| With the construction of the "Belt and Road" interconnection and open channel,the layout focus of the highway network will shift towards the western regions and areas of low latitude.The Tibet Plateau is the core area of the "Belt and Road," characterized by its perennial extreme low temperatures,vast diurnal temperature variations,and intense ultraviolet radiation.These conditions accelerate the aging process of asphalt pavement,leading to early damages such as low-temperature cracking,greatly shortening the service life of pavement.However,the current research primarily involves the aging mechanisms and aging properties of asphalt under the effect of thermal oxygen,ultraviolet,or thermos-photo-oxygen,but the depth of research on the mechanism of ultraviolet aging is not insufficient.The strong ultraviolet radiation in high-altitude and cold regions often accompanies large temperature variations.Therefore,elucidating the aging mechanisms under the combined effects of intense ultraviolet radiation and large temperature variations,as well as mastering the evolution pattern of macro rheological properties,is beneficial for accurately analyzing the causes of asphalt pavement diseases in high-altitude and cold regions,which better guides the design and use of asphalt pavement structures as well as subsequent pavement maintenance design and decision-making.Based on these key issues,the following studies are as follows:(1)Based on the investigation of national radiation,temperature differences,and relative average humidity patterns,this study particularly analyzed the climatic environmental characteristics of high-altitude and cold regions(the Tibet Plateau area),determining the environmental parameters for laboratory intense ultraviolet and large temperature variations coupled aging experiments.A high-pressure mercury lamp was chosen as the light source.Due to the release of a significant amount of heat by the high-pressure mercury lamp during radiation in indoor simulation experiments,traditional environmental chambers were unable to meet the requirements for cooling rate and maintaining low temperatures under strong ultraviolet radiation.Therefore,relying on the advanced technology of the artificial variable environment box owned by the National Bridge and Tunnel Key Laboratory at Chongqing Jiaotong University,a set of indoor intense UV radiation and large temperature variations combining accelerated aging experimental device was designed and assembled using high-pressure mercury lamps and other equipment.(2)The asphalt aging mechanism under the combined effects of intense ultraviolet radiation and large temperature variations was revealed.The analysis of the chemical components,microscopic morphology,and micro-mechanical changes of the base asphalt and SBR modified asphalt revealed that the aging mechanism under the combined effects differs significantly from the traditional aging mechanisms(thermal oxygen aging,ultraviolet aging,and large temperature variations effects).(1)Asphalt exhibited a distinct "gradient aging" phenomenon under combined aging.It was found that asphaltenes played a crucial role throughout the aging process,and the aging pathway could be described with asphaltenes as the main thread.The surface asphalt initially underwent photo-oxidative reactions,forming hydroxyl,carboxylic acid,ketones,esters,and anhydrides.Subsequently,photocondensation and photocrosslinking reactions dominated,with the main products being asphaltenes and their photo-oxidized cross-linked polymers,ultimately forming a thin film layer that is non-melting,brittle,and hard at high temperatures.The sub-surface layer of asphalt experienced the reorganization and aggregation of asphaltenes,while in the lower layers,new asphaltenes were generated under the coupled effects,along with the decomposition of large molecular asphaltenes and the "migration" of small molecular asphaltenes to the sub-surface layer,and the decomposition of large molecular asphaltenes was mainly influenced by the large temperature variations.(2)The addition of SBR modifiers resulted net-like structure in colloidal systems,adsorbed g a large number of asphaltene molecules in the early stages of aging,thus reducing the decomposition rate of large molecules and the "migration" rate of small molecules.During the aging process,the degradation of the SBR compensated for the loss of medium and small molecules,thereby delaying the aging process.(3)The "gradient aging" of asphalt under combined effects led to significant differences in mechanical properties,which are crucial for the initiation and development of surface cracks of asphalt.The study suggested that a "modulus ratio" to quantitatively assess the risk of surface cracking.A higher modulus ratio indicates a greater risk of cracking or a higher crack rate,and there is a peak value of the "modulus ratio" in the later stages of aging,indicating the most unfavorable point in time for cracking.(3)A detailed analysis of the rheological behaviors of asphalt under the combined effects of intense ultraviolet(UV)radiation and large temperature variations was conducted.The evolution law of rheological properties(full temperature viscoelasticity,high temperature stability,low temperature cracking resistance and medium temperature fatigue properties)was studied under the combined effects of intense UV radiations and large temperature variations,which mainly represents the characteristics of the lower layer asphalt.It was found that the sensitivity of macro properties of the lower layer asphalt to the combined effects of intense UV radiations and large temperature variations is as follows: high temperature properties > fatigue properties > low temperature properties.Notably,the high temperature stability with the aging time,showing a "first increase and then decrease" law,indicating that the high temperature performance of asphalt deteriorated in the later stages of aging.This pattern provides a good explanation for the phenomenon that rutting distress is a common issue for asphalt pavements in high-altitude cold regions,ranking second only to cracking distress.Furthermore,high-temperature stability(rutting factor)and fatigue performance(the VECD model)were proposed as key indicators for evaluating the rheological performance of the overall asphalt under the combined effects.Meanwhile the changes in the parameters of SBR asphalt under the combined effects was significantly reduced,indicating that the SBR modifier effectively mitigated the aging process.(4)Research on anti-aging enhancement measures for asphalt under the combined effects of intense ultraviolet radiation and large temperature variations was conducted.Based on the aging mechanism of asphalt under the combined effects of intense ultraviolet radiation and large temperature variations,the evolution of macro rheological properties and the comprehensive consideration of asphalt pavement materials,high temperature rutting resistance,low temperature cracking resistance,the selection of PPA modified SBR asphalt in order to improve combined aging resistance.The macroscopic rheological performance analysis indicated that polyphosphoric acid(PPA)significantly enhanced the high-temperature stability and fatigue performance of asphalt,reducing the impact of the combined effects on the rheological properties.Chemical composition and microscopic morphology analysis results showed that PPA led to a more uniform distribution within the asphalt system,improving the compatibility of the SBR modifier with the asphalt.Additionally,it effectively reduced the degradation effects of the asphaltene and SBR modifier under the combined effects of intense UV radiation and large temperature variations,thus enhancing the stability of SBR modified asphalt during the aging process.Additionally,the Polyphosphoric Acid(PPA)reacted with the base asphalt to create phosphate esters,which served to decompose hydroperoxides into alcohols,diminishing free radical activity,and thereby effectively improving the asphalt’s resistance to photo-oxidative aging.This is a distinct difference from the SBR mechanism,which passively mitigates aging through its own degradation.Therefore,anti-aging effect of SBR is considered "passive".PPA contributes to the "active" anti-aging for the asphalt.Meanwhile PPA significantly reduces the "modulus ratio" of aged SBR modified asphalt,considerably lowering the risk of surface cracking and crack rate,and enhancing the crack resistance of SBR-modified asphalt under the combined effects of UV radiations and temperature variations. |
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