
weight molecules after aging. During the aging process of SBS molecules, degradation into
smaller molecules also occurs [4,5,7,44,45].
In summary, while scholars have conducted extensive research on the viscoelasticity
of asphalt mixtures under aging conditions, the results have varied significantly, and
there is a lack of microscopic analysis on the viscoelasticity of CMA. To address this gap,
this study conducted indoor aging tests on CMA mixtures using RTFOT and PAV aging
protocols to obtain CMA binders and mixtures at different stages of aging. Microscopic
analysis was performed using FTIR and GPC to qualitatively and quantitatively analyze the
characteristic functional groups and molecular weight of CMA. Additionally, rheological
performance analysis included basic performance testing and temperature scanning tests.
For the viscoelastic analysis of the mixtures, dynamic modulus testing was performed under
dynamic loading conditions. Through the integration of microscopic analysis, rheological
performance analysis, and viscoelastic analysis, this study reveals the evolution patterns of
the viscoelasticity of CMA materials under various aging conditions.
2. Materials and Methods
2.1. Aging Tests
2.1.1. RTFOT Test
The RTFOT origin is Hangzhou City, Zhejiang Province, China. The BA and CMA
were placed on the aging tray to simulate short-time thermal oxidative aging of the asphalt
(163
◦
C, 5 h). BA is denoted as
BARTFOT
after the RTFOT, while CMA after the RTFOT is
referred to as CMARTFOT.
2.1.2. PAV Test
The PAV 20-h and PAV 40-h aging tests were carried out to simulate asphalt aging after
5–8 years and 8–12 years of service life, respectively [
46
], under experimental conditions
(2.1 MPa, 100
◦
C). BA after 20 h and 40 h of PAV is designated as
BAPAV20
and
BAPAV40
,
respectively. Similarly, CMA after 20 h and 40 h of PAV is referred to as
CMAPAV20
and
CMAPAV40, respectively.
2.2. Asphalt and Asphalt Mixture
2.2.1. Asphalt
This study utilized one type of base asphalt (BA) and one type of CMA, PetroChina
Karamay Petrochemical Co., Ltd. 90# (Grade A) road asphalt was chosen as the BA and
served as the control group. Based on prior research [
12
], CMA was obtained by modifying
BA. The results of the main performance indicators for the two types of asphalt before and
after RTFOT are presented in Figure 1.
2.2.2. Asphalt Mixture
Asphalt Mixture Design and Forming
Crushed stone and manufactured sand were sourced from Rongxiang Piez Stone Mine
in Yanqi County, Xinjiang, China, while the mineral powder was produced by Shuo Zheng
Mining Co., Ltd. in Hezhuo County, Xinjiang, China. The raw material testing results
were conducted in accordance with the requirements of the Technical Standard for Testing

Polymers 2025,17, 698 4 of 20
Aggregates in Highway Engineering (JTG 3432-2024 [
47
]) and the Technical Specifications
for Construction of Asphalt Pavements (JTG F40-2004 [
48
]). A summary of the test results
for crushed stone, manufactured sand, and mineral powder is presented in Tables 1and 2.
Polymers 2025, 17, x FOR PEER REVIEW 4 of 21
Figure 1. Main performance indexes of BA and CMA before and after RTFOT.
2.2.2. Asphalt Mixture
Asphalt Mixture Design and Forming
Crushed stone and manufactured sand were sourced from Rongxiang Piez Stone
Mine in Yanqi County, Xinjiang, China, while the mineral powder was produced by Shuo
Zheng Mining Co., Ltd. in Hezhuo County, Xinjiang, China. The raw material testing re-
sults were conducted in accordance with the requirements of the Technical Standard for
Testing Aggregates in Highway Engineering (JTG 3432-2024 [47]) and the Technical Spec-
ifications for Construction of Asphalt Pavements (JTG F40-2004 [48]). A summary of the
test results for crushed stone, manufactured sand, and mineral powder is presented in
Tables 1 and 2.
Table 1. Test results of crushed stone, machine-made sand, and mineral powder.
Detecting Parameter Apparent Rela-
tive Density
Relative Density
of Gross Volume
Water Absorption
(%)
Detection
Result
Sample Name
Gravel (15–20) mm 2.871 2.842 0.28
Qualified
Gravel (10–15) mm 2.862 2.827 0.43
Gravel (5–10) mm 2.855 2.803 0.64
Gravel (3–5) mm 2.849 2.794 0.69
Machine sand (0–3) mm 2.821 2.755 /
Mineral powder 2.718 / /
Technical requirements
Gravel ≮ 2.60
/ ≯2.0 / Machine sand
≮ 2.50
Table 2. Test results of mineral powder.
Lime-
stone Property Unit Tested Values Code Values
Mineral
powder
Apparent density(kg·m−3) g/cm3 2.718 ≥2.50
Water carrying capacity (%) % 0.104 ≤1
Passing percentage (%)
0.6 mm 100 100
<0.15 mm 98.1 90~100
<0.075 mm 87.2 75~100
Appearance / No agglomerates No agglomerates
Hydrophilic coefficient / 0.4 <1
Plasticity index (%) % 2.56 <4
Figure 1. Main performance indexes of BA and CMA before and after RTFOT.
Table 1. Test results of crushed stone, machine-made sand, and mineral powder.
Detecting Parameter Apparent Relative
Density
Relative Density
of Gross Volume
Water Absorption
(%) Detection Result
Sample Name
Gravel (15–20) mm 2.871 2.842 0.28
Qualified
Gravel (10–15) mm 2.862 2.827 0.43
Gravel (5–10) mm 2.855 2.803 0.64
Gravel (3–5) mm 2.849 2.794 0.69
Machine sand (0–3) mm 2.821 2.755 /
Mineral powder 2.718 / /
Technical requirements Gravel <2.60 />2.0 /
Machine sand <2.50
Table 2. Test results of mineral powder.
Limestone Property Unit Tested Values Code Values
Mineral powder
Apparent density(kg·m−3) g/cm32.718 ≥2.50
Water carrying capacity (%) % 0.104 ≤1
Passing percentage (%)
0.6 mm 100 100
<0.15 mm 98.1 90~100
<0.075 mm 87.2 75~100
Appearance / No agglomerates No agglomerates
Hydrophilic coefficient / 0.4 <1
Plasticity index (%) % 2.56 <4
The asphalt mixture proportions included two types of asphalt (BA, CMA) and two
gradations (AC-13, AC-16). The road performance of the CMA mixture was evaluated
through water stability tests, rutting tests, and low-temperature cracking resistance tests,
and subsequently compared with that of the BA mixture. Two types of BA mixtures are
denoted as
BAAC−13
and
BAAC−16
, while CMA is referred to as
CMAAC−13
and
CMAAC−16
.
The asphalt mixture was initially formed using a rotary compactor to achieve the
specimen specification (
Φ
150 mm
×
h170 mm). After demolding, the mixture was placed

Polymers 2025,17, 698 5 of 20
at room temperature for 48 h, then cored and cut into specimens (
Φ
100 mm
×
h150 mm)
(Figure 2).
Polymers 2025, 17, x FOR PEER REVIEW 5 of 21
The asphalt mixture proportions included two types of asphalt (BA, CMA) and two
gradations (AC-13, AC-16). The road performance of the CMA mixture was evaluated
through water stability tests, ruing tests, and low-temperature cracking resistance tests,
and subsequently compared with that of the BA mixture. Two types of BA mixtures are
denoted as BA and BA , while CMA is referred to as CMA and
CMA.
The asphalt mixture was initially formed using a rotary compactor to achieve the
specimen specification (Φ150 mm × h170 mm). After demolding, the mixture was placed
at room temperature for 48 h, then cored and cut into specimens (Φ100 mm × h150 mm)
(Figure 2).
(a) (b)
Figure 2. The designed gradation curves: (a) AC-13; (b) AC-16.
Asphalt Mixture Viscoelasticity Test
The viscoelastic properties of the CMA mixture were tested using a UTM-130 testing
machine. The dynamic modulus test was employed to characterize the viscoelastic behav-
ior. Five experimental temperatures were used (−10, 4.4, 21.1, 37.8, and 54.4 °C), and six
loading frequencies were used (0.1, 0.5, 1, 5, 10, and 25 Hz) [31,36]. The dynamic modulus
test is a non-destructive testing method (Figure 3).
Figure 3. Dynamic modulus test.
2.3. Dynamic Shear Rheometer (DSR) Test
DSR tests were analyzed using the CV0150 AR1500ex manufactured by TA Instru-
ments, focusing primarily on the variations in G∗ and δ, as well as G∗/sinδ.
2.4. Structural Characterization
2.4.1. FTIR Test
Figure 2. The designed gradation curves: (a) AC-13; (b) AC-16.
Asphalt Mixture Viscoelasticity Test
The viscoelastic properties of the CMA mixture were tested using a UTM-130 testing
machine. The dynamic modulus test was employed to characterize the viscoelastic behavior.
Five experimental temperatures were used (
−
10, 4.4, 21.1, 37.8, and 54.4
◦
C), and six loading
frequencies were used (0.1, 0.5, 1, 5, 10, and 25 Hz) [
31
,
36
]. The dynamic modulus test is a
non-destructive testing method (Figure 3).
Polymers 2025, 17, x FOR PEER REVIEW 5 of 21
The asphalt mixture proportions included two types of asphalt (BA, CMA) and two
gradations (AC-13, AC-16). The road performance of the CMA mixture was evaluated
through water stability tests, ruing tests, and low-temperature cracking resistance tests,
and subsequently compared with that of the BA mixture. Two types of BA mixtures are
denoted as BA and BA , while CMA is referred to as CMA and
CMA.
The asphalt mixture was initially formed using a rotary compactor to achieve the
specimen specification (Φ150 mm × h170 mm). After demolding, the mixture was placed
at room temperature for 48 h, then cored and cut into specimens (Φ100 mm × h150 mm)
(Figure 2).
(a) (b)
Figure 2. The designed gradation curves: (a) AC-13; (b) AC-16.
Asphalt Mixture Viscoelasticity Test
The viscoelastic properties of the CMA mixture were tested using a UTM-130 testing
machine. The dynamic modulus test was employed to characterize the viscoelastic behav-
ior. Five experimental temperatures were used (−10, 4.4, 21.1, 37.8, and 54.4 °C), and six
loading frequencies were used (0.1, 0.5, 1, 5, 10, and 25 Hz) [31,36]. The dynamic modulus
test is a non-destructive testing method (Figure 3).
Figure 3. Dynamic modulus test.
2.3. Dynamic Shear Rheometer (DSR) Test
DSR tests were analyzed using the CV0150 AR1500ex manufactured by TA Instru-
ments, focusing primarily on the variations in G∗ and δ, as well as G∗/sinδ.
2.4. Structural Characterization
2.4.1. FTIR Test
Figure 3. Dynamic modulus test.
2.3. Dynamic Shear Rheometer (DSR) Test
DSR tests were analyzed using the CV0150 AR1500ex manufactured by TA Instru-
ments, focusing primarily on the variations in G∗and δ, as well as G∗/sinδ.
2.4. Structural Characterization
2.4.1. FTIR Test
FTIR was employed to analyze the functional groups of the binder before and after
aging. The FTIR test was performed using the Nicolet iS20 Fourier-transform infrared
spectrometer produced by Thermo Scientific in the USA. The infrared spectra wavenumber
range was set as 400~4000 cm−1(4 cm−1, 32 scans).
In order to quantitatively analyze the changes in the characteristic functional groups
of rubber asphalt recycled binder before and after aging, the area of the C-H stretching
vibration peaks within the ranges of 1325~1480 cm
−1
and 2740~3000 cm
−1
was used as
a baseline. The carbonyl factor (
IC=O
), sulfoxide factor (
IS=O
), long-chain factor (
ILC
),

Polymers 2025,17, 698 6 of 20
branched-chain factor (
IBC
), aromatic factor (
IAroma
), and naphthenic factor (
IAli ph a
) are
defined as follows, where the area A represents the integral of the infrared spectrum at the
specified wavenumbers before and after aging [49].
IC=O=A1673.0−1718.5/A1325.0−1480.0, 2740.0−3000.0, (1)
IS=O=A981.7−1057.0/A1325.0−1480.0, 2740.0−3000.0, (2)
ILC=A704.0−734.8/A1325.0−1480.0, 2740.0−3000.0, (3)
IBC=A1325.1−1392.6/A1325.0−1480.0, 2740.0−3000.0, (4)
IAroma=A1517.9−1637.5/A1325.0−1480.0, 2740.0−3000.0, (5)
IAli ph a=A1325.1−1392.6,1392.6−1479.4/A1325.0−1480.0, 2740.0−3000.0, (6)
2.4.2. GPC Test
Asphalt analysis was conducted using the Agilent 1260 Infinity II Gel Permeation
Chromatograph and associated software manufactured by Agilent Technologies in the
Santa Clara, CA, USA. Tetrahydrofuran (THF) was used as the mobile phase, and three
chromatographic columns were connected in series to wash asphalt molecules based on
their molecular sizes.
The chromatograms were divided into 13 segments and categorized into three groups,
large molecule size compounds (LMS, sections 1–5), medium molecule size compounds
(MMS, sections 6–9), and small molecule size compounds (SMS, sections 9–13), for further
analysis of the compositional changes of the two types of asphalt before and after aging.
Asphalt molecules are often dispersed in nature. To characterize their molecular
weight and molecular weight distribution, the commonly used measures include the
number-average molecular weight (
Mn
), weight-average molecular weight (
Mw
), and
polydispersity index (PDI). Here,
Ni
represents the number of molecules with molecular
weight
Mi
(g/mol), while
Wi
denotes the quantity of components with molecular weight
Mi(g/mol).
Mn=∑Wi/∑Wi∗Mi−1, (7)
Mw=∑Wi∗Mi/∑Wi, (8)
PDI =Mw/Mn, (9)
3. Results
3.1. Test Results of Asphalt Rheological Properties
The complex modulus (
G∗
) represents the capability of a material to resist shear
deformation, while the phase angle (
δ
) signifies the ratio of viscous (non-recoverable) to
elastic (recoverable) components. The
G∗
aging index
CMAI
and
δ
aging index
PAAI
were
used to evaluate the aging degree of the two types of asphalt. A higher
CMAI
and a lower
PAAI indicate more severe aging. The calculations for CMAI and PAAI are as follows:
CMA I =G∗1/G∗2, (10)
PAAI =δ1/δ2, (11)
Equation: G∗1, G∗2: G∗before and after aging; δ1,δ1:δbefore and after aging.
Figure 4a–c depict the
G∗
,
δ
, and
G∗/sinδ
of BA and CMA before and after aging.
It can be observed from Figure 4a–c that the
G∗
of both asphalt samples decreases with
increasing temperature. Contrastingly, the
G∗
values of both asphalt samples significantly

Polymers 2025,17, 698 7 of 20
increased after RTFOT aging compared to the unaged samples, with CMA showing a more
pronounced increase in
G∗
values than BA. Following RTFOT aging, the
δ
curves of the
two asphalt samples exhibited distinct trends. A reduction in
δ
signifies a decrease in the
ratio of viscous to elastic components, indicating an increase in the stiffness of the asphalt.
Post-RTFOT aging, BA demonstrated a higher δcompared to CMA
Polymers 2025, 17, x FOR PEER REVIEW 7 of 21
The complex modulus (G∗) represents the capability of a material to resist shear de-
formation, while the phase angle (δ) signifies the ratio of viscous (non-recoverable) to elas-
tic (recoverable) components. The G∗ aging index C and δ aging index P were
used to evaluate the aging degree of the two types of asphalt. A higher C and a lower
PAAI indicate more severe aging. The calculations for 𝐶 and 𝑃
 are as follows:
𝐶 = 𝐺∗/𝐺∗ , (10)
𝑃
 =𝛿/𝛿, (11)
Equation: G∗, G∗: G∗ before and after aging; δ, δ
: δ before and after aging.
Figure 4a–c depict the G∗, δ, and G∗/sinδ of BA and CMA before and after aging. It
can be observed from Figure 4a–c that the G∗ of both asphalt samples decreases with in-
creasing temperature. Contrastingly, the G∗ values of both asphalt samples significantly
increased after RTFOT aging compared to the unaged samples, with CMA showing a
more pronounced increase in G∗ values than BA. Following RTFOT aging, the δ curves
of the two asphalt samples exhibited distinct trends. A reduction in δ signifies a decrease
in the ratio of viscous to elastic components, indicating an increase in the stiffness of the
asphalt. Post-RTFOT aging, BA demonstrated a higher δ compared to CMA
(a) (b)
(c) (d)
Figure 4. Rheological properties before and after RTFOT aging of BA and CMA. (a) δ; (b) G∗; (c)
G∗/sinδ; (d) CMAI and PAAI.
Figure 4d displays the changes in C and P for BA and CMA before and after
aging. It is evident from Figure 4d that at the same testing temperature, CMA exhibited
lower values within the range of 46–76 °C. After RTFOT aging, the BA C ranged be-
tween 1.1 and 1.8, while CMA ranged between 1.1 and 2.0, with a more pronounced
Figure 4. Rheological properties before and after RTFOT aging of BA and CMA. (a)
δ
; (b)
G∗
; (c)
G∗/sinδ; (d) CMAI and PAAI.
Figure 4d displays the changes in
CMAI
and
PAAI
for BA and CMA before and after
aging. It is evident from Figure 4d that at the same testing temperature, CMA exhibited
lower values within the range of 46–76
◦
C. After RTFOT aging, the BA
CMAI
ranged
between 1.1 and 1.8, while CMA ranged between 1.1 and 2.0, with a more pronounced
difference post-RTFOT aging. The trend for BA showed an initial increase followed by
stabilization in
CMAI
with temperature variation. However, CMA was followed by an
increase after 46 ◦C, indicating its superior aging resistance at high temperatures.
Figure 4d illustrates the changes in
PAAI
for BA and CMA before and after aging. It
is evident from Figure 4d that BA overall exhibited lower
PAAI
values compared to CMA.
Post-RTFOT aging, the
PAAI
values of BA slightly increased with temperature, whereas
the
PAAI
values of CMA post-RTFOT aging did not strictly increase with temperature and
showed a slight decrease within the 46–64
◦
C range, followed by a slight increase within
the 64–76 ◦C range [16,50].
Based on the observed changes in
G∗
and
δ
, it was determined that the degree of
hardening for both asphalt types was greater following RTFOT aging than prior to aging.
Both types of asphalt demonstrated strong long-term aging resistance, with CMA exhibiting
superior aging resistance at elevated temperatures compared to BA. This observation is

Polymers 2025,17, 698 8 of 20
consistent with previous studies that suggest polymer-modified additives improve aging
resistance.
3.2. FTIR Analysis
To investigate the changes in viscoelastic properties of the two types of asphalt at the
molecular structure level, FTIR spectroscopy was employed for quantitative analysis of
the carbonyl index (
Ic=o
), sulfoxide index (
Is=o
), long-chain index (
ILC
), branched index
(
IBC
), aromatic index (
Iar
), and aliphatic index (
Ial
) during the aging process of the asphalt
(Figure 5).
Polymers 2025, 17, x FOR PEER REVIEW 8 of 21
difference post-RTFOT aging. The trend for BA showed an initial increase followed by
stabilization in C with temperature variation. However, CMA was followed by an in-
crease after 46 °C, indicating its superior aging resistance at high temperatures.
Figure 4d illustrates the changes in P for BA and CMA before and after aging. It
is evident from Figure 4d that BA overall exhibited lower P values compared to CMA.
Post-RTFOT aging, the P values of BA slightly increased with temperature, whereas
the P values of CMA post-RTFOT aging did not strictly increase with temperature and
showed a slight decrease within the 46–64 °C range, followed by a slight increase within
the 64–76 °C range [16,50].
Based on the observed changes in G∗ and δ, it was determined that the degree of
hardening for both asphalt types was greater following RTFOT aging than prior to aging.
Both types of asphalt demonstrated strong long-term aging resistance, with CMA exhib-
iting superior aging resistance at elevated temperatures compared to BA. This observation
is consistent with previous studies that suggest polymer-modified additives improve ag-
ing resistance.
3.2. FTIR Analysis
To investigate the changes in viscoelastic properties of the two types of asphalt at the
molecular structure level, FTIR spectroscopy was employed for quantitative analysis of
the carbonyl index (𝐼), sulfoxide index (𝐼), long-chain index (𝐈𝐋𝐂), branched index
(I), aromatic index (𝐈𝐚𝐫), and aliphatic index (𝐈𝐚𝐥) during the aging process of the asphalt
(Figure 5).
(a)
Figure 5. FTIR spectra of BA and CMA with different aging conditions. (a) FTIR spectra of BA and
CMA with different aging conditions; (b) BA and CMA; (c) RTFOT; (d) PAV20; (e) PAV40.
The
Ic=o
and
Is=o
indices were used to characterize the degree of aging of the asphalt,
while the variations in
ILC
,
IBC
,
Iar
, and
Ial
were attributed to the evolution of the viscoelastic
properties of the asphalt.
Figure 6illustrates the trend of changes in the
Ic=o
and
Is=o
as a function of aging
degree. During the aging process, carbon oxidation occurs in the carbon atoms of the
aromatic rings near the side chains, resulting in the formation of
Ic=o
, while the sulfur
present in the asphalt undergoes oxidation to generate
Ic=o
. As the aging time increases,
the oxidation reactions in the asphalt become more pronounced, leading to higher concen-
trations of
Ic=o
and
Is=o
. The test results and analyses presented in Figure 6are consistent

Polymers 2025,17, 698 9 of 20
with these findings. Furthermore, the variations in
Ic=o
and
Is=o
for the binders modified
with SBS and SBR show a trend of initially decreasing followed by increasing, indicating a
certain degree of mitigation of the aging process of the binder. This may be attributed to the
absorption of lighter components by the SBS and SBR, indirectly slowing down the aging
of the constituents within the asphalt. Additionally, SBS and SBR can gradually release
these lighter components during the aging process, compensating for the components lost
due to aging. The polymer chains in SBS and SBR also act to obstruct oxygen penetration
into the asphalt, thereby reducing the rate of aging [51].
Polymers 2025, 17, x FOR PEER REVIEW 9 of 21
Figure 5. FTIR spectra of BA and CMA with different aging conditions. (a) FTIR spectra of BA and
CMA with different aging conditions; (b) BA and CMA; (c) RTFOT; (d) PAV20; (e) PAV40.
The 𝐼 and 𝐼 indices were used to characterize the degree of aging of the as-
phalt, while the variations in 𝐼, 𝐼, 𝐼, and 𝐼 were aributed to the evolution of the
viscoelastic properties of the asphalt.
Figure 6 illustrates the trend of changes in the 𝐼 and 𝐼 as a function of aging
degree. During the aging process, carbon oxidation occurs in the carbon atoms of the aro-
matic rings near the side chains, resulting in the formation of 𝐼, while the sulfur present
in the asphalt undergoes oxidation to generate 𝐼. As the aging time increases, the oxi-
dation reactions in the asphalt become more pronounced, leading to higher concentrations
of 𝐼 and 𝐼. The test results and analyses presented in Figure 6 are consistent with
these findings. Furthermore, the variations in 𝐼 and 𝐼 for the binders modified
with SBS and SBR show a trend of initially decreasing followed by increasing, indicating
a certain degree of mitigation of the aging process of the binder. This may be aributed to
the absorption of lighter components by the SBS and SBR, indirectly slowing down the
aging of the constituents within the asphalt. Additionally, SBS and SBR can gradually re-
lease these lighter components during the aging process, compensating for the compo-
nents lost due to aging. The polymer chains in SBS and SBR also act to obstruct oxygen
penetration into the asphalt, thereby reducing the rate of aging [51].
(a)
(b)
Figure 6. Changes in functional group indicators: (a) variation of 𝐼; (b) variation of 𝐼.
In addition to the formation of 𝐼 and 𝐼, the aging process of asphalt is accom-
panied by condensation and addition reactions, wherein small molecules aggregate to
form larger molecules. This leads to a reduction in the number of branched chains, result-
ing in longer molecular chains and an increased proportion of long chains (Figure 7). Fig-
ure 8 illustrates the trends of changes in the 𝐼 and 𝐼 during the aging process. With
increasing aging severity, 𝐼 rises while 𝐼 decreases. For 𝐼, the incorporation of SBS
and SBR has a mitigating effect on its increase, indicating that the addition of SBS and SBR
can effectively reduce the polymerization of long chains during the aging process. Con-
versely, the incorporation of SBS and SBR also slows down the reduction in I, leading
to an overall shortening of the asphalt molecular chains. As a result, the asphalt molecules
with SBS and SBR additives exhibit greater flexibility compared to those without these
modifiers.
(a)
Figure 6. Changes in functional group indicators: (a) variation of Ic=o; (b) variation of Is=o.
In addition to the formation of
Is=o
and
Ic=o
, the aging process of asphalt is accom-
panied by condensation and addition reactions, wherein small molecules aggregate to
form larger molecules. This leads to a reduction in the number of branched chains, re-
sulting in longer molecular chains and an increased proportion of long chains (Figure 7).
Figure 8illustrates
the trends of changes in the
ILC
and
IBC
during the aging process. With
increasing aging severity,
ILC
rises while
IBC
decreases. For
ILC
, the incorporation of SBS
and SBR has a mitigating effect on its increase, indicating that the addition of SBS and
SBR can effectively reduce the polymerization of long chains during the aging process.
Conversely, the incorporation of SBS and SBR also slows down the reduction in
IBC
, leading
to an overall shortening of the asphalt molecular chains. As a result, the asphalt molecules
with SBS and SBR additives exhibit greater flexibility compared to those without these
modifiers.
Polymers 2025, 17, x FOR PEER REVIEW 9 of 21
Figure 5. FTIR spectra of BA and CMA with different aging conditions. (a) FTIR spectra of BA and
CMA with different aging conditions; (b) BA and CMA; (c) RTFOT; (d) PAV20; (e) PAV40.
The 𝐼 and 𝐼 indices were used to characterize the degree of aging of the as-
phalt, while the variations in 𝐼, 𝐼, 𝐼, and 𝐼 were aributed to the evolution of the
viscoelastic properties of the asphalt.
Figure 6 illustrates the trend of changes in the 𝐼 and 𝐼 as a function of aging
degree. During the aging process, carbon oxidation occurs in the carbon atoms of the aro-
matic rings near the side chains, resulting in the formation of 𝐼, while the sulfur present
in the asphalt undergoes oxidation to generate 𝐼. As the aging time increases, the oxi-
dation reactions in the asphalt become more pronounced, leading to higher concentrations
of 𝐼 and 𝐼. The test results and analyses presented in Figure 6 are consistent with
these findings. Furthermore, the variations in 𝐼 and 𝐼 for the binders modified
with SBS and SBR show a trend of initially decreasing followed by increasing, indicating
a certain degree of mitigation of the aging process of the binder. This may be aributed to
the absorption of lighter components by the SBS and SBR, indirectly slowing down the
aging of the constituents within the asphalt. Additionally, SBS and SBR can gradually re-
lease these lighter components during the aging process, compensating for the compo-
nents lost due to aging. The polymer chains in SBS and SBR also act to obstruct oxygen
penetration into the asphalt, thereby reducing the rate of aging [51].
(a)
(b)
Figure 6. Changes in functional group indicators: (a) variation of 𝐼; (b) variation of 𝐼.
In addition to the formation of 𝐼 and 𝐼, the aging process of asphalt is accom-
panied by condensation and addition reactions, wherein small molecules aggregate to
form larger molecules. This leads to a reduction in the number of branched chains, result-
ing in longer molecular chains and an increased proportion of long chains (Figure 7). Fig-
ure 8 illustrates the trends of changes in the 𝐼 and 𝐼 during the aging process. With
increasing aging severity, 𝐼 rises while 𝐼 decreases. For 𝐼, the incorporation of SBS
and SBR has a mitigating effect on its increase, indicating that the addition of SBS and SBR
can effectively reduce the polymerization of long chains during the aging process. Con-
versely, the incorporation of SBS and SBR also slows down the reduction in I, leading
to an overall shortening of the asphalt molecular chains. As a result, the asphalt molecules
with SBS and SBR additives exhibit greater flexibility compared to those without these
modifiers.
(a)
Polymers 2025, 17, x FOR PEER REVIEW 10 of 21
(b)
Figure 7. Changes in functional group indicators: (a) variation of 𝐼𝐿𝐶 ; (b) variation of 𝐼𝐵𝐶.
During the aging process, the aliphatic components in asphalt gradually convert to
aromatic compounds, resulting in a decrease in aliphatic content and an increase in aro-
matic content, as shown in Figure 8. The aromatic formation can be demonstrated as a
combined result of two processes: the full hydrogenation of aromatic rings and the alkyl
substitution of cycloalkanes. Notably, the incorporation of SBS and SBR slows the rate of
reduction of aliphatic molecules during aging, thereby mitigating the aromatic conversion
compared to binders without these additives. This indicates that during thermal oxidative
aging, the absorption of lighter components by SBS and SBR effectively slows down part
of the aromatic conversion process of aliphatic molecules. Furthermore, the long polymer
chains of SBS and SBR provide effective barriers against the further penetration of oxygen,
thereby reducing the conversion of aliphatic to aromatic compounds [51,52].
(a)
(b)
Figure 8. Changes in functional group indicators: (a) variation of Iar; (b) variation of Ial.
Figure 9 illustrates the trends in changes of 𝐼, 𝐼, 𝐼 , 𝐼, 𝐼, and 𝐼 before
and after aging. The 𝐼 of BA decreased by 13.6%, while the 𝐼 of CMA also decreased
by 2%. Additionally, the 𝐼 of CMA decreased by 6.9%. The growth rates of the other
parameters for both BA and CMA before and after aging exceeded 9.3%, with CMA dis-
playing a remarkable increase of 136% in 𝐼 .
Figure 9. Changes in functional group indicators at different aging stages.
3.3. GPC Analysis
By using GPC, the changes in number-average molecular weight (𝑀), weight-aver-
age molecular weight (𝑀 ), and polydispersity index (PDI) of BA and CMA samples
Figure 7. Changes in functional group indicators: (a) variation of ILC ; (b) variation of IBC .
During the aging process, the aliphatic components in asphalt gradually convert
to aromatic compounds, resulting in a decrease in aliphatic content and an increase in
aromatic content, as shown in Figure 8. The aromatic formation can be demonstrated as a

Polymers 2025,17, 698 10 of 20
combined result of two processes: the full hydrogenation of aromatic rings and the alkyl
substitution of cycloalkanes. Notably, the incorporation of SBS and SBR slows the rate of
reduction of aliphatic molecules during aging, thereby mitigating the aromatic conversion
compared to binders without these additives. This indicates that during thermal oxidative
aging, the absorption of lighter components by SBS and SBR effectively slows down part
of the aromatic conversion process of aliphatic molecules. Furthermore, the long polymer
chains of SBS and SBR provide effective barriers against the further penetration of oxygen,
thereby reducing the conversion of aliphatic to aromatic compounds [51,52].
Polymers 2025, 17, x FOR PEER REVIEW 10 of 21
(b)
Figure 7. Changes in functional group indicators: (a) variation of 𝐼𝐿𝐶 ; (b) variation of 𝐼𝐵𝐶.
During the aging process, the aliphatic components in asphalt gradually convert to
aromatic compounds, resulting in a decrease in aliphatic content and an increase in aro-
matic content, as shown in Figure 8. The aromatic formation can be demonstrated as a
combined result of two processes: the full hydrogenation of aromatic rings and the alkyl
substitution of cycloalkanes. Notably, the incorporation of SBS and SBR slows the rate of
reduction of aliphatic molecules during aging, thereby mitigating the aromatic conversion
compared to binders without these additives. This indicates that during thermal oxidative
aging, the absorption of lighter components by SBS and SBR effectively slows down part
of the aromatic conversion process of aliphatic molecules. Furthermore, the long polymer
chains of SBS and SBR provide effective barriers against the further penetration of oxygen,
thereby reducing the conversion of aliphatic to aromatic compounds [51,52].
(a)
(b)
Figure 8. Changes in functional group indicators: (a) variation of Iar; (b) variation of Ial.
Figure 9 illustrates the trends in changes of 𝐼, 𝐼, 𝐼 , 𝐼, 𝐼, and 𝐼 before
and after aging. The 𝐼 of BA decreased by 13.6%, while the 𝐼 of CMA also decreased
by 2%. Additionally, the 𝐼 of CMA decreased by 6.9%. The growth rates of the other
parameters for both BA and CMA before and after aging exceeded 9.3%, with CMA dis-
playing a remarkable increase of 136% in 𝐼 .
Figure 9. Changes in functional group indicators at different aging stages.
3.3. GPC Analysis
By using GPC, the changes in number-average molecular weight (𝑀), weight-aver-
age molecular weight (𝑀 ), and polydispersity index (PDI) of BA and CMA samples
Figure 8. Changes in functional group indicators: (a) variation of Iar; (b) variation of Ial.
Figure 9illustrates the trends in changes of
Ic=o
,
Is=o
,
ILC
,
IBC
,
Iar
, and
Ial
before and
after aging. The
Iar
of BA decreased by 13.6%, while the
Iar
of CMA also decreased by 2%.
Additionally, the
IBC
of CMA decreased by 6.9%. The growth rates of the other parameters
for both BA and CMA before and after aging exceeded 9.3%, with CMA displaying a
remarkable increase of 136% in ILC .
Polymers 2025, 17, x FOR PEER REVIEW 10 of 21
(b)
Figure 7. Changes in functional group indicators: (a) variation of 𝐼𝐿𝐶 ; (b) variation of 𝐼𝐵𝐶.
During the aging process, the aliphatic components in asphalt gradually convert to
aromatic compounds, resulting in a decrease in aliphatic content and an increase in aro-
matic content, as shown in Figure 8. The aromatic formation can be demonstrated as a
combined result of two processes: the full hydrogenation of aromatic rings and the alkyl
substitution of cycloalkanes. Notably, the incorporation of SBS and SBR slows the rate of
reduction of aliphatic molecules during aging, thereby mitigating the aromatic conversion
compared to binders without these additives. This indicates that during thermal oxidative
aging, the absorption of lighter components by SBS and SBR effectively slows down part
of the aromatic conversion process of aliphatic molecules. Furthermore, the long polymer
chains of SBS and SBR provide effective barriers against the further penetration of oxygen,
thereby reducing the conversion of aliphatic to aromatic compounds [51,52].
(a)
(b)
Figure 8. Changes in functional group indicators: (a) variation of Iar; (b) variation of Ial.
Figure 9 illustrates the trends in changes of 𝐼, 𝐼, 𝐼 , 𝐼, 𝐼, and 𝐼 before
and after aging. The 𝐼 of BA decreased by 13.6%, while the 𝐼 of CMA also decreased
by 2%. Additionally, the 𝐼 of CMA decreased by 6.9%. The growth rates of the other
parameters for both BA and CMA before and after aging exceeded 9.3%, with CMA dis-
playing a remarkable increase of 136% in 𝐼 .
Figure 9. Changes in functional group indicators at different aging stages.
3.3. GPC Analysis
By using GPC, the changes in number-average molecular weight (𝑀), weight-aver-
age molecular weight (𝑀 ), and polydispersity index (PDI) of BA and CMA samples
Figure 9. Changes in functional group indicators at different aging stages.
3.3. GPC Analysis
By using GPC, the changes in number-average molecular weight (
Mn
), weight-average
molecular weight (
Mw
), and polydispersity index (PDI) of BA and CMA samples before
and after aging with RTFOT, PAV20, and PAV40 were obtained and are depicted in the
curve shown in Figure 10. Studies have shown that
Mw
is sensitive to compounds with
high molecular weights; a higher
Mw
indicates a higher amount of high-molecular-weight
substances. On the other hand,
Mn
is sensitive to compounds with low molecular weights;
a larger
Mn
indicates a higher amount of low-molecular-weight substances. The polydisper-
sity coefficient PDI represents the distribution of both high- and low-molecular-weight sub-
stances, with a larger PDI indicating a more dispersed distribution of all
molecules [53–55]
.

Polymers 2025,17, 698 11 of 20
Polymers 2025, 17, x FOR PEER REVIEW 11 of 21
before and after aging with RTFOT, PAV20, and PAV40 were obtained and are depicted
in the curve shown in Figure 10. Studies have shown that M is sensitive to compounds
with high molecular weights; a higher 𝑀 indicates a higher amount of high-molecular-
weight substances. On the other hand, 𝑀 is sensitive to compounds with low molecular
weights; a larger 𝐌𝐧 indicates a higher amount of low-molecular-weight substances. The
polydispersity coefficient PDI represents the distribution of both high- and low-molecu-
lar-weight substances, with a larger PDI indicating a more dispersed distribution of all
molecules [53–55].
Figure 10a,c show the 𝑀 distribution of BA and CMA during the aging process.
The x-axis represents the logarithmic weight-average molecular weight of asphalt, while
the y-axis represents the relative content of asphalt. Figure 10a,c demonstrate that after
RTFOT aging, PAV20 aging, and PAV40 aging, the overall molecular weight of asphalt
shifted towards larger molecules. During aging, the lighter components in asphalt volati-
lize and transform into colloids and asphaltenes. Aging ultimately leads to a decrease in
the content of lighter components in asphalt and an increase in the content of asphaltenes.
Asphaltenes are high-molecular-weight substances. Consequently, after aging, the molec-
ular weight distribution curve of asphalt gradually shifts to the right, indicating a reduc-
tion in polymer molecular weight due to the degradation of long-chain polymers [56].
(a) (b)
(c) (d)
Figure 10. GPC results of BA and CMA: (a) 𝑀 distribution of BA; (b) 𝑀 of BA of different aging
conditions; (c) 𝑀 distribution of CMA; (d) 𝑀 of CMA of different aging conditions.
Figure 10b,d reveal that after RTFOT aging and PAV20 aging, BA and CMA show
varying degrees of increase in 𝑀, 𝑀, and PDI values, indicating an increase in molec-
ular weight and dispersity of aged asphalt. This indicates a reduction in low 𝑀 compo-
nents and an increase in high 𝑀 components following RTFOT and PAV20 aging,
which enhances the shear resistance of aged asphalt and improves its high-temperature
Figure 10. GPC results of BA and CMA: (a)
Mw
distribution of BA; (b)
Mw
of BA of different aging
conditions; (c)Mwdistribution of CMA; (d)Mwof CMA of different aging conditions.
Figure 10a,c show the
Mw
distribution of BA and CMA during the aging process. The
x-axis represents the logarithmic weight-average molecular weight of asphalt, while the
y-axis represents the relative content of asphalt. Figure 10a,c demonstrate that after RTFOT
aging, PAV20 aging, and PAV40 aging, the overall molecular weight of asphalt shifted
towards larger molecules. During aging, the lighter components in asphalt volatilize and
transform into colloids and asphaltenes. Aging ultimately leads to a decrease in the content
of lighter components in asphalt and an increase in the content of asphaltenes. Asphaltenes
are high-molecular-weight substances. Consequently, after aging, the molecular weight
distribution curve of asphalt gradually shifts to the right, indicating a reduction in polymer
molecular weight due to the degradation of long-chain polymers [56].
Figure 10b,d reveal that after RTFOT aging and PAV20 aging, BA and CMA show
varying degrees of increase in
Mn
,
Mw
, and PDI values, indicating an increase in molecular
weight and dispersity of aged asphalt. This indicates a reduction in low
Mw
components
and an increase in high
Mw
components following RTFOT and PAV20 aging, which en-
hances the shear resistance of aged asphalt and improves its high-temperature performance.
The polydispersity coefficient PDI increases for both asphalt types after RTFOT aging
and PAV20 aging, indicating a more dispersed molecular distribution after these aging
processes. However, after PAV40 aging, the
Mn
,
Mw
, and PDI values of BA and CMA show
varying degrees of decrease, mainly due to degradation of long-chain polymers during the
aging process [53].
Figure 11 shows that before aging, the content of LMS in base asphalt was 36%, MMS
content was 52%, and SMS content was 12%. The content of LMS, MMS, and SMS in CMA
was 42%, 48%, and 10%, respectively. During the aging process, both types of asphalt

Polymers 2025,17, 698 12 of 20
showed a decrease in MMS and SMS content, with an increase in LMS content. This was
due to oxidation reactions during aging, leading to the conversion of non-polar components
into polar components, as well as the evaporation of SMS at high temperatures, resulting
in a reduction of lower molecular weight components in the asphalt [
4
,
57
,
58
]. This is
corroborated by the analysis in Figure 11. After RTFOT aging, the content of LMS, MMS,
and SMS in base asphalt was 40%, 50%, and 10%, respectively, while for CMA it was 43%,
47%, and 10%. Following PAV20 aging, the content of LMS, MMS, and SMS in BA was 43%,
46%, and 11%, and for CMA it was 44%, 45%, and 11%. After PAV40 aging, the content of
LMS, MMS, and SMS in BA was 46%, 44%, and 10%, and in CMA it was 51%, 41%, and
8%. The molecular weight of base asphalt and CMA increased after RTFOT aging and PAV
aging. This indicates that the aging level of asphalt is significantly more severe after PAV
aging than RTFOT aging, and CMA exhibits superior resistance to long-term hot oxygen
aging. The decrease in peak values of the SBS and SBR molecular curve after aging suggests
a reduction in the number of SBS and SBR molecules after aging, indicating that the SBS
and SBR molecules have already played a role and delayed the aging process of modified
asphalt, resulting in an overall increase in asphalt molecular mass [
57
]. The changes in
asphalt molecular weight lead to an increase in the complex modulus of asphalt at the
macro level, consistent with the changes in rheological properties of aged asphalt in this
experiment [2].
Polymers 2025, 17, x FOR PEER REVIEW 12 of 21
performance. The polydispersity coefficient PDI increases for both asphalt types after
RTFOT aging and PAV20 aging, indicating a more dispersed molecular distribution after
these aging processes. However, after PAV40 aging, the 𝑀, 𝑀, and PDI values of BA
and CMA show varying degrees of decrease, mainly due to degradation of long-chain
polymers during the aging process [53].
Figure 11 shows that before aging, the content of LMS in base asphalt was 36%, MMS
content was 52%, and SMS content was 12%. The content of LMS, MMS, and SMS in CMA
was 42%, 48%, and 10%, respectively. During the aging process, both types of asphalt
showed a decrease in MMS and SMS content, with an increase in LMS content. This was
due to oxidation reactions during aging, leading to the conversion of non-polar compo-
nents into polar components, as well as the evaporation of SMS at high temperatures, re-
sulting in a reduction of lower molecular weight components in the asphalt [4,57,58]. This
is corroborated by the analysis in Figure 11. After RTFOT aging, the content of LMS, MMS,
and SMS in base asphalt was 40%, 50%, and 10%, respectively, while for CMA it was 43%,
47%, and 10%. Following PAV20 aging, the content of LMS, MMS, and SMS in BA was
43%, 46%, and 11%, and for CMA it was 44%, 45%, and 11%. After PAV40 aging, the con-
tent of LMS, MMS, and SMS in BA was 46%, 44%, and 10%, and in CMA it was 51%, 41%,
and 8%. The molecular weight of base asphalt and CMA increased after RTFOT aging and
PAV aging. This indicates that the aging level of asphalt is significantly more severe after
PAV aging than RTFOT aging, and CMA exhibits superior resistance to long-term hot
oxygen aging. The decrease in peak values of the SBS and SBR molecular curve after aging
suggests a reduction in the number of SBS and SBR molecules after aging, indicating that
the SBS and SBR molecules have already played a role and delayed the aging process of
modified asphalt, resulting in an overall increase in asphalt molecular mass [57]. The
(a) (b)
Figure 11. Changes in molecular weight of BA and CMA before and after aging: (a) division of LMS,
MMS, and SMS; (b) molecular weight distribution results.
The aforementioned study reveals that after aging, the number of high-molecular-
weight molecules increases in CMA, while the number of low and medium-molecular-
weight molecules decreases [2–4,57].
3.4. High and Low Temperature Performance of Asphalt Mixture
3.4.1. Marshall Test
Figure 11. Changes in molecular weight of BA and CMA before and after aging: (a) division of LMS,
MMS, and SMS; (b) molecular weight distribution results.
The aforementioned study reveals that after aging, the number of high-molecular-
weight molecules increases in CMA, while the number of low and medium-molecular-
weight molecules decreases [2–4,57].
3.4. High and Low Temperature Performance of Asphalt Mixture
3.4.1. Marshall Test
The Marshall test results for the optimal asphalt-to-aggregate ratios of the
BAAC−13
,
BAAC−16, CMAAC−13 , and CMAAC−16 asphalt mixtures are presented in Figure 12.
As shown in Figure 12, the voids ratio of the asphalt mixture specimen (VV), aggregate
voids ratio of the asphalt mixture specimen (VMA), optimal asphalt content (OAC), and
the incorporation of SBS and SBR increased the stability value and OAC value of the
BA mixture, decreasing the VV value and VMA value. As previously mentioned, this
is attributed to the addition of polymers, which enhances the adhesion of BA, thereby
reducing the stability values [
41
,
42
,
44
] and increasing the maximum load that the CMA
mixture can bear, which is reflected in the stability value. Additionally, an increase in

Polymers 2025,17, 698 13 of 20
asphalt content may lead to a higher proportion of free asphalt, which could also decrease
the Marshall stability value of the asphalt mixture.
Polymers 2025, 17, x FOR PEER REVIEW 13 of 21
The Marshall test results for the optimal asphalt-to-aggregate ratios of the BA,
BA, CMA, and CMA asphalt mixtures are presented in Figure 12.
Figure 12. Marshall test results.
As shown in Figure 12, the voids ratio of the asphalt mixture specimen (VV), aggre-
gate voids ratio of the asphalt mixture specimen (VMA), optimal asphalt content (OAC),
and the incorporation of SBS and SBR increased the stability value and OAC value of the
BA mixture, decreasing the VV value and VMA value. As previously mentioned, this is
aributed to the addition of polymers, which enhances the adhesion of BA, thereby re-
ducing the stability values [41,42,44] and increasing the maximum load that the CMA
mixture can bear, which is reflected in the stability value. Additionally, an increase in as-
phalt content may lead to a higher proportion of free asphalt, which could also decrease
the Marshall stability value of the asphalt mixture.
3.4.2. Analysis of Road Performance
A comparative performance analysis was conducted between the CMA mixture and
the BA mixture, focusing on their high-temperature stability (dynamic stability (DS)), low-
temperature cracking resistance (bending tensile strength (𝑅); maximum bending tensile
strain (𝜀), flexural stiffness modulus (𝑆)), water stability (residual stability (𝑀), and
freeze–thaw spliing strength ratio (TSR)), among other road performance characteristics.
Figure 13 presents the results of road performance tests for four types of asphalt mix-
tures. The DS values for the four mixtures were 1135, 1055, 3365, and 3172 times/mm, all
of which meet the highest requirements specified in the JTGE20-2011 standard [59],
namely, greater than 800 times and 2800 times/mm. The results indicate that the polymer
network crosslinking structure inherent in the CMA system enhances the stiffness and
high-temperature stability of the CMA mixture, resulting in excellent high-temperature
ruing resistance.
Figure 12. Marshall test results.
3.4.2. Analysis of Road Performance
A comparative performance analysis was conducted between the CMA mixture and
the BA mixture, focusing on their high-temperature stability (dynamic stability (DS)), low-
temperature cracking resistance (bending tensile strength (
RB
); maximum bending tensile
strain (
εB
), flexural stiffness modulus (
SB
)), water stability (residual stability (
MS0
), and
freeze–thaw splitting strength ratio (TSR)), among other road performance characteristics.
Figure 13 presents the results of road performance tests for four types of asphalt
mixtures. The DS values for the four mixtures were 1135, 1055, 3365, and 3172 times/mm,
all of which meet the highest requirements specified in the JTGE20-2011 standard [
59
],
namely, greater than 800 times and 2800 times/mm. The results indicate that the polymer
network crosslinking structure inherent in the CMA system enhances the stiffness and
high-temperature stability of the CMA mixture, resulting in excellent high-temperature
rutting resistance.
The
RB
of
CMAAC−16
is 1.3 times that of
BAAC−16
. Additionally, the
εB
for the four as-
phalt mixtures is ranked from smallest to largest as follows:
BAAC−13
<
BAAC−16
<
CMAAC−13
<
CMAAC−16
. The results indicate that the low-temperature cracking resistance of the CMA
mixtures is superior to that of the BA mixtures. The incorporation of SBS and SBR has im-
proved the low-temperature performance of the CMA mixtures, achieving an enhancement of
133% compared to the BA mixtures, thereby meeting the highest requirement specified in the
standards, which is greater than 2800.

Polymers 2025,17, 698 14 of 20
Polymers 2025, 17, x FOR PEER REVIEW 14 of 21
The 𝑅 of CMA is 1.3 times that of BA. Additionally, the 𝜀 for the four
asphalt mixtures is ranked from smallest to largest as follows: BA < BA <
CMA < CMA. The results indicate that the low-temperature cracking resistance
of the CMA mixtures is superior to that of the BA mixtures. The incorporation of SBS and
SBR has improved the low-temperature performance of the CMA mixtures, achieving an
enhancement of 133% compared to the BA mixtures, thereby meeting the highest require-
ment specified in the standards, which is greater than 2800.
The 𝑀 and the TSR for all four asphalt mixtures meet the standard’s highest re-
quirements, specifically, residual stability greater than 85% and a TSR greater than 80%.
The 𝑀 value for CMA is notably higher, exceeding that of BA by 10%. Fur-
thermore, a comparison of TSR indicates that the 𝑀 of CMA asphalt mixtures is signif-
icantly greater than 10% in the BA mixtures. These results demonstrate that CMA mix-
tures exhibit excellent water stability, and the addition of SBS and SBR significantly en-
hances the adhesion and aggregate coating performance of CMA. This, in turn, reduces
the susceptibility of the modified CMA asphalt system to moisture and improves the re-
sistance of the CMA mixtures to water damage and frost.
Figure 13. Results of the analysis of road performance indicators.
3.5. Viscoelasticity
For the dynamic modulus (E∗) of the four types of asphalt mixtures, an improved
sigmoid model was used to establish the master curve of E∗ at the reference temperature
[34,38]. The sigmoidal model is expressed as follows in Equation (12):
𝑙𝑔|𝐸∗|=𝜑+𝛼/1+𝑒
󰇛
󰇜, (12)
Equation: |E∗| is the dynamic modulus, MPa; 𝜑 is the logarithm of the minimum
value of the E∗; 𝛼 is the logarithm of the difference between the maximum and minimum
Figure 13. Results of the analysis of road performance indicators.
The
MS0
and the TSR for all four asphalt mixtures meet the standard’s highest require-
ments, specifically, residual stability greater than 85% and a TSR greater than 80%. The
MS0
value for
CMAAC−16
is notably higher, exceeding that of
BAAC−16
by 10%. Furthermore, a
comparison of TSR indicates that the
MS0
of CMA asphalt mixtures is significantly greater
than 10% in the BA mixtures. These results demonstrate that CMA mixtures exhibit excel-
lent water stability, and the addition of SBS and SBR significantly enhances the adhesion
and aggregate coating performance of CMA. This, in turn, reduces the susceptibility of
the modified CMA asphalt system to moisture and improves the resistance of the CMA
mixtures to water damage and frost.
3.5. Viscoelasticity
For the dynamic modulus (
E∗
) of the four types of asphalt mixtures, an im-
proved sigmoid model was used to establish the master curve of
E∗
at the reference
temperature [34,38]. The sigmoidal model is expressed as follows in Equation (12):
lg|E∗|=φ+α/1+eβ+(γl g f r), (12)
Equation: |
E∗
| is the dynamic modulus, MPa;
φ
is the logarithm of the minimum
value of the
E∗
;
α
is the logarithm of the difference between the maximum and minimum
E∗
values;
β
and
γ
are parameters describing the shape of the master curve. The
l g f r
as
shown in Equation (13):
l g f r=lg f +lgαT, (13)
where
l g f r
is the reduced frequency at the reference temperature, Hz;
lgαT
is the shift
factor, a function of temperature T; and fis the frequency, Hz.
Figures 14 and 15 show that the
E∗
of asphalt mixtures gradually increase with fre-
quency at the same temperature, while at the same frequency, the
E∗
decreases as temper-

Polymers 2025,17, 698 15 of 20
ature rises. As the temperature increases, the elasticity of the asphalt mixture weakens,
and its viscosity enhances, causing the material properties to lean more towards viscous
behavior. Consequently, the
E∗
of the asphalt mixture decreases, making it less capable of
resisting permanent deformation. Under dynamic loading conditions, the simultaneous
incorporation of SBS and SBR into CMA mixtures increases the dynamic modulus in the 25
Hz and
−
10
◦
C range by 24.3% (AC-13) and 15.4% (AC-16), and reduces the
φ
by 55.8%
(AC-13) and 40% (AC-16).
Polymers 2025, 17, x FOR PEER REVIEW 15 of 21
E∗ values; 𝛽 and 𝛾 are parameters describing the shape of the master curve. The 𝑙𝑔𝑓

as shown in Equation (13):
𝑙𝑔𝑓
= 𝑙𝑔𝑓 + 𝑙𝑔𝛼, (13)
where 𝑙𝑔𝑓
 is the reduced frequency at the reference temperature, Hz; 𝑙𝑔𝛼 is the shift
factor, a function of temperature T; and f is the frequency, Hz.
Figures 14 and 15 show that the E∗ of asphalt mixtures gradually increase with fre-
quency at the same temperature, while at the same frequency, the E∗ decreases as tem-
perature rises. As the temperature increases, the elasticity of the asphalt mixture weakens,
and its viscosity enhances, causing the material properties to lean more towards viscous
behavior. Consequently, the E∗ of the asphalt mixture decreases, making it less capable
of resisting permanent deformation. Under dynamic loading conditions, the simultaneous
incorporation of SBS and SBR into CMA mixtures increases the dynamic modulus in the
25 Hz and −10 °C range by 24.3% (AC-13) and 15.4% (AC-16), and reduces the 𝜑 by 55.8%
(AC-13) and 40% (AC-16).
(a) (b)
(c) (d)
Figure 14. E∗: (a) BAAC−13 ; (b) BAAC−16; (c) CMAAC−13; (d) CMAAC−16.
Taking CMA as an example, the variation of E∗ with temperature exhibits a
significant nonlinear characteristic. Taking 0.5 Hz as an example, the E∗ value at 54.4 °C
is only 8.6% of that at 21.1 °C. As the temperature increases (or the frequency decreases),
the asphalt mixture softens and approaches a more viscous state, resulting in a decrease
in E∗ and an increase in the phase angle δ. Under high-temperature conditions (or low-
frequency conditions), the influence of asphalt on the mixture diminishes; at this point,
the primary factor driving the modulus variation of the asphalt mixture is the interlocking
Figure 14. E∗: (a) BAAC−13 ; (b) BAAC−16; (c) CMAAC−13; (d) CMAAC−16.
Taking
CMAAC−16
as an example, the variation of
E∗
with temperature exhibits a
significant nonlinear characteristic. Taking 0.5 Hz as an example, the
E∗
value at 54.4
◦
C is
only 8.6% of that at 21.1
◦
C. As the temperature increases (or the frequency decreases), the
asphalt mixture softens and approaches a more viscous state, resulting in a decrease in
E∗
and an increase in the phase angle
δ
. Under high-temperature conditions (or low-frequency
conditions), the influence of asphalt on the mixture diminishes; at this point, the primary
factor driving the modulus variation of the asphalt mixture is the interlocking force of the
aggregates. Further increases in temperature (or reductions in frequency) lead to a decrease
in φ, as well as a reduction in E∗, although the values begin to stabilize.
At the same temperature and frequency, the
E∗
of BA is lower than that of CMA, and
the change rate of the
E∗
of the CMA mixture in response to temperature is smaller than
that of BA. In the low-frequency range of 0.1 Hz to 1 Hz, there is a significant increase in
the
E∗
values; however, this increase becomes more gradual between 1 Hz and 25 Hz. This
is mainly because, at higher loading frequencies, the asphalt pavement approaches elastic
deformation, preventing further increases in the
E∗
. Between
−
10
◦
C and 21.1
◦
C, the
E∗

Polymers 2025,17, 698 16 of 20
of the asphalt mixture decreases rapidly, indicating that the E∗is considerably affected by
temperature in the low-temperature range, while the impact diminishes after 21.1 ◦C.
Polymers 2025, 17, x FOR PEER REVIEW 16 of 21
force of the aggregates. Further increases in temperature (or reductions in frequency) lead
to a decrease in φ, as well as a reduction in E∗, although the values begin to stabilize.
At the same temperature and frequency, the E∗ of BA is lower than that of CMA,
and the change rate of the E∗ of the CMA mixture in response to temperature is smaller
than that of BA. In the low-frequency range of 0.1 Hz to 1 Hz, there is a significant increase
in the E∗ values; however, this increase becomes more gradual between 1 Hz and 25 Hz.
This is mainly because, at higher loading frequencies, the asphalt pavement approaches
elastic deformation, preventing further increases in the E∗. Between −10 °C and 21.1 °C,
the E∗ of the asphalt mixture decreases rapidly, indicating that the E∗ is considerably
affected by temperature in the low-temperature range, while the impact diminishes after
21.1 °C.
(a) (b)
(c) (d)
Figure 15. φ: (a) BAAC−13; (b) BAAC−16; (c) CMAAC−13; (d) CMAAC−16.
Analyzing the variation of E∗ with temperature (frequency) reveals that at higher
temperatures (or lower frequencies), the E∗ value decreases and approaches the mini-
mum value E
∗ . Conversely, at lower temperatures (or higher frequencies), the E∗
value increases and approaches the maximum value E
∗. The generalized logarithmic
sigmoidal model effectively captures the variation characteristics of E∗.
The sigmoid model demonstrates a good fiing performance, with an average coef-
ficient of determination (R) exceeding 0.9445. Figures 14–16 illustrate the E∗, φ, and mas-
ter curve of E∗. As the test frequency decreases, the phase angle initially increases and
then decreases, indicating the presence of an inflection point in the master curve. This
behavior is aributed to the greater viscosity of the binder at low frequencies and high
Figure 15. φ: (a) BAAC−13 ; (b) BAAC−16; (c) CMAAC−13; (d) CMAAC−16.
Analyzing the variation of
E∗
with temperature (frequency) reveals that at higher
temperatures (or lower frequencies), the
E∗
value decreases and approaches the minimum
value
E∗
min
. Conversely, at lower temperatures (or higher frequencies), the
E∗
value in-
creases and approaches the maximum value
E∗
max
. The generalized logarithmic sigmoidal
model effectively captures the variation characteristics of E∗.
The sigmoid model demonstrates a good fitting performance, with an average coeffi-
cient of determination (
R2
) exceeding 0.9445. Figures 14–16 illustrate the
E∗
,
φ
, and master
curve of
E∗
. As the test frequency decreases, the phase angle initially increases and then
decreases, indicating the presence of an inflection point in the master curve. This behavior
is attributed to the greater viscosity of the binder at low frequencies and high temperatures,
where the aggregate skeleton responds to external stress and strain. As the test frequency
gradually increases, the binder exhibits more elastic behavior under low-temperature and
high-frequency conditions, leading to a gradual decrease in phase angle and an increase in
dynamic modulus. The generalized logarithmic sigmoidal model can be utilized to com-
pute the master curve of dynamic viscoelastic parameters for the CMA mixture, enabling a
comprehensive expression of the dynamic mechanical characteristics of the CMA mixture.

Polymers 2025,17, 698 17 of 20
Polymers 2025, 17, x FOR PEER REVIEW 17 of 21
temperatures, where the aggregate skeleton responds to external stress and strain. As the
test frequency gradually increases, the binder exhibits more elastic behavior under low-
temperature and high-frequency conditions, leading to a gradual decrease in phase angle
and an increase in dynamic modulus. The generalized logarithmic sigmoidal model can
be utilized to compute the master curve of dynamic viscoelastic parameters for the CMA
mixture, enabling a comprehensive expression of the dynamic mechanical characteristics
of the CMA mixture.
Figure 16. Master curve of E∗: (a) BAAC−13; (b) BAAC−16; (c) CMAAC−13; (d) CMAAC−16 .
4. Conclusions
This paper investigates the viscoelastic evolution of CMA materials through micro-
scopic analysis, rheological testing, and the viscoelastic analysis of mixtures, leading to
the following key conclusions:
(1) In terms of molecular structure, the findings indicate that the incorporation of SBS
and SBR increases the levels of carbonyl and sulfoxide factors while reducing the level of
long-chain factors, which slows the rate of change in large molecule content and enhances
the aging resistance of the asphalt. Regarding molecular weight components, the addition
of SBS and SBR reduces the rate of change of LMS by more than 6%, while also mitigating
the rate of change in overall molecular weight distribution to below 50%.
(2) Under dynamic loading conditions, the simultaneous incorporation of SBS and
SBR into CMA mixtures increases the dynamic modulus in the 25 Hz and −10 °C range by
24.3% (AC-13) and 15.4% (AC-16), and reduces the 𝜑 by 55.8% (AC-13) and 40% (AC-16).
(3) This study clarifies the evolution paerns of the characteristic functional groups
and molecular weight of CMA. However, there remain limitations in revealing the visco-
elastic evolution of the binder from a compositional perspective. Future research could
Figure 16. Master curve of E∗: (a) BAAC−13 ; (b) BAAC−16 ; (c) CMAAC−13; (d) CMAAC−16.
4. Conclusions
This paper investigates the viscoelastic evolution of CMA materials through micro-
scopic analysis, rheological testing, and the viscoelastic analysis of mixtures, leading to the
following key conclusions:
(1) In terms of molecular structure, the findings indicate that the incorporation of SBS
and SBR increases the levels of carbonyl and sulfoxide factors while reducing the level of
long-chain factors, which slows the rate of change in large molecule content and enhances
the aging resistance of the asphalt. Regarding molecular weight components, the addition
of SBS and SBR reduces the rate of change of LMS by more than 6%, while also mitigating
the rate of change in overall molecular weight distribution to below 50%.
(2) Under dynamic loading conditions, the simultaneous incorporation of SBS and
SBR into CMA mixtures increases the dynamic modulus in the 25 Hz and
−
10
◦
C range by
24.3% (AC-13) and 15.4% (AC-16), and reduces the φby 55.8% (AC-13) and 40% (AC-16).
(3) This study clarifies the evolution patterns of the characteristic functional groups and
molecular weight of CMA. However, there remain limitations in revealing the viscoelastic
evolution of the binder from a compositional perspective. Future research could conduct a
four-component analysis of CMA, quantitatively characterizing the contents of asphaltenes,
aromatics, saturated fractions, and resins to evaluate the impact of compositional changes
on the viscoelastic properties of CMA.
Author Contributions: Methodology, Z.Y. and Z.M.; Formal analysis, X.L.; Investigation, Z.F. and
Y.Z.; Resources, Z.Y.; Funding acquisition, Z.Y. All authors have read and agreed to the published
version of the manuscript.

Polymers 2025,17, 698 18 of 20
Funding: This research was sponsored by Natural Science Foundation of Xinjiang Uygur Au-
tonomous Region (2023D01B26).
Institutional Review Board Statement: Not applicable.
Data Availability Statement: The original contributions presented in this study are included in the
article. Further inquiries can be directed to the corresponding author.
Conflicts of Interest: Authors Z.Y., X.L., Z.F., Y.Z. were employed by the company Xinjiang Institute
of Transportation Sciences Co., Ltd., Author Z.M. was employed by the company Xinjiang Highway
and Bridge Testing and Inspection Center Co., Ltd. The remaining authors declare that the research
was conducted in the absence of any commercial or financial relationships that could be construed as
a potential conflict of interest.
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