Research On ECC Cracking Behavior And Chlorine Corrosion Resistance Of Loaded Continuous Beam At Negative Bending Moment

Concrete continuous girder bridge has the advantages of good structural stiffness,small deformation,fewer expansion joints,smooth running and so on.It has become the most widely used bridge type at present.However,due to the low tensile strength of ordinary concrete,the concrete in the negative moment zone is easy to be opened and cracked in continuous beam bridge,which affects the service performance of the bridge.PVA fiber-reinforced Cemetitious Composite（ECC）is an ultra-high toughness composite material emerging in recent years with good mechanical properties and durability（good crack,permeability and erosion resistance）,which has a wide range of applications in Bridges.In addition,there is a lot of snow in cold areas,so in order not to affect the traffic,chlorine salt snowmelt is often used for snow removal,and chloride ion intrusion into concrete is easy to cause corrosion of steel bars,thus affecting the durability of the structure.In view of this situation,the ECC cracking behavior and chlorine corrosion resistance of continuous beam in negative bending moment area under load condition were studied by combining experimental research with theoretical analysis.The ECC cracking behavior in the negative moment zone of continuous beam under the load level of 0.3,0.5 and 0.7 is studied,and the chloride ion erosion rule under the load-bearing cracking ECC for 25,50 and 70 dry-wet cycles,as well as the chloride ion diffusion model considering the influence of cracks.The main results obtained in this paper are as follows:（1）The ECC cracking behavior in the negative moment zone of the continuous beam with load is studied experimentally and the maximum crack width is calculated and analyzed.The crack morphology in the negative moment zone of the continuous beam under static and load conditions is analyzed,and the development law of crack width,crack number,crack spacing and crack depth in the negative moment zone under different load levels is studied.The ECC maximum crack width calculation model suitable for load-bearing continuous beam in negative moment zone is presented.The results show that,compared with ordinary concrete,the ECC of continuous beam in negative bending moment zone presents multi-crack,and the crack width is smaller under the same load level.With the increase of load holding level,the crack width of ECC in the negative bending moment zone of continuous beam gradually increases,the crack number increases first and then remains unchanged,the crack spacing is small,and the crack depth gradually increases with the increase of load holding level and crack width.Based on the bond slip theory,the ECC model of maximum crack width in the negative moment zone of continuous beam with load is consistent with the experimental value,and the model has good applicability.（2）An experimental study was carried out on the chloride ion erosion resistance in the negative bending moment zone of the continuous beam with load after 25,50 and 70 dry and wet cycles.The influence of crack width and maximum crack width on the chloride ion erosion depth and chloride ion content distribution was explored,the chloride ion erosion rule under different crack numbers was studied,and the phase characteristics of cracking ECC after chloride salt erosion were analyzed.The results show that the increase of crack width promotes the increase of erosion depth,and the number of wet and dry cycles also has an effect on chloride ion erosion depth,but the effect is less than that of crack width.With the increase of crack width,the chloride ion content in ECC gradually increases,but when the crack width is greater than the upper limit,the chloride ion content growth rate becomes stable.At the maximum crack width,with the increase of depth,the chloride content decreases in two stages.The first stage has a smaller depth,and the decrease rate of chloride ion is larger;the second stage has a larger depth,and the difference of chloride content at each depth is small,and the decrease rate of chloride ion content is smaller with the increase of depth.The number of cracks on the top surface is greater than the number of erosion paths,and only 45%of cracks extend into the ECC to form chloride ion erosion paths.As the number of cracks in the top surface increased,the content of chloride ions invading ECC gradually increased,and the effect of the number of cracks in the top surface on the content of chloride ions was more obvious at the high holding level.By analyzing the XRD diffraction pattern,it is found that chloride ions intruding into ECC will produce Friedel salt,which reduces the content of Ca（OH）₂ in the matrix and increases the content of Ca（CO）₃.With the increase of crack width,the diffraction peak of Friedel salt is enhanced,which promotes the generation of Ca（CO）₃.The microstructure of ECC cracks is compacted.（3）Based on the basic equation of Fick’s second diffusion law,a cracking ECC chloride ion erosion model in the negative moment zone of a continuous beam with load was established.The influence of cracks on the cracking ECC chloride ion diffusion coefficient in the negative moment zone of a continuous beam was studied,and the applicability of the model was verified.The results show that the diffusion coefficient of chloride ions increases with the increase of erosion depth,and the increase of crack width also promotes the increase of chloride ion diffusion coefficient.The slope change K of linear fitting between chloride ion content and depth can be used as an indicator of the influence of the number of dry and wet cycles on chloride ion content.An ECC chloride ion diffusion model for cracking in the negative moment zone of a continuous beam with load is developed considering the influence coefficients of crack width and dry and wet cycles.