| The rapid development of highway engineering demands a large number of high-quality asphalt materials,the non-renewability of petroleum resources determines the unsustainability of asphalt materials.The partial replacement of asphalt by bio-oil and the asphalt modification by crumb rubber(CR)have shown enormous potential to prompt the sustainability and high-performance of asphalt pavement materials,however,the asphalt obtained by a single modification method cannot simultaneously meet the new requirements of highway engineering for pavement materials sustainable and high-performance development.Hence,bio-oil/CR composite modified asphalt(CMA)is prepared by combining bio-oil with CR modification technology,the high dosage and high-performance application of solid waste and alternative asphalt materials in road engineering can be realized,through the complementary advantages and synergistic modification between the bio-oil and CR.To do so,the compatibility among components of CMA was firstly explored,the properties evolution of CMA was investigated based on the swelling-degradation behavior and the composite modification mechanism was revealed.Then,the technological parameters of CMA preparation were proposed,and the modification effect was analyzed by pavement performance evaluation.Moreover,the durability in service of CMA were emphatically studied.Finally,the pre-treatment of CR with bio-oil was proposed to enhance the storage stability and swelling properties of CMA.Molecular interaction at nano scale directly affects and determines the material’s macro properties.The compatibility among components of CMA was investigated by molecular simulation.The simulation results show that the bio-oil molecules inserted and filled into the asphalt molecules;the aromatics,resins,and asphaltenes are mixable with the bio-oil,and showed good compatibility;meanwhile,the diffusion coefficient of the bio-oil molecule is 1.9 times as much as that of asphalt in the diffusion-fusion process due to the smaller molecular size and molecular polarity,and finally,a homogeneous system was formed.Comparing to the asphalt,bio-oil is filled in the gaps of the rubber molecules and can significantly swell the rubber.Besides,the overall performance of CMA was codetermined by the supplement of bio-oil to the light components of asphalt,CR swelling,degradation,and asphalt aging.In the early stage,the overall stiffness of asphalt increased because of the elastic semisolid network that was formed by the gel film on the swollen CR surface;in the medium stage,the fatigue property of asphalt was improved obviously because of the three-dimensional network structure that was formed by the released polymer from degraded CR;in the later stage,however,the overall aging of the asphalt aggravated the properties deterioration of the CMA,and the modification mechanism of the CMA was revealed.Carrying out bio-oil modification first to supplement the light components in the asphalt can meet the subsequent demand for CR swelling,and the corresponding CMA has better pavement performance.The optimum blending parameters of bio-oil and asphalt were 140℃,30 min,5000 r/min,which was determined based on the compatible blending theory and orthogonal experimental design.The bio-oil modified asphalt that was prepared by the aforementioned method was a compatible system both in thermodynamics and at the micro scale.The CMA was prepared by three stages of CR swelling,shear dispersion,and breeding.The influence of bio-oil and CR modification on the CMA properties were analyzed and found that bio-oil can significantly improve the low-temperature cracking resistance of asphalt through dilution,swelling,and increase of viscous response,the low-temperature ductility,and fracture energy was increased by 72.3% and 292.3%,respectively,when the bio-oil dosage is 20%;the CR mainly improved the high-temperature performance with its micro-skeleton and elastic response,the softening point was increased by 45.3%,and non-recoverable creep compliance was reduced to 4.1% of that without CR,when the CR dosage is 20%.Terms thermal-oxidative and photo-oxidative aging characteristics of CMA also were investigated to evaluate the durability in service.The results show that the special groups in the bio-oil penetrated into the asphaltenes nanoaggregates,thus reducing theπ-π conjugate interaction among the fused polycyclic aromatics of asphaltenes by disturbance of electron,inhibit the aggregation of asphaltenes during the aging effectively.After thermal-oxidative aging,the properties degradation of CMA was lower than that of the CRM.The fatigue factor of CMA was 76.4% of that of unaged CRM,and crossover frequency was 10.5 times as much as that of aged CRM,the glass transition temperature was 2.9℃ lower than that of unaged CRM.The chemical characteristics analysis indicated that the effect of thermal-oxidative aging on the chemical properties of CMA was lower than that of CRM.Photo-oxidative aging testing results show that the aging resistance of CMA significantly depends on the source of the bio-oil,the properties deterioration of castor-based CMA was significantly lower than that of other asphalt after long-term UV exposure due to the abundant light components and unsaturated groups.The inhibition of bio-oil molecules on asphalt aggregation,and the light components in bio-oil,act as the stabilizer of asphalt colloidal structure,could effectively compensate for the loss of light components during the asphalt aging,and the unsaturated groups,act as active free-radical scavengers,could effectively protect asphalt molecules from the attack of free-radicals produced by the aging reaction.Moreover,the absorption of the light components by CR swelling,and the slow release of light components by CR degradation will further enhance the anti-aging performance of the CMA.The feasibility of CR pre-treatment by bio-oil to improve the storage stability and swelling properties of CRM was explored.It was found that bio-oil molecules could quickly migrate to the rubber interface and invade the rubber structure compared with asphalt molecules,the interaction energy of bio-oil and rubber was 24.6% higher than that of the asphalt and rubber,showing obvious adsorption priority.Hence,CR was directly mixed with the bio-oil firstly,surface activated rubber was prepared by grafting ester molecules on the rubber surface through chemisorption with microwave assistance,pre-swelled rubber was prepared by making the bio-oil molecules diffusion into the rubber through physisorption with heat-treatment.The properties testing results showed that the segregation index and softening point difference of the surface grafting activated CMA was reduced by 41.0% and 44.0%,respectively,comparing to that of CRM.For pre-swelled CMA,the swollen rubber phase area was 6.6 times as much as that of CRM,and the percent recovered of CMA was increased by 156.0%.The mechanism of CR pre-treatment using bio-oil to improve the CMA performance was analyzed.The surface activated rubber particles can anchor and overlap each other thus distributing in the asphalt matrix more stably,then the storage stability of CR modified asphalt was improved.The swollen rubber particles acted as an elastic micro-skeleton in the modified asphalt system like elastic balls,so the CRM has great resistance to permanent deformation. |
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