Early Crack Analysis Of π-type Concrete Girder Of Cable-stayed Bridge

π-type concrete main beams are increasingly used in cable-stayed bridges.At home and abroad,there are more studies on cracks in the use of such bridges,but less research on the early cracks.Based on the existing research results,this dissertation studies the early cracks of π-type concrete main girder of a cable-stayed bridge during the actual construction process by the method of on-site monitoring combined with finite element calculation and analysis.The main research contents and conclusions are as follows:(1)The distribution position,size,and occurrence time of early cracks at the flange plate and beam of the π-type concrete main beam of the cable-stayed bridge are introduced.In addition,it summarizes the common types of early cracks.Based on the field measurement and monitoring results and combined with the existing research results of early cracks,it analyzes the possible causes of cracking,such as the reinforcement ratio,prestressed steel bars,heat of hydration,and concrete shrinkage.The first two factors were basically eliminated through preliminary analysis,and the effects of hydration heat effect and concrete shrinkage effect may lead to the formation of early cracks.(2)Combined with the actual construction situation,the physical and mechanical parameters,thermodynamic parameters and boundary conditions of the main beam concrete are determined.The three-dimensional solid model of some main beam segments is established by using FEA software.And analyze the distribution and change of temperature field of whole beam,main rib,flange plate and beam due to hydration heat during curing.The results show that the temperature of the whole and all parts of the main beam rises and then decreases during the curing,but there are temperature gradients between the main ribs,flange plates and beams,and there are temperature gradients at the same location in different directions.(3)The temperature stress changes of the main rib,flange plate and beam in the process of hydration heat of concrete are analyzed respectively,and the stress distribution is considered to analyze the of possible position of cracks.By changing the ambient temperature and the type of the template,the sensitivity of hydration heat is studied.The results show that during the process of hydration heat,the surface of the main rib is under tension and the internal of the rib is under pressure.The upper and lower surfaces of the flange plate will appear tensile stress that exceeds the tensile strength of the same age.Tensile stresses will also appear at both ends of the beam,but the value is small.Higher ambient temperature and template with better thermal insulation properties help to reduce the temperature stress caused by the hydration heat and improve the structural stress.(4)Compare and analyze the existing concrete shrinkage prediction models,and select appropriate models based on the actual engineering situation,and analyze the changes of the tensile stress of the flange plate and beam under the conditions of bridge deck shrinkage,main rib shrinkage,and beam shrinkage.In addition,the stress of flange plate and beam under the combined action of hydration heat and concrete shrinkage is analyzed.The results show that the shrinkages of the main ribs and the bridge deck have a small effect on the stress of the flange plate and the beam,while the shrinkage of the beam has almost no effect on the flange plate and has a great effect on itself,which will produce transverse tensile stress in the middle of beam that exceeds the tensile strength of the same age.