Study On Optimization Of Construction Period Of Prestressed Concrete Continuous Box Girder Layered Pouring

In the modern foundation bridge construction,the prestressed concrete cast-in-place continuous box girder bridge has been widely used due to its manifold advantages.However,with the continuous development of prestressed continuous box girder,some problems are highlighted.Among them,the most noteworthy is the cracking of the box girder structure.Some cast-in-place girder bridges have cracks during the construction process.These cracks not only affect the aesthetic appearance of the structure,but also damage the bearing capacity of the box girder bridge and affect the durability of the bridge.A large number of studies have found that in the cast-in-place box girder constructed by layered pouring,because the shrinkage of the two layers of concrete has a certain degree of difference,coupled with the difference in temperature drop in the temperature drop stage of hydration,the early transverse cracks are prone to occur in the roof.Currently,there is relatively little research on the early cracks in the roof of the box girder under the joint action of the two.In this paper,combining with two layered construction of cast-in-situ box girder bridge of prestressed concrete of a project,through the analysis of the construction process of cast-in-situ box girder of early hydration heat and strain variation trend to determine the impact of the early construction of crack parameters,and simulate the stress and plastic damage of concrete box girder under different parameters.Through simulation analysis,we can get the possibility of cracks on the roof of cast-in-place concrete box girder,focus on prevention and control during construction,make corresponding optimization recommendations for construction period,and guide the construction.The research contents are shown as follows:(1)The early hydration heat,strain test plan and implementation process of pre-stressed cast-in-situ concrete box girders bridged by two layers are introduced.And analyze and summarize the field test data.The study found that the heat of hydration of the box girder basically dissipated about 9 days after pouring.After the age difference of layered pouring exceeds 9 days,the joint action of hydration of the top plate and bottom web tends to be stable,with only minor changes.The tensile strain of the middle section of the continuous box girder is obviously smaller than the maximum tensile strain near the continuous end section,which is basically consistent with the location of cracks in the actual layered pouring box girder.(2)The space finite element model of box girder is established by Midas FEA,the hydration heat temperature field of cast-in-place box girder is analyzed,and the calculated results are in good agreement with the measured ones.The different box-girder models with different ages were analyzed,and the temperature rise and temperature drop values of the top and bottom webs of different box beams were obtained.(3)The finite element model of prestressed concrete continuous box girder was established by ABAQUS,and the influence of the difference of layered pouring age on the early cracks of the roof was analyzed.It is known that with the increase of age difference of layered pouring,the influence of shrinkage difference between new and old concrete and hydration temperature drop on the early cracks of roof will be more serious.Shortening the age difference of layered pouring can effectively reduce the number of early cracks in the roof and reduce the early crack width of the roof.It is recommended that the age difference of cast-in-place box girders for layered pouring is 5-9 days as the best construction period.