| Permeable asphalt mixture is a pavement material with a void ratio greater than 18%and a large number of connected voids.Because of its excellent performance in drainage,anti-skid,noise reduction and improvement of urban heat island effect,which are not available in traditional dense-graded asphalt,it has become a focus of attention in the construction of functional roads in China.However,compared with the macro damage of traditional pavement such as cracks,bumps and pits,the frost heaving and meso-structure decay of ice crystals caused by changes in temperature,moisture and other factors have become the main factors hindering the popularization and application of pervious asphalt mixture in the service environment of cold regions.In order to further understand the low temperature performance decay mechanism of permeable asphalt mixture in cold regions,this paper,relying on the project of Jilin Provincial Department of Communications "Research on Key Technologies to Improve the Road Usability of Asphalt Pavement in Seasonal Frozen Regions",discusses the evolution law of the internal structure of permeable asphalt mixture in the service environment in cold regions from the macro and micro perspectives through tests.At the same time,on the basis of the previous quantitative evolution of voids,the dynamic development process of ice crystals under the influence of water migration and the meso damage mechanism are visually analyzed by using the finite element numerical simulation technology,and targeted improvement measures are proposed for the design of permeable asphalt mixture under the effect of ice crystal frost heaving.The specific research contents and conclusions of this paper are as follows:1、In the macro damage study of permeable asphalt mixture(OGFC-13 and OGFC-16)based on the service environment in cold regions,it was found that after thousands of times of dynamic water pressure washing,the strength of 13-III and 16-III specimens with 25%voids decreased by about 30%and 40%respectively,significantly greater than those with 20%and 23%voids.In the 10min extreme plugging test,the maximum plugging depth of the plug is about 20mm,and the permeability coefficient of all test pieces after plugging will not exceed 0.06cm/s.In addition,OGFC-13 has better frost resistance than OGFC-16 in continuous frost heave simulation and the damage degree of the two mixtures is 0.96%～32%and 5.88%～42.57%respectively.2、In the research on the evolution law of the meso-structure of permeable asphalt mixture,it is found that after the dynamic water scouring,the internal voids of asphalt mixture with a void ratio of 20%mainly become smaller after the fine aggregate or asphalt peeling.The internal voids of asphalt mixture with 23%and 25%voids are mainly connected and expanded.Compared with the original structure and erosion damage,the number and size of voids after plugging test increase,and the contour of voids becomes more complex.In the quantitative analysis,the order of void fraction under the influence of different service environments is:frost heaving state>scouring state>original state>plugging state.In terms of void size,the equivalent diameter of voids under all working conditions is mainly 1.18～4.75mm,accounting for about 50%of the total number of voids.In the fractal dimension of the void profile,the fractal values before and after void evolution remain around 1.1,indicating that asphalt mixture damage does not have a significant impact on the internal void morphology.3、Based on the grey theory,the internal relations among the external influencing factors,void evolution and the macro characteristics of the mixture are analyzed.The results show that the correlation degree of OGFC-13 is above 0.6 when the aggregate size is greater than 0.6 mm in terms of influencing factors of void structure evolution.OGFC-16 has a significant effect on void evolution only when the aggregate size is greater than 1.18mm.In terms of the influence of service environment on the evolution and strength of void structure,the influence degree of frost heaving>the influence degree of scouring>the influence degree of blocking has become a factor that must be considered in the construction of permeable asphalt mixture pavement in cold regions.4、From the simulation results of temperature field and water separation field in the early frost heaving stage of pervious asphalt mixture in cold regions,in November and December in winter,the asphalt mixture is mainly damaged by in-situ frost heaving of water,and in January,the asphalt mixture is mainly damaged by segregation frost heaving of water migration.During the dynamic development of ice crystals,the correlation between the freezing rate of ice crystals and the void size and the ambient temperature is greater than that between the freezing rate of ice crystals and the grading type.At the same time,unfrozen water is more likely to gather to the front edge of ice crystals in the heating stage of the mixture matrix,and the condensation becomes larger in the cooling stage.5、From the perspective of the whole dynamic development of ice crystals,the attenuation rate of frost heaving stress of ice crystals with diameters of 2.36mm and 4.75mm is significantly higher than that of ice crystals with diameters of 9.5mm.The former has a frost heaving influence range of about 8mm,while the latter has a frost heaving influence range of about 10mm.In the influence analysis of void shape,the difference between the simplified void with the same area and the actual void in the maximum frost heaving force,frost heaving influence range and frost heaving stress deceleration rate is small.It shows that the simplified void model,which is convenient for automatic generation of programs,fully meets the needs of frost heaving damage research on ice crystals of pervious asphalt mixture in cold regions.6、In the comprehensive performance evaluation analysis and optimization design,the comprehensive score of OGFC-13 is more than 0.6 in two groups,which is better than the comprehensive score of OGFC-16.Combined with the analysis results of main influencing factors(porosity>asphalt binder>aggregate gradation>aggregate type),it is finally determined that the target porosity of permeable asphalt mixture in cold regions should be controlled between 20%and 23%.The pass rate range of key particle size 2.36mm and 4.75mm is 13.3%～16.3%and 16.2%～20.5%respectively.Conventional road building materials can be used for coarse aggregate under the condition that the index parameters meet the specification requirements.Star shaped SBS modified asphalt or rubber powder modified asphalt with better low-temperature adhesion and deformation capacity can be selected as asphalt. |
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