Introduction

    With the rapid development of highway construction, polymer modified asphalt plays an increasingly important role in pavement. At present, over 90% of pavement in highway construction in China's expressways and provincial capitals uses SBS modified asphalt and achieves better road performance. However, its high cost also limits its use and promotion.
    In recent years, rubber powder made from discarded tires is widely used as a modified material of asphalt at home and abroad. It not only has low price, but also can obtain good modification effect, turning waste into treasure and eliminating "black pollution." The use of rubber powder modified asphalt can significantly increase the viscosity of asphalt, reduce the penetration of asphalt, improve the ductility and softening point, improve high temperature performance; and thermoplastic elastomer styrene-butadiene-styrene tri-embedding SBS segment copolymer ,because of its own plastic section and rubber section, , unlike conventional vulcanized rubber, it can increase the softening point of bitumen and make the bitumen still flexible below the brittle point temperature, maintain the toughness of bitumen and improve the low temperature effect obviously. In summary, SBS modified bitumen has obvious advantages in low temperature and high temperature performance, and rubber bitumen also has unparalleled advantages in durability and environmental protection, so the composite modified bitumen technology has drawn people's attention in recent years . Different modifiers are used to compound the different modification effects of high and low temperature properties of asphalt, and the advantages among the modifiers are fully exerted to promote and supplement each other so as to improve the high and low temperature performance of ordinary asphalt. Based on this, this article has carried on the research to the application technology of compound modified rubber asphalt.

Analysis of Modification Mechanism
    There is no unified understanding about the modification mechanism of polymer modified asphalt, but there are mainly three kinds of views as follows.
1. Compatibility
    Polymer modified bitumen is a two-phase blend composed of polymer and bitumen, which can be divided into three basic types according to the continuity of phase: The asphalt phase is the continuous phase ,the polymer phase is the single continuous structure of the dispersed phase; the polymer phase is the continuous phase , the asphalt phase is the single continuous structure of the dispersed phase; the two-phase continuous structure or the two-phase interlock structure, namely the asphalt phase and the polymer are all continuous.
    Sam Maccarrone believes that the ideal state of the polymer in the asphalt - polymer system is fine distribution rather than compatibility. There are two theories to explain this phenomenon. Firstly, when polymers are added to bitumen, according to the principle of lowest energy, the system has a tendency to automatically reduce the surface energy. This tendency is realized by reducing the surface area or adsorbing certain substances with similar structures to reduce the surface energy. Because the disperse phase polymer particle size is small, the surface energy is large, it can absorb similar structure in the asphalt material, so the components able to reduce the surface energy will be adsorbed on the polymer surface to form an interface adsorption layer to reduce the surface energy. Secondly, the material solubility parameter is used as a quantitative index to evaluate the compatibility (good and bad). According to the principle of similarity and compatibility, the closer the polarities of the two substances are, the smaller their difference in solubility parameter is, the easier they are miscible with each other.
2. Swelling
    The study on the swelling behavior of polymers in asphalt shows that the polymer does not generally undergo chemical reaction after addition into asphalt, but the volume of polymer expands, namely swells under the action of light weight components of asphalt. In order to ensure their own stability, the polymer absorbs the oil in the asphalt and expands in volume by 5 to 10 times of the original volume. The light weight components in the asphalt penetrate into the polymer under the chemical position , the polymer absorbs only a limited amount aromatic phenols, saturated phenols and waxes and swells rather than dissolves. Asphalt itself is a colloidal structure in equilibrium, the polymer’s absorption of bitumen aromatic phenol or saturated phenol (or both) will inevitably undermine the equilibrium state, thus forms the situation of asphalt micelles and polymers fight for light components , eventually they reach a new balance system ---- the new asphalt colloidal structure balance and polymer swelling balances.
3. Critical dosage
There is a critical value for the amount of polymer in bitumen, which Collins, the U.S. scientist, calls the critical mass fraction. When the critical volume is reached, the polymer forms a stable network structure. When the value exceeds, the modified system undergoes phase transition, the polymer changes from discontinuous phase to continuous phase, and the asphalt phase changes from continuous phase to discontinuous phase .
4. Compound modification mechanism
    In this study, rubber powder modified and SBS modified are compounded, the use of the advantages of the two modifiers, synergism and complement each other, to access high and low temperature properties improved modified asphalt.
    According to the rubber powder modification mechanism and polymer modification mechanism analysis, in the composite modification system, the two mechanisms will work synergistically with each other. After adding asphalt powder and polymer, both start swelling and compatibility effect; rubber powder and polymer absorb the oil in asphalt, expand volume, and form a stable network structure, which increases the viscosity of asphalt, improve its Low temperature performance.

Discussion on Test plan and result
Raw materials
1. Base asphalt
    Base asphalt is a mixture of a variety of complex hydrocarbons and non-metallic derivatives of these hydrocarbons. Bitumen classified by 4-component method includes asphaltenes, gums, saturated phenols and aromatic phenols.
    The performance of matrix asphalt directly affects the performance of composite modified rubber asphalt. In this study, SK90 # matrix asphalt was used as the composite modification target.
2. Rubber powder
    The main chemical composition of rubber powder is natural rubber and synthetic rubber (such as styrene butadiene rubber, butadiene rubber), and sulfur, carbon black, Silica, iron oxide, calcium oxide and other additives, these ingredients are good asphalt modified Agent.
3. SBS
    SBS is a styrene-butadiene-styrene block copolymer, divided into two types : linear and star structure. S segment → polystyrene → hard phase → disperse phase → island phase in SBS molecular chain, plays the role of restraint the sulfide point of soft segment; segment B → poly-butadiene → soft phase → continuous phase → marine phase in SBS molecular chain. In the assembled state, the poly-butadiene segments form a continuous rubbery phase while the polystyrene segments are dispersed in the continuous rubbery phase to form the plastic micro-dissection zone. Because the two segments are linked by unvulcanized chemical bonds, the polystyrene micro-domains function as cross-linking sites for poly-butadiene segments over a certain range of temperatures. When the temperature is higher than the Tg of polystyrene, the S-segment is released and the cross-link is opened. The SBS shows plasticity. When the temperature is lower than Tg of polystyrene, the physical cross-link reappears, showing the comprehensive properties of the vulcanizate, no longer play the role of restraint sulfide. Therefore, it can be seen that SBS has the reversibility of chemical or physical cross-linking properties.
Preparation Process
    Using 20% rubber powder, composite with SBS, prepares composite modified rubber asphalt. The process is as follows: firstly, slowly heated the matrix asphalt to about 150 degree centigrade, and then slowly added the well-mixed SBS and rubber powder mixture into the asphalt liquid, and using a high-speed shearer with a speed of 3000rmin- , stopped the shearer after fully mixing for 2 hours. Shear process must be applied to the composite modified rubber asphalt ,because it is difficult for SBS to melt into the matrix asphalt through agitation.
Analysis of test results
    Adopt 20% mixed rubber powder. Add 0.5%, 1%, 1.5%, 2% SBS for composite modification. Due to the shearing process, the index of 20% rubber powder mixed asphalt is different from that of the above-mentioned hot-melt method.
(1) penetration. With the increase of the content of SBS, the penetration decreased gradually and tended to be stable, which showed that SBS was beneficial to the improvement of temperature sensitivity of rubber asphalt, and its temperature sensitivity is improved obviously with the increase of dosage .
(2) softening point. The improvement of high temperature performance of rubber asphalt by SBS is very significant. With the increase of the dosage, the softening point of rubber modified asphalt is significantly increased. Therefore, in order to improve the high temperature performance of rubber asphalt, the most common practice is to add SBS.
(3) Durability. With the increase of SBS content, the low temperature performance of composite modified rubber asphalt is obviously improved. Ordinary rubber asphalt has low temperature ductility, which often can not meet the specifications; when the SBS dosage is up to 1%, the low temperature ductility of composite modified rubber asphalt can reach more than 8cm, durability is up to 10cm or more when the amount is 2%.
(4) elastic recovery. Compared with no SBS addition, the initial addition of SBS did not improve the elastic recovery obviously, but with the increase of SBS content, the elastic recovery of composite modified asphalt gradually increased.
(5) 180 ℃ rotational viscosity. Although with the SBS content increases, the composite modified rubber asphalt 180 °C rotational viscosity increases. From 0% to 2%, the viscosity increased from 0.9Pa•s to 1.35Pa•s, but the viscosity value increased only slightly and the overall value was still low. It is visible that viscosity improvement by adding SBS rubber asphalt is not significant.

Conclusion

(1) Adding SBS can significantly improve the temperature sensitivity of rubber asphalt, the ductility index, and the low temperature performance.
(2) Softening point index of composite modified rubber asphalt is higher than that of ordinary rubber asphalt. Composite modified rubber asphalt can achieve better high temperature performance than ordinary rubber asphalt.
(3) From the viscosity index, we can see that the most critical factor affecting the viscosity is the amount of rubber powder. The viscosity of the compound modified rubber asphalt is low, but it can be increased by increasing the amount of cement powder.
(4) Through indoor tests, it is suggested that the content of rubber powder in composite modified rubber asphalt is 20% and the content of SBS modifier is 2%.

(Article from: http://www.asphaltpavement.cn)