## 4 Results and Discussion  
### Low Temperature Characteristics of Bitumens  
Indicators of low temperature resistance of bitumens stiffness S and m-value at load duration of 60 s, determined on BBR at the temperatures of \(-\)24 \({}^{\circ}\)C, \(-\)30 \({}^{\circ}\)C, and \(-\)36 \({}^{\circ}\)C, are represented in Figures 10-15. The Superpave technical specifications require that bituminous binder stiffness at calculated minimal temperature and at load duration of 60 s should not exceed 300 MPa and m-value should not exceed 0.3 [10].  
Figure 8: Equipment for testing of asphalt concrete sample at direct tension.  
Figure 9: Hamburg wheel tracking machine.  
Figure 11: Bitumens m-value at the temperature of \(-30\)\({}^{\circ}\)C.  
Figure 10: Bitumens m-value at the temperature of \(-24\)\({}^{\circ}\)C.  
Figure 12: Bitumens m-value at the temperature of \(-\)36 \({}^{\circ}\)C.  
Figure 13: Bitumens stiffness at the temperature of \(-\)24 \({}^{\circ}\)C.  
It is seen from Figures 10-12 that except for the bitumen with polymer Elvaloy after 50 cycles of FT at the temperatures of \(-\)30 \({}^{\circ}\)C and \(-\)36 \({}^{\circ}\)C all the considered bitumens satisfy the Superpave requirements. The decrease of bitumen m-value is observed with the number increase of FT cycles. However, noticeable change for the bitumens m-value was not found out depending on a temperature within the considered range of temperature.  
Stiffness for all the considered bitumens is considerably increased with the temperature decrease and increase of number for FT cycles (Figures 13-15). Only neat bitumen does not satisfy Superpave requirements after 50 cycles of FT at the temperature of \(-\)36 \({}^{\circ}\)C, all other bitumens have stiffness lower than 300 MPa at all the considered temperatures.  
It is clear that an important characteristic for bitumens is the value for its stiffness increase at different negative temperatures after various cycles of FT. Bar charts showing the values of stiffness increase for the considered bitumens are represented in Figures 16-18. It is seen that the stiffness of  
Figure 14: Bitumens stiffness at the temperature of \(-\)30 \({}^{\circ}\)C.  
Figure 15: Bitumens stiffness at the temperature of \(-\)36 \({}^{\circ}\)C.  
the neat bitumen, the bitumens with polymers Butonal and Titan+SBS has increased after 50 cycles of FT and at the temperature of \(-24\)\({}^{\circ}\)C for 66.4%, 54.9%, and 55.8% respectively, and stiffness increase for the bitumens with polymers Butonal and Kraton at the temperature of \(-36\)\({}^{\circ}\)C was 87.0% and 88.8% respectively.  
Figure 16: Bitumens stiffness increase after 25 and 50 cycles of FT at the temperature of \(-24^{\circ}\)C.  
Figure 17: Bitumens stiffness increase after 25 and 50 cycles of FT at the temperature of \(-30\)\({}^{\circ}\)C.  
The following binders are found as the most resistant to cyclic FT: bitumen with polymer SBS at the temperature of \(-24\)\({}^{\circ}\)C (stiffness increase is 3.1% and 15.9% respectively); bitumen with polymer Calprene at the temperature of \(-30\)\({}^{\circ}\)C (19.6% and 27.6% respectively); bitumen with polymers Calprene, SBS, Zydex+ SBS, and Titan+SBS at the temperature of \(-36\)\({}^{\circ}\)C (from 14.7% to 34.6%).  
Thus, when selecting a suitable bituminous binder considering climatic conditions it is necessary to take into account as the number of FT cycles as well as a characteristic low temperature.