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Effects of aging on the properties of modified asphalt binder with flame retardants
Construction and Building Materials 24 (2010) 2554–2558
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of aging on the properties of modified asphalt binder with flame retardants Peiliang Cong a,b,*, Shuanfa Chen a, Jianying Yu b, Shaopeng Wu b a b
Engineering Research Center of Transportation Materials of Ministry of Education, Chang’an University, Xi’an 710064, PR China Key Laboratory of Silicate Materials Science and Engineering of Ministry of Education, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 30 September 2009 Received in revised form 8 May 2010 Accepted 31 May 2010 Available online 25 June 2010 Keywords: Asphalt Aging Flame retardancy Rheological property
a b s t r a c t Effects of aging on the properties of asphalt binders modified by incorporating Styrene–Butadiene– Styrene (SBS) and flame retardants (FR) were studied. Asphalt binders were artificially aged in the rolling thin film oven (RTFOT) and Pressure Aging Vessel (PAV). The flame retardancy of modified asphalt binders were characterized using limited oxygen index (LOI), and the effects of aging on the properties of asphalt binders were studied using Brookfield viscometer test and dynamic shear rheometer test. Experimental results indicated that the flame retardancy of asphalt binder was increased after aging. But the increasing amount of LOI is low when the modified asphalt binder containing more flame retardants. The variation of the LOI, softening point, penetration, ductility and viscosity of asphalt binder decreased with flame retardant content increasing, meaning the flame retardants can improve the thermo-oxidative aging resistance of asphalt binder. Furthermore, the G/sin d, stiffness and m-value of flame retardant modified asphalt binders display smaller changes after two different aging. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The oxidation of asphalt is one of the principal factors causing the deterioration of asphalt pavements [1–4]. But the long term aging is a very complex process, such as the sunshine especially ultraviolet radiation and rainwater have different effect on asphalt binder in different zone. The mechanical properties and chemical structures of asphalt binders change with aging time [5,6]. The aging include short term aging that occurs during the mixing, paving, compacting and long term aging during the service life in the pavement. The rolling thin film oven test (RTFOT), as described by the ASTM Standard Methods D2872, has been accepted as a reliable procedure to simulate the short term aging. Further aging was carried out on the RTFOT residue using the Pressure Aging Vessel (PAV) following the standard practice outlined by AASHTO to simulate the long term aging [7–10]. The aging properties of asphalt binders were normally characterized by measuring physical and rheological properties (e.g. softening point, penetration, viscosity and complex modulus) before and after artificial aging in the laboratory [11–13]. The asphalt binders with good flame retardancy have been prepared successfully by the addition of mix flame retardants such as decabromodiphenyl ether, antimony trioxide and zinc borate as
* Corresponding author at: Engineering Research Center of Transportation Materials of Ministry of Education, Chang’an University, Xi’an 710064, PR China. Tel.: +86 029 82337349. E-mail address: [email protected] (P. Cong). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.05.022
previous research [14,15]. But the effects of flame retardants on the aging resistance have not focused in previous research. This paper investigates flame retardant modified asphalt binders’ aging properties. The rolling thin film oven test (RTFOT) and Pressure Aging Vessel (PAV) were employed to simulate the short term and long term aging. The effects of aging on flame retardancy, physical and rheological properties of asphalt binders were evaluated. The relatively properties of asphalt binders for both flame retardant modified asphalt binder and pure asphalt binder was compared in order to assess the influence of flame retardants on aging resistance of asphalt binder.
2. Experimental 2.1. Raw materials Commercial asphalt was used, and the physical properties of the asphalt were listed in Table 1. Styrene–Butadiene–Styrene (SBS), Grade 1301, was produced by Yueyang Petrochemical Co., Ltd., Hunan Province, China. It was a linear-like SBS, containing 30 wt.% styrene, and the average molecular weight of SBS was 120,000. All of flame retardants were commercial product. 2.2. Methods Asphalt binder was heated to 175 ± 5 °C in an oil-bath heating container until it flowed fully and 3% SBS was added slowly. The
2555
P. Cong et al. / Construction and Building Materials 24 (2010) 2554–2558 Table 1 Physical properties of asphalt.
28
Penetration (25 °C, 0.1 mm) Softening point (°C) Ductility (15 °C) Ductility (5 °C) Viscosity (135 °C, Pa s)
75 46.5 150 21 0.45
26 24 22
Unaging RTFOT aging PAV aging
20
3. Results and discussion
18 0
2
4
6
8
10
Flame Retardant Content (wt%) Fig. 1. Effect of aging on flame retardancy.
when modified asphalt containing the same contents of flame retardants. The improvement of flame retardancy of asphalt binder maybe explained that small molecules in asphalt transform to larger molecules and form the stronger chemical bonds, which pyrolyzing and broken needs high temperature. Consequently, the flame retardancy of asphalt binder was improved by aging. 3.2. Effect of aging on the conventional parameters Softening point increment after aging can reflect the susceptive degree of aging. It can be expressed as DS, and it was calculated as Eq. (1).
DS ¼ Saged Sunaged
ð1Þ
where DS is the softening point increment after aged. Saged and Sunaged is softening point of asphalt binder before and after aged, respectively. The effect of aging on softening point of flame retardant modified asphalt binder and unmodified asphalt binder is shown in Fig. 2. The results showed that softening point of the modified and unmodified asphalt binders have increased after the two different aging, meaning an inherent hardening process of the material in aging. Compared with the asphalt binder without flame retardant, the flame retardant modified asphalt binder displayed lower softening point increment (DS), and the lower DS, the more flame retardant content. Generally, the content of fractions with large molecules in asphalt increases and the content of the small
6
16 RTFOT aging PAV aging
5
ΔS ( ºc )
blends were sheared under 5000 rpm rotation speed for 30 min to ensure the blends became essentially homogenous. The appropriate amounts of flame retardant and stabilizer were added into the blends and mixed for 30 min with a lab mixer set fast enough (usually 1000 rpm) to create a small vortex, without whipping excessive air into the sample. The Rolling Thin Film Oven (Model B1, CONTROL Co., Ltd., Italy) was employed to simulate short term aging according to ASTM D2872. In this test, the 35 g asphalt sample was placed in a glass bottle, which has a narrow top opening. The glass containers were placed in a carriage such that the axis of revolution was horizontal and the container opening was facing a jet of air. The oven was kept at 163 °C and the carriage was rotated in the oven at a rate of 15 rpm for 85 min. The Pressure Aging Vessel (Model PR9300, Prentex Co., Ltd., America) was used for the accelerated aging of asphalts. As soon as the time of RTFOT of asphalt was over, 50 ± 0.5 g of each asphalt sample was immediately poured into two marked pans and placed together in the same PAV for a single test run. When the temperature inside the PAV was within ±2 °C of the aging temperature (109 °C), an air pressure of 2.1 ± 0.1 Map was applied and maintained for 20 h ± 10 min. After 20 h, the pans were removed and poured into different containers for different physical and chemical tests. Flame retardancy of flame retardants modified asphalt binder was evaluated by the limited oxygen index (LOI) using HC-2 oxygen index tester according to ASTM D2863. The physical properties of asphalt binders, including softening point, penetration and ductility, were tested in accordance with ASTM D36, ASTM D5 and ASTM D113-86, respectively. Dynamic shear properties were carried out using AR2000 dynamic shear rheometer (TA Co., Ltd., USA) in a parallel plate configuration with a gap width of 1 mm according to ASTM D6648. Measurements were conducted at fixed frequencies 10 rad/s and temperature from 30 to 95 °C with rate of heating 1 °C/min. The strain value was 0.1%, which was chosen to be as small as possible to ensure measurement in the linear region, but large enough to allow sufficient torque readings. Brookfield viscometer (Model DV-II+, Brookfield Engineering Inc., USA) was employed to measure the rotational viscosity of flame retardant SBS modified asphalt binders in according to ASTM D4402.
15
4
14
3
13
2
12
1
11
3.1. Effect of aging on the flame retardancy Fig. 1 showed the effects of aging on LOI of asphalt binders with different flame retardant content. The results indicated that the aging can increase LOI of asphalt binders. The increasing amount of LOI decreased when the contents of flame retardants was increased, meaning that flame retardant improve the aging resistance of asphalt binders. Compared with RTFOT aging, PAV aging led asphalt binder to further aging. The results showed that the LOI of asphalt binders after PAV aging was largest than others
0
10 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 2. Effect of aging on the softening point of asphalt binder.
ΔS ( ºc )
Measured values
LOI (%)
Physical properties
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P. Cong et al. / Construction and Building Materials 24 (2010) 2554–2558
molecules decreased during the aging. It leads to increase of the DS. However, flame retardant restricts the oxidation of asphalt, which decreased the hardening process of the asphalt. Thus, the results indicated that the flame retardant can improve the thermooxidative aging resistance of asphalt. Penetration ratio (PR) of the aged asphalt with their unaged asphalt can also be able to reflect the change of properties of asphalt binder during aging, and it is calculated as Eq. (2).
Paged 100 Punaged
ð2Þ
where PR is penetration ration of asphalt. Paged and Punaged is penetration of aged and unaged asphalt binder. The effects of aging on the penetration ratio of flame retardant modified asphalt binder and unmodified asphalt binder is shown in Fig. 3. The results showed that penetration of the modified and unmodified asphalt binders have decreased after the two different aging. But the penetration ratio increased with the contents of flame retardant increasing, meaning the degree of aging was decreased. The results also indicated that the thermo-oxidative aging resistance of asphalt was improved when flame retardant was added. Ductility retention rate (DRR) was another parameter that reflect the change of properties of asphalt binder during aging, and it was calculated as Eq. (3).
Daged 100 Dunaged
ð3Þ
where DRR is ductility retention rate of asphalt. Daged and Dunaged is ductility of aged and unaged asphalt binder. The effect of aging on the ductility of asphalt binder is shown in Fig. 4. It can be seen that ductility of asphalt binder clearly decreased after the two different aging, especially PAV aging. But ductility retention rate (DRR) of the asphalt binder increased with the content of flame retardant increasing. The DRR of asphalt binder without flame retardant was 34.8% after RTFOT aging, but it was 57.9% when 10 wt.% flame retardant was added. The results indicated that the addition of flame retardant can reduce deterioration in ductility of asphalt during aging. The ductility result was corresponded with softening point and penetration test results. The viscosity aging index (VAI) is very important parameter to evaluate aging resistance properties of asphalt, and it was calculated as Eq. (4).
40 30 20 10 0 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 4. Effect of aging on the ductility of asphalt binder.
VAI ¼
V aged V unaged 100 V unaged
ð4Þ
where VAI is viscosity aging index of asphalt. Vaged and Vunaged is viscosity of aged and unaged asphalt binder. Fig. 5 showed the effects of aging on the viscosity of asphalt after RTFOT and PAV aging. It was easily found that viscosity aging index (VAI) was gradually decreased with the contents of flame retardant increasing. Those suggested that flame retardant can decreased the viscosity increment during aging and improve the thermo-oxidative aging resistance. During the aging of the asphalt, the naphthene aromatics were converted partly to polar aromatics and polar aromatics turned to asphaltenes. This composition changes make the viscosity of asphalt binder increasing. The results maybe due to the flame retardant obstruct large molecular formation and volatile components loss in asphalt at high temperature. Therefore, flame retardant led VAI to decrease and improve the aging resistance of asphalt binder. 3.3. Effect of aging on the rheological properties The dynamic shear rheometer (DSR) was currently being used by the asphalt community for determination of the viscoelastic properties of asphalt binders according to the recommendations of the Strategic Highway Research Program (SHRP). The Superpave specification parameter G/sin d was identified as the term to be used for high temperature performance grading of paving asphalts
30
100 RTFOT aging PAV aging
80
65 RTFOT aging
25
60
PAV aging
20
VAI (%)
Penetration ratio (%)
RTFOT aging PAV aging
50
60
55 15 50
40 10 20
45
5 0
0 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 3. Effect of aging on the penetration of asphalt binder.
40 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 5. Effect of aging on the viscosity of asphalt binder (at 135 °C).
VAI (%)
DRR ¼
Ductility retention rate (%)
PR ¼
60
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P. Cong et al. / Construction and Building Materials 24 (2010) 2554–2558
DG = sin d ¼ ðG = sin dÞaged ðG = sin dÞunaged
ð5Þ
where DG = sin d is the G /sin d increment after aged. ðG = sin dÞaged and ðG = sin dÞunaged is the G/sin d of asphalt binder before and after aged, respectively. The G/sin d increment (at 70 °C) after two different aging is shown in Fig. 7. Compared with asphalt binder without flame retardant, the G/sin d increment decreased as the flame retardant was mixed. The G/sin d increment of asphalt binder containing 0 wt.% and 10 wt.% flame retardant was 0.6 MPa and 0.49 MPa after RTFOT aging. The PAV aging showed almost similar effects. The 1.9 MPa and 1.47 MPa of G/sin d increment was obtained for the asphalt binder containing 0 wt.% and 10 wt.% flame retardant. Thus, flame retardant decreased the oxidation of asphalt, which reduced the hardening process of asphalt during aging. The bending beam rheometer (BBR) is used to accurately evaluate asphalt binder properties at low temperatures at which asphalt binders were too stiff to reliably measure rheological properties using the parallel plate geometry of the DSR equipment [17,18].
3.5
3.5
3.0 Unaging RTFOT aging PAV aging
2.5
RTFOT aging PAV aging
2.5 2.0 1.5
2.0
1.0
Δ G*/sinδ (MPa)
3.0
G*/sinδ (MPa)
in rating the binders for their rutting resistance [16]. It was known that the temperature of the asphalt binders when G/sin d is equal to 1 kPa is defined as a criterion for the high temperature performance of asphalt. Fig. 6 showed the effects of RTFOT and PAV aging on G/sin d for the 6.5 wt.% flame retardant modified and unmodified asphalt binders. It was clearly observed that the G/sin d of asphalt binder increased when the 6.5 wt.% flame retardant was added and after two different aging. However, compared with the unmodified asphalt binder, the G/sin d of flame retardant modified asphalt binder decreased remarkably with temperature increasing. As a result, the slop of the G/sin d master curve increased with temperature increasing. The increase degree of the G/sin d after aging decreased slightly for flame retardant modified asphalt binders at high temperature. Thus, flame retardant was beneficial to the thermo-oxidative aging resistance of asphalt. Fig. 7 showed that the effects of aging on G/sin d of asphalt binder containing different contents of flame retardant at 70 °C. The results exhibited that the G/sin d of asphalt binder containing different content flame retardant increased after two different aging, especially after PAV aging. But, the slop of the G/sin d decreased with increasing flame retardant concentration, meaning that flame retardant made the asphalt better thermo-oxidation aging resistance. The G/sin d increment after aging can reveal the susceptive degree of aging. It can be expressed as DG/sin d, and it was calculated as Eq. (5).
1.5 0.5 1.0
0.0 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 7. Effect of aging on G/sin d of asphalt binder containing different content of flame retardant (at 70 °C).
The BBR test measured creep stiffness (S) and m-value of asphalt binders. Stiffness indicated the susceptibility to low-temperature cracking as designated by SHRP. The Superpave technology specified that the creep stiffness must not exceed 300 MPa to prevent low-temperature cracking. Since low-temperature cracking occurs only after the pavement has been in-service for some time, this specification addresses these properties using binder aged. The rate of change of asphalt binder stiffness with loading time was represented by the m-value. A high m-value was desired. A minimum m-value of 0.3 after 60 s was required by the Superpave PG binder specification. The effect of aging on stiffness and m-value of asphalt binder at 12 °C is shown in Fig. 8. The results revealed that the stiffness of all asphalt binder satisfies the SHRP Superpave specifications at 12 °C, and all asphalt binders show higher than the required minimum m-value. The high stiffness value was obtained when flame retardant was added into asphalt binder. Those indicate that the asphalt concrete using flame retardant modified asphalt binders maybe susceptible to low-temperature cracking, especially in cold areas and/or weather. Nonetheless, the stiffness and m-value of the flame retardant modified asphalt binders meets the SHRP Superpave specifications at 12 °C. The change of stiffness and m-value of flame retardant modified asphalt binders was not remarkably different with the asphalt without flame retardant. Therefore, the flame retardant does not deteriorate the thermo-oxidation aging resistance of asphalt binders.
100 0.5
360
1
No Flame retardants (NFR) RTFOT aging (NFR) PAV aging (NFR) Flame retardant modified (FRM) RTFOT aging (FRM) PAV aging (FRM)
0.1 48
56
64
72
80
270
0wt% 6.5wt% 10wt%
0.4
0.3
180
0.2
90
0.1
m value
0wt% 6.5wt% 10wt%
10
Stiffness (MPa)
G*/sinδ (MPa)
450
88
Temperature ( ºc )
0.0
0 Unaging
Fig. 6. Effect of aging on G/sin d of asphalt binder (6.5 wt.% flame retardants modified).
RTFOT
PAV
Fig. 8. Effect of aging on stiffness and m-value of asphalt binder (at 12 °C).
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P. Cong et al. / Construction and Building Materials 24 (2010) 2554–2558
4. Conclusions The effects of the thermo-oxidation aging containing RTFOT and PAV aging on the flame retardancy, conventional parameters and rheological properties of asphalt binder with different flame retardant contents were investigated. The flame retardancy of asphalt binder increased after the RTFOT and PAV. The increasing amount of LOI decreased with the flame retardants contents increasing. The softening point and viscosity of asphalt binder increased and penetration and ductility of asphalt binder decreased after two different aging. But the variation of the softening point, penetration, ductility and viscosity of asphalt binders decreased with flame retardant contents increasing, meaning the thermo-oxidative aging resistance of asphalt binder was improved when flame retardant was added. Furthermore, the effects of aging on rheological behaviors of the asphalt binder containing different content of flame retardant were investigated. The flame retardant modified asphalt binders displayed smaller changes in G/sin d after two different aging. The results revealed that the stiffness of all asphalt binder satisfies the SHRP Superpave specifications at 12 °C, and all asphalt binders showed higher than the required minimum m-value. The change of stiffness and m-value of flame retardant modified asphalt binders was not remarkably different with the asphalt without flame retardant. Therefore, the flame retardant does not deteriorate the thermo-oxidation aging resistance of asphalt binders. On the contrary, flame retardant can improve the aging resistance of asphalt binders. Acknowledgements Supported by the Special Fund for Basic Scientific Research of Central Colleges, Chang’an University (Number: CHD 2009JC133). References [1] Xiaohu Lu, Isacsson Ulf. Effect of ageing on bitumen chemistry and rheology. Constr Build Mater 2002;16:15–22.
[2] Stastna J, Zanzotto L, Vacin OJ. Viscosity function in polymer–modified asphalts. J Colloid Interf Sci 2003;259:200–7. [3] Sengoz Burak, Isikyakar Giray. Analysis of styrene–butadiene–styrene polymer modified bitumen using fluorescent microscopy and conventional test methods. J Hazard Mater 2008;150:424–32. [4] Zora V, Chaminda W, Jiri S, Ludo Z. Creep characteristics of asphalt modified by radial styrene–butadiene–styrene copolymer. Constr Build Mater 2007;21:567–77. [5] Giovanni P, Jiri S, Dario B, Ludovit Z. Relation between polymer architecture and nonlinear viscoelastic behavior of modified asphalts. Curr Opin Colloid Interf Sci 2006;11:230–45. [6] Gawel I, Baqinska K. Effect of chemical nature on the susceptibility of asphalt to aging. Petrol Sci Technol 2004;22:1261–71. [7] Bahia HU, Anderson DA. Glass transition behaviour and physical hardening of asphalt binders. J Assoc Asphalt Paving Technol 1993;62:93–129. [8] Simon A, Hesp M. Reversible aging in asphalt binders. Energy Fuel 2007;21:1112–21. [9] Iswandaru Widyatmoko, Richard Elliott. Characteristics of elastomeric and plastomeric binders in contact with natural asphalts. Constr Build Mater 2008;22:239–49. [10] Ruan YH, Richard RD, Charles JG. The effect of long-term oxidation on the rheological properties of polymer modified bitumens. Fuel 2003;82:1763–73. [11] Mastrofini D, Scarsella M. The application of rheology to the evaluation of bitumen ageing. Fuel 2000;79:1005–15. [12] Lee Soon-Jae, Amirkhanian Serji N, Shatanawi Khaldoun, Kim Kwang W. Shortterm aging characterization of asphalt binders using gel permeation chromatography and selected Superpave binder tests. Constr Build Mater 2008;22:2220–7. [13] Wu Shaopeng, Cong Peiliang, Yu Jianying, Luo Xiaofeng. Experimental investigation of related properties of asphalt binders containing various flame retardants. Fuel 2006;85:1298–304. [14] Cong Peiliang, Yu Jiangying, Wu Shaopeng, Luo Xiaofeng. Laboratory investigation of the properties of asphalt and its mixtures modified with flame retardant. Constr Build Mater 2008;22:1037–42. [15] Yu Jiangying, Cong Peiliang, Wu Shaopeng. Investigation of the properties of asphalt and its mixtures containing flame retardant modifier. Constr Build Mater 2009;23:2277–82. [16] Choquet FS, Verhasselt AF. Ageing of bitumen: from the road to the laboratory and vice versa. In: Proceedings of strategic highway research program (SHRP) and traffic safety on two continents, Hague; 1994. p. 193–213. [17] Vlachovicova Zora, Wekumbura Chaminda, Stastna Jiri, Zanzotto Ludo. Creep characteristics of asphalt modified by radial styrene–butadiene–styrene copolymer. Constr Build Mater 2007;21:567–77. [18] Shashidhar N, Shenoy A. On using micromechanical models to describe dynamic mechanical behavior of asphalt mastics. Mech Mater 2002;34:657–69.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of aging on the properties of modified asphalt binder with flame retardants Peiliang Cong a,b,*, Shuanfa Chen a, Jianying Yu b, Shaopeng Wu b a b
Engineering Research Center of Transportation Materials of Ministry of Education, Chang’an University, Xi’an 710064, PR China Key Laboratory of Silicate Materials Science and Engineering of Ministry of Education, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 30 September 2009 Received in revised form 8 May 2010 Accepted 31 May 2010 Available online 25 June 2010 Keywords: Asphalt Aging Flame retardancy Rheological property
a b s t r a c t Effects of aging on the properties of asphalt binders modified by incorporating Styrene–Butadiene– Styrene (SBS) and flame retardants (FR) were studied. Asphalt binders were artificially aged in the rolling thin film oven (RTFOT) and Pressure Aging Vessel (PAV). The flame retardancy of modified asphalt binders were characterized using limited oxygen index (LOI), and the effects of aging on the properties of asphalt binders were studied using Brookfield viscometer test and dynamic shear rheometer test. Experimental results indicated that the flame retardancy of asphalt binder was increased after aging. But the increasing amount of LOI is low when the modified asphalt binder containing more flame retardants. The variation of the LOI, softening point, penetration, ductility and viscosity of asphalt binder decreased with flame retardant content increasing, meaning the flame retardants can improve the thermo-oxidative aging resistance of asphalt binder. Furthermore, the G/sin d, stiffness and m-value of flame retardant modified asphalt binders display smaller changes after two different aging. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The oxidation of asphalt is one of the principal factors causing the deterioration of asphalt pavements [1–4]. But the long term aging is a very complex process, such as the sunshine especially ultraviolet radiation and rainwater have different effect on asphalt binder in different zone. The mechanical properties and chemical structures of asphalt binders change with aging time [5,6]. The aging include short term aging that occurs during the mixing, paving, compacting and long term aging during the service life in the pavement. The rolling thin film oven test (RTFOT), as described by the ASTM Standard Methods D2872, has been accepted as a reliable procedure to simulate the short term aging. Further aging was carried out on the RTFOT residue using the Pressure Aging Vessel (PAV) following the standard practice outlined by AASHTO to simulate the long term aging [7–10]. The aging properties of asphalt binders were normally characterized by measuring physical and rheological properties (e.g. softening point, penetration, viscosity and complex modulus) before and after artificial aging in the laboratory [11–13]. The asphalt binders with good flame retardancy have been prepared successfully by the addition of mix flame retardants such as decabromodiphenyl ether, antimony trioxide and zinc borate as
* Corresponding author at: Engineering Research Center of Transportation Materials of Ministry of Education, Chang’an University, Xi’an 710064, PR China. Tel.: +86 029 82337349. E-mail address: [email protected] (P. Cong). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.05.022
previous research [14,15]. But the effects of flame retardants on the aging resistance have not focused in previous research. This paper investigates flame retardant modified asphalt binders’ aging properties. The rolling thin film oven test (RTFOT) and Pressure Aging Vessel (PAV) were employed to simulate the short term and long term aging. The effects of aging on flame retardancy, physical and rheological properties of asphalt binders were evaluated. The relatively properties of asphalt binders for both flame retardant modified asphalt binder and pure asphalt binder was compared in order to assess the influence of flame retardants on aging resistance of asphalt binder.
2. Experimental 2.1. Raw materials Commercial asphalt was used, and the physical properties of the asphalt were listed in Table 1. Styrene–Butadiene–Styrene (SBS), Grade 1301, was produced by Yueyang Petrochemical Co., Ltd., Hunan Province, China. It was a linear-like SBS, containing 30 wt.% styrene, and the average molecular weight of SBS was 120,000. All of flame retardants were commercial product. 2.2. Methods Asphalt binder was heated to 175 ± 5 °C in an oil-bath heating container until it flowed fully and 3% SBS was added slowly. The
2555
P. Cong et al. / Construction and Building Materials 24 (2010) 2554–2558 Table 1 Physical properties of asphalt.
28
Penetration (25 °C, 0.1 mm) Softening point (°C) Ductility (15 °C) Ductility (5 °C) Viscosity (135 °C, Pa s)
75 46.5 150 21 0.45
26 24 22
Unaging RTFOT aging PAV aging
20
3. Results and discussion
18 0
2
4
6
8
10
Flame Retardant Content (wt%) Fig. 1. Effect of aging on flame retardancy.
when modified asphalt containing the same contents of flame retardants. The improvement of flame retardancy of asphalt binder maybe explained that small molecules in asphalt transform to larger molecules and form the stronger chemical bonds, which pyrolyzing and broken needs high temperature. Consequently, the flame retardancy of asphalt binder was improved by aging. 3.2. Effect of aging on the conventional parameters Softening point increment after aging can reflect the susceptive degree of aging. It can be expressed as DS, and it was calculated as Eq. (1).
DS ¼ Saged Sunaged
ð1Þ
where DS is the softening point increment after aged. Saged and Sunaged is softening point of asphalt binder before and after aged, respectively. The effect of aging on softening point of flame retardant modified asphalt binder and unmodified asphalt binder is shown in Fig. 2. The results showed that softening point of the modified and unmodified asphalt binders have increased after the two different aging, meaning an inherent hardening process of the material in aging. Compared with the asphalt binder without flame retardant, the flame retardant modified asphalt binder displayed lower softening point increment (DS), and the lower DS, the more flame retardant content. Generally, the content of fractions with large molecules in asphalt increases and the content of the small
6
16 RTFOT aging PAV aging
5
ΔS ( ºc )
blends were sheared under 5000 rpm rotation speed for 30 min to ensure the blends became essentially homogenous. The appropriate amounts of flame retardant and stabilizer were added into the blends and mixed for 30 min with a lab mixer set fast enough (usually 1000 rpm) to create a small vortex, without whipping excessive air into the sample. The Rolling Thin Film Oven (Model B1, CONTROL Co., Ltd., Italy) was employed to simulate short term aging according to ASTM D2872. In this test, the 35 g asphalt sample was placed in a glass bottle, which has a narrow top opening. The glass containers were placed in a carriage such that the axis of revolution was horizontal and the container opening was facing a jet of air. The oven was kept at 163 °C and the carriage was rotated in the oven at a rate of 15 rpm for 85 min. The Pressure Aging Vessel (Model PR9300, Prentex Co., Ltd., America) was used for the accelerated aging of asphalts. As soon as the time of RTFOT of asphalt was over, 50 ± 0.5 g of each asphalt sample was immediately poured into two marked pans and placed together in the same PAV for a single test run. When the temperature inside the PAV was within ±2 °C of the aging temperature (109 °C), an air pressure of 2.1 ± 0.1 Map was applied and maintained for 20 h ± 10 min. After 20 h, the pans were removed and poured into different containers for different physical and chemical tests. Flame retardancy of flame retardants modified asphalt binder was evaluated by the limited oxygen index (LOI) using HC-2 oxygen index tester according to ASTM D2863. The physical properties of asphalt binders, including softening point, penetration and ductility, were tested in accordance with ASTM D36, ASTM D5 and ASTM D113-86, respectively. Dynamic shear properties were carried out using AR2000 dynamic shear rheometer (TA Co., Ltd., USA) in a parallel plate configuration with a gap width of 1 mm according to ASTM D6648. Measurements were conducted at fixed frequencies 10 rad/s and temperature from 30 to 95 °C with rate of heating 1 °C/min. The strain value was 0.1%, which was chosen to be as small as possible to ensure measurement in the linear region, but large enough to allow sufficient torque readings. Brookfield viscometer (Model DV-II+, Brookfield Engineering Inc., USA) was employed to measure the rotational viscosity of flame retardant SBS modified asphalt binders in according to ASTM D4402.
15
4
14
3
13
2
12
1
11
3.1. Effect of aging on the flame retardancy Fig. 1 showed the effects of aging on LOI of asphalt binders with different flame retardant content. The results indicated that the aging can increase LOI of asphalt binders. The increasing amount of LOI decreased when the contents of flame retardants was increased, meaning that flame retardant improve the aging resistance of asphalt binders. Compared with RTFOT aging, PAV aging led asphalt binder to further aging. The results showed that the LOI of asphalt binders after PAV aging was largest than others
0
10 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 2. Effect of aging on the softening point of asphalt binder.
ΔS ( ºc )
Measured values
LOI (%)
Physical properties
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P. Cong et al. / Construction and Building Materials 24 (2010) 2554–2558
molecules decreased during the aging. It leads to increase of the DS. However, flame retardant restricts the oxidation of asphalt, which decreased the hardening process of the asphalt. Thus, the results indicated that the flame retardant can improve the thermooxidative aging resistance of asphalt. Penetration ratio (PR) of the aged asphalt with their unaged asphalt can also be able to reflect the change of properties of asphalt binder during aging, and it is calculated as Eq. (2).
Paged 100 Punaged
ð2Þ
where PR is penetration ration of asphalt. Paged and Punaged is penetration of aged and unaged asphalt binder. The effects of aging on the penetration ratio of flame retardant modified asphalt binder and unmodified asphalt binder is shown in Fig. 3. The results showed that penetration of the modified and unmodified asphalt binders have decreased after the two different aging. But the penetration ratio increased with the contents of flame retardant increasing, meaning the degree of aging was decreased. The results also indicated that the thermo-oxidative aging resistance of asphalt was improved when flame retardant was added. Ductility retention rate (DRR) was another parameter that reflect the change of properties of asphalt binder during aging, and it was calculated as Eq. (3).
Daged 100 Dunaged
ð3Þ
where DRR is ductility retention rate of asphalt. Daged and Dunaged is ductility of aged and unaged asphalt binder. The effect of aging on the ductility of asphalt binder is shown in Fig. 4. It can be seen that ductility of asphalt binder clearly decreased after the two different aging, especially PAV aging. But ductility retention rate (DRR) of the asphalt binder increased with the content of flame retardant increasing. The DRR of asphalt binder without flame retardant was 34.8% after RTFOT aging, but it was 57.9% when 10 wt.% flame retardant was added. The results indicated that the addition of flame retardant can reduce deterioration in ductility of asphalt during aging. The ductility result was corresponded with softening point and penetration test results. The viscosity aging index (VAI) is very important parameter to evaluate aging resistance properties of asphalt, and it was calculated as Eq. (4).
40 30 20 10 0 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 4. Effect of aging on the ductility of asphalt binder.
VAI ¼
V aged V unaged 100 V unaged
ð4Þ
where VAI is viscosity aging index of asphalt. Vaged and Vunaged is viscosity of aged and unaged asphalt binder. Fig. 5 showed the effects of aging on the viscosity of asphalt after RTFOT and PAV aging. It was easily found that viscosity aging index (VAI) was gradually decreased with the contents of flame retardant increasing. Those suggested that flame retardant can decreased the viscosity increment during aging and improve the thermo-oxidative aging resistance. During the aging of the asphalt, the naphthene aromatics were converted partly to polar aromatics and polar aromatics turned to asphaltenes. This composition changes make the viscosity of asphalt binder increasing. The results maybe due to the flame retardant obstruct large molecular formation and volatile components loss in asphalt at high temperature. Therefore, flame retardant led VAI to decrease and improve the aging resistance of asphalt binder. 3.3. Effect of aging on the rheological properties The dynamic shear rheometer (DSR) was currently being used by the asphalt community for determination of the viscoelastic properties of asphalt binders according to the recommendations of the Strategic Highway Research Program (SHRP). The Superpave specification parameter G/sin d was identified as the term to be used for high temperature performance grading of paving asphalts
30
100 RTFOT aging PAV aging
80
65 RTFOT aging
25
60
PAV aging
20
VAI (%)
Penetration ratio (%)
RTFOT aging PAV aging
50
60
55 15 50
40 10 20
45
5 0
0 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 3. Effect of aging on the penetration of asphalt binder.
40 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 5. Effect of aging on the viscosity of asphalt binder (at 135 °C).
VAI (%)
DRR ¼
Ductility retention rate (%)
PR ¼
60
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P. Cong et al. / Construction and Building Materials 24 (2010) 2554–2558
DG = sin d ¼ ðG = sin dÞaged ðG = sin dÞunaged
ð5Þ
where DG = sin d is the G /sin d increment after aged. ðG = sin dÞaged and ðG = sin dÞunaged is the G/sin d of asphalt binder before and after aged, respectively. The G/sin d increment (at 70 °C) after two different aging is shown in Fig. 7. Compared with asphalt binder without flame retardant, the G/sin d increment decreased as the flame retardant was mixed. The G/sin d increment of asphalt binder containing 0 wt.% and 10 wt.% flame retardant was 0.6 MPa and 0.49 MPa after RTFOT aging. The PAV aging showed almost similar effects. The 1.9 MPa and 1.47 MPa of G/sin d increment was obtained for the asphalt binder containing 0 wt.% and 10 wt.% flame retardant. Thus, flame retardant decreased the oxidation of asphalt, which reduced the hardening process of asphalt during aging. The bending beam rheometer (BBR) is used to accurately evaluate asphalt binder properties at low temperatures at which asphalt binders were too stiff to reliably measure rheological properties using the parallel plate geometry of the DSR equipment [17,18].
3.5
3.5
3.0 Unaging RTFOT aging PAV aging
2.5
RTFOT aging PAV aging
2.5 2.0 1.5
2.0
1.0
Δ G*/sinδ (MPa)
3.0
G*/sinδ (MPa)
in rating the binders for their rutting resistance [16]. It was known that the temperature of the asphalt binders when G/sin d is equal to 1 kPa is defined as a criterion for the high temperature performance of asphalt. Fig. 6 showed the effects of RTFOT and PAV aging on G/sin d for the 6.5 wt.% flame retardant modified and unmodified asphalt binders. It was clearly observed that the G/sin d of asphalt binder increased when the 6.5 wt.% flame retardant was added and after two different aging. However, compared with the unmodified asphalt binder, the G/sin d of flame retardant modified asphalt binder decreased remarkably with temperature increasing. As a result, the slop of the G/sin d master curve increased with temperature increasing. The increase degree of the G/sin d after aging decreased slightly for flame retardant modified asphalt binders at high temperature. Thus, flame retardant was beneficial to the thermo-oxidative aging resistance of asphalt. Fig. 7 showed that the effects of aging on G/sin d of asphalt binder containing different contents of flame retardant at 70 °C. The results exhibited that the G/sin d of asphalt binder containing different content flame retardant increased after two different aging, especially after PAV aging. But, the slop of the G/sin d decreased with increasing flame retardant concentration, meaning that flame retardant made the asphalt better thermo-oxidation aging resistance. The G/sin d increment after aging can reveal the susceptive degree of aging. It can be expressed as DG/sin d, and it was calculated as Eq. (5).
1.5 0.5 1.0
0.0 0
2
4
6
8
10
Flame retardant content (wt%) Fig. 7. Effect of aging on G/sin d of asphalt binder containing different content of flame retardant (at 70 °C).
The BBR test measured creep stiffness (S) and m-value of asphalt binders. Stiffness indicated the susceptibility to low-temperature cracking as designated by SHRP. The Superpave technology specified that the creep stiffness must not exceed 300 MPa to prevent low-temperature cracking. Since low-temperature cracking occurs only after the pavement has been in-service for some time, this specification addresses these properties using binder aged. The rate of change of asphalt binder stiffness with loading time was represented by the m-value. A high m-value was desired. A minimum m-value of 0.3 after 60 s was required by the Superpave PG binder specification. The effect of aging on stiffness and m-value of asphalt binder at 12 °C is shown in Fig. 8. The results revealed that the stiffness of all asphalt binder satisfies the SHRP Superpave specifications at 12 °C, and all asphalt binders show higher than the required minimum m-value. The high stiffness value was obtained when flame retardant was added into asphalt binder. Those indicate that the asphalt concrete using flame retardant modified asphalt binders maybe susceptible to low-temperature cracking, especially in cold areas and/or weather. Nonetheless, the stiffness and m-value of the flame retardant modified asphalt binders meets the SHRP Superpave specifications at 12 °C. The change of stiffness and m-value of flame retardant modified asphalt binders was not remarkably different with the asphalt without flame retardant. Therefore, the flame retardant does not deteriorate the thermo-oxidation aging resistance of asphalt binders.
100 0.5
360
1
No Flame retardants (NFR) RTFOT aging (NFR) PAV aging (NFR) Flame retardant modified (FRM) RTFOT aging (FRM) PAV aging (FRM)
0.1 48
56
64
72
80
270
0wt% 6.5wt% 10wt%
0.4
0.3
180
0.2
90
0.1
m value
0wt% 6.5wt% 10wt%
10
Stiffness (MPa)
G*/sinδ (MPa)
450
88
Temperature ( ºc )
0.0
0 Unaging
Fig. 6. Effect of aging on G/sin d of asphalt binder (6.5 wt.% flame retardants modified).
RTFOT
PAV
Fig. 8. Effect of aging on stiffness and m-value of asphalt binder (at 12 °C).
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P. Cong et al. / Construction and Building Materials 24 (2010) 2554–2558
4. Conclusions The effects of the thermo-oxidation aging containing RTFOT and PAV aging on the flame retardancy, conventional parameters and rheological properties of asphalt binder with different flame retardant contents were investigated. The flame retardancy of asphalt binder increased after the RTFOT and PAV. The increasing amount of LOI decreased with the flame retardants contents increasing. The softening point and viscosity of asphalt binder increased and penetration and ductility of asphalt binder decreased after two different aging. But the variation of the softening point, penetration, ductility and viscosity of asphalt binders decreased with flame retardant contents increasing, meaning the thermo-oxidative aging resistance of asphalt binder was improved when flame retardant was added. Furthermore, the effects of aging on rheological behaviors of the asphalt binder containing different content of flame retardant were investigated. The flame retardant modified asphalt binders displayed smaller changes in G/sin d after two different aging. The results revealed that the stiffness of all asphalt binder satisfies the SHRP Superpave specifications at 12 °C, and all asphalt binders showed higher than the required minimum m-value. The change of stiffness and m-value of flame retardant modified asphalt binders was not remarkably different with the asphalt without flame retardant. Therefore, the flame retardant does not deteriorate the thermo-oxidation aging resistance of asphalt binders. On the contrary, flame retardant can improve the aging resistance of asphalt binders. Acknowledgements Supported by the Special Fund for Basic Scientific Research of Central Colleges, Chang’an University (Number: CHD 2009JC133). References [1] Xiaohu Lu, Isacsson Ulf. Effect of ageing on bitumen chemistry and rheology. Constr Build Mater 2002;16:15–22.
[2] Stastna J, Zanzotto L, Vacin OJ. Viscosity function in polymer–modified asphalts. J Colloid Interf Sci 2003;259:200–7. [3] Sengoz Burak, Isikyakar Giray. Analysis of styrene–butadiene–styrene polymer modified bitumen using fluorescent microscopy and conventional test methods. J Hazard Mater 2008;150:424–32. [4] Zora V, Chaminda W, Jiri S, Ludo Z. Creep characteristics of asphalt modified by radial styrene–butadiene–styrene copolymer. Constr Build Mater 2007;21:567–77. [5] Giovanni P, Jiri S, Dario B, Ludovit Z. Relation between polymer architecture and nonlinear viscoelastic behavior of modified asphalts. Curr Opin Colloid Interf Sci 2006;11:230–45. [6] Gawel I, Baqinska K. Effect of chemical nature on the susceptibility of asphalt to aging. Petrol Sci Technol 2004;22:1261–71. [7] Bahia HU, Anderson DA. Glass transition behaviour and physical hardening of asphalt binders. J Assoc Asphalt Paving Technol 1993;62:93–129. [8] Simon A, Hesp M. Reversible aging in asphalt binders. Energy Fuel 2007;21:1112–21. [9] Iswandaru Widyatmoko, Richard Elliott. Characteristics of elastomeric and plastomeric binders in contact with natural asphalts. Constr Build Mater 2008;22:239–49. [10] Ruan YH, Richard RD, Charles JG. The effect of long-term oxidation on the rheological properties of polymer modified bitumens. Fuel 2003;82:1763–73. [11] Mastrofini D, Scarsella M. The application of rheology to the evaluation of bitumen ageing. Fuel 2000;79:1005–15. [12] Lee Soon-Jae, Amirkhanian Serji N, Shatanawi Khaldoun, Kim Kwang W. Shortterm aging characterization of asphalt binders using gel permeation chromatography and selected Superpave binder tests. Constr Build Mater 2008;22:2220–7. [13] Wu Shaopeng, Cong Peiliang, Yu Jianying, Luo Xiaofeng. Experimental investigation of related properties of asphalt binders containing various flame retardants. Fuel 2006;85:1298–304. [14] Cong Peiliang, Yu Jiangying, Wu Shaopeng, Luo Xiaofeng. Laboratory investigation of the properties of asphalt and its mixtures modified with flame retardant. Constr Build Mater 2008;22:1037–42. [15] Yu Jiangying, Cong Peiliang, Wu Shaopeng. Investigation of the properties of asphalt and its mixtures containing flame retardant modifier. Constr Build Mater 2009;23:2277–82. [16] Choquet FS, Verhasselt AF. Ageing of bitumen: from the road to the laboratory and vice versa. In: Proceedings of strategic highway research program (SHRP) and traffic safety on two continents, Hague; 1994. p. 193–213. [17] Vlachovicova Zora, Wekumbura Chaminda, Stastna Jiri, Zanzotto Ludo. Creep characteristics of asphalt modified by radial styrene–butadiene–styrene copolymer. Constr Build Mater 2007;21:567–77. [18] Shashidhar N, Shenoy A. On using micromechanical models to describe dynamic mechanical behavior of asphalt mastics. Mech Mater 2002;34:657–69.