Research On The Cooling Effect And Its Improvement Method Of The Thermochromism-Based Asphalt Pavement

The light absorption nature shown by asphalt material makes asphalt pavement absorb a large amount of solar radiation energy and then present excessively high pavement surface temperature in hot summertime.On one hand,the higher pavement surface temperature easily causes the viscous flow of asphalt and leads to the high-temperature pavement distresses like rutting.On the other hand,the photo and thermal oxidation of paving asphalt is accelerated by higher pavement surface temperature,which can further induce other pavement distresses such as cracking and moisture damage.Environmentally,the higher pavement surface temperature also aggravates the urban heat island effect,resulting in the urban microclimate deterioration as well.As a result,taking eff ective measures to lower the service temperature of asphalt pavement in summertime is not only beneficial to prolonging the lifespan of pavement but also improving the urban microclimate.The reflective pavement is able to reduce the amount of heat flux in to the pavement structure through enhancing the reflectance of pavement against solar radiation.Differing from the traditional heat reflective coating materials that exhibit fixed high reflectivity to solar radiation,reversible thermochromic materials pr esent dynamically varying reflection abilities in response to ambient temperature variation,which can attenuate the undesirably cooling effects of traditional heat reflective coating materials on pavement in cold winter.At present,the research work conc erning the application of reversible thermochromic materials in asphalt pavement is still in the preliminary stage.The research topics including the reliability,durability and improvement methods with regard to cooling effect of thermochromic asphalt mat erial need to be further conducted.This is because the systematic research on abovementioned topics can further promote the engineering application of such pavement material,and then contribute to the development of green pavement and the improvement of urban microclimate.In this dissertation,with the aid of optical property characterization,outdoor verification test of cooling performance and finite element numerical simulation,the mechanism behind the cooling effect of thermochromic asphalt materi al on pavement in summertime was revealed,and the cooling pavement efficiency of such material in different regions was explored.Then,the engineering performance differences between thermochromic asphalt material and pure asphalt material were compared with and without consideration of cooling effect of thermochromic asphalt material,so the influence mechanism of cooling effect of thermochromic asphalt material on its engineering performance was revealed.During the photo and thermal aging process,the cooling effect durability of thermochromic asphalt material was analyzed from perspectives of thermochromic microcapsule and its modified asphalt binder and mixture.The differences in engineering and cooling performance of thermochromic asphalt mixtures prepared by different processes were compared,and the influence mechanism of different preparation processes on cooling efficiency of thermochromic asphalt mixture was explored from the integrity and dispersion state of microcapsules in asphalt mixture.The main conclusions are presented as follows:(1)The structural change of chromophore above and below thermochromic temperature makes the reflectivity of thermochromic microcapsule thermally respond in visible light region,and the reflectivity above therm ochromic temperature is larger than that below thermochromic temperature.In addition,the thermochromic microcapsule maintains fixed high reflectivity in infrared region whether above or below thermochromic temperature.The optical properties are consiste nt between thermochromic microcapsule and its modified asphalt mixture.Compared with the solar radiation absorptivity of 0.95 shown by pure asphalt mixture,the solar radiation absorptivity of thermochromic asphalt mixture is reduced to 0.7 0 when the service temperature of thermochromic asphalt mixture is larger than the thermochromic temperature(31℃)of microcapsule,which demonstrates the thermochromic microcapsule can improve the reflection ability of asphalt mixture against solar radiation at the high-temperature duration of summer day.(2)In outdoor cooling performance verification test,the thermochromic asphalt mixture presents lower inner temperatures than pure asphalt mixture,especially at the high-temperature duration of summer day,which illus trates the cooling pavement effect of thermochromic asphalt mixture in summertime.In different regions of China,an average decrease in maximum pavement temperature by about 6 ℃ can be achieved in summer by paving thermochromic asphalt material.(3)On account of the cooling pavement effect of thermochromic asphalt material,the moduli of in-field asphalt binder and mixture at the high-temperature duration of summer day can be largely enhanced and the permanent deformation of asphalt mixture can be obviously reduced.Moreover,the decrease in pavement service temperature,due to the cooling pavement effect of thermochromic asphalt material,can alleviate the aging hardening phenomenon of asphalt binder and mixture during high-temperature service period in summer,which is conducive to improving the relaxation and low-temperature deformation abilities of asphalt mixture that serves in the field for a long period.Nevertheless,the cooling pavement effect of thermochromic asphalt material has little influence on the improvement of moisture damage resistance of asphalt mixture.(4)During the long-term photo oxidation aging process,the structure of thermochromic microcapsule is seriously damaged,and the methyl stearate outflows from the broken microcapsules and penetrates into the aged asphalt matrix.As a result,the volume concentration of methyl stearate in asphalt matrix increases,while the concentration of aging products in asphalt matrix no longer increases monotonously with the aging process due to the dilution effect of methyl stearate.Differing from the pure asphalt that presents an aging hardening phenomenon,the thermochromic asphalt is gradually softened with an increase in aging duration macroscopically and presents a reversing-aging phenomenon.(5)The indoor accelerated ultraviolet radiation aging for twenty days reduces the cooling effect of thermochromic asphalt mixture by more than 50%,indicating the photo and thermal impacts during pavement service process cause serious damage to the cooling effect sustainability of thermochromic asphalt material.Based on the equivalent relationship between indoor accelerated ultraviolet radiation aging effect and outdoor weathering aging effect,it can be inferred that the thermochromic asphalt material can effectively play its role in cooling pavement within about one and a half years of service in Changsha region.(6)Compared with hot-mix asphalt mixture preparation process,the warm-mix asphalt mixture preparation process can improve the high-temperature stability of thermochromic asphalt mixture,but cause adverse effects on the low-temperature performance and moisture damage resistance of thermochromic asphalt mixture.The wet process is superior to the dry process in terms of guaranteeing the high-temperature stability and low-temperature cracking resistance of thermochromic asphalt mixture.As for the effect on moisture damage resistance of thermochromic asphalt mixture,the wet process is similar to dry process.In the dry preparation process of thermochromic asphalt mixture,different material addition orders have a significant impact on the engineering performance of thermochromic asphalt mixture.(7)In the dry process,the later the order of adding thermoch romic microcapsules,the higher the reflectivity of the prepared thermochromic asphalt mixture.In comparison with the dry process,the wet process is more conducive to the uniform dispersion of thermochromic microcapsules in asphalt mixture.Among the three dry preparation processes with different material addition orders,the shorter the mixing time between thermochromic microcapsules and other materials,the more serious the agglomeration of microcapsules in asphalt mixture becomes.Based on the reflection spectrum,fluorescence image and outdoor cooling performance verification test results,it can be found that the cooling effect of thermochromic asphalt mixture is not only influenced by the integrity of microcapsules,but also related to the dispersion of microcapsules in asphalt mixture.(8)The warm-mix process can reduce the preparation temperature of thermochromic asphalt mixture,which can improve the short-term cooling effect but have no obvious effect on the long-term cooling effect of thermochromic asphalt mixture.On the contrary,the dry process cannot improve the short-term cooling effect of thermochromic asphalt mixture due to the agglomeration of microcapsules in its prepared asphalt mixture,but the dry process can improve the cooling effect durability of thermochromic asphalt mixture by reducing the damage degree of microcapsules after long-term aging process of asphalt mixture.