Research On Construction Measures And Temperature Field And Stress Field Of Frame Box Culvert Based On Crack Control

With the construction of China's railways and highways,the perfection of criss-crossing traffic networks has led to the need for new railways to be worn under existing railways.The way in which frame culverts are prefabricated after construction is increasingly used in engineering practice.In the construction process of the frame box culvert,cracks are easily formed due to its structural characteristics,which seriously affects the waterproofness,integrity and safety of the frame box culvert.Cracks have occurred in the frame box culvert during the pouring construction,and the causes of the cracks are complicated.At present,the research on the mechanism of the cracks in the frame box culvert and the related research on the crack control measures are not enough.In the past,the crack control was carried out according to similar engineering experience,and the effect was not First,it is of great significance to study the crack control measures and the changes of temperature field and stress field in the box culvert during the early stage of frame box culvert.To this end,this paper relies on Ningbo under the railway frame box culvert concrete crack control technology research project,carry out actual engineering comparison test on frame box culvert crack control measures,analyzes the variation of temperature field and stress field before and after the improvement of crack control measures,and combines limited The numerical simulation is used to analyze the influence of the steel formwork pouring,setting the cooling water pipe,the pre-stressing tendon and the curing control measures on the temperature and stress strain of the frame box culvert for the frame culvert analysis.The crack control effect is compared and analyzed.The conclusions are as follows:(1)The temperature change trend of frame box culvert concrete after pouring is consistent with the trend of hydration heat release.After adopting comprehensive crack control measures in combination with steel formwork,cooling water pipe and curing environment,the maximum temperature of frame box culvert concrete Significantly reduced,the time to reach the maximum temperature after concrete pouring is prolonged.At this time,the strength of the concrete is already high,and the time to reach the maximum temperature is prolonged to facilitate the control of the crack.The temperature at the center of the box culvert is the highest,and the temperature ratio inside the box culvert is higher.The outside temperature is slightly higher,the surface of the top plate is not affected by the ambient temperature,and the temperature of the side wall of the box culvert is greatly affected by the ambient temperature.The middle of the box culvert is basically not affected by the change of the ambient temperature.The existence and maintenance environment has a significant effect on the concrete temperature stability of the box culvert surface.(2)The main stress of the frame culvert concrete surface is mainly the main tensile stress after demoulding,and the stress change trend is consistent in all directions.The environmental temperature change has great influence on the concrete stress variation on the box culvert surface.From the middle of the box culvert to the entrance of the box,the strain change gradually increases.This is because the closer the hole is,the less constraining the structure is,and the strain changes are larger.At the middle section,the restraining effect of the surrounding concrete structure is more obvious,and the strain change is more obvious.small.In the same section,the upper strain is larger,and the closer to the bottom plate,the smaller the strain change.This is because the casting time of the bottom plate is earlier,and the bottom plate has a constraining effect on the upper concrete,which is caused by the self-weight.(3)The strain on the inner side of the side wall of the frame culvert is generally larger than that on the outer side.The strain increases rapidly during the period of 24h-48 h after concrete pouring,which is characterized by a rapidly increasing tensile stress,and at this stage,the hydration heat is also more radiated.During the period when the internal temperature growth changes the most,the internal temperature of the concrete reaches the maximum temperature during this period,and the temperature difference between the inside and the outside is the largest.Therefore,the control of the crack mainly needs to be concentrated in this period.By adopting steel formwork and setting the cooling water pipe,etc.,it can effectively reduce The internal temperature of the concrete,and the heat generated in the concrete is dissipated in time,and the outside is insulated to prevent cracks from occurring due to excessive temperature difference between the inside and the outside.Due to the different internal and external temperature of the side wall,the highest temperature point generated by the hydration heat is biased to the inside.The displacement causes the temperature deformation between the inner and outer sides of the side wall,and the temperature difference-bending crack effect is generated,which provides a basis for the same condition maintenance measures inside and outside the box culvert.(4)Combined with the finite element method,the numerical simulation of the crack control measures is carried out to compare the influence of the steel formwork and the wood formwork.The steel formwork can promote the heat loss of the concrete,and the wood formwork has a certain heat preservation effect on the concrete,and the concrete hydration heat station The heat generated is unfavorable,and the concrete can be kept at a higher temperature for a longer period of time.After setting the cooling water pipe,the internal temperature of the structure is obviously improved,and the temperature difference between the inside and the outside is reduced,and the main tensile stress in the structure is reduced.The main stress is mainly the main compressive stress around the water pipe position,and the surface layer of the structure is mainly The main tensile stress is also significantly reduced,and the maximum position is below the allowable stress of the concrete tensile strength,and the effect is obvious.The application of prestressed steel bars can effectively reduce the maximum value of the main tensile stress on the surface of the box culvert.The simulation analysis of the crack by the damage model of concrete is consistent with the crack observed in the field.(5)The width of the cracks in the box culvert is less than 0.5mm.The cracks are generally oriented and the number is determined.However,the faults are small and the spacing is small.The cracks are only small at the two ends and the width is slightly wider.After adopting a series of crack control measures such as steel form pouring,setting cooling water pipe,mold temperature control,pouring with color strip cloth,strictly controlling curing temperature and humidity,controlling demoulding time,etc.,the effect is obvious after the box culvert inside and outside without one crack.(6)The influence of concrete pouring formwork was analyzed by water migration model.The finite difference numerical solution was carried out on the theoretical model of water migration under the condition of concrete surface boundary considering the water absorption and health effects of the template.The theoretical solution of different formwork pouring was compared and analyzed.Under the joint action of concrete and formwork,the influence of different formwork pouring on crack control is obtained.The steel formwork can effectively control the generation and expansion of concrete cracks.