J. Cent. South Univ. Technol. (2008) 15(s1): 111-114
DOI: 10.1007/s11771-008-326-y
Effect of ageing on fatigue properties of asphalt
WANG Ji(王 佶), PANG Ling(庞 凌), WU Shao-peng(吴少鹏), LIU Quan-tao(刘全涛), CHEN Zheng(陈 筝)
(Key Laboratory of Silicate Materials Science and Engineering, Ministry of Education,
Wuhan University of Technology, Wuhan 430070, China)
Abstract: The fatigue properties of asphalts were investigated after various laboratory simulation ageing tests and outdoor natural exposure ultraviolet radiation ageing, by dynamic shear rheometer (DSR) time sweep fatigue test in constant strain model and a new type of specimen which was introduced to avoid the problem of adhesion failure between rotor and asphalt binder. The results show that outdoor natural exposure ageing (NEA) causes the decrease of retained fatigue life distinctly, and photodegradation caused by outdoor NEA of 1 250 ?m thin films asphalt for three months, is found to be severer than pressure ageing vessel (PAV) with respects to retained fatigue life. The effect of photodegradation increases as the time of outdoor NEA increases. DSR time sweep fatigue test in constant strain indicates that the aged styrene-butadiene-styrene (SBS) modified asphalt still displays better fatigue properties than the corresponding base asphalt after ageing.
Key words: asphalt; SBS modified asphalt; ageing; time sweep test; fatigue property
1 Introduction
The hardening of asphalt is a result of ageing in the process of asphalt mixture construction (short-term ageing) and in the service (long-term ageing), which can result in the changes of the fatigue properties of road asphalts[1-2]. In laboratory simulation ageing tests, rolling thin film oven test (RTFOT) is used for examining the thermal degradation during short-term ageing, and the pressure ageing vessel (PAV) test is used for simulating long-term ageing[3-4]. Besides heating and oxidation, the effects of ultraviolet radiation on asphalt pavement materials, which are relative to conditions in service, have gained attention in recent years[5-7]. Our objective is to study the evolution of fatigue properties of asphalts under different ageing conditions, especially photo degradation of asphalts. Superpave fatigue parameter of asphalts, G*sinδ, has been applied to evaluate the fatigue properties of aged asphalt binders. However, which is received considerable criticism as a specification requirement for its defect in reflecting the accumulation process of fatigue damage[8-9].
In this paper, the effects of ageing on asphalt fatigue properties were investigated using dynamic shear rheometer (DSR) time sweep fatigue test on a new type of specimen in constant strain model. The new type of specimen was introduced to avoid the problem of adhesion failure between rotor and binder and to test the fatigue response. Special attention was given to ageing methods that affect the fatigue properties of asphalt binders. Two standard methods, RTFOT and PAV were used to age asphalt. The results of both these artificial types of ageing were compared to those obtained from outdoor natural exposure ageing (NEA) during the summer time to determine the effects of different oxidation conditions on the fatigue properties of asphalts.
2 Experimental
The SBS modified asphalt PG76, PG70 and their base asphalt AH70 used in this study came from Koch Asphalt Co.Ltd, China. For AH70, the penetration at 25 ℃ is 730.1 mm, with ductility of more than 150 cm at 15 ℃ and softening point of 48 ℃; For PG76, the penetration at 25 ℃ is 520.1 mm, with ductility of 37 cm at 5 ℃ and softening point of 86 ℃; For PG70, the penetration at 25 ℃ is 620.1 mm, with ductility of 33 cm at 5 ℃ and softening point of 92 ℃.
PG76, PG70 and AH70 were aged in RTFOT (75 min, 163 ℃ and air flow at 4 L/min) according to ASTM D2872, which is supposed to simulate the ageing during mixing, reserving and transporting. The aged asphalts obtained from RTFOT were further subjected to age in PAV (20 h, 100 ℃ and air pressure at 2.1 MPa) according to AASHTO PP1 for field ageing.
For the NEA test, the asphalt specimens with film thickness of 1 250 μm were flatly mounted on the roof of a building, located in Wuhan of China (Location: latitude 30?35′ north, Longitude 114?19′ east; Temperature: (20±15) ℃). The specimens were exposed to natural sunlight and atmospheric conditions from Aug. 2006. A glass plate with UV transmittance of 70% in 300-400 nm region was set with a 5 mm gap above the specimen to prevent the contamination from dirt and water that would damage the samples. The maximum surface UV intensity of the specimens was around 12 W/m2. The natural exposure had been carried out for one year and the specimens were analyzed at intervals of 3 months.
To avoid the problem of adhesion failure between rotor and bituminous binder, a new type of specimen was introduced into the fatigue test. The process of sample molding can be illustrated as follows: first heating to 170 ℃ for SBS modified asphalt, or to 160 ℃ for base asphalt, then molding the specimen in a knock-down molding clamp, finally knocking down the clamp to obtain the asphalt specimen. The total height of the specimen is 20 mm, in which a column with 10 mm in height and 6 mm in diameter is the effective section for testing. At each end, a steel ring with inter-diameter of 7 mm, outer-diameter of 8 mm and height of 4 mm was placed for the purpose of fastening. Considering that the elastic-visco characteristics of asphalt binders would result in contact loose due to relaxation effect, the clamps designed did not touch the asphalt specimen directly, but two steel rings at both ends. The failure way of asphalt specimen is shown in Fig.1. It can be found that the adhesion between asphalt binder and steel ring was strong enough and no such adhesion failure was observed during all tests.
A DSR (MCR101, Anton Paar, made in Germany) time sweep fatigue test was used to measure fatigue properties of asphalt specimens by means of the drop of complex modulus. Measurements were conducted at 10 ℃ and 10 Hz, fitted with a suit of self-made clamp. All of the tests were conducted under strain-controlled mode and the applied strain was kept within the linear viscoelastic range. For pure asphalt, a 0.5% strain was used. For SBS modified asphalt, a 0.5% or 2% strain was used. The decrease of complex modulus with load repetitions can be used as an indicator of fatigue. The fatigue failure point was defined as the point where the complex modulus decreased to 50% of the initial complex modulus.
Fig.1 Photo of failure specimen
3 Results and discussion
In this study, DSR time sweep fatigue test was applied to monitor the evolutions of fatigue properties of base asphalt AH70 and its SBS modified asphalt PG76, PG70 before and after RTFO, PAV and NEA ageing.
3.1 Evolution of retained fatigue life of base asphalt AH70
The complex modulus changes of AH70 with load time are shown in Fig.2. It can be seen that the load time decreases with the sequence of the conditions of un- ageing, RTFOT ageing, PAV ageing and NEA ageing, respectively. Obviously, lesser load time appeared after ageing, especially after NEA ageing.
Fig.2 Complex modulus as function of load time for AH70 before and after ageing
The retained fatigue life was obtained by calculating the product of load time and frequency. Fig.3 shows the data about the retained fatigue life changes of aged asphalts. It can be seen that for base asphalt AH70, the retained fatigue life decreases to 50.5% after RTFOT, to 36.9% after PAV, which indicates that the effect of ageing on fatigue properties of asphalt are distinct, and the hardening of asphalt results in the decrease of fatigue life in strain controlled mode. The more severely the oxidation reaction is carried out, the shorter the retained fatigue life is.
Fig.3 also indicates that the fatigue life after 3- month NEA ageing is nearly the same as that after PAV ageing, and the fatigue life decreases with the extension of ageing time and reaches a minimum value at 12-month NEA ageing. In other words, NEA ageing in the service has great importance on the fatigue life of asphalts.
Fig.3 Retained fatigue life of AH70 before and after ageing
3.2 Evolution of retained fatigue life of SBS modified asphalt
At strain of 0.5%, SBS modified asphalts PG76 and PG70 took much more load times to the fatigue failure point than AH70 before and after ageing. It means that the SBS modified asphalts have better fatigue properties than their corresponding base asphalt before and after ageing. This observation is consistent with BAHIA’s report [10-11].
Fig.4 indicates the result of retained fatigue life for PG76 before and after ageing at 2% strain. It can be observed that the retained fatigue life gradually decreased with the extension of ageing. From the result of Fig.4, it is clear that the retained fatigue life decreases with the sequence of the conditions of un-ageing, RTFOT ageing, PAV ageing and NEA ageing, respectively.
Fig.4 Retained fatigue life of PG76 before and after ageing
Fig.4 shows the detailed data about the changes of retained fatigue life of PG76. For PG76, the fatigue life decreases to 23.4% after RTFOT, to 7.1% after PAV, and the fatigue life of PG76 after 3 months NEA ageing is shorter than that after PAV ageing. The fatigue life decreases with the extension of ageing time and reaches a minimum value at 12 months NEA ageing, which indicates that the effect of NEA ageing on fatigue properties of SBS modified asphalt is distinct.
From the change tendency of retained fatigue life of SBS modified asphalt it can be found that the retained fatigue life decreases with the extension of oxidation reaction. Obviously, the retained fatigue life of the SBS reduced with asphaltenes increase in base asphalt and SBS degradation.
Fig.5 reveals the retained fatigue life for PG70 before and after ageing. The similar tendency can be obtained as PG76, and the decreasing extent of PG70 is nearly the same as that for PG76 after RTFOT, PAV, NEA. The detailed data were calculated and summarized in Table 1.
Fig.5 Retained fatigue life of PG70 before and after ageing
Table 1 Changes of retained fatigue life of SBS modified asphalts before and after ageing
4 Conclusions
1) A DSR time sweep fatigue test was used to measure the effect of ageing on the fatigue properties of asphalt specimens by means of a constant repeated strain. And a new type of specimen was introduced to avoid the problem of adhesion failure between rotor and bituminous binder.
2) The hardening of asphalt resulted in decrease of the fatigue life in strain controlled mold. The severer the oxidation reaction is carried out, the shorter the retained fatigue life is. The effect of NEA ageing on fatigue properties of asphalt is distinct.
3) The fatigue properties of SBS modified asphalt excel those of base asphalt before and after ageing.
References
[1] WHITEOAK O. Handbook of shell asphalts [M]. London: London Shell Asphalt UK, 1991.
[2] RUAN Yong-hong, DAVISON R R, GLOVER C J. Oxidation and viscosity hardening of polymer-modified asphalts [J]. Fuels, 2003, 17(4): 991-998.
[3] Annual Book of ASTM Standards D 6373. Standard specification for performance graded asphalt binder [M]. 1999.
[4] SHRP superior performing asphalt pavements (superpave): the product of SHRP asphalt research program[M]. Washington, DC: National Research Council, 1994.
[5] DURRIEU F, FARCAS F, MOUILLET V. The influence of UV ageing of a SBS modified asphalt: Comparison between laboratory and on site aging [J]. Fuel, 2007, 86(10/11): 1446-1451.
[6] BOCCI M, CERNI G. The ultraviolet radiation in short-term and long-term aging of asphalt[C]// Proc 2nd Eurasphalt & Eurobitume Congress, Session 1: Performance Testing and Specifications for Binder and Mixtures. Barcelona, 2000: 49-58.
[7] GLOTVOA N A, KATS B I, GORSHKOV V S. Photooxidation of asphalts in thin films[J]. Khimiya i Tekhnologiya Topliv i Mase, 1974, 11: 45-48.
[8] BAHIA H U, HANSON D I, ZENG M, ZHAI H. Characterization of modified asphalt binders in superpave mix design[M]. National Academy Press, 2001: 60-65.
[9] ANDERSON D A, et al. Prediction of fatigue damage in binders. Western Research Institute Fatigue Symposium. Laramie: Wyoming, 2001.
[10] ANDERSON D A, CHRISTENSEN D W, BAHIA H U, DONGRE R, SHARMA M G, BUTTON J J. Binder characterization and evaluation.Volume 3: Physical characterization, strategic highway research program[M]. Washington, DC: National Research Council, 1994.
[11] BAHIA H U, HANSON D I, ZENG M, ZHAI H, KHATRI M A, ANDERSON R M. Characterization of modified asphalt binders in superpave mix design[R]. NCHRP Report 459, Transportation Research Board, Washington, D.C., 2001.
(Edited by LONG Huai-zhong)
Foundation item: Projects(2006BAB04A05) supported by the Eleventh Five-Year Plan of the National Key Technology Research and Development Program
Received date: 2008-06-25; Accepted date: 2008-08-05
Corresponding author: PANG Ling, PhD Candidate; Tel: +86-27-87162595; E-mail: hmywj@263.net
