J. Cent. South Univ. Technol. (2008) 15(s1): 434-437

DOI: 10.1007/s11771-008-394-z

Shear resistance properties of TPS modified bitumen binders and

 asphalt mixtures

CAO Ting-wei(曹庭维), WU Shao-peng(吴少鹏), LIU Cong-hui(刘聪慧), ZHANG Tao(张 涛)

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

Wuhan University of Technology, Wuhan 430070, China)

Abstract: Shear resistance properties of the virgin bitumen and modified bitumen binders with Tafpack Super (TPS) modifier and SBS modified bitumen were discussed. Dynamic shear rheometer (DSR) was used to measure the laboratory creep data for these binders over a wide range of constant shear stresses at 20 ℃ to characterize the shear creep behaviors of all kinds of asphalt binders, and the rutting test system was used to investigate the permanent deformation of porous asphalt mixtures using the above bitumen binders for a fixed compressive stress. The shear strain rate and shear creep modulus were used to characterize the shear creep behavior of the TPS modified bitumen, and the rutting test results were used to show the consistency of porous asphalt mixtures with the bitumen binders. Results indicate that a distinction of shear creep strain can be made among different contents of TPS modified bitumen at the same stress level, where the shear creep strain—time response curve of the SBS modified bitumen binder is between the curves of the 8% TPS and 12% TPS modified bitumen binders. The shear strain rate and the shear creep modulus of the TPS modified bitumen binders are obtained to compare with those of the SBS modified bitumen binder which results in the same trend as the shear creep strain-time response curve. Permanent deformation results of all the porous asphalt mixtures from the rutting test show reasonable agreement with the findings of the shear strain rates and shear creep modulus over the range of shear stress levels.

Key words: TPS modifier; bitumen binders; asphalt mixtures; shear resistance properties

1 Introduction

In southern China, one of the most common forms of road failure is rutting (permanent deformation), where material from the wheel path of a heavy vehicle flows and compacts to form a groove or rut^[1]. Rutting is, by far, the most important distress that should be taken into consideration when designing asphalt concrete pavements^[2]. Permanent deformation in bituminous materials is mainly due to densification and shear displacements, although there may also be some densification under initial traffic, particularly for those pavements not adequately compacted during construction^[3].

To improve the rutting performance, the modified bitumen and bitumen additives were used^[4]. Tafpack super (TPS) is one bitumen modifier, in the form of pellet with 2-3 mm diameter that makes possible to modify the straight asphalt into the high viscosity However, carrying out a polymer blending together with an adhesion resin, a plasticizer, it becomes possible to be dissolved into asphalt straight and to modify it into high viscosity asphalt of high quality^[5].

The objective of this paper is to investigate the use of dynamic shear rheometer (DSR) and rutting test system to characterize the sheer resistance properties^[6] of TPS modified asphalt binders and asphalt mixtures with TPS modifiers, respectively. The virgin bitumen and SBS modified bitumen were also tested to compare with the modified bitumen binders and asphalt mixture using TPS. Shear strain rates and shear creep modulus from the creep test were measured to evaluate the effect of the TPS modifier. The rutting performance of the asphalt mixtures using TPS as an additive was carried out to investigate the permanent deformation of the porous asphalt mixtures at high temperature.

2 Bitumen binders and asphalt mixtures

Five bitumen types, i.e. the original “AH-70” with 60/80 penetration grade, 8% TPS, 12% TPS, 16% TPS modified bitumen and the “PG 76-22” SBS modified bitumen, were used. The virgin bitumen was used to modify into the TPS bitumen binder, using the high shear emulsified machine, with one shaft stirrer with blades at a rotational rate of 5 000 r/min and 170-180 ℃ for 1 h.

The results of conventional bitumen test were

Table 1 Conventional properties of TPS modified bitumen

presented in Table 1. It can be seen from Table 1 that there is a significant large decrease of the penetration values and a considerable increase of the softening point temperatures of TPS modified bitumen, respectively. The increase of softening point is favorable since bitumen with higher softening point may be less susceptible to the permanent deformation (rutting). The increasing viscosities give an indication of the stiffening effect of TPS modification which can result in significant change on the shear resistance properties of the bitumen and asphalt mixture.

The asphalt mixture tested in this experiment is chosen as porous asphalt mixture type with (20±2)% air void, because it is taken into account that the TPS modifier is mainly used in porous asphalt pavement for improving the relative performance of porous asphalt mixture. It should be noted that the TPS modifier is directly blended with the hot aggregate and fiber instead of modifying the bitumen whereas the total mass of the bitumen and TPS modifier binder is the same for all the asphalt mixtures.

3 Apparatus and specimens

An 8 mm diameter spindle was firmly mounted as the upper test plate of the DSR. A zero gap between the upper and lower test plates was established. Finally, a gap setting of 1 mm was established by moving the plates apart. The loading mechanism and the upper plate were lowered to the required gap width plus 50 mm, squeezing out the excess bitumen between the two plates. By moving a heated trimming tool around the upper and lower plate perimeters, the excess bitumen was trimmed from the specimen. When the trimming was completed, the gap was decreased to the desired testing gap. The test was started after the temperature remained at the desired temperature 20 ℃ for at least 10 min.

4 Results and analysis

4.1 Shear creep rate and modulus of TPS modified bitumen binders

Typical test results from the creep test over a wide range of constant shear stresses at 20 ℃ on the 16% TPS modified bitumen binder is shown in Fig.1, where the shear creep strain is plotted as a function of time elapsed after the application of the stress (results from the other bitumen binders shows similar behavior). It should be noted that in these DSR tests, shear strain is approximately linear with time (shear strain rate is nearly constant). Since the shear creep strain covers a wide range due to the wide range of shear stress, the relationship between shear creep strain and time is on a log-log scale, where each curve consists of two regions: creep strain increasing linearly on the loading region for 200 s and creep strain recovering on the unloading region for another 200 s.

Fig.1 Strain—time response curves at 20 ℃

Fig.2 presents the shear creep strain for various kind of bitumen binders. The creep schematics show that a distinction of shear creep strain rate and recoverable strain rate can be made among different bitumen binders. The shear creep strain shows clear characteristic of shear deformation decreases with the increase of TPS additive content, whereas the shear creep strain of SBS modified bitumen is between the strain of 8% TPS and 12% TPS modified bitumen binders.

Shear creep strains and recoverable strains for each bitumen binder can be seen in Table 2. Shear creep strain for TPS modified bitumen decreases from 1.02% to 0.181% with the content of TPS additive increase from 8% to 16%, whereas the ratio of recoverable strain to shear creep strain increases with the increase of TPS additive. However, the ratio for SBS modified bitumen is close to 16% TPS modified bitumen binder.

Fig.2 Comparison of shear creep strain for each bitumen binder at 20 ℃ (Shear stress is 100 Pa)

Table 2 Shear creep strains and recoverable strains for each bitumen binder at 20 ℃

As it is just mentioned from Fig.1 that for all the bitumen binders, the shear creep rate is approximately constant and the behavior is dominated by viscous effects in the loading region. The steady-state creep region where viscous effects dominate has been used to define a steady-state shear strain rate corresponds to a particular value of applied shear stress. It should be noted that in these DSR tests the shear stain and shear strain rate are calculated by Eqns.(1) and (2):

                                     (1)

                                     (2)

where   and  are DSR shear strain and shear strain rate, respectively; R is radius of parallel disk;  and  are angular rotation and angular velocity; h is the gap between parallel plates; t is the test time. Since the shear strain rate  is approximately constant from the analysis of Fig.1, the shear strain rate  can be obtained as follows:

                                      (3)

Steady-state shear strain rates observed over a wide range of constant shear stresses at 20 ℃ for each bitumen binder are presented in Fig.3. The figure shows that the steady-state creep behavior of all the bitumen binders is similar. The date for all the bitumen binders is reasonable well where the shear strain rate shows relationship with shear stress as follows:

                                   (4)

where   and  are shear strain rate and shear stress, respectively; a is material regression coefficient and b is shear creep exponent.

Fig.3 Shear strain rate of each bitumen binder at 20 ℃

The parameters of the power equations for each bitumen binder are given in Table 3. It can be seen that the shear creep exponents of all the bitumen binders are around 1.000 0 which indicates the shear creep behavior of all the bitumen types is linear at all the shear stress level chosen, whereas the material regression coefficient shows the shear resistance properties for different bitumen binders. It should be noted that the strain rate of SBS modified bitumen binder is also between the value of 8% TPS and 12% TPS modified bitumen binders.

Table 3 Parameters of power equations for each bitumen binder at 20 ℃

Shear creep modulus which is defined as the ratio of constant shear stress to ultimate shear creep strain, is also developed to compare the shear resistance properties of each bitumen type which is presented in Fig.4. It can be seen that 12% TPS modified bitumen binder has the relatively higher shear creep modulus than that of the SBS modified bitumen binder.

Fig.4 Shear creep modulus of each bitumen binder at 20 ℃

4.2 Rutting deformation of porous asphalt mixture using TPS modifier

Since the permanent deformation of asphalt mixture is a good response of shear displacement, rutting test for porous asphalt mixture using each type of bitumen binder is conducted at 60 ℃ using the rutting test system. Similar results are observed from the rutting testing for each asphalt mixture (see Fig.5). The results exhibit the same trend with the result of bitumen binders from the DSR testing.

Fig.5 Rutting deformation of porous asphalt mixture at 60 ℃

The rutting depth and dynamic stability are presented in Table 4. It is noteworthy that the results of permanent deformation for porous asphalt mixtures are consistent with those of shear creep behaviors of bitumen. It can be concluded that a good agreement is found between the bitumen binders and porous asphalt mixtures using TPS modifier on the shear resistance properties.

Table 4 Rutting results of porous asphalt mixtures at 60 ℃

5 Conclusions

1) TPS modifier has a significant effect on reducing the shear strain, and at the same shear stress level, the 12% TPS modified bitumen binder behaves better performance of shear creep resistance than SBS modified bitumen binder.

2) Shear creep modulus obtained from the experiment is found to have the same level for all the shear stress level, and the 12% TPS modified bitumen binder also has the higher creep modulus than SBS modified bitumen binder.

3) The results from the shear creep test by DSR and the rutting test show good agreement with the effect of TPS additive on the shear deformation resistance properties of both modified bitumen and asphalt mixture.

References

[1] THROWER E N, MORTAZAVI S, DOUGILL J W. Methods for predicting permanent deformation in flexible pavements: Contractor Report No. CR38[R]. Transportation Research Laboratory, Crowthorne, UK, 1986.

[2] WITCZAK M, KALOUSH K, PELLINEN T, EL-BASYOUNY M, Von QUINTUS H. Simple performance test for superpave mix design[R]. National Cooperative Highway Research Program, Report 465, Transportation Research Board, DC, 2002.

[3] HOSFRA A, KLOMP A. Permanent deformation of flexible pavements under simulated road traffic conditions[C]//Proceedings of the Third International Conference on the Structural Design of Asphalt Pavements. 1972: 613-621.

[4] NAKANISHI H, TAKEI S, GOTO K. Suggestion to the improvement in durability of the function of porous asphalt pavements[R]. Engineering Research Laboratory, Taiyu Kensetsu Co. Ltd, Japan, 1985.

[5] Taiyu Kensetsu Co Ltd. Asphalt modifier for porous asphalt pavements-tafpack super[R]. Japan, Taiyu Kensetsu Co Ltd.

[6] COLLOP A, ATREY G D, KHANZADA S. Creep test of bitumens using the dynamic shear rheometer[J]. The International Journal of Pavement Engineering, 2002, 3(2): 107-116.

(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: CAO Ting-wei, Doctoral candidate; Tel: +86-27-87162595; E-mail: caotw@whut.edu.cn