Research On Rheological Property And Low Temperature Performance Of Hard Grade Bitumen And Its Mixture

Hard grade bitumen is a low-penetration, high-siffness binder with good rutting resistance. It is widely used for high-grade roads in European countries, such as Britain and France. In China, hard bitumen with penetration 20/40 starts to be applied in road engineering. But the investigation about its rheological propery and low temperature performance was incomplete which inhibited the otherwise wide use of this high performance bitumen. A systematic study on the rhelogical and low temperature performance of hard bitumen and its mixture is essential. This paper investigates the rheolgical performance and low temerature property of hard bitumen based on the rheology theory of viscoelasitc materials by static testing and dynamic testing. The main contents of this paper include:The typical hard bitumen is studied in this paper, including seven 20/40 penetration bitumen and one lower penetration bitumen. These binders are from different oil sources, and produced by different methods, such as air blown, solvent deasphalting and blending. Based on the chemical theory of heavy oil, this paper investigates the chemical composition of eight hard bitumen obtained from different oil sources and processing methods, such as group composition, elemental composition and structure group composition. Influences of these compositions on the basic properties of hard bitumen are also discussed, based on the Gray Correlation theory.This paper investigated the rheological properties of hard bitumen within a wide temperature range including the high temperature dynamic shear rheological characteristic and the low temperature creep characteristic. The dynamic shear mode, stable flowing mode, multi-stress creep mode of dynamic shear rheometer(DSR) are used to study the high temperature rheological properties. Complex modulus G*, phase angle δ, master curve are obtained at different temperature and frequency for five hard bitumen. Zero shear viscosity(ZSV) is predicted based on the Cross model and Carreu model. The rutting factor, non-recovery creep compliance Jnr, and ZSV are evaluated and compared for five hard bitumen. Results indicate that the high temperature grading for five hard bitumen is higher than PG 76 ℃. Jnr at 64℃ also indicates the good rutting resistance of hard bitumen. However, the rheological property is significantly different between different oil sources, even at the same penetration level. The low temperature rheological property of hard bitumen is investigated by bending beam rheometer(BBR). Creep stiffness S curve and power law parameter m value are obtained from-3℃~-18℃. The glass transition temperature Tg is investigated by differential scanning calorimeter(DSC). Based on Burgers model, the viscoelasticproperty at low temperature is analyzed for hard bitumen. The comprehensive creep compliance index J is obtained and compared for five hard bitumen. The correlation between chemical composition and low temperature performance is investigated by Correlation Theory. Results indicate that asphaltene and resin have significant effect on high temperature and low temperature performance, whilst saturation and aromatics have very little effect on binder performance. Among chemical structure group compositions, HAU/CA、σ have notable effects on low temperature property of hard bitumen, whilst structure unit number n and slide chain length L have the least effect. At the same time, for the low temperature property, including ductility, S, m, Tg, breaking point and J, the influence of chemical group composition and structure group composition on ductility is the weakest, So evaluating the low temperature performance of hard bitumen by ductility is not adequate.After mix design, the performance of hard bitumen was tested for two aggregate gradations, i.e. skeleton structure and suspension structure, and the overall performance of hard bituminous mixture are compared with crumb rubber modified bitumen and SBS modified bitumen. Results indicate that the high temperature property of hard bitumen mixture is similar to crumb rubber modified bitumen mixture and SBS modified bitumen mixture. The maximum strain εmax of hard bitumen mixture meets the Chinese specifications on low temperature property for Highway Climate Region 1-3. In addition, the low temperature performance of hard bitumen mixture may be improved by mineral gradation. Then the viscoelastic property of hard bitumen mixture is investigated by dynamic modulus test with compression mode. Seven temperatures and five frequencies are trialed in this paper. Based on the sigmoid function and CAM model, the viscoelastic behavior of hard bitumen mixture is analyzed, and the viscoelastic parameters such as Ge*, Gg*, fc, R, are also obtained. Results confirmed that mineral gradation may improve the viscoelastic characteristic for hard bituminous mixture, especially the viscous performance at low temperature. The viscoelastic parameter fc may explain the change of phase state.This paper adopted the indirect tensile creep test and thermal stress of restricted specimen test(TSRST) to investigate the shrinkage and creep property of hard bitumen mixture at low temperatures. Creep compliance with-10℃, 0℃, and 10℃, are obtained for two gradation. Based on the power law function and time-temperature superposition principle, Creep compliance master curves are constructed for these hard bituminous mixtures. Creep compliance gradient m values are obtained from these creep compliance master curves. Fracture temperature TF, Fracture force FF, glass transition temperature Tg and thermal stress gradient Ds/ T are compared between bituminous mixtures made with different bitumen binders and mineral gradations.After the low temperature properties of binder and mixture are tested, this paperinvestigates the correlation between the low temperature performance indexes of hard bitumen based on rheology, and the low temperature property f hard bitumen mixture. This study indicates that the bending stiffness S(60s), m value and Fraass breaking point temperature have better correlation with low temperature index of hard bitumen mixture, namely εmax, TF, FF, TSRST-Tg and temperature stress gradient Ds/ T. However, the ductility of hard bitumen is different. Thus, ductility cannot scientifically represent the low temperature performance of hard bitumen. The linear equations between εmax of hard bituminous mixture and stiffness modulus S, m value and Fraass breaking point of hard bitumen are constructed based on the correlation coefficient. According to the requirement of 2000με of maximum strain εmax for 1-3 Climate Region, the thresholds of stiffness S, m value and Fraass breaking point are determined by back-calculation.The use of hard bitumen in asphalt pavement maintenance on Chang’an Street has proved the excellence of hard bitumen and the effectiveness of the specifications suggested in this study for low temperature performance of hard bitumen. Findings of this paper will hopefully promote the use of hard bitumen in Chinese Highway Climate Region 1-3 and the warmer climate regions in China.