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Enhanced Self-healing Process of Sustainable Asphalt Materials Containing Microcapsules Daquan Sun, Qi Pang, Xingyi Zhu, Yang Tian, Tong Lu, and Yang Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01850 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017
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Enhanced Self-healing Process of Sustainable Asphalt Materials Containing Microcapsules
1
Daquan Sun1, Qi Pang1, Xingyi Zhu1,2*, Yang Tian1, Tong Lu1, Yang Yang1 Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, Shanghai 200092 , P. R. China 2 State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian 116024, P. R. China
Abstract Asphalt is a typical self-healing material, but the healing process is rather inefficient. Therefore, melamine urea formaldehyde (MUF) microcapsules containing rejuvenator were fabricated to enhance the self-healing ability of asphalt. Optical and Scanning Electronic Microscope (SEM) morphologies showed that the prepared microcapsules were intact and the outer surface of the microcapsule was rough, both of which were beneficial for the interaction between asphalt and microcapsules. Microcapsules were mixed with asphalt by a proportion of 3%wt, and almost all the microcapsules were kept intact even when experiencing 160◦C high temperature and mechanical agitation. It is noted that microcapsules were distributed homogeneously, which were highly likely to release rejuvenator after meeting with micro-cracks. Ductility self-healing test, along with fluorescence microscope observation, was conducted to demonstrate how MUF microcapsules performed in asphalt. The test found that microcapsules were broken by the fracture energy at the tip of crack, thus a rejuvenator channel among microcapsules and cracks was formed to let the rejuvenator capillary flow into the crack before the closure of micro-cracks. DSR fatigue-healing-fatigue test and direct tensile test were further carried out to evaluate the healing efficiency of asphalt binder containing different content of microcapsules. Key words: Polymeric composites; Functional; Microcapsules; Asphalt; Self-healing.
Introduction Asphalt is a typical self-healing material [1] but with low healing efficiency, especially at a relatively low temperature. Moreover, the aging problem due to environmental conditions further weakens its self-healing ability. Nowadays both active and passive enhancement technologies are recognized to be helpful to enhance the healing rate of asphalt materials. The active enhancement technology increases the self-healing rate by using softer binder or by incorporating additives into the binder to stimulate the self-healing potential [2-4]. Shen [5] found that polymer modifiers enhance the self-healing *
Corresponding author. E-mail:
[email protected] 1
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ability of asphalt binders, and modified binders outperform neat binders in terms of self-healing capability. Bhairampally [6] assumed that the increased self-healing ability of asphalt is correlated with the added amount of hydrated lime. However, there is also evidence showing that polymers could not improve the self-healing ability [7] but sometimes even incur adverse effects [8]. Ganjei and Aflaki[9] mixed the asphalt and nano silica together by ultrasonic high shear mixer to obtain modified asphalt. It is found that the self-healing behavior of asphalt mixture is enhanced as silica content
increases.
Moreover,
they
also
found
the
application
of
nano-silica
and
styrene-butadiene-styrene(SBS) together has a better efficient on self-healing behavior[10]. Sayadi and Hesami[11] found the asphalt mastic with electric arc furnace dust (EAFD) has a better self-healing
performance.
Passive
enhancement
technology
repairs
micro-cracks
by
energy-supplying-based or substance-filling-based technology. Induction heating is a widely used method of energy-supplying-based technology. By incorporating conductive fibers [12] or graphite [13] into asphalt mixtures, asphalt is capable of melting and flowing into micro-cracks with the induction energy. The deficiencies of this healing method are great electricity consumption and inevitable human intervention. Microcapsule method [14-16] is a typical substance-filling-based technology, which has been widely used in the polymer science. White et al. [17] prepared self-healing epoxy by incorporating Grubbs’ catalyst and microencapsulated dicyclopentadiene (DCPD) with a urea-formaldehyde shell. The fracture tests using a tapered double-cantilever beam demonstrated 75% toughness recovery because of the leakage of filling materials from the penetrated microcapsules. Brown et al. [18] further studied healing efficiency of DCPD-filled urea-formaldehyde shell microcapsules. The results showed the addition of microcapsules yields up to 127% increasing in fracture toughness. Wang et al. [19] prepared melamine-formaldehyde resin-walled microcapsules containing styrene, and the main diameter of microcapsules is in the range of 20~71 µm. They concluded that the average diameter and properties of microcapsules were influenced by dispersion rate and core/shell ratio. Liu et al. [20] produced microcapsules with a melamine-urea-formaldehyde (MUF) polymer wall containing 5-ethylidene-2-norbornene (ENB) by in situ polymerization method. These MUF microcapsules exhibited more thermally stable than urea-formaldehyde (UF) microcapsules, and MUF microcapsules with thicker shell exhibited improved mechanical strength and storage properties. With the successful applications of microencapsulation techniques in polymers [21-23], the application of microcapsules to promote asphalt self-healing behavior has become a hot research field. Once a micro-crack appears, microcapsules containing rejuvenator with asphalt materials will be ruptured by the fracture energy at the tip of the crack and the rejuvenator will be released. The released oily-liquid will be mixed with the surrounding aged asphalt because of the capillary 2
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action. Thus, the asphalt will be softened, which leads to the enhancement of the material’s self-healing ability. Considering the self-healing process of asphalt binder, the high mixing temperature of asphalt concrete and the compaction process of asphalt pavement, the microcapsules applied in asphalt pavement should have several characteristics as follows: Firstly, core material should be healing agent of low viscosity and free-flowing. Secondly, microcapsules need to be thermal stable and strong enough to keep shell structure intact under high mixing temperature and compaction. Meanwhile, embedded microcapsules should be sensitive enough to be ruptured by approaching cracks. Su et al. [24] investigated thermal stability of microcapsules by TGA analyzer, and reported that microcapsules perform better thermal stability with lower drop speed of pre-polymer and lower core/shell ratio. With appropriate core/shell ratio and drop speed, microcapsules incorporated in asphalt enable to keep shell structure at 180℃. Lee et al. [25] reported that microcapsules with larger diameters had higher elastic modulus. Chung et al. [16] fabricated microcapsules with xylenol-filled urea-formaldehyde shell by in situ polymerization method. The asphalt mixture containing microcapsules showed a great self-healing capability since the recovered beam had a better impact strength than undamaged one. Micaelo et al.[26] manufactured calcium-alginate capsules by encapsulating sunflower oil, and they found the self-healing behavior of the asphalt mixture with calcium-alginate capsules is enhanced. Xue et al. [27] prepared microcapsules by in-situ polymerization method and investigated the fatigue and low temperature performance of asphalt containing microcapsules. They found that the microcapsules had good healing efficiency for the asphalt under conditions of low temperature and fatigue load, but the healing efficiency increased first and then decreased with the addition of microcapsules, and the optimal amount of microcapsules was 0.3–0.5 wt% of the asphalt. Shirzad et al.[28] fabricated microcapsules containing sunflower oil, and investigated the effect of microcapsules on the performance of asphalt mixtures. They found the microcapsules decreased the low temperature stiffness of asphalt binder but increased the m-value of the binder blends. They also found the healing efficiency of mixtures with microcapsules is better than neat mixture, but lower than mixtures with pure sunflower oil, which indicated that not all the sunflower oil was released from the microcapsule so that the self-healing performance was limited. In summary, microencapsulation method seems to be an intelligent and promising technology to help prolong the lifespan of asphalt pavement. Some breakthroughs have been made on self-healing (micro)capsules, but there are still several problems remained to be solved: the capsule in asphalt mixtures is found to have an inhomogeneous distribution[15] and the self-healing process and efficiency of microcapsules in asphalt have not been clearly demonstrated[14,16]. In this study, melamine urea formaldehyde (MUF) microcapsules containing 3
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rejuvenator were fabricated via an in-situ polymerization method. The distribution of microcapsules in asphalt was observed based on the Fluorescence Microscope (FM) and the Scanning Electronic Microscope (SEM), and the problem of whether the microcapsules can enhance the self-healing ability was discussed. Finally, the healing efficiency of asphalt mixtures containing different content of microcapsules was evaluated based on the fatigue-healing-fatigue test and strength recovery test.
Experimental Section Materials The neat asphalt binder (penetration grade 70, AH-70) was used in this study. The basic properties of neat asphalt binder are shown in Table 1. The rejuvenator used as the core material is transparent yellow liquid with pungent smell, its main component is light oil, and its properties are given in Table2. Table 1. Properties of the asphalt binder Asphalt type
Penetration (25℃, 100g, 5s), 0.1mm
Apparent Viscosity(135℃/Pa·s)
Ductility(15℃)
Neat asphalt
66.2
0.549
>100cm
Table 2. Properties of the rejuvenator used as core materials Apparent Viscosty(60℃/pa·s)
Kinematic Viscosity(60℃/cSt)
Flash Point(/℃)
Saturates (%)
Aromatics (%)
Density (15℃/g·cm-3)
0.264
121
245
28.4
70.9
0.806
Synthesis of MUF microcapsules containing rejuvenator Fig. 1 shows the synthesis process of microcapsules containing rejuvenator based on in situ polymerization. The process is stated as follows: (1) 0.7g Sodium dodecyl sulfonate (SDS) was added to 100ml deionized water and stirred for about 30min at a stirring rate of 300r/min under room temperature. Then about 10g of rejuvenator was emulsified in the SDS aqueous solution by a high-speed disperse machine at a speed of 1500r/min for about 1h. (2) At room temperature, 5g mixture of melamine, urea and formaldehyde was added to 15g deionized water. After the urea dissolved, the PH value of the solution was adjusted to 8-9 with a 10%wt sodium hydroxide (NaOH) solution and then the temperature of the solution was slowly increased to 70◦C. One hour later, the pre-polymer was obtained. (3) The pre-polymer was added by drops to the prepared oil in water emulsion under agitation (350r/min by a mechanical blender). At the same time, the temperature was heated to 65◦C at a rate of 5◦C/min, and the PH value was adjusted to 3 in 90min 4
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by a 10%wt citric acid solution. After 4 hours reaction, the microcapsules were filtered carefully and washed with deionized water for several times. Finally, microcapsules were dried in an oven at 40◦C for about 24h. The morphology of microcapsules was observed by an FM (Olympus BX-41) and an SEM (Nova Nano-SEM 450). The size distribution of more than 250 microcapsules was investigated by using the FM.
Fig. 1 Schematic of synthesis process microcapsules containing rejuvenator by in situ polymerization The morphologies of the prepared microcapsules are presented in Fig. 2. Fig. 2(a) exhibits the optical microscope morphologies of the microcapsules with spherical shapes. By investigating the diameters of at least 250 microcapsules randomly, it is found that the diameter ranges from 10µm to 330µm, and the average size of the production is about 130µm. Fig. 2(b) shows the SEM surface morphology of microcapsules. Fig. 2(c) is the partial amplification SEM surface morphology of a microcapsule. It can be concluded that the inner surface of the microcapsules is smooth and flawless and that the rejuvenator is not likely to be leaking unless the microcapsule is 5
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broken. The rough outer surface of the microcapsule may result from the deposition of pre-polymer and it does good to the interaction between asphalt and microcapsules. The thickness of shell is around 950nm (see Fig.2(d)), the inner surface of the shell is very smooth and intact, which ensures the long-term storage of core materials.
Fig. 2 Surface morphologies of prepared microcapsules: (a) optical microscope image of microcapsules; (b) SEM morphology of microcapsules; (c) partial amplification SEM surface morphology of a microcapsule; (d) SEM morphology of shell structure.
Asphalt binder containing microcapsules To observe the self-healing process induced by microcapsules in asphalt binder, the asphalt binder containing microcapsules was prepared. The preparation process is illustrated as follows: (1) Four samples of AH-70 with equal mass were heated in the oven at temperature of 160℃. (2) 1%wt, 3%wt and 5%wt (to asphalt binder weight) dried microcapsules were slowly added into asphalt and stirred for 30 min at a speed of 300 r/min. Then, asphalt specimens were cooled down at room temperature. (3) The asphalt specimen with 3%wt shell material was prepared according to the above method. A drop of prepared hot asphalt was extracted and spread on a clean glass slide (2cm×2cm), and then the distribution of microcapsules in asphalt can be observed via FM, see Fig. 3. Different 6
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material can be distinguished and located by fluorescence of different color. Fig. 3 shows that microcapsules are luminous yellow while asphalt is yellow green. Apparently, different content of microcapsules is homogeneously distributed in asphalt specimens. Microcapsules still keep intact after experiencing mechanical stir and 160℃ high temperature treatments, which indicates that the microcapsules have high thermal stability and enough strength.
Fig. 3 FM morphologies of microcapsules in asphalt binder
To investigate the chemical structure of asphalt specimens, asphalt binders with 0%wt, 1%wt, 3%wt, 5%wt microcapsules and 3%wt shell material were identified based on Fourier transform infrared spectrometer (FTIR). FTIR spectra of rejuvenator, shell material, microcapsules, neat asphalt and neat asphalt with 3% microcapsules are shown in Fig. 4. Fig. 5 gives FTIR spectra of asphalt specimens with different content of microcapsules and shell materials.
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Fig. 4 FTIR spectra of microcapsules and related substances
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Fig. 5 FTIR spectra of asphalt specimen with different mass ratio of microcapsules and shell material Based on Fig. 4, it can be concluded that the spectrum of MUF shell exhibits absorption peak of N-H group at 3338 cm-1. For rejuvenator, the spectrum reveals absorption peak of C=O at 1744 cm-1. The absorption peak at 1600 cm-1 represents the benzene ring and hydroxyl of neat asphalt. The absorption peak of N-H group and C=O at 3338 cm-1, 1744 cm-1 can be observed in microcapsules, which suggests the rejuvenator has been successfully encapsulated into MUF shell. The spectra of neat asphalt and neat asphalt with 3% microcapsules are similar, and no new absorption peak in asphalt with 3% microcapsules is found. It suggests that the mixing of neat asphalt and microcapsules is totally a physical process. Compared with Fig.4, Fig. 5 shows that all asphalt specimens containing microcapsules are identified with absorption peak at 3338 cm-1 of MUF shell (shown as Fig. 5 (a)) and absorption peak at 1744 cm-1 of rejuvenator core material (shown as Fig. 5 (b)), which proves the existence of MUF and rejuvenator oil in the microcapsules. Fig. 5 (a) and Fig. 5 (b) also reveal that all absorption peaks at 3338 cm-1 and 1744 cm-1 are enhanced with the increasing number of microcapsules. Asphalt mixture containing microcapsules To establish that the microcapsules can withstand the shear stresses induced by the mixing process with mineral aggregate particles in the asphalt mixture without breaking, the asphalt concrete mixture with and without microcapsules were prepared. The aggregate gradation of asphalt mixture is shown in Table 3. 9
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Table 3 Gradation of AC-10 Sieve size/mm
13.2
9.5
4.75
2.36
1.18
0.6
0.3
0.15
0.075
Passing rate/%
100
95
60
44
32
22.5
16
11
6
70# virgin asphalt and 70# virgin asphalt with 3wt% microcapsules were selected respectively to prepare asphalt mixture (AC-10) according to the aggregate gradation shown in Table 3. The 400mm×300mm×75mm asphalt concrete sample was formed by vibratory roller, and the sample was cooled at room temperature at least 12h after molding. The formed sample was cut into 380mm×63mm×50mm standard four-point bending small beam, and then the small beam specimen was insulated at 15℃ for at least 4h, so as to ensure that the specimen reaches the required test temperature before the test. After that, the flexural stiffness based on four-point bending fatigue test was measured. It can be seen from Fig. 6 that the initial stiffness modulus of the asphalt mixture with microcapsules was smaller than that of the asphalt mixture without microcapsules (decreased 7%), which was possibly because that a small portion of microcapsules was ruptured during the mixing and compaction process, and the rejuvenator leaked out from the broken microcapsules and then softened the asphalt mixture. However, the difference is not distinct, which indicates that most of microcapsules can remain intact during the mixing and compaction process. Besides, it also can be concluded from the number of fatigue load that the addition of microcapsules enhanced the resistance to fatigue damage of the asphalt mixture and doubled the fatigue life. Above idea indicates that the microcapsules are gradually ruptured by the repeated loadings, and thus gradually increase the fatigue life of the asphalt concrete.
Fig. 6 The flexural stiffness modulus of the asphalt mixture specimen with and without microcapsules Of course, above observation can only indirectly demonstrate the most of microcapsule still keep intact after mixing it with asphalt binder and solid aggregate particles. We are planning to find 10
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some methods which could more intuitively reflect the existing situation of microcapsules in asphalt mixtures, such as SEM or CT. Above research outcomes will be reported in our future work.
Fatigue-healing-fatigue test of asphalt binder The Dynamic Shear Rheometer (DSR) has been proved to be a major test to evaluate the fatigue behavior of asphalt materials [29], which has been written in AASHTO [30]. Meanwhile, many researchers have adopted this method to evaluate the fatigue behavior of asphalt materials [31-35]. Therefore, the time sweep test by DSR was conducted in this study to simulate the fatigue process. By using a Dynamic Shear Rheometer (DSR, AR1500EX, TA Instruments Company, UAS), the fatigue-healing-fatigue test was conducted to evaluate the self-healing efficiency of asphalt binders containing different contents of microcapsules. The test was controlled by time-sweep program under 3% strain loading level. The loading rate was 10 Hz and test temperature was kept at 20℃. All tests were conducted by DSR with 8 mm diameter and 2 mm height specimens. Based on former experience [36, 37], recovery of 30% damaged asphalt binder (namely complex shear modulus (G*) drops to 70% of the initial modulus) could characterize the healing property of asphalt binder better. Therefore, the damage degree was set as 30%, namely the loading would stop when G* dropped to 70% of the initial value, and the specimen was allowed to rest for 1h, after that the shear was continued until the G* dropped to 70% of the initial value again. The whole procedure can be illustrated in Fig. 7.
Fig. 7 Illustration of fatigue-healing-fatigue test
Healing efficiency of asphalt specimen can be evaluated by Eqs. (1)-(3): 11
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1
HI =
2
HI =
G *healing − G *termianl
3
G
HI =
* initial
Wafter Wbefore
(1)
G *initial − G *termianl
G * healing − G * termianl −G
*
×
N after − N before N before
termianl
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(2)
n
2 * , W = ∑ DEi , DEi = πεi Gi sin δi
(3)
i =1
where G*initial is the initial complex shear modulus of asphalt binder, G*terminal is the complex shear modulus of asphalt binder before rest, and G*healing is the complex shear modulus of asphalt binder after rest. Nbefore is the fatigue life of asphalt binder before healing, and Nafter is the fatigue life of asphalt binder after healing. HI3 is the ratio of dissipated energy (DE) before and after rest, which can also indicate the healing capacity of asphalt binder [38, 39]. In Eq. (3), DEi is the dissipated energy by the ith loading cycle, and εi，δi，Gi* are strain, phase angle and complex shear modulus under ith loading cycle, respectively. Wbefore and Wafter are the accumulative dissipated energy before and after rest, respectively. Besides, the heal efficiency relative to neat asphalt can be calculated by the following equation:
ܫܪ = ܫܪ ൗܫܪ where
ܫܪ
is the relative healing efficiency to neat asphalt;
(4) ܫܪ
is the healing efficiency of ith
asphalt estimated by healing indictor j (i=1 to 4, representing neat asphalt with 1%, 3% ,5% wt microcapsules and 3%wt shell).
Strength recovery test of asphalt binder 5 parallel tests in each group were conducted in this test, and the variability was acceptable. The prepared hot asphalt was then poured into the asphalt ductility mold to get test specimens (about 75mm length× 10mm thickness), the surfaces of which were evened by a scraper after being cooled down for at least 2 hours at room temperature. Then the specimens were placed in a 40℃ oven for about 1 hour to get smoother surfaces. After being cooled down again, the specimens were placed in an environmental chamber at -18℃ for 24h. Finally, both ends of the specimens were stretched by a ductility testing equipment at a rate of 5cm/min till the specimen was fractured, and the tensile strength of the specimen was recorded. Then, the fractured surfaces were immediately kept in full contact with each other and the healing process was observed by the FM and recorded by a screen recording software. After FM observation, the specimens were kept in environment cabinet at 10℃ (to simulate the relative low healing temperature) for 48h (to ensure the ruptured sample has enough time to heal itself) and then kept in environment cabinet at -18℃ 12
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for 24h. After that, the ductility test was conducted again and the recovery tensile strength after healing was measured. It should be mentioned that asphalt could flow and diffuse into microcrack because of the different pressure between contact point and crack [40]. Then the asphalt molecules wet the crack faces, diffuse from one face to the other when crack faces in rather close contact, and finally put both crack faces in contact. Therefore, during the test, we only need to make sure that the two fractured faces of the specimen keep into fully contact with each other. Thus, only a small force was applied via the metal mold, then the force was unloaded to let the crack be naturally healed. The similar findings can be found in Refs [36,41,42]. According to many self-healing theories, for asphalt materials, crack can be healed when the temperature is above the glass transition temperature (but healing time varies) [40]. Therefore, self-healing behavior of asphalt will occur in a wide temperature range. In general, with temperature increases, the self-healing efficiency of asphalt materials will increase [43-45]. To simulate the self-healing behavior of asphalt pavement in service, the room temperature was chosen in this study. The strength recovery test is to completely fracture the specimen and then bring it together as mentioned above. Therefore, it is convenient to observe whether the homogeneously distributed microcapsules could work when only one crack exists. The result shows that the microcapsules on fracture face could enhance the self-healing behavior of asphalt, though there appears only one crack.
Fig. 8 asphalt specimen in ductility test Results and discussion Self-healing process Fig. 9 presents the FM morphologies of microcapsules in asphalt with the help of light characters. In Fig. 9(a, b), the luminous dots are microcapsules with the background of asphalt. The microcapsules distribute so homogeneously in asphalt that cracks can meet microcapsules 13
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more easily when the damage appears in the asphalt pavement, which is certainly beneficial for the self-healing process of asphalt. Moreover, most microcapsules were kept intact after 160◦C high temperature treatment and mechanical agitation. Fig. 9(c, d) shows the profiles of a ruptured asphalt specimen. Most of the microcapsules are ruptured when the asphalt specimen is torn apart whereas some still stay intact. The ruptured microcapsules indicate that the microcapsules will release the rejuvenator by approaching cracks in asphalt pavement.
Fig. 9 Fluorescence microscopic morphologies of microcapsules in asphalt: (a) Distribution of microcapsules in asphalt; (b) Enlarger version of microcapsules in asphalt; (c) The profile of a asphalt specimen; (d) The profile of a single crashed microcapsule.
Fig. 10 shows the self-healing process of asphalt containing microcapsules. A distinct micro-crack can be observed although fractured asphalt specimen surfaces are fully kept in contact. Fig. 10 (a) is the original appearance of the specimen with a crack width of nearly 100 µm. 3 microcapsules are ruptured with the rejuvenator leaking out, and then a channel contained rejuvenator is formed along the micro-crack among the 3 ruptured microcapsules. The channel in Fig. 10 (b) is broadened as more rejuvenators are released. With time passing by, the asphalt is softened after being mixed with rejuvenators, and then the crack is narrowed and the rejuvenator channel gradually disappears (Fig. 10 (b-e)). In Fig. 10 (d), the crack between two adjacent microcapsules is closed since the two microcapsules are getting closer in distance and plenty of rejuvenators are supplied. In Fig. 10 (e), the cracks closer to microcapsules are healed prior to those distant ones. In this case, it is the rejuvenator released from the microcapsules instead of temperature that heals the cracks. After 30 minutes, the crack is fully closed (Fig. 10 (f)).
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Fig. 10 The self-healing process of asphalt mixed with microcapsules: (a) Fluorescence microscopic images of microcapsules in asphalt with cracks (0min); (b~e) The crack is disappearing; (f) The crack disappears in 30 minutes. Fatigue-healing-fatigue test results Fatigue-healing-fatigue test was conducted by DSR to investigate the healing efficiency of asphalt binder containing different contents of microcapsules (0%wt, 1%wt, 3%wt and 5%wt). To consider the influence of shell material on healing capacity, the asphalt specimen with 3%wt shell material was tested also. Because the initial complex shear moduli (G*initial) of different specimens are different, G*/G*initial is employed to indicate the variation of modulus during fatigue process. Fig. 11 shows the variation of G*/G*initial of asphalt binder before/after healing with loading times. Table 4 shows G*, fatigue life and healing indicators of specimens in different stage. 15
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Fig. 11 Variation of G*/G*initial with loading times of asphalt with/without microcapsules Table 4 Complex shear modulus and healing indicators of asphalt specimens with/without microcapsules Asphalt type
G*initial /MPa
G*terminal /MPa
G*healing /MPa
Nbefore
Nafter
HI1
HI2
HI3
neat
7.89
5.53
6.19
15020
1640
0.2797
0.0326
0.0763
neat+1%wt microcapsule neat+3%wt microcapsule neat+5%wt microcapsule
7.78
5.4
6.14
17000
1810
0.3109
0.0331
0.0803
7.71
5.38
6.2
16000
1930
0.3519
0.0425
0.0889
7.4
5.22
6.1
15400
2010
0.4037
0.0527
0.0982
neat+3%wt shell
8.85
6.28
6.95
14130
1490
0.2607
0.0275
0.0741
It can be found in Table 4 that adding microcapsules in asphalt binder may lower the value of
G
*
initial,
and G*initial decreased with the increasing contents of microcapsules. However, the
decrease tendency was not distinct, which indicated that most of microcapsules were still intact and had certain strength to resist mechanical agitation. Besides, it can be observed that G*initial of asphalt specimen with 3%wt MUF-shell was increased might because shell material reinforced neat asphalt. Fig. 11 shows that, in initial stage, G* of asphalt specimen containing microcapsules decreased more rapidly than neat asphalt, and G* curves fluctuated more drastically with the increasing content of microcapsules. The main reason is that during the loading process, the microcracks were gradually initiated, and microcapsules were ruptured and more rejuvenator was released. Although G* of asphalt with microcapsules decreased sharply at initial stage, with the increase of 16
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the loading cycles, G* curves began to flatten, which revealed that partial micro-cracks were healed since the crack faces were bonded by the releasing rejuvenator. G* of neat asphalt and asphalt with 3%wt shell decreased slowly at initial stage, but the descending tendency was more distinct with the increase of the loading cycles, which implied the low healing efficiency. When complex shear modulus dropped to 70% of the initial value, the corresponding loading cycles of asphalt with microcapsules were higher than neat asphalt, which indicated that microcapsules can prolong the fatigue life of asphalt. The fatigue loading cycles of asphalt with 1%wt microcapsules was the highest, followed by the asphalt with 3%wt microcapsules, and then the asphalt with 5%wt microcapsules. With the increasing releasing rejuvenator, although more micro-crack faces would be bonded, it was impossible for the strength of asphalt binder to be recovered within such a short time, meanwhile the asphalt binder became softer because of more releasing rejuvenator, so the fatigue loading cycles would be decreased with the increase of the content of microcapsules. It also can be found that the loading cycle of asphalt specimen with 3%wt shell material was the lowest, which manifested that it was rejuvenator rather than shell material enhanced the fatigue life of the asphalt binder, and the shell material alone may even impede fatigue life of asphalt binder. After 1h resting, the G*healing/G*initial and the fatigue life of asphalt binder increased with the increase of microcapsules. The self-healing indicators (HI1, HI2 and HI3) of asphalt binder with microcapsules were higher than neat asphalt, and the indicators were also increased with the increase of microcapsules content (see Fig. 12). An obvious fatigue life extension phenomenon can be observed when microcapsules were added into the asphalt, i.e., the healing index was increased 40% (for the case of 5%wt microcapsules) compared with the neat asphalt. It can be concluded that microcapsules can enhance the healing efficiency of asphalt binder. Besides, the healing indicators of asphalt with shell material were even lower than the neat asphalt, which revealed that it was rejuvenator instead of shell material contributed to the healing efficiency.
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Fig. 12 Healing efficiency of asphalt binders with different content of microcapsules In addition, the relative healing efficiency increased dramatically with the increase of microcapsule content, as shown in table 5. The relative healing efficiency of neat+5%wt microcapsule could reach 1.44(ܫܪଵ ), 1.66ሺܫܪଶ ) and 1.28(ܫܪଷ ), which means microcapsule enhanced the self-healing behavior of asphalt significantly. Table 5 Relative healing efficiency ܫܪଵ
ܫܪଶ
ܫܪଶ
neat+1%wt microcapsule
1.1115
1.0153
1.0524
neat+3%wt microcapsule
1.2581
1.3037
1.1651
neat+5%wt microcapsule
1.4433
1.6166
1.2870
neat+3%wt shell
0.9321
0.8436
0.9712
After the fatigue-healing-fatigue test, the asphalt specimens were observed by fluorescence microscope. As shown in Fig. 13, all microcapsules were ruptured and shell materials were left as highlight dots in the pictures. The amount of left shell materials increased with the increase of microcapsules.
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Fig. 13 The fluorescence morphologies of asphalt specimens after fatigue-healing-fatigue test Strength recovery test results Fig. 14 shows the healing process of asphalt specimen with 3%wt microcapsules. The original asphalt ductility sample can be found in Fig. 14 (a). After ductility test, the fracture face was in a hard and brittle state, see Fig. 14 (b). The fractured face was located in middle of the sample. The sectional profile of the fractured face can be found in Fig.14. However, after 3 days healing (for 48h at 10℃ and for 24h at -18℃), Fig. 14 (c) indicates that the asphalt sample has already had a certain strength. Table 6 and Fig. 16 present tensile strength of asphalt specimens before and after healing. It can be observed that the tensile strength values of neat asphalt and asphalt specimens with 1%wt microcapsules were approximately the same. The tensile strength of asphalt decreased with the increase of the microcapsules content, and the tensile strength of asphalt with 5%wt microcapsules was the lowest. The small microcapsules may sometimes be regarded as the small defects appearing in the asphalt binder, which may reduce the strength of asphalt binder, but such reduction was not distinct. However, the tensile strength after 3 days healing and strength recovery ratio increased with the increase of the microcapsules content. The strength recovery ratio of asphalt with 5%wt microcapsules even reached up to 83.8%, which was the highest among all the samples. Because of high content of microcapsules, more microcapsules were ruptured at the fractured faces, more releasing rejuvenator softened the asphalt, promoting the asphalt molecular diffusion rate across the fractured faces, and therefore the healing rate was greatly improved.
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Fig. 14 The fracture and healing morphologies of asphalt with 3%wt microcapsules mold
Fig. 15 Fractured faces of deformed specimens Table 6 The tensile strength of asphalt specimens with different content of microcapsules before and after healing specimen
Tensile strength/N
Tensile strength after recovery/N
Strength recovery ratio（ （%） ）
Neat asphalt
52.2
27.9
53.45
neat+1% microcapsule
51.5
31.4
60.97
neat+3% microcapsule
49.1
36.3
73.93
neat+5% microcapsule
46.3
38.8
83.80
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Fig. 16 The tensile strength and recovery ratio of asphalt specimens with different dosage of microcapsules before and after healing
The self-healing ability is influenced by several factors, such as ambient temperature, healing time, damage degree and so on. In general, the asphalt would gradually heal itself under suitable healing temperature and enough healing time. That is the reason why the healing due to the heat treatment (10℃ for 48 hours) results in a 53.45% strength recovery for the neat binder without any microcapsule. However, the additional strength recovery ratio for asphalt with microcapsules does exist due to the use of microcapsules. The additional strength recovery ratios are 7.52%, 20.48% and 30.35% to asphalt with 1%, 3% and 5%wt microcapsules respectively. Therefore, we can conclude that the addition of microcapsules can promote the healing efficiency of asphalt materials. Besides, from an application point of view, the asphalt pavement in service has no sufficient healing time to heal the cracks due to the continuous repeated loadings. However, the cracks can be repaired when microcapsules meet with the cracks. As a result, under the same situation without enough healing time, the asphalt mixture with microcapsules could still heal the cracks in some extent. Therefore, it is true that we can say the use of microcapsules implies they are sustainable especially for long-term performance of asphalt mixtures for pavement applications.
Conclusions Self-healing MUF microcapsules containing rejuvenator have been fabricated successfully. The fluorescence microscope images and the SEM morphologies indicate that the microcapsules have 21
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spherical shapes with an average size of 130µm diameter. Mixed with hot asphalt, nearly all the microcapsules remain intact even at 160 ℃ high temperature and mechanical agitation. A homogeneous distribution of microcapsules in asphalt is found and microcapsules are indeed broken when cracks appear. Moreover, based on the fatigue-healing-fatigue test and the strength recovery test, it can be observed from the test results that the microcapsules we fabricated can promote the healing efficiency of asphalt binder. The following conclusions can be drawn: (1) FTIR test was conducted on asphalt specimen to identify the characteristic peak of microcapsules. The FTIR test showed that the rejuvenator oil has been successfully encapsulated in the MUF shell. (2) FM morphologies was carried out to observe distribution of microcapsules. It revealed that the distribution was homogeneously. Shell structures were kept intact after high temperature stir, which proved that the microcapsules had great thermal stability and enough strength to resist the mechanical agitation. (3) According to the observation of FM morphologies in the fractured faces of asphalt, it can be confirmed that the microcapsules were ruptured by the fracture energy at the tip of crack. Moreover, the observation of healing process induced by microcapsules revealed that a rejuvenator channel among microcapsules and cracks was formed to let the rejuvenator capillary flow into the crack before the closure of micro-cracks. (4) DSR fatigue-healing-fatigue test showed that microcapsules enhanced the fatigue life and self-healing capacity of asphalt. The self-healing efficiency was increased with the increase of microcapsules content, and the self-healing index is increased 40% (for the case of 5%wt microcapsules) compared with the neat asphalt. (5) Direct tensile test was conducted to investigate the influence of microcapsules on the tensile strength recovery of asphalt binder. The strength recovery ratio of asphalt with 5%wt microcapsules can even reach up to 83.8%, which indicated that the microcapsules we fabricated can effectively enhance the healing efficiency of asphalt binder. Further studies (which include laboratory and field investigations on asphalt concrete mixtures) are necessary for the evaluation of the effectiveness of the microcapsule for long-term sustainability of asphalt pavements. Above lines will be discussed in our future studies. Acknowledgements
The work described in this paper is supported by the National Natural Science Foundation of China (Nos. 51378393 and U1633116), the Innovation Program of Shanghai Municipal Education Commission (No.15ZZ017), the Fundamental Research Funds for the Central Universities, and Open Foundation of State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, China.
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SUPPORTING INFORMATION. There is no supporting information. References [1] Bazin, P., Saunier, J. (1967). Deform-ability, fatigue, and healing properties of asphalt mixes, in: Proceedings of the Second International Conference on the Structural Design of Asphalt Pavements. [2] Su, J. F., Han, S., Wang, Y. Y., Schlangen, E., Han, N. X., Liu, B., ... & Li, W. (2017). Experimental observation
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For Table of Contents Use Only Synopsis: The self-healing process of asphalt binder mixed with microcapsules (see (a)-(d)), which gives evidence for long lifespan sustainable asphalt pavement.
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