Crumb Rubber Modified Asphalt Binder Thermal-oxygen Aging Mechanism And Performance Evaluation

Many waste tires accumulate to pollute the environment and occupy land resources.The waste tires are crushed into powder to be used as asphalt modifier to improve the durability of asphalt pavement.During the service of crumb rubber-modified asphalt（CRMA）pavement,its performance is changed by aging.CRMA is a multi-phase composite material,which leads to a more complex performance evolution,and the understanding of the aging mechanism of CRMA is also limited.Currently,the aged CRMA pavement faces regeneration,the classification of the CRMA aging stage is the premise of aging asphalt regeneration,but the current research is less relevant.Because of the above key issues,this paper mainly carried out the following research:（1）In this study,the aging states of the“asphalt phase”and“Crumb rubbe-r modifier（CRM）”of the aged CRMA were characterized by microscopic testi-ng methods（SEM,GC-MS,FTIR,GPC）.For the“asphalt phase”,the content o-f carbonyl,sulfoxide,and polybutadiene functional groups in the CRMA increas-ed during the aging process.For the“CRM”,the CRM degrades and releases macromolecular polymers into the asphalt phase and further degrades into small molecule components.Degradation and cross-linking of“CRM”in aged CRMA co-occur.With the degradation of“CRM”,the rubber hydrocarbon content in th-e“CRM”is reduced,and carbon black and other components are released into the asphalt phase.CRM degradation products in the asphalt phase and aging to produce 4-（2-thienyl methylamino）-methyl benzoic acid,followed by 4-hydroxy-6-methyl-1-thiophene-2-ylmethyl-1-hydro-pyridin-2-one,and 1hydrogen-indole-2-carb-oxylic acid,6-（4-methoxyphenyl）-3-methyl-4-oxo-4,5,6,7-tetrahydroisobutyl ester.The degradation of CRM reduces the particle effect of CRM and the gradual lo-ss of its fluffy internal structure,which causes the reduction of CRM-asphalt p-hase interaction.CRMA does not have a“bee-type structure”.Aging leads to a-n increase in microscopic roughness and microscopic modulus and a decrease i-n surface energy and adhesion.Compared with the base asphalt,CRMA has a smaller roughness and greater adhesion.（2）Based on the rheological indexes,the viscoelastic properties and performance evolution of aged CRMA were studied,and a performance prediction model considering aging time and CRM admixture was established.During the aging,the complex modulus and phase angle of CRMA in the low-frequency domain of the master curves show a“stages”evolution pattern.The“plateau area”appears in the middle-frequency domain of the master curves of the phase angle but disappears gradually with the increase of the degradation of the CRM.The appearance or disappearance of the plateau area and the Black space plot curve,and the slope of the Han curve can be used as indicators to evaluate the degradation of CRM.The fatigue and low-temperature properties of aged CRMA are monotonically decreasing,while its high-temperature properties have a“stages”evolution pattern.Asphalt relaxation and ductility related indicators for evaluating the low-temperature performance of CRMA have limitations.The parameters related to asphalt ductility also have limitations for evaluating the fatigue performance of aged CRMA.The high-temperature performance index G^*/sinδ,low-temperature performance index creep deformationε,and fatigue performance index N_fhave a strong correlation with the microstructure of aged CRMA,which can be used as the performance index to reflect its rheological performance evolution characteristics,and a BP neural network prediction mathematical model was established based on the above indexes.（3）The rheological properties of aged CRMA have a stage evolution pattern,and the non-monotonicity of the rheological indexes can cause a wrong evaluation of its aging degree.This paper evaluates the sensitivity of the aging indexes of CRMA using the entropy value and gray correlation analysis methods.Based on the stages evolution characteristics of CRMA properties,combined with the degradation degree of CRM,the aging stages of CRMA are divided,and the initial guidelines for regeneration of aging CRMA are proposed.Asphalt carbonyl index I_C=O,the microscopic modulus is more sensitive to changes in the aging degree of CRMA.It can be used as an indicator to evaluate the aging of CRMA.The aging process of CRMA can be divided into three stages:StageⅠ（Neat-PAV 20h）,aged CRMA is mainly aging in the“asphalt phase”,regeneration to supplement the light component.StageⅡ（PAV 20h-PAV 60h or PAV20h-PAV 80h）,the CRM has a large degree of degradation,resulting in asphalt softening.Replenishment of appropriate amounts of light components and polymer modifiers during regeneration.StageⅢ（PAV 80h-PAV 100h）,most of the rubber hydrocarbons in the CRM are degraded.During regeneration,light components are supplemented with sufficient polymer modifiers.（4）The BP neural network model was used to establish a quantitative model of the microstructure and properties of aged CRMA,and the microstructural factors affecting the properties of aged CRMA were obtained based on the gray correlation analysis method.CRM-asphalt phase interaction and residual CRM particle effect have significant effects on the high-temperature performance index G^*/sinδand fatigue-performance index N_fof aged CRMA.The aging degree of asphalt phase and the broken chain structure of polymer generated by degradation of CRM have significant effects on the low-temperature creep deformationεof aged CRMA.