| Early-age cracking of cast-in-situ concrete track bed of ballastless track in high-speed railway occurs regularly,due to material performance,construction environment,construction technology and other factors.The crack obviously affects the overall stress state and durability of the track bed.The early-age cracking of concrete track bed is a complex time-varying chemo-thermo-hygro-mechanical coupled problem,which is lacking of systematic research in both time-varying behavior and cracking mechanism up to now.Furthermore,the research on the influence of material performance,environment and construction factors on early-age cracking are pretty insufficient.As a result,it is difficult to put forward targeted risk management and control measures on early-age cracking of the track bed.Firstly,this paper deeply summarized the existing theoretical research and test results of early-age concrete hydration field,temperature field,humidity field and mechanical field.In addition,this paper systematically expounded and defined the field coupling relationship of early-age concrete track bed.Based on the degree of concrete hydration,the field equations of the whole process for concrete beginning from its early stage were uniformly described.According to relevant research and experiments,the characterization of each field variables and parameters,as well as the value range of the parameters were comprehensively sorted out and checked.The characteristic description and treatment method of convective heat transfer、radiative heat transfer on the surface of the track bed were purposed,which subjected to the association of atmospheric environmental factors such as solar radiation,wind speed,ambient temperature and humidity,as well as early-age concrete curing measures.Moreover,the early-age creep of concrete caused by various factors was thoroughly considered.And based on the chemo-thermo-hygro-mechanical coupling theory,prediction model for early-age track bed cracking of ballastless track was established.Secondly,the research proposed a feasible subsystem decoupling scheme,a multi-field hybrid finite element order selection method,a multi-field coupling finite element coordinated grid division rule,and a solution process error control standard.Using of secondary development of COMSOL Multiphysics platform,the calculation and prediction program for early-age track bed cracking risk were completed.A new early-age concrete stress ring restraint test was designed and completed by using quartz glass and invar alloy as restraint ring materials.Comparing the results between theoretical calculation and ring restraint test,as well as the test data of 20 working conditions of early-age concrete,the prediction model calculation procedure were fully verified for early-age track bed cracking of ballastless track.Subsequently,the time-varying behaviors for the early-age track bed were systematically analyzed,such as hydration field,temperature field,humidity field and mechanical field(include ing cracking risk and cracking form).Through the comparative study,the influence mechanism and modeling optimization of key factors such as twin-block sleepers,humidity field and creep on the cracking of the early-age track bed were clarified.The research illustrates that,in terms of hydration field,the maximum hydration rate of the track bed occurs at about 7 hours after concrete pouring;the final setting time of the track bed is about 9.5 hours after pouring;the hydration process of the track bed on the 7th day can reach to 90%of the final degree of hydration.In terms of temperature field,convective heat transfer and solar radiation are the main environmental factors affecting the early-age temperature field of the track bed.The temperature of the track bed at first zero stress state is about 26.6°C,and it reaches to the highest temperature about 16 hours after pouring,which is about 15°C higher than the mold temperature.In terms of humidity field,there is local uneven humidity area near the twin-block sleeper in the early-age track bed,and the maximum humidity gradient can reach 117%/m.In terms of mechanism field,In the 30th hour,the cracking risk factor of the new and old concrete joint around the sleeper exceeds 1.0;on the 7th day,the cracking risk of the splay cracks of the track bed near the sleeper reaches the maximum,with a risk factor of about 0.75;temperature deformation is the dominant factor for the cracking of the surface layer of the track bed,and the humidity deformation is the main driving force for the cracking of the bottom of the track bed.Through the comparative study of sleeper,humidity and creep,it is found that the twin-block sleeper is the main structural factor that induces cracking of the track bed,and its related structural details should be considered in the calculation and modeling.The humidity field mainly affects the cracking of the track bed by changing the temperature and producing humidity deformation,which cannot be ignored in the calculation and modeling.If the early-age concrete creep of the track bed is not considered,the risk of cracking will be greatly overestimated(the risk factor will increase by about 1 times).Therefore,the creep should be considered in the modeling,through a simplified creep model was allowed.Finally,this paper focuses on series factors which can influence the early-age cracking of the track bed,including the material factors represented by the ultimate heat of hydration and exothermic peak time,the construction environment factors represented by the temperature difference between day and night,the average daily cooling rate and the solar radiation intensity,and the construction technology factors represented by the concrete mold temperature,pouring time and insulation coefficient.As well as,the comprehensive control measures on early-age cracking of the track bed purposed.The research shows that,in terms of material factors,the cracking risk increases rapidly with the increase of the limit hydration heat.When the limit hydration heat exceeds 430 k J/kg,the maximum cracking risk coefficient of the surface of the track bed exceeds the limit value of 0.7;The cracking risk has a concave function relationship with the exothermic peak time,the optimal exothermic peak time is about9 hours after concrete pouring.The faster or slower hydration process will significantly increase the cracking risk of track bed.In terms of environmental and atmospheric factors,the cracking risk increases linearly with the temperature varies from day to night.When the temperature difference between day and night is 18°C,the cracking risk of the surface of the track bed is close to 0.9.Additionally,the cracking risk factor increases linearly with the average daily cooling rate.When the cooling rate is 0.75°C/d,the cracking risk factor of each layer of the track bed is above 0.87.Consequently,the track bed has a greatly overall cracking risk during the continuous cooling process of atmospheric temperature;the solar radiation intensity has the great impact on the surface cracking risk.The surface cracking risk increases linearly with the radiation intensity,and when the intensity reaches 800(W/m~2·K),the cracking risk of the surface layer is about 0.77.In terms of construction technology factor,lower temperature is necessary owing to the conclusion cracking risk increases linearly with the increasing of mold temperature.Meanwhile,the cracking risk is greatly affected by the molding time.20:00PM is the most beneficial molding time to control the cracking of the track bed;Besides,cracking risk increases with the thermal insulation coefficient,when the thermal insulation coefficient is greater than 0.5,the thermal insulation layer can effectively reduce the cracking risk of the surface layer of track bed.When it comes to comprehensive early-age track bed cracking risk control measures,it is recommended to select early-age track bed cracking control measures in the order of reducing hydration heat→optimizing exothermic peak time→enhancing thermal insulation measures→selecting reasonable pouring time→controlling molding temperature. |
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