| Plant-mixed hot recycled asphalt mixture is one of the most widely used recycling technologies due to its stable quality.The seasonally frozen areas in China account for more than50% of the total area,and the asphalt pavement in this area has high requirements for lowtemperature crack resistance.However,due to the defect of low-temperature crack resistance of recycled asphalt mixture,its promotion and application in this area are limited.This paper focuses on the optimization of the low-temperature performance of hot recycled asphalt mixture in the seasonally frozen areas.First of all,taking the climate characteristics of Changchun City as an example,the typical road structure of asphalt pavement in the seasonal frozen area was selected,the temperature field model of asphalt pavement was established,the characteristics of the temperature field of asphalt pavement was analyzed,and the temperature stress damage model of asphalt pavement was established,the main causes of low-temperature cracking of asphalt pavement in the seasonal frozen area and the impact of the characteristics of asphalt mixture materials on the temperature stress damage were analyzed,the optimization method of low temperature anti-cracking performance of recycled asphalt mixture was also proposed and verified based on the existing standard test methods and evaluation indicators.The results showed: the temperature change rate of the surface in seasonal is the largest,and the cooling rate is usually about 1.0℃/h,which can reach 2℃/h when the temperature drops suddenly;The direct cause of low-temperature cracking in the first year is the continuous drop or sudden drop of temperature;Modulus is the main factor affecting the low-temperature crack resistance of asphalt mixture,and relaxation time and temperature sensitivity are the main factors affecting the modulus of asphalt mixture.The method of increasing the fine RAP content to increase the total asphalt aggregate ratio was proposed to improve the low-temperature crack resistance of recycled asphalt mixture and test results showed that this method can improve the low-temperature crack resistance of recycled asphalt mixture.Then,based on the problem that the fracture energy cannot well predict the low-temperature cracking condition of the pavement on site,the concept of energy absorption rate was proposed to calculate the fracture energy,and the problem that the fracture energy is used as an evaluation index is analyzed;The geometric and physical significance of fracture energy of restrained specimen thermal stress test(TSRST),three-point beam bending test(3PBB),semicircular bending test(SCB)and dis-shaped compact tensile test(DCT)were studied,and the factors affecting the fracture energy were analyzed;Based on the energy analysis of the lowtemperature cracking of asphalt pavement,a new evaluation index of low temperature cracking resistance of asphalt mixture based on energy is proposed.The results show that the asphalt mixture with higher fracture energy does not mean its freezing breaking temperature is lower;The fracture energy is mainly determined by the load-displacement curve near the peak load;The main component of fracture energy is surface energy.The fracture energy of asphalt mixture with more aggregates fractured at low-temperature or produced with high surface energy aggregates is higher.Dissipation energy,which is consistent with the evaluation result of fracture temperature of asphalt pavement and has better distinguish different asphalt mixtures,is recommended as the evaluation index of low-temperature crack resistance of asphalt mixture.Then,based on the SCB test,the calculation method of dissipated energy was proposed,and the test is designed to evaluate the low temperature cracking resistance of asphalt mixture;ABAQUS was used to analyze the stress characteristics of the not-notched SCB creep test(NNSCB),and the calculation method of dissipated energy of NN-SCB creep test was proposed.The test was designed to evaluate the low-temperature cracking resistance of asphalt mixture;Based on the characteristics of cylinder sample,seven creep test methods were designed.ABAQUS was used to analyze the stress characteristics of each test specimen,an appropriate test method was selected,and its test conditions were determined.The test was designed to evaluate the low-temperature cracking resistance of asphalt mixture.The results showed: the fracture energy of asphalt mixture first increases and then decreases with the increase of test temperature,which is caused by the change rule of dissipation energy with temperature.The dissipation energy of asphalt mixture can be estimated by the difference of SCB test fracture energy under two test temperatures.When the dissipation energy of asphalt mixture is calculated at 0℃ and-12℃,the evaluation result is reasonable,but the variability is large;The percentage of tensile dissipated energy in NN-SCB creep test is 31.5%,and the evaluation result of tensile dissipated energy in NN-SCB creep test is reasonable,but its energy consumption is mainly the tensile area near the specimen indenter,with insufficient volume representation,and the variability of test results may be large;The tensile stress and tensile dissipated energy of the arc specimen tensile creep test(ASTCT)are evenly distributed.ASTCT test is recommended to be used as the low temperature crack resistance test method.As t tensile creep dissipated energy is high in ASTCT test,its calculation method is simple,and is also highly distinguishable to different asphalt mixtures,and sensitive to asphalt aggregate ratio and recycled agent,it is recommended to be used as the low temperature crack resistance evaluation index of ASTCT creep test.Finally,the distribution of asphalt in asphalt mixture was analyzed by combining the results of electronic scanning electron microscope and nano-indentation test,and the influence of different asphalt distribution on low temperature crack resistance was analyzed by ABAQUS,and the concept of invalid free asphalt was proposed;The activation method of invalid free asphalt was proposed,the idea of grading optimization was analyzed,and the fine grading was optimized based on the compressible accumulation model.Several groups of fine candidate gradation were selected for ASTCT test to determine the optimal fine aggregate gradation;Based on the particle reinforcement theory,a meso model of asphalt mixture was established.The influence of different volume fractions of coarse aggregate on low temperature crack resistance was analyzed by finite element simulation.The coarse aggregate volume fraction and the optimized gradation of recycled asphalt mixture were determined.The ASTCT test was used to verify the high and low temperature performance of the optimized gradation;According to the climate characteristics of seasonal freezing area and the performance requirements of asphalt pavement,the mix design method of optimized gradation recycled asphalt mixture was determined.The results showed that the asphalt inside the asphalt mixture can be divided into three states: the asphalt film adhering to the aggregate surface(including structural asphalt and free asphalt)and the filled free asphalt.The thicker the asphalt film on the aggregate surface,the better the low temperature crack resistance.The filled free asphalt in the void has little effect on the low temperature crack resistance,which can be regarded as ineffective free asphalt;The low temperature crack resistance of asphalt mixture based on G2 fine gradation was 67.6%higher than the AC-20;The optimum volume fraction of coarse aggregate for low temperature crack resistance of asphalt mixture is 30%.Based on the high temperature performance requirements and the performance characteristics of recycled materials,the volume fraction of coarse aggregate was determined as 50%;The low temperature crack resistance of the optimized gradation at 40% RAP content is 1.2 times higher than that of the AC-20 with 40% RAP content,and the high temperature performance meets the specification requirements;The rotary compaction method was recommended as the method to determine the optimum asphalt aggregate ratio for optimizing gradation.The design void ratio is recommended to be 3%,VMA > 12%,69% < VFA< 79%,and the powder cement ratio is 0.9~1.6;It is suggested that the freeze-thaw cycle of salt solution should be used as the verification condition for the water stability and low temperature crack resistance of recycled asphalt mixture;The low temperature crack resistance of optimized gradation recycled asphalt mixture with 40% RAP content is better than that of AC-20 with 20% RAP content,and its water stability is equivalent. |
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