Research on Improving the Durability of Bridge Pavement Using a High-Modulus Asphalt Mixture

In order to improve the durability of the asphalt pavement on a cement concrete bridge, this study investigated the effect of the modulus of the asphalt mixture at the bottom layer on the mechanical response of bridge pavement, along with a type of emerging bridge pavement structure. In addition, the design method and pavement performance of a high-modulus asphalt mixture were investigated using laboratory and field tests, and the life expectancy of the deck pavement structure was predicted based on the rutting deformation. The results showed that the application of a high-modulus asphalt mixture as the bottom asphalt layer decreased the stress level of the pavement structure. The new high-modulus asphalt mixture displayed excellent comprehensive performance, i.e., the dynamic stability reached 9632 times/mm and the fatigue life reached 1.65 million cycles. Based on the rutting depth prediction, using high-modulus mixtures for the bridge pavement prolonged the service life from the original 5 years to 10 years, which significantly enhanced the durability of the pavement structure. These research results could be of potential interest for practical applications in the construction industry.


Introduction
During the 21st century, bridge construction has made considerable progress in China. By the end of 2018, the total number of national highway bridges had reached 851.5 thousand [1], among which, the cement concrete bridge is the most commonly used. After more than ten years of scientific research, a system of pavement materials with stone mastic asphalt (SMA), epoxy asphalt concrete, and waterproof adhesive layer materials with thermal sprayed modified asphalt, rubberized asphalt, and epoxy-based binder has formed [2][3][4][5]. However, in recent years, due to the increase in traffic volumes, there still exist many pavement diseases, for example, rutting, slippage, and potholes [6]. There is still a lot of improvement potential for the design of bridge pavement structures, pavement materials, and construction technology.
The existing research results showed that the high-modulus asphalt mixture had a high modulus, high rutting resistance, and low sensitivity to low-temperature cracking and fatigue cracking [7,8]. The above advantages could reduce the plastic deformation of the pavement structure, especially the rutting resistance at high temperatures when used in the middle and bottom pavement layers, and improve the pavement fatigue performance to extend its service life [9]. Based on the rerouting project of national highway G312 in the Zhenjiang urban area, this study systematically investigated the high-modulus asphalt mixture used in bridge pavement using mechanical analysis, laboratory and field tests, and a fatigue life prediction. The objective of this research was to improve the deformation resistance and antifatigue performance and durability of the bridge deck pavement structure, and finally, prolong the service life.

Bridge Pavement Structure Scheme
In China, the current cement concrete bridge deck asphalt pavements usually utilize double-layer pavement, which coincides with the intermediate and surface pavement layer of the connected roads [10]. The surface layer was 4 cm of modified asphalt SMA-13-M, and the lower layer was 6 cm of unmodified asphalt AC-20-U or modified asphalt AC-20-M. This paper presents a novel pavement structure scheme in which a high-modulus asphalt mixture (EME, for short; see below for the asphalt mix design) was used for the lower layer. The rutting test was used to evaluate the high-temperature stability of the durable and safe bridge deck pavement structure. The dynamic stability of the specimens with different structures is shown in Table 1.  ABAQUS was used to calculate the effect of using the high-modulus mixtures on the stress response of the pavement structure [11]. When using the finite element method to divide the mesh, in order to save time and improve the accuracy of the calculation, the C3D8R type was used in a box girder, a hollow slab girder, and concrete pavement. Through segmentation, adjusting the pattern of the mesh control to "structured" ensured the stability of the mesh division.

Development of Bridge Deck Pavement Model
According to the actual bridge structure used in the rerouting project of national highway G312 in the Zhenjiang urban area, this study selected the hollow slab girder as a representative bridge structure. The span, width, and other major size parameters of the bridge are shown in Table 2. For design temperatures from −80 to 200 • C, the 3D model of the hollow slab structure and the asphalt pavement layer was drawn according to the actual size using the finite element analysis software ABAQUS, as shown in Figure 1. In the ABAQUS modeling process, the transient elastic parameter "Moduli time scale" was selected as "Instantaneous", and the viscoelastic parameter was defined as the "Prony series" for the time.

Material and Load Parameter Selection
Asphalt pavement materials are considered uniform, isotropic viscoelastic mixtures [12]. A generalized Maxwell model was selected to represent the viscoelastic mechanical behav- ior of asphalt mixtures [13]. The viscoelastic parameters of SMA-13-M, AC-20-U, AC-20-M, and EME were determined on the basis of dynamic modulus tests (ASTM D3497) [14]. The Poisson's ratio of the asphalt mixture was taken as 0.25. A cement bridge deck and a waterproof bonding layer were used in the linear elastic model. The Poisson's ratio of the concrete bridge deck plate was 0.35 and the elastic modulus of the concrete was 32.5 GPa. The thickness of the waterproof bonding layer was 0.2 cm and the elastic modulus was 150 MPa. The physical properties of the asphalt mixture are shown in Table 3.

Material and Load Parameter Selection
Asphalt pavement materials are considered uniform, isotropic viscoelastic mixtures [12]. A generalized Maxwell model was selected to represent the viscoelastic mechanical behavior of asphalt mixtures [13]. The viscoelastic parameters of SMA-13-M, AC-20-U, AC-20-M, and EME were determined on the basis of dynamic modulus tests (ASTM D3497) [14]. The Poisson's ratio of the asphalt mixture was taken as 0.25. A cement bridge deck and a waterproof bonding layer were used in the linear elastic model. The Poisson's ratio of the concrete bridge deck plate was 0.35 and the elastic modulus of the concrete was 32.5 GPa. The thickness of the waterproof bonding layer was 0.2 cm and the elastic modulus was 150 MPa. The physical properties of the asphalt mixture are shown in Table 3.

Analysis of the Most Unfavorable Load Position
Due to the various positive and negative bending moment factors, the load position has an important influence on the mechanical response of a concrete deck pavement system. Selecting the most unfavorable load position is the key to accurately calculating the maximum load stress in the pavement system [15].
Transverse load position: The rerouting project adopted a two-way, six-lane layout, where the single width of lane was set to 12.5 m. In order to research the influence of the lateral arrangement of a vehicle on the pavement layer stress, by considering the bridge symmetry and the actual situation of a vehicle moving on the bridge, the analysis started from the middle of the slab, moving 1 m as the analysis step length toward the margin, which resulted in seven positions.
Longitudinal loading position: By considering the symmetry of the longitudinal load along the bridge, the loading positions were selected at the bridge support, L/8, L/4, 3L/8, and L/2, where L was the length of the bridge.
By considering the pavement damage mechanism, the selected mechanical control indexes were the maximum shear stress and the horizontal tensile stress in the pavement, the vertical tensile stress between the asphalt concrete and the cement concrete layer, and the maximum horizontal shear stress between the asphalt concrete and the cement concrete layers.

Most Unfavorable Transverse Position
The mechanical analysis results of the deck pavement with different lateral load positions are shown in Figure 2. It can be seen that the maximum shear stress and the horizontal tensile stress in the pavement gradually increased with the loading position shifting from the bridge's center (position 1) to the bridge's edge (position 7). Furthermore, the maximum value appeared at the edge of the bridge. The vertical tensile stress at the bridge's edge (position 7) increased significantly, while the other stress indexes did not change like this. Therefore, the bridge's edge (position 7) was selected as the most unfavorable position in the transverse direction.

Analysis of the Most Unfavorable Load Position
Due to the various positive and negative bending moment factors, the load p has an important influence on the mechanical response of a concrete deck pa system. Selecting the most unfavorable load position is the key to accurately calc the maximum load stress in the pavement system [15].
Transverse load position: The rerouting project adopted a two-way, six-lane where the single width of lane was set to 12.5 m. In order to research the influenc lateral arrangement of a vehicle on the pavement layer stress, by considering the symmetry and the actual situation of a vehicle moving on the bridge, the analysis from the middle of the slab, moving 1 m as the analysis step length toward the m which resulted in seven positions.
Longitudinal loading position: By considering the symmetry of the longi load along the bridge, the loading positions were selected at the bridge support, L 3L/8, and L/2, where L was the length of the bridge.
By considering the pavement damage mechanism, the selected mechanical indexes were the maximum shear stress and the horizontal tensile stress in the pav the vertical tensile stress between the asphalt concrete and the cement concrete lay the maximum horizontal shear stress between the asphalt concrete and the ceme crete layers.

Most Unfavorable Transverse Position
The mechanical analysis results of the deck pavement with different later positions are shown in Figure 2. It can be seen that the maximum shear stress a horizontal tensile stress in the pavement gradually increased with the loading p shifting from the bridge's center (position 1) to the bridge's edge (position 7). F more, the maximum value appeared at the edge of the bridge. The vertical tensil at the bridge's edge (position 7) increased significantly, while the other stress inde not change like this. Therefore, the bridge's edge (position 7) was selected as th unfavorable position in the transverse direction.

Most Unfavorable Longitudinal Position
The mechanical analysis results of the deck pavement with different longitudinal load positions are shown in Figure 3. The maximum shear stress and the horizontal tensile stress in the pavement, as well as the maximum horizontal shear stress in the interlayer, at the fulcrum were obviously higher than that at the other positions. It can be seen from the three indicators above that the most unfavorable longitudinal position was at the bridge support, mainly due to the strong bearing support at the bridge's end, which was prone to generating a greater negative bending moment, which produced greater stress in the layer. Therefore, the support position was selected as the most unfavorable position in the longitudinal direction. The mechanical analysis results of the deck pavement with different long load positions are shown in Figure 3. The maximum shear stress and the horizon sile stress in the pavement, as well as the maximum horizontal shear stress in th layer, at the fulcrum were obviously higher than that at the other positions. It can from the three indicators above that the most unfavorable longitudinal position the bridge support, mainly due to the strong bearing support at the bridge's end was prone to generating a greater negative bending moment, which produced stress in the layer. Therefore, the support position was selected as the most unfa position in the longitudinal direction.

Mechanical Response Analysis of the Deck Pavement with Different Paving Materia
The dynamic modulus and phase angle of several asphalt mixtures at 20 shown in Table 4. The order of the dynamic moduli from small to large was AC AC-20-M< EME. On the basis of the analysis of the most unfavorable loads on the hollow sla longitudinal and transverse directions, the AC-20-U, AC-20-M, and EME were the lower pavement layer. The results of mechanical responses were analyzed

Mechanical Response Analysis of the Deck Pavement with Different Paving Materials
The dynamic modulus and phase angle of several asphalt mixtures at 20 • C are shown in Table 4. The order of the dynamic moduli from small to large was AC-20-U < AC-20-M < EME. On the basis of the analysis of the most unfavorable loads on the hollow slab in the longitudinal and transverse directions, the AC-20-U, AC-20-M, and EME were used in the lower pavement layer. The results of mechanical responses were analyzed and are shown in Figure 4.  It can be seen from Figure 4 that with the increase of the asphalt mixture m the maximum shear stress in the pavement was almost unchanged. The transvers stress in the pavement, the horizontal shear stress at the interlayer, and the vert sile stress at the interlayer showed a downward trend. This was attributed to the with the modulus increase of the lower layer pavement, the stiffness was also inc which alleviated the effect of the bridge deck's deformation on the pavement st which decreased the lateral tensile stress within the pavement structure [16,17] other hand, since the difference between the modules of the pavement and the bridge structure was reduced, the maximum horizontal shear stress and vertica stress between the layers were reduced [18]. The high-modulus asphalt pavem ture made the whole structure of the deck pavement system more coordinated achieving the purpose of improving the pavement stress state.

Design of the High-Modulus Asphalt Mixture
The concept of high-modulus asphalt mixtures was first proposed in France The mixture was designed based on both performance indexes and mechanical ties, using low-grade asphalt or hard asphalt, a high oil-stone ratio, and particu dation to produce a smaller interspace. The gyratory compaction method was a Antimoisture damage tests and wheel tracking tests were used to represent t performance [21,22]. In addition, dynamic modulus tests and fatigue resistance te used to measure the mechanical properties [23].
The asphalt content was selected based on an indicator named the richness m in the French EME design method, which is different from the design methods ac to volumetric indicators used in the France and China [24,25].

Selection of Raw Materials
Low-grade asphalt or hard asphalt is the key material that is required to a It can be seen from Figure 4 that with the increase of the asphalt mixture modulus, the maximum shear stress in the pavement was almost unchanged. The transverse tensile stress in the pavement, the horizontal shear stress at the interlayer, and the vertical tensile stress at the interlayer showed a downward trend. This was attributed to the fact that with the modulus increase of the lower layer pavement, the stiffness was also increasing, which alleviated the effect of the bridge deck's deformation on the pavement structure, which decreased the lateral tensile stress within the pavement structure [16,17]. On the other hand, since the difference between the modules of the pavement and the concrete bridge structure was reduced, the maximum horizontal shear stress and vertical tensile stress between the layers were reduced [18]. The high-modulus asphalt pavement mixture made the whole structure of the deck pavement system more coordinated, hence achieving the purpose of improving the pavement stress state.

Design of the High-Modulus Asphalt Mixture
The concept of high-modulus asphalt mixtures was first proposed in France [19,20]. The mixture was designed based on both performance indexes and mechanical properties, using low-grade asphalt or hard asphalt, a high oil-stone ratio, and particular gradation to produce a smaller interspace. The gyratory compaction method was adopted. Antimoisture damage tests and wheel tracking tests were used to represent the road performance [21,22]. In addition, dynamic modulus tests and fatigue resistance tests were used to measure the mechanical properties [23].
The asphalt content was selected based on an indicator named the richness modulus in the French EME design method, which is different from the design methods according to volumetric indicators used in the France and China [24,25].

Selection of Raw Materials
Low-grade asphalt or hard asphalt is the key material that is required to achieve a high-modulus asphalt mixture [26]. Based on existing experience, unmodified asphalt and hard particles can be blended to produce low-grade asphalt. Hard particles are highviscosity asphalt products that are made from heavy crude oil as the raw material through special processing that involves high-vacuum distillation, removing the solvent, oxidation, and a blending process. The weight ratio of 7:3 was chosen as the ratio of unmodified asphalt to hard particles. The test results are shown in Table 5, which met the requirements of EN 13924 for hard asphalt of grades 15~25.   Table 5, which met the requirements of EN 13924 for hard asphalt of grades 15~25.   Using the above gradation, the abundance coefficient K was 3.59 when the asphalt/stone ratio was 5.7%, which met the requirement of the abundance coefficient K of 3.4.
The French gyratory compaction test was carried out in accordance with the French specification (NF P 98-252) using the gradation and asphalt-stone ratio described above [27,28]. The results of the gyratory compaction tests are compiled in Table 6. According to the result of the rotary compaction test, the porosity (2.8%) of the high-modulus asphalt mixture met the design requirements. Using the above gradation, the abundance coefficient K was 3.59 when the asphalt/stone ratio was 5.7%, which met the requirement of the abundance coefficient K of 3.4.
The French gyratory compaction test was carried out in accordance with the French specification (NF P 98-252) using the gradation and asphalt-stone ratio described above [27,28]. The results of the gyratory compaction tests are compiled in Table 6. According to the result of the rotary compaction test, the porosity (2.8%) of the high-modulus asphalt mixture met the design requirements.
where VV-porosity (%), h min -the minimum specimen height at which the porosity was zero (mm), and h i -specimen height after the gyratory compaction of i cycles (mm).

Moisture Susceptibility
According to the freeze-thaw splitting test JTG E20 T 0729-2000 [29], the split strength ratio of the mixture specimens before and after the moisture damage was used to evaluate the moisture susceptibility of the high-modulus asphalt mixture. It can be seen from Table 7 that the freeze-thaw splitting strength ratio (TSR) of the high-modulus asphalt mixture was 86.2%, which satisfied the specification technical requirements. Meanwhile, the splitting strength of the high-modulus asphalt mixture was 2 to 3 times higher than the ordinary asphalt mixture, the strength of which was 0.8~1.0 kN.

High-Temperature Stability
Chinese Rutting Test The high-temperature stability of the high-modulus asphalt mixture was tested using the Chinese wheel tracking test method JTG E20 T 0719-2011 [29], which was compared with the traditional modified asphalt mixture AC-20. The test conditions were 60 ± 1 • C and 0.7 ± 0.05 MPa.
It can be seen from Table 8 that the dynamic stability of the high-modulus asphalt mixture was 9632 times/mm, which was 30% higher than that of the modified asphalt mixture AC-20. The durability of the high-modulus asphalt mixture was demonstrated by its excellent high-temperature resistance to rutting [30].

French Rutting Test
The high-temperature stability of the high-modulus asphalt mixture was determined using the French wheel tracking test NF P 98-252 (corresponding to EN 12697-22) [31]. The results of the test are shown in Table 9. It can be seen that the rutting ratio gradually increased with the increase of the number of load cycles. When the number of load cycles was 30,000, the rutting rate still met the requirements, which shows that the high-modulus asphalt mixture displayed excellent high-temperature stability.  [32] to measure the fatigue behavior of the high-modulus asphalt mixture. It was tested under the following conditions: 10 • C, 25 Hz, and 130 µε.
It can be seen from Table 10 that the fatigue life of the durable high-modulus asphalt mixture reached 1.65 million cycles at 130 µε, far more than the technical requirements of 1 million cycles. Therefore, the high-modulus asphalt mixture displayed excellent fatigue properties.  [33]. The test temperature and frequency were 15 • C and 10 Hz, respectively. It can be seen from Table 11 that the complex modulus of the high-modulus asphalt mixture reached 19,620 MPa, which satisfied the technical requirement of more than 14,000 MPa. In summary, in order to improve the durability and rutting resistance of the pavement structure under a load, a high-modulus asphalt mixture with high strength, a high modulus, and excellent high-temperature stability and fatigue resistance can be used for the lower layer of the bridge pavement.

Validation of the Practical Project
In the rerouting project of national highway G312 in Zhenjiang urban area, in order to further improve the construction quality and promote the application of new technologies and new materials, a trial pavement structure with high-modulus mixtures was constructed in 2016 at the Shengyuan Road interchange to enhance the durability of the bridge pavement.
In order to demonstrate the actual construction effect, the water permeability test and compaction degree test were carried out on the test section. Table 12 reveals that the qualified rate found using the water permeability test was 100%. Moreover, both the theoretical compaction degree and the rotational compaction degree of the core samples met the technical requirements. As shown in Figure 6, the high-modulus asphalt mixture pavement had a smooth surface, and the compaction, water permeability, and other indicators met the requirements. The practical engineering application proved that the construction effect of the highmodulus mixture pavement was successful. rials 2021, 14, x FOR PEER REVIEW Figure 6. Effect of paving using the high-modulus asphalt mixture.

Life Prediction of the Deck Pavement Based on the Rutting
In order to verify the durability of the high-modulus asp lower layer of bridge pavement, the rutting deformation was ta Figure 6. Effect of paving using the high-modulus asphalt mixture.

Life Prediction of the Deck Pavement Based on the Rutting Deformation
In order to verify the durability of the high-modulus asphalt mixture used in the lower layer of bridge pavement, the rutting deformation was taken as the control index and the calculation formula for the rutting depth in the Specifications for Design of Highway Asphalt Pavement (JTG D50-2017) was used in this paper [34]. When the lower layer was made of durable high-modulus EME-14 and AC-20-M, the permanent deformation of the asphalt pavement structure reached the design-allowable value of 10 mm According to the layering method, each asphalt pavement structure was divided into five layers according to the following layer thicknesses: 40 mm thick rubber asphalt SMA-13 upper layer was divided into 20 + 20 mm and a 60 mm thick durable high-modulus EME-14 or 60 mm AC-20-M bottom layer was divided into 20 + 20 + 20 mm.
The vertical compressive stresses at the top of each layer under the designed load were calculated. The results are provided in Table 13. The latest data found that the average daily equivalent axle load and annual growth rate of vehicle flow of the G312 highway were 6086 times/day and 6.19%. Thus, the cumulative number of axle loads for different design years was calculated [35]. The permanent deformation of the pavement structure corresponding to different design years is shown in Table 14. According to the results, for the traditional pavement structure using AC-20-M, the rutting depth will reach 10.34 mm after 5 years. However, for the durable pavement structure using EME-14, the rutting depth will reach 10.67 mm after 10 years. Therefore, the service lives are expected to be 5 and 10 years, respectively, when a rutting depth of 10 mm is taken as the requirement.
The high-modulus asphalt mixture technology can improve the durability of the deck pavement, effectively reduce the probability of rutting, and consequently, it will extend the maintenance interval and reduce the traffic delays due to maintenance work during the operation period.

Conclusions
In this study, through mechanical analysis, laboratory and field tests, and life prediction, a high-modulus asphalt mixture that was designed and used as the bottom layer of a cement concrete bridge pavement system was investigated; the main conclusions are as follows: (1) When the high-modulus mixture was used for the lower layer of the bridge deck pavement, the difference in modulus between the pavement layer and the concrete bridge was reduced. As a result, the whole structure of the deck pavement system was more coordinated, achieving the purpose of improving the pavement stress state.
(2) The road performance test results showed that with its high strength and modulus and excellent high-temperature stability and fatigue resistance, the high-modulus asphalt mixture can be used for the lower layer of deck pavement to improve the ability to resist rutting, reducing the permanent deformation under loading.
(3) The results of the practical engineering application suggested that the new highmodulus asphalt mixture had excellent comprehensive performance, i.e., the dynamic stability reached 9632 times/mm and the fatigue life reached 1.65 million cycles. However, the long-term performance needs to be further tracked and evaluated.
(4) The life prediction based on the rutting deformation revealed that when the deck pavement structures with EME and AC-20-M were compared, the times to reach 10 mm rutting depth are expected to be 10 and 5 years, respectively. The application of the highmodulus asphalt mixture can significantly improve the durability of bridge pavement.