Fatigue Properties of Rubber Modified SMA Asphalt Mixture

To evaluate fatigue properties of rubber modified SMA asphalt mixture, based on optimizing the preparation process of rubber asphalt, the splitting fatigue tests of three kinds of asphalt mixture is carried out including ordinary asphalt, modified asphalt with rubber powder and SBS modified asphalt. And the freezing and thawing conditions were considered in the tests. The results show that rubber powder can improve the fatigue properties of SMA asphalt mixture, but the modified effect of rubber powder is not as good as SBS modifier, the fatigue properties of asphalt mixture are deteriorated by freeze-thaw conditions, the deterioration degree of rubber modified asphalt mixture is less than SBS modified asphalt mixture, but rubber modified asphalt mixture has better freeze-thaw durability.


Introduction
The accumulation of waste tire not only occupies land, but also seriously pollutes environment. At present, rubber bitumen as a kind of environmental-friendly bitumen has been widely used [1][2][3] .The results of relevant research indicate that adding rubber powder grinded from waste tire into asphalt mixture can obviously improve pavement performance and prolong useful life of asphalt pavement [4][5][6] .
Due to the existence of crumb rubber, the compatibility between rubber bitumen and gradation should be considered during its use. Li Peili [7] carried out volumetric parameters test and pavement performance test for gap gradation, continuous gradation and American gradation.
Through comparison and selection, gap-graded rubber asphalt mixture has better pavement performance because of its excellent skeleton interlocking structure. SMA asphalt mixture has been widely used in the surface layer of bituminous pavement because of its excellent pavement performance. Therefore, gap-graded SMA asphalt mixture which rubber powder is mixed into has a good application prospect.
Relevant research results show that [8,9] rubber modified SMA asphalt mixture shows excellent performance. But the durability of rubber modified SMA asphalt mixture is referred less, and there is no relevant report on the effect of freeze-thaw conditions on its fatigue properties. Therefore, considering freeze-thaw conditions, the influence of rubber powder on the fatigue properties was evaluated by comparing SMA asphalt mixture which rubber powder is added into or not.

Design of Test Plan
In order to fully evaluate the influence of rubber powder on the fatigue properties of SMA asphalt mixture, the performance comparison of three kinds of asphalt To consider the influence of freeze-thaw conditions on the fatigue properties of various asphalt mixtures, the fatigue tests were conducted after 0 (non-condition), 1, 3 and 5 freeze-thaw cycles respectively. The freeze-thaw circulation conditions are as follows: the specimen is put into a plastic bag after vacuum water-retention; 10 mL water is added; the specimen is put into the mouth of the bag and kept in thermostatic water tank at -18℃ for 16 hours. Then the specimen is taken out, immediately put into the 60℃ thermostatic water tank, removed the plastic bag, and kept warm for 24 hours.

Manufacturing Technology of Rubber bitumen
Stirring process of rubber bitumen can be divided into dry method and wet method. The performance of rubber asphalt mixture prepared by wet method is better than that by dry method. Therefore, this study employed wet method to prepare rubber bitumen. When using highspeed shearing equipment to produce rubber bitumen, the viscosity of rubber bitumen will decrease with the increase of stirring time. At the high-speed shearing process, the shearing equipment will further refine the rubber powder, making crumb rubber continue to decline, so the viscosity continues to decline. Therefore, high-speed shearing equipment is not suitable for rubber bitumen production. Simple blade stirring is used in this study. The rotational speed is 250r/min when stirring.
In the production of rubber modified bitumen, crumb powder must be fully swelled in bitumen to give full play to the effect of modification. The swelling degree of rubber powder in bitumen is closely related to stirring temperature and mixing time. Therefore, stirring temperature and mixing time are important indexes which affect the performance of rubber bitumen. Although there have been many research results on the production process of rubber modified bitumen [7][8][9] , considering the compatibility between bitumen and rubber powder, this study evaluated the influence of mixing time and stirring temperature on the performance of rubber modified bitumen, to determine the production process of indoor rubber modified bitumen.
In this study, the AH-70 oil bitumen and 60 mesh oblique tire rubber powder to prepare rubber bitumen is used, in which the rubber powder content was 18%. The three indexes (penetration, softening point and ductility) of rubber modified bitumen prepared under different stirring temperature and mixing time are shown in Figure 1 and  In summary, the preparation process of rubber modified bitumen is as follows: heat the bitumen to 170 ~180℃ and then add the rubber powder with 18% by weight of asphalt mixture, then stir 40min.
After determining the manufacturing technology of rubber modified bitumen, the rubber modified bitumen produced, the base bitumen and SBS modified bitumen used in this study were tested for conventional indicators. The test result is shown in Table 1: It can be seen from the test results in Table 1

Preparation of specimen
The framework-dense structure of SMA asphalt mixture can ensure that it has good pavement performance. This study selected the gradation as shown in Figure 4 for the preparation of asphalt mixture.

Figure 4 Gradation of SMA-13
Based on the gradation of Figure 4, the composition of SMA-13 asphalt mixture is designed by Marshall test. The key test indexes are shown in Table 2.  As is shown in the Table 2, the optimal asphalt content of rubber modified asphalt mixture is 0.5% higher than that of SBS modified asphalt mixture. The Marshall stability is higher than that of ordinary asphalt mixture, but is lower than that of SBS modified asphalt mixture. was tested, as shown in Table 3.   Table 8. As is shown in Figures 5-7 and Table 4, the test results are as follows:

Analysis and result of indirect tensile fatigue test
(1) For unconditional specimens, among the three asphalt mixtures, SBS modified asphalt mixture has the best fatigue properties. Rubber modified asphalt mixture takes the second place. Ordinary asphalt mixture is the worst. It shows that rubber powder can improve the fatigue properties of asphalt mixture, but the modified effect is not as good as SBS modifier.
(2) The fatigue life of three kinds of asphalt mixture decreases with the increase of freeze-thaw cycles. The influence of 1 freeze-thaw cycle is small, and the fatigue life is obviously reduced after three freeze-thaw cycles.

Conclusion
In the study, the fatigue characteristic of rubber modified SMA-13 asphalt mixture was investigated and compared with ordinary and SBS modified SMA-13 asphalt mixture.
Based on the outcome of the experimental study, the following conclusions can be drawn: (1) The rubber modified asphalt produced by mechanical stirring for 40 minutes at the temperature of 170℃ ~180℃ has better technical properties.
(2) Compared with SBS modified asphalt mixture, the optimal asphalt content of rubber modified asphalt mixture is increased by 0.5%, and the Marshall stability is higher than that of ordinary asphalt mixture, but lower than that of SBS modified asphalt mixture.
(3) Rubber powder can improve the fatigue performance of asphalt mixture, but the improvement effect is not as good as that of SBS modifier.
(4) Freeze-thaw cycles have adverse effects on the fatigue properties of various asphalt mixtures. The fatigue properties of rubber modified asphalt mixtures reduction significantly especially after 3 freeze-thaw cycles. The reduction degree of rubber modified asphalt mixtures is less than SBS modified asphalt mixtures and rubber modified asphalt mixture has better freeze-thaw durability.