Rheological properties and micro-characteristics of polyurethane composite modified asphalt

Construction and Building Materials 234 (2020) 117395

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Rheological properties and micro-characteristics of polyurethane composite modified asphalt Xin Jin a, Naisheng Guo a,⇑, Zhanping You b, Lin Wang a, Yankai Wen a, Yiqiu Tan c a

College of Transportation Engineering, Dalian Maritime University, Dalian, Liaoning 116026, China Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, MI 49931-1295, USA c School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150090, China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A newly-developed composite

modified asphalt was prepared with polyurethane (PU) and rock asphalt (RA).
Performance and its influencing factors of PU composite modified asphalt were revealed.
PU can improve the low temperature performance of base asphalt.
The chemical reaction mechanism in PU composite modified asphalt was studied through the FTIR test.
The optimum additive amount of the PU and RA were put forward.

a r t i c l e

i n f o

Article history: Received 31 July 2019 Received in revised form 23 October 2019 Accepted 25 October 2019

Keywords: Polyurethane Rheological properties Micro-characteristics Rock asphalt

a b s t r a c t The objective of this study aims to investigate the rheological properties and micro-characteristics of polyurethane (PU) composite modified asphalt. In this study, the mixture of PU and rock asphalt (RA) was incorporated into base asphalt to prepare PU composite modified asphalt through a selfdetermined laboratory process. The properties of PU composite asphalt with different PU and RA contents was analyzed by the base performance tests. The dynamic shear rheology (DSR), bending beam rheology (BBR), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermal gravity (TG) were used to clarify rheological properties, chemical and microstructure of the binders, respectively. The results indicated that the PU composite modified asphalt with 5% PU and 5% RA exhibited a favorable performance in terms of the penetration, ductility, softening point and rotational viscosity, respectively. The PU composite modified asphalt with 3%, 5% PU and 15% RA showed the best high temperature performance grade (PG). Both the PU and RA indicated a preferable compatibility with the base asphalt. The unsaturated bond in PU crosslinked with the S-S bond in asphalt, which is helpful to improve the resistance to low temperature deformation. It can be found that the composite asphalt containing 5% PU and 5% RA performed a preferable laboratory performance investigated based on rheological properties tests and the microstructure observation. Also, the isocyanate respectively reacted with the phenol and carboxylic acid in the PU composite modified asphalt leading to the performance improvement. This study also showed

⇑ Corresponding author. E-mail addresses: [email protected] (X. Jin), [email protected] (N. Guo). https://doi.org/10.1016/j.conbuildmat.2019.117395 0950-0618/Ó 2019 Published by Elsevier Ltd.

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

that the PU has promising properties, and the PU and RA can be used as composite modification to increase the durability and service life of asphalt pavement. Ó 2019 Published by Elsevier Ltd.

1. Introduction The practical use of common asphalt as a binder for mixing materials has been affected seriously due to the limitations of its chemical composition and structure. In the past ten years, waterinduced damage and traffic load were the two reasons for causing the pavement distress including rutting, raveling, peeling and cracking. Surrounding above adversely problems will affect pavement performance and even endanger drivers, which also have resulted in a significant influence on the performance and sustainable development of highway [1–4]. Consequently, it is highly difficult for the traditional common asphalt to meet the requirements of highway construction [5]. In addition, quite a lot of asphalt pavement with special performance requirements were put forward, such as the pavement of highway toll gate, airport runway, steel bridge deck, etc. In order to effectively deal with the damage mentioned above and improve the quality and service life of asphalt pavement, the polymer modified asphalt is a key technology to enhance the performance of asphalt pavement depending on physicochemical modification [6]. In general, polymer modified asphalt technology can be traced back to the early 19th century, and the technology of rubber modified asphalt was first proposed by British researcher [7]. In 1920, the rubber modified asphalt pavement had been built in France [8]. According to statistics concerned, there were 116 patents and papers published on the polymer modified asphalt technology before 1943 [9]. Japan and the United States have started the work of polymer modified asphalt successively since the 1950s. However, the production, storage and performance of the traditional polymer modified asphalt are still unsatisfactory for pavement construction. Therefore, it is urgent to discover a new asphalt modifier which can effectively make up for the deficiencies referred to above. Currently, polyurethane (PU) as a new kind of organic polymer material is rising rapidly in engineering [10]. However, it has been widely used in coating, electronic appliances, building waterproofing, automobile industry, shoe sole materials and medical devices as a result of flexible and superior performance. The PU can be used as a new kind of asphalt modifier performed properties stronger than traditional polymer modifier in terms of durability and elastic recovery [11–13]. The addition of PU modifier can greatly enhance the elastic properties of common asphalt, and increased the resistance to plastic deformation suffered from environmental load and other factors [14]. Therefore, the use of PU in the asphalt can remarkably prolong the service life, and effectively improve the service quality of the pavement [15]. Although PU has been applied in a lot of different fields, a small quantity of studies had investigated the PU as polymer modifier to prepare the maintenance materials of asphalt pavement. Li [16] used a synthetic thermoplastic PU as an additive of asphalt, the test results showed that the PU modified asphalt mixture exhibited a favorable low temperature resistance, nonetheless the water stability of the asphalt mixture needed to be further improved. Sun et al. [17] pointed out that the optimum content of PU was 5%, the PU modified asphalt had an inferior low-temperature performance as compared to SBS modified asphalt, and the PU modified asphalt mixtures were a cost-effective material. Bazmara et al. [5] found that PU can improve the low-temperature cracking resistance of asphalt, the FTIR test results showed that the

characteristic peaks of PU in modified asphalt confirmed that new chemical bonds were formed after PU as modifier reacted with asphalt. In summary, the researches concerned with PU modified asphalt were reported less presently, and mainly focusing on performance evaluation of PU modified asphalt mixture. In addition, there are a few scholars detected the chemical reactions occurring in PU modified asphalt and investigate chemical modification mechanism using microscopic morphology tests. However, the study on the mechanism of internal chemical reaction of PU modified asphalt still need to be conducted. On the other hand, poor high temperature resistance and relatively weak water stability are the two major problems have exposed in the conventional thermoplastic PU modified asphalt mixture on the market. Therefore, some researchers made efforts to seek a compound modification method to improve the technical properties of PU modified asphalt. Bu et al. [18] utilized polyurethane/epoxy resin composite modified asphalt, it was found that the use of PU in the asphalt was <30%, the composite modified asphalt showed a favorable compatibility, and with the increase of PU content, the failure elongation of the composite modified asphalt, the resistance to the high-temperature rutting and low temperature cracking can be improved. Yu [19] proposed a composite modified asphalt mixture, which significantly improved the high and low temperature performance owing to the using of PU and Graphene Oxide (GO). This means the PU alloying improved the elastic modulus and failure strength of asphalt, and the compounding of GO can maintain the toughness of PU composite modified asphalt mixture by improving the modulus of the material. Additionally, the use of epoxy resin and GO can effectively meliorate the high temperature performance of PU modified asphalt. Also, the development and application of PU in asphalt pavement construction was limited due to their high price. Therefore, an effective way to solve the above problem is to find a costeffective asphalt modifier synergy with PU together, and then acquire the PU composite modified asphalt. Rock asphalt (RA) including the advantages of high temperature resistance, anti-aging, effective improvement of water stability of the mixture, convenient construction and low cost. Moreover, the RA and base asphalt have a better compatibility. However, RA modified asphalt presents the poor or even negative effect on the improvement of low-temperature cracking resistance of asphalt mixture [20–24]. In order to avert the effects arising from traffic loading and climate, and provide driving comfort, pavement materials should be designed to achieve a certain level of performance. Consequently, a new kind of PU composite modified asphalt with superior high and low-temperature performance can be prepared by adding PU and RA into the base asphalt. Until now, a large number of scholars have done plentiful of researches on the rheological properties of RA modified asphalt, and few studies have been focused on using PU and RA composite modified asphalt. Moreover, the studies on PU modified asphalt are rarely reported in literatures, researchers have not yet made a comprehensive and systematic evaluation, especially in regard to microstructure and rheological properties of the PU composite modified asphalt need to be investigated. Herein, in this study, the preparation process of PU composite modified asphalt was given, and then the base performance tests of prepared modified asphalt with different PU and RA contents were tested to confirm whether they can meet the technical requirements. The rheological

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properties of the composite modified asphalt with different PU and RA contents through dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests, the multiple stress creep recovery (MSCR) test was performed to evaluate the resistance to permanent deformation of the PU composite modified asphalt, and then the performance grade (PG) of the PU composite modified asphalt was determined. After that, the effects of RA on the phase microstructures and network structure of modified asphalt was investigated by using X-ray diffraction (XRD) to reveal the modification mechanism of RA in the asphalt. Also, the Fourier transform infrared spectroscopy (FTIR) was employed to characterize effects of PU on chemical compositions and functional groups of asphalt, combined with discussing the compatibility and the reaction mechanism between PU and RA. Additionally, influences of modified asphalt with different PU and RA contents on morphology were explored by using scanning electron microscope (SEM). Finally, influences of PU and RA contents on thermal stability, endothermic and exothermic reactions of the composite modified asphalt were analyzed by using differential scanning calorimeter (DSC) and thermal gravity (TG) tests.

2. Materials and experiment method 2.1. Materials 2.1.1. Polyurethane Polyurethane (PU) containing –NHCOO– macromolecular structures, as a new kind of organic polymer material, which is known as the fifth largest plastic. The chemical synthesis formula is shown in Fig. 1. The formula of PU is diverse and synthetic material used with plenty of purposes. A quite lot PU products with different properties and appearance can be prepared by varying the type of functional groups in asphalt [25–27]. Hence, PU as a new type of modifier has attracted more attention [28]. The PU used in this

study was produced by BASF (Germany) Company Ltd, and its main technical properties are shown in Table 1. 2.1.2. Rock asphalt In this study, the technical properties of the RA used are shown in Table 2. 2.1.3. Asphalt The grade of base asphalt labeled as 90# produced at Pan-jin in China was used in this study, and its technical properties are presented in Table 3. Table 1 The technical properties of PU. Technical properties

Test result

Standards

Density (g/cm3) Friction loss (mm3) Modulus in tension (300%, N/mm2)

1.12 25 10

Tear strength (N/mm2) Tensile strength (N/mm2)

70 45

Bump impact strength (23 °C, kJ/m2) Hardness (Shore D) Elongation at break (%)

unbroken 36 600

Modulus in tension (20%, N/mm2)

2.5

Compression deformation rate (25 °C, %) Bump impact strength (-30 °C, kJ/m2) Elongation at break (80 °C in water for 21 days, %) Compression deformation rate (70 °C, %) Tear strength (80 °C in water for 21 days, N/ mm2) Modulus in tension (100%, N/mm2)

25 unbroken 600

DIN 53479 [29] DIN 53516 [30] DIN 53504-S2 [31] DIN 53515 [32] DIN 53504-S2 [31] DIN 53453 [33] DIN 53505 [34] DIN 53504-S2 [31] DIN 53504-S2 [31] DIN 53517 [35] DIN 53453 [33] DIN 53054-S2 [31] DIN 53517 [35] DIN 53504-S2 [31] DIN 53504-S2 [31] DIN 53505 [34]

Hardness (Shore A)

Fig. 1. Chemical synthesis of PU [10].

Fig. 2. Flowchart of preparation of PU composite modified asphalt.

45 32 6 87

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

Table 2 The technical properties of RA. Technical properties 3

Density (15 °C, g/cm ) Flash point ( °C) Softening point ( °C) Mesh number Ash content (%)

Table 4 The content of additives of PU composite modified asphalt.

Test result

Standard in China (JTG E20-2011) [36]

1.02 235 170–200 80 8

T0603-2011 T0611-2011 T0606-2011 – T0614-2011

PU/ (%)

0 5 10 15

2.2. Experimental methods 2.2.1. Developed preparation method of PU composite modified asphalt In this study, the PU composite modified asphalt was prepared using a BME 100L laboratory shear device. Firstly, the base asphalt was heated to 145 °C, and then was fully mixed with PU for 10 min through using a high-speed shear instrument at 3000 r/min. After that, the temperature of the composite system was heated to 150 °C, and the shear mixing was continued for another 30 min to obtain PU modified asphalt. Then the RA was added into the PU modified asphalt and mixed for 30 min at 150 °C at a speed of 5000 r/min. According to the same preparing procedure, the PU composite modified asphalt samples were prepared, containing PU contents of 1%, 3% and 5% by weight of base asphalt, and the addition of RA was filled with 5%, 10% and 15%, respectively. The content of additives of the PU composite modified asphalt was determined (by weight of base asphalt), as shown in Table 4. For instance, 1% PU/5% RA was represented by a simplified 1P/5R, and the same simplification of the rest of PU composite modified asphalt was the presented in this study.

2.2.2. Basic performance tests The technical requirements for the PU composite modified asphalt are still undefined since it is a new type of modified asphalt. In this study, the basic performance tests of the prepared PU composite modified asphalt were tested, including penetration (25 °C), ductility (5 °C) and softening point tests according to technical requirements of polymer modified asphalt in Standard Test Methods of Bitumen and Bituminous Mixture in Highway Engineering (JTG E20-2011) in China. The penetration (25 °C) is a method to determine the consistency of asphalt, also, show the rheological properties of the asphalt investigated. The formula of the penetration index of asphalt calculated from the value of penetration (25 °C) and softening point as follows:

1952  500lgP25  20T R&B PI ¼ 50lgP25  T R&B  120

RA/ (%)

ð2:1Þ

where lgP25 —— Penetration index; TR&B —— Softening point The rotary viscosity of asphalt was tested by Brookfield viscosity meter, the viscosity test for asphalt was performed at 135 °C

0

1

3

– – p

– p p p

– p

5 p p

p

p

–

10 p – – –

and 175 °C, respectively, and the experimental method also can be found in JTG E20-2011. 2.2.3. Rheological properties (1) DSR test The PU composite modified asphalt was measured using a DSR Gemini II ADS machine according to AASHTO T 315. The high temperature performance of modified asphalt was evaluated by calculating the complex shear modulus (G*), phase angle (d) and rut factor (G*/sind). In this study, the DSR test was mainly carried out in the temperature sweep mode. In the temperature sweep analysis, the test temperature was set at 46 °C, 52 °C, 58 °C, 64 °C, 70 °C, 76 °C and 82 °C, respectively. Using strain control mode, the strain was 12%, the loading frequency was set at 10 rad/s. (2) MSCR test In this study, the multiple stress creep recovery (MSCR) test was used to evaluate the anti-permanent deformation ability of the PU composite modified asphalt. According to AASTO TP70-09 the percent recovery (R) reflecting the elasticity of asphalt binder and non-recoverable creep compliance (Jnr) were calculated as a measure of the binder’s contribution to permanent deformation behavior. Firstly, the composite modified asphalt samples were aged through RTFOT and then the stress-recovery control mode of DSR was conducted, for 1 s loading time, unloading time was 9 s, and testing temperature was kept at 64 °C. During the test, the stress was applied at 1.0 kPa and repeated for 10 times, then the stress was increased to 3.2 kPa and repeated for 10 times. As shown in Fig. 3, In MSCR test, two key parameters can be obtained: R and Jnr were calculated by using Eqs. (2.2) and (2.3), respectively [37]. 

R ¼ crec ¼



J nr ¼

10 1 X  crec ðsÞ 10 1

ð2:2Þ

10 1 X J nr ðsÞ  10 1

ð2:3Þ

(3) BBR test To evaluate the low-temperature performance of the PU composite modified asphalt, the BBR manufactured by Cannon Instrument Company was used to measure the creep stiffness (S) and m-value at low temperatures based on the ASTM D 6648 and

Table 3 Physical properties of base asphalt. Technical properties

Test result

Specification limits

Standard in China (JTG E20-2011) [36]

Penetration (25 °C, 0.1 mm) Softening point ( °C) Ductility (5 °C, cm) Flash point ( °C) Density (15 °C, g/cm3) Solution (Chloral, %) RTFOT Residuum

80 48 8.6 254 1.003 99.87 0.05 73.2 2.3

80–100 44 – 245 – 99.5 ±0.8 57 8

T T T T T T T T T

Mass loss rate (%) Penetration ratio (25 °C, %) Ductility (5 °C, cm)

0604–2011 0606–2011 0605–2011 0611–2011 0603–2011 0607–2011 0610–2011 0610–2011 0610–2011

X. Jin et al. / Construction and Building Materials 234 (2020) 117395

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system spectrophotometer (Frontier, PerkinElmer Co., United States) to discuss the compatibility between PU and asphalt with different contents, and then the modification mechanism of composite modified asphalt was speculated. The spectra were collected in the wave number ranged from 400 cm1–4000 cm1 with a resolution of 4 cm1, the scanning was performed 32 times. Each sample was prepared by casting film onto a potassium bromide (KBr) thin plate. (3) SEM test

Fig. 3. Scheme of MSCR test.

AASHTO T 313. The testing temperature were chosen respectively at 14 °C, 16 °C, 18 °C, 20 °C, –22 °C and 24 °C. Moreover, the rheological tests were employed on the PU composite modified asphalt to determine the high and low temperatures performance grades (PG). 2.2.4. Microscopic morphology analysis (1) XRD test An XRD (X’ Pert PRO MPD, PANalytical Co., Netherlands) was used to investigate the physical phase of RA with Cu-K radiation (k = 0.15406 nm), The accelerating voltage and applied current were 40 kV and 40 mA, respectively. The XRD pattern was recorded in the 2h range from 5° to 80°, at a rate of 2°/min in the step scanning mode.

As shown in Fig. 5, SEM (SUPRA 55 SAPPHIRE instrument, ZEISS Co., Germany) analysis equipment was used to characterize visually the compatibility between PU modifier and asphalt binder, The SEM test was employed to obtain microscopic image of the cross sections of the PU composite modified asphalt magnified 500 times and 50 times. Since the asphalt material is not conductive. Firstly, all samples were fixed on an aluminum sample stub and sputtered with gold under vacuum conditions (JFC-1100 model instrument, JEOL Co., Japan). Then the sample chamber was opened to place samples. Finally, the microstructure characteristics of the samples were observed by using SEM. (4) TG test TG (Q 50 instrument, TA Co., United States) analysis was used to characterize the impact of the thermal stability of PU composite modified asphalt. The mass of each sample was between 3 mg and 5 mg, and then heated the samples from room temperature to 800 °C at a rate of 20 °C /min using nitrogen as the protection gas. (5) DSC test

FTIR test is able to determine the qualitative, semi-quantitative, and quantitative information of chemical structures and physical characteristics for polymer, either solid, liquid, or gas. As illustrated in Fig. 4, in order to explore the effects of PU modifier on functional groups and chemical compositions of composite modified asphalt, the mid infrared spectra were acquired using a FTIR

A DSC (Q 20 instrument, TA Co., United States) was used to analyze the effects of PU and RA with different contents on thermal stability. A scale (1/10000g) was adopted to measure the weighing due to the high precision requirement of the sample that is not more than 5 ± 1 mg. Nitrogen as the protection gas, the sample was heated from room temperature to 130 °C at heating rate of 10 °C /min, constant temperature for 2 min, then cooling to 0 °C at a rate of 10 °C /min, at last constant temperature for 1 min after rising to 150 °C at heating rate of 10 °C /min, and then the temperature DSC curves were obtained. The glass transition was analyzed by the DSC testing results, the glass transition temperature (Tg) can be obtained by the DSC testing curve of PU composite modified asphalt, and the DSC testing curve for glass transition is illustrated in Fig. 4.

Fig. 4. The schematic diagram of glass transition in DSC testing curve.

Fig. 5. The penetration of modified asphalt.

(2) FTIR test

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

3. Results and discussion 3.1. PU and RA effects on basic properties of asphalt The effect of PU and RA contents on physical properties of the composite modified asphalts are shown in Figs. 5–7, and the test results of the prepared PU composite modified asphalt samples can meet technical requirements of polymer modified asphalt in China. It should be noted that with the amounts of PU was added from 1 to 5%, for the use of identical amount of RA, the penetration and ductility increased, and the softening point exhibited a slight variation. The softening point of the PU composite modified asphalt with the same amount of PU increased with the increasing content of RA, whereas, the penetration and ductility decreased. The results showed that the increased addition of RA into the asphalt specimens can lead to an improved viscosity of asphalt. However, the reduction of penetration should be more considerable for a higher RA percentage at each group, which indicates that RA can make PU composite modified asphalt becoming harder and leading to a better high-temperature stability. Also, the rutting resistance can be improved with the incorporation of RA, which is crucial for the stability of the material. This can be attributed that the use of RA was more than 5% resulting in a decreased penetration in terms of the PU composite modified asphalt. It was also worth noting that the addition of RA hardened the PU composite modified asphalt and improved the deformation resistance of asphalt. The influence of PU content on low-temperature cracking resistance of the PU composite modified asphalt can be evaluated by the ductility testing. The ductility of the PU composite modified asphalt increased significantly with the increase of PU. What indicates that the PU played a main role to enhance the lowtemperature performance of PU composite modified asphalt. These three experimental results demonstrate that the RA dosage has critical values in regard to the economic and engineering concerns. In conclusion, the PU composite modified asphalt labeled 5P/5R performed the best high and low temperature performance. The colloidal structure of the PU composite modified asphalt is Sol-Gel with certain elasticity and thixotropy. However, its colloidal structure may vary with the change of temperature. Usually, the high-temperature area above the softening point is gelatinous along with non-Newtonian (composite) flow characteristics. This means that the penetration index (PI) of the PU composite modified asphalt can exhibit a variation with the temperature.

The PI of all the specimens are depicted in Fig. 8. For the used uniform content of PU, an increased content of RA from 0 to 15% led to the PI increased. Similarly, the content of PU adding from 1 to 5%. When the identical content of the RA was employed, the PI also increased. What suggests that both PU and RA can reduce the temperature sensitivity of asphalt. As illustrated in Table 5, when the content of PU was fixed, the viscosity of the PU composite modified asphalt increased as the content of RA increasing from 0 to 15%. The viscosity of the PU composite modified asphalt did not increase obviously with the increase of PU content in terms of the same amount of RA. This indicates that RA can be the main factor affecting the viscosity of the PU composite modified asphalt. According to Technical Specifications for Construction of Highway Asphalt Pavements (JTG F402004) in China and SHRP standards. It was found that the test results of the rotary viscosity of the polymer modified asphalt generally did not exceed 3 Pas at 135 °C. The PU composite modified asphalt met the requirements of the specification. This implies that the PU composite modified asphalt subjected to external load can exhibit a strong resistance to flow deformation. Based on the colloid structure theory, it can be deduced that the dispersed phase should be RA likely in the PU composite modified asphalt. To be more precise, the larger percentage of RA, the higher the volume concentration coefficient will be. The volume

Fig. 6. The ductility of modified asphalt.

Fig. 8. The penetration index (PI).

Fig. 7. The softening point of modified asphalt.

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

the 5P/15R. This indicates that the addition of the RA can increase G*/sind. In detail, the G*/sind increased with the increase of the RA percentage. However, when the content of PU was more than 3%, the G*/sind of the PU composite modified asphalt decreased significantly. Taking into account the above result, the RA played a crucial role in improving the high temperature performance of the PU composite modified asphalt, the 3P/15R exhibited the best hightemperature performance in terms of the medium and high temperature conditions.

Table 5 Viscosity of PU composite modified asphalt. Samples

Rotational viscosity (135 °C, Pas)

Rotational viscosity (175 °C, Pas)

10R 5P 10P 1P/5R 1P/10R 1P/15R 3P/5R 3P/10R 3P/15R 5P/5R 5P/10R 5P/15R

1.032 0.581 1.023 0.597 0.965 1.975 0.684 1.025 2.430 0.683 1.143 2.593

0.167 0.102 0.153 0.118 0.149 0.349 0.106 0.144 0.270 0.112 0.171 0.213

(2) MSCR test

concentration coefficient increased with the increase of RA. Whereas, a large amount of RA used in the composite modified asphalt reduced the free asphalt (light components and oils) in the dispersion medium. Correspondingly, the light components (alkanes, cycloalkanes, aromatics) in the free asphalt might also decreased. Therefore, the increasing viscosity of free asphalt resulting in a significant increase in the viscosity of the whole dispersion medium. It can be seen that the superfluous content of RA should not be used in the composite modified asphalt. The analysis above showed that the PU composite modified asphalt with 5% RA was beneficial to enhance the base testing performance investigated. 3.2. Rheological properties 3.2.1. High temperature performance (1) DSR test The temperature sweep test results of the PU composite modified asphalt are summarized in Fig. 9, a rapid fall at start stage in the G*/sind of the PU composite modified asphalt samples were observed with the increasing temperature, and then the temperature curves smoothed as temperature increasing. In addition, it is displayed that the G*/sind of the RA modified asphalt was slightly higher than the PU modified asphalt. The 3P/15R and 5P/15R increased the maximum temperature limit to 76 °C and 82 °C after the RA was used, respectively. Although the high temperature performance grade of both 3P/15R and 5P/15R were 82 °C, the 3P/15R performed higher G*/sind compared with

Fig. 10 displays the MSCR test results of the PU composite modified asphalt at shear stress level of 0.1 and 3.2 kPa, respectively. Short-term aging of the PU composite modified asphalt samples were achieved using the rolling thin-film oven test (RTFOT) to simulate aging during construction according to AASHTO T 240, and the fatigue resistance of the specimens was obtained from the time-strain curves at 64 °C. The 5P/15R showed a smallest unrecoverable strain with regard to the all specimens tested, and the 3P/15R showed a similar unrecoverable strain to the 5P/15R. When the same content of the modifier was used, the unrecoverable strain of RA modified asphalt was much lower than that of the PU modified asphalt. For the base asphalt was modified by PU alone, with an increase in the PU content, the unrecoverable strain showed a slightly decrease, and the base asphalt performed a significant increase as compared to the PU composite modified asphalt in terms of the unrecoverable strain. Additionally, the result from the MSCR test presented a superior consistency with the DSR test. Based on the Jnr results of the modified asphalt at 64 °C in Table 6, the comparison of Jnr0.1 (the corresponding unrecoverable compliance at 0.1 kPa), Jnr3.2 (the corresponding unrecoverable compliance at 3.2 kPa) and the high-temperature grade are shown in Table 6. The irrecoverable compliance of the PU modified asphalt reduced with an increase in the PU content. The addition of the PU can improve the permanent deformation resistance of the PU composite modified asphalt at high-temperature. Nevertheless, when the content of the PU was identical, the irrecoverable compliance increased as the RA increasing. It was found that the ability of the PU composite modified asphalt to resist permanent deformation gradually weakened at high-temperature with the increase of PU. This indicates that the addition of the PU can enhance the permanent deformation resistance of asphalt.

120000

1000

10% RA 5% PU 10% PU 1P/5R 1P/10R 1P/15R 3P/5R 3P/10R 3P/15R 5P/5R 5P/10R 5P/15R

G*/sinδ (Pa)

80000

60000

40000

20000

100

10

Stain (%)

100000

BA 10% RA 5% PU 10% PU 1P/5R 1P/10R 1P/15R 3P/5R 3P/10R 3P/15R 5P/5R 5P/10R 5P/15R

1

0.1

0.01 0 45

50

55

60

65

70

75

Temperature (°C) Fig. 9. G*/sind of PU composite modified asphalt.

80

85

0

50

100

150

Time (s) Fig. 10. Time-strain curve at 64 °C.

200

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

Table 6 Comparison of non-recoverable compliance and the high-temperature grade. Samples

Classification of high temperature grade

Jnr0.1 (kPa1)

Jnr3.2 (kPa1)

BA 10%RA 5%PU 10%PU 1P/5R 1P/10R 1P/15R 3P/5R 3P/10R 3P/15R 5P/5R 5P/10R 5P/15R

64 76 64 64 70 70 76 70 76 82 70 76 82

2.34  105 1.33  105 9.95  105 1.60  104 6.75  105 6.06  105 5.95  105 1.05  104 9.84  105 8.82  105 1.10  104 9.98  105 9.55  105

1.01  104 3.02  105 3.53  104 4.35  104 1.96  104 1.61  104 1.53  104 2.10  104 1.69  104 1.59  104 2.18  104 1.75  104 1.69  104

In addition, the R and Jnr calculated from the experimental results are shown in Fig. 11, for an identical RA content, the elastic components of asphalt system increased with the increase of PU, the R of the PU composite modified asphalt can be improved remarkably. The Jnr value decreased gradually with an increase of PU in the PU composite modified asphalt. Moreover, the deformation recovery ability of the PU composite asphalt can also be improved, it was concluded that the PU performed a better resistance to permanent deformation than the RA.

3.2.2. Low temperature performance The relationship between creep stiffness (S) and temperature of the PU composite modified asphalt are shown in Fig. 12. The S of the PU composite modified asphalt exhibited a rapid increase with the decreasing temperature when the temperature was in the range from 16 to 24 °C. Whereas, the temperature range between 14 and 16 °C, the S of the PU composite modified asphalt gradually leveled off. For an identical PU content, the increase in the S value of RA was observed as the increasing temperature. This indicates that the higher content of RA, the worse low-temperature crack resistance of the PU composite modified asphalt will be. In addition, among all of samples, the 5P/5R showed the smallest S, indicating that the low-temperature crack resistance of the PU composite modified asphalt can be improved with the increase of PU. The m-value of the PU composite modified asphalt are determined from Fig. 13, the results imply that the m-value of the PU composite modified asphalt decreased with the decrease of temperature. When the content of the PU remained the same, the increase of RA with the m-value decreased. Therefore, it can be concluded that the low-temperature ductility and fatigue resistance of the PU composite modified asphalt can be reduced by

1.0

0.1 kPa 3.2 kPa 0.8

R (%)

0.6

0.4

0.2

0.0 BA

10%RA 5%PU 10%PU 1P/5R 1P/10R 1P/15R 3P/5R 3P/10R 3P/15R 5P/5R 5R/10R 5P/15R

Samples

Fig. 12. S of PU composite modified asphalt.

(a) R % of PU composite modified asphalt. 0.0006

0.1 kPa 3.2 kPa

0.0005

-1

Jnr (kPa )

0.0004

0.0003

0.0002

0.0001

0.0000 BA

10%RA 5%PU 10%PU 1P/5R 1P/10R 1P/15R 3P/5R 3P/10R 3P/15R 5P/5R 5R/10R 5P/15R

Samples

(b) Jnr of PU composite modified asphalt. Fig. 11. MSCR-based evaluation of PU composite modified asphalt.

Fig. 13. m-value of PU composite modified asphalt.

X. Jin et al. / Construction and Building Materials 234 (2020) 117395

9

the increasing content of RA [37]. Nevertheless, the m-value increased with an increase in the PU content. When the content of RA reached 5%, at 14 °C, 18 °C and –22 °C, respectively, the m-value of the PU composite modified asphalt decreased with the increase of PU. The m-value of the RA modified asphalt might be improved with the addition of PU. Also, the PU can effectively release the generating internal stress. Consequently, the addition of PU can inhibit the plasticity deformation of asphalt, which significantly improved the low-temperature performance of the PU composite modified asphalt. In a conclusion, the 5P/5R performed the best low-temperature performance. 3.2.3. Pg Based on the DSR and BBR test results, the PG of the PU composite modified asphalt was conducted according to the method of SHRP asphalt performance specification (G*/sind  1 kPa, S  300 MPa, m-value  0.3), and the results are shown in Table 7. The 3P/15R and 5P/15R displayed the best high-temperature grade, and the 5P/5R showed the best low-temperature grade. In a word, the PU played an leading role in the modification system, but the added percentage of RA should be restricted, since the RA not only weaken the low-temperature performance of PU composite modified asphalt, but also reduced the stress sensitivity at high temperatures. Besides, the 5% RA added into the PU composite modified asphalt was recommended, because a bit amount of RA cannot cause significant improvement of high-temperature performance of the PU composite modified asphalt. To further explore the micro-mechanism of PU and RA in the PU composite modified asphalt, the XRD, FITR, SEM, TG and DSC testing analysis were conducted in this study. 3.3. Microstructural characterization The influences of PU and RA dosages on the micro morphology of the composite modified asphalt were investigated to reveal the micro-mechanism of the PU composite modified asphalt. For the understanding and direct characterization of the micro-structure of the composite modified asphalt with different dosages of modifier, the amount of PU was chosen as 1%, 3% and 5%, and the RA was used as 5%, 10% and 15%, respectively. 3.3.1. XRD characterization of RA In order to qualitatively expose the mineral composition of the RA used, the XRD test result is illustrated in Fig. 14. The physical phase analysis result of the XRD spectra of the RA was obtained by using the software named High Score. The chemical components of these diffraction peaks corresponded to calcite (CaCO3), quartz (SiO2) and plagioclase in the RA, respectively. It is implied that the content of calcite in RA was the highest in all of mineral phases. And, there was also a certain amount of quartz

Fig. 14. XRD pattern obtained for RA.

and plagioclase. In addition, there existed a bit contents of sulfate, silica, alumina, magnesium carbonate and iron oxide in the RA, and a few amounts of metal-oxide such as sodium, strontium and titanium, which showed that the alkaline mineral element appeared in the RA. The physical adsorption and chemical adsorption can occur between acidic petroleum asphalt and alkaline minerals. Therefore, if the RA is mixed into the asphalt mixture, the adhesion and antistripping performance of asphalt and aggregate will be further improved due to the presence of the mineral element. 3.3.2. FTIR test The FTIR was used to further analyze the mechanism of the PU modifier on the composite modified asphalt. On account of the test results described above indicated that the addition of the PU and RA can improve the performance of the base asphalt, but it was unknown whether PU and RA reacted with base asphalt in the process of modification. Therefore, FTIR tests of several PU composite modified asphalts were conducted to confirm the existence of chemical reaction between the PU and the base asphalt. The FTIR test results of two modifiers and composite modified asphalt with different contents of PU and RA are shown in Figs. 15 and 16, respectively. As can be seen that the spectra presented quite different absorption bands. The broad absorption band centered at approximately 3634 cm1, which was caused either by —OH stretching.

Table 7 PG of PU composite modified asphalt. Samples

PG

G*/sind (kPa)

S (MPa)

m – value

10%RA 5%PU 10%PU 1P/5R 1P/10R 1P/15R 3P/5R 3P/10R 3P/15R 5P/5R 5P/10R 5P/15R

70–4 58–24 58–28 70–16 70–16 76–16 70–18 76–18 82–16 70–22 76–18 82–16

1.44 1.99 2.04 1.03 1.80 1.83 1.26 1.16 1.18 1.20 1.86 1.12

274 296 271 150 172 189 207 270 187 299 197 208

0.323 0.351 0.319 0.429 0.359 0.331 0.369 0.351 0.361 0.301 0.397 0.341 Fig. 15. FTIR spectra of PU and RA.

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

Fig. 16. FTIR spectra of base asphalt and PU composite modified asphalt.

A strong stretching in the C–H bond of CH2 group occurred at frequencies between 2920 and 2848 cm1. The absorption peak at 1551 cm1 can be assigned to the stretching vibration of C = C (benzene ring skeleton vibration) in the aromatic ring. The absorption peak at 1453 cm1 showed the presence of C = O (carbonyl in the inorganic carbonate). The peaks between 1300 and 1000 cm1 were assigned to the vibration of C—O bond in the aromatic ring. A wide absorption peak at 1159 cm1 corresponding to Si—O bond compounds’ modes of vibration. In the fingerprint region, the peaks between 650 and 910 cm1 were assigned to the vibration of H in the aromatic ring. Moreover, different substitution absorption positions of the benzene ring would be displayed in this region, and all of the peaks in this region can be assigned to the oscillation and vibration outside the C–H bond of the benzene ring framework. The conducted analysis above demonstrated that the main constituent groups of the RA include carbonyl, silicate, silica, unsaturated carbon chain, carbonate, etc., and the characteristic groups were consistent with the phase detection result of XRD. Additionally, it should be noted that the high content of asphaltene hetero-atomic groups, strong aromaticity and polarity of the RA may effectively enhance the adsorption capacity of asphalt on the surface of aggregate, resulting in the improvement of the anti-water stripping ability of asphalt [24]. The characteristic groups in the PU molecules are ammonia ester group, hydrocarbon group, phenyl group, ester group, ether group, hydroxyl group, isocyanate ester group, urea group, amide group, biuret group, allophanate, etc. [38–40]. The absorption peak at 3078 cm1 was the characteristic absorption peak of —NH (free

Fig. 17. Reaction between isocyanates and phenol [38].

NH stretching band). The wide absorption peak was about 2652 cm1 associated with —CH2 or —CH3 stretching. In addition, the strong peaks between 1300 and 1500 cm1 showed the introduction of alkyl groups. The absorption peak at approximately 2258 cm1 was formed by —NCO (isocyanate) group in the PU. The bands near 1480 and 1435 cm1 were assigned to the C = O stretching vibration. The broad absorption band centered at 1265 cm1 can be associated with C—O bond or C stretching in the ester group. The absorption peak at 1153 cm1 was produced by —O— (ether group) in urethane group. In the range of 500 to 900 cm1, many absorption peaks were the vibration of benzene ring in isocyanate. Since new functional groups did not appear, the process of the RA modified base asphalt can be regarded as a simple physical mix [41]. From Fig. 16, the functional groups from the spectroscopy derivatives associated with the PU, RA and base asphalt were clearly shown in FTIR results, it was observed that the absorption peak of —NH disappeared at the band of 3078 cm1 after the base asphalt modified by the PU and RA, and the band at 3634 cm1 was not found on FTIR spectra of the PU composite modified asphalt samples, which was related to —OH (free carboxylic acid vibration peak) stretching. As a same RA content, the symmetric and asymmetric stretching vibration absorption peaks of —CH3 and —CH2 were slightly shifted with the increasing additive amount of PU. When the PU content reached 5%, at around 1590 cm1, the peak (polycyclic aromatic hydrocarbons) disappeared, and the 3%PU was used, a new C = O stretching vibration absorption peak was found at the band of 1160 cm1 due to the binder’s oxidation. Generally, on account of the phenyl group is the electron-withdrawing group the reaction activity of isocyanate between phenol is weak, and the equation can be depicted in Fig. 17. Additionally, it was noted that a strong stretching in the C–H bond occurred at frequencies between 2920 and 2840 cm1. Band at 1590 cm1 correspond to C = C and C–C stretching vibration of the polycyclic aromatic hydrocarbons. Two new absorption peaks were found at the band of 1453 and 1375 cm1, which was C–H plane bending vibration absorption. Below 1000 cm1, there existed mainly C–H (=C–H) out-of-plane bending vibration absorption peaks in the unsaturated benzene ring. The weak absorption peak of S—S was found at 477 cm1. Therefore, it is known that the molecules of Liaohe petroleum asphalt produced from Panjin in China is mainly composed of carbonyl compounds, aromatic hydrocarbons, unsaturated and saturated hydrocarbons and a bit amounts of sulfur-containing compounds [42]. The schematic diagram of chemical reaction is shown in Fig. 18, it was found that the stretching vibration peak at the band of 3634 cm1 disappeared, which was attributed to the —OH (free carboxylic acid) of RA with less stretching. Partal [43] confirmed that the remained water in asphalt can react with cyanate to produce urea and release carbon dioxide. By contrast, the characteristic peaks of sulfur-containing compounds disappeared with the incorporation of PU modifier. This might likely be due to the reaction between the unsaturated bond in the PU and the S-S bond in the asphalt, which eventually formed the crosslink between macromolecules. In this case, X in Sx was 1– 2, since it’s difficult to make a sulfur bridge with more than two sulfur atoms, the schematic diagram of chemical reaction is displayed as follows (See Fig. 19):

Fig. 18. Reaction of isocyanate with carboxylic acid [38].

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

Fig. 19. Isocyanate react with sulfur [38].

On account of FTIR is used to obtain the absorbance/transmittance spectrum of solid, liquid, and gas materials, therefore, the compositions of chemical elements can be seen from the FTIR spectra. According to the test result, the petroleum asphalt is a complicated mixture of hydrocarbons including nonmetal derivatives of sulfur, oxygen, etc. In the process of preparation of the composite modified asphalt, the alkyl group linked with benzene ring was oxidized due to the combined action of thermal oxygen. In addition, the reaction occurrence of condensation dehydrogenation led to the aromatics converted into colloids continually. It is known that the properties of asphalt depend on the content of components in asphalt [44]. The addition of PU into the asphalt can increase the content of saturated fraction and colloid, also, reduce the viscosity of asphalt resulting in the improvement of the plasticity of asphalt. With an increase of asphaltene in RA, the temperature sensitivity of asphalt showed a decrease, and then the mass percentage of asphaltene and gum increased, which induced the improvement of viscosity of asphalt. Combined with the base performance tests results, it can be concluded that the formation and structure of PU in asphalt had an extremely important influence on the properties of the modified asphalt. The PU and RA with different mass fractions in the composite modified asphalt performed similar positions of peaks in

the FTIR spectra. It was found that there was a microfilamentous connection between the PU and asphalt by FTIR spectrum analysis, the reason is that the unsaturated bond in the PU can combine with the S-S bond in asphalt to form a threedimensional cross-linked structure. The PU was coated with asphalt and folded together, thereby, enlarging the scope of viscoelastic domain and improving the resistance of asphalt to lowtemperature cracking. Furthermore, the strong effect of the crosslinked structure restricts the transfer and the fluidity of asphalt colloids in asphalt, and enhances the resistance to external forces, only when a large external force is applied can the asphalt produce relative displacement. Therefore, the low-temperature resistance performance of the PU composite modified asphalt will be further improved with the higher inclusion of PU.

3.3.3. SEM test The SEM images of the PU composite modified asphalt magnified 500 times and 50 times are presented in Figs. 20 and 21, respectively. It can be seen from Fig. 20 that the surface morphology of the samples investigated were compacted, in which were homogeneous integral structure. In order to expose the modification mechanism of PU in asphalt. The microscopic morphologies of the samples were observed at 500 times magnification. It can be found from Fig. 20(a) that the section of asphalt was a relatively flat homogeneous structure. The micropore in Fig. 20(b) was caused by the residual water in the synthesis process of PU, resulting in the appearance of micropores in the material. As shown in Fig. 20(b) and (d), when PU was added from 1% to 5%, the surface of asphalt became rough gradually. However, the RA used in this study cannot be observed in these images. Combined with the XRD analysis results, it was speculated that the components of asphalt should be similar with

10μm

10μm (a)BA

(b)1P/5R

10μm (c)3P/5R

10μm (d)5P/5R

Fig. 20. The SEM images of base asphalt and PU composite modified asphalt magnified 500 times.

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

200μm

200μm

(a)1P/5R

(b)1P/10R

200μm (c)1P/15R Fig. 21. SEM images of 1P with different RA contents.

the RA, and the RA should disperse in asphalt in the form of physical blending. On the other hand, as the asphalt is non-conductive, it needs to be treated with gold spraying before the testing. It was possible that gold dust had covered the inorganic components in the RA. It can be seen that the base asphalt exhibited strong interaction with the RA and the PU. When the composite modified asphalt is damaged by external forces, the PU can absorb part of the energy and achieved the effect of stress transfer [47,48]. It was found that the content of PU reached 1 or 3%, the surface of the modified asphalt was brittle fracture interface without any accumulation. It can be observed that the PU particles of the composite modified asphalt with 5%PU was evenly wrapped by asphalt, and the surface morphology of asphalt was fuzzy. The SEM tests showed that the composite modified asphalt was a heterogeneous system, and the PU cannot be completely dissolved in asphalt, however, it was filled in asphalt as elastic particles. Moreover, it can be speculated from the coating degree of the PU and asphalt that the chemical reaction between the PU and asphalt occurred, and the crosslinking network structure was formed between the PU and asphalt, this will be helpful to change the mechanical properties of asphalt. It can be presented from Fig. 21 that the base asphalt, PU and RA exhibited a superior compatibility. For the composite modified asphalt containing 1%PU, as the content of RA increasing, the damage marks of the composite modified asphalt subjected to external forces was deepened. The inorganic components in the RA can strengthen the viscoelasticity of the base asphalt and made the

modified asphalt difficult to deform at high temperature. From Fig. 21(b) and (c) that the incorporation of RA should not be more than 10%, combined with the test results in Table 5, it was found that the increase in the content of RA can gradually increase the viscosity of the composite modified asphalt resulting in the improvement of the high-temperature performance. Nevertheless, the excessive addition of RA may limit the mobility of the asphalt and increase the modulus of the material, accordingly, and weakening the stress resistance of the composite system. 3.3.4. TG test The thermal behavior of two modifiers were evaluated by TG test and the results are shown in Fig. 22 and Table 8. There was no weight loss in the temperature range of 100–260 °C. The initial stage weight loss appeared over the temperature around 260 °C was primarily caused by the small molecule combustion, cracking and release of oxidation decomposition in asphalt. The largest weight loss that was occurred over the temperature range of 340–490 °C, which was attributed to the macromolecules split into small molecules and eventually become gaseous and volatile substance. The mass of RA residual accounts for only about 51.5% of the total weight at 500 °C. The thermal decomposition of the PU was divided into two stages, on account of the PU modifier used in this experiment belongs to two-phase structure. The second stage of degradation was more distinct than the first stage. Combined with the FTIR spectrum of PU, it was speculated that urea and allophanate might

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

Fig. 22. The TG curve of the modifiers.

Fig. 23. TG curves of PU composite modified asphalt.

Table 8 The ratio of thermal decomposition temperature of modifier to residual mass. Type of modifier

T5% (°C)

T10% (°C)

Residual mass ratio (%)

RA PU

340.7 321.6

416.3 331.5

51.30 2.37

cause differences in the melting peaks of the hard phase crystallization zone. In the vicinity of 321.6 °C, the PU started to break down. Around 331.5 °C, the degradation rate reached the maximum, this is mainly because of the decomposition of carbamate in the first stage resulted in the crystallization of the hard phase, leading to the discrepancy between the decomposition temperature of other groups and carbamate groups. Hence, it was easy to cause the difference in the degradation temperature and degradation rate. The second phase was mainly for the degradation of oligomer glycol, then carbon chain and the degradation of aromatic ring [45], after the temperature reached 650 °C, the mass of remains basically unchanged with the temperature continue to raise, and formed the thermal hysteresis. The thermal decomposition temperature of the RA modifier was higher compared to the PU modifier, and the residual mass ratio showed that the thermal stability of the PU modifier was worse than the RA. This result indirectly indicates that the RA can effectively improve the high temperature performance of asphalt. As shown in Fig. 23, from room temperature to 255.1 °C, the weightlessness curve of the base asphalt was a straight line, which suggests that there was no significant mass loss of asphalt in the range of temperature. Moreover, the physical and chemical reactions did not occurr in this stage. As the temperature increasing, the light component in asphalt converted to asphaltene and the weight of the base asphalt gradually decreased. The largest weight loss occurred over the temperature range of 265.7 to 550 °C. The mass of the residue was only about 9.47% at 700 °C. In addition, it was found that the thermal decomposition of all materials only had one stage. The order of initial decomposition temperature can be depicted as: 1P/5R < BA < 3P/5R < 5P/5R < 5P/10R < 5P/15R, the bond of molecules decreased with the increase of temperature, and hereby the original composition and formation of base asphalt had changed. With increasing contents in the PU and RA, the PU composite modified asphalt formed a polyphase system gradually including asphalt phase and polymer phase, due to its physical structure and chemical components change, resulting in the shown decrease of heat absorption peak area and improvement of the temperature stability significantly in the composite modified asphalt.

It can be seen from the enlarged image of the initial decomposition section that when the less amount of PU was used, the content of carbamate showed a decrease due to the crosslinking of molecular chains, which was also the reason that the 1P/5R and 3P/5R were worse thermal stability than the base asphalt, and the 1P/5R and 3P/5R exhibited worse thermal stability as compared to the base asphalt. However, the initial decomposition temperature of the composite modified asphalt increased with the increase of RA. The higher early loss temperature and the increased decomposition temperature of the crosslinked appearance led to improvement of the heat resistance of the composite modified asphalt when a large amount of RA was introduced. This suggests that the sheet structure in the RA did not act as a block of heat conduction, instead, improved the heat resistance of the modified asphalt. Moreover, the PU exhibited a similar improvement compared with the RA in terms of the thermal stability. As presented in Table 9, the thermal decomposition temperature of the composite modified asphalt with 5% PU increased gradually with the increase of RA. It was of great significance to explore the high-temperature resistance and thermal decomposition properties of the composite modified asphalt to expand the application of modified asphalt materials. 3.3.5. DSC test As shown in Fig. 24, both modifiers began to absorb heat as the temperature increasing. The result indicates that the chain segment starts to move and the material may melt as the temperature proceed to rise. As can be seen from the DSC curve of the PU, there were two peaks in the melting zone. The first peak appearance was due to incomplete hard segment crystallization of PU, which may cause the irregular arrangement of hard segment and the decrease of the hydrogen bonds and cohesion in the PU, the result of this

Table 9 Thermal decomposition temperature and residual mass ratio of PU composite modified asphalt. Samples

T5% (°C)

T10% (°C)

Residual mass ratio (%)

BA 1P/5R 3P/5R 5P/5R 5P/10R 5P/15R

255.1 254.1 255.7 292.1 307.6 329.2

265.7 262.4 268.9 294.2 309.7 332.3

9.47 4.39 6.25 9.19 12.14 11.06

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X. Jin et al. / Construction and Building Materials 234 (2020) 117395

Fig. 25. DSC test curves of PU composite modified asphalt. Fig. 24. DSC curves of the two modifiers.

phenomenon depended on the melting occurrence first followed by the start of melting in the neat region of the crystal. The heat peak on the DSC test curve represents the agglomeration state of materials is the proportion of solid phase and liquid phase. In the polymerization state, the thermal stability decreases with the increase of peak area. The peak area of RA was slightly larger than that of the PU, indicating that the PU showed higher temperature stability compared to the RA, and the temperature sensitivity of modified asphalt was reduced by the incorporation of PU. The glass transition temperature (Tg) of the RA and PU was determined as 0 °C, and 24.3 °C using equidistant method, respectively. It was further confirmed that there can be more hydrogen bonds because of the higher RA content. The hydrokeys formed can make the modified asphalt stiff and brittle due to the lower temperature sensitivity of the PU, and it was very helpful to improve the mechanical properties of asphalt. To brief, an appropriated amount of PU used in the composite modified asphalt can effectively make up the deficiency of the RA modifier. Asphalt is a low molecular weight material, in generally, its molecular weight is no more than 6000. Due to the small molecular weight and weak connection between molecules, it is quite difficult for asphalt suffered from load to maintain original state. Moreover, asphalt is a complex polymer and it often presents morphological changes at different temperatures. It can be seen from the value of Tg that the proportion of saturates content and asphaltene in asphalt may exhibit certain regularity, namely asphaltene and Tg decreased with increasing saturates. While the midpoint of Tg can display the average situation of the entire glassy transition region, hence, the Tg is suitable for the glassy transition analysis. The purpose of this study is mainly to investigate the influence of the amount of RA and PU on the thermal properties of asphalt. The DSC curves of the composite modified asphalt with different PU and RA contents are shown in Fig. 25, as the temperature increasing, the DSC curves showed a peak in terms of Tg, indicating that the phase states of all the asphalt samples used had changed. Asphalt is composed of a variety of complex components that has different phase transition temperature, peak size and temperature range. Based on the theory of thermal characteristics, the temperature causes the aggregation state of different molecules and components of the molecular structure in asphalt to change from glass state to rubber state and finally to viscous flow state. The above views were further verified by experiments. There was no significant difference in the heat flow between the base asphalt and the PU composite modified asphalt, what suggests that

the structure of composite modified asphalt did not change significantly in nature. However, for a same RA content, the 3P/5R performed the smallest heat flow rate with regard to the all samples, showing that the 3P/5R presented best thermal stability. The results of rheological properties were verified from the perspective of thermal characteristics. During the development from viscous state to viscoelastic state, the base asphalt showed similar variation trend in the DSC curve as compared to the PU composite modified asphalt. The Tg of the 1P/5R was inferior to that of base asphalt. Therefore, the RA played a dominant role in improving the ratio of viscoelasticity to elasticity of asphalt, because of the less amount of PU added. When the content of PU was higher than 3%, the PU composite modified asphalt showed a decrease in regard to Tg, also, the 5P/5R performed 1.47% lower than the base asphalt (See Fig. 26). In addition, the Tg of the composite modified asphalt containing 5% PU increased with an increase in the RA content, which suggests that the addition of the RA reduced the ratio of viscoelasticity to elasticity of the PU composite modified asphalt. This is because the inorganic components in the RA combining with the saturated and aromatic components with small molecular weight and strong fluidity in asphalt to increase the content of asphaltene, accelerate the phase change of asphalt components, and thus increased the component of vitrification transformation. In fact, vitrification

Fig. 26. Tg variation at different PU and RA contents.

X. Jin et al. / Construction and Building Materials 234 (2020) 117395

transformation is the phenomenon of molecular chain fracture in asphalt. Asphalt would show viscoelasticity above Tg and brittle fracture properties below Tg. As the same RA content, the Tg decreased with increasing PU content. This means that the low temperature performance of asphalt improved with the addition of PU. For an identical PU content, the Tg decreased with the increase of RA. This suggests that a reduced low-temperature performance was exhibited with the addition of RA, in which also verified the results of rheological properties. 4. Conclusions In this study, the PU composite modified asphalt samples were successfully produced by a newly developed preparation process, and then the laboratory performance including rheological properties and micro-characteristics were investigated and discussed. Based on the study conducted, the following conclusions were drawn from the results of this study. (1) For the use of identical amount of RA in the composite asphalt, an increased content of PU from 1% to 5%, the penetration increased 25%-42%, and the ductility increased 6%19%, respectively, however, the softening point exhibited a slight variation. As the RA increasing from 5% to 15%, the softening point of the PU composite modified asphalt with the same amount of PU increased 4%-7%, whereas, the penetration and ductility decreased 25%-42% and 6%-19%, respectively. The composite modified asphalt with 5% PU and 5% RA exhibited a favorable performance in terms of the penetration, ductility, softening point and rotational viscosity. Overall consideration, the PU composite modified asphalt with 5% PU and 5% RA exhibited a favorable performance in terms of the penetration, ductility, softening point and rotational viscosity. (2) The RA modified asphalt can still exhibit an excellent hightemperature performance when the same content of PU was used in the composite modified asphalt. The PU composite modified asphalt with 5% RA and the use of PU was more than 3% can effectively reduce the low-temperature performance of base asphalt. The 3P/15R and 5P/15R showed the best high-temperature grade and the 5P/5R displayed the best low-temperature grade among the dataset. (3) The permanent deformation resistance of the composite modified asphalt was improved with the inclusion of PU, and the PU performed a better resistance to permanent deformation than the RA. The unrecoverable creep compliance at 3.2 kPa stress combining with the high temperature PG can more accurately evaluate the resistance to permanent deformation of the composite modified asphalt. The 5P/15R and 3P/15R showed excellent and similar resistance to permanent deformation. (4) The mineral phase in the RA was dominated by calcium carbonate, and the physical and chemical adsorption can occur between the acidic petroleum asphalt and the alkaline mineral leading to the improvement of adhesion and antistripping performance of asphalt and aggregate due to the incorporation of RA into the asphalt. The phase ingredient of RA was similar to that of the base asphalt. (5) The PU and RA used in the investigation performed a good compatibility with the base asphalt. The composite modified asphalt containing 5% RA can significantly strengthen the crosslinked structure, and the anti-low temperature cracking ability increased with an increase in the PU content. However, when the content of RA was greater than 5%, the composite modified asphalt exhibited a decrease in terms of the low-temperature cracking resistance.

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(6) The FTIR test further proved that isocyanate in the composite modified asphalt reacted with phenol and carboxylic acid in asphalt, respectively. And the unsaturated bond of PU cross-linked with S-S bond in asphalt. In the composite modified asphalt, the PU coated asphalt was folded and overlapped together leading to the increase of the viscoelastic domain and the improvement of the low temperature cracking resistance of the asphalt. (7) The PU exhibited a similar improvement as compared to the RA with regard to the thermal stability. However, the PU can increase the low-temperature performance. The addition of RA into the PU modified asphalt reduced the lowtemperature performance and increased the formation of hydro-keys, which also verified the investigation results exhibited for the rheological properties. The composite modified asphalt with 5% PU and 5% RA, as can be regarded as an appropriate composition, performed a favorable performance based on the investigation conducted in this study.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was funded by the Liaoning Highway Administration Bureau under Grant No. 201701 and National Natural Science Funding of China under Grant No. 51308084. References [1] M. Sun, M. Zheng, G. Qu, et al., Performance of polyurethane modified asphalt and its mixtures, Constr. Build. Mater. 191 (2018) 386–397. [2] P. Cong, N. Liu, Y. Tian, Y. Zhang, Effects of long-term aging on the properties of asphalt binder containing diatoms, Constr. Build. Mater. 123 (2016) 534–540. [3] J. Zhang, Z. Fan, D. Hu, et al., Evaluation of asphalt-aggregate interaction based on the rheological properties, Int. J. Pavement Eng. 19 (7) (2018) 586–592. [4] J. Chen, T. Wang, C. Lee, Evaluation of a highly-modified asphalt binder for field performance, Constr. Build. Mater. 171 (2018) 539–545. [5] B. Bazmara, M. Tahersima, A. Behravan, Influence of thermoplastic polyurethane and synthesized polyurethane additive in performance of asphalt pavements, Constr. Build. Mater. 166 (2018) 1–11. [6] X. Cao, Z. Zhang, P. Hao, Research of the effects of PPA on high and low temperature properties of asphalt mixture, J. Wu Han Univ. Technol. 36 (6) (2014) 47–53. [7] D. Lesueur, The colloidal structure of bitumen: consequences on the rheology and on the mechanisms of bitumen modification, Adv. Colloid. Interfac. 145 (1–2) (2009) 42–82. [8] L.H. Lewandowski, Polymer modification of paving asphalt binders, Rubber. Chem. Technol. 67 (3) (1994) 447–480. [9] A.H. Fawcett, T.M. McNally, A dynamic mechanical and thermal study of various rubber-bitumen blends, J. Appl. Polym. SCI. 76 (2000) 586–601. [10] P. Król, Synthesis methods, chemical structures and phase structures of linear polyurethanes. Properties and applications of linear polyurethanes in polyurethane elastomers, copolymers and ionomers, Prog. Mater. SCI. 52 (6) (2007) 915–1015. [11] D. Hermida-Merino, B. O’Driscoll, P.J. Harris, et al., Enhancement of microphase ordering and mechanical properties of supramolecular hydrogen-bonded polyurethane networks, Polym. Chem-UK 9 (2018) 3406–3414. [12] W. Gao, M. Bie, Y. Quan, et al., Self-healing, reprocessing and sealing abilities of polysulfide-based polyurethane, Polymer 151 (2018) 27–33. [13] M.H. Jomaa, L. Roiban, D.S. Dhungana, J. Xiao, et al., Quantitative analysis of grafted CNT dispersion and of their stiffening of polyurethane (PU), Compos. SCI. Technol 171 (2019) 103–110. [14] A. Zhou, C. Li, S. Li, Polyurethane industry status and application, China Syn. Fib. Ind. 36 (2) (2013) 46–49. [15] A.A. Cuadri, M. García-Morales, F.J. Navarro, et al., Effect of transesterification degree and post-treatment on the in-service performance of NCOfunctionalized vegetable oil bituminous products, Chem. Eng. SCI. 111 (2014) 126–134. [16] C. Li, Preparation of polyurethane modified asphalt and research on road performance evaluation of mixture, J. Wu Han Univ. Technol. 41 (6) (2017) 958–963.

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