Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9881-9893
Enhanced Self-Healing Process of Sustainable Asphalt Materials Containing Microcapsules Daquan Sun,† Qi Pang,† Xingyi Zhu,*,†,‡ Yang Tian,† Tong Lu,† and Yang Yang† †
Key Laboratory of Road and Traffic Engineering of Ministry of Education, Tongji University, Shanghai 200092, P. R. China 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 microscopy (SEM) morphologies showed that the prepared microcapsules were intact, and the outer surface of the microcapsule was rough, with both observations 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 microcracks. A ductility self-healing test, along with fluorescence microscopy observations, 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 the crack, and thus a rejuvenator channel among microcapsules and cracks was formed to let the rejuvenator capillary-flow into the crack before the closure of microcracks. The DSR (dynamic shear rheometer) fatigue−healing−fatigue test and direct tensile test were further carried out to evaluate the healing efficiency of asphalt binder containing different contents of microcapsules. KEYWORDS: Polymeric composites, Functional, Microcapsules, Asphalt, Self-healing
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INTRODUCTION Asphalt is a typical self-healing material1 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 Shen5 found that polymer modifiers enhance the self-healing ability of asphalt binders, and modified binders outperform neat binders in terms of self-healing capability. Bhairampally6 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 ability7 but sometimes even incur adverse effects.8 Ganjei and Aflaki9 mixed the asphalt and nanosilica together by an ultrasonic high-shear mixer to obtain © 2017 American Chemical Society
modified asphalt. It is found that the self-healing behavior of the asphalt mixture is enhanced as silica content increases. Moreover, they also found that the application of nanosilica and styrene−butadiene−styrene (SBS) together is more efficient for the self-healing behavior.10 Sayadi and Hesami11 found that the asphalt mastic with electric-arc furnace dust (EAFD) has a better self-healing performance. Passive enhancement technology repairs microcracks by energysupplying-based or substance-filling-based technology. Induction heating is a widely used method of energy-supplying-based technology. With the incorporation of conductive fibers12 or graphite13 into asphalt mixtures, asphalt is capable of melting and flowing into microcracks with the induction energy. The deficiencies of this healing method are great electricity consumption and inevitable human intervention. Received: June 8, 2017 Revised: September 13, 2017 Published: September 20, 2017 9881
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
Research Article
ACS Sustainable Chemistry & Engineering The microcapsule method14−16 is a typical substance-fillingbased technology, which has been widely used in 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 that the addition of microcapsules yields up to a 127% increase 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 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 thermal stability than urea−formaldehyde (UF) microcapsules, and MUF microcapsules with a 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 microcrack 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 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: First, the core material should be a healing agent of low viscosity and free-flowing. Second, microcapsules need to be thermally stable and strong enough to keep the 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 a TGA analyzer, and reported that microcapsules perform with better thermal stability with a lower drop speed of prepolymer and lower core/shell ratio. With an appropriate core/shell ratio and drop speed, microcapsules incorporated in asphalt enable keeping the shell structure at 180 °C. Lee et al.25 reported that microcapsules with larger diameters had a higher elastic modulus. Chung et al.16 fabricated microcapsules with xylenol-filled urea−formaldehyde shell by an 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 the undamaged one. Micaelo et al.26 manufactured calcium-alginate capsules by encapsulating sunflower oil, and they found that the self-healing behavior of the asphalt mixture with calcium-alginate capsules is enhanced. Xue et al.27 prepared microcapsules by the 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; 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 that the microcapsules decreased the low-temperature stiffness of asphalt binder but increased the m value of the binder blends. They also found that the healing efficiency of mixtures with microcapsules is better than that of the neat mixture, but lower than that of 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, the 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 that remain 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 rejuvenator were fabricated via an in situ polymerization method. The distribution of microcapsules in asphalt was observed on the basis of fluorescence microscopy (FM) and scanning electronic microscopy (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 contents of microcapsules was evaluated on the basis of the fatigue−healing−fatigue test and strengthrecovery test.
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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 a transparent yellow liquid with a pungent smell; its main component is light oil, and its properties are given in Table 2.
Table 1. Properties of the Asphalt Binder asphalt type
penetration (25 °C, 100 g, 5 s), 0.1 mm
apparent viscosity (135 °C; Pa s)
ductility (15 °C)
neat asphalt
66.2
0.549
>100 cm
Synthesis of MUF Microcapsules Containing Rejuvenator. Figure 1 shows the synthesis process of microcapsules containing rejuvenator based on in situ polymerization. The process is stated as follows: (1) a 0.7 g portion of sodium dodecyl sulfonate (SDS) was added to 100 mL of deionized water and stirred for about 30 min at a stirring rate of 300 rpm under room temperature. Then, about 10 g of rejuvenator was emulsified in the SDS aqueous solution by a highspeed disperse machine at a speed of 1500 rpm for about 1 h. (2) At room temperature, a 5 g mixture of melamine, urea, and formaldehyde was added to 15 g of 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. The prepolymer was obtained 1 h later. (3) The prepolymer was added by drops to the prepared oil-in-water emulsion under agitation (350 rpm by a mechanical blender). At the same time, the temperature was heated to 65 °C at a rate of 5 °C min−1, and the pH value was adjusted to 3 in 90 min by a 10 wt % citric acid solution. After 4 h of reaction, the microcapsules were filtered carefully and washed with deionized water several times. 9882
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
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ACS Sustainable Chemistry & Engineering Table 2. Properties of the Rejuvenator Used as Core Materials apparent viscosity (60 °C; Pa s)
kinematic viscosity (60 °C; cSt)
flash point (°C)
saturates (%)
aromatics (%)
density (15 °C; g cm−3)
0.264
121
245
28.4
70.9
0.806
Figure 1. Schematic of the synthesis process of microcapsules containing rejuvenator by in situ polymerization. Finally, microcapsules were dried in an oven at 40 °C for about 24 h. The morphology of microcapsules was observed by FM (Olympus BX41) and SEM (Nova Nano-SEM 450). The size distribution of more than 250 microcapsules was investigated by using FM. The morphologies of the prepared microcapsules are presented in Figure 2. Figure 2a 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 10 to 330 μm, and the average size of the production is about 130 μm. Figure 2b shows the SEM surface morphology of microcapsules. Figure 2c 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 leak unless the microcapsule is broken. The rough outer surface of the microcapsule may result from the deposition of prepolymer, and it benefits the interaction between asphalt and microcapsules. The thickness of the shell is around 950 nm (see Figure 2d), and the inner surface of the shell is very smooth and intact, which ensures the long-term storage of core materials. Asphalt Binder Containing Microcapsules. For observation of the self-healing process induced by microcapsules in asphalt binder, the asphalt binder containing microcapsules was prepared. The preparation process is illustrated as follows:
min at a speed of 300 rpm; 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 (2 cm × 2 cm), and then, the distribution of microcapsules in asphalt can be observed via FM (see Figure 3). Different materials can be distinguished and located by different fluorescence colors. Figure 3 shows that microcapsules are luminous yellow while asphalt is yellow-green. Apparently, different contents of microcapsules are homogeneously distributed in asphalt specimens. Microcapsules still remain intact after experiencing mechanical stirring and 160 °C hightemperature treatments, which indicates that the microcapsules have high thermal stability and sufficient strength. For investigation of the chemical structure of asphalt specimens, asphalt binders with 0, 1, 3, and 5 wt % microcapsules and 3 wt % shell material were identified on the basis of Fourier transform infrared spectrometry (FTIR). FTIR spectra of the rejuvenator, shell material, microcapsules, neat asphalt, and neat asphalt with 3% microcapsules are shown in Figure 4. Figure 5 gives FTIR spectra of asphalt specimens with different contents of microcapsules and shell materials. On the basis of Figure 4, it can be concluded that the spectrum of the MUF shell exhibits an absorption peak of the NH group at 3338 cm−1. For the rejuvenator, the spectrum reveals an absorption peak of CO at 1744 cm−1. The absorption peak at 1600 cm−1 represents the benzene ring and hydroxyl of neat asphalt. The absorption peak of the NH group and CO at 3338, and 1744 cm−1 can be observed in microcapsules, which suggests that the rejuvenator has been
(1) Four samples of AH-70 with equal mass were heated in the oven at a temperature of 160 °C. (2) Portions of 1, 3, and 5 wt % (to asphalt binder weight) dried microcapsules were slowly added into asphalt and stirred for 30 9883
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
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ACS Sustainable Chemistry & Engineering
Figure 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 the shell structure. successfully encapsulated into the 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. This suggests that the mixing of neat asphalt and microcapsules is totally a physical process. Compared with Figure 4, Figure 5 shows that all asphalt specimens containing microcapsules are identified with an absorption peak at 3338 cm−1 of the MUF shell (shown as Figure 5a) and an absorption peak at 1744 cm−1 of the rejuvenator core material (shown as Figure 5b), which proves the existence of MUF and rejuvenator oil in the microcapsules. Figure 5a,b also reveal that all absorption peaks at 3338 and 1744 cm−1 are enhanced with the increasing number of microcapsules. Asphalt Mixture Containing Microcapsules. For confirmation 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 mixtures with and without microcapsules were prepared. The aggregate gradation of the asphalt mixture is shown in Table 3. Samples of 70# virgin asphalt and 70# virgin asphalt with 3 wt % microcapsules were selected, respectively, to prepare the asphalt mixture (AC-10) according to the aggregate gradation shown in Table 3. The 400 mm × 300 mm × 75 mm asphalt concrete sample was formed by vibratory roller, and the sample was cooled at room temperature at least 12 h after molding. The formed sample was cut into a 380 mm × 63 mm × 50 mm standard four-point bending small beam, and then, the small beam specimen was insulated at 15 °C for at least 4 h, to ensure that the specimen reaches the required test temperature before the test. After that, the flexural stiffness based on the four-point bending fatigue test was measured. It can be seen from Figure 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 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 the microcapsules can remain intact during the mixing and compaction process. In addition, it also can be concluded from the number of fatigue loading cycles that the addition of microcapsules enhanced the
resistance to fatigue damage of the asphalt mixture and doubled the fatigue life. The above idea indicates that the microcapsules are gradually ruptured by the repeated loadings, and thus gradually increase the fatigue life of the asphalt concrete. Of course, the above observation can only indirectly demonstrate that most of the microcapsules still remain intact after mixing with asphalt binder and solid aggregate particles. We are planning to find some methods which could more intuitively reflect the existing situation of microcapsules in asphalt mixtures, such as SEM or CT. The above research outcomes will be reported in our future work. Fatigue−Healing−Fatigue Test of Asphalt Binder. The dynamic shear rheometer (DSR) test has been proven 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), 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 a time-sweep program under 3% strain loading level. The loading rate was 10 Hz, and test temperature was kept at 20 °C. All tests were conducted by DSR with 8 mm diameter and 2 mm height specimens. On the basis of former experience,36,37 recovery of 30% damaged asphalt binder [namely, complex shear modulus (G*) drops to 70% of the initial modulus] could better characterize the healing property of asphalt binder. 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 1 h; after that, the shear was continued until the G* dropped to 70% of the initial value again. The whole procedure is illustrated in Figure 7. Healing efficiency of the asphalt specimen can be evaluated by eqs 1−3: HI1 = 9884
G*healing − G*terminal G*initial − G*terminal
(1) DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
Research Article
ACS Sustainable Chemistry & Engineering
Figure 3. FM morphologies of microcapsules in asphalt binder.
HI2 =
G*healing − G*terminal N − Nbefore × after Nbefore G*initial − G*terminal
HI3 =
Wafter , Wbefore
Strength-Recovery Test of Asphalt Binder. Five 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 obtain test specimens (about 75 mm length × 10 mm thickness), the surfaces of which were made even by a scraper after being cooled down for at least 2 h at room temperature. Then, the specimens were placed in a 40 °C oven for about 1 h to obtain smoother surfaces. After being cooled down again, the specimens were placed in an environmental chamber at −18 °C for 24 h. Finally, both ends of the specimens were stretched by ductility testing equipment (see Figure 8) at a rate of 5 cm min−1 until 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 an environmental cabinet at 10 °C (to simulate the relatively low healing temperature) for 48 h (to ensure that the ruptured sample has enough time to heal itself) and then kept in an environmental cabinet at −18 °C for 24 h. 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 the microcrack because of the different pressures between the contact point and crack.40 Then, the asphalt molecules wet the crack faces, diffuse from one face to the other when the crack faces are 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 remain fully in contact with each other. Thus, only a
(2)
n
W=
∑ DEi ,
DEi = πεi2Gi* sin δi
i=1
(3) Here, the terms are as follows: G*initial is the initial complex shear modulus of the asphalt binder, G*terminal is the complex shear modulus of the asphalt binder before rest, and G*healing is the complex shear modulus of the asphalt binder after rest. Nbefore is the fatigue life of the asphalt binder before healing, and Nafter is the fatigue life of the asphalt binder after healing. HI3 is the ratio of dissipated energy (DE) before and after rest, which can also indicate the healing capacity of the asphalt binder.38,39 In eq 3, DEi is the dissipated energy by the ith loading cycle, and εi, δi, and Gi* are the strain, phase angle, and complex shear modulus under the ith loading cycle, respectively. Wbefore and Wafter are the accumulative dissipated energy before and after rest, respectively. In addition, the heal efficiency relative to neat asphalt can be calculated by the following equation:
HIrij = HIij /HInej
(4)
HIjri
is the relative healing efficiency to neat asphalt, and HIji is where the healing efficiency of the ith asphalt estimated by healing indictor j (i = 1 to 4, representing neat asphalt with 1, 3, and 5 wt % microcapsules and 3 wt % shell). 9885
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
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Figure 4. FTIR spectra of microcapsules and related substances. small force was applied via the metal mold; then, the force was unloaded to let the crack be naturally healed. Similar findings can be found in refs 36, 41, and 42. According to many self-healing theories, for asphalt materials, cracks 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 For simulation of the self-healing behavior of asphalt pavement in service, 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 the fracture face could enhance the self-healing behavior of asphalt, though only one crack appears.
capsules distribute so homogeneously in asphalt that cracks can meet microcapsules more easily when the damage appears in the asphalt pavement, which is certainly beneficial for the selfhealing process of asphalt. Moreover, most microcapsules were kept intact after 160 °C high-temperature treatment and mechanical agitation. Figure 9c,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. Figure 10 shows the self-healing process of asphalt containing microcapsules. A distinct microcrack can be observed although fractured asphalt specimen surfaces are fully kept in contact. Figure 10a is the original appearance of the specimen with a crack width of nearly 100 μm. Three microcapsules are ruptured with the rejuvenator leaking out, and then, a channel containing rejuvenator is formed along the microcrack among the 3 ruptured microcapsules. The channel in Figure 10b is broadened as more rejuvenator is released. With time passing by, the asphalt is softened after being mixed with rejuvenator; then, the crack is narrowed, and the
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RESULTS AND DISCUSSION Self-Healing Process. Figure 9 presents the FM morphologies of microcapsules in asphalt with the help of light characteristics. In Figure 9a,b, the luminous dots are microcapsules with the background of asphalt. The micro9886
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
Research Article
ACS Sustainable Chemistry & Engineering
Figure 5. FTIR spectra of an asphalt specimen with different mass ratios of microcapsules and shell material.
Table 3. Gradation of AC-10 sieve size (mm) passing rate (%)
13.2 100
9.5 95
4.75 60
2.36 44
1.18 32
0.6 22.5
0.3 16
0.15 11
0.075 6
Figure 7. Illustration of the fatigue−healing−fatigue test. Figure 6. Flexural stiffness modulus of the asphalt mixture specimen with and without microcapsules.
rejuvenator channel gradually disappears (Figure 10b−e). In Figure 10d, the crack between two adjacent microcapsules is closed since the two microcapsules are becoming closer in distance, and plenty of rejuvenator is supplied. In Figure 10e, the cracks closer to microcapsules are healed prior to the distant ones. In this case, it is the rejuvenator released from the microcapsules instead of temperature that heals the cracks. After 30 min, the crack is fully closed (Figure 10f). Fatigue−Healing−Fatigue Test Results. The fatigue− healing−fatigue test was conducted by DSR to investigate the healing efficiency of asphalt binder containing different contents of microcapsules (0, 1, 3, and 5 wt %). For the consideration of the influence of shell material on healing capacity, the asphalt specimen with 3 wt % shell material was
Figure 8. Asphalt Specimen in Ductility Test.
tested also. Because the initial complex shear modulus (G*initial) values of different specimens are different, G*/G*initial is employed to indicate the variation of modulus during the fatigue process. Figure 11 shows the variation of G*/G*initial of 9887
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
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ACS Sustainable Chemistry & Engineering
Figure 9. Fluorescence microscopic morphologies of microcapsules in asphalt. (a) Distribution of microcapsules in asphalt. (b) Enlarged version of microcapsules in asphalt. (c) Profile of an asphalt specimen. (d) Profile of a single crashed microcapsule.
Figure 10. Self-healing process of asphalt mixed with microcapsules. (a) Fluorescence microscopic images of microcapsules in asphalt with cracks (0 min). (b−e) The crack is disappearing. (f) The crack disappears in 30 min.
addition, it can be observed that G*initial of the asphalt specimen with 3 wt % MUF shell was increased, and this might be because the shell material reinforced neat asphalt. Figure 11 shows that, in the initial stage, G* of the asphalt specimen containing microcapsules decreased more rapidly than that of 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, microcapsules were ruptured, and
asphalt binder before and after healing with loading times. Table 4 shows G*, fatigue life, and healing indicators of specimens in different stages. 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 the microcapsules were still intact and had a certain strength to resist mechanical agitation. In 9888
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
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ACS Sustainable Chemistry & Engineering
Figure 11. Variation of G*/G*initial with loading times of asphalt with and without microcapsules.
Table 4. Complex Shear Modulus and Healing Indicators of Asphalt Specimens with and without Microcapsules asphalt type neat neat neat neat neat
+ + + +
1 3 5 3
wt wt wt wt
% % % %
microcapsules microcapsules microcapsules shell
G*initial (MPa)
G*terminal (MPa)
G*healing (MPa)
Nbefore
Nafter
HI1
HI2
HI3
7.89 7.78 7.71 7.4 8.85
5.53 5.4 5.38 5.22 6.28
6.19 6.14 6.2 6.1 6.95
15 020 17 000 16 000 15 400 14 130
1640 1810 1930 2010 1490
0.2797 0.3109 0.3519 0.4037 0.2607
0.0326 0.0331 0.0425 0.0527 0.0275
0.0763 0.0803 0.0889 0.0982 0.0741
Figure 12. Healing efficiency of asphalt binders with different contents of microcapsules.
more rejuvenator was released. Although G* of asphalt with microcapsules decreased sharply at the initial stage, with the increase of the loading cycles, G* curves began to flatten, which revealed that partial microcracks were healed since the crack faces were bonded by the released rejuvenator. G* of neat asphalt and asphalt with 3 wt % shell decreased slowly at the 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 that of 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 that of the asphalt with 3 wt % microcapsules, and then that of the asphalt with 5 wt % microcapsules. With the increasing rejuvenator being released, although more microcrack faces would be bonded, it was impossible for the strength of asphalt binder to recover within such a short time; meanwhile, the asphalt binder became softer because of more rejuvenator being released, 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 the asphalt specimen with 3 wt % shell material was the 9889
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
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ACS Sustainable Chemistry & Engineering
After the fatigue−healing−fatigue test, the asphalt specimens were observed by fluorescence microscopy. As shown in Figure 13, all microcapsules were ruptured, and shell materials were left as highlighted dots in the pictures. The amount of remaining shell materials increased with the increase of microcapsules. Strength-Recovery Test Results. Figure 14 shows the healing process of an asphalt specimen with 3 wt % microcapsules. The original asphalt ductility sample can be found in Figure 14a. After the ductility test, the fracture face was in a hard and brittle state (see Figure 14b). The fractured face was located in the middle of the sample. The sectional profile of the fractured face can be found in Figure 15. However, after 3 days of healing (for 48 h at 10 °C and for 24 h at −18 °C), Figure 14c indicates that the asphalt sample already has a certain strength. Table 6 and Figure 16 present the 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 microcapsule 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 of healing and the strengthrecovery ratio increased with the increase of the microcapsule 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 the high content of microcapsules, more microcapsules were ruptured at the fractured faces, and more released rejuvenator softened the asphalt, promoting the asphalt molecular diffusion rate across
lowest, which showed that it was rejuvenator rather than shell material that enhanced the fatigue life of the asphalt binder, and the shell material alone may even impede the fatigue life of asphalt binder. After 1 h of 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 those of neat asphalt, and the indicators also increased with the increase of microcapsule content (see Figure 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 that of the neat asphalt. It can be concluded that microcapsules can enhance the healing efficiency of asphalt binder. In addition, the healing indicators of asphalt with shell material were even lower than those of the neat asphalt, which revealed that it was rejuvenator instead of shell material that contributed to the healing efficiency. 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 Table 5. Relative Healing Efficiency neat neat neat neat
+ + + +
1 3 5 3
wt wt wt wt
% % % %
microcapsules microcapsules microcapsules shell
HI1r
HI2r
HI2r
1.1115 1.2581 1.4433 0.9321
1.0153 1.3037 1.6166 0.8436
1.0524 1.1651 1.2870 0.9712
% microcapsules could reach 1.44 (HI1r ), 1.66 (HI2r ), and 1.28 (HI3r ), which means that microcapsules enhanced the selfhealing behavior of asphalt significantly.
Figure 13. Fluorescence morphologies of asphalt specimens after the fatigue−healing−fatigue test. 9890
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
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Figure 14. Fracture and healing morphologies of asphalt with the 3 wt % microcapsule mold. (a) Original asphalt ductility sample. (b) After the ductility test, the fracture face was in a hard and brittle state. (c) Indication that the asphalt sample already has a certain strength after 3 days of healing (for 48 h at 10 °C and for 24 h at −18 °C).
that the addition of microcapsules can promote the healing efficiency of asphalt materials. In addition, from an application point of view, the asphalt pavement in service has no sufficient healing time to heal the cracks because of 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 to some extent. Therefore, it is true that the use of microcapsules implies that they are sustainable especially for the long-term performance of asphalt mixtures for pavement applications.
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Figure 15. Fractured faces of deformed specimens.
CONCLUSIONS Self-healing MUF microcapsules containing rejuvenator have been fabricated successfully. The fluorescence microscopy images and the SEM morphologies indicate that the microcapsules have spherical shapes with an average size of 130 μm diameter. Mixed with hot asphalt, nearly all of the microcapsules remain intact even at 160 °C high temperature and with mechanical agitation. A homogeneous distribution of microcapsules in asphalt is found, and microcapsules are indeed broken when cracks appear. Moreover, on the basis of 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) An FTIR test was conducted on an 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 were carried out to observe the distribution of microcapsules. They revealed that the distribution was homogeneous. Shell structures were kept intact after high-temperature stirring, 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
Table 6. Tensile Strength of Asphalt Specimens with Different Contents of Microcapsules before and after Healing specimen neat asphalt neat + 1% microcapsules neat + 3% microcapsule neat + 5% microcapsule
tensile strength (N)
tensile strength after recovery (N)
strength-recovery ratio (%)
52.2 51.5
27.9 31.4
53.45 60.97
49.1
36.3
73.93
46.3
38.8
83.80
the fractured faces; therefore, the healing rate was greatly improved. 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 a suitable healing temperature and enough healing time. That is the reason why healing due to the heat treatment (10 °C for 48 h) results in a 53.45% strength recovery for the neat binder without any microcapsules. However, the additional strengthrecovery ratio for asphalt with microcapsules does exist because of the use of microcapsules. The additional strength-recovery ratios are 7.52%, 20.48%, and 30.35% for asphalt with 1, 3, and 5 wt % microcapsules, respectively. Therefore, we can conclude 9891
DOI: 10.1021/acssuschemeng.7b01850 ACS Sustainable Chem. Eng. 2017, 5, 9881−9893
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Figure 16. Tensile strength and recovery ratio of asphalt specimens with different dosages of microcapsules before and after healing.
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the tip of the crack. Moreover, the observation of the 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 microcracks. (4) The DSR fatigue−healing−fatigue test showed that microcapsules enhanced the fatigue life and self-healing capacity of asphalt. The self-healing efficiency increased with the increase of microcapsule content, and the selfhealing index increased 40% (for the case of 5 wt % microcapsules) compared with that of the neat asphalt. (5) A 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 longterm sustainability of asphalt pavements. The items above will be discussed in our future studies.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xingyi Zhu: 0000-0002-0822-6261 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work described in this paper is supported by the National Natural Science Foundation of China (51378393 and U1633116), the Innovation Program of Shanghai Municipal Education Commission (15ZZ017), the Fundamental Research Funds for the Central Universities, Open Foundation of State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, China, and China Scholarship Council. 9892
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