| In order to increase the crack resistance of concrete bridge decks in the negative bending moment area of steel-concrete composite bridges and simplify site construction process,a new ultra-high performance concrete(UHPC)joi nt scheme of the negative bending moment area of steel-concrete composite bridges was proposed.Compared with the micro expansion fiber C50 concrete(NC)wet joint scheme,the UHPC wet joint scheme has the same economy,good durability,at the same time,t he field welding amount is reduced by 36.5%,and the construction is convenient.It is expected to solve the problem of concrete cracking in the negative moment area of composite bridges and realize rapid construction.The whole finite element calculation was carried out based on the background project on a bridge in Hunan province.Based on this,in order to study the flexural behavior of UHPC wet joint in negative moment area of steel-concrete composite bridges,this article mainly from the following aspe cts to carry out the research:(1)According to the key section stress equivalence principle,a 1:2 scale model bending test,as well as a theoretical calculation,was carried out to ascertain whether the anti-crack performance and bending capacity of the new UHPC joint scheme can meet the design requirements.The test results satisfied the engineering requirements,and the strength of the joint is higher than that of the preform.Under the action of negative bending moment,the strain along the beam height of the specimen satisfies the plane-section assumption,the interlaminar slippage between the steel beam and concrete slab and between UHPC and normal concrete(NC)is small.The ultimate failure was the buckling failure of the bottom flange of the prefab ricated steel beam.(2)Matlab programs were developed to calculate the reduction of stiffness before and after cracking of composite beams.The reduction rate of the section stiffness of UHPC-NC composite section in the negative moment area is 38%,which is lower than 59% of the NC section.After redistribution of internal force,the increase of positive bending moment in midspan and the decrease of negative bending moment at the fulcrum of continuous composite beam with UHPC-NC composite section is smaller.Based on the section moment-curvature program,modified the existing NC crack width formula,and a suggested formula for the maximum crack width of NC in the negative moment area of composite beams considering the restriction of UHPC was put forward.(3)For the supporting engineering of applying the UHPC wet joint proposed in this paper,the actual bridge load test was carried out.The test results showed that the stress of bridge deck,steel beam,reinforcement,and section deflection increases with the increase of loading stages.Under overload conditions,there is no crack in the bridge deck,and the stress of steel beam,reinforcement,and mid-span deflection have a significant safety reserve.Under the same condition,the closer the centerline of th e loaded vehicle is,the higher the stress of the deck,steel beam,and reinforcement of the same section of the different single beam is.(4)The shrinkage data of the actual bridge concrete were analyzed,and results showed that the law of expansion and shrinkage of UHPC in the actual bridge and the contrast UHPC slab is generally similar,which can be divided into three stages: 1)expansion,2)expansion and shrinkage balance each other,3)shrink and reach stability.Besides,no slippage between reinforcement and UHPC under shrinkage.Reinforcement in the layer and NC in the lower layer restrained the shrinkage of UHPC.The closer is the layer to the NC;the smaller is the shrinkage.At 180 days,compared with the peak value of expansion,the longitudinal and transverse shrinkage of the actual bridge was about 66% and 41% of that of the unreinforced UHPC contrast slab.The "T" type joint showed the advantages of retarding shrinkage.When the shrinkage tended to be stable,the maximum shrinkage strain at the interface indicated that the section was the weak section of UHPC joint,which should be paid more attention in design.In a certain period,the pouring of UHPC in the upper layer will lead to additional tensile strain of micro expansion NC,but it does n ot affect on the development of long-term expansion and shrinkage of micro expansion NC.(5)Simulation analysis of the test beam and the actual bridge load test was carried out by using Abaqus,and the correctness of the finite element method was verified.Parameter analysis revealed that the longitudinal length of the joint NC beam can be 0.35 m.The longitudinal length of UHPC layer should be 0.2 times the standard span.The thickness of UHPC layer is determined according to the actual engineering design requirements.The crack resistance of concrete in the negative moment area can be increased by increasing the diameter of reinforcement in the bridge deck. |
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