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Temperature induced self-healing capability transition phenomenon of bitumens
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Full Length Article
Temperature induced self-healing capability transition phenomenon of bitumens ⁎
Guoqiang Sun, Mingjun Hu, Daquan Sun , Yue Deng, Jianmin Ma, Tong Lu Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, Shanghai 200092, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Bitumen Self-healing capability transition Healing cross temperature (HCT) Healing peak temperature (HPT) Phase transition (PT)
The temperature may induce the inconsistent order of self-healing capability between two bituminous materials, which could be called as the temperature induced self-healing capability transition phenomenon. To confirm this phenomenon, two base bitumens and four different styrene–butadiene-styrene polymer modified bitumen (SPMB) binders are selected to systematically investigate their self-healing capability through establishing the healing rate temperature (HR-T) curves and healing index temperature (HI-T) curves. Fluorescence microscopy (FM) is applied to study the internal morphology of different SPMB binders. The phase transition (PT) temperature range is determined by the differential scanning calorimeter (DSC) test. Within the PT temperature range, three kinds of special temperatures are discovered, i.e. healing peak temperature (HPT), healing critical temperature range (HCTR) and healing cross temperature (HCT). The HPT and HCTR are both lower than the high temperature PT range, followed by the softening point, and can be applied as the optimum healing temperature to guide the formulation of energy efficient pavement induction healing strategies. The HCT clearly divides the HR-T curves or HI-T curves of bitumens into different parts, thus leading to an opposite order of selfhealing capability between two bitumens, which confirms the self-healing capability transition phenomenon. Therefore, the self-healing capability can not be evaluated comprehensively and accurately at only one given temperature. Most notably, it can be inferred that the temperature induced phase structure and property transition of bitumen is the essential reason for the self-healing capability transition phenomenon, which is significantly affected by the bitumen type, polymer type and content.
1. Introduction Bituminous concrete is a typical viscoelastic material, which has self-healing capability. The self-healing capability of bituminous materials have been a focus of research in both laboratory and field since 1960s [1,2]. On the one hand, stress relaxation in the crack region and spontaneous interface healing will occur to reduce surface free energy [3–7]. On the other hand, bitumen has the abilities of interfacial wetting and diffusion to close the cracks [1,8–11]. These characteristics provide the basic conditions for self-healing of bituminous materials. Temperature is the principal external factor affecting the self-healing efficiency of bituminous materials, and polymer modification is an important internal factor. It is of great significance to investigate the effect of these key factors on the self-healing capability for the design of long-life self-healing pavement with zero maintenance. However, researchers have different understandings on the influence of temperature and polymer modification on the self-healing capability of bitumen.
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Especially, the conclusions on the influence of temperature on the self-healing capability of bituminous materials are not uniform. A typical opinion is that the self-healing capability of bitumen binder will increase monotonously with temperature. It was well reported that the self-healing behaviour below the Glass transition temperature will disappears since the bitumen molecule interdiffusion between crack faces almost ceases at such low temperatures [12]. As the temperature rises to the medium temperature range (roughly within 5 ℃–35 ℃, depending on the bitumen types), the bitumen molecule will move from freezing state to moving state, thus the healing ability increases markedly at the medium temperature [4,13–16]. Most of self-healing articles have focused on the healing behaviour of bitumen materials at this temperature range. Another opinion is that there exists a critical threshold temperature for bitumen self-healing, and the optimum healing temperature obtained by different scholars is different [17–20]. Tang et al. thought that the softening point temperatures of bitumens are the optimal temperatures to heal fatigue damage [17]. García et al. and Zhanget al. revealed that the binder at the flow threshold
Corresponding author. E-mail address: [email protected] (D. Sun).
https://doi.org/10.1016/j.fuel.2019.116698 Received 15 August 2019; Received in revised form 3 November 2019; Accepted 19 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Guoqiang Sun, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116698
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Table 1 Physical properties of bitumen binders used in the investigation. Binders’ Code
Description
Penetration (25 ℃, 100 g, 5 s), 0.1 mm
Softening point (R&B), ℃
Rotational viscosity at 135 ℃, mPa.s
Bit-A Bit-B Bit-A-3%S Bit-A-4%S Bit-A-6%S Bit-A-4%L Bit-B-4%S
Bitumen Bitumen Bitumen Bitumen Bitumen Bitumen Bitumen
69.8 103.2 49.8 47.1 42.2 49.0 66.5
49.6 42.2 64.2 69.3 73.4 66.2 63.5
468.6 301.3 1036.2 1477.6 1714.8 1125.8 1000.4
A B A + 3% star SBS A + 4% star SBS A + 6% star SBS A + 4% linear SBS B + 4% star SBS
temperature will become a near-Newtonian fluid and can flow into the cracks and heal them rapidly (usually ranging from 30 ℃ to 70 ℃, depending on the type of bitumen and aging level) [18–20]. However, Sun et al. thought that the molecular diffusion rate and diffusion range are more distinct especially within the phase transition temperature range of base bitumens, and determined the optimal healing temperature range of bitumens is around 40.3 ℃–48.7 ℃ [11]. Therefore, it can be assumed that the self-healing capability of bitumen binder may have a very complex nonlinear relationship within a wide temperature range. Especially, the research on promoting the self-healing of bituminous materials by microwave/electromagnetic induction heating is very extensive in recent years [7,21–24], and a key issue is how to determine the real optimum healing temperature. Hence, it is necessary to conduct a more in-depth investigation on the influence mechanism of temperature on the bitumen healing ability, thus helping formulate an energy efficient pavement induction healing strategy. With regard to the polymer modification, some contradictory understandings on the effect of SBS polymer modifiers on the healing capability of bituminous materials are widespread as yet [7,21]. More results show that polymer modified bituminous materials possess superior healing capability to base bitumen materials. Shen and Carpenter found that the healing rate of polymer modified asphalt mixtures at 20 ℃ was much better than that of the neat asphalt mixtures [25]. The further work from Shen et al. showed that the healing rate of the polymer modified asphalt PG 70-28 was greater than that of the neat asphalt PG64-28 based on the DSR fatigue test (at 15 ℃ and 25 ℃) [14]. Sun et al. concluded that the SBS modified asphalt (linear SBS, 5%wt) exhibited the most excellent healing efficiency than other neat bitumens with different penetration grade values (20, 50, 70, 100) by FM observation at room temperature, attributable to the SBS-rich elastic network structure [10]. However, some researchers considered that the polymer modification might have a small or even negative effect on the healing capability of base bitumen. Little et al. indicated that the addition of SBS acted as a filler system to interrupt the ability of pure asphalt healing [26]. They thought that polymer networks in bitumen partially absorb more compatible components and thus swell, leading to a higher asphaltene (highly interactive) left in the rest of the bitumen. The bitumen with a higher asphaltene concentration is less likely to flow and heal. Lv et al. also found that the increased modifier content deteriorated the healing capability of the base binder regardless of the type of additive employed by the Binder Bond Strength (BBS) test at 25 ℃ [27]. Furthermore, Qiu et al. investigated the effect of SBS modifier on the healing behavior of asphalt materials at 10 ℃, 20 ℃ and 40 ℃ by the DDT-based fracture-healing-re-fracture (FHR) test [28]. They found that the SBS modified mastics had higher healing rate than base bitumen mastics at 10 ℃, but showed much lower healing rate at 20 ℃ and 40 ℃. Hence, it can be assumed that the order of self-healing capability between SBS modified bitumen and base bitumen may be different at different temperatures. Overall, it may be biased to evaluate the self-healing capability of bitumens at only one given temperature. It can be assumed that the temperature may induce the inconsistent order of self-healing capability between two bituminous materials, which could be called as the temperature induced self-healing capability transition phenomenon.
Hence, to confirm this phenomenon, two base bitumens and four different SBS polymer modified bitumen (SPMB) binders are selected to systematically investigate their self-healing capability at different temperatures through establishing the healing rate temperature (HR-T) curves and healing index temperature (HI-T) curves. In addition, the basic physical properties, morphology and phase transition temperature range of different bitumen binders are analyzed. The analysis of selfhealing capability transition can provide a more accurate and comprehensive method to evaluate and distinguish the self-healing capability of different bitumens (especially for polymer modified bitumen and base bitumen), so as to guide the selection of bitumen binders with high self-healing capability. Moreover, the investigations on the wide temperature self-healing behaviour can help formulate an energy efficient pavement induction healing strategy. 2. Experiments 2.1. Materials Two base bitumens and two commercial SBS block copolymers (star-branched structure and linear structure) are used to produce SBS polymer modified bitumen (SPMB) samples investigated in this study. Some basic properties of the bitumen samples are listed in Table 1. SPMB samples are prepared by blending SBS block copolymers and bitumen at 175 ℃ for 1 h using a high shear mixer. The SBS concentration is varied in the range of 3–6 wt% (by weight of bitumen) as detailed in Table 1. 2.2. Fluorescence microscopy (FM) test The morphology of the SPMB samples is studied using an Olympus Fluorescence microscope, equipped with a digital camera. FM micrographs are taken at original magnifications of 4, 20 and 100. FM specimens are prepared by the hot melt method and the micrographs are taken at room temperature. 2.3. Differential scanning calorimeter (DSC) test To determine the phase transition (PT) temperature ranges of different bitumen binders, the Differential scanning calorimeter (DSC) test is employed to measure the heat amount required to increase the temperature of a sample. The test conditions are listed in the following: 1) temperature range: −50 ℃–100 ℃; 2) heating rate: 10 ℃/min; 3) sample weight: 2–5 mg; 4) test instrument: DSC Q2000; 5) reference material: indium (a kind of metal with high-purity). 2.4. Self-healing behaviour characterization based on fatigue-healingfatigue (FHF) test The DSR FHF test was performed at different temperatures (15 ℃, 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃, 45 ℃ and 50 ℃) by DSR (AR1500EX, TA Instruments Company, USA). Based on our previous study [11,16], recovery of 30% damaged bitumen binder could characterize the healing property of bitumen binder better. It would take much time to heal the 2
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depicted in Fig. 3. It could be clearly observed the polymer network is gradually connected into a continuous network structure with the increase of SBS content. When the SBS content is low (3%wt), SBS is not uniformly distributed in bitumen phase, the polymers tend to form aggregates with diameters of 2–8 µm according to the enlarged micrograph. When the SBS content is increased to 4%wt, SBS in bitumen phase begin to form polymer networks, but does not connected completely. Notably, the star SBS in bitumen seems to have coarser network structure than that in linear SBS modified bitumen, which may explain that Bit-A-4%S has higher softening point and viscosity than Bit-A-4%L. When the SBS content is increased to 6%wt, a dense continuous network is formed in bitumen phase. The formation of continuous network is also reflected in the physical/mechanical properties. For example, a marked increase in the softening point and rotational viscosity and a pronounced decrease in the penetration value are observed when the SBS content is increased from 3 to 6 wt% (Table 1).
Fig. 1. DSR FHF test process.
fatigue cracks generated in bitumen binder once the damage degree is more than 30% (like 50%), thus leading to a poor accuracy when evaluating the healing efficiency of the seriously damaged bitumen. Hence, the damage degree is set as 30%, namely the loading would stop when the initial modulus drops to 70% of the initial value. Then the test is stopped for different time to heal the cracks. After that, the DSR test is continued until the modulus dropped to 70% of the initial modulus again. The FHF test process of is shown in Fig. 1. The healing index HIG is shown in Eq. (1) is used to estimate the self-healing efficiency of bitumen binder:
HIG =
3.2. Phase transition (PT) temperature determination The PT temperature ranges are determined based on the DSC curve. DSC curve can reflect the change of aggregate states of bitumen with temperature. The DSC test results of all investigated binders are shown in Fig. 4. Two heat absorption peaks can be clearly observed in the range of test temperature regardless of the bitumen types. As shown in Fig. 4(a), the thermal analysis software (TA Universal Analysis) is applied to calibrate the marked endothermic peak. The temperature ranges with the largest slope and the corresponding PT peak temperature will be automatically identified by the software, i.e. medium temperature PT range and high temperature PT range. Near the peak PT temperature, the solid–liquid transition of aggregates in bitumen will be more intense, which will lead to significant changes in the macroscopic properties of bitumen [34–37]. Two PT temperature ranges can be determined for each bitumen type, as listed in Table 2. It could be found that different bitumen binders have different PT temperature range. Actually, the bitumen is a mixture of substances with different molecular sizes, chemical compositions and structures. Due to the difference of chemical composition and molecular structure, the PT temperatures of different bitumens are inevitably different. For base bitumen binders, it appears that Bit-B has lower peak PT temperature than Bit-A. Bit-B is softer than Bit-A, which means that Bit-B contains more light components (liquids state), so the PT temperature of Bit-B is lower. When the bitumen is modified by SBS polymer, the phase structure in bitumen will change a lot. On the one hand, the polymer will absorb the light components of bitumen, thereby increasing the relative content of large molecule components in bitumen, like asphaltene (solid state), thus finally inducing the increase of the PT temperature of bitumen. On the other hand, the PT temperature of SBS polymer itself is much higher than that of bitumen, so the PT temperature of SPMB is higher than that of base bitumen. In addition, the peak PT temperature of modified bitumen gradually increases with the increase of polymer content, which is just the same order of softening point ranked in Table 1. It can be analyzed that the medium temperature PT peak temperature of the bitumen binders is about 13 ℃ (based on their arithmetic average), and the high temperature PT peak temperature is about 43 ℃. Hence, the PT temperature range of the bitumen binders can be roughly determined as 13 ℃–43 ℃, in which the bitumen will undergo frequent solid–liquid transition. Seven critical temperatures around the PT temperature range, i.e. 15 ℃, 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃ and 45 ℃, are selected for the FHF test.
∗ ∗ Ghealing − Gter min al ∗ ∗ Ginitial − Gter min al
(1)
∗ where Ginitial
is the initial complex modulus after one minute of shearing ∗ to get the stable value. Ghealing is the complex modulus after healing, and ∗ ∗ Gter min al = 70% × Ginitial . The healing process contains two vital stages, i.e. crack closure and strength gain [29]. Some micromechanical and phenomenological healing models in polymers have been proposed to describe these two stages [30–32]. The molecular diffusion model proposed by Wool and O’Connor is one of the most well-known models. This model revealed independently a straight-line relationship with an exponent of 0.25 for fracture stress and healing time using double cantilever beam tests on polybutadiene [30]. According to the extensive work done in polymer healing by Wool and O’Connor et al. [30], some studies also pointed out two stages for bitumen materials including [8,10,16,29,33]: (1) the instantaneous strength gained due to crack closure and cohesion driven by wetting, and (2) long term strength gained due to diffusion and randomization of molecules from one face to the other. The observed macroscopic healing recovery at temperature T and time t, HI (T, t), can be written as:
HI (T , t ) = HI0 (T ) + HR (T )·t 0.25
(2)
where HI (T, t) is the observed macroscopic healing recovery at temperature T and time t. HI0 (T) represents the instantaneous strength gain. HR(T) is a temperature-dependent parameter, which indicates the strength gain rate at temperature T, i.e. healing rate (HR). In this study, two kinds of healing indexes, i.e. healing index (HI) and healing rate (HR), are used to characterize the self-healing capability of different bitumens. Fig. 2 exhibits the framework for the whole experimental program of this study. 3. Results and discussion 3.1. Internal morphology analysis
3.3. Self-healing behaviour equation of different bitumens Fig. 3 shows the evolution of the SPMB morphology with increasing SBS polymer content. In these FM micrographs, the SBS-rich phase appears light yellow, whereas the bitumen-rich phase appears dark. FM micrographs with original magnifications of 4, 20 and 100 are clearly
In order to explore the self-healing behaviour evolution of different bitumens with temperature, the healing behaviour curves of two base bitumens and their modified bitumens at different temperatures are 3
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Penetration, Softening point, Rotational viscosity Basic physical prperties
Base Bitumens Internal morphology analysis of different SPMB binder
SBS Polymer
FM
DSC
Phase transition temperature range determination
SPMB binders DSR
Self‐healing transition phenomenon based on FHF test: HI‐T curve, HR‐T curve Fig. 2. Experimental program.
fitted according to Eq. (2), as shown in Fig. 5. As Fig. 5 exhibits, it can be clearly observed that there is a good linear fitting relationship between the healing index (HI) of different bitumens and the 0.25 power of healing time (R2 > 0.95), which further proves that Wool’s diffusion healing theory is also applicable to modified bitumens [30]. As discussed in Section 2.4, the slope of the healing curve can represent the healing rate (HR) of bitumen. Obviously, the temperature has a significant effect on the healing capability of different bitumens. It is noted that the SPMB binders (Bit-A4%S and Bit-B-4%S) exhibits higher healing capability (HI and HR) than base bitumens at 15 ℃ and 20 ℃, but then with the increase of temperature (> 25 ℃), their healing capability gradually begins to be lower than that of base bitumens. This phenomenon reveals that there is healing cross temperature (HCT) between different bitumens. Most notably, Bit-A-4%S has the highest healing recovery index (HI) at 15 ℃ and 20 ℃ (around medium temperature PT range), but its HI value becomes the lowest when the temperature increases to 30 ℃, 35 ℃ and 40 ℃ (around high temperature PT range). On the contrary, Bit-B has lower HI value at 15 ℃ and 20 ℃, but its HI value becomes the highest at 25 ℃, 30 ℃, 35 ℃ and 40 ℃. Another interesting observation from Fig. 5 is that there is healing peak temperature (HPT) for different bitumens. For example, the HR value of Bit-B increases from 0.026 at 15 ℃ to a peak value 0.184 at 30 ℃, then decreases to 0.120 at 40 ℃. This particular temperature also exists in other bitumens. Based on the analysis of the self-healing behaviour curves, it is fully confirmed that the self-healing behaviour of the bitumens is a very sensitive process to temperature. Hence, it is biased to evaluate the selfhealing capability of bitumens at only one given temperature. In the vicinity of PT temperature range (13 ℃–43 ℃), two kinds of special temperatures in bitumen, i.e. HCT and HPT, are discovered, ultimately leading to the self-healing capability transition of different bitumens. Further analysis on the temperature induced self-healing capability transition of different bitumens will be presented in next section.
curve, but there may be more than one HCT point at the HR-T curves. By the way, the HCT exists at the cross point between two HR-T curves of two different bitumens. According to healing flow and diffusion theory in bituminous materials [10,11,17–20], when the bitumen is heated to its glass transition temperature, it will gradually liquefy and enter into the cracked face, flowing sluggishly and diffusing into the fractured matrix driven by gradients of pressure or of concentration, and entangling with the matrix molecules by surface energy difference or thermodynamic driving force over long time periods. However, less researches focused on the self-healing evolution process in the whole PT temperature zone. The existence of HPT and HCT in the PT temperature range leads to self-healing capability transition between different bitumens, and demonstrates the complexity of the self-healing behaviour of bitumen evolving with temperature. Moreover, it can be observed that the HI values of different bitumens will reach to 0.95–1.00 at higher temperature range, which means that the bitumens are almost healed completely at this temperature range. This temperature range is called as the healing critical temperature range (HCTR). Evidently, the HCTR of each bitumen is lower than its high temperature PT range, followed by its softening point. The HPT exists at the peak point of each HR-T curve. From Fig. 6(a), it can be found that the HPT values of Bit-A, Bit-B, Bit-A-4%S and Bit-B4%S are around 35 ℃, 30 ℃, 40 ℃ and 35 ℃, respectively, which is close to the HCTR. The HPT is within the PT temperature range, in which the bitumen will undergo a very intense solid–liquid transition, so that the flow healing rate of bitumen will also change, and reach a peak state at a certain temperature, namely HPT. Once exceeding this temperature (HPT), the solid–liquid transition rate of bitumen tends to be stable, which makes the flow healing rate of bitumen also begin to decrease. Hence, the HPT value will change with the change of bitumen phase structure, so the HPT value of different types of bitumen will also be different. For Bit-A and Bit-B, Bit-B is softer than Bit-A since Bit-B contains more light components (liquids state) , so the solid–liquid transition in Bit-B will occur more easily than that in Bit-A at the same temperature. As a result, the HPT of Bit-B is lower than Bit-A. Similarly, the SBS polymer will change the phase structure of bitumen and hinder the solid–liquid transition of bitumen, thus reducing the flow rate of bitumen phase in the SPMB binder, so the HPT of Bit-A-4%S is higher than that of Bit-A, and the HPT of Bit-B-4%S is higher than that of Bit-B. Correspondingly, the HCTR of bitumen also changes with the change of bitumen phase structure. The HCTR of Bit-B is lower than that of Bit-A, and the HCTR of base bitumen is lower than that of their modified bitumen. Actually, the HCTR represents the temperature range in which the mechanical properties of bitumen (HI) almost completely recover, while HPT represents the temperature when the bitumen healing rate is
3.4. Temperature induced self-healing capability transition of different bitumens 3.4.1. Effect of bitumen type on self-healing capability transition To further explore the effect of bitumen type on the self-healing capability transition, the relationship curves of HI-T and HR-T are established in Fig. 6. The HI value at 60 min is selected to build the HI-T curve. As described in Section 3.4, the HCT and HPT for four bitumens can be easily marked at the HR-T and HI-T curves, as shown in Fig. 6. It can be seen that each bitumen will have only one HPT point at the HR-T 4
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Fig. 3. The effect of SBS content and type on the morphology of SPMB: (a) BitA -3%S, (b) Bit-A-4%S, (c) Bit-A-4%L, (d) Bit-A-6%S.
Bit-A and Bit-B in HI-T curve (as shown in Fig. 6(b)) since Bit-B enters the HCTR much earlier than Bit-A. For Bit-A, the HCT between Bit-A and Bit-A-4%S is around 27 ℃. Bit-A-4%S shows a higher HR value than Bit-A below 27 ℃, but shows a much lower HR value than Bit-A above 27 ℃. According to the above flow theory, it is easy to explain Bit-A-4%S has lower HR above 27 ℃ since the SBS polymer will hinder the flow of bitumen phase [26–29], but it is hard to explain why Bit-A-4%S has higher HR than Bit-A below 27 ℃. Actually, the SBS polymer elastic network structure also promotes the recovery of mechanical properties of bitumen according to many previous studies [10,14,25], so that Bit-A-4%S has higher HR than Bit-A below 27 ℃. It can be inferred that SBS polymer modification has dual effects on bitumen healing. On the one hand, the SBS elastic network structure in bitumen phase promotes the healing recovery of bitumen especially around the medium temperature PT range. On the other hand, the SBS polymer absorbs light components and hinders the flow of bitumen, thus reducing the healing rate especially around the high temperature PT range. For Bit-B, it can be found that there are two HCT points between Bit-B and Bit-B-4%S. The first HCT point is around
at the highest value. Therefore, the HCTR of bitumen is similar to or slightly higher than HPT. As for the HCT phenomenon, it exists between the healing temperature curves of different bitumens (HR-T curve or HI-T curve), as shown in Fig. 6(a) and (b). Due to the differences in the molecular composition and structure of different bitumens, the temperature induced PT in different bitumens is different, resulting in the temperature induced healing flow rate differences in different bitumens. Hence, the healing-temperature curves of different bitumens will inevitably cross at a certain temperature. It is exhibited that the HCT of Bit-A and Bit-B is around 34 ℃, and Bit-B shows a higher HR value than Bit-A below 34 ℃, but shows a much lower HR value than Bit-A above 34 ℃. Since Bit-B is softer than Bit-A, the healing flow rate of Bit-B is higher than Bit-A. However, once exceeding the HPT of Bit-B (i.e. around 30 ℃), the HR of Bit-B begins to decrease, and the HR of Bit-A is still growing until reaching to the HPT of Bit-A (i.e. around 35 ℃). Hence, Bit-B shows a much lower HR value than Bit-A above 34 ℃. For the convenience of explanation, this phenomenon of HPT induced healing capacity reduction can be called “HPT effect”. Notably, there is no HCT between 5
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temperature range of bitumens. Combining with the analysis of PT temperature of different bitumens, it can be found that the self-healing capability transition behaviour of bitumens is closely related to the temperature induced PT process in bitumen. It can be inferred that the temperature induced phase structure and property transition of bitumen is the essential reason for the self-healing capability transition of bitumens. 3.4.2. Effect of SBS polymer content on self-healing capability transition To study the effect of SBS polymer content on the self-healing capability transition, the HI-T and HR-T curves of Bit-A, Bit-A-3%S, BitA-4%S and Bit-A-6%S are shown in Fig. 7. From Fig. 7(a), the HPT values of SPMB binders with different polymer contents are all around 40 ℃, which means that the HR will decrease after 40 ℃. Moreover, their HPTs enter the HCTR except for Bit-A-6%S. It can be inferred that the dual effect of SBS polymer elastic network structure on healing is the most significant in Bit-A-6%S since a very complete and dense polymer network is formed and continuously distributed in its bitumen phase (as shown in Fig. 3). The more complete the polymer network structure, the greater the flow healing resistance of bitumen, thus the higher the HCTR of bitumen. Therefore, when the temperature exceeds HCT between Bit-A and Bit-A-6%S, the HR of Bit-A-6%S increases slowly due to the obstruction of flow by polymer. Furthermore, Bit-A-6%S does not has an obvious healing peak, and the increase of HR of Bit-A-6%S is the smallest relative to that of Bit-A-3%S and Bit-A-4%S. Hence, Bit-A-6%S has a highest HCTR, followed by Bit-A-4%S, Bit-A-3%S and Bit-A according to the results of HIT curve. Correspondingly, the HCT values of Bit-A-3%S, Bit-A-4%S and Bit-A-6%S with Bit-A increases in turn whether in HR-T curve or HI-T curve. It is noticed that the HCT at HI-T curve is higher than that at HRT curve. HR represents the healing rate and HI represents the healing recovery degree after a certain time. Therefore, the change of HR with temperature will not be reflected on the HI-T curve synchronously, but there is a certain lag. This also explains that the HCT at HR-T curve is always close to or lower than the HCTR at HI-T curve. Once again, due to the dual effects of SBS polymer networks, the SPMB binders show more excellent healing capability than base bitumens below the HCT, but show poorer healing capability than base bitumens above the HCT. In addition, there is one HCT point among BitA-3%S, Bit-A-4%S and Bit-A-6%S. Due to the lag effect between HR-T curve and HI-T curve, its value is around 29 ℃ at HR-T curve, whereas is around 35 ℃ at HI-T curve. Further observations illustrated in the Fig. 7(a) and (b), clearly reveals that the healing capability of SPMB binders increases with the increase of polymer content below HCT due to the increase of polymer structure, but decreases with the increase of polymer content above HCT due to the decrease in flow ability.
Fig. 4. Relationship between heat capacity and temperature: (a) Bit-A; (b) all test bitumen samples. Table 2 PT temperature ranges of the bitumen binders measured by DSC. Binders’ codes
Bit-A Bit-B Bit-A-3%S Bit-A-4%S Bit-A-6%S Bit-A-4%L Bit-B-4%S
Medium temperature PT ( ℃)
High temperature PT ( ℃)
Range
Peak temperature
Range
Peak temperature
6.28–16.90 5.53–13.18 6.88–18.07 8.34–19.02 10.5–21.08 7.91–18.75 7.30–17.86
12.25 8.53 13.89 14.11 15.51 14.02 13.21
40.64–45.76 40.00–45.11 40.23–47.48 40.75–47.59 40.3–47.88 40.33–47.71 39.72–47.65
42.56 42.01 43.25 44.05 44.31 44.01 43.18
3.4.3. Effect of polymer type on self-healing capability transition To study the effect of SBS polymer type on the self-healing capability transition, the HI-T and HR-T curves of Bit-A, Bit-A-4%S and BitA-4%L are shown in Fig. 8. From Fig. 8, Bit-A-4%L has a lower HPT than Bit-A-4%S, and its HPT is lower than the HCTR at HI-T curve. Meanwhile, it can be seen that the HCT between Bit-A and Bit-A-4%L is lower than the HCT between Bit-A and Bit-A-4%S. In addition, it can be observed that there are two HCT points at the HR-T curve due to the dual effects of SBS polymer networks on bitumen healing and the “HPT effect”. The first HCT point is around 28 ℃, and the second HCT point is around 40 ℃. These two points divide the HR-T curve into three stages: i) For stage I (below 28 ℃), Bit-A-4%L has lower HR than Bit-A-4%S since it has weaker polymer structure, as shown in Fig. 2; ii) For stage II (28 ℃–40 ℃), BitA-4%L shows higher HR than Bit-A-4%S because it has higher flow ability than (or lower viscosity, as shown in Table 1); iii) For stage III (above 35 ℃), Bit-A-4%L has lower HR than Bit-A-4%S again because of the “HPT effect”.
20 ℃, and the second HCT point is around 35 ℃. These two points divide the HR-T curve into three stages: i) For stage I (below 20 ℃), BitB-4%S has higher HR than Bit-B due to the healing promotion of SBS elastic polymer network structure; ii) For stage II (20 ℃–35 ℃), Bit-B shows higher HR than Bit-B-4%S since Bit-B shows larger flow ability than Bit-B-4%S with the increase of temperature; iii) For stage III (above 35 ℃), Bit-B-4%S has higher HR than Bit-B again because of the “HPT effect”. The same interpretation can be applied to three-stage analysis of HR-T curves between Bit-A-4%S and Bit-B-4%S. Correspondingly, the HCT also exists on HI-T curve, the HCT between Bit-A and Bit-A-4%S is around 28 ℃, and the HCT between Bit-B and Bit-B4%S is around 20 ℃. Bit-B contains more light components and is softer than Bit-A, so it reaches the HCTR at lower temperature than Bit-A, thus its HCT is lower than Bit-A’s HCT. It is obvious that the HCT, HPT and HCTR all occur in the PT 6
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Fig. 5. Healing behaviour curves of two base bitumens and their modified bitumens at different temperatures: (a) 15 ℃; (b) 20 ℃; (c) 25 ℃; (d) 30 ℃; (e) 35 ℃; (f) 40 ℃.
4. Conclusions
HI-T curves. The HPT and HCTR are both lower than the high temperature PT range, followed by the softening point. The HCT clearly divides the HR-T curves or HI-T curves of bitumens into different parts, thus leading to an opposite order of self-healing capability between two bitumens, which proves the existence of the self-healing capability transition phenomenon. Due to this phenomenon of bitumens, the self-healing capability can not be evaluated comprehensively and accurately at only one given temperature. (2) The HPT exists at the peak point of each HR-T curve, which shows that the healing rate of bitumen will reach its peak at this temperature point. The HTCR represents the critical temperature range in which the bitumens are almost healed completely, and is close to the HPT of bitumens. The HPT and HCTR can be applied as the optimum healing temperature to guide the formulation of energy efficient pavement induction healing strategies. The HCT points are
In this work, the temperature induced self-healing capability transition of different bitumen binders is systematically investigated through establishing the HR-T curve and HI-T curve based on the DSR FHF test. The internal morphology of different SPMB binders are compared by FM observation. The PT temperature of each bitumen is determined by the DSC test. Based on the test results, the main conclusions in this study are summarized as follows: (1) The self-healing capability of bitumen binder is significantly affected by temperature and has a complex nonlinear relationship within PT temperature range. In the PT temperature range (13 ℃–43 ℃), three kinds of special temperatures, i.e. healing cross temperature (HCT), healing peak temperature (HPT) and healing critical temperature range (HCTR), are discovered at HR-T curves or 7
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Fig. 7. Healing verse temperature curves of SPMB binders with different polymer contents: (a) HR-T relationship curve; (b) HI-T relationship curve.
Fig. 6. Healing verse temperature curves of SPMB binders with different bitumen types: (a) HR-T relationship curve; (b) HI-T relationship curve.
polymer dramatically influences the solid–liquid transition process in bitumen. The PT temperature of SPMB is higher than that of base bitumen, and the peak PT temperature of modified bitumen gradually promotes with the increase of polymer content, attributable to the gradual formation of a continuous SBS-rich network structure.
found to be ubiquitous in the HR-T curve or HI-T curve between different bitumens. (3) The self-healing capability transition phenomenon is significantly influenced by the bitumen type, polymer type and content. The HPT, HCT and HCTR of softer base bitumen is lower than that of harder base bitumen. The HPT, HCT and HCTR of SPMB binders increase with the polymer content and the modified bitumens with star structure SBS polymer are all higher than that of the modified bitumens with linear structure SBS polymer. Most notably, SBS polymer modification has dual effects on bitumen healing. On the one hand, the SBS elastic network structure in bitumen phase promotes the healing recovery of bitumen especially around the medium temperature PT range. On the other hand, the SBS polymer absorbs light components and hinders the flow of bitumen, thus reducing the healing rate especially around the high temperature PT range. (4) Based on the PT temperature range determined by DSC test, the influence mechanism of temperature on self-healing ability is deeply understood. The self-healing capability transition phenomenon of bitumens is closely related to the temperature induced PT process in bitumen phase. The temperature induced phase structure and property transition in bitumen is the essential reason for the self-healing capability transition of bitumens, which is markedly affected by the bitumen type, polymer type and content. At the same temperature, the solid–liquid transition in softer base bitumen appears to occur more easily than that in harder bitumen, thus resulting in a lower PT temperature range. The existence of SBS
Further studies are recommended to investigate the effect of aging on the self-healing capability transition of bitumens since the aging has a pronounced influence on the phase components of bitumen. Also, the self-healing capability transition may occur in the bituminous mastics and mixtures, which requires more systematic research. Especially, a complex changing temperature field in the field pavement may induce the self-healing capability transition of bituminous pavement due to the coupling effect of season, day and night and depth, thus affecting the design and assessment of bituminous pavement service life.
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 The work described in this paper is supported by the National Natural Science Foundation of China (Nos. 51878500 and 51378393). 8
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[8] Bommavaram RR, Bhasin A, Little DN. Determining intrinsic healing properties of asphalt binders. Trans Res Rec 2009;2126(1):47–54. [9] Sun D, Lin T, Zhu X, Tian Y, Liu F. Indices for self-healing performance assessments based on molecular dynamics simulation of asphalt binders. Comput Mater Sci 2016;114:86–93. [10] Sun D, Sun G, Zhu X, Pang Q, Yu F, Lin T. Identification of wetting and molecular diffusion stages during self-healing process of asphalt binder via fluorescence microscope. Constr Build Mater 2017;132:230–9. [11] Sun D, Sun G, Zhu X, Ye F, Xu J. Intrinsic temperature sensitive self-healing character of asphalt binders based on molecular dynamics simulations. Fuel 2018;211:609–20. [12] Sun G, Sun D, Guarin A, Ma J, Chen F, Ghafooriroozbahany E. Low temperature self-healing character of asphalt mixtures under different fatigue damage degrees. Constr Build Mater 2019;223:870–82. [13] Mazzoni G, Stimilli A, Canestrari F. Self-healing capability and thixotropy of bituminous mastics. Int J Fatigue 2016;92:8–17. [14] Shen S, Chiu HM, Huang H. Characterization of fatigue and healing in asphalt binders. J Mater Civ Eng 2010;22:846–52. [15] Shan L, Tan Y, Underwood S, Kim YR. Application of thixotropy to analyze fatigue and healing characteristics of asphalt binder. Trans Res Rec 2010;2179:85–92. [16] Sun D, Lin T, Zhu X, Cao L. Calculation and evaluation of activation energy as a selfhealing indication of asphalt mastic. Constr Build Mater 2015;95:431–6. [17] Tang J, Liu Q, Wu S, Ye Q, Sun Y, Schlangen E. Investigation of the optimal selfhealing temperatures and healing time of asphalt binders. Constr Build Mater 2016;113:1029–33. [18] Zhang L, Liu Q, Wu S, Rao Y, Sun Y, Xie J, et al. Investigation of the flow and selfhealing properties of UV aged asphalt binders. Constr Build Mater 2018;174:401–9. [19] García A. Self-healing of open cracks in asphalt mastic. Fuel 2012;93:264–72. [20] García A, Bueno M, Norambuena-Contreras J, Partl MN. Induction healing of dense asphalt concrete. Constr Build Mater 2013;49:1–7. [21] Ayar P, Moreno-Navarro F, Rubio-Gámez MC. The healing capability of asphalt pavements: a state of the art review. J Clean Prod 2016;113:28–40. [22] García A, Norambuena-Contreras J, Bueno M, Partl MN. Influence of steel wool fibers on the mechanical, termal, and healing properties of dense asphalt concrete. J Test Eva 2014;42(5):1107–18. [23] Menozzi A, García A, Partl MN, Tebaldi G, Schuetz P. Induction healing of fatigue damage in asphalt test samples. Constr Build Mater 2015;74:162–8. [24] Zhu X, Ye F, Cai Y, Birgisson B, Lee K. Self-healing properties of ferrite-filled opengraded friction course (OGFC) asphalt mixture after moisture damage. J Clean Prod 2019;232:518–30. [25] Shen S, Carpenter SH. Dissipated energy concepts for HMA performance: Fatigue and healing. Tech Rep Res Supported by Federal AVIATION Administration. 2006. [26] Little DN, Lytton RL, Chairl B, Williams D, Texas A. An analysis of the mechanism of microdamage healing based on the application of micromechanics first principles of fracture and healing. J Assoc Asph Pav Tech 1999:68. [27] Lv Q, Huang W, Xiao F. Laboratory evaluation of self-healing properties of various modified asphalt. Constr Build Mater 2017;136:192–201. [28] Qiu J, Van de Ven M, Wu S, Yu J, Molenaar A. Evaluating self healing capability of bituminous mastics. Exp Mech 2012;52(8):1163–71. [29] Qiu J, Van de Ven M, Molenaar A. Crack-healing investigation in bituminous materials. J Mater Civ Eng 2012;25(7):864–70. [30] Wool RP, Oconnor KM. A theory crack healing in polymers. J Appl Phys 1981;52(10):5953–63. [31] Schapery R. On the mechanics of crack closing and bonding in linear viscoelastic media. Int J Fract 1989;39(1-3):163–89. [32] Wool RP. Self-healing materials: a review. Soft Matter 2008;4(3):400–18. [33] Bhasin A, Little DN, Bommavaram R, Vasconcelos K. A framework to quantify the effect of healing in bituminous materials using material properties. Road Mater Pav Des 2008;9(suppl 1):219–42. [34] Ma F, Zhang C, Fu Z. DSC Study of Nano-CaCO3 Modified Asphalt. J Zhengzhou Uni Eng Ed 2006;27(4):49–52. [35] Ward IM, Klein PG. Polymer Physics. Encyclopedia of Magnetic Resonance. John Wiley & Sons Ltd; 2007. [36] Hua YQ, Jin RG. Polymer Physics (the 4th Edition). Beijing: Chemical Industry Press; 2013. [37] Zhang D, Chen M, Wu S, Riara M, Wan J, Li Y. Thermal and rheological performance of asphalt binders modified with expanded graphite/polyethylene glycol composite phase change material (EP-CPCM). Constr Build Mater 2019;194:83–91.
Fig. 8. Healing verse temperature curves of SPMB binders with different polymer types: (a) HR-T relationship curve; (b) HI-T relationship curve.
References [1] Bazin P, Saunier JB. Deformability, fatigue and healing properties of asphalt mixes. Proceedings of the 2nd International Conference on the Structural Design of Asphalt Pavements, Michigan. 1967. [2] Raithby K, Sterling A. The effect of rest periods on the fatigue performance of a hotrolled asphalt under reversed axial loading and discussion. Assoc Asphalt Paving Technol Proc 1970;39:134–52. [3] Bonnaure FP, Huibers A, Boonders A. A laboratory investigation of the influence of rest periods on the fatigue characteristics of bituminous mixes (with discussion). Assoc Asphalt Paving Technol Proc 1982;51:104–28. [4] Canestrari F, Virgili A, Graziani A, Stimilli A. Modeling and assessment of selfhealing and thixotropy properties for modified binders. Int J Fatigue 2015;70(1):351–60. [5] Shen S, Airey G, Carpenter S, Huang H. A dissipated energy approach to fatigue evaluation. Road Mater Pavement Des 2006;7(1):47–69. [6] Sun D, Yu F, Li L, Lin T, Zhu X. Effect of chemical composition and structure of asphalt binders on self-healing. Constr Build Mater 2017;133:495–501. [7] Sun D, Sun G, Zhu X, Guarin A, Li B, Dai Z, et al. A comprehensive review on selfhealing of asphalt materials: mechanism, model, characterization and enhancement. Adv Col Inter Sci 2018;256:65–93.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Temperature induced self-healing capability transition phenomenon of bitumens ⁎
Guoqiang Sun, Mingjun Hu, Daquan Sun , Yue Deng, Jianmin Ma, Tong Lu Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, Shanghai 200092, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Bitumen Self-healing capability transition Healing cross temperature (HCT) Healing peak temperature (HPT) Phase transition (PT)
The temperature may induce the inconsistent order of self-healing capability between two bituminous materials, which could be called as the temperature induced self-healing capability transition phenomenon. To confirm this phenomenon, two base bitumens and four different styrene–butadiene-styrene polymer modified bitumen (SPMB) binders are selected to systematically investigate their self-healing capability through establishing the healing rate temperature (HR-T) curves and healing index temperature (HI-T) curves. Fluorescence microscopy (FM) is applied to study the internal morphology of different SPMB binders. The phase transition (PT) temperature range is determined by the differential scanning calorimeter (DSC) test. Within the PT temperature range, three kinds of special temperatures are discovered, i.e. healing peak temperature (HPT), healing critical temperature range (HCTR) and healing cross temperature (HCT). The HPT and HCTR are both lower than the high temperature PT range, followed by the softening point, and can be applied as the optimum healing temperature to guide the formulation of energy efficient pavement induction healing strategies. The HCT clearly divides the HR-T curves or HI-T curves of bitumens into different parts, thus leading to an opposite order of selfhealing capability between two bitumens, which confirms the self-healing capability transition phenomenon. Therefore, the self-healing capability can not be evaluated comprehensively and accurately at only one given temperature. Most notably, it can be inferred that the temperature induced phase structure and property transition of bitumen is the essential reason for the self-healing capability transition phenomenon, which is significantly affected by the bitumen type, polymer type and content.
1. Introduction Bituminous concrete is a typical viscoelastic material, which has self-healing capability. The self-healing capability of bituminous materials have been a focus of research in both laboratory and field since 1960s [1,2]. On the one hand, stress relaxation in the crack region and spontaneous interface healing will occur to reduce surface free energy [3–7]. On the other hand, bitumen has the abilities of interfacial wetting and diffusion to close the cracks [1,8–11]. These characteristics provide the basic conditions for self-healing of bituminous materials. Temperature is the principal external factor affecting the self-healing efficiency of bituminous materials, and polymer modification is an important internal factor. It is of great significance to investigate the effect of these key factors on the self-healing capability for the design of long-life self-healing pavement with zero maintenance. However, researchers have different understandings on the influence of temperature and polymer modification on the self-healing capability of bitumen.
⁎
Especially, the conclusions on the influence of temperature on the self-healing capability of bituminous materials are not uniform. A typical opinion is that the self-healing capability of bitumen binder will increase monotonously with temperature. It was well reported that the self-healing behaviour below the Glass transition temperature will disappears since the bitumen molecule interdiffusion between crack faces almost ceases at such low temperatures [12]. As the temperature rises to the medium temperature range (roughly within 5 ℃–35 ℃, depending on the bitumen types), the bitumen molecule will move from freezing state to moving state, thus the healing ability increases markedly at the medium temperature [4,13–16]. Most of self-healing articles have focused on the healing behaviour of bitumen materials at this temperature range. Another opinion is that there exists a critical threshold temperature for bitumen self-healing, and the optimum healing temperature obtained by different scholars is different [17–20]. Tang et al. thought that the softening point temperatures of bitumens are the optimal temperatures to heal fatigue damage [17]. García et al. and Zhanget al. revealed that the binder at the flow threshold
Corresponding author. E-mail address: [email protected] (D. Sun).
https://doi.org/10.1016/j.fuel.2019.116698 Received 15 August 2019; Received in revised form 3 November 2019; Accepted 19 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Guoqiang Sun, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116698
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Table 1 Physical properties of bitumen binders used in the investigation. Binders’ Code
Description
Penetration (25 ℃, 100 g, 5 s), 0.1 mm
Softening point (R&B), ℃
Rotational viscosity at 135 ℃, mPa.s
Bit-A Bit-B Bit-A-3%S Bit-A-4%S Bit-A-6%S Bit-A-4%L Bit-B-4%S
Bitumen Bitumen Bitumen Bitumen Bitumen Bitumen Bitumen
69.8 103.2 49.8 47.1 42.2 49.0 66.5
49.6 42.2 64.2 69.3 73.4 66.2 63.5
468.6 301.3 1036.2 1477.6 1714.8 1125.8 1000.4
A B A + 3% star SBS A + 4% star SBS A + 6% star SBS A + 4% linear SBS B + 4% star SBS
temperature will become a near-Newtonian fluid and can flow into the cracks and heal them rapidly (usually ranging from 30 ℃ to 70 ℃, depending on the type of bitumen and aging level) [18–20]. However, Sun et al. thought that the molecular diffusion rate and diffusion range are more distinct especially within the phase transition temperature range of base bitumens, and determined the optimal healing temperature range of bitumens is around 40.3 ℃–48.7 ℃ [11]. Therefore, it can be assumed that the self-healing capability of bitumen binder may have a very complex nonlinear relationship within a wide temperature range. Especially, the research on promoting the self-healing of bituminous materials by microwave/electromagnetic induction heating is very extensive in recent years [7,21–24], and a key issue is how to determine the real optimum healing temperature. Hence, it is necessary to conduct a more in-depth investigation on the influence mechanism of temperature on the bitumen healing ability, thus helping formulate an energy efficient pavement induction healing strategy. With regard to the polymer modification, some contradictory understandings on the effect of SBS polymer modifiers on the healing capability of bituminous materials are widespread as yet [7,21]. More results show that polymer modified bituminous materials possess superior healing capability to base bitumen materials. Shen and Carpenter found that the healing rate of polymer modified asphalt mixtures at 20 ℃ was much better than that of the neat asphalt mixtures [25]. The further work from Shen et al. showed that the healing rate of the polymer modified asphalt PG 70-28 was greater than that of the neat asphalt PG64-28 based on the DSR fatigue test (at 15 ℃ and 25 ℃) [14]. Sun et al. concluded that the SBS modified asphalt (linear SBS, 5%wt) exhibited the most excellent healing efficiency than other neat bitumens with different penetration grade values (20, 50, 70, 100) by FM observation at room temperature, attributable to the SBS-rich elastic network structure [10]. However, some researchers considered that the polymer modification might have a small or even negative effect on the healing capability of base bitumen. Little et al. indicated that the addition of SBS acted as a filler system to interrupt the ability of pure asphalt healing [26]. They thought that polymer networks in bitumen partially absorb more compatible components and thus swell, leading to a higher asphaltene (highly interactive) left in the rest of the bitumen. The bitumen with a higher asphaltene concentration is less likely to flow and heal. Lv et al. also found that the increased modifier content deteriorated the healing capability of the base binder regardless of the type of additive employed by the Binder Bond Strength (BBS) test at 25 ℃ [27]. Furthermore, Qiu et al. investigated the effect of SBS modifier on the healing behavior of asphalt materials at 10 ℃, 20 ℃ and 40 ℃ by the DDT-based fracture-healing-re-fracture (FHR) test [28]. They found that the SBS modified mastics had higher healing rate than base bitumen mastics at 10 ℃, but showed much lower healing rate at 20 ℃ and 40 ℃. Hence, it can be assumed that the order of self-healing capability between SBS modified bitumen and base bitumen may be different at different temperatures. Overall, it may be biased to evaluate the self-healing capability of bitumens at only one given temperature. It can be assumed that the temperature may induce the inconsistent order of self-healing capability between two bituminous materials, which could be called as the temperature induced self-healing capability transition phenomenon.
Hence, to confirm this phenomenon, two base bitumens and four different SBS polymer modified bitumen (SPMB) binders are selected to systematically investigate their self-healing capability at different temperatures through establishing the healing rate temperature (HR-T) curves and healing index temperature (HI-T) curves. In addition, the basic physical properties, morphology and phase transition temperature range of different bitumen binders are analyzed. The analysis of selfhealing capability transition can provide a more accurate and comprehensive method to evaluate and distinguish the self-healing capability of different bitumens (especially for polymer modified bitumen and base bitumen), so as to guide the selection of bitumen binders with high self-healing capability. Moreover, the investigations on the wide temperature self-healing behaviour can help formulate an energy efficient pavement induction healing strategy. 2. Experiments 2.1. Materials Two base bitumens and two commercial SBS block copolymers (star-branched structure and linear structure) are used to produce SBS polymer modified bitumen (SPMB) samples investigated in this study. Some basic properties of the bitumen samples are listed in Table 1. SPMB samples are prepared by blending SBS block copolymers and bitumen at 175 ℃ for 1 h using a high shear mixer. The SBS concentration is varied in the range of 3–6 wt% (by weight of bitumen) as detailed in Table 1. 2.2. Fluorescence microscopy (FM) test The morphology of the SPMB samples is studied using an Olympus Fluorescence microscope, equipped with a digital camera. FM micrographs are taken at original magnifications of 4, 20 and 100. FM specimens are prepared by the hot melt method and the micrographs are taken at room temperature. 2.3. Differential scanning calorimeter (DSC) test To determine the phase transition (PT) temperature ranges of different bitumen binders, the Differential scanning calorimeter (DSC) test is employed to measure the heat amount required to increase the temperature of a sample. The test conditions are listed in the following: 1) temperature range: −50 ℃–100 ℃; 2) heating rate: 10 ℃/min; 3) sample weight: 2–5 mg; 4) test instrument: DSC Q2000; 5) reference material: indium (a kind of metal with high-purity). 2.4. Self-healing behaviour characterization based on fatigue-healingfatigue (FHF) test The DSR FHF test was performed at different temperatures (15 ℃, 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃, 45 ℃ and 50 ℃) by DSR (AR1500EX, TA Instruments Company, USA). Based on our previous study [11,16], recovery of 30% damaged bitumen binder could characterize the healing property of bitumen binder better. It would take much time to heal the 2
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depicted in Fig. 3. It could be clearly observed the polymer network is gradually connected into a continuous network structure with the increase of SBS content. When the SBS content is low (3%wt), SBS is not uniformly distributed in bitumen phase, the polymers tend to form aggregates with diameters of 2–8 µm according to the enlarged micrograph. When the SBS content is increased to 4%wt, SBS in bitumen phase begin to form polymer networks, but does not connected completely. Notably, the star SBS in bitumen seems to have coarser network structure than that in linear SBS modified bitumen, which may explain that Bit-A-4%S has higher softening point and viscosity than Bit-A-4%L. When the SBS content is increased to 6%wt, a dense continuous network is formed in bitumen phase. The formation of continuous network is also reflected in the physical/mechanical properties. For example, a marked increase in the softening point and rotational viscosity and a pronounced decrease in the penetration value are observed when the SBS content is increased from 3 to 6 wt% (Table 1).
Fig. 1. DSR FHF test process.
fatigue cracks generated in bitumen binder once the damage degree is more than 30% (like 50%), thus leading to a poor accuracy when evaluating the healing efficiency of the seriously damaged bitumen. Hence, the damage degree is set as 30%, namely the loading would stop when the initial modulus drops to 70% of the initial value. Then the test is stopped for different time to heal the cracks. After that, the DSR test is continued until the modulus dropped to 70% of the initial modulus again. The FHF test process of is shown in Fig. 1. The healing index HIG is shown in Eq. (1) is used to estimate the self-healing efficiency of bitumen binder:
HIG =
3.2. Phase transition (PT) temperature determination The PT temperature ranges are determined based on the DSC curve. DSC curve can reflect the change of aggregate states of bitumen with temperature. The DSC test results of all investigated binders are shown in Fig. 4. Two heat absorption peaks can be clearly observed in the range of test temperature regardless of the bitumen types. As shown in Fig. 4(a), the thermal analysis software (TA Universal Analysis) is applied to calibrate the marked endothermic peak. The temperature ranges with the largest slope and the corresponding PT peak temperature will be automatically identified by the software, i.e. medium temperature PT range and high temperature PT range. Near the peak PT temperature, the solid–liquid transition of aggregates in bitumen will be more intense, which will lead to significant changes in the macroscopic properties of bitumen [34–37]. Two PT temperature ranges can be determined for each bitumen type, as listed in Table 2. It could be found that different bitumen binders have different PT temperature range. Actually, the bitumen is a mixture of substances with different molecular sizes, chemical compositions and structures. Due to the difference of chemical composition and molecular structure, the PT temperatures of different bitumens are inevitably different. For base bitumen binders, it appears that Bit-B has lower peak PT temperature than Bit-A. Bit-B is softer than Bit-A, which means that Bit-B contains more light components (liquids state), so the PT temperature of Bit-B is lower. When the bitumen is modified by SBS polymer, the phase structure in bitumen will change a lot. On the one hand, the polymer will absorb the light components of bitumen, thereby increasing the relative content of large molecule components in bitumen, like asphaltene (solid state), thus finally inducing the increase of the PT temperature of bitumen. On the other hand, the PT temperature of SBS polymer itself is much higher than that of bitumen, so the PT temperature of SPMB is higher than that of base bitumen. In addition, the peak PT temperature of modified bitumen gradually increases with the increase of polymer content, which is just the same order of softening point ranked in Table 1. It can be analyzed that the medium temperature PT peak temperature of the bitumen binders is about 13 ℃ (based on their arithmetic average), and the high temperature PT peak temperature is about 43 ℃. Hence, the PT temperature range of the bitumen binders can be roughly determined as 13 ℃–43 ℃, in which the bitumen will undergo frequent solid–liquid transition. Seven critical temperatures around the PT temperature range, i.e. 15 ℃, 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃ and 45 ℃, are selected for the FHF test.
∗ ∗ Ghealing − Gter min al ∗ ∗ Ginitial − Gter min al
(1)
∗ where Ginitial
is the initial complex modulus after one minute of shearing ∗ to get the stable value. Ghealing is the complex modulus after healing, and ∗ ∗ Gter min al = 70% × Ginitial . The healing process contains two vital stages, i.e. crack closure and strength gain [29]. Some micromechanical and phenomenological healing models in polymers have been proposed to describe these two stages [30–32]. The molecular diffusion model proposed by Wool and O’Connor is one of the most well-known models. This model revealed independently a straight-line relationship with an exponent of 0.25 for fracture stress and healing time using double cantilever beam tests on polybutadiene [30]. According to the extensive work done in polymer healing by Wool and O’Connor et al. [30], some studies also pointed out two stages for bitumen materials including [8,10,16,29,33]: (1) the instantaneous strength gained due to crack closure and cohesion driven by wetting, and (2) long term strength gained due to diffusion and randomization of molecules from one face to the other. The observed macroscopic healing recovery at temperature T and time t, HI (T, t), can be written as:
HI (T , t ) = HI0 (T ) + HR (T )·t 0.25
(2)
where HI (T, t) is the observed macroscopic healing recovery at temperature T and time t. HI0 (T) represents the instantaneous strength gain. HR(T) is a temperature-dependent parameter, which indicates the strength gain rate at temperature T, i.e. healing rate (HR). In this study, two kinds of healing indexes, i.e. healing index (HI) and healing rate (HR), are used to characterize the self-healing capability of different bitumens. Fig. 2 exhibits the framework for the whole experimental program of this study. 3. Results and discussion 3.1. Internal morphology analysis
3.3. Self-healing behaviour equation of different bitumens Fig. 3 shows the evolution of the SPMB morphology with increasing SBS polymer content. In these FM micrographs, the SBS-rich phase appears light yellow, whereas the bitumen-rich phase appears dark. FM micrographs with original magnifications of 4, 20 and 100 are clearly
In order to explore the self-healing behaviour evolution of different bitumens with temperature, the healing behaviour curves of two base bitumens and their modified bitumens at different temperatures are 3
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Penetration, Softening point, Rotational viscosity Basic physical prperties
Base Bitumens Internal morphology analysis of different SPMB binder
SBS Polymer
FM
DSC
Phase transition temperature range determination
SPMB binders DSR
Self‐healing transition phenomenon based on FHF test: HI‐T curve, HR‐T curve Fig. 2. Experimental program.
fitted according to Eq. (2), as shown in Fig. 5. As Fig. 5 exhibits, it can be clearly observed that there is a good linear fitting relationship between the healing index (HI) of different bitumens and the 0.25 power of healing time (R2 > 0.95), which further proves that Wool’s diffusion healing theory is also applicable to modified bitumens [30]. As discussed in Section 2.4, the slope of the healing curve can represent the healing rate (HR) of bitumen. Obviously, the temperature has a significant effect on the healing capability of different bitumens. It is noted that the SPMB binders (Bit-A4%S and Bit-B-4%S) exhibits higher healing capability (HI and HR) than base bitumens at 15 ℃ and 20 ℃, but then with the increase of temperature (> 25 ℃), their healing capability gradually begins to be lower than that of base bitumens. This phenomenon reveals that there is healing cross temperature (HCT) between different bitumens. Most notably, Bit-A-4%S has the highest healing recovery index (HI) at 15 ℃ and 20 ℃ (around medium temperature PT range), but its HI value becomes the lowest when the temperature increases to 30 ℃, 35 ℃ and 40 ℃ (around high temperature PT range). On the contrary, Bit-B has lower HI value at 15 ℃ and 20 ℃, but its HI value becomes the highest at 25 ℃, 30 ℃, 35 ℃ and 40 ℃. Another interesting observation from Fig. 5 is that there is healing peak temperature (HPT) for different bitumens. For example, the HR value of Bit-B increases from 0.026 at 15 ℃ to a peak value 0.184 at 30 ℃, then decreases to 0.120 at 40 ℃. This particular temperature also exists in other bitumens. Based on the analysis of the self-healing behaviour curves, it is fully confirmed that the self-healing behaviour of the bitumens is a very sensitive process to temperature. Hence, it is biased to evaluate the selfhealing capability of bitumens at only one given temperature. In the vicinity of PT temperature range (13 ℃–43 ℃), two kinds of special temperatures in bitumen, i.e. HCT and HPT, are discovered, ultimately leading to the self-healing capability transition of different bitumens. Further analysis on the temperature induced self-healing capability transition of different bitumens will be presented in next section.
curve, but there may be more than one HCT point at the HR-T curves. By the way, the HCT exists at the cross point between two HR-T curves of two different bitumens. According to healing flow and diffusion theory in bituminous materials [10,11,17–20], when the bitumen is heated to its glass transition temperature, it will gradually liquefy and enter into the cracked face, flowing sluggishly and diffusing into the fractured matrix driven by gradients of pressure or of concentration, and entangling with the matrix molecules by surface energy difference or thermodynamic driving force over long time periods. However, less researches focused on the self-healing evolution process in the whole PT temperature zone. The existence of HPT and HCT in the PT temperature range leads to self-healing capability transition between different bitumens, and demonstrates the complexity of the self-healing behaviour of bitumen evolving with temperature. Moreover, it can be observed that the HI values of different bitumens will reach to 0.95–1.00 at higher temperature range, which means that the bitumens are almost healed completely at this temperature range. This temperature range is called as the healing critical temperature range (HCTR). Evidently, the HCTR of each bitumen is lower than its high temperature PT range, followed by its softening point. The HPT exists at the peak point of each HR-T curve. From Fig. 6(a), it can be found that the HPT values of Bit-A, Bit-B, Bit-A-4%S and Bit-B4%S are around 35 ℃, 30 ℃, 40 ℃ and 35 ℃, respectively, which is close to the HCTR. The HPT is within the PT temperature range, in which the bitumen will undergo a very intense solid–liquid transition, so that the flow healing rate of bitumen will also change, and reach a peak state at a certain temperature, namely HPT. Once exceeding this temperature (HPT), the solid–liquid transition rate of bitumen tends to be stable, which makes the flow healing rate of bitumen also begin to decrease. Hence, the HPT value will change with the change of bitumen phase structure, so the HPT value of different types of bitumen will also be different. For Bit-A and Bit-B, Bit-B is softer than Bit-A since Bit-B contains more light components (liquids state) , so the solid–liquid transition in Bit-B will occur more easily than that in Bit-A at the same temperature. As a result, the HPT of Bit-B is lower than Bit-A. Similarly, the SBS polymer will change the phase structure of bitumen and hinder the solid–liquid transition of bitumen, thus reducing the flow rate of bitumen phase in the SPMB binder, so the HPT of Bit-A-4%S is higher than that of Bit-A, and the HPT of Bit-B-4%S is higher than that of Bit-B. Correspondingly, the HCTR of bitumen also changes with the change of bitumen phase structure. The HCTR of Bit-B is lower than that of Bit-A, and the HCTR of base bitumen is lower than that of their modified bitumen. Actually, the HCTR represents the temperature range in which the mechanical properties of bitumen (HI) almost completely recover, while HPT represents the temperature when the bitumen healing rate is
3.4. Temperature induced self-healing capability transition of different bitumens 3.4.1. Effect of bitumen type on self-healing capability transition To further explore the effect of bitumen type on the self-healing capability transition, the relationship curves of HI-T and HR-T are established in Fig. 6. The HI value at 60 min is selected to build the HI-T curve. As described in Section 3.4, the HCT and HPT for four bitumens can be easily marked at the HR-T and HI-T curves, as shown in Fig. 6. It can be seen that each bitumen will have only one HPT point at the HR-T 4
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Fig. 3. The effect of SBS content and type on the morphology of SPMB: (a) BitA -3%S, (b) Bit-A-4%S, (c) Bit-A-4%L, (d) Bit-A-6%S.
Bit-A and Bit-B in HI-T curve (as shown in Fig. 6(b)) since Bit-B enters the HCTR much earlier than Bit-A. For Bit-A, the HCT between Bit-A and Bit-A-4%S is around 27 ℃. Bit-A-4%S shows a higher HR value than Bit-A below 27 ℃, but shows a much lower HR value than Bit-A above 27 ℃. According to the above flow theory, it is easy to explain Bit-A-4%S has lower HR above 27 ℃ since the SBS polymer will hinder the flow of bitumen phase [26–29], but it is hard to explain why Bit-A-4%S has higher HR than Bit-A below 27 ℃. Actually, the SBS polymer elastic network structure also promotes the recovery of mechanical properties of bitumen according to many previous studies [10,14,25], so that Bit-A-4%S has higher HR than Bit-A below 27 ℃. It can be inferred that SBS polymer modification has dual effects on bitumen healing. On the one hand, the SBS elastic network structure in bitumen phase promotes the healing recovery of bitumen especially around the medium temperature PT range. On the other hand, the SBS polymer absorbs light components and hinders the flow of bitumen, thus reducing the healing rate especially around the high temperature PT range. For Bit-B, it can be found that there are two HCT points between Bit-B and Bit-B-4%S. The first HCT point is around
at the highest value. Therefore, the HCTR of bitumen is similar to or slightly higher than HPT. As for the HCT phenomenon, it exists between the healing temperature curves of different bitumens (HR-T curve or HI-T curve), as shown in Fig. 6(a) and (b). Due to the differences in the molecular composition and structure of different bitumens, the temperature induced PT in different bitumens is different, resulting in the temperature induced healing flow rate differences in different bitumens. Hence, the healing-temperature curves of different bitumens will inevitably cross at a certain temperature. It is exhibited that the HCT of Bit-A and Bit-B is around 34 ℃, and Bit-B shows a higher HR value than Bit-A below 34 ℃, but shows a much lower HR value than Bit-A above 34 ℃. Since Bit-B is softer than Bit-A, the healing flow rate of Bit-B is higher than Bit-A. However, once exceeding the HPT of Bit-B (i.e. around 30 ℃), the HR of Bit-B begins to decrease, and the HR of Bit-A is still growing until reaching to the HPT of Bit-A (i.e. around 35 ℃). Hence, Bit-B shows a much lower HR value than Bit-A above 34 ℃. For the convenience of explanation, this phenomenon of HPT induced healing capacity reduction can be called “HPT effect”. Notably, there is no HCT between 5
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temperature range of bitumens. Combining with the analysis of PT temperature of different bitumens, it can be found that the self-healing capability transition behaviour of bitumens is closely related to the temperature induced PT process in bitumen. It can be inferred that the temperature induced phase structure and property transition of bitumen is the essential reason for the self-healing capability transition of bitumens. 3.4.2. Effect of SBS polymer content on self-healing capability transition To study the effect of SBS polymer content on the self-healing capability transition, the HI-T and HR-T curves of Bit-A, Bit-A-3%S, BitA-4%S and Bit-A-6%S are shown in Fig. 7. From Fig. 7(a), the HPT values of SPMB binders with different polymer contents are all around 40 ℃, which means that the HR will decrease after 40 ℃. Moreover, their HPTs enter the HCTR except for Bit-A-6%S. It can be inferred that the dual effect of SBS polymer elastic network structure on healing is the most significant in Bit-A-6%S since a very complete and dense polymer network is formed and continuously distributed in its bitumen phase (as shown in Fig. 3). The more complete the polymer network structure, the greater the flow healing resistance of bitumen, thus the higher the HCTR of bitumen. Therefore, when the temperature exceeds HCT between Bit-A and Bit-A-6%S, the HR of Bit-A-6%S increases slowly due to the obstruction of flow by polymer. Furthermore, Bit-A-6%S does not has an obvious healing peak, and the increase of HR of Bit-A-6%S is the smallest relative to that of Bit-A-3%S and Bit-A-4%S. Hence, Bit-A-6%S has a highest HCTR, followed by Bit-A-4%S, Bit-A-3%S and Bit-A according to the results of HIT curve. Correspondingly, the HCT values of Bit-A-3%S, Bit-A-4%S and Bit-A-6%S with Bit-A increases in turn whether in HR-T curve or HI-T curve. It is noticed that the HCT at HI-T curve is higher than that at HRT curve. HR represents the healing rate and HI represents the healing recovery degree after a certain time. Therefore, the change of HR with temperature will not be reflected on the HI-T curve synchronously, but there is a certain lag. This also explains that the HCT at HR-T curve is always close to or lower than the HCTR at HI-T curve. Once again, due to the dual effects of SBS polymer networks, the SPMB binders show more excellent healing capability than base bitumens below the HCT, but show poorer healing capability than base bitumens above the HCT. In addition, there is one HCT point among BitA-3%S, Bit-A-4%S and Bit-A-6%S. Due to the lag effect between HR-T curve and HI-T curve, its value is around 29 ℃ at HR-T curve, whereas is around 35 ℃ at HI-T curve. Further observations illustrated in the Fig. 7(a) and (b), clearly reveals that the healing capability of SPMB binders increases with the increase of polymer content below HCT due to the increase of polymer structure, but decreases with the increase of polymer content above HCT due to the decrease in flow ability.
Fig. 4. Relationship between heat capacity and temperature: (a) Bit-A; (b) all test bitumen samples. Table 2 PT temperature ranges of the bitumen binders measured by DSC. Binders’ codes
Bit-A Bit-B Bit-A-3%S Bit-A-4%S Bit-A-6%S Bit-A-4%L Bit-B-4%S
Medium temperature PT ( ℃)
High temperature PT ( ℃)
Range
Peak temperature
Range
Peak temperature
6.28–16.90 5.53–13.18 6.88–18.07 8.34–19.02 10.5–21.08 7.91–18.75 7.30–17.86
12.25 8.53 13.89 14.11 15.51 14.02 13.21
40.64–45.76 40.00–45.11 40.23–47.48 40.75–47.59 40.3–47.88 40.33–47.71 39.72–47.65
42.56 42.01 43.25 44.05 44.31 44.01 43.18
3.4.3. Effect of polymer type on self-healing capability transition To study the effect of SBS polymer type on the self-healing capability transition, the HI-T and HR-T curves of Bit-A, Bit-A-4%S and BitA-4%L are shown in Fig. 8. From Fig. 8, Bit-A-4%L has a lower HPT than Bit-A-4%S, and its HPT is lower than the HCTR at HI-T curve. Meanwhile, it can be seen that the HCT between Bit-A and Bit-A-4%L is lower than the HCT between Bit-A and Bit-A-4%S. In addition, it can be observed that there are two HCT points at the HR-T curve due to the dual effects of SBS polymer networks on bitumen healing and the “HPT effect”. The first HCT point is around 28 ℃, and the second HCT point is around 40 ℃. These two points divide the HR-T curve into three stages: i) For stage I (below 28 ℃), Bit-A-4%L has lower HR than Bit-A-4%S since it has weaker polymer structure, as shown in Fig. 2; ii) For stage II (28 ℃–40 ℃), BitA-4%L shows higher HR than Bit-A-4%S because it has higher flow ability than (or lower viscosity, as shown in Table 1); iii) For stage III (above 35 ℃), Bit-A-4%L has lower HR than Bit-A-4%S again because of the “HPT effect”.
20 ℃, and the second HCT point is around 35 ℃. These two points divide the HR-T curve into three stages: i) For stage I (below 20 ℃), BitB-4%S has higher HR than Bit-B due to the healing promotion of SBS elastic polymer network structure; ii) For stage II (20 ℃–35 ℃), Bit-B shows higher HR than Bit-B-4%S since Bit-B shows larger flow ability than Bit-B-4%S with the increase of temperature; iii) For stage III (above 35 ℃), Bit-B-4%S has higher HR than Bit-B again because of the “HPT effect”. The same interpretation can be applied to three-stage analysis of HR-T curves between Bit-A-4%S and Bit-B-4%S. Correspondingly, the HCT also exists on HI-T curve, the HCT between Bit-A and Bit-A-4%S is around 28 ℃, and the HCT between Bit-B and Bit-B4%S is around 20 ℃. Bit-B contains more light components and is softer than Bit-A, so it reaches the HCTR at lower temperature than Bit-A, thus its HCT is lower than Bit-A’s HCT. It is obvious that the HCT, HPT and HCTR all occur in the PT 6
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Fig. 5. Healing behaviour curves of two base bitumens and their modified bitumens at different temperatures: (a) 15 ℃; (b) 20 ℃; (c) 25 ℃; (d) 30 ℃; (e) 35 ℃; (f) 40 ℃.
4. Conclusions
HI-T curves. The HPT and HCTR are both lower than the high temperature PT range, followed by the softening point. The HCT clearly divides the HR-T curves or HI-T curves of bitumens into different parts, thus leading to an opposite order of self-healing capability between two bitumens, which proves the existence of the self-healing capability transition phenomenon. Due to this phenomenon of bitumens, the self-healing capability can not be evaluated comprehensively and accurately at only one given temperature. (2) The HPT exists at the peak point of each HR-T curve, which shows that the healing rate of bitumen will reach its peak at this temperature point. The HTCR represents the critical temperature range in which the bitumens are almost healed completely, and is close to the HPT of bitumens. The HPT and HCTR can be applied as the optimum healing temperature to guide the formulation of energy efficient pavement induction healing strategies. The HCT points are
In this work, the temperature induced self-healing capability transition of different bitumen binders is systematically investigated through establishing the HR-T curve and HI-T curve based on the DSR FHF test. The internal morphology of different SPMB binders are compared by FM observation. The PT temperature of each bitumen is determined by the DSC test. Based on the test results, the main conclusions in this study are summarized as follows: (1) The self-healing capability of bitumen binder is significantly affected by temperature and has a complex nonlinear relationship within PT temperature range. In the PT temperature range (13 ℃–43 ℃), three kinds of special temperatures, i.e. healing cross temperature (HCT), healing peak temperature (HPT) and healing critical temperature range (HCTR), are discovered at HR-T curves or 7
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Fig. 7. Healing verse temperature curves of SPMB binders with different polymer contents: (a) HR-T relationship curve; (b) HI-T relationship curve.
Fig. 6. Healing verse temperature curves of SPMB binders with different bitumen types: (a) HR-T relationship curve; (b) HI-T relationship curve.
polymer dramatically influences the solid–liquid transition process in bitumen. The PT temperature of SPMB is higher than that of base bitumen, and the peak PT temperature of modified bitumen gradually promotes with the increase of polymer content, attributable to the gradual formation of a continuous SBS-rich network structure.
found to be ubiquitous in the HR-T curve or HI-T curve between different bitumens. (3) The self-healing capability transition phenomenon is significantly influenced by the bitumen type, polymer type and content. The HPT, HCT and HCTR of softer base bitumen is lower than that of harder base bitumen. The HPT, HCT and HCTR of SPMB binders increase with the polymer content and the modified bitumens with star structure SBS polymer are all higher than that of the modified bitumens with linear structure SBS polymer. Most notably, SBS polymer modification has dual effects on bitumen healing. On the one hand, the SBS elastic network structure in bitumen phase promotes the healing recovery of bitumen especially around the medium temperature PT range. On the other hand, the SBS polymer absorbs light components and hinders the flow of bitumen, thus reducing the healing rate especially around the high temperature PT range. (4) Based on the PT temperature range determined by DSC test, the influence mechanism of temperature on self-healing ability is deeply understood. The self-healing capability transition phenomenon of bitumens is closely related to the temperature induced PT process in bitumen phase. The temperature induced phase structure and property transition in bitumen is the essential reason for the self-healing capability transition of bitumens, which is markedly affected by the bitumen type, polymer type and content. At the same temperature, the solid–liquid transition in softer base bitumen appears to occur more easily than that in harder bitumen, thus resulting in a lower PT temperature range. The existence of SBS
Further studies are recommended to investigate the effect of aging on the self-healing capability transition of bitumens since the aging has a pronounced influence on the phase components of bitumen. Also, the self-healing capability transition may occur in the bituminous mastics and mixtures, which requires more systematic research. Especially, a complex changing temperature field in the field pavement may induce the self-healing capability transition of bituminous pavement due to the coupling effect of season, day and night and depth, thus affecting the design and assessment of bituminous pavement service life.
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 The work described in this paper is supported by the National Natural Science Foundation of China (Nos. 51878500 and 51378393). 8
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Fig. 8. Healing verse temperature curves of SPMB binders with different polymer types: (a) HR-T relationship curve; (b) HI-T relationship curve.
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