Abstract
As a highway pavement maintenance technology, the road performance of the ultra-thin overlay depends heavily on the performance of asphalt cement. For the preparation of high-performance-modified asphalt suitable for ultrathin overlay, the reactive blending technology was used based on SBS modified asphalt and combined with a penetration test, softening point test, ductility test, dynamic viscosity test, and viscous toughness test to study the optimal blending ratio of high-viscosity modified asphalt. Further, the rolling thin-film oven test (RTFOT), dynamic shear rheological (DSR) test, and Brookfield viscosity test are used to evaluate the technical properties of SBS/PTW modified asphalt under the best mixing ratio. Finally, the SMA-13 asphalt mixture was prepared with SBS/PTW high-viscosity modified asphalt and evaluated through a rutting test, low-temperature bending test, immersion Marshall test, freeze-thaw splitting test, structural depth test (TD), and British pendulum number (BPN) test. These results showed that the best blending mass ratio of high-viscosity modified asphalt is 6% SBS + 4% solubilizer + 0.2% PTW + 0.2% nano-ZnO. All technical property indexes of the prepared SBS/PTW high-viscosity modified asphalt meet the requirements of an ultra-thin overlay binder. The dynamic viscosity at 60°C reaches 64204 Pa·s, and the viscosity and toughness are 25.4 N·m and 20.9 N·m, respectively. SBS/PTW high-viscosity modified asphalt under dynamic shear rheology (DSR) test is classified as PG88 at high temperature. The rutting factor changes the smallest before and after aging, and it has excellent anti-aging performance. Furthermore, the asphalt mixture prepared by SBS/PTW high-viscosity modified asphalt has excellent high and low-temperature performance, water stability, and skid resistance performance. All properties meet the requirements of the ultra-thin overlay. This study demonstrated that the SBS/PTW high-viscosity modified asphalt possessed high viscosity properties, which can be used in the ultra-thin overlay and other highly demanding working environments.
1. Introduction
The total mileage of highways completed in China was about 501.25 × 104 km by the end of 2019, including 14.96 × 104 km of asphalt concrete pavements on highway [1]. With the repeated action of traffic load, the international friction index of asphalt concrete pavement decreases continuously, which weakens the anti-sliding performance of the pavement surface, resulting in a significant reduction in the safety of high-speed traffic [2]. As a kind of technology for asphalt pavement surface layer combined with asphalt mixture and modified emulsified asphalt binder, the ultra-thin overlay can effectively and reliably solve the problems of moderate and mild cracks and loose pavement anti-skid failure [3, 4]. The technology can reduce the thickness of the traditional asphalt overlay to 1/3–1/2 (≤25 mm) under the premise of achieving the same performance of the pavement, significantly reduce the cost of pavement maintenance engineering, delay the attenuation of the original pavement structure strength, and prolong the service life of the pavement structure [5, 6]. However, at the same time, as a critical part of direct contact with wheel load, rain, and other environmental factors, ultra-thin overlay not only needs to meet the performance of road surfaces such as anti-sliding and wear resistance but also needs to have specific structural strength, bond strength, and fatigue durability. Therefore, higher requirements should be required in the design of raw materials.
After implementing ultra-thin overlay technology, the pavement diseases mainly composed of slippage, looseness, and spalling. It can be seen that the damage of ultra-thin overlay mainly comes from the shear stress and tensile stress generated within the structure layer under the wheel loads. Therefore, the modified asphalt with high viscosity should be used as far as possible to constrain the skeleton deformation of the mixture [7–9]. Then, the dynamic viscosity at 60°C, which strongly correlates with the permanent deformation resistance of asphalt pavement, should be used as an essential control index of the ultra-thin overlay. In addition, it is suggested that high-viscosity modified asphalt, rubber asphalt, and high content SBS modified asphalt can be used in the hot-mixed ultra-thin overlay by the Technical Specifications for Maintenance of Highway Asphalt Pavement (JTG 5142-2019) [10]. The appropriate binder can be selected according to the pavement damage condition, climatic conditions, traffic volume, and composition. High content SBS modified asphalt is widely used because of its excellent performance. However, with the increase of polymer SBS content, the prepared high-viscosity modified asphalt is prone to phase separation, elastic index reduction, and storage stability deterioration [11, 12]. Therefore, many scholars have gradually researched the technical properties of compound high-performance asphalt based on polymer SBS. Tian et al. [13] developed a new SBS-PU high viscosity and high-elasticity modified asphalt with SBS and polyurethane (PU) as modifiers, analyzed its modification mechanism, rheological properties, and storage stability, and compared it with TPS and SINOTPS high-viscosity modified asphalt. Zhang et al. [14] carried out dynamic shear rheological test, thin film oven test, and beam bending rheological tests on three kinds of nanomaterials/polymer modified asphalt. Combined with the scanning electron microscopy test and infrared spectroscopy test, it was concluded that nanomaterials could improve polymer dispersion in asphalt and improve the high and low-temperature performance of matrix asphalt. Zhang and Hu [15] prepared high viscosity-modified asphalt based on polymer SBS, plasticizer, and crosslinking agent and revealed its modification mechanism. It was found that plasticizers can effectively improve the dispersion of SBS. Zhou and Chen [16] selected polymer SBS, tackifying resin, and plasticizer to prepare high viscosity-modified asphalt and evaluated its performance. The prepared high viscosity modified asphalt has excellent high-temperature rutting resistance and permanent deformation resistance.
Therefore, there is a need to further improve the comprehensive performance of high content SBS modified asphalt for ultra-thin overlay. In this study, based on 6% SBS modified asphalt, SBS/PTW high-viscosity modified asphalt was prepared using solubilizer, toughening agent, and nano-ZnO as modifiers combined with a series of physical properties tests. Then, the physical properties, rheological properties, and economic benefits of the prepared SBS/PTW high viscosity modified asphalt are analyzed with three kinds of high-performance modified asphalt. Finally, the corresponding SMA-13 asphalt mixture is prepared to evaluate the road performance, which provides a reference for expanding the engineering application of SBS compound high viscosity modified asphalt in the ultra-thin overlay.
2. Materials
2.1. Raw Material
The matrix asphalt AH-70 in this study was provided by Maoming Asphalt Co. Ltd. (Guangdong Province, China), and its main properties are listed in Table 1 according to the standard test methods in China test specification JTG E 20-2011 [17]. The modifier used the linear SBS modifier, C9 petroleum resin, furfural extraction oil as a solubilizer, and ternary copolymer PTW as a plasticizer. The fundamental performance indexes of material are listed in Tables 2 to 4. In addition, three commonly used high-performance modifiers are selected for comparative study, and their types and contents in matrix asphalt are listed in Table 5.
2.2. Preparation of SBS/PTW High-Viscosity Modified Asphalt
The preparation process of SBS/PTW high-viscosity modified asphalt is shown in Figure 1 and involves the following steps: (1) the solubilizer and polymer SBS were added to the AH-70 matrix asphalt, and the mixture was stirred at a speed of 500 r/min for 20 minutes at 165°C by an electric mixer. (2) The toughening agent PTW was added to the mixture, and then the mixture was stirred with a shear rotation speed of 4000 r/min for 25 min at 180°C by a high shear emulsifier to obtain the modified asphalt. (3) The nano-ZnO was added to the modified asphalt, and the mixture was continued to stir with a shear rotation speed of 4000 r/min for 15 min at 180°C by a high shear emulsifier to obtain the composite-modified asphalt. (4) The composite-modified asphalt was put into an oven for 30 min at 180°C to mix the SBS/PTW high-viscosity modified asphalt well. (5) The preparation of high-performance modified asphalt (HMA)-I, II, and III involves the following steps: the corresponding modifier is added to the matrix asphalt, and then the mixture was stirred with a shear rotation speed of 4000 r/min for 30 min at 180°C by a high shear emulsifier to obtain the modified asphalt. Finally, the high-performance modified asphalt was put into an oven for 30 min at 180°C to mix well.
2.3. Asphalt Mixture Design
The aggregates and mineral filler were processed from basalt, and the basalt fiber content was 0.4%. The aggregate gradation of SMA-13 was used, and the aggregate gradation is shown in Figure 2. The properties of aggregate and basalt fiber are listed in Tables 6 and 7.
3. Experiment Methods
3.1. Conventional Physical Properties Test
The conventional physical properties test of the modified asphalt, including the softening point, ductility (5 cm/min, 5°C), penetration (25°C), toughness and tenacity (25°C, 500 mm/min), dynamic viscosity at 60°C, Brookfield viscosity (135°C), and rolling thin film oven, were tested according to the standard test methods in China test specification JTG E20-2011 [17].
3.2. Dynamic Shear Rheometer (DSR) Test
The DSR test can be used to measure both the viscous and elastic behaviors of asphalt binder. A dynamic shear rheometer DSR-MCR102 from Anton Paar Company, Austria, was used to characterize the high-temperature grading of SBS/PTW high-viscosity modified asphalt samples from the rutting factor through temperature sweep according to ASTM D 7175 [18]. The temperature sweep test was set to be between 50°C and 90°C with a fixed frequency of 1.59 Hz and used the strain-controlled mode (2%).
3.3. Wheel Tracking Test
The dynamic stability (DS) was used to evaluate the high-temperature performance of the SBS/PTW high-viscosity modified asphalt mixture, The asphalt mixture samples (300 mm × 300 mm × 50 mm) were prepared by a wheel rolling machine, and the test temperature was set to 60°C with the wheel load of 0.7 MPa according to the standard test methods in China test specification JTG E20-2011 [17].
3.4. Low-Temperature Beam Bending Test
In this study, according to JTG E20-2011 [17], the low-temperature performance of modified asphalt mixture was evaluated by the small beam specimen bend at low temperature, the test temperature was −10°C, and the universal testing machine will load through mid-point loading at a speed of 50 mm/min [19]. The bending tensile strength and maximum bending tensile strength were calculated and employed as evaluation indices for modified asphalt mixture low-temperature crack resistance [20, 21], and the tensile strength and maximum bending tensile strength are calculated by the following equations:where is the span of the specimen, is the maximum load of the specimen failure, is the width of the specimen, is the height of the specimen, and is the mid-span deflection of the specimen failure.
3.5. Immersion Marshall Test
The immersion Marshall test was used to evaluate the anti-stripping ability of the asphalt mixture when damaged by water. In this study, the Marshall stability and flow value of asphalt mixtures were tested according to JTG E20-2011 [17]. The Marshall specimen (101.6 mm ± 0.2 mm in diameter and 63.5 ± 1.3 mm high) was prepared by the standard compaction method, and the load was applied to the specimen at a rate of 50 mm/min until the specimen was destroyed. The retained stability of the specimen is calculated according to the following formula:where is the stability of the test specimen after immersion for 48 h at 60°C and is the stability of the test specimen after immersion for 0.5 h at 60°C.
3.6. Freeze-Thaw Splitting Test
The freeze-thaw splitting test was used to determine the strength ratio of splitting failure of mixture specimens before and after water damage and evaluate the water stability of the asphalt mixture. According to JTG E20-2011 [17], the prepared cylinder specimen (101.6 mm ± 0.25 mm in diameter and 63.5 ± 1.3 mm high) was kept in a vacuum for 15 min and then placed in water for 0.5 h. After taking out the specimen, 10 mL of water was added and kept at −18°C for 16 h, then the specimen was placed in a 60°C thermostatic water tank for 24 h, and finally immersed in a 25°C thermostatic water tank for 2 h. The tensile strength ratio is calculated as follows:where is the average value of splitting strength before freezing and thawing and is the average value of splitting strength after freezing and thawing.
3.7. Skid Resistance Tests
In this study, the structural depth test (TD) and the British pendulum number (BPN) test were used to determine the skid resistance of the SBS/PTW high–viscosity modified asphalt mixture. The structural depth test is a commonly used method for the measurement of skid resistance of pavement surface according to JTG E20-2011 [17]. First, a certain amount of 0.15 mm–0.3 mm particle size of sand is prepared, and then the sand is poured on the asphalt mixture samples (300 mm × 300 mm × 50 mm) and slowly pushed into a circle from inside to outside, so that the sand was filled into the void on the surface of the specimen. Finally, the average diameter of the circle is measured in two vertical directions. The TD is calculated as follows:where is the volume of sand and is the average diameter.
The British pendulum number test can simulate the antiskid performance of the road in a wet state. The test was conducted by measuring the low-speed friction at about 10 km/h according to DIN EN 13036-4, and the obtained friction coefficient is named as British pendulum number (BPN) [22].
4. Performance of SBS/PTW High-Viscosity Modified Asphalt
4.1. Influence of Solubilizer Type on Performance of SBS-Modified Asphalt
When the content of polymer SBS in base asphalt reaches 6%, the technical requirements of high-viscosity modified asphalt can be met by composite modification technology [15]. However, due to the molecular difference between polymer and asphalt, it is easy to cause segregation of modified asphalt, and the compatibility can be promoted by adding oil or petroleum resin rich in aromatic components [23]. Therefore, petroleum resin-C9 and furfural extraction oil were selected as solubilizers, and the effect of the two types of solubilizers on the performance of SBS-modified asphalt is determined based on penetration, softening point, and ductility tests. The test results are listed in Table 8.
Table 8 lists that the SBS modified asphalt shows different modification effects after adding the same content of petroleum resin-C9 or furfural extraction oil based on 6% SBS modified asphalt. After adding petroleum resin-C9, the penetration, softening point, and ductility of modified asphalt are reduced. The maximum decrease is 80%, which is related to the incorporation amount of rigid side chain-benzene ring that petroleum resin contains, resulting in the decrease of light component content and the weakening of SBS swelling degree [24]. After adding the furfural extraction oil, the penetration, softening point, and ductility of SBS-modified asphalt increased, with the maximum increase of 70.8%, indicating that the addition of furfural extraction oil has a positive impact on the performance of SBS modified asphalt, thus significantly greatly improving its low-temperature performance. The main components of furfural extraction oil are saturates, aromatics, and colloids, especially the aromatic content reaching more than 68.3% [17], and the saturates and aromatics could increase the penetration of asphalt as light components. The increase of aromatics promotes the swelling degree of PB segment in the SBS modifier, thereby improving SBS dispersion and improving the softening point and ductility of modified asphalt [23]. Therefore, the improvement effect of furfural extraction oil on SBS modified asphalt is better than that of petroleum resin-C9.
4.2. Determining the Content of PTW
Related studies have shown that the epoxy group in the toughening agent (PTW) molecule can react with the free carboxylic acid group and phenolic hydroxyl group in the asphalt molecule, improving the asphalt stability and having a toughening effect, which can effectively improve the toughness of the asphalt [24]. To further improve the comprehensive performance of modified asphalt, by controlling the content of PTW, the effects of adding 0.1%, 0.2%, and 0.3% PTW on the penetration, softening point, ductility, dynamic viscosity at 60°C, and toughness of SBS-modified asphalt based on scheme F (matrix asphalt-6% SBS-4% furfural extract oil) are studied. Then, the optimum content of PTW is determined, and the test results are shown in Figure 3.
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It can be seen from Figure 3 that compared with Scheme F (matrix asphalt-6% SBS-4% furfural extraction oil), the softening point of the composite-modified asphalt does not change significantly after adding PTW, and the maximum decrease is only 4.3%, indicating that PTW has little effect on the high-temperature performance of the composite-modified asphalt. However, the penetration, ductility, and dynamic viscosity at 60°C show a trend of increase at first then decrease with the increase of PTW content, and the maximum increases of penetration, ductility, and dynamic viscosity at 60°C were 10.8%, 13.1%, and 104.5%, respectively, and all reached the optimum when the content of PTW was 0.2%. In particular, the dynamic viscosity at 60°C has a significant correlation with the dynamic stability and dispersion loss of the asphalt mixture [25]. The dynamic viscosity value of modified asphalt increases most significantly, reaching 5.2 × 104 Pa·s when the content of PTW was 0.2%, which is about 2.6 times that required by the Chinese specification-GB/T 30516-2014 [26], indicating that SBS/PTW composite-modified asphalt meets the technical requirements of high-viscosity modified asphalt.
The properties of toughness and tenacity about asphalt are important indexes to evaluate the effect of asphalt modification. As shown in Figure 3, with the increase of PTW content, the toughness and tenacity of modified asphalt increase first and then tend to be stable. Compared with before adding the PTW, the toughness increased by 11.1%, 27.2%, and 30.9%, and the tenacity increased by 12.0%, 38.6%, and 39.9%, respectively. The reason for this phenomenon is that PTW is an ethylene terpolymer containing epoxy groups, and the epoxy group can react with the carboxyl group and phenolic hydroxyl liberation in the asphalt molecule, which reduces the interfacial and surface tension between the asphalt phase and the SBS phase and effectively improves the swelling degree of the polybutadiene (PB) of the SBS modifier in the matrix asphalt.
It can be concluded that after adding an appropriate amount of toughening agent (PTW), the high-temperature performance of SBS/PTW high-viscosity modified asphalt changes little, while the low-temperature performance, toughness, and dynamic viscosity increase significantly. When the content of PTW was 0.2%, the SBS/PTW high-viscosity modified asphalt was better in the comprehensive performance.
4.3. Influence of Nano-ZnO Content on Performance of SBS/PTW High-Viscosity Modified Asphalt
To further optimize the comprehensive performance of SBS/PTW high-viscosity modified asphalt, based on 6% SBS, 4% furfural extraction oil, and 0.2% PTW, combined with the unique scale effect of nanomaterials, according to the research results of Su et al. [27], polymer SBS can be adsorbed on the surface of nanomaterials and reduce the surface free energy, and nano-ZnO can effectively improve the dispersion effect and stability of SBS in asphalt. Therefore, 0.2%, 0.5%, and 1% nano-ZnO were selected to mixed with SBS/PTW high-viscosity modified asphalt, respectively. The change rate of the performance index with the content of nano-ZnO is shown in Figure 4.
The change rate of performance index in Figure 4 shows that with the increase of nano-ZnO content, the softening point and dynamic viscosity at 60°C of SBS/PTW high-viscosity modified asphalt increase gradually, and the dynamic viscosity at 60 oC changes most significantly, which increases by 25.5%, 29.5%, and 35.5%, respectively. This phenomenon shows that nano-ZnO particles are easy to react with asphalt molecules due to their high specific surface and high chemical action activity [28, 29], and after adding nano-ZnO, the high-temperature deformation resistance of SBS/PTW high-viscosity modified asphalt is effectively improved. However, the penetration, ductility, and toughness decreased with the increase of nano-ZnO content. Among them, ductility decreased by 5.5%, 12%, and 22.1%, respectively, which shows that the bonds between matrix asphalt and nanoparticles can debond under pull force and stress concentration is generated due to the rigidity of inorganic nanoparticles. Therefore, the ductility of SBS/PTW high-viscosity modified asphalt is decreased [30, 31], but all indexes exceeded the requirements of high-viscosity modified asphalt.
Therefore, the most obvious improvement effect of nano-ZnO on SBS/PTW high-viscosity modified asphalt is high-temperature stability, and its influence on other properties is limited. With careful consideration of physical properties and economic benefits, the optimal content of nano-ZnO added in SBS/PTW high-viscosity modified asphalt was 0.2%. The optimum proportion of the prepared high-viscosity modified asphalt was 6% SBS, 4% furfural extraction oil, 0.2% PTW, and 0.2% nano-ZnO. To further analyze the technical properties of SBS/PTW high-viscosity modified asphalt, the performance evaluation was carried out with three commonly used high-performance modified asphalts. The test results are listed in Table 9.
As listed in Table 9, compared with HMA-I, II, and III, the softening point and dynamic viscosity at 60°C of SBS/PTW high-viscosity modified asphalt have apparent advantages. The softening points are 1.14, 1.19, and 1.15 times higher than those of the three modified asphalts, and the dynamic viscosities at 60°C are 2.75, 5.26, and 3.25 times higher than those of the three modified asphalts, which are more than three times higher than those required by the specification, indicating that SBS/PTW high-viscosity modified asphalt has an excellent high-temperature performance. The toughness and tenacity of SBS/PTW high-viscosity modified asphalt are 1.27 and 1.44 times of HMA-I, respectively, which are close to the performance of HMA-II and III. The ductility at 5°C is 1.1 times that of HMA-III, 19.8% and 26.7% lower than HMA-I and II, respectively, but still much higher than the specification requirements.
4.4. Antiaging Properties
The short-term aging behavior of SBS/PTW high-viscosity modified asphalt and three kinds of modified asphalt were simulated by a rolling thin film oven test, then the high and low-temperature performance of modified asphalt before and after aging was evaluated through a series of physical properties tests and rheological test. The test results are listed in Table 10.
Table 10 lists that the quality change of each modified asphalt is not apparent after the rolling thin film oven test, and the penetration, softening point, and ductility are highly affected by aging. The penetration ratio of SBS/PTW high-viscosity modified asphalt residue after aging was the biggest, which is 1.09, 1.24, and 1.36 times that of HMA-I, II, and III, respectively, and its reducing rate of ductility was the least. While, the softening point of modified asphalt all increased after aging, and the increment of softening point of SBS/PTW high-viscosity modified asphalt was 1.06°C, which is only 62%, 43%, and 34.4% of HMA-I, II, and III, respectively, which could be mutual verification by the test results of penetration and ductility. It fully illustrates that SBS/PTW high-viscosity modified asphalt has a lower degree of hardening after aging and better short-term aging resistance.
4.5. High-Temperature Performance Grade
The dynamic shear rheological (DSR) tests of SBS/PTW high-viscosity modified asphalt and HMA-I, II, and III were carried out to evaluate the high-temperature performance grade (PG), and the asphalt rutting factor G/sin (δ) used to determine the PG classification of modified asphalt [32]. The original sample asphalt G/sin (δ) ≥ 1.0 kPa, TFOT residual asphalt G/sin (δ) ≥ 2.2 kPa, and the test results are listed in Table 11.
From Table 11, it can be seen that the rutting factors of four modified asphalts show the same trend with the temperature change and gradually decrease with the temperature. Among them, the rutting factor of the SBS/PTW high-viscosity modified asphalt is the largest, and the greater the rutting factor, the better the high-temperature stability of the asphalt mixture. After rotating film aging, the changing trend of the rutting factor of modified asphalt with temperature is the same as before aging, but the value of the rutting factor increases significantly. This phenomenon is due to cracking some light components of asphalt molecules during aging, which reduces the flow medium in asphalt and makes the asphalt harden, which is more sensitive to temperature [33]. Compared with HMA-I, II, and III, the rutting factor of SBS/PTW high-viscosity modified asphalt is least affected by aging, and the increase of rutting factor after aging is 9.6%, 24.3%, and 46.1% lower than that of HMA-I, II, and III, respectively. It indicates that SBS/PTW high-viscosity modified asphalt has good antiaging performance. According to the PG classification of modified asphalt, the high-temperature grade of SBS/PTW high-viscosity modified asphalt is 88, which is 1, 2, and 1 grade higher than that of HMA-I, II, and III, respectively.
4.6. Paving Temperature
To evaluate the pumping performance of asphalt in the process of construction and the construction workability of asphalt mixture, the Brookfield viscosity test of modified asphalt at 135°C was carried out [17]. Combined with the technical requirements of the ASTM D6373-07 specification of Superpave system, the viscosity of modified asphalt at 135°C should not exceed 3 Pa·s [34]. The test results are shown in Figure 5.
From the comparative analysis in Figure 5, compared with the specification of SHRP that the viscosity of asphalt binder should not exceed 3 Pa·s, the viscosity values of HMA-I and III exceed the specification requirements of 43.3% and 6.7%, respectively, which indicates that the two kinds of modified asphalts need the higher paving temperature to ensure sufficient fluidity in construction mixing. The viscosity values of HMA-II and SBS/PTW high-viscosity modified asphalt are 60% and 83% of the standard limit, respectively, which can still have sufficient fluidity at relatively low temperatures, effectively reduce carbon emissions, conform to the concept of green development, and have good social and economic benefits.
5. Results of SBS/PTW High-Viscosity-Modified Asphalt Mixture Road Performance Test
5.1. High-Temperature Stability
The Marshall method was used to design the optimal asphalt-aggregate ratio, and the optimal asphalt-aggregate ratios of HMA-I, II, and III were 5.5%, 6.0%, and 5.7%, respectively. The SBS-modified asphalt-D and SBS/PTW high-viscosity modified asphalt mixtures were 5.6% and 5.9%, respectively. The rutting test of modified asphalt mixture is carried out under the optimum oil-aggregate ratio [35], and the test results are shown in Figure 6.
According to the results of the rutting test, SBS/PTW high-viscosity modified asphalt mixture has the best high-temperature performance, and the dynamic stability reaches 10227 times/mm, which is 1.18, 1.82, and 1.21 times that of HMA-I, II, and III mixtures, respectively. Moreover, compared with the SBS modified asphalt mixture, the dynamic stability was increased by 28.5%, which fully shows that the introduction of solubilizer, toughening agent, and nano-ZnO synergistic effect on the high-temperature rutting resistance of SBS modified asphalt mixture has improved significantly. The high-temperature stability of the SBS/PTW high-viscosity modified asphalt mixture is significantly better than that of the HMA-I, II, and III mixtures, far beyond the specification of not less than 3000 times/mm.
5.2. Low-Temperature Performance
The low-temperature performance of modified asphalt mixture was evaluated by the small beam specimen bend at low temperature, and the test results are shown in Figure 7.
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Figure 7 shows that the bending tensile strength of the SBS/PTW high-viscosity modified asphalt mixture is only slightly lower than the HMA-II mixture, which is 1.17 and 1.52 times that of the HMA-I and III mixtures, respectively, and the bending tensile strength is 11.4% higher than that of the SBS modified asphalt mixture. The maximum bending tensile strength value of the SBS/PTW high-viscosity modified asphalt mixture is 28.7% and 41.7% higher than the HMA-I and III mixtures, respectively, which is less than 1% lower than the HMA-II mixture and is 1.11 times of the SBS modified asphalt mixture. It shows that the compatibility between SBS modifier and asphalt under the action of solubilizer has a significant improvement on the low-temperature performance of asphalt mixture.
5.3. Water Stability Performance
The water stability of the SBS/PTW high-viscosity modified asphalt mixture was evaluated by immersion Marshall and freeze-thaw splitting tests [17]. The test results are shown in Figure 8.
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Figure 8 shows that the residual stability of the SBS/PTW high-viscosity modified asphalt mixture is 97.3%, and the freeze-thaw splitting strength ratio is 95.4%, which are better than those of SBS modified asphalt mixture. Compared with HMA-I, II, and III mixtures, the residual stability increased by 4.0%, 7.9%, and 6.2%, respectively, and the freeze-thaw splitting strength ratio increased by 2.8%, 7.3%, and 10%, respectively. Furthermore, SBS/PTW high-viscosity modified asphalt mixture meets the technical requirements of freeze-thaw splitting strength ratio of not less than 80% and residual stability of not less than 85% in the specification, which fully illustrates that the SBS/PTW high-viscosity modified asphalt has higher viscosity after compounding. The increase of viscosity can enhance the bonding ability between asphalt and aggregate [36], effectively reduce the emulsification of asphalt under the coupling effect of temperature and water, and reduce the shedding of asphalt film.
5.4. Skid Resistance
The skid resistance of the SBS/PTW high-viscosity modified asphalt mixture was evaluated by the structural depth test and British pendulum number test [17]. The test results are shown in Figure 9.
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From Figure 9(a), it can be seen that the SBS/PTW high-viscosity modified asphalt mixture has a higher structural depth than the other asphalt mixture. Compared with SBS modified asphalt mixture and HMA-I, II, and III mixtures, the structural depth increased by 10.7%, 5.6%, 17.5%, and 9.6%, respectively. Figure 9(b) shows that the value of BPN of SBS/PTW high-viscosity modified asphalt mixture is 77.4, which is slightly lower than the HMA-I mixture. Compared with SBS modified asphalt and HMA-II and HMA-III mixtures, the value of BPN increased by 11.8%, 7.8%, and 4.2%, respectively.
Moreover, if the TD and BPN of the ultra-thin overlays are over 0.6 mm and 45, the pavement is considered satisfactory skid resistance according to the Technical Specifications for Maintenance of Highway Asphalt Pavement (JTG 5142-2019). In this study, the TD and BPN of SBS/PTW high-viscosity modified asphalt mixture are 1.9 times and 1.72 times of the technical standard, respectively. It fully illustrates that SBS/PTW high-viscosity modified asphalt mixture has good skid resistance, improving road driving safety and reducing traffic accidents.
6. Conclusion
In this study, combined with the technical performance requirements of ultra-thin overlay asphalt binder, SBS/PTW high-viscosity modified asphalt was prepared based on the blending composite modification technology, and its performance and road performance of its mixture were evaluated through laboratory tests. The main conclusions are as follows:(1)The optimal ratio of SBS/PTW high-viscosity modified asphalt was determined by the penetration, softening point, ductility, toughness, and 60°C dynamic viscosity test using the control variable method as follows: matrix asphalt + 6% polymer SBS + 4% furfural extraction oil + 0.2% PTW + 0.2% nano-ZnO. The prepared high-viscosity modified asphalt met the performance requirements of ultra-thin overlay asphalt binder.(2)The high-temperature grading of SBS/PTW high-viscosity modified asphalt can reach 88, which is conducive to improving the high-temperature deformation resistance of asphalt mixture, and the variation of rutting factor is the smallest after the rolling thin film oven test, which has good anti-aging performance. Furthermore, the viscosity at 135°C is reduced by 16.7% compared with the standard requirement of not more than 3 Pa·s, which effectively reduces the paving temperature of the asphalt mixture and carbon emissions during the construction period and has excellent social and economic benefits.(3)The dynamic stability of SBS/PTW high-viscosity modified asphalt mixture can reach 10227 times/mm; the residual stability and freeze-thaw splitting strength ratio are 97.3% and 95.4%, respectively. The low-temperature performance is better than modified asphalt-H and J mixtures, and the skid resistance is beyond specification requirements. Thus, the SBS/PTW high-viscosity modified asphalt mixture has a better high-low temperature performance, water stability, and skid resistance.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors appreciate the financial support from the National Natural Science Foundation of China (grant no. 51378474), Fund of Leading Talent in Science and Technology Innovation (grant no. 194200510015), Science and Technology Department of Henan Province (grant no. 192102210047), and Fund of Zhengzhou University of Aeronautics Graduate Education Innovation Plan (grant nos. 2021CX59 and 2021CX60).