| In recent years,with the increasing heavy traffic volume of asphalt pavement on national and provincial highways in Tibet,and at the same time under the adverse climatic conditions,various kinds of diseases have been gradually produced on the asphalt pavement of national and provincial highways in Tibetan areas.In order to ensure the good performance of Tibetan national highways,relying on the project of Tibetan national and provincial highway asphalt pavement maintenance,this study explores the application of Ultra Thin Overlay maintenance technology on asphalt pavement of national and provincial highways in Tibetan areas based on such factors as topography,climatic features,structural features of pavement,and incentives of typical diseases in Tibetan areas.Firstly,this study analyzes the elements of Ultra Thin Overlay design by making a field investigation on the topography,climatic environment and asphalt pavement condition national and provincial highways in Tibetan areas with combination of major points of Ultra Thin Overlay maintenance technology design.In Tibetan areas,It is found that the average altitude is higher,with large land fall,and many long,upward,and downward slopes and curves are set in the linear design process of national and provincial highways;In this area,the average annual temperature is low,with a large difference between day and night;the dry and wet seasons are distinct,with concentrated precipitation and night rain from mid-June to mid-September every year;And the average annual sunshine hours are long and solar radiation is intensive;In addition,the pavement structure design is single,mainly based on semi-rigid subgrade,the overall pavement condition presents high PCI and low RQI;Typical diseases are temperature cracks,load fatigue cracks,pit slot and loose.Combining the key points of Ultra Thin Overlay maintenance technology design with the results of asphalt pavement in Tibetan national and provincial highways,it is proposed that the design of Ultra Thin Overlay in Tibetan areas should focus on the low temperature crack resistance,water stability performance and anti-slip performance of the asphalt mixture.Secondly,according to the features of Ultra Thin Overlay asphalt mixes in Tibetan areas,raw materials were selected and conventional index tests were carried out to ensure that all properties satisfy the specification requirements.Comparing the existing asphalt mix design methods at home and abroad and combining with engineering experience,the initial grade range of Ultra Thin Overlay was determined by existing design experience of Superpave,and the variation trend between the key sieve passage rate and road performance was analyzed by orthogonal design method with extreme difference to further determine the grade range.From the orthogonal test polar difference analysis,the recommended range of 9.5mm screen passage rate is 91%-95%,4.75 mm screen passage rate is 34%-48%,2.36 mm screen passage rate is 23%-28%,0.075 mm screen passage rate is 5%-7%.In accordance with these results,balanced mix design is used to propose the Ultra Thin Overlay ratio that is suitable for Tibetan areas with the balance of low-temperature water temperature and low-temperature skid resistance,based on the balance of SCB test,water immersion Marshall test,freeze-thaw splitting test,TD test and BPN test as the balance index of low-temperature performance,water stability performance and skid resistance,and the corresponding grade and asphalt dosage optimization design according to the performance combination chart.The design is based on the performance combination diagram.Thirdly,the two mixes recommended in the study were compared with the three thin layer overlay mixes commonly used in Tibetan areas for low and high temperature performance,water stability performance and slip resistance.The results of the test show that: in the beam bending test,the low-temperature anti-slip graded asphalt mix has the highest bending tensile strain and the lowest bending modulus,and its low-temperature crack resistance is the best,followed by the low-temperature water-stable graded mix;in the high-temperature rutting test,compared with the AC-10 graded thin-layer asphalt mix,the high-temperature performance of the low-temperature water-stable graded mix and the low-temperature anti-slip graded mix improved by31.15% and 37.29%;in the water immersion Marshall test and freeze-thaw splitting test,the water stability performance of the low temperature water-stable and low temperature skid-resistant graded asphalt mixes was improved by 3.90% and 1.85% respectively compared to the Novachip-B graded asphalt mixes;in the construction depth TD and friction coefficient BPN tests,the low temperature skid-resistant and low temperature skid-resistant graded asphalt mixes were improved by 3.90% and 1.85% respectively compared to the AC-10 graded mixes.Compared with the AC-10 grade,the low-temperature skid resistance and low-temperature water-stable grade improved by124.62%,101.54% and 12.92%,3.19% respectively.In other words,the two mixes recommended in this study confirm to the requirements of the specifications for each road performance index,and the low-temperature performance,water stability and skid resistance are better than the other three commonly used thin layer overlays,which meet the needs of asphalt pavement maintenance in Tibetan national and provincial highways.Finally,to further verify the construction feasibility of Ultra Thin Overlay maintenance technology in asphalt pavement maintenance project on national and provincial highways in Tibetan areas,a test road was paved in Shannan City,Tibet,and hrough the application of Ultra Thin Overlay maintenance technology from the selection of road construction materials,asphalt mixture mixes ratio design,quality control in various aspects during construction,and completion acceptance and other sets of technical solutions. |
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