| Waste crumb rubber is an internationally recognized way of recycling waste tire and harmless processing and utilization.Using waste crumb rubber as asphalt modifier can improve the high and low temperature performance of asphalt as a road material and a waterproof material for buildings on the one hand,and has great significance for sustainable development and environmental protection on the other hand.Starting from the stability and compatibility problems of waste crumb rubber in asphalt system,a new clean production technology — power ultrasonic is sought to solve the problem of actual production of crumb rubber modified asphalt through acoustics,mechanical vibration,material science,chemistry and other cross-disciplines,which has a broad prospect.Power ultrasonic is a kind of production technology that intensifies the treatment of liquid medium,changes chemical properties,and improves the uniform mixing of heterogeneous substances.It generates enough energy on the ultrasonic vibration radiation surface,resulting in the movement,deformation and collapse of the micro cavitation nucleus formed in the liquid under the action of negative pressure.It will release huge energy,produce powerful bubble wall shock velocity and extremely high instantaneous temperature.This cavitation effect can reduce the viscosity and degrade the high viscosity materials.Up to now,domestic and foreign scholars have done a lot of work on viscosity reduction in the field of petrochemical industry with power ultrasonic technology as well as ultrasonic cavitation bubble dynamics model construction and optimization.The relevant theory and experimental system have a certain foundation.However,the degradation and in-situ compatibilization of ultrasonic enhanced asphalt and modified asphalt are almost blank,and the ultrasonic process parameters and the optimization of modified asphalt preparative technological process remains to be explored.In view of the definite physical property parameters of complex mixtures,the distribution of liquid cavitation cloud with high viscosity,and the research on the cavitation dynamics equation coupled with various types of cavitation bubble groups driven by ultrasonic field is obviously insufficient.In addition,considering that the experimental cost is expensive and the experimental results are difficult to analyze the microscopic mechanism of the material on the molecular scale,it is very important to analyze the influence of ultrasonic on the compatibility and system stability of the crumb rubber modified asphalt in collaboration with macro performance test,microscopic mechanism characterization and molecular dynamics simulation.Therefore,in this paper,a cavitation bubble dynamic equation under the coupling action of ultrasonic cavitation bubble group in the asphalt modified by complex liquid medium was proposed,the cavitation effect released when the cavitation bubble collapses in the asphalt under the optimization equation based on several typical cavitation bubble dynamics models was explored.Using basic knowledge of ultrasonic cavitation theory,materials science,molecular dynamics,acoustics,mechanical vibration and other disciplines,the influence mechanism of ultrasonic wave on the interfacial compatibility of asphalt and crumb rubber modifiers was analyzed through macroscopic/mesoscopic/microscopic/nano scale collaboration,and the reliability of the method was confirmed.The main work included the following aspects:(1)The physical and cavitation parameters of waste rubber modified asphalt under ultrasonic conditions were studied.The actual power and sound intensity under different ultrasonic amplitudes were tested and calculated by calorimetry.Molecular models of base asphalt and modified asphalt were constructed to simulate molecular dynamics and calculate fluctuation parameters,adiabatic compression rate,density and sound velocity.A cavitation bubble dynamic model considering the coupling effect of saturated vapor pressure and multibubble spherical bubble group was constructed,as well as the change of temperature,pressure and energy of cavitation bubble under adiabatic state.Through numerical analysis and highspeed camera system,the cavitation effect released during cavitation bubble collapse in asphalt was explored.It was found that superimposed cavitation bubble collapse energy under the action of bubble group coupling can theoretically destroy chemical bonds and change the colloidal structure of materials.(2)An experimental device for ultrasound-assisted preparation of modified asphalt was set up.The preparation and strengthening treatment of base asphalt and crumb rubber modified asphalt were carried out according to two different technological parameters and processes including ultrasonic time and collaborative high-speed shear.The basic physical properties of the asphalt prepared under different parameters,such as penetration,softening point,ductility and Brookfield viscosity at 135℃,were tested.Ultrasonic treatment can effectively reduce the Brookfield viscosity of asphalt and improve the viscosity of asphalt compatible with the modifier.In addition,the effect of the process on the aging resistance of asphalt was verified by the short-term aging of simulated asphalt.The results of functional group change determined by FTIR and statistical analysis show that the asphalt prepared by stirring for 30 min,shearing for a certain time and ultrasonic for 20-30 min has the best comprehensive performance.(3)The molecular models of asphalt and crumb rubber modified asphalt were constructed before and after aging at different temperatures.The molecular dynamics simulations were carried out under NVT and NPT ensembles.The reliability of the molecular models was verified by radial distribution function and density results.Perl script was used to apply sinusoidal external forces to the molecular model to simulate ultrasonic vibration.The mean square displacement,diffusion coefficient,free volume fraction,dipole moment,cohesion energy density,solubility parameters,total energy of the system,radius of rotation,viscosity and so on were calculated for all kinds of asphalt after molecular simulation.The asphalt crack model was constructed to simulate the self-healing state of asphalt and compare the self-healing ability of different asphalt.Different asphalt-acid/alkaline aggregate models were constructed,and it was found that the adhesion between asphalt and acid aggregate was stronger.Different asphalt-water-acid/alkaline aggregate models were constructed to calculate the debonding work,energy ratio,hydrogen bond changes and relative concentration distribution changes.Asphaltnano-droplet wetting aggregate surface models were constructed to calculate the contact angle,which jointly revealed the moisture damage resistance ability when different asphalts adhere aggregates.(4)Optical microscope with a large depth-of-field was used to observe the microscopic dispersion of asphalt prepared by different ultrasonic processes.It was found that the agglomeration phenomenon of crumb rubber particles was more serious without collaborative treatment by high-speed shearing process,and the dispersion of crumb rubber modified asphalt after collaborative high-speed shearing process was still good after aging.The high temperature stability segregation test of the prepared asphalt was carried out,the softening point difference of the upper and lower sections was calculated,and the dispersion of crumb rubber particles in the upper and lower sections was observed under LSCM.It was found that the crumb rubber particles in the upper section were almost absent,and the segregation phenomenon was obvious.Through the crack simulation of asphalt samples prepared by different ultrasonic processes,the crack depth under different self healing time at 25℃ was scanned by LSCM to construct the crack healing rate curve.The adhesion between asphalt and coarse aggregate was tested by boiling method.Combined with the comparison of the wetting angle of asphalt on granite surface,the performance of ultrasonic modified asphalt was explored on a multi-scale.Finally,it was found that ultrasonic combined with high-speed shearing process had a positive effect on the stability of crumb rubber modified asphalt,but it still had a certain aging effect. |
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