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Study of the mechanical properties and self-healing ability of asphalt mixture containing calcium-alginate capsules
Construction and Building Materials 123 (2016) 734–744
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Study of the mechanical properties and self-healing ability of asphalt mixture containing calcium-alginate capsules R. Micaelo a,b,c,⇑, T. Al-Mansoori c, A. Garcia c a
DEC, FCT, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal CERIS, CESUR, Universidade de Lisboa, 1049-001 Lisboa, Portugal c Nottingham Transportation Engineering Centre, School of Civil Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK b
h i g h l i g h t s Calcium-alginate capsules with a
maximum payload of 75% were produced. Capsules without hard shell resist mixing and compaction. Capsules deformed and broke with loading. Oil diffused in the bitumen in less than 24 h. Large reduction of cracks around the capsules after resting 4 days at 20 °C.
g r a p h i c a l a b s t r a c t Cracked asphalt pavement
a Shell
3 mm Core
Capsule Acc.V Spot Magn Det WD 10.0 kV 4.0 66x BSE 9.6
Capsules
i n f o
Article history: Received 23 April 2016 Received in revised form 4 July 2016 Accepted 17 July 2016 Available online 21 July 2016 Keywords: Self-healing Rejuvenators encapsulation Mechanical testing
Crack
Aggregate
Crack healing around the capsules
2 mm
a b s t r a c t The natural self-healing ability of asphalt mixtures can be enhanced with encapsulated rejuvenators: when crack damage appears the capsules release healing agents, which dissolve bitumen and drain into the cracks. In this study, the effect of a new type of capsules in the mechanical properties and the selfhealing ability of asphalt mixtures is investigated. Sunflower oil was encapsulated in calcium-alginate, and protected with a hard shell made of epoxy-cement composite. Results show that the hard shell was not required for these capsules to resist mixing and compaction procedures. Capsules deformed and broke with loading, releasing oil that diffused in the bitumen in less than 24 h. Healing of cracks in asphalt mixture led to an increase of stiffness. However, asphalt specimens with capsules had lower deformation resistance. Computer tomography scanning of specimens showed large reductions in cracks around the capsules, after resting 4 days at 20 °C. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Asphalt mixtures are the most worldwide used material to build roads. Although pavement structures with asphalt layers ⇑ Corresponding author at: DEC, FCT, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal. E-mail addresses: [email protected] (R. Micaelo), [email protected] (T. Al-Mansoori), [email protected] (A. Garcia). http://dx.doi.org/10.1016/j.conbuildmat.2016.07.095 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
1 mm
Broken capsule
Healing agent coming out of the capsule
a r t i c l e
Capsules heal the cracks
Asphalt contains capsules
are designed for a 10–30 years of service life, replacing the top layer with new asphalt materials is required about every eight years [1]. Damage in asphalt mixture starts at the microstructure level and evolves continuously up to the macro-level that is seen by road drivers and causes disturbances in comfort and safety of driving. Ultraviolet radiation, temperature and moisture contribute altogether to the binder’s ageing [2,3], which becomes stiffer with time, and loses flexibility and adhesion. Hence, the binder is not able to support temperature and/or
R. Micaelo et al. / Construction and Building Materials 123 (2016) 734–744
traffic induced stresses anymore, and microcracks occur in the material [4,5]. However, bitumen is a self-healing material [6]. When a crack occurs in an asphalt road, bitumen tends to drain into the crack and, if adequate time and temperature conditions are given, the crack may be fully closed. This process takes a very long time at normal temperatures, making it extremely ineffective [7]. In addition, asphalt self-healing is highly influenced by the viscosity and surface energy of bitumen [8–11]. In most countries a large portion of the asphalt materials reclaimed from old pavements are recycled and incorporated into new pavement layers [12]. Moreover, virgin binder, additives and aggregates are added to recycled mixtures in order to achieve minimum quality for using recycled asphalt materials in roads. Additives are softening agents or rejuvenators that modify the chemical composition of the old binder, rebalancing the proportion of light and heavy elements, and also lowering the viscosity [13,14]. Ideally, asphalt layers should be rehabilitated in situ without milling or scarifying interventions. One technique that has been proven in the laboratory is to heat asphalt mixture using induction energy to promote the flow of bitumen into the cracks [15]. A different approach to accelerate self-healing is the incorporation of capsules containing rejuvenators which break and release their content when damage occurs in their vicinity [16]. Then, the rejuvenators diffuse into bitumen reducing its viscosity and it can easily drain into the cracks [17]. Up to now, two different methods for the fabrication of capsules for asphalt self-healing have been proposed: (1) saturating porous aggregates with rejuvenators and sealing them with a shell made of epoxy resin and cement [17]; (2) microencapsulation of the rejuvenator by in situ polymerization of urea-formaldehyde [18], methanol-melamine-formaldehyde [19] or phenol–formaldehyde [20]. The first type of capsules resist mixing and compaction operations [17] and release the rejuvenator in asphalt specimens due to loading [21]. In addition, microcapsules release their content in presence of cracks [22,23] but have not been yet tested in asphalt mixture. Moreover, crack healing induced by capsules has been shown in bitumen [24], but never before in asphalt mixture. This paper describes an experimental study aimed at investigating the ability of a new type of capsules containing rejuvenators to induce self-healing in asphalt. The rejuvenator, sunflower oil, was encapsulated in a polymeric structure created by the ionotropic gelation of sodium alginate in the presence of calcium ions. Moreover, certain batches of capsules were additionally coated with a hard shell made of epoxy resin and cement. The research analysed the effect of (1) capsules on the mechanical behaviour of asphalt specimens subjected to cyclic loading and (2) introducing rest periods on the self-healing properties of asphalt mixture containing capsules. Finally, asphalt self-healing was visualised using computer tomography scans.
2. Materials and experiments 2.1. Materials To fabricate the capsules the following materials were used: (1) sodium alginate (C6H7O6Na) (Sigma-Aldrich), which is an anionic polysaccharide widely distributed in the cell walls of brown algae; (2) calcium chloride (CaCl2), provided by Sigma-Aldrich as anhydrous, granular pellets of 7 mm diameter and 93% purity; (3) epoxy resin (AralditeÒ 506 epoxy resin, Sigma-Aldrich), obtained by combining the epoxy resin with a fast hardener; (4) micro-cement (MICROCEM 550Ò, Tarmac), with average particle size ranged from 5 to 10 lm, provided by; (5) sunflower oil (East End, UK).
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Limestone aggregates (Tunstead quarry, Derbyshire, UK) and paving grade bitumen 40/60 were used to fabricate asphalt mixture. 2.2. Capsules The core of the capsules was composed by a polymeric structure made of calcium-alginate that encapsulates the rejuvenator. Moreover, in some batches the core was coated with a shell made of epoxy-cement. The polymeric core results from the reaction of alginate with calcium ions, creating a porous structure, often named as ‘‘egg-box” [25]. The fluid (rejuvenator) is entrapped in the structure during the reaction. The rejuvenator was a commercial sunflower oil, selected because it does not require special health and safety measures at the lab and is thermally stable. Vegetable oils can be used as bitumen rejuvenators [26,27], and they have also been encapsulated with self-healing purposes before [21,28]. The fabrication procedure is illustrated in Fig. 1. The left-hand side flowchart shows the procedure to fabricate the polymeric capsules, defined in [29], and the right-hand side flowchart shows the procedure to fabricate the hard-shell, defined from [17]. All tasks were performed at room temperature. First, 600 ml of deionized water and oil were introduced in 1 L glass container. Two types of emulsions were built with oil/water (o/w) ratio 1.0 (300 g of oil and 300 g of water) and 0.5 (200 g of oil and 400 g of water), respectively. Oil and water were homogenised using a laboratory gear drive mixer at 400 rpm during 1 min. Then, 15 g of sodium alginate were added and stirred until complete solution at 400 rpm, for 10 min. The alginate acted as an emulsifier, stabilizing the emulsion. Simultaneously, a calcium chloride solution was prepared by mixing 600 ml of water with 12 g of calcium chloride in 1 L glass water container. Capsules were formed by letting the oil-in-water emulsion drop into the calcium chloride emulsion from a 1000 ml pressure-equalizing dropping funnel with 3 mm socket size. During the capsule formation process, the calcium solution was gently agitated using a magnetic stirrer. Capsules were allowed to stay in the solution until the end of the encapsulation process, which lasted for approximately 4 h. Finally, capsules were decanted and washed with deionized water. They were dried during 12 h under the constant movement of air produced by a fan. Then, selected batches of dry capsules were covered with an outer shell made of epoxy-cement to increase their strength. The process of making the shell started by covering the calciumalginate capsules with epoxy in a mass ratio of 14% (14 g of epoxy for each 100 g of capsules). Later, seven steel balls of 20 mm diameter were placed in a plastic container, together with approximately 1 kg of cement. After, 250 g of capsules and epoxy were added, the container was closed, and shaken vigorously by hand for 15 s. After this, capsules, cement, and steel balls were separated in a 2 mm sieve, and let to rest for 12 h. The epoxy-cement coating process could be repeated various times for each batch of capsules to improve their strength. Three types of capsules were used in this study: (I) o/w 1.0, with three epoxy-cement coatings; (II) o/w 0.5, with two epoxy-cement coatings; (III) o/w 0.5, with no epoxy-cement coatings. Fig. 2 displays these capsules, which were selected because of the different morphology, size and strength characteristics. 2.3. Asphalt mixture Asphalt mixture for base courses, AC 20 base 40/60 (EN 131081), was used. Table 1 describes the composition of the mixture. Furthermore, asphalt mixture was pre-fabricated and aged in the laboratory before adding the capsules to simulate the natural
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(a) Encapsulation
(b) External shell
Mix oil and water, 400 rpm, 1 min
Polymeric capsules without shell
Sodium alginate (15g)
Epoxy resin
Mix, 400 rpm, 10 min
Hand-mix capsules with resin Cement Repeat
Fill dropper CaCl solution (2%)
Agitate by hand, 15 s
Emulsion drops in CaCl solution
Sieve, #2 mm
Decant capsules
Dry, 24h, room temp
Wash w/ deionized water
Capsules with shell
Dry, 24 h, air ventilation, room temp
Polymeric capsules without shell
Fig. 1. Flowchart of the capsules’ fabrication process: (a) polymeric (calcium-alginate) capsules; (b) hard shell.
Type I
Type II
Type III
10 mm
Fig. 2. Images of capsules. Table 1 Asphalt mixture design properties. AC 20 base 40/60 Aggregate gradation (% passing) 20 mm 10 mm 2 mm 0.063 mm Binder content (%M) Bulk density (kg/m3) Air voids (%) Voids in mineral aggregate (%) Voids filled with bitumen (%)
100.0 30.3 2.2 1.8 4.5 2384 4.5 14.9 69.8
ageing process of roads. The ageing procedure consisted in storing the mixture for 12 days in an air-ventilated oven at 85 °C. Before re-mixing and compacting, the aged asphalt mixture was heated at 160 °C. The capsules were added to the aged mixture and stirred for a short time (15 s) at the end of the re-mixing process. The mass proportion of capsules type I and II in asphalt was 6%. Moreover, the mass proportion of capsules type III in asphalt was 3%. Less capsules type III were used to compensate the high proportion of oil in the capsules. Finally, cylindrical asphalt specimens with diameter 100 mm and final height of approximately 50 mm
were compacted using a gyratory compactor with a maximum of 250 gyrations. For comparison purposes, asphalt specimens without capsules were fabricated with aged and non-aged asphalt. 2.4. Imaging techniques Three different imaging techniques were used in this research: optical microscopy (OM), scanning electron microscopy (SEM) and X-ray computed tomography (CT) scans. To determine the size of the capsules, these were observed with a stereoscopic microscope and the images taken were processed with the software ImageJ. The inner structure of the capsules was analysed with SEM (Philips XL30FEG SEM), in capsules casted in an epoxy resin matrix and then abraded until the inner structure was exposed. CT was performed on some asphalt specimens with an XRadia Versa XRM-500 scanner operated at 160 kV and 63 lA. The samples were assembled on a rotational table at a distance of 80.6 mm from the X-ray source and of 40.0 mm from the X-ray detector. To do this, small specimens of 40 40 50 mm3 were cut from the cylindrical specimens. These were adequate to obtain a pixel resolution of 45 lm. Later, the different materials that compose asphalt mixture were separated from the grayscale images using intensity thresholding in ImageJ.
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2.5. Capsule compression testing
K¼
The mechanical strength of individual capsules was measured with uniaxial compression testing. Testing was done on an Instron Model 5969 machine, with a loading rate of 0.2 mm/min. Ten capsules of each type studied were tested at 20 °C and at 130 °C, respectively. 2.6. Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) testing was performed to determine the thermal resistance and composition of capsules. Testing was done on a NETZSCH, TG 449 F3 Jupiter ThermoMicrobalance, using nitrogen atmosphere and a heating rate of 10 °C/min from room temperature up to 1000 °C. One capsule was inserted in the aluminium crucible in each test. 2.7. Mechanical testing of asphalt mixture
(a) 3.E+04 Load Strain
2.8. Chemical analysis of bitumen Fourier Transform Infrared spectroscopy (FTIR) was used to assess the modification of bitumen in asphalt mixture when the capsules release their content. Bitumen samples were collected directly from the surface of asphalt mixture test specimens with a hot knife. For comparison purposes, bitumen was recovered from aged asphalt mixture and mixed directly with oil in a 5% and 10% by mass. Furthermore, pure bitumen and oil were also analysed. The FTIR spectrum was obtained in the absorption mode within the wavenumber range of 400–4000 cm1, with a resolution of 4 cm1. The effect of the sunflower oil on the bitumen was evaluated from the changes in the absorption peak at the wavenumber ranged from 1700 to 1800 cm1. This range was selected because vegetable oils have a distinct peak at 1743 cm1 (C-O stretch) [30]. 3. Results and discussion 3.1. Characterization of capsules Fig. 4 shows the capsules’ internal structure, obtained with scanning electron microscopy. The capsules are approximately spherical and the oil is contained in the micropores of the core (see Fig. 4(b)). Furthermore, a gap between the shell and the core is observed in Fig. 4(a). The main characteristics of the capsules: size, strength, composition, are influenced by the o/w ratio and the number of shell coatings. Fig. 5 shows the diameter and compressive strength of the capsules. Their average size varies from 4.6 mm (type III) to 6.8 mm (type I) and increases linearly with the number of coatings. Furthermore, the strength of capsules was highly affected by the temperature. At 20 °C, the average compressive strength of
5%
(b)
4%
1.E+04
εiv
Strain (%)
Load (N)
2.E+04
5.E+03
6.E-03 P
2.E+04
Pi
ð1Þ
where Pi is the load applied in cycle i (N), r is the average radius of the specimen cross-section (m), and evi is the vertical strain in cycle i (m/m). In every loading cycle there is a small fraction of the deformation that is not recovered. The evolution of the permanent deformation (d) with the loading cycles (n) is illustrated in Fig. 3(b). The slope of the accumulated permanent deformation (DS) is determined from fitting a linear model to the region with nearly constant increase rate.
Permanent Deformation (m)
Asphalt specimens were tested under cyclic uniaxial compression without lateral confinement, in load-control mode using an universal testing machine (MAND 100kN). The specimens’ vertical deformation was determined from the cylinder cross-head displacement. Testing was performed at 20 °C. The testing protocol had three stages: initial loading, resting and re-loading. During initial loading a significant level of crack damage was induced in the test specimens, and the effect of the resting period was evaluated from the stiffness of the test specimens during the re-loading period. The resting period was introduced to give time to the oil to diffuse and change the properties of the bitumen in asphalt mixture. To assess the time for the oil to diffuse, different resting periods were used: 3 h, 6 h, 12 h, 24 h and 48 h. This procedure was performed with all asphalt specimens, with and without capsules. The maximum loading level during cyclic loading for asphalt containing capsules type I and type II was 20 kN. This value was chosen because it corresponds to 28% of the peak load obtained from uniaxial compression of specimens without capsules, measured at the constant deformation rate of 5 mm/min. In case of test specimens containing capsules type III, 20 kN caused that the test specimens break during initial loading. For this reason, the loading level was reduced to 14 kN. Moreover, the loading frequency was 1 Hz with 0.5 s of rest between loading cycles. The variation of the load and strain during one loading cycle is illustrated in Fig. 3(a). The vertical stiffness K (Pa) was determined in every cycle from the loading path as:
Pi
p r2 evi
Top plate Specimen
4.E-03
Base plate
2.E-03 DS Δn
3%
0.E+00 0
2
Time (s)
4
Δd
0.E+00 0
2000
Cycles
Fig. 3. Mechanical test results: (a) load and strain variation; (b) vertical deformation accumulation.
4000
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Calcium-alginate structure
Shell Core
(a)
(b)
(a) 8
(b)
60
Compressive strength (N)
Fig. 4. Images of the inner structure of capsules type I. (a) Capsule cross-section; (b) calcium-alginate structure in core.
50
test 20ºC
Diameter (mm)
7
6
5
4
test 130ºC
40 30 20 10 0
I
II
III
I
Capsule type
II
III
Capsule type
Fig. 5. (a) Size and (b) strength of capsules.
capsules with three shell coatings (type I) was approximately twice the strength of other types of capsules. Besides, capsules type II and III had very similar strength values despite the epoxycement shell of type II. On the other hand, at 130 °C, the average strength of capsules was 12.0 N, 8.4 N and 6.1 N for capsules type I, II and III, respectively. Moreover, capsules can survive the high temperatures used in hot-mix asphalt production. The TGA test results obtained for the capsules are depicted in Fig. 6(a), and the materials used in their fabrication in Fig. 6(b). Moreover, the capsules suffered only minor mass loss (64%) in the temperature range used for asphalt production (<200 °C), which was caused by degradation of calciumalginate and evaporation of water from the capsules. The composition of capsules can be determined from the TGA test results analysis and from the volumetric proportion of the core and shell, determined from the SEM images. A four-equation system with the unknowns being the weight proportions of the four components can be built as follows:
8 > > > > > < > > > > > :
Mo þ Mp þ Mc þ Me ¼ 1 qcap Mqoo þ Mqpp ¼ Voþp qcap Mq c þ Mq e ¼ Vcþe c
ð2Þ
e
Mrc Mc þ Mrp Mp þ Mre Me ¼ Mrcap
where Mo, Mp, Mc and Me are the mass proportion of oil, calciumalginate, cement and epoxy resin, respectively; Mrcap, Mrc, Mrp and Mre, are the remaining mass of capsule, cement, calcium-alginate
and epoxy resin, respectively, after heating to 1000 °C; qcap is the density of the capsules; qo is the density of the oil; qp is the density of calcium-alginate; qc is the density of the cement; qe is the density of the epoxy resin; Vo+p is the volume proportion of the core in capsules and Vc+e is the volume proportion of the epoxycement shell in capsules. Moreover, Table 2 presents the composition of the capsules, in terms of mass proportion. The oil content in capsules type III doubled that of capsules type I and type II. This is the reason why the amount of capsules type III in asphalt was halved. 3.2. Incorporation of capsules into asphalt mixture If the capsules would release the rejuvenators during mixing and compaction, bitumen would become softer and permanent deformation would be likely to occur. Thus, the capsules should not affect the ability to achieve a compact aggregate interlocking during compaction. Fig. 7 shows images of asphalt specimens with capsules type II (a) and III (b). A visual examination of the specimens showed that capsules resisted mixing and compaction. Capsules were not completely coated with bitumen because they were added at the end of the mixing process (see Fig. 7(a)). This situation was even more aggressive for the capsules than adding them at the beginning of the mixing process, because the bitumen coating could help particles sliding and re-arranging during compaction. For this reason, most of the capsules type I and II presented the epoxy-cement shell cracked. To evaluate the core condition when the shell was broken,
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(a) 100%
(b) 100% Capsule I Capsule II
80%
Capsule III
Remaining mass
Remaining mass
80% 60% 40% 20%
Oil Polymer Cement Epoxy
60% 40% 20%
0%
0% 0
200
400
600
800
1000
0
Temperature (ºC)
200
400
600
800
1000
Temperature (ºC)
Fig. 6. TGA test results: a) capsules; b) basic components of capsules.
Table 2 Mass composition of capsules. Type
Oil (%)
Polymer (%)
Epoxy (%)
Cement (%)
I II III
30 37 75
6 12 25
12 15 0
52 36 0
the shell of some capsules at the surface of the test specimens was removed by hand. It was observed that the core was intact. In addition, capsules type III, without epoxy-cement shell, resisted also mixing and compaction (see Fig. 7(b)). Moreover, the condition of the capsules inside asphalt test specimens was evaluated using CT Scans. Fig. 8 shows cross-sections of the two specimens in Fig. 7. It can be observed that (1) all capsules
were deformed, (2) voids appeared between the cracked epoxycement shells and the capsules (see Fig. 8(a)), and (3) capsules type III, in Fig. 8(b), had adapted their shape to the aggregates around them. As referred previously, at high temperatures the polymer structure is soft and allows large deformations with small oil release. Moreover, it can be concluded that a compressive strength of 6 N at high temperature is sufficient for these capsules to survive mixing and compaction. Furthermore, the evolution of the test specimens’ height during compaction is shown in Fig. 9(a), where it can be observed that the final height was similar for the three types of materials tested, but that mixtures with capsules type II and type III were easier to compact. The reason for this is that some of the weaker capsules may have broken during compaction. In addition, Fig. 9(b) shows the air voids content of compacted specimens. The air voids content
Capsules with cracked shell
Intact capsules
Capsules
(a)
(b) Fig. 7. Images of asphalt specimens with capsules type (a) II and (b) III.
Voids Coarse aggregate particles
Shell
Capsule Polymer core
(a)
2 mm
(b)
Fig. 8. CT images of asphalt specimens with capsules type (a) II and (b) III.
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R. Micaelo et al. / Construction and Building Materials 123 (2016) 734–744
(a)
65
Unaged (No cap) Cap I Cap II Cap III
Height (mm)
60
55
(b)
10
8
Air voids (%)
740
6
4
50 2 45 0
50
100
150
200
0
250
Unaged Aged (no cap) (no Cap)
Number of gyrations
Cap I
Cap II
Cap III
Fig. 9. (a) Compaction evolution with number of gyrations. (b) Air voids.
3.3. Stiffness and deformation evolution of asphalt mixture
10
Aged (No cap) Cap III
Air voids (%)
8
6
4
2
0 10 (Top)
20
30
Vertical distance (mm)
40 (Down)
Fig. 10. Variation of air voids with height.
of specimens with capsules type I is approximately 3% higher than that of specimens with capsules types II and III. Finally, Fig. 10 compares the variation of air voids with height, obtained from CT Scans, for test samples of asphalt mixture without capsules and containing capsules type III, without epoxy-cement shell. It can be observed that the compaction level and voids content are similar in the two specimens. However, the specimen with capsules showed very low air voids content at the bottom.
(a) 5.E+08
Fig. 11(a) shows the stiffness evolution of a selected test sample without and with capsules type III. The stiffness of test samples without capsules increased constantly, which is a sign that the packing level of aggregates increased during cycling loading. On the other hand, the stiffness of the test samples with capsules decreased after the first 500 cycles, which shows that capsules broke during testing. After the resting period, the stiffness of the test samples without capsules remained constant, while the stiffness of test samples containing capsules increased, which may be due to self-healing of cracks in the test specimens. The vertical accumulated deformation evolution of asphalt containing capsules type III is shown in Fig. 11(b). During the first cycles there was approximately 1–2 mm of imposed deformation. Then, the test specimens with and without capsules had different deformation trends. The specimens containing capsules showed higher deformation rate than the test specimens without capsules. The reason for this is that capsules broke during cycling loading and the oil was released in the asphalt mixture. Moreover, the test specimens without capsules showed a similar deformation rate before and after the rest period. However, the deformation rate of test specimens containing capsules was higher after the rest period, which shows that the oil in the capsules reduced the viscosity of asphalt mixture. Moreover, Fig. 12 shows the relationship between the average stiffness before (K1st) and after (K2nd) the resting period. Fig. 12(a) shows the relationship for aged and un-aged test samples, without
(b) 6.E-03 Resting period (48h)
Resting period (48h) 5.E-03
4.E+08
d (m)
K (Pa)
4.E-03 3.E+08 3.E-03
2.E+08 2.E-03 1.E+08
Aged (No cap) Cap III 0.E+00
1.E-03
Aged (No cap) Cap III
0.E+00 0
2000
4000
6000
Number of Cycles
8000
0
2000
4000
6000
8000
Number of Cycles
Fig. 11. Variation of (a) stiffness (K) and (b) permanent deformation (d) with loading cycles in asphalt mixture with and without capsules.
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(a)
(b) 1.2
1.2
Unaged (No cap)
Cap I
Aged (No cap)
Cap II Cap III
1.1
K1st/K2nd
K1st/K2nd
1.1
1.0
1.0
0.9
0.9 3
6
12
24
48
3
6
Rest time (h)
12
24
48
Rest time (h)
Fig. 12. Variation of stiffness with rest time for test specimens (a) without capsules and (b) with capsules.
capsules, while Fig. 12(b) shows the relationship for aged test samples containing capsules type I, II and III. It can be observed that the stiffness ratio did not change with the resting time for test specimens without capsules and it was higher for aged test specimens. The reason for this is that 48 h were not enough for bitumen to flow into and repair cracks in asphalt mixture. On the other hand, the stiffness ratio of asphalt mixture containing capsules type III increased approximately 15% after 48 h, which is a clear sign that the oil was released and cracks were repaired. Moreover, the stiffness increased progressively with the resting period. Note that the stiffness of asphalt mixture containing capsules type I, the strongest, did not change. This shows that these capsules were too strong and did not break and release their content. Moreover, changes in test specimens containing capsules type III, without cement-epoxy shell, are higher than in test specimens containing capsules type II, with two layers of cement-epoxy around the calcium-alginate core. This shows that more oil was released from capsules type III and that these were more able to change the properties of asphalt mixture. Besides, Fig. 13 shows the relationship between the average deformation of test samples before (DS1st) and after (DS2nd) the resting period. Fig. 13(a) shows the relationship for aged and unaged test samples, without capsules, while Fig. 13(b) shows the relationship for aged test samples containing capsules type I, II and III. It can be observed that the deformation levels of test specimens without capsules did not change with the resting period, while in general they increased with the resting period for test specimens containing capsules type II and III. The reason for this
(a)
could be that the viscosity of bitumen in asphalt mixture containing capsules type II and III was modified by the oil coming out and diffusing from the capsules. 3.4. Assessment of asphalt healing To assess the amount of oil released from the capsules before and after loading, mastic samples were obtained from the test specimens and tested using FTIR spectrometry. For comparison purposes, oil, aged bitumen extracted from asphalt test samples and aged bitumen mixed with 5% and 10% of oil by mass were also tested using FTIR. These amounts were chosen because they are representative of the amount of oil contained in the capsules. Fig. 14(a) shows the normalized absorbance curves of the oil and binders for the wavenumbers ranged from 1700 to 1800 cm1. This wavenumber range was selected because bitumen does not show a peak whereas the oil shows a clear one at 1741 cm1. Moreover, the binders modified with oil show a peak at 1746 cm1, with the absorbance value being proportional to the oil content. Furthermore, Fig. 14(b) compares the area under the absorbance curve for the different materials studied. Mastic was obtained from asphalt test specimens containing capsules that (1) had been loaded and let to rest for 48 h and (2) had not been loaded. It was obtained that bitumen from test samples that had not been loaded contained oil, because some capsules broke during compaction. Moreover, the peak of capsules type II and III was higher than the peak of capsules type I, which showed that less capsules type II and III resisted mixing and compaction. In addition,
(b)
6
Unaged (No cap)
6
Cap I Cap II Cap III
4
DS2nd/DS1st
DS2ns/DS1st
Aged (No cap)
2
0
4
2
0 3
6
12
Rest time (h)
24
48
3
6
12
24
48
Rest time (h)
Fig. 13. Variation of deformation slope with rest time for test specimens (a) without capsules and (b) with capsules.
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(b) 10
0.1
Oil bit A bit A + oil(5%) bit A + oil(10%)
1 Loading, rest 48h
Area
Normalized absorbance
(a)
0.05
No loading
0.1
0.01
0 1700
1750
bit A
1800
Oil
Wavenumber (cm-1)
bit A + bit A + Cap I oil(5%) oil(10%)
Cap II Cap III
Fig. 14. (a) FTIR testing results. (b) Area of the normalized absorbance curve in the wavenumber range 1700–1800 cm1.
dV ¼
Vx Vd Vd
ð3Þ
where Vd is the percentage of air voids, including cracks, in the test specimens cross section after loading and, Vx is the percentage of air voids, including cracks, in the test specimens cross section after x days resting. A negative value for dV means that there was a decrease of the air voids during the resting period and can be interpreted as cracks closing. As it can be observed, there was a global reduction of air voids along the test specimen, which occurs mainly during the first 4 days. The main reductions in air voids were located 50
10
Variation in air voids (dV)
Rest 4 days Rest 16 days
Rest 9 days Capsules
25
5
0
0
-5
-25
-50
10
(Top)
15
20
25
30
35
Vertical distance (mm)
Fig. 15. Percentage of capsules and variation in air voids vs. vertical distance.
40
-10
(Bottom)
Percentage capsules (%)
approximately 3 mm to the bottom of the section containing capsules. This may be due to the oil and bitumen flowing down by effect of gravity. To confirm that changes in the air voids content were due to the reduction of crack size Figs. 16 and 17 present selected sections of the computed tomography scans of asphalt mixture, after mechanical testing, and after 4 days resting at 20 °C. Fig. 16 shows cracks without surrounding capsules while Fig. 17 shows cracks near capsules. It can be observed that cracks partially healed in the areas around the capsules, while they did not heal in areas without capsules. Authors consider that the reason for crack closure was that the oil coming out of the capsules reduced the viscosity of bitumen, which favoured the drain of bitumen into the cracks. Finally, a 3-Dimensional representation of a single crack before and after 4 days resting at 20 °C was reconstructed from CT-Scan images, see Fig. 18. This crack had a total length of about 20 mm and a maximum width of 500 lm. Moreover, at each end of the crack there were capsules (coloured yellow). It can be observed that before resting the crack was continuous between the capsules, while after the rest period the crack was partially filled around the capsules. Moreover, a closer look at the planar CT images shows that the crack width was reduced in approximately 60 lm in its widest point. These results prove that capsules are able to induce crack healing in asphalt. Note that these cracks were exceptionally wide (approximately 600 lm) and that in a real road situation capsules will start healing cracks at an early state (shorter and narrower), with what the healing efficiency is expected to be much higher [7]. For this reason,
the absorbance curve of all the test samples containing capsules shows higher area after the loading and resting periods and capsules type III released substantially more oil than the other capsules. In addition, Fig. 15 shows the percentage of capsules along the vertical axis of a selected test specimen of 40 40 50 mm3 containing capsules type III. This test specimen was damaged under cyclic loading and cracks with average width 600 lm were created. It can be observed that the concentration of capsules was higher at the bottom of the test specimen. This may be due to capsules being mixed in the last place and not uniformly distributed in the mixture. Moreover, it was determined the change in air voids (dV) during the resting period. The CT-Scanned asphalt specimens were divided in a stack of cross sectional images and the relationship between the air voids before and after the resting period was calculated as:
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(a) Immediately after loading
(b) After 4 days resting 4mm
4mm
4mm
4mm
Fig. 16. Examples of asphalt mixture without capsules, (a) after mechanical testing, (b) after 4 days rest at 20 °C.
(a) Immediately after loading
(b) After 4 days resting 4mm
4mm
3mm
3mm
Fig. 17. Examples of asphalt mixture with capsules, (a) after mechanical testing, (b) after 4 days rest at 20 °C.
After 4 days resting at 20°C
After cyclic loading
4 mm
Capsule
(a)
4 mm
Crack
Capsule
Capsule
Crack
Capsule
(b)
Fig. 18. A crack healing due to the effect of capsules (a) after cyclic loading, (b) after 4 days resting at 20 °C.
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R. Micaelo et al. / Construction and Building Materials 123 (2016) 734–744
future research should focus on the maximum width of crack that can be healed at the environment temperature by the action of capsules. 4. Conclusions The study presented in this paper investigates a new type of capsules for asphalt self-healing purposes. Sunflower oil was encapsulated in calcium-alginate, and protected with a hard shell made of epoxy-cement composite. After, capsules were added into asphalt mixture and the healing properties tested. The main findings can be summarized as follows: Calcium-alginate capsules with a maximum payload of 75% were produced. Later, capsules were coated with an epoxycement composite shell. The calcium-alginate core and the epoxy-cement shell were not bonded. The epoxy-cement shell was not necessary to resist asphalt mixing and compaction, in fact the epoxy-cement shell cracked in every occasion. Moreover, capsules without epoxy-cement adapted their shape to the surrounding aggregates. All the types of capsules studied released a small amount of oil during asphalt mixture fabrication. This did not affect the compaction process, but affected the permanent deformation resistance of asphalt mixture. In the future capsules have to be redesigned to avoid oil losses during mixing and compaction. Capsules broke and released their content during cyclic loading. Capsules without shell released the largest amount of oil during cyclic loading. Capsules with epoxy-cement shell released a smaller amount of oil. Oil diffused in the bitumen at 20 °C and modified its chemical properties. Asphalt specimens containing capsules showed significant increases of stiffness after damaging them under cyclic loading and letting them rest. On the contrary, the deformation resistance reduced in specimens containing capsules. The stiffness and deformation resistance of asphalt mixture after the resting period depended on the type of capsule analysed. For example asphalt mixture containing the strongest capsules, with epoxy-cement shell, had the same stiffness after the resting period. On the other hand, the stiffness of asphalt mixture containing capsules without epoxy-cement shell increased approximately 15% after resting for 48 h. Imaging analysis of cracked asphalt mixture showed large reductions of the area of voids and cracks near the capsules location: oil and/or modified bitumen flowed into and filled the cracks to the bottom of the capsules. In some places, 50% of the air voids disappeared after the resting period. These results are very promising for the development of asphalt mixtures with self-healing ability. However further research is necessary before this technology be applied in roads. Following, it is intended to investigate the increase of strength with crack closure and how this mechanism is affected by capsules and cracks characteristics. Acknowledgments The authors would like to acknowledge the financial support of the FCT – Fundação para a Ciência e a Tecnologia, IP, through grant SFRH/BSAB/113646/2015 financed by the Portuguese Government budget, the Higher Committee of Education Development in Iraq for the PhD scholarship of the second author, and the EPSRC project EP/M014134/1, Induction heating for closing cracks in asphalt concrete.
References [1] S. Burningham, N. Stankevich, Why road maintenance is important and how to get it done, Transp. Notes (2005). no. TRN-4. [2] S. Wu, L. Pang, L. Mo, J. Qiu, G. Zhu, Y. Xiao, UV and thermal aging of pure bitumen-comparison between laboratory simulation and natural exposure aging, Road Mater. Pavement Des. 9 (sup1) (Jan. 2008) 103–113. [3] J. Read, D. Whiteoak, The Shell Bitumen Handbook, 5th ed., Thomas Telford Publishing, London, 2003. [4] G.D. Airey, State of the art report on ageing test methods for bituminous pavement materials, Int. J. Pavement Eng. 4 (3) (Sep. 2003) 165–176. [5] U. Isacsson, H. Zeng, Cracking of asphalt at low temperature as related to bitumen rheology, J. Mater. Sci. 33 (8) (Apr. 1998) 2165–2170. [6] J. Qiu, Self-Healing of Asphalt Mixes: Literature Review, Delft University of Technology, Delft, 2008. Report no 7-08-183-1. [7] Á. García, Self-healing of open cracks in asphalt mastic, Fuel 93 (Mar. 2012) 264–272. [8] R. Bhairampally, R. Lytton, D. Little, Numerical and graphical method to assess permanent deformation potential for repeated compressive loading of asphalt mixtures, Transp. Res. Rec. J. Transp. Res. Board 1723 (Jan. 2000) 150–158. [9] J. Qiu, Self-healing of asphalt mixtures, towards a better understanding PhD thesis, Delft University of Technology, Delft, 2012. [10] A.K. Apeagyei, J.R.A. Grenfell, G.D. Airey, Observation of reversible moisture damage in asphalt mixtures, Constr. Build. Mater. 60 (Jun. 2014) 73–80. [11] A.T. Pauli, Chemomechanics of damage accumulation and damage-recovery healing in bituminous asphalt binders PhD thesis, Delft University of Technology, Delft, 2014. [12] EAPA, Asphalt in Figures 2014, European Asphalt Pavement Association, 2014. [13] Z. Sun, J. Yi, Y. Huang, D. Feng, C. Guo, Properties of asphalt binder modified by bio-oil derived from waste cooking oil, Constr. Build. Mater 102 (Part 1) (2016) 496–504. [14] S.-H. Yang, L.-C. Lee, Characterizing the chemical and rheological properties of severely aged reclaimed asphalt pavement materials with high recycling rate, Constr. Build. Mater. 111 (May 2016) 139–146. [15] Á. García, E. Schlangen, M. van de Ven, G. van Bochove, Optimization of composition and mixing process of a self-healing porous asphalt, Constr. Build. Mater. 30 (May 2012) 59–65. [16] Swapan Kumar Ghosh, Self-Healing Materials: Fundamentals, Design Strategies, and Applications, WILEY-VCH, Weinheim, 2008. [17] Á. García, E. Schlangen, M. van de Ven, G. Sierra-Beltrán, Preparation of capsules containing rejuvenators for their use in asphalt concrete, J. Hazard. Mater. 184 (1–3) (Dec. 2010) 603–611. [18] R. Li, T. Zhou, J. Pei, Design, preparation and properties of microcapsules containing rejuvenator for asphalt, Constr. Build. Mater. 99 (Nov. 2015) 143– 149. [19] J.-F. Su, E. Schlangen, Synthesis and physicochemical properties of high compact microcapsules containing rejuvenator applied in asphalt, Chem. Eng. J. 198–199 (Aug. 2012) 289–300. [20] L. Lv, Z. Yang, G. Chen, G. Zhu, N. Han, E. Schlangen, F. Xing, Synthesis and characterization of a new polymeric microcapsule and feasibility investigation in self-healing cementitious materials, Constr. Build. Mater. 105 (Feb. 2016) 487–495. [21] A. Garcia, J. Jelfs, C.J. Austin, Internal asphalt mixture rejuvenation using capsules, Constr. Build. Mater. 101 (1) (2015) 309–316. [22] J.-F. Su, E. Schlangen, Y.-Y. Wang, Investigation the self-healing mechanism of aged bitumen using microcapsules containing rejuvenator, Constr. Build. Mater. 85 (Jun. 2015) 49–56. [23] J.-F. Su, Y.-Y. Wang, N.-X. Han, P. Yang, S. Han, Experimental investigation and mechanism analysis of novel multi-self-healing behaviors of bitumen using microcapsules containing rejuvenator, Constr. Build. Mater. 106 (Mar. 2016) 317–329. [24] J.-F. Su, J. Qiu, E. Schlangen, Stability investigation of self-healing microcapsules containing rejuvenator for bitumen, Polym. Degrad. Stab. 98 (6) (Jun. 2013) 1205–1215. [25] S.D. Mookhoek, H.R. Fischer, S. van der Zwaag, Alginate fibres containing discrete liquid filled vacuoles for controlled delivery of healing agents in fibre reinforced composites, Compos. Part Appl. Sci. Manuf. 43 (12) (Dec. 2012) 2176–2182. [26] M. Zargar, E. Ahmadinia, H. Asli, M.R. Karim, Investigation of the possibility of using waste cooking oil as a rejuvenating agent for aged bitumen, J. Hazard. Mater. 233–234 (Sep. 2012) 254–258. [27] H. Asli, E. Ahmadinia, M. Zargar, M.R. Karim, Investigation on physical properties of waste cooking oil – Rejuvenated bitumen binder, Constr. Build. Mater. 37 (Dec. 2012) 398–405. [28] J.-F. Su, J. Qiu, E. Schlangen, Y.-Y. Wang, Investigation the possibility of a new approach of using microcapsules containing waste cooking oil: In situ rejuvenation for aged bitumen, Constr. Build. Mater. 74 (Jan. 2015) 83–92. [29] C. Peniche, I. Howland, O. Carrillo, C. Zaldıìvar, W. Argüelles-Monal, Formation and stability of shark liver oil loaded chitosan/calcium alginate capsules, Food Hydrocoll. 18 (5) (2004) 865–871. [30] P. Liang, H. Wang, C. Chen, F. Ge, D. Liu, S. Li, B. Han, X. Xiong, S. Zhao, The use of fourier transform infrared spectroscopy for quantification of adulteration in virgin walnut oil, J. Spectrosc. 2013 (Dec. 2012) e305604.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Study of the mechanical properties and self-healing ability of asphalt mixture containing calcium-alginate capsules R. Micaelo a,b,c,⇑, T. Al-Mansoori c, A. Garcia c a
DEC, FCT, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal CERIS, CESUR, Universidade de Lisboa, 1049-001 Lisboa, Portugal c Nottingham Transportation Engineering Centre, School of Civil Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK b
h i g h l i g h t s Calcium-alginate capsules with a
maximum payload of 75% were produced. Capsules without hard shell resist mixing and compaction. Capsules deformed and broke with loading. Oil diffused in the bitumen in less than 24 h. Large reduction of cracks around the capsules after resting 4 days at 20 °C.
g r a p h i c a l a b s t r a c t Cracked asphalt pavement
a Shell
3 mm Core
Capsule Acc.V Spot Magn Det WD 10.0 kV 4.0 66x BSE 9.6
Capsules
i n f o
Article history: Received 23 April 2016 Received in revised form 4 July 2016 Accepted 17 July 2016 Available online 21 July 2016 Keywords: Self-healing Rejuvenators encapsulation Mechanical testing
Crack
Aggregate
Crack healing around the capsules
2 mm
a b s t r a c t The natural self-healing ability of asphalt mixtures can be enhanced with encapsulated rejuvenators: when crack damage appears the capsules release healing agents, which dissolve bitumen and drain into the cracks. In this study, the effect of a new type of capsules in the mechanical properties and the selfhealing ability of asphalt mixtures is investigated. Sunflower oil was encapsulated in calcium-alginate, and protected with a hard shell made of epoxy-cement composite. Results show that the hard shell was not required for these capsules to resist mixing and compaction procedures. Capsules deformed and broke with loading, releasing oil that diffused in the bitumen in less than 24 h. Healing of cracks in asphalt mixture led to an increase of stiffness. However, asphalt specimens with capsules had lower deformation resistance. Computer tomography scanning of specimens showed large reductions in cracks around the capsules, after resting 4 days at 20 °C. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Asphalt mixtures are the most worldwide used material to build roads. Although pavement structures with asphalt layers ⇑ Corresponding author at: DEC, FCT, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal. E-mail addresses: [email protected] (R. Micaelo), [email protected] (T. Al-Mansoori), [email protected] (A. Garcia). http://dx.doi.org/10.1016/j.conbuildmat.2016.07.095 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
1 mm
Broken capsule
Healing agent coming out of the capsule
a r t i c l e
Capsules heal the cracks
Asphalt contains capsules
are designed for a 10–30 years of service life, replacing the top layer with new asphalt materials is required about every eight years [1]. Damage in asphalt mixture starts at the microstructure level and evolves continuously up to the macro-level that is seen by road drivers and causes disturbances in comfort and safety of driving. Ultraviolet radiation, temperature and moisture contribute altogether to the binder’s ageing [2,3], which becomes stiffer with time, and loses flexibility and adhesion. Hence, the binder is not able to support temperature and/or
R. Micaelo et al. / Construction and Building Materials 123 (2016) 734–744
traffic induced stresses anymore, and microcracks occur in the material [4,5]. However, bitumen is a self-healing material [6]. When a crack occurs in an asphalt road, bitumen tends to drain into the crack and, if adequate time and temperature conditions are given, the crack may be fully closed. This process takes a very long time at normal temperatures, making it extremely ineffective [7]. In addition, asphalt self-healing is highly influenced by the viscosity and surface energy of bitumen [8–11]. In most countries a large portion of the asphalt materials reclaimed from old pavements are recycled and incorporated into new pavement layers [12]. Moreover, virgin binder, additives and aggregates are added to recycled mixtures in order to achieve minimum quality for using recycled asphalt materials in roads. Additives are softening agents or rejuvenators that modify the chemical composition of the old binder, rebalancing the proportion of light and heavy elements, and also lowering the viscosity [13,14]. Ideally, asphalt layers should be rehabilitated in situ without milling or scarifying interventions. One technique that has been proven in the laboratory is to heat asphalt mixture using induction energy to promote the flow of bitumen into the cracks [15]. A different approach to accelerate self-healing is the incorporation of capsules containing rejuvenators which break and release their content when damage occurs in their vicinity [16]. Then, the rejuvenators diffuse into bitumen reducing its viscosity and it can easily drain into the cracks [17]. Up to now, two different methods for the fabrication of capsules for asphalt self-healing have been proposed: (1) saturating porous aggregates with rejuvenators and sealing them with a shell made of epoxy resin and cement [17]; (2) microencapsulation of the rejuvenator by in situ polymerization of urea-formaldehyde [18], methanol-melamine-formaldehyde [19] or phenol–formaldehyde [20]. The first type of capsules resist mixing and compaction operations [17] and release the rejuvenator in asphalt specimens due to loading [21]. In addition, microcapsules release their content in presence of cracks [22,23] but have not been yet tested in asphalt mixture. Moreover, crack healing induced by capsules has been shown in bitumen [24], but never before in asphalt mixture. This paper describes an experimental study aimed at investigating the ability of a new type of capsules containing rejuvenators to induce self-healing in asphalt. The rejuvenator, sunflower oil, was encapsulated in a polymeric structure created by the ionotropic gelation of sodium alginate in the presence of calcium ions. Moreover, certain batches of capsules were additionally coated with a hard shell made of epoxy resin and cement. The research analysed the effect of (1) capsules on the mechanical behaviour of asphalt specimens subjected to cyclic loading and (2) introducing rest periods on the self-healing properties of asphalt mixture containing capsules. Finally, asphalt self-healing was visualised using computer tomography scans.
2. Materials and experiments 2.1. Materials To fabricate the capsules the following materials were used: (1) sodium alginate (C6H7O6Na) (Sigma-Aldrich), which is an anionic polysaccharide widely distributed in the cell walls of brown algae; (2) calcium chloride (CaCl2), provided by Sigma-Aldrich as anhydrous, granular pellets of 7 mm diameter and 93% purity; (3) epoxy resin (AralditeÒ 506 epoxy resin, Sigma-Aldrich), obtained by combining the epoxy resin with a fast hardener; (4) micro-cement (MICROCEM 550Ò, Tarmac), with average particle size ranged from 5 to 10 lm, provided by; (5) sunflower oil (East End, UK).
735
Limestone aggregates (Tunstead quarry, Derbyshire, UK) and paving grade bitumen 40/60 were used to fabricate asphalt mixture. 2.2. Capsules The core of the capsules was composed by a polymeric structure made of calcium-alginate that encapsulates the rejuvenator. Moreover, in some batches the core was coated with a shell made of epoxy-cement. The polymeric core results from the reaction of alginate with calcium ions, creating a porous structure, often named as ‘‘egg-box” [25]. The fluid (rejuvenator) is entrapped in the structure during the reaction. The rejuvenator was a commercial sunflower oil, selected because it does not require special health and safety measures at the lab and is thermally stable. Vegetable oils can be used as bitumen rejuvenators [26,27], and they have also been encapsulated with self-healing purposes before [21,28]. The fabrication procedure is illustrated in Fig. 1. The left-hand side flowchart shows the procedure to fabricate the polymeric capsules, defined in [29], and the right-hand side flowchart shows the procedure to fabricate the hard-shell, defined from [17]. All tasks were performed at room temperature. First, 600 ml of deionized water and oil were introduced in 1 L glass container. Two types of emulsions were built with oil/water (o/w) ratio 1.0 (300 g of oil and 300 g of water) and 0.5 (200 g of oil and 400 g of water), respectively. Oil and water were homogenised using a laboratory gear drive mixer at 400 rpm during 1 min. Then, 15 g of sodium alginate were added and stirred until complete solution at 400 rpm, for 10 min. The alginate acted as an emulsifier, stabilizing the emulsion. Simultaneously, a calcium chloride solution was prepared by mixing 600 ml of water with 12 g of calcium chloride in 1 L glass water container. Capsules were formed by letting the oil-in-water emulsion drop into the calcium chloride emulsion from a 1000 ml pressure-equalizing dropping funnel with 3 mm socket size. During the capsule formation process, the calcium solution was gently agitated using a magnetic stirrer. Capsules were allowed to stay in the solution until the end of the encapsulation process, which lasted for approximately 4 h. Finally, capsules were decanted and washed with deionized water. They were dried during 12 h under the constant movement of air produced by a fan. Then, selected batches of dry capsules were covered with an outer shell made of epoxy-cement to increase their strength. The process of making the shell started by covering the calciumalginate capsules with epoxy in a mass ratio of 14% (14 g of epoxy for each 100 g of capsules). Later, seven steel balls of 20 mm diameter were placed in a plastic container, together with approximately 1 kg of cement. After, 250 g of capsules and epoxy were added, the container was closed, and shaken vigorously by hand for 15 s. After this, capsules, cement, and steel balls were separated in a 2 mm sieve, and let to rest for 12 h. The epoxy-cement coating process could be repeated various times for each batch of capsules to improve their strength. Three types of capsules were used in this study: (I) o/w 1.0, with three epoxy-cement coatings; (II) o/w 0.5, with two epoxy-cement coatings; (III) o/w 0.5, with no epoxy-cement coatings. Fig. 2 displays these capsules, which were selected because of the different morphology, size and strength characteristics. 2.3. Asphalt mixture Asphalt mixture for base courses, AC 20 base 40/60 (EN 131081), was used. Table 1 describes the composition of the mixture. Furthermore, asphalt mixture was pre-fabricated and aged in the laboratory before adding the capsules to simulate the natural
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(a) Encapsulation
(b) External shell
Mix oil and water, 400 rpm, 1 min
Polymeric capsules without shell
Sodium alginate (15g)
Epoxy resin
Mix, 400 rpm, 10 min
Hand-mix capsules with resin Cement Repeat
Fill dropper CaCl solution (2%)
Agitate by hand, 15 s
Emulsion drops in CaCl solution
Sieve, #2 mm
Decant capsules
Dry, 24h, room temp
Wash w/ deionized water
Capsules with shell
Dry, 24 h, air ventilation, room temp
Polymeric capsules without shell
Fig. 1. Flowchart of the capsules’ fabrication process: (a) polymeric (calcium-alginate) capsules; (b) hard shell.
Type I
Type II
Type III
10 mm
Fig. 2. Images of capsules. Table 1 Asphalt mixture design properties. AC 20 base 40/60 Aggregate gradation (% passing) 20 mm 10 mm 2 mm 0.063 mm Binder content (%M) Bulk density (kg/m3) Air voids (%) Voids in mineral aggregate (%) Voids filled with bitumen (%)
100.0 30.3 2.2 1.8 4.5 2384 4.5 14.9 69.8
ageing process of roads. The ageing procedure consisted in storing the mixture for 12 days in an air-ventilated oven at 85 °C. Before re-mixing and compacting, the aged asphalt mixture was heated at 160 °C. The capsules were added to the aged mixture and stirred for a short time (15 s) at the end of the re-mixing process. The mass proportion of capsules type I and II in asphalt was 6%. Moreover, the mass proportion of capsules type III in asphalt was 3%. Less capsules type III were used to compensate the high proportion of oil in the capsules. Finally, cylindrical asphalt specimens with diameter 100 mm and final height of approximately 50 mm
were compacted using a gyratory compactor with a maximum of 250 gyrations. For comparison purposes, asphalt specimens without capsules were fabricated with aged and non-aged asphalt. 2.4. Imaging techniques Three different imaging techniques were used in this research: optical microscopy (OM), scanning electron microscopy (SEM) and X-ray computed tomography (CT) scans. To determine the size of the capsules, these were observed with a stereoscopic microscope and the images taken were processed with the software ImageJ. The inner structure of the capsules was analysed with SEM (Philips XL30FEG SEM), in capsules casted in an epoxy resin matrix and then abraded until the inner structure was exposed. CT was performed on some asphalt specimens with an XRadia Versa XRM-500 scanner operated at 160 kV and 63 lA. The samples were assembled on a rotational table at a distance of 80.6 mm from the X-ray source and of 40.0 mm from the X-ray detector. To do this, small specimens of 40 40 50 mm3 were cut from the cylindrical specimens. These were adequate to obtain a pixel resolution of 45 lm. Later, the different materials that compose asphalt mixture were separated from the grayscale images using intensity thresholding in ImageJ.
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2.5. Capsule compression testing
K¼
The mechanical strength of individual capsules was measured with uniaxial compression testing. Testing was done on an Instron Model 5969 machine, with a loading rate of 0.2 mm/min. Ten capsules of each type studied were tested at 20 °C and at 130 °C, respectively. 2.6. Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) testing was performed to determine the thermal resistance and composition of capsules. Testing was done on a NETZSCH, TG 449 F3 Jupiter ThermoMicrobalance, using nitrogen atmosphere and a heating rate of 10 °C/min from room temperature up to 1000 °C. One capsule was inserted in the aluminium crucible in each test. 2.7. Mechanical testing of asphalt mixture
(a) 3.E+04 Load Strain
2.8. Chemical analysis of bitumen Fourier Transform Infrared spectroscopy (FTIR) was used to assess the modification of bitumen in asphalt mixture when the capsules release their content. Bitumen samples were collected directly from the surface of asphalt mixture test specimens with a hot knife. For comparison purposes, bitumen was recovered from aged asphalt mixture and mixed directly with oil in a 5% and 10% by mass. Furthermore, pure bitumen and oil were also analysed. The FTIR spectrum was obtained in the absorption mode within the wavenumber range of 400–4000 cm1, with a resolution of 4 cm1. The effect of the sunflower oil on the bitumen was evaluated from the changes in the absorption peak at the wavenumber ranged from 1700 to 1800 cm1. This range was selected because vegetable oils have a distinct peak at 1743 cm1 (C-O stretch) [30]. 3. Results and discussion 3.1. Characterization of capsules Fig. 4 shows the capsules’ internal structure, obtained with scanning electron microscopy. The capsules are approximately spherical and the oil is contained in the micropores of the core (see Fig. 4(b)). Furthermore, a gap between the shell and the core is observed in Fig. 4(a). The main characteristics of the capsules: size, strength, composition, are influenced by the o/w ratio and the number of shell coatings. Fig. 5 shows the diameter and compressive strength of the capsules. Their average size varies from 4.6 mm (type III) to 6.8 mm (type I) and increases linearly with the number of coatings. Furthermore, the strength of capsules was highly affected by the temperature. At 20 °C, the average compressive strength of
5%
(b)
4%
1.E+04
εiv
Strain (%)
Load (N)
2.E+04
5.E+03
6.E-03 P
2.E+04
Pi
ð1Þ
where Pi is the load applied in cycle i (N), r is the average radius of the specimen cross-section (m), and evi is the vertical strain in cycle i (m/m). In every loading cycle there is a small fraction of the deformation that is not recovered. The evolution of the permanent deformation (d) with the loading cycles (n) is illustrated in Fig. 3(b). The slope of the accumulated permanent deformation (DS) is determined from fitting a linear model to the region with nearly constant increase rate.
Permanent Deformation (m)
Asphalt specimens were tested under cyclic uniaxial compression without lateral confinement, in load-control mode using an universal testing machine (MAND 100kN). The specimens’ vertical deformation was determined from the cylinder cross-head displacement. Testing was performed at 20 °C. The testing protocol had three stages: initial loading, resting and re-loading. During initial loading a significant level of crack damage was induced in the test specimens, and the effect of the resting period was evaluated from the stiffness of the test specimens during the re-loading period. The resting period was introduced to give time to the oil to diffuse and change the properties of the bitumen in asphalt mixture. To assess the time for the oil to diffuse, different resting periods were used: 3 h, 6 h, 12 h, 24 h and 48 h. This procedure was performed with all asphalt specimens, with and without capsules. The maximum loading level during cyclic loading for asphalt containing capsules type I and type II was 20 kN. This value was chosen because it corresponds to 28% of the peak load obtained from uniaxial compression of specimens without capsules, measured at the constant deformation rate of 5 mm/min. In case of test specimens containing capsules type III, 20 kN caused that the test specimens break during initial loading. For this reason, the loading level was reduced to 14 kN. Moreover, the loading frequency was 1 Hz with 0.5 s of rest between loading cycles. The variation of the load and strain during one loading cycle is illustrated in Fig. 3(a). The vertical stiffness K (Pa) was determined in every cycle from the loading path as:
Pi
p r2 evi
Top plate Specimen
4.E-03
Base plate
2.E-03 DS Δn
3%
0.E+00 0
2
Time (s)
4
Δd
0.E+00 0
2000
Cycles
Fig. 3. Mechanical test results: (a) load and strain variation; (b) vertical deformation accumulation.
4000
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Calcium-alginate structure
Shell Core
(a)
(b)
(a) 8
(b)
60
Compressive strength (N)
Fig. 4. Images of the inner structure of capsules type I. (a) Capsule cross-section; (b) calcium-alginate structure in core.
50
test 20ºC
Diameter (mm)
7
6
5
4
test 130ºC
40 30 20 10 0
I
II
III
I
Capsule type
II
III
Capsule type
Fig. 5. (a) Size and (b) strength of capsules.
capsules with three shell coatings (type I) was approximately twice the strength of other types of capsules. Besides, capsules type II and III had very similar strength values despite the epoxycement shell of type II. On the other hand, at 130 °C, the average strength of capsules was 12.0 N, 8.4 N and 6.1 N for capsules type I, II and III, respectively. Moreover, capsules can survive the high temperatures used in hot-mix asphalt production. The TGA test results obtained for the capsules are depicted in Fig. 6(a), and the materials used in their fabrication in Fig. 6(b). Moreover, the capsules suffered only minor mass loss (64%) in the temperature range used for asphalt production (<200 °C), which was caused by degradation of calciumalginate and evaporation of water from the capsules. The composition of capsules can be determined from the TGA test results analysis and from the volumetric proportion of the core and shell, determined from the SEM images. A four-equation system with the unknowns being the weight proportions of the four components can be built as follows:
8 > > > > > < > > > > > :
Mo þ Mp þ Mc þ Me ¼ 1 qcap Mqoo þ Mqpp ¼ Voþp qcap Mq c þ Mq e ¼ Vcþe c
ð2Þ
e
Mrc Mc þ Mrp Mp þ Mre Me ¼ Mrcap
where Mo, Mp, Mc and Me are the mass proportion of oil, calciumalginate, cement and epoxy resin, respectively; Mrcap, Mrc, Mrp and Mre, are the remaining mass of capsule, cement, calcium-alginate
and epoxy resin, respectively, after heating to 1000 °C; qcap is the density of the capsules; qo is the density of the oil; qp is the density of calcium-alginate; qc is the density of the cement; qe is the density of the epoxy resin; Vo+p is the volume proportion of the core in capsules and Vc+e is the volume proportion of the epoxycement shell in capsules. Moreover, Table 2 presents the composition of the capsules, in terms of mass proportion. The oil content in capsules type III doubled that of capsules type I and type II. This is the reason why the amount of capsules type III in asphalt was halved. 3.2. Incorporation of capsules into asphalt mixture If the capsules would release the rejuvenators during mixing and compaction, bitumen would become softer and permanent deformation would be likely to occur. Thus, the capsules should not affect the ability to achieve a compact aggregate interlocking during compaction. Fig. 7 shows images of asphalt specimens with capsules type II (a) and III (b). A visual examination of the specimens showed that capsules resisted mixing and compaction. Capsules were not completely coated with bitumen because they were added at the end of the mixing process (see Fig. 7(a)). This situation was even more aggressive for the capsules than adding them at the beginning of the mixing process, because the bitumen coating could help particles sliding and re-arranging during compaction. For this reason, most of the capsules type I and II presented the epoxy-cement shell cracked. To evaluate the core condition when the shell was broken,
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(a) 100%
(b) 100% Capsule I Capsule II
80%
Capsule III
Remaining mass
Remaining mass
80% 60% 40% 20%
Oil Polymer Cement Epoxy
60% 40% 20%
0%
0% 0
200
400
600
800
1000
0
Temperature (ºC)
200
400
600
800
1000
Temperature (ºC)
Fig. 6. TGA test results: a) capsules; b) basic components of capsules.
Table 2 Mass composition of capsules. Type
Oil (%)
Polymer (%)
Epoxy (%)
Cement (%)
I II III
30 37 75
6 12 25
12 15 0
52 36 0
the shell of some capsules at the surface of the test specimens was removed by hand. It was observed that the core was intact. In addition, capsules type III, without epoxy-cement shell, resisted also mixing and compaction (see Fig. 7(b)). Moreover, the condition of the capsules inside asphalt test specimens was evaluated using CT Scans. Fig. 8 shows cross-sections of the two specimens in Fig. 7. It can be observed that (1) all capsules
were deformed, (2) voids appeared between the cracked epoxycement shells and the capsules (see Fig. 8(a)), and (3) capsules type III, in Fig. 8(b), had adapted their shape to the aggregates around them. As referred previously, at high temperatures the polymer structure is soft and allows large deformations with small oil release. Moreover, it can be concluded that a compressive strength of 6 N at high temperature is sufficient for these capsules to survive mixing and compaction. Furthermore, the evolution of the test specimens’ height during compaction is shown in Fig. 9(a), where it can be observed that the final height was similar for the three types of materials tested, but that mixtures with capsules type II and type III were easier to compact. The reason for this is that some of the weaker capsules may have broken during compaction. In addition, Fig. 9(b) shows the air voids content of compacted specimens. The air voids content
Capsules with cracked shell
Intact capsules
Capsules
(a)
(b) Fig. 7. Images of asphalt specimens with capsules type (a) II and (b) III.
Voids Coarse aggregate particles
Shell
Capsule Polymer core
(a)
2 mm
(b)
Fig. 8. CT images of asphalt specimens with capsules type (a) II and (b) III.
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R. Micaelo et al. / Construction and Building Materials 123 (2016) 734–744
(a)
65
Unaged (No cap) Cap I Cap II Cap III
Height (mm)
60
55
(b)
10
8
Air voids (%)
740
6
4
50 2 45 0
50
100
150
200
0
250
Unaged Aged (no cap) (no Cap)
Number of gyrations
Cap I
Cap II
Cap III
Fig. 9. (a) Compaction evolution with number of gyrations. (b) Air voids.
3.3. Stiffness and deformation evolution of asphalt mixture
10
Aged (No cap) Cap III
Air voids (%)
8
6
4
2
0 10 (Top)
20
30
Vertical distance (mm)
40 (Down)
Fig. 10. Variation of air voids with height.
of specimens with capsules type I is approximately 3% higher than that of specimens with capsules types II and III. Finally, Fig. 10 compares the variation of air voids with height, obtained from CT Scans, for test samples of asphalt mixture without capsules and containing capsules type III, without epoxy-cement shell. It can be observed that the compaction level and voids content are similar in the two specimens. However, the specimen with capsules showed very low air voids content at the bottom.
(a) 5.E+08
Fig. 11(a) shows the stiffness evolution of a selected test sample without and with capsules type III. The stiffness of test samples without capsules increased constantly, which is a sign that the packing level of aggregates increased during cycling loading. On the other hand, the stiffness of the test samples with capsules decreased after the first 500 cycles, which shows that capsules broke during testing. After the resting period, the stiffness of the test samples without capsules remained constant, while the stiffness of test samples containing capsules increased, which may be due to self-healing of cracks in the test specimens. The vertical accumulated deformation evolution of asphalt containing capsules type III is shown in Fig. 11(b). During the first cycles there was approximately 1–2 mm of imposed deformation. Then, the test specimens with and without capsules had different deformation trends. The specimens containing capsules showed higher deformation rate than the test specimens without capsules. The reason for this is that capsules broke during cycling loading and the oil was released in the asphalt mixture. Moreover, the test specimens without capsules showed a similar deformation rate before and after the rest period. However, the deformation rate of test specimens containing capsules was higher after the rest period, which shows that the oil in the capsules reduced the viscosity of asphalt mixture. Moreover, Fig. 12 shows the relationship between the average stiffness before (K1st) and after (K2nd) the resting period. Fig. 12(a) shows the relationship for aged and un-aged test samples, without
(b) 6.E-03 Resting period (48h)
Resting period (48h) 5.E-03
4.E+08
d (m)
K (Pa)
4.E-03 3.E+08 3.E-03
2.E+08 2.E-03 1.E+08
Aged (No cap) Cap III 0.E+00
1.E-03
Aged (No cap) Cap III
0.E+00 0
2000
4000
6000
Number of Cycles
8000
0
2000
4000
6000
8000
Number of Cycles
Fig. 11. Variation of (a) stiffness (K) and (b) permanent deformation (d) with loading cycles in asphalt mixture with and without capsules.
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(a)
(b) 1.2
1.2
Unaged (No cap)
Cap I
Aged (No cap)
Cap II Cap III
1.1
K1st/K2nd
K1st/K2nd
1.1
1.0
1.0
0.9
0.9 3
6
12
24
48
3
6
Rest time (h)
12
24
48
Rest time (h)
Fig. 12. Variation of stiffness with rest time for test specimens (a) without capsules and (b) with capsules.
capsules, while Fig. 12(b) shows the relationship for aged test samples containing capsules type I, II and III. It can be observed that the stiffness ratio did not change with the resting time for test specimens without capsules and it was higher for aged test specimens. The reason for this is that 48 h were not enough for bitumen to flow into and repair cracks in asphalt mixture. On the other hand, the stiffness ratio of asphalt mixture containing capsules type III increased approximately 15% after 48 h, which is a clear sign that the oil was released and cracks were repaired. Moreover, the stiffness increased progressively with the resting period. Note that the stiffness of asphalt mixture containing capsules type I, the strongest, did not change. This shows that these capsules were too strong and did not break and release their content. Moreover, changes in test specimens containing capsules type III, without cement-epoxy shell, are higher than in test specimens containing capsules type II, with two layers of cement-epoxy around the calcium-alginate core. This shows that more oil was released from capsules type III and that these were more able to change the properties of asphalt mixture. Besides, Fig. 13 shows the relationship between the average deformation of test samples before (DS1st) and after (DS2nd) the resting period. Fig. 13(a) shows the relationship for aged and unaged test samples, without capsules, while Fig. 13(b) shows the relationship for aged test samples containing capsules type I, II and III. It can be observed that the deformation levels of test specimens without capsules did not change with the resting period, while in general they increased with the resting period for test specimens containing capsules type II and III. The reason for this
(a)
could be that the viscosity of bitumen in asphalt mixture containing capsules type II and III was modified by the oil coming out and diffusing from the capsules. 3.4. Assessment of asphalt healing To assess the amount of oil released from the capsules before and after loading, mastic samples were obtained from the test specimens and tested using FTIR spectrometry. For comparison purposes, oil, aged bitumen extracted from asphalt test samples and aged bitumen mixed with 5% and 10% of oil by mass were also tested using FTIR. These amounts were chosen because they are representative of the amount of oil contained in the capsules. Fig. 14(a) shows the normalized absorbance curves of the oil and binders for the wavenumbers ranged from 1700 to 1800 cm1. This wavenumber range was selected because bitumen does not show a peak whereas the oil shows a clear one at 1741 cm1. Moreover, the binders modified with oil show a peak at 1746 cm1, with the absorbance value being proportional to the oil content. Furthermore, Fig. 14(b) compares the area under the absorbance curve for the different materials studied. Mastic was obtained from asphalt test specimens containing capsules that (1) had been loaded and let to rest for 48 h and (2) had not been loaded. It was obtained that bitumen from test samples that had not been loaded contained oil, because some capsules broke during compaction. Moreover, the peak of capsules type II and III was higher than the peak of capsules type I, which showed that less capsules type II and III resisted mixing and compaction. In addition,
(b)
6
Unaged (No cap)
6
Cap I Cap II Cap III
4
DS2nd/DS1st
DS2ns/DS1st
Aged (No cap)
2
0
4
2
0 3
6
12
Rest time (h)
24
48
3
6
12
24
48
Rest time (h)
Fig. 13. Variation of deformation slope with rest time for test specimens (a) without capsules and (b) with capsules.
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(b) 10
0.1
Oil bit A bit A + oil(5%) bit A + oil(10%)
1 Loading, rest 48h
Area
Normalized absorbance
(a)
0.05
No loading
0.1
0.01
0 1700
1750
bit A
1800
Oil
Wavenumber (cm-1)
bit A + bit A + Cap I oil(5%) oil(10%)
Cap II Cap III
Fig. 14. (a) FTIR testing results. (b) Area of the normalized absorbance curve in the wavenumber range 1700–1800 cm1.
dV ¼
Vx Vd Vd
ð3Þ
where Vd is the percentage of air voids, including cracks, in the test specimens cross section after loading and, Vx is the percentage of air voids, including cracks, in the test specimens cross section after x days resting. A negative value for dV means that there was a decrease of the air voids during the resting period and can be interpreted as cracks closing. As it can be observed, there was a global reduction of air voids along the test specimen, which occurs mainly during the first 4 days. The main reductions in air voids were located 50
10
Variation in air voids (dV)
Rest 4 days Rest 16 days
Rest 9 days Capsules
25
5
0
0
-5
-25
-50
10
(Top)
15
20
25
30
35
Vertical distance (mm)
Fig. 15. Percentage of capsules and variation in air voids vs. vertical distance.
40
-10
(Bottom)
Percentage capsules (%)
approximately 3 mm to the bottom of the section containing capsules. This may be due to the oil and bitumen flowing down by effect of gravity. To confirm that changes in the air voids content were due to the reduction of crack size Figs. 16 and 17 present selected sections of the computed tomography scans of asphalt mixture, after mechanical testing, and after 4 days resting at 20 °C. Fig. 16 shows cracks without surrounding capsules while Fig. 17 shows cracks near capsules. It can be observed that cracks partially healed in the areas around the capsules, while they did not heal in areas without capsules. Authors consider that the reason for crack closure was that the oil coming out of the capsules reduced the viscosity of bitumen, which favoured the drain of bitumen into the cracks. Finally, a 3-Dimensional representation of a single crack before and after 4 days resting at 20 °C was reconstructed from CT-Scan images, see Fig. 18. This crack had a total length of about 20 mm and a maximum width of 500 lm. Moreover, at each end of the crack there were capsules (coloured yellow). It can be observed that before resting the crack was continuous between the capsules, while after the rest period the crack was partially filled around the capsules. Moreover, a closer look at the planar CT images shows that the crack width was reduced in approximately 60 lm in its widest point. These results prove that capsules are able to induce crack healing in asphalt. Note that these cracks were exceptionally wide (approximately 600 lm) and that in a real road situation capsules will start healing cracks at an early state (shorter and narrower), with what the healing efficiency is expected to be much higher [7]. For this reason,
the absorbance curve of all the test samples containing capsules shows higher area after the loading and resting periods and capsules type III released substantially more oil than the other capsules. In addition, Fig. 15 shows the percentage of capsules along the vertical axis of a selected test specimen of 40 40 50 mm3 containing capsules type III. This test specimen was damaged under cyclic loading and cracks with average width 600 lm were created. It can be observed that the concentration of capsules was higher at the bottom of the test specimen. This may be due to capsules being mixed in the last place and not uniformly distributed in the mixture. Moreover, it was determined the change in air voids (dV) during the resting period. The CT-Scanned asphalt specimens were divided in a stack of cross sectional images and the relationship between the air voids before and after the resting period was calculated as:
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(a) Immediately after loading
(b) After 4 days resting 4mm
4mm
4mm
4mm
Fig. 16. Examples of asphalt mixture without capsules, (a) after mechanical testing, (b) after 4 days rest at 20 °C.
(a) Immediately after loading
(b) After 4 days resting 4mm
4mm
3mm
3mm
Fig. 17. Examples of asphalt mixture with capsules, (a) after mechanical testing, (b) after 4 days rest at 20 °C.
After 4 days resting at 20°C
After cyclic loading
4 mm
Capsule
(a)
4 mm
Crack
Capsule
Capsule
Crack
Capsule
(b)
Fig. 18. A crack healing due to the effect of capsules (a) after cyclic loading, (b) after 4 days resting at 20 °C.
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future research should focus on the maximum width of crack that can be healed at the environment temperature by the action of capsules. 4. Conclusions The study presented in this paper investigates a new type of capsules for asphalt self-healing purposes. Sunflower oil was encapsulated in calcium-alginate, and protected with a hard shell made of epoxy-cement composite. After, capsules were added into asphalt mixture and the healing properties tested. The main findings can be summarized as follows: Calcium-alginate capsules with a maximum payload of 75% were produced. Later, capsules were coated with an epoxycement composite shell. The calcium-alginate core and the epoxy-cement shell were not bonded. The epoxy-cement shell was not necessary to resist asphalt mixing and compaction, in fact the epoxy-cement shell cracked in every occasion. Moreover, capsules without epoxy-cement adapted their shape to the surrounding aggregates. All the types of capsules studied released a small amount of oil during asphalt mixture fabrication. This did not affect the compaction process, but affected the permanent deformation resistance of asphalt mixture. In the future capsules have to be redesigned to avoid oil losses during mixing and compaction. Capsules broke and released their content during cyclic loading. Capsules without shell released the largest amount of oil during cyclic loading. Capsules with epoxy-cement shell released a smaller amount of oil. Oil diffused in the bitumen at 20 °C and modified its chemical properties. Asphalt specimens containing capsules showed significant increases of stiffness after damaging them under cyclic loading and letting them rest. On the contrary, the deformation resistance reduced in specimens containing capsules. The stiffness and deformation resistance of asphalt mixture after the resting period depended on the type of capsule analysed. For example asphalt mixture containing the strongest capsules, with epoxy-cement shell, had the same stiffness after the resting period. On the other hand, the stiffness of asphalt mixture containing capsules without epoxy-cement shell increased approximately 15% after resting for 48 h. Imaging analysis of cracked asphalt mixture showed large reductions of the area of voids and cracks near the capsules location: oil and/or modified bitumen flowed into and filled the cracks to the bottom of the capsules. In some places, 50% of the air voids disappeared after the resting period. These results are very promising for the development of asphalt mixtures with self-healing ability. However further research is necessary before this technology be applied in roads. Following, it is intended to investigate the increase of strength with crack closure and how this mechanism is affected by capsules and cracks characteristics. Acknowledgments The authors would like to acknowledge the financial support of the FCT – Fundação para a Ciência e a Tecnologia, IP, through grant SFRH/BSAB/113646/2015 financed by the Portuguese Government budget, the Higher Committee of Education Development in Iraq for the PhD scholarship of the second author, and the EPSRC project EP/M014134/1, Induction heating for closing cracks in asphalt concrete.
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