J. Cent. South Univ. Technol. (2008) 15(s1): 118-121

DOI: 10.1007/s11771-008-328-9

Investigation of rheological properties of TPS modified bitumen

LIU Quan-tao(刘全涛), WU Shao-peng(吴少鹏), LIU Cong-hui(刘聪慧), WANG Jin-gang(王金刚)

(Key Laboratory of Silicate Materials Science and Engineering of Ministry of Education,

Wuhan University of Technology, Wuhan 430070, China)

Abstract: Rheological properties of the virgin bitumen and TPS modified bitumen binders with several percentages of TPS additives were studied. All TPS modified bituminous binders were prepared on a laboratory scale. Dynamic shear rheometer (DSR) strain sweep test was made to measure the linear viscoelasticity areas of various bitumen binders at -20-70 ℃, then temperature sweep test and frequency sweep test were made in the linear viscoelasticity areas. Complex modulus master curves were drawn to analyze and compare various bitumen binders’ rheological properties. Based on the test results, the ideal percentage of TPS additive was brought forward. The results show that TPS modified bitumen binders have more excellent properties at high, medium and low temperatures compared with original bitumen. The dosages of TPS additive are vital to their properties.

Key words: TPS modified bitumen binders; temperature sweep test; frequency sweep test; rheological properties

1 Introduction

TPS contains a thermoplastic elastomer as a main gradient, which, in general, is hard to dissolve in asphalt. However, carrying out a polymer blending together with an adhesion resin, a plasticizer, becomes possible to be dissolved into straight asphalt and to modify it into high viscosity asphalt of high quality^[1]. TPS modified bitumen has wide applications in recent years to improve the rutting performance of pavement^[2]. The objective of this paper is to investigate the high temperature rutting resisting properties and low temperature cracking resisting properties of the TPS modified bitumen with several percentages of TPS modifier through the rheological testing using dynamic shear rheometer^[3-5]. The virgin bitumen and SBS modified bitumen were also tested to make comparisons with the TPS modified bituminous binders. Firstly, strain sweep tests were conducted to measure the linear viscoelasticity areas of various bituminous binders, and then investigated the high temperature rutting resisting properties, medium temperature fatigue properties and low temperature cracking resisting properties of all bituminous binders in the linear viscoelasticity areas by temperature sweep tests and frequency sweep tests^[6-11].

2 Bituminous binders

Five types of bitumen, i.e., the original, 8% TPS, 12% TPS, 16% TPS modified and the SBS modified PG76-22, were used. The penetration grades of them were 72.5, 49.4, 43.6, 40.2 and 50.2, respectively. The virgin bitumen AH-70 and the SBS modified bitumen come from Nanyue Ltd., Co. in Guangdong Province and Shell Asphalt (Ezhou) Ltd., Co. in Hubei Province respectively. The TPS modified bitumen was prepared on a laboratory scale. The virgin bitumen was used to prepare the TPS modified bitumen binders by adding 8%, 12% and 16% of the TPS modified bitumen binder, respectively, by a constant mixing at 160 ℃ using the high shear emulsifying machine. To obtain the homogenous bitumen-TPS mastics, the TPS was added slowly into the preheated pure bitumen and mixing them for 2 h. The SBS modified PG76-22 bitumen was introduced in the tests to make a comparison with the properties of TPS modified bituminous binders.

3 Preparation of apparatus and specimens

The rheological test involves determination of the complex shear modulus and phase angle of each binder in various conditions. The dynamic shear rheometer (DSR, Austria, Anton Paar) with parallel plate geometry was used to determine the rheological characteristics of bituminous binders. The tests at temperature from -20 ℃ to 40 ℃ and from 30 ℃ to 80 ℃ were conducted on the 8 mm plate with the gap of 2 mm and on the 25 mm plate with the gap of 1 mm respectively in strain controlled mode. About 0.6 g or 0.3 g sample was put onto the lower plate with diameter of 8 mm or 25 mm respectively. After the sample was heated to flow, the upper parallel plate was lowered to contact tightly with the sample for trimming the sample, finally reached the right position to get the desired gap.

4 Laboratory tests and analysis

4.1 Strain sweep test

The stress—strain relationship tests were conducted in dynamic shear rheometer (DSR) strain sweep mode to measure the linear viscoelasticity areas of various bituminous binders so as to make sure all the following temperature sweep tests and frequency sweep tests were conducted in the linear viscoelasticity areas.

Fig.1 shows the stress-strain relationship of original bitumen from -20 ℃ to 40 ℃. The same tests conducted on the modified bitumen show that they have broader linear viscoelasticity areas than that of original bitumen. It is clear that high temperature leads to a broader linear viscoelasticity area for each bituminous binder. Based on the chart, the linear viscoelasticity area of original bitumen and ascertain the proper strain level can be found for each temperature sweep test and frequency sweep test.

Fig.1 Stress—strain relationship curves

Table 1 shows the strain level of frequency sweep test for each temperature.

Table 1 Strain level of frequency sweep test for each temperature

To make the tests much easier, the same strain level was chosen for both original bitumen and modified bituminous binders at the same temperature.

4.2 Temperature sweep test

The temperature sweep tests were conducted at strain of 0.2% and 10% for lower temperature(from -20 ℃ to 40 ℃) and higher temperature(from 30 ℃ to 80 ℃) respectively at a fixed frequency of 10 rad/s.

Figs.2-3 indicate that the complex modulus and rutting parameter of binders show greater difference at higher temperature and increase with the increase of the addition of TPS modifier, that is to say, the dosage of TPS modifier has great importance on the high temperature properties of TPS modified bituminous binders, and the rutting-resisting properties of bitumen can be greatly improved by adding TPS modifier. Concretely, the high temperature property of SBS modified bitumen is between the property of 8% TPS modified bitumen binder and the property of 16% TPS modified bitumen binder, and the high temperature properties of 16% and 20% TPS modified bitumen binders are close to each other, which shows very little difference when temperature is higher than 60 ℃. So 16% is the ideal percentage to improve the rutting- resisting property of bitumen.

Fig.2 Complex modulus—temperature curves

Fig.3 Rutting parameter—temperature curves

Figs.4-5 show that the complex modulus and storage modulus of binders decrease with the increase of the percentage of TPS modifier at the temperature blew 20 ℃, which indicates that the elastic property of bituminous binders can be significantly improved by the modification of TPS modifier, especially at low temperatures. It can be seen that the storage modulus of original bitumen, SBS modified bitumen and 8% TPS modified bitumen show little difference in low temperature, however, the storage modulus of 16% TPS modified bitumen and 20% TPS modified bitumen has a sharply decrease, which indicates that 16% TPS modified bitumen and 20% TPS modified bitumen have better low temperature cracking-resisting property. It should be noted that the low temperature property of SBS modified bituminous binder is also between the value of the 8% TPS and 16% TPS modified bitumen binders. The difference of the low temperature property between the 16% TPS modified bitumen and 20% TPS modified bitumen is very little, so 16% is also an adequate percentage to improve the cracking-resisting property of bitumen.

Fig.4 Complex modulus—temperature curves

Fig.5 Storage modulus—temperature curves

4.3 Frequency sweep test

The frequency sweep tests were conducted over a range of test frequencies (from 100 rad/s to 0.1 rad/s) and temperatures( from -20 ℃ to 70 ℃) at a constant strain for each temperature as is given above. Fig.6 shows the relationship between complex and angular frequency at different temperatures, and Fig.7 shows the complex modulus master curves gained according to the time temperature equivalence principle. Based on the complex modulus master curves, it can be evaluated that the properties over a much wider span of frequency ranging from 10^-6 rad/s to 10⁶ rad/s.

Fig.6 Complex modulus—angular frequency curves at different temperatures

Fig.7 Complex modulus curves

It is clear that the TPS modified bitumen and SBS modified bitumen show less frequency dependence than original bitumen. Generally, compared with the original bitumen, the TPS and SBS modified bituminous binders show higher complex modulus at the low angular frequency end and lower complex modulus at the high angular frequency end. According to the time temperature equivalence principle, low angular frequency corresponds to high temperature and high angular frequency corresponds to low temperature, so it can be concluded that the modified bitumen has both better rutting-resisting property at high temperature and better cracking-resisting property at lower temperature. This conclusion accords with the results of temperature tests. Particularly, the frequency dependence of binders decrease with the increase of the addition of TPS modifier and the properties of SBS modified bitumen binder are also between those of 8% TPS and 16% TPS modified bitumen binders. The difference of properties between 16% TPS modified bitumen and 20% TPS modified bitumen is inconspicuous, so 16% can be served as an adequate percentage to improve the cracking-resisting and rutting-resisting properties of bitumen.

Fig.8 shows the relationship between loss modulus of various bituminous binders and testing frequency, which can reflect the fatigue properties of binders to some extent. It can be seen that the loss modulus of bituminous binders decrease with the increase of the addition dosage of TPS modifier at all frequencies, that is to say, the lowest temperature at which the binders have fatigue-resisting properties can greatly decline by adding TPS modifier and this fatigue property strengthening effect tends to be more distinct at high frequency or low temperature. Once again, the fatigue property of SBS modified bitumen binder is also better than that of    8% TPS bituminous binders and worse than that of 16% TPS modified bituminous binders, and the difference of properties between 16% TPS modified bitumen and 20% TPS modified bitumen is inconspicuous.

Fig.8 Loss modulus—angular frequency curves at 10 ℃

5 Conclusions

1) TPS Modifier can improve the high temperature rutting-resisting property, medium temperature fatigue-resisting property and low temperature property of bitumen.

2) All the high temperature property, medium temperature property and low temperature property of SBS modified PG76-22 bitumen are between those of 8% TPS and 16% TPS modified bituminous binders.

3) 16% is an adequate percentage to improve the cracking-resisting and rutting-resisting properties of bitumen.

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(Edited by CHEN Can-hua)

Foundation item: Project(NCET-05-0656) supported by Education Ministry for the New Century Excellent Talents, China

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: LIU Quan-tao, Doctoral candidate; Tel: +86-15927195410; E-mail: liuqt@whut.edu.cn