Study On Temperature Control And Curing Techniques Of Long-span PC Continuous Rigid Frame Bridge In Construction

The cantilever cast box girder is limited by the maintenance space,the outer surface of the box girder is difficult to maintain,and the complex environmental factors after demoulding lead to a large number of early cracks on the web and floor of the box girder.In addition,with the appearance of high performance concrete,the autogenous shrinkage of concrete is large and the cracking sensitivity is increased.The traditional maintenance method of box girder can not fully guarantee the quality of concrete box girder.Therefore,in order to ensure proper temperature and humidity of concrete and prevent early cracking of concrete surface,it is necessary to adopt scientific and reasonable early maintenance technology.This paper takes the continuous rigid frame bridge with span(80 + 140 + 80)m ?Rongba Bridge as the engineering background,this paper uses Midas/FEA to calculate the temperature field of concrete structure of the bridge,in order to find a concrete maintenance technology suitable for the cantilever construction section of the bridge.The following research work has been done in this paper:(1)This paper summarizes the research status of concrete temperature field and Bridge early maintenance methods,and briefly describes the basic theory of temperature field.(2)With the aid of simulation software Midas/FEA,the actual construction process of 1# segment is simulated,and the coupling effect of hydration heat temperature and sunshine temperature is studied.At the same time,the effects of various parameters(template material,template thickness,template thermal conductivity,ambient temperature,injection temperature and wind speed)on the temperature field are analyzed,and reasonable maintenance control parameters are put forward.The results show that the temperature of concrete is higher and the temperature difference between inside and outside is larger when the section size is larger.The choice of material and thickness of formwork is based on the bearing capacity of formwork.For the selection of formwork,when the bearing capacity is satisfied,the wood formwork and plastic formwork with better thermal insulation performance should be selected when the thickness of section is large,such as web and bottom;when the thickness of section is thin,such as roof,the steel formwork with better heat dissipation performance should be selected.When the thickness of the template is 20 mm,the thermal conductivity is suggested to be in the range of 1~2 kJ/m2·h·?.Mould entry temperature and ambient temperature have great influence,so it should be reasonably controlled according to the actual situation.When the wind speed is high,the wind resistance measures should be well done.(3)The heating and maintenance method of electric heating furnace is put forward.Midas/FEA software is used to establish the calculation model of the closure section in winter under the maintenance of the electric heating furnace.The accuracy of finite element modeling and the effectiveness of the maintenance of the electric heating furnace are verified by the temperature monitoring of the actual project.Finally,the maintenance process system of the electric heating furnace is modified and improved.The analysis results show that the errors between the measured and calculated values are within 10%,which is in good agreement.The peak value of temperature difference between inside and outside of box girder is 9.2 ?,and the maintenance meets the requirements of the code.By reducing the temperature difference between inside and outside of box girder maintenance,the maintenance effect of box girder can be further optimized.(4)From the aspect of economy and maintenance effect,the current maintenance methods in summer and winter are compared and analyzed,and the best maintenance methods in summer and winter are put forward respectively.At the same time,Midas/FEA software is used to verify the method.The results show that the compound maintenance method of B is recommended in summer: "pasting method + automatic spray method".For winter: large and medium-sized steam curing greenhouses,steam consumption is large.When using high-power electric boilers,steam curing method of gas-fired boilers is recommended;for box girder segments,concrete volume is small,greenhouse space is small,steam consumption is small,electric heating furnace curing method is recommended.