Stress Analysis In Construction Process For Concrete Wide Box Girder Of Hybrid Girder Cable-stayed Bridge

The phenomenon of cracking of prestressed concrete box girder bridges is more common and has become a technical issue attached to bridge construction.Based on the construction of the main bridge of the Jiayu Yangtze River Highway Bridge,this paper studies the stress state and cracking control of the concrete box girder in order to provide technical support for solving the problem.This paper combines the Ministry of Transport’s science and technology plan project "Research on the key technology of PC wide box beam quality improvement of hybrid beam cable-stayed bridge"(project number: 2018-MS1-017),based on detailed field test,The hydration heat and thermal effect of the common C55 concrete box girder and RPC(Reactive Powder Concrete)steel-concrete composite segement and the early-stage stress performance of the cast-in-place prestressed concrete box girder segment were studied.The main contents include:1.The hydration heat and thermal effect on PC wide box beam(1)The strength of large span PC box girder bridge concrete at early age was tested.The results show that the curing conditions have a great influence on the development of concrete early age strength,and the intensity difference increases with the increase of age.(2)The hydration heat during the construction of the concrete box girder on the north side of Jiayu Changjiang Highway Bridge was tested,and the development law of hydration heat of box girder was obtained.Based on the measured results of mechanical properties of early age of concrete,a time-varying model was established to analyze the hydration heat temperature of box girder.Field and stress field.The results show that the box girder concrete reaches the peak temperature about 1d after pouring,and will experience a faster temperature rise process in the first 1~2d.During this period,the position of the box girder is prone to large internal and external temperature difference,which may lead to concrete cracking.2.The hydration heat and thermal effect on RPC steel-concrete composite segement(1)The hydration heat during the construction of the steel-concrete composite segement of RPC(Reactive Powder Concrete)and the nearby section of the Jiayu Yangtze River Highway Bridge was tested,and the hydration heat development lawwas obtained.The test results show that the thermal properties of RPC and C55 ordinary concrete are quite different.The RPC section and the mixed zone of RPC and C55 reach the peak in 2D and reach stability in 12 d,while the C55 ordinary concrete reaches the peak in 1D,in Figure 9d.Stable.(2)Based on the experimental results of early mechanical properties of RPC and mixed-area concrete and the measured results of hydration heat temperature field,the time-varying model was used to analyze the temperature field and stress field during the hydration process of RPC steel-concrete cpmposite segment.The calculation results show that the RPC and mixed zone hydration heat temperature is higher,and the internal measuring point temperature reaches 110.4 °C at the 2d wind fairing.During this period,the temperature difference between the inside and outside of the wind fairing is large,up to 40~50 °C.Without steel box girder constraint,due to the excessive temperature difference between the inside and outside,the area will generate large stress on the concrete surface,which may cause concrete cracking.At the same time,due to the influence of the overall temperature field,the ordinary concrete section will cause large temperature-induced tensile stress due to excessive temperature difference between different concrete sections.Therefore,in the actual construction,measures such as mixing ratio of raw materials,lowering the temperature of the mold and cooling water pipes should be optimized to reduce the temperature of the concrete in the RPC section and the mixed zone to prevent excessive temperature-induced stress in the structure.3.Stress Analysis of Early Casting Section of Cast-in-place Prestressed Concrete Box Girder(1)Based on the prestressed tensile test of the concrete box girder on the north side of Jiayu Yangtze River Highway Bridge,the finite element model of prestressed concrete box girder was established,and the structural performance of standard segment box girder during early age tension was studied.The influence of the age difference of adjacent sections and the prestressed tensile age on the mechanical performance of concrete box girder at early age.The results show that the box girder will cause large tensile stress due to the concrete shrinkage effect,and a tensile stress zone will be generated near the joint of the adjacent beam section.The prestressing tension of the early age can form prestressing stress on the structure to resist the construction period.Poor tensile stress caused by shrinkage,creep,or temperature.However,if the standard section is prematurely overstretched and prestressed,it will cause cracks in the inside of the box girder and in the manhole.Inaddition,premature tension prestressing also increases the prestress loss of the box girder.Therefore,it is recommended that the concrete age should be greater than 5d when prestressed tension is applied.(2)In order to study and analyze the shrinkage stress caused by the adjacent segment constraints during the construction of the wet joints,the finite element model of the wet joints and adjacent beam sections of the Jiayu Yangtze River Highway Bridge was established,and the box girder under the concrete contraction was studied.The stress field is analyzed and different prestressed tensioning schemes are analyzed.The results show that the wet joints under the contraction of concrete and adjacent segments,the tensile stress caused by shrinkage at the joint surface on the third day after concrete pouring reaches 1.8MPa,which is the tensile strength of the concrete at this age.87%,so it is necessary to carry out prestressing tension at this time to reduce the tensile stress of concrete to prevent concrete from cracking at early age;if the wet joint is tensioned according to the conventional scheme,the maximum main pull of wet joint early age The stress is less than the instantaneous tensile strength of concrete,but its maximum tensile stress at 28 d is2.75 MPa,which is 93% of the tensile strength of concrete at this age,and there is risk of cracking.Under the tension scheme proposed in this paper,the wet joint is early.The maximum main tensile stress in the age is reduced by 22.2%~32%compared with the conventional scheme,which effectively guarantees the crack resistance requirement of the post-wet wet joint in the early age.