Analysis Of Early Cracks In Cast-in-situ Construction Of Prestressed Wide Box Girder Support

With the development of super-long-span hybrid girder cable-stayed bridges,wide-width box girder is also widely used in hybrid girder bridges.However,due to the large width of the wide box beam and the solid section of the outer web belong to mass concrete,the early cracking of the wide box beam has become a key issue for scholars at home and abroad.Based on the actual engineering background,this paper analyzes and studies the causes of early cracks in prestressed wide box girder,and proposes corresponding crack prevention and treatment measures to provide experience for the same type of box girder construction in the future.The main research contents and related conclusions of this paper are as follows:(1)This paper introduces the cast-in-place construction method of the bracket of wide box girder and the design process of concrete mix ratio,and carries out early temperature field monitoring on the standard beam section to obtain the development law of early hydration heat temperature field of prestressed wide box girder.According to the results of testing,It was found that the temperature of each part reached the highest after 20?24h of pouring,and then the temperature of each position began to decrease gradually.Because of the lower cooling rate,the whole early hydration heat reaction lasted about 4d.In addition,since the solid part of the outer web belongs to mass concrete,it is affected by the early hydration heat,and the temperature difference between inside and outside is large,and cracks are prone to occur.(2)Based on the related theory of early temperature field and stress field of concrete,the finite element software Midas FEA was used to establish a three-dimensional solid model.By comparison,it was found that the measured data and the theoretical calculation data maintained good consistency.In addition,the finite element model is used to simulate the early temperature sag and the concrete shrinkage.The results show that the external temperature sag will cause large tensile stress on the inner surface of the outer web and the surface of the diaphragm,and the local tensile stress near the diaphragm manhole exceeds the allowable tensile strength of the material.Under the dual influence of self-hydration heat and temperature dip,the tensile stress on the inner surface of the outer web and the surface of the diaphragm are over-limit.The calculation results are basically consistent with the on-site observations.At the same time,the influence of the thickness of the outer web on the surface stress is analyzed.The results show that the smaller the thickness of the outer web,the smaller the tensile stress generated on the inner surface of the outer web during the early hydration heat.(3)According to the field measured data and theoretical analysis results,combined with the structural characteristics of the wide box beam,the corresponding crack prevention measures are proposed.First,in the design of concrete mix proportion,the early crack resistance of concrete can be improved by adding appropriate crack resistant fibers.Secondly,according to the local climate characteristics in winter and summer,different maintenance methods are adopted,and through finite element simulation calculation and analysis,it is found that demoulding after 4 days of box girder pouring is beneficial to reduce cracks.Finally,using the more mature crack treatment method,the existing cracks will be treated in time to avoid the cracks from continuing to expand,and provide experience for crack prevention of wide box girder in the same type of engineering in the future.