Flexural Behavior And Design Method Of Weathering Steel-UHPC Composite Girder With Waffle-slab System

Since 1950s,there was a rapid development of composite bridges.The composite girders have been applied in various types of bridges,including the girder bridges with medium and small span of 20 m～50 m,cable-stayed bridges,and suspension bridges with span of nearly 1000 m.After decades of operation,many composite girder bridges constructed have suffered from multiple diseases due to the effects of loads,natural environment,and material degradation.In order to solve the problems such as severe corrosion of steel beams,easy cracking of bridge deck,and corrosion of internal reinforcement bars in composite girder bridges,the weathering steel(WS)-UHPC composite girder with waffle-slab system was proposed to improve the durability and to enhance the bearing capacity of structure.Based on the problems existing during the service life of WS-UHPC composite beam with waffle slab constructed in marine atmosphere,the following aspects were studied around the flexural behavior and design method of WS-UHPC composite beam with waffle slab:1.An experimental investigation into the corrosion behavior and mechanical properties of WS(Q420q NH and Q420q NHY)in marine atmosphere was presented.And the carbon steel(Q420q)was chosen as contrast.Firstly,the neutral salt-spray accelerated corrosion test was conducted to compare and analyze the corrosion characteristics of three types of steels.Secondly,tensile testing on corroded specimens was carried out to investigate the mechanical property degradation of WS.Finally,the degradation mechanism of mechanical properties was revealed by analyzing the surface strain of corroded specimens.The results show that Q420q NHY steel with much higher corrosion resistance was more applicable to the marine atmosphere.However,Q420q NH steel was less applicable to the marine atmosphere.Q420q NH and Q420q NHY steels presented a similar degradation degree of mechanical property after corrosion,which was less than that of Q420q steel.The local corrosion accelerated the development of strain at the fracture section,which eventually resulted in the reduction of elongation with increasing corrosion degree.It should be noted that the different corrosion behavior and mechanical properties of WS should be considered simultaneously during the bridge design.2.An investigation into the adhesion capability of rust layer of WS under tension during initial stages of corrosion in marine atmosphere was presented.And the corrosion behavior of WS substrate after the damage of rust layer was explored.Firstly,the WS specimens were pre-rusted through the field exposure corrosion test.Secondly,tensile testing on corroded specimens was conducted to capture the damage process of rust layer and record the strain field of rust layer on the surface of WS during the loading process.And the cracking strain or stress of rust layer was analyzed.The electrochemical measurements were performed on corroded specimens after different tensile stresses.Finally,the critical stress and strain of rust layer corresponding to the serviceability limit state were proposed.Results show that the rust layer formed on Q420q NH exhibited the stronger adhesion capability than Q420q NHY after 30 d field exposure corrosion.The increase of external load accelerated the corrosion process of WS.The 0.9f_y for Q420q NH and 0.8f_y for Q420q NHY were reasonably suggested as the critical stress of rust layer cracking.3.A three-dimensional(3D)cellular automata(CA)based method was proposed to reproduce the atmospheric corrosion process of WS.The diffusion process based on Fick′s law and the electrochemical reaction process based on Faraday′s law were all involved in the CA model.The randomness of diffusion and electrochemical reaction,and the uncertainties of environmental parameters were considered by introducing probability into the CA model.Firstly,based on the characteristics of CA model,the basic assumptions were made with the purpose of simplifying the complicated corrosion process.Secondly,the rules of electrochemical reactions and oxygen diffusion were proposed and the MATLAB program was developed to simulate the corrosion evolution process of steel in atmosphere.The results obtained from the CA model,including corrosion kinetics,corrosion morphology,and distribution characteristics of pits for WS in atmosphere,were compared with the basic corrosion laws verified by lots of experiments.The results show that the basic hypotheses and evolution rules were reasonable and the model was reliable.The parametric analysis was performed to investigate the influences of initial conditions on corrosion characteristics of WS.Finally,by setting the dimension of time and size,the relationship between CA model and atmospheric exposure test was established to achieve the model application.Based on the relationship,the CA model can be used to continuously predict the corrosion behaviors of WS during the long-term atmospheric corrosion process.4.Based on the understanding of basic mechanical system of steel-UHPC composite beam with waffle slab,taking the ratio of rib height to upper panel thickness,ratio of width to spacing for longitudinal rib,ratio of transverse rib spacing to longitudinal rib spacing,waffle-slab height and layout of studs as variables,six steel-UHPC composite beams with waffle slab were designed and their flexural behaviors were investigated experimentally.The effects of different parameters on the flexural behaviors of composite beam were analyzed by comparing the test results which included the crack propagation of UHPC,failure mode,load-deflection relationship,strain distribution and development,and relative slip between steel beam and UHPC waffle slab.The results show that the steel-UHPC composite beam with waffle slab exhibited obvious bending failure characteristic and good ductility.The composite beam can fully develop its flexural strength without significant longitudinal splitting cracks on the waffle slab even when the transverse reinforcement ratio is 0.33%.The longitudinal strain on the lower flange of steel beam and on the top of waffle slab at mid-span,all obtained the full development at fracture state.The UHPC waffle slab matches the steel beam well.The composite beam with UHPC waffle slab avoids the fact of"weak steel beam and strong UHPC slab"for composite beam with UHPC solid slab.The increase of ratio of width to spacing for longitudinal rib and ratio of transverse rib spacing to longitudinal rib spacing will weaken the stiffness and integrity of waffle slab.Thus it results in the earlier longitudinal cracking or the more obvious cracks on the top of waffle slab.5.A non-linear finite element model of steel-UHPC composite beam with waffle slab was established to investigate the flexural behaviors.By comparing simulation results with test results,the rationality and validity of model were verified.Then,the flexural working mechanism of steel-UHPC composite beam with waffle slab was analyzed in depth and the extensive parametric analysis was performed to investigate the effect of each parameter on the flexural behaviors of steel-UHPC composite beam with waffle slab and to determine the main influencing factors.Based on the above analyses and research achievements of traditional composite beam,the formula for calculating the flexural capacity of steel-UHPC composite beam with waffle slab was put forward,and the design method was further proposed.Finally,according to the research results of WS,the corrosion allowance of WS for the designed service life of100 years was calculated and the design method of WS-UHPC composite beam with waffle slab was proposed.The results show that the reduction of thickness of upper panel for waffle slab will significantly weaken its integrity and increase the risk of longitudinal splitting failure of steel-UHPC composite beam with waffle slab when comparing with reducing the number of ribs.The waffle slab with low ribs should be adopted as the bridge deck.The increase of transverse reinforcement ratio has little effect on improving the ultimate flexural capacity of steel-UHPC composite beam with waffle slab.But it is obvious for improving the ultimate deformation capacity.The transverse reinforcement ratio of 0.5%is suggested for steel-UHPC composite beam with waffle slab.The contribution of longitudinal and transverse ribs is small for flexural capacity but is large for ensuring ultimate deformation capacity.The elastic flexural design method and the calculation formula of ultimate flexural capacity proposed have high reliability and accuracy.The flexural design recommendations proposed can provide reference for the design of steel-UHPC composite girder bridge.The flexural design method of WS-UHPC composite beam with waffle slab provides theoretical and technical support for the design and construction of such girder bridge in the future.