1、Mechanical and Self-Healing Properties of Stone Mastic Asphalt Containing Encapsulated Rejuvenators Jose Norambuena-Contreras1; Erkut Yalcin2; Robin Hudson-Griffiths3; and Alvaro Garca4 Abstract: This paper presents an experimental study to evaluate the mechanical and crack-healing properties of sto

2、ne mastic asphalt (SMA) mixtures with encapsulated rejuvenators. With this goal, calcium alginate capsules with encapsulated sunflower oil as the rejuvenating agent have been manufactured and added into the SMA mixtures. Physical and mechanical properties of SMAwith and without capsules have been ev

3、aluated following the British standard tests. Healing properties of SMA by the action of capsules have been assessed using three-point bending (3PB) tests applied on test beams conditionedat different healing times, from 5 to 216 h. The spatial distribution of the capsules in the SMA mixtures was ev

4、aluated by using X-ray computed microtomography. Results showed that the capsules can resist the manufacturing process without significantly reducing their properties. Additionally, testing of the mechanical properties of SMA mixtures with and without encapsulated rejuvenators presented similar resu

5、lts. Moreover, capsules showed a good spatial distribution inside the SMA samples. It was found that capsules with encapsulated oil increase the crack-healing properties of SMA when compared to mixtures without encapsulated rejuvenators. Overall, the results proved that the capsules with asphalt cra

6、ck-healing purposes can be safely used in asphalt pavement con- struction without affecting its properties. DOI: 10.1061/(ASCE)MT.1943-5533.0002687. 2019 American Society of Civil Engineers. Author keywords: Stone mastic asphalt; Rejuvenating agent; Calcium alginate capsules; Capsule influence; Stif

7、fness modulus; Fatigue behavior; Self-healing capability; Capsule distribution. Introduction Stone mastic asphalt (SMA) mixture is a road construction material widely used in many countries around theworld (Wu et al. 2007). It is a composite material formed by a coarse aggregate skeleton and a high

8、binder content (Woodward et al. 2016). The coarse aggregate skeleton provides the mixture with stone-on-stone contact, giving it strength, whereas the high-binder-content grout enhances durabil- ity (Ahmadinia et al. 2011). SMA mixtures should be designed and installed ensuring a strong coarse aggre

9、gate skeleton because the high concentration of aggregate particles will make the mixture more resistant to wear (Vaiana et al. 2013). Nevertheless, environmental conditions combined with dy- namic traffic loads contribute to premature deterioration of SMA pavements, reducing their mechanical streng

10、th and durability over time (Norambuena-Contreras et al. 2016). The main environmental factors affecting durability of SMA mixtures are (Pratic and Vaiana 2015) rutting, water damage, thermal cracking, fatigue, and aging. These factors mainly depend on the bituminous binders used because the aging d

11、amage of bitumen has an adverse effect on the stiffness of SMA mixtures. The aging of bitumen is caused by the oxidation and the loss of volatiles from bitumen composition (Ylmaz et al. 2013). Aged bitumen has higher viscosity and is stiffer than fresh bitumen, presenting recognizable changes in its

12、 chemical composition (Lu and Isacsson 1998). These changes in the bitumen lead to raveling and cracking in the pavement surface, especially reflective cracking (Norambuena-Contreras and Gonzalez-Torre 2015), that occur because aged bitumen is more brittle (Holleran et al. 2006). One recent techniqu

13、e to solvethe problem of the maintenance of aged asphalt pavements is the use of capsules containing rejuvenat- ing agents to renovate the properties of the aged bitumen (Chung et al. 2015; Garca et al. 2010a; Su et al. 2013; Garca et al. 2016). A rejuvenating agent is a low-viscosity oil, or a heal

14、ing agent used to recover the original properties of the aged bitumen. Capsules containing rejuvenating agents have been added in the asphalt mix- tures to promote crack healing (Garca et al. 2010a). This technol- ogy is based on the fact that these capsules will remain inactive for years in the asp

15、halt pavement until an external mechanical trigger happens on the road (Su et al. 2013; Garca et al. 2010b, 2016; Norambuena-Contreras et al. 2018). Hence, when a microcrack starts to occur in the asphalt pavement, the crack reaches a capsule and breaks the capsules shell at the appropriate time, le

16、ading to the release of the rejuvenator agent into the asphalt pavement, reducing the bitumen viscosity and letting it flow into the existing micro- cracks (Garca et al. 2016). Several experimental methods have been used to prepare micro- capsules containing rejuvenator agents with asphalt self-heal

17、ing purposes. In the research of Garca et al. (2010b) different capsule types were manufactured by encapsulating rejuvenators using sa- turated porous aggregates with a shell made of epoxy-cement 1Postdoctoral Researcher, Nottingham Transportation Engineering Centre, School of Civil Engineering, Uni

18、v. of Nottingham, Nottingham NG7 2RD, UK; Assistant Professor, Dept. of Civil and Environmental Engineering, LabMAT, Univ. of Bo-Bo, Concepci on 4051381, Chile (corresponding author). ORCID: /0000-0001-8327-2236. Email: jnorambuenaubiobio.cl 2Visiting Researcher, Nottingham Transport

19、ation Engineering Centre, School of Civil Engineering, Univ. of Nottingham, Nottingham NG7 2RD, UK; Associate Researcher, Faculty of Engineering, Dept. of Civil Engineering, Firat Univ., Elazig 23119, Turkey. Email: erkutyalcin .tr 3Senior Advisor,HighwaysEngland,Safety,Engineeringand Stand

20、ardsPavement Materials, 199 Wharfside St., Birmingham B1 1RN, UK. Email: Robin.Hudson-Griffithshighwaysengland.co.uk 4Lecturer, Nottingham Transportation Engineering Centre, School of Civil Engineering, Univ. of Nottingham, Nottingham NG7 2RD, UK. Email: Alvaro.Garcianottingham.ac.uk Note. This manu

21、script was submitted on March 1, 2018; approved on November 5, 2018; published online on March 14, 2019. Discussion period open until August 14, 2019; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, ASCE, ISSN 0899-15

22、61. ASCE04019052-1J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2019, 31(5): 04019052 Downloaded from by Changsha University of Science and Technology on 02/13/20. Copyright ASCE. For personal use only; all rights reserved. matrix and cement. Garca et al. (2011) encapsulated four differen

23、t types of rejuvenators and studied their effect on the properties of the capsules. Likewise, Li et al. (2015) designed microcapsules containing urea-formaldehyde resin as a core material based on an in situ polymerization method, and Su and Schlangen (2012) synthesized core-shell microcapsules by i

24、n situ polymerization of urea-formaldehyde with a shell made of methanol-melamine- formaldehyde (MMF). Furthermore, previous works have studied the use of heavy aromatic oils (Su et al. 2015) and sunflower oil (Micaelo et al. 2016) as encapsulated rejuvenators in asphalt mix- tures.Based ontheworkof

25、Garca et al. (2016) it was concludedthat the use of heavy aromatic oils may present risks in terms of human health if they are not perfectly encapsulated, whereas sunflower oil can be used without human health risk, proving a positive influence on the aged pavement (binder) rejuvenation. Authors lik

26、e Al-Mansoori et al. have recently published studies in which polymeric capsules with sunflower oil as the encapsulated agent were prepared to be evaluated in dense asphalt mixtures (Al-Mansoori et al. 2017) and asphalt mastic (Al-Mansoori et al. 2018). They were made using the encapsulation techniq

27、ue of ionic gelation of sodium alginate in the presence of calcium chloride solution. Overall, the works of Al-Mansoori et al. demonstrated that the calcium alginate capsules with sunflower oil are resistant to asphalt mastic/mixture manufacturing and release the rejuvena- tor agent by the action of

28、 external loading. Also, they have a pos- itive effect on the physical and mechanical properties of aged and unaged bituminous materials. Likewise, these polymeric capsules proved the ability to self-heal open microcracks and rejuvenate aged asphalt mixture, although these studies did not evaluate t

29、he effect of the addition of capsules with sunflower oil inside stone mastic asphalt mixtures. This paper aims to evaluate the effect of the capsule addition on the mechanical and self-healing properties of SMA mixtures. These polymeric capsules were designed byAl-Mansoori et al. (2017) and recently

30、 improved by Norambuena-Contreras et al. (2018). With this purpose, this study evaluated several mechanical properties of SMA mixtures with and without capsules by measuring the stiff- ness modulus, indirect tensile strength and fatigue resistance, and water sensitivity. Physical properties such as

31、bulk density, air void content, and skid resistance on SMA mixtures with and without capsules have also been reported. Furthermore, the self-healing properties of SMA at different healing times (from 5 to 216 h) were quantifiedthroughthree-pointbendingtestsoncrackedSMAbeams with and without capsules

32、. Finally, the spatial distribution and the integrity of the calcium alginate capsules inside the SMA sam- ples were evaluated using X-ray computed tomography tests. Materials and Methods Fig. 1 presents a schematic representation of the experimental test methods used on the stone mastic asphalt sam

33、ples with and without encapsulated rejuvenators. Raw Materials A stone mastic asphalt mixture SMA 14 surf 40/60 according to BS EN 13108-5 (BSI 2006) and calcium alginate capsules were used. SMA mixtures were manufactured by using bitumen 40/60 pen (density 1.030 g=cm3) and graded Tunstead lime- sto

34、ne aggregate (density 2.700 g=cm3), both provided by Tarmac. Aggregate gradation and SMA design properties are shown in Fig. 2 and Table 1, respectively. Polymeric capsules with density of 1.116 g=cm3and average size of 2.5 mm were prepared, allowing for 25% by volume calcium alginate polymer encaps

35、ulating 75% by volume rejuvenator agent; see Fig. 3. The used rejuvenator was sunflower oil provided by East End (UK) with density, smoke point, and flash point of 0.92 g=cm3, 227C and 315C, respec- tively (Micaelo et al. 2016; Norambuena-Contreras et al. 2018). The polymeric structure of the capsul

36、es was made of sodium SMA14 surf 40/60 Rejuvenator release from capsules Stiffness modulus Fatigue testing Water sensitivity Skid resistance Crack healing X-ray tomography Without capsules 0.5% of capsules With capsules Fig. 1. Experimental program used in this study. 0 10 20 30 40 50 60 70 80 90 10

37、0 0.010.1110100 gn i s saP % Sieve Size (mm) Upper limit SMA gradation Lower limit Fig. 2. Aggregate gradation of the stone mastic asphalt. Table 1. Design properties of the stone mastic asphalt mixture PropertyValueStandard limitsa Binder content (%M)5.95.0 Bulk density (g=cm3)2.243 Maximum density

38、 (g=cm3)2.370 Air void content (%)5.51.514 Voids in mineral aggregate (%)14 Voids filled with bitumen (%)757192 Note: %M= percent by mass. aStandard limits according to BS EN 13108-5 (BSI 2006). ASCE04019052-2J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2019, 31(5): 04019052 Downloaded from ascelibrary.

39、org by Changsha University of Science and Technology on 02/13/20. Copyright ASCE. For personal use only; all rights reserved. alginate (C6H7O6Na in powder) and calcium chloride (CaCl2in granular pellets) provided by Sigma-Aldrich, UK. Main properties of used capsules are shown in Table 2. Encapsulat

40、ion Procedure of Capsules Capsules were prepared at 20C by ionic gelation of sodium algi- nate in the presence of calcium ions (Micaelo et al. 2016). The step-by-step procedure for preparation of the capsules containing sunflower oil was previously described in Norambuena-Contreras et al. (2018). Th

41、is procedure consisted of the following three steps: (1) preparation of the sodium alginate emulsion, (2) preparation of the calcium chloride solution and capsule formation, and (3) drying and storage of the capsules. A schematic description of the encap- sulation procedure is shown in Fig. 4. Prepa

42、ration of SMA Specimens Prismatic and cylindrical SMA test specimens were prepared in this study. Mixing and compaction of the SMA test specimens were developed according to the standards BS EN 12697-35 (BSI 2016) and BS EN 12697-33 (BSI 2003b), respectively. Likewise, the material design was manufa

43、ctured according to the standard BS EN 13108-5 (BSI 2006). The capsule content added to the SMA test specimens was 0.5% by total mass of the mixture. This amount corresponds to an oil-to- bitumen content of 5.23% by mass of bitumen, which is set within the range recommended by Ji et al. (2016) with

44、the aim of recov- ering the aged bitumen properties. SMA mixtures were manufac- tured in 14 kg batches using a laboratory mixer. SMA mixtures with and without capsules can be described as follows: 1. SMAwithcapsules(W/C):Thebitumenandaggregateswerepre- heated at 160C for 4 and 12 h, respectively, wh

45、ile the capsules were preconditioned for 2 h at 20C. Aggregates and bitumen were mixed for 3 min at 125 rpm at 160C. After that, the cap- suleswereaddedtotheSMAmixtureandmixedfor20additional seconds, ensuring their adequate distribution in the asphalt. 2. SMA without capsules (WO/C): The bitumen and

46、 aggregates were preheated at 160C for 4 and 12 h, respectively. Then, the raw materials were mixed for 3 min at 125 rpm at 160C. Then, the SMA mixtures were transferred to the steel molds for their compaction and cutting following the procedure described by Norambuena-Contreras et al. (2018). First

47、, SMA slabs of 306 306 60 mm were compacted using a roller compactor to reach the properties presented in Table 1. Then, samples with dimensions 150 100 60 mm were cut from asphalt slabs, and a transverse notch of 5 5 mm was made at the midpoint on their bottom sur- face. Furthermore, cylindrical sp

48、ecimens (100-mm diameter and 60-mm height) were manufactured using a gyratory compactor (inclination angle of 1.25 and static pressure of 600 kPa), applying a maximum of 250 gyrations. The horizontality of the faces of the SMA test specimens after manufacturing was verified using a level. Finally, d

49、ifferent SMA cylindrical cores (35-mm diameter and 60-mm height) were drilled from asphalt cylindrical specimens with encapsulated rejuvenators for the computed tomography (CT) scan tests. Indirect Tensile Stiffness Modulus The stiffness properties of the SMA mixtures with and without capsules were

50、measured by the indirect tensile stiffness modulus (ITSM) test. Stiffness modulus was measured at 5C, 20C, 25C, and 30C following the methodology defined in the BS EN 12697- 26 (BSI 2004). Thereby, the stiffness modulus of each specimen was calculated according to Sm F 0.27 z h 1 where Sm= measured

51、stiffness modulus (MPa); F = maximum vertical load applied (N); z = horizontal strain amplitude (mm); h = average thickness of the specimen (mm); and = Poissons ratio 0.35 was adopted according to BS EN 12697-26 (BSI 2004). In total, more than 40 SMA specimens were tested. Indirect Tensile Fatigue T

52、est The indirect tensile fatigue tests (ITFT) were performed on the SMA specimens with and without capsules under a controlled stress mode according to BS DD ABF (BSI 2003c). Fatigue tests were carried out at 20C, and maximum stress levels of 200, 250, 300, 350, and 400 kPa (max x ) were used in the

53、 study. The indirect tensile fatigue test was sustained until the breakage of SMA spec- imens. The maximum horizontal tensile strain (max x ) at the center of the cylindrical specimen can be defined as follows: max x max x 1 3 Sm 2 where Sm= stiffness modulus (MPa); max x = maximum tensile stress at

54、 the center of the specimen (MPa); and = Poissons ratio of 0.35. In total, more than 30 SMA specimens were tested. 10 mm Fig. 3. Calcium alginate capsules with encapsulated sunflower oil. Table 2. Main properties of the calcium alginate capsules PropertyValue Diameter (mm)2.5 Rejuvenator content (%v

55、ol)75 Polymer content (%vol)25 Bulk density (g=cm3)1.116 Thermal expansion coefficient (m/mC)187.10 Mass loss (%) at 160C4.2 200C6.3 Compressive strength (N) at 20C45.5 160C19.4 Note: %vol= percent by volume. ASCE04019052-3J. Mater. Civ. Eng. J. Mater. Civ. Eng., 2019, 31(5): 04019052 Downloaded fro

56、m by Changsha University of Science and Technology on 02/13/20. Copyright ASCE. For personal use only; all rights reserved. Water Sensitivity Analysis Based on previous studies, the rehydration of the capsules may affect the preservation of the encapsulated sunflower oil (Vreeker et

57、al. 2008). Likewise, swelling of encapsulated rejuvenators may affect the capsule/mastic bonding. The A-method defined in standard BS EN 12697-12 (BSI 2008) was used to evaluate the influence of saturation and accelerated water conditioning of SMA mixtures with and without capsules. The test specime

58、ns were divided in two groups and conditioned as follows: 1. Dry SMA specimens: conditioned at 20C for 72 h in a temperature-controlled room. 2. Wet SMA specimens: conditioned in a temperature-controlled water bath at 40C for 72 h. The specimens were saturated in a vacuum container for 30 min before

59、 placing them in the bath, with a residual pressure of 6.7 kPa (BSI 2008). Water sensitivity of the SMA mixtures was evaluated by the indirect tensile strength (ITS) method of the cylindrical asphalt specimens, according to the methodology defined in the BS EN 12697-23 (BSI 2003a), at the test temperatures of 20C and 25C. The indirect tensile strength (ITS) in units of kPa was calculated by applying the following equation: ITS 2 F L D 3 where F = peak value of the appli