Research On The Influence In Fatigue Characteristics Of Asphalt Concrete Subgrade Protecting Layer In High-speed Railway Induced By Freeze-thaw Cycles

It is a key technical issue to research the evolvement rule of fatigue characteristic parameters and fatigue life of asphalt concrete subgrade protecting layer in high-speed railway enduring environmental load in extreme long-term service life.Based on newly-built test section of asphalt concrete subgrade protecting layer in Beijing-zhangjiakou high-speed railway project,this paper studies the evolvement rule of asphalt concrete fatigue characteristic under series of freeze-thaw cycles,proposes fatigue life prediction model of asphalt concrete protecting layer considering freeze-thaw cycles and provides technical support for durability design of asphalt concrete subgrade protecting layer in high-speed railway.Firstly,according to historical meteorological data in site,simulation test of domestic freeze-thaw cycles was designed and implemented on field specimens.Besides,four-point beam fatigue test in control-strain mode was determined to be in utilization as test method in accord with dynamic response of asphalt concrete subgrade protecting layer.Moreover,fatigue tests of asphalt concrete endured series of freeze-thaw cycles were carried out.Secondly,based on dissipated energy method including Ratio of Dissipated Energy Change（RDEC）,Hopman Energy Ratio,Pronk Energy Ratio and Rowe Energy Ratio,test data obtained from four-point beam fatigue test was analyzed and fatigue parameters including Plateau Value（PV）,change law of N1 Point_（HP）representing load cycle in accord with slope changing point of linear rising period on Hopman Energy Ratio curve,N1 Point_（P）representing load cycle in accord with slope changing point of linear rising period on Pronk Energy Ratio curve and NF Point representing load cycle in accord with peak of Pronk Energy Ratio curve were obtained.Then evolvement rule of those four fatigue parameters with unmber of freeze-thaw cycles were drawn as follows:rising speed of PV increases with increment of freeze-thaw cycles in exponential function and PV is increasingly more sensitive to freeze-thaw cycles with increment of freeze-thaw cycles.Descending velocities of N1 Point_（HP）,N1 Point_（P）and NF Point values of asphalt concrete decrease with increment of freeze-thaw cycles and they three are in exponential function.After comparsion of N1 Point（HP）,N1 Point（P）and NF Point,average value of NF Point were selected for validating both fatigue life predition models considering freeze-thaw cycles.Then,based on the relationship of PV-N_f50,where N_f50 is known as 50%initial stiffness reduction as the fatigue failure point,general fatigue life prediction model of asphalt concrete subgrade protecting layer was raised as follows:PV（28）0.0374N_f^-0.829.Furthermore,in utilization of this general life prediction model and change law of PV with number of freeze-thaw cycles,fatigue life prediction models considering freeze-thaw cycles were raised,both of which were verified by average value of NF Point and exerted excellent validity.Lastly,by introduction of Equivalent Number of Free-Thaw Cycle,freeze-thaw cycles are connected with service time.Accounting for the excitation test result conducted in site,fatigue limit of asphalt concrete and engineering reliability,150μεwas selected as the strain value to predict the fatigue life of asphalt concrete subgrade protecting layer in pratical service condition,after which the PV of asphalt concrete endured full service-life freeze-thaw cycles was gained through fatigue test in 150μεcondition.Based on the general fatigue life prediction model mentioned above,service life checking computation of Beijing-zhangjiakou high-speed railway asphalt concrete protecting layer was implemented and result showed that Beijing-zhangjiakou high-speed railway asphalt concrete subgrade protecting layer could meet the requirement of train loading times throughout service life.