Effects of SBS Configuration on Performance of High Modulus Bitumen Based on Dynamic Mechanical Analysis

High modulus bitumens modified by polystyrene-block-polybutadiene-block-polystyrene (SBS) with different molecular structure were investigated on dynamic shear rheometer and fluorescence microscopy to evaluate viscoelastic properties and morphology of binders. The results shows that storage modulus (G’) is obviously less than loss modulus (G”), which means viscous behaviour of bitumen is dominant, and anti-rutting factor (G* ⁄ sin δ) is markedly enhanced by star SBS than by linear SBS. The morphology indicated that star SBS improved the softening point more obviously, tending to form a cross-linked network in bitumen. As for linear SBS, it is dispersed in bitumen in the form of globules and enhances the ductility of binder.


Introduction
In recent years, pavement distress, such as rutting, fatigue and cracks, have become a problem with the increase in traffic volume, number of heavy vehicles and the influence of climate change. 1,2In semirigid and flexible pavement, bitumen mixtures support most vehicle loads, and bitumen has a great effect on the function of the bitumen mixture. 3he results of the study show that the main cause of serious rutting in China is that the penetration of selected bitumen is high and the modulus is low. 4,5gh modulus bitumen has excellent ability to resist deformation, because it produces small strain when subjected to stress.Therefore, the development and application of high modulus bitumen is important for improving pavement's ability to resist rutting and prolonging life span of pavement. 6The earliest attempt in HMBC (high modulus bitumen concrete) had been carried out in France.The French standard NFP98-140 gives the definition of HMBC, whose complex modulus is higher than 14 000 MPa at 15 °C and 10 Hz. 7 However, there are no specific standards related to high modulus bitumen in China.In engineering applications, we usually regard bitumen whose complex modulus is greater than 10 000 MPa at 60 °C and 10 Hz as high modulus bitumen.
Nowadays, the solvent de-asphalting process has become a widely applied heavy oil process.When asphaltene, resin, and metal are removed from residue to produce light oils, it can also produce de-oiled bitumen with high asphaltene, low wax, and high softening point.In recent years, the issue of the utilization of de-oiled bitumen has become an important subject in the solvent de-asphalting process.De-oiled bitumen, as main material, was mixed with soft bitumen components and SBS with different structure to produce high modulus bitumen, which achieved a win--win situation for making effective use of de-oiled bitumen and producing pavement bitumen with high added-value.In this paper, high modulus bitumen modified by SBS with different molecular structure were carried out on dynamic shear rheometer by means of temperature sweep and fluorescence microscopy to evaluate anti-rutting performance, viscoelastic properties and morphology of binders.

Materials
Oil slurry and aromatic oil were obtained from Jingmen Petrochemical Company, while the base bitumen was prepared by blending de-oiled bitumen with oil slurry and aromatic oil.The base bitumen was then modified by SBS.The basic properties of the three materials are presented in Table 1.The author selected star Daogai 2# SBS and linear YH-791H, both of SBS with 30 ⁄ 70 in block ratio styrene/ polybutadiene, from Yueyang Baling Petrochemical Company.Base bitumen was prepared by de-oiled bitumen, oil slurry and aromatic oil at the ratio of 53 : 37 : 10.SBS with different structure was added into the base bitumen with FLUKE FM300-digital high-shear, and then samples were sheared for 10 min at 170 °C and 3000 -4000 rpm.Finally, the samples were stirred for 2 h in order to prepare HMA.The dosage (in mass) of SBS was 3 % and 5 %, and that of stabilizer was 0.3 %.

Performance evaluation
Conventional properties, such as ductility, penetration, softening point, were analysed according to T0606-2000, T0604-2000 and T0605-2000 in JTJ052-2000 "Experimental regulation of highway project bitumen and bitumen mixture".Dynamic viscosity was measured by RVDV-II+P Brookfield viscosity instrument produced by Brookfield engineering laboratory.Morphology was detected by BX51 microscope produced by Olympus, and magnification was 100.Oscillation tests were carried out on rheometer AR2000EX produced by US TA Company in LVR (linear viscoelasticity region).25 mm parallel-plate in diameter and 1 mm gap was selected.LVR was previously tested for all samples based on stress sweep.

Conventional performance
Conventional properties, including low temperature ductility, penetration, softening point, are utilized to measure low temperature performance, consistency and high temperature performance of bitumen.Conventional performance of high modulus bitumen produced by SBS with different structure are shown in Fig. 1.The structure of SBS has great effect on conventional performance of bitumen, especially for softening point and ductility.Star Daogai2# has a greater softening point and linear YH-791H has greater ductility.However, the structure of SBS has little effect on penetration.This is due to the fact that three-dimensional network structure formed by polybutadiene has more crosslinking domains. 8Thus, high modulus bitumen possesses better high-temperature properties.However, high modulus bitumen modified by linear YH-791H has better low-temperature properties, and its ductility is higher at low temperatures.
The relation between temperature and viscosity of bitumen is used to evaluate bitumen's susceptiveness to temperature.Characterization of viscosity-temperature properties of high modulus bitumen with different structure SBS is displayed in Fig. 2. The Saal formula was used, to describe the relation between temperature and viscosity of high modulus bitumen, where η denotes viscosity of bitumen, and m and n are coefficients.The results in Fig. 2 show that the relation between viscosity and temperature is a good linear relation in a relatively wide temperature range, and the curve fitted fairly well.Correlation coefficients obtained by Saal formula are displayed in Table 2.The changes in viscosity with temperature of bitumen is more obvious when n is greater.The data in Table 2 show that n of high modulus bitumen produced by star Daogai SBS is larger than linear YH-791H.This indicates that the changes in viscosity with temperature of high modulus bitumen produced by star SBS is more obvious.The n of w = 5 % SBS is larger than w = 3 % SBS with the same type of SBS.This indicates that a larger dosage of SBS produces a more obvious change in viscosity with temperature.Thus, it is less susceptible to temperature.

Dynamic mechanical properties
The permanent deformation which developed under heavy loads at a high environmental temperature are mainly distress in pavement. 9,10In order to describe the permanent deformation, dynamic mechanical parameters were obtained on DSR (dynamic shear rheometer) by temperature sweep tests.Elasticity and viscosity of bitumen was evaluated by the relation between G*, G', G" and δ, while anti-rutting factor G* ⁄ sin δ represents anti-rutting performance.
Variation in G' and G" with temperature of high modulus bitumen prepared by different SBS are displayed in Fig. 3.
The results in Fig. 3 show that G' and G" will be lower in high-temperature regions.Storage modulus G' together with loss modulus G" decreased by three to four magnitudes when temperature increased from 30 °C to 90 °C.This indicates that temperature has great effect on modulus of bitumen.Storage modulus G' is usually less than loss modulus G".Thus, high modulus bitumen mainly presents viscous properties.G' decreased more obviously than G" when the temperature increased.This shows that temperature influences the storage modulus G' more easily.When the dosage of SBS was the same, it was observed that G' as well as G" for star SBS is higher than that of linear SBS.This is because the star SBS easily formed a crosslinking system within the bitumen, which accounted for the increased viscosity of the bitumen.Anti-rutting performance improved because the crosslinking system was able to maintain the original structure and morphology with imposed external force.G' together with G" is higher when the dosage of SBS is greater at the same temperatures.This indicates that the anti-distortion capacity of bitumen will be stronger.Evolutions of phase angle for high modulus bitumens are shown in Fig. 4. As can be seen, the phase angle increased drastically with temperature for both SBS with the dosage of 3 %.However, the phase angle almost kept constant in a wide temperature range, indicating less temperature susceptibility.The change in G* ⁄ sin δ with temperature of high modulus bitumen prepared by different SBS is pre-sented in Fig. 5.The results in Fig. 5 show that G* ⁄ sin δ decreased when temperature increased.G* ⁄ sin δ of bitumen prepared by star SBS is higher than linear SBS.This shows that the structure of star SBS had a great effect on the modulus of bitumen.Owing to special structure of star SBS, the three-dimensional network structure formed by polybutadiene in star SBS had more crosslinking points, and star SBS easily formed a structure in the bitumen medium.Therefore, the modulus and high temperature of will be improved obviously.Density and stability of network structure formed by bitumen in linear SBS are far worse than that in star SBS.

Relation between micro-morphology and dynamic mechanical properties
Fluorescence optical microscopy structures of high modulus bitumen with different SBS are presented in Fig. 6.The result in Fig. 6 shows that linear YH-791H was dispersed in the form of globules in colloid system of bitumen.But star Daogai2# formed a crosslinking structure within the bitumen with the help of a stabilizer, and the crosslinking structure improved the high-temperature properties and anti-rutting performance of the bitumen.This phenomenon reflected in fluorescence optical microscopy structures is in line with the result in Fig. 1.Comparing Fig.  (B) and Fig. 6 (D), it was found that bitumen was a continuous phase, and SBS generates a slight improvement in the softening point when the dosage of SBS was small.SBS particle distance decreased and polymers tended to form a network structure when the mixing amount of SBS increased.The network confined the ability of flowing and deformation of bitumen, which consequently improved softening point, viscosity and anti-rutting performance. 11

Table 1 -
Properties and compositions of raw materials

Table 2 -
Coefficients of viscosity-temperature curve for various SBS modified bitumen Tablica 2 -Rezultat regresijske analize ovisnosti visokoznost-temperatura bitumena modificiranih različitim SBS-ovima Temperature sweep of high modulus bitumen with different SBS was carried out on dynamic shear rheometer.The conventional tests indicate that high-temperature performance for star SBS is better than the linear one, while the ductility of star SBS is lower than that of linear SBS.Moreover, the linear SBS has lower viscosity compared to star SBS at the same temperature, which indicates that former has better mixing ability.The dynamic test results show that G' together with G" decreases when temperature increases, and that storage modulus G' is less than loss modulus G".G', G" and G* ⁄ sin δ for star SBS is higher than that of linear SBS in the same concentration.The morphology indicates that star SBS improve the softening point more obviously compared to linear SBS because of the dense cross-linked network formed by SBS molecule in bitumen phase.Linear SBS dispersed in bitumen presents spherical particles, which enhances the ductility of binder.