Research On Low Temperature Cracking Characteristics Of Composite Modified Asphalt Mixture Based On Macro And Mesoscopic Method

Asphalt pavement is easy to crack in the low temperature season,which seriously affects the service life of the pavement.Relevant studies have found that SBS modified asphalt has excellent road performance,but its cost is high and its wide application is limited.Rubber powder modified asphalt has good low temperature performance,but it has problems such as poor construction workability and easy segregation,which also has a greater impact on its application.In order to improve the above problems and obtain a modified asphalt with better low-temperature crack resistance,two modifiers were compounded to prepare SBS/rubber powder composite modified asphalt and its mixture（CCRMA）,which were combined with SBS modified asphalt and the mixture（SBSMA）was compared and studied.As asphalt mixture is a composite material,its macro-mechanical behavior is inherently related to its micro-structure characteristics.Therefore,this paper compares the low-temperature performance,asphalt-aggregate adhesion,and low-temperature cracking characteristics of the asphalt mixture based on the macro-and meso-scale test method.The main research contents are as follows:First,the bending beam rheology（BBR）and infrared spectroscopy（FTIR）tests were performed on SBS modified asphalt（SBS）and SBS/rubber powder composite modified asphalt（CCR）before and after aging and freeze-thaw cycles.The low-temperature rheological properties and chemical characteristics of asphalt were analyzed by m/S value,infrared spectrogram and chemical functional group index.The results show that the effects of aging and freeze-thaw cycles both reduce the low-temperature rheological properties of asphalt;as the number of freeze-thaw cycles increases,the low-temperature performance of asphalt decreases rapidly between 0-5freeze-thaw cycles,and slows down after 5 times.Compared with the water freezing cycle,the low temperature performance under the salt freezing cycle is poor;No obvious chemical reaction occurred when SBS and rubber powder modified the base asphalt by single or compound modification;Both asphalts undergo chemical reactions during the aging process.During the freeze-thaw cycle,the asphalt reacts with the oxygen dissolved in the water,while the salt does not react significantly with the asphalt;In addition,the changes of hydroxyl and aromatic ring index under different conditions have a strong correlation with the rheological properties of asphalt,which further explains the results of the macro-BBR test from the perspective of micro-chemical characteristics,compared with the aromatic ring,the hydroxyl index can more accurately reflect the low-temperature rheological properties of asphalt.Secondly,based on the contact angle measurement and AFM test,the surface energy and adhesion work are used to analyze the adhesion between the two asphalts and aggregates under different conditions from the macro and microscopic perspective;the average DMT modulus of the asphalt is measured from the microscopic scale.(A_DMT)and DMT modulus root mean square error R_q are analyzed;and the macro-and meso-mechanical indexes are correlated.The results show that the effects of aging and freeze-thaw cycles both reduce the adhesion of SBS and CCR modified asphalt to aggregates,and reduce the anti-cracking ability of the asphalt-aggregate interface;compared with water freezing cycles,salt freezing cycles Adhesion is poor;under any conditions,CCR has better bonding performance with aggregate,that is,it has strong crack resistance at the interface with aggregate.The decrease in macroscopic rheological properties of asphalt under different conditions is manifested in the increase of A_DMTvalue and Rq from the perspective of micromechanics,that is to say,the elasticity of asphalt increases and the deformation ability decreases.Under the action of external load,stress concentration is more likely to occur on its surface,which promotes the formation of cracks.The mechanical properties of SBS and CCR modified asphalt under different conditions show a strong correlation at the two scales,which proves that the microscopic properties of asphalt will affect its macroscopic properties.Then,SBSMA and CCRMA were subjected to temperature stress test of constrained specimens and three-point bending test of trabecular beam based on digital imaging technology（DIC）under different conditions,using freezing fracture temperature and fracture mechanics indicators（fracture energy density G_d,critical fracture toughness J_c）)And the index defined based on the strain field（critical strain Ez,strain derivative Eˊ（t））,the low-temperature cracking characteristics of SBSMA and CCRMA are analyzed from the macro and meso scales,and the gray correlation analysis is carried out on the macro and meso low-temperature performance evaluation indicators and the freeze-off temperature.The results show that the effects of aging and freeze-thaw cycles reduce the anti-cracking properties of asphalt mixtures;compared with water-freezing cycles,the low-temperature anti-cracking properties of the mixtures under salt-freezing cycles are poor;no matter what the conditions,the low-temperature anti-cracking performance of CCRMA,Anti-aging and freeze-thaw resistance are better than SBSMA;in the freeze-break test,the freeze-break temperature and the transition point temperature can better reflect the changing law of the low-temperature cracking characteristics of the asphalt mixture under different conditions;through the gray correlation analysis,it is recommended to G_d is used at the macro level and E_z at the meso level as the evaluation index of low-temperature cracking characteristics of asphalt mixture.Finally,through correlation analysis,the effect of asphalt low-temperature performance and asphalt-aggregate adhesion on the low-temperature cracking characteristics of asphalt mixtures is studied.the results show that the low-temperature rheology of asphalt and asphalt-aggregate adhesion have an important impact on the low-temperature crack resistance of asphalt mixtures,and the asphalt-aggregate adhesion has a greater impact on it.