Effect of Ultraviolet Aging on Asphalt Rheological Properties

Wang Jiani; Xue Zhongjun; Tan Yiqiu; Zhang Lei

(1. School of Ciνil Engineering, Beijing Jiaotong Uniνersity, Beijing 100044; 2. Beijing Major Projects Construction Headquarters Office, Beijing 100029; 3. School of Communications Science & Engineering, Harbin Institute of Technology, Harbin 150090; 4. Beijing Road Engineering Quality Superνision Station, Beijing 100076)

Effect of Ultraviolet Aging on Asphalt Rheological Properties

Wang Jiani1,2,3; Xue Zhongjun4; Tan Yiqiu3; Zhang Lei3

(1. School of Ciνil Engineering, Beijing Jiaotong Uniνersity, Beijing 100044; 2. Beijing Major Projects Construction Headquarters Office, Beijing 100029; 3. School of Communications Science & Engineering, Harbin Institute of Technology, Harbin 150090; 4. Beijing Road Engineering Quality Superνision Station, Beijing 100076)

This paper simulated the ultraviolet aging process of asphalt and used dynamic mechanic analysis (DMA) method to evaluate the effect of ultraviolet aging on the asphalt rheological properties. After having experienced ultraviolet aging, the low temperature performance of asphalt binder decreased significantly, with its complex modulus increased and phase angle decreased along with changing rheological properties as compared to the performance of original asphalt binder. The ultraviolet aging process would make asphalt binder more sensitive to brittle and fatigue failure. On the basis of the time-temperature superposition principle (TTSP), the viscoelastic transition frequency (ωT) is proposed to evaluate the effect of ultraviolet aging. It is found that with the increase in ultraviolet aging time, theωTmoves to the lower frequency range gradually. Since the viscoelastic transition frequency is sensitive to the effect of aging, it can be used as an indicator of ultraviolet aging.

asphalt; ultraviolet aging; rheological properties; viscoelastic transition frequency

1 Introduction

Asphalt binder is easily to be aged in the field, especially under thermal and UV radiation conditions[1-3]. The aging degree depends on many factors, such as ultraviolet radiation intensity, temperature, oxygen, moisture and chemical composition of the asphalt binder. Photo-oxidation and thermal oxidation are two quite different aging types[4]. The aging of asphalt binder caused by ultraviolet radiation and oxygen is defined as ultraviolet aging. Intensive UV radiation can aggravate the aging of asphalt binder to affect the performance of asphalt significantly[5]. As a result, the durability of pavement is decreased.

The length of solar radiation vary with the altitude, which provides plateaus with a unique climate and environmental features because they are endowed with remarkably more and stronger radiation, longer daylight time, and much higher percentage of UV content, ranging from 20% to 25% of the total solar light, which is five times as that obtained by plains[6-7]. On the other hand, because of the greenhouse effect, the ultraviolet radiation intensity arriving at the earth surface increases year after year. Actually, reduction of ozone concentration happens successively on the globe. Since 1980, the ozone concentration has attenuated by 3%—6% on the north latitude of 25°—60°, and the UV-B intensity has increased by 4%—7% on the earth surface.

In the 1960s, people who worked on asphalt binder aging research already observed sun radiation, particularly ultraviolet radiation, and considered it as one of important factors leading to asphalt binder aging in the service period. However, the current binder performance evaluation system in China takes little consideration on UV aging caused by photo-oxidation. In recent years, both the Harbin Institute of Technology and the Tongji University have developed the accelerated aging equipment to study the aging process of asphalt, and one of their research achievements can simulate ultraviolet aging process. Tan, et al.[8-9]pointed out that asphalt binder shows different sensitivities to thermal ageing and UV radiation ageing. Thermal aging of asphalt could not reflect the influence of ultraviolet aging, and there would be some limitations on substituting thermal ageing for ultraviolet aging. It isfound out that ultraviolet aging has more serious effect than thermal aging, especially with respect to the low temperature performance of asphalt binder. Yamaguchi, et al.[10]separated components of asphalt binders, which were irradiated with ultraviolet (UV) rays to examine the aging mechanism. The results con firmed that saturates are converted to resins and asphaltenes. The observation also suggested that aromatics are converted to resins and asphaltenes by a mechanism differing from that of saturates and found the addition of carbon black could improve the durability of asphalt materials and asphalt pavements. Ortiz and Tauta[11]determined UV radiation’s influence on the mechanical and dynamic property of a dense asphalt mixture and found that the dynamic module of the mixture increased between 90% and 132% with the permanent deformation decreased by 57%, making it a more fragile structure. Zhang, et al.[12]investigated the effect of UV aging on montmorillonite (MMT)/SBS modified bitumen and found that after UV aging, both viscosity aging index and softening point increment of SBS modified bitumen decreased due to the introduction of Na(+)-MMT, which could be further reduced under the in fluence of OMMT.

Although some researches on UV aging have been carried out, most of them only evaluated the effect of UV aging on the asphalt properties with traditional indexes. In this study, the rheological properties of asphalt binders were investigated and a new indictor for evaluating UV aging degree was proposed and discussed.

2 Experimental

2.1 Materials and test methods

Three asphalt binders with a penetration grade of #90 (labeled as 1-9, 2-9, and 3-9, respectively) collected from different refineries were chosen. The basic properties are summarized in Table 1.

Table 1 Basic properties of asphalt binders

2.2 Ultraviolet aging procedure

To simulate the ultraviolet aging of samples in the field environment, the ultraviolet radiation equipment, which has been developed and discussed in another paper[8]by the same group, was used. The ultraviolet aging time was set at 110 hours and 220 hours in laboratory, respectively, making the radiation energy equivalent to the total received energy within 2, 4, 8 and 16 months in the outdoor of Tibet, China.

2.3 Rheological measurements

The low temperature performance of asphalt binder were evaluated with direct tension (DT) test by means of an Interlaken DTT equipment, with the test temperature equating to -6 ℃. The test was carried out according to the speci fication ASTM D6723.

All samples for DMA tests were conducted on a strain controlled rheometer Gemini-150 according to the specifications: ASTM D7175 and ASTM D7552.

The details of tests are described as follows:

1) Strain sweeps at 0.1 and 500 rad/s were performed for each sample, in order to determine the strain level to be used in the frequency sweep test.

2) High temperature rheological tests at 65 ℃ and 10 rad/s were performed on the three binders with different ultraviolet aging times by using the 25-mm diameter parallel plate and 1 000 μm testing gap geometry.

3) The time sweeps (fatigue performance test) were conducted at a constant frequency of 10 rad/s and a temperature of 25 ℃, using the 8-mm diameter parallel plate and 2 000 μm testing gap geometry.

4) The frequency sweep tests were performed under controlled strain loading conditions using frequencies between 0.1 to 100 rad/s with the 8-mm diameter parallel plate and 2 000 μm testing gap geometry at intermediate temperatures such as 15 ℃ and 25 ℃, respectively, and using frequencies between 0.1 to 100 rad/s with the 25-mm diameter and 1 000 μm testing gap geometry at high temperature, such as 35 ℃, 45 ℃, 55 ℃, and 65 ℃, respectively. Based on these frequency sweeps curves,G*Master curves were obtained at 25 ℃ in compliance with the time-temperature superposition principle (TTSP).

3 Results and Discussion

3.1 Performance and rheological properties

3.1.1 Low temperature performance

The typical DT test result is shown in Figure 1.

Figure 1 Typical DT test results

Based on the DT test, the strain energy density can be obtained. The area below the strain–stress curve is defined as strain energy density, which can be calculated from the strain–stress curve shown above. The higher the strain energy density is, the better the low temperature performance will be.

The critical values of strain energy density of the above three kinds of asphalt binders under different aging times are shown in Table 2.

Table 2 Strain energy density of asphalt binderkJ/m2

It can be seen from Table 2 that strain energy density of asphalt binders 1-9 and 2-9 decreased gradually with an increasing aging time. The strain energy density value of asphalt binder 3-9 drastically reduced after aging for 220 h. It can be inferred from Table 2 that after long-term ultraviolet radiation aging, the low temperature performance of asphalt binder decreased significantly.

3.1.2 High temperature performance

G*/sinδ, which is known as the rutting factor, is selected to evaluate the high temperature performance of asphalt binder in this part. The higher theG*/sinδis, the better the high temperature performance of the asphalt binder would be. The test results are shown in Figure 2.

Figure 2 G*/sinδof asphalt binders

It can be found from Figure 2 thatG*/sinδof three aged asphalt binders all increased after ultraviolet aging, which indicates that high temperature rheological properties are changed by ultraviolet aging.

To further investigate the ultraviolet aging effect on the high temperature property of asphalt binders, the changing rate after ultraviolet aging is calculated and shown in Figure 3.

Figure 3 Changing rate after ultraviolet aging of asphalt binders

It can be found from Figure 3 that the aging rate varied with the type of binders at varying aging times. It can be seen from Figure 3 that the aging rate of binder 3-9 was the smallest among the three binders at the first 110 h, and the aging rate of the binder 1-9 was the largest. However, after 220 h of ultraviolet aging, the aging rate of the binder 3-9 increased significantly compared with the other two binders and became the biggest one. Another interesting result was the change rate of the binder 1-9 which was the biggest at the first 110 h, but after 220 h, the change rater of this binder became the smallest one. So the test results demonstrated that different binders showed different antiultraviolet-aging ability that varied with the aging time. To predicate the anti-ultraviolet-aging properties of binderwhich can accurately assess the performance of binders after UV irradiation, the long term ultraviolet aging test should be considered.

3.1.3 Fatigue performance

The fatigue parameter DER (cumulative dissipated energy ratio) is recommended in NCHRP 9-10 project reports for the first time. DER is applied in this paper to analyze the rheological property of ultraviolet aged asphalt binders at intermediate temperature, and DER curves are obtained from the time sweep tests. The index Np20 is defined as the value of abscissa for damage point of DER curve when the DER curve deviates from the straight line (DER=N) by 20%. Np20 reflects the fatigue life of asphalt binder, and denotes the loading time when the asphalt binder is damaged.

After ultraviolet aging, asphalt binder becomes hard and brittle coupled with changes in its mechanical behavior. The performance of the asphalt binder would degrade at low and intermediate temperature, leading to a worse fatigue property. Np20 is a suitable index used to evaluate the fatigue properties of asphalt binder. Figures 4 to 6 show changes in Np20 values of UV aged binders at 25 ℃.

Figure 4 Np20 of asphalt binder 1-9 under time sweeps at 25℃

Figure 5 Np20 of asphalt binder 2-9 under time sweeps at 25℃

Figure 6 Np20 of asphalt binder 3-9 under time sweeps at 25℃

Ultraviolet aging decreases the asphalt fatigue resistant ability. The value of Np20 becomes lower after UV aging. Figures 4 to 6 indicate that the fatigue performance is down for all asphalt binders. It is learned from test results and the above-mentioned analysis that rheological properties of asphalt binder samples degrade with an increasing ultraviolet aging time. And the comparative analyses of the three parameters for the asphalt binder samples are summarized in Table 3.

Table 3 Performance decreasing with ultraviolet aging time

Ultraviolet radiation enhances aging of asphalt binders and also changes its deformability. With an increasing aging time, its low-temperature performance and fatigue performance become worse and worse, which make materials prone to brittle failure. Test results also show that asphalt samples produced by different refineries have different ultraviolet aging sensitivity.

3.2 Effect of ultraviolet aging on viscoelastic behavior

As a complicated macromolecular colloid, the physical and mechanical characteristics of asphalt have close connection with its chemical components. Ultraviolet radiation can make some chemical bonds broken, and change the components of asphalt binder, which results in the change of rheological properties of asphalt binder. The master curves of phase angle (δ) generated by TTSP are shown in Figures 7—9, respectively.

Figure 7 Phase angle master curves of asphalt binder 1-9

Figure 8 Phase angle master curves of asphalt binder 2-9

Figure 9 Phase angle master curves of asphalt binder 3-9

It can be seen from the curves that phase angle reduces with the increase of aging time. The value ofG″/G′ becomes smaller, which means that the elasticity increases while the viscosity decreases gradually. Accordingly, the ultraviolet aged binders need longer time to release the thermal and loading stress which makes asphalt binder more easily damaged at low temperature than those without being subjected to ultraviolet aging.

3.3 Ultraviolet aging evaluation index

Asphalt binders exhibit different mechanical behaviors that are dependent on aging time and temperature, when they are working in elastic, visco-elastic and viscous regions, respectively. Asphalt binder shows stress relaxation related mechanical behavior in the elastic region at low temperature or under high speed load, shows creeping mechanical behavior in the viscous region at high temperature or under repeated load, and shows both stress relaxation and creeping mechanical behavior in the viscoelastic transition region under repeated load at medium temperature. The master curve ofG*for an asphalt binder is given in Figure 10, in which the three mechanical behaviors are shown clearly.

Figure 10 Complex modulus master curve based on Time-Temperature Equivalence Principle

3.3.1 Viscosity-elastic transition frequency (ωT)

Extending tangents ofG*curve in the elastic and viscous region will cross the viscoelastic region. The frequency corresponding to the junction of two tangents is named the viscoelastic transition frequency (denoted asωT, as shown in Figure 10). The tangent ofG*in the elastic region is equivalent to the complex modulus when the loading action time is equal to zero, while the tangent ofG*in the viscous region is equivalent to the complex modu-lus when the loading action time is close to permanent. Therefore,ωTas the junction of these two tangents of complex modulus in asphalt binder’s working region reflects the transition of asphalt binder from viscous region to elastic region, and can be used to evaluate binder’s basic viscoelastic mechanical behavior in combination with the shape ofG*master curve.

3.3.2 Transformation ofωTafter ultraviolet aging

It is validated that asphalt binder becomes much more stiff and brittle after UV aging as evidenced by many tests and investigations in the field. The viscoelastic transition frequency (ωT) provides a theoretic method to evaluate the mechanical behavior of ultraviolet aged asphalt binder.

In order to analyze the effect of ultraviolet aging onG*master curve and evaluate the visco-elastic mechanical behavior of asphalt binder with different aging times, this paper carried out frequency sweeps at different temperatures. Master curves were obtained at 25 ℃ by using the time-temperature superposition principle (TTSP), as shown in Figures 11—13, respectively. And theωTvalues of the asphalt samples are shown in these figures as well. It can be seen from the data of three figures that the shapes ofG*master curves become more flat with an increasing aging time as compared with unaged asphalt binder, which means that the thermal susceptibility of asphalt binders becomes lower after ultraviolet aging.

Figure 11 Complex modulus master curves of asphalt 1-9 obtained at different aging times

Figure 12 Complex modulus master curves of asphalt 2-9 obtained at different aging times

Figure 13 Complex modulus master curves of asphalt 3-9 obtained at different aging times

It can be found from the above-mentioned figures thatωTgradually moves from high frequency region to low frequency region with an increasing aging time, denoting that the asphalt binder shifted from the viscous state to the elastic state in the low frequency region after ultraviolet aging.

Judging from a viscoelastic point of view, low frequency is equal to high temperature, which gives a good explanation on why the ultraviolet aged binder is easier to be destroyed at low temperature than the unaged binder.

Table 4 TheωTof asphalts binder aged with different ultraviolet exposure time

The calculatedωTvalues for the asphalt binder samples are given in Table 4. It is noted that the values ofωTfor three asphalt binders are more similar when ultraviolet aging is equal to 110 hours. When the ultraviolet aging time increases to 220 h, the value ofωTof asphalt binder 1-9 is the biggest, andωTof asphalt binder 2-9 is the smallest. The results show thatωTof three asphalt bind-ers decreases at different extent after ultraviolet aging for 220 h, which means that different asphalt binder has different anti-ultraviolet aging ability. These results are consistent with DT test results. It is proved thatωTcan be used to evaluate the aging degree and can well explain the viscoelastic behavior of ultraviolet aged asphalt binder.

4 Conclusions

1) The rheological properties of asphalt binder decrease significantly after ultraviolet aging. The asphalt performance changes with increase of aging time. Deformation ability is damaged seriously, making ultraviolet aged asphalt more brittle than the unaged one under the same condition.

2) Viscoelastic behavior of asphalt binder changed by ultraviolet aging: the viscosity-elastic transition frequency (ωT) gradually moves to low frequency range (which corresponds to high temperature), which means that asphalt binder’s properties are transformed from the viscous state to the elastic state at lower frequency after ultraviolet aging, which makes asphalt more easily damaged at low temperature.

3) The results of rheology study are consistent with ASTM tests, denoting that the rheology methodology index,ωT, can also be used to evaluate the ultraviolet aging effect on asphalt binder.

Acknowledgements:The study described in this paper is supported by the National Natural Science Foundation of China (NNSFC, Grant No.50808058), the New Century Excellent Talents in University, 2007(NCETU), the National Science Foundation for Post-doctoral Scientists of China (NSFPSC, Grant No.20080430925), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (SRFDPHEC, Grant No.200902403). The authors are very grateful for their supports on this study.

[1] Huang S C, Grimes W. Influence of aging temperature on rheological and chemical properties of asphalt binders [J]. Transportation Research Record, 2010, 2179(1): 39-48

[2] Lee S J, Amirkhanian SN, Kim K W. Laboratory evaluation of the effects of short-term oven aging on asphalt binders in asphalt mixtures using HP-GPC [J]. Construction and Building Materials, 2009, 23(9): 3087-3093

[3] Colbert B, You Z P. The properties of asphalt binder blended with variable quantities of recycled asphalt using short term and long term aging simulations [J]. Construction and Building Materials, 2012, 26(1): 552-557

[4] Martin K G. Photo-oxidation of asphalt [R]. American Chemical Society, 1971: 85

[5] Zhang F, Yu J Y, Han J. Effects of thermal oxidative ageing on dynamic viscosity, TG/DTG, DTA and FTIR of SBS- and SBS/sulfur-modified asphalts [J]. Construction and Building Materials, 2011, 25(1): 129-137

[6] Zhou Yunhua. Ultraviolet solar radiation in China [J]. Acta Geographica Sinica, 1986, 41(2): 132-146 (in Chinese)

[7] Xiao Yide, Wang Guangyong, Li Xiaogang. Corrosion behavior of atmospheric environment and corrosion feature of materials in our western area [J]. Journal of Chinese Society for Corrosion and Protection, 2003, 23(4): 248-255 (in Chinese)

[8] Tan Yiqiu, Wang Jiani, Feng Zhongliang, et al. Ultraviolet aging mechanism of asphalt binder [J]. China Journal of Highway and Transport, 2008, 21(1): 19-24 (in Chinese)

[9] Tan Yiqiu, Feng Zhongliang, Zhou Xingye. Evaluation method study of anti-ultraviolet aging ability of asphalt [EB/ OL]. Science Paper Online, 2003, http://www.paper.edu.cn/ releasepaper/content/2003-10-31

[10] Yamaguchi K, Sasaki L, Meiarashi S. Mechanism of asphalt binder aging by ultraviolet irradiation and aging resistance by adding carbon black [J]. Journal of the Japan Petroleum Institute, 2004, 47(4): 266-273

[11] Ortiz O, Tauta J. Effect of ultraviolet radiation on an asphalt mixture’s mechanical and dynamic properties [J]. Revista Ingenieriae Investigacion, 2008, 28(3): 22-27

[12] Zhang H L, Yu J Y, Wu S P. Effect of montmorillonite organic modification on ultraviolet aging properties of SBS modified bitumen [J]. Construction and Building Materials, 2012, 27(1): 553-559

Recieved date: 2013-07-01; Accepted date: 2013-08-26.

Zhang Lei, E-mail: hit.andy@foxmail.com.