Study On Permeation And Influence Of Chloride Ion Dry-wet Cycle On UHPC Beams With Cracks

With the deepening of research on ultra-high performance concrete,more projects choose to use ultra-high performance concrete to simplify the structure of buildings and reduce the self-weight of structures.While benefiting from the steel fiber,the flexural capacity of ultra-high performance concrete components has been improved,the development of cracks has been limited,but the rebar UHPC components are also working with cracks under the normal conditions,which provides chlorine ions and other corrosive ions a path that can penetrate inside the UHPC matrix,This also results in a decrease in the durability of the rebar UHPC component.This paper relies on the provincial-level project "Application Research of Ultra-High Performance Concrete in Railway Engineering Repair",13 reinforced steel UHPC beams and 2 ordinary reinforced concrete beams were designed and manufactured,and cracks were made by actual loading.Focusing on the permeability of chloride ions to the components of UHPC beams with cracks under the chloride ion dry-wet cycle condition;The reason and degree of the change of the bearing capacity of the components before and after the chloride ion dry-wet cycle;and the formula for calculating the maximum crack width of the steel bar UHPC beam under different conditions is modified.The main research results are as follows:(1)The ultimate load-bearing capacity and stffness of cracked reinforced UHPC beams are improved after chloride ion dry-wet cycling treatment.Under the same load,the maximum crack width,midspan deflection and concrete strain of the treated members are less than those of the untreated members.The number of cracks in UHPC beams is less than that in ordinary concrete beams.When UHPC beams are damaged,only one crack extends obviously,while ordinary concrete beams exhibit uniform development of multple cracks.(2)Four-point bending loads were carried out on 13 reinforced UHPC beams,and the related data of the maximum crack width was collected.Based on the calculation formula of the maximum crack width under the existing specification,the average crack width correction coefficient A is introduced,and the formula for calculating the maximum crack width of the rebar UHPC beam is proposed.Then,on the basis of the new formula,the formula for calculating the maximum crack width of UHPC beams after chloride ion dry-wet cycle is given.(3)By testing and analyzing the chloride ion content in the cracks and on both sides of the cracks,it is found that the chloride ions content at the cracks in UHPC is much higher than that of the complete cross section,and the chloride ion content around the crack will rapidly return to the same level of chloride ion concentration at the full cross section regardless of the UHPC crack width.As the crack deepens,however,the reduction of chloride ion content at UHPC cracks is much faster than that of ordinary concrete.(4)By means of the rehydration test,the causes of the change of the bearing capacity on the UHPC beam were explored and analyzed.Modeling with software ABAQUS,the degree of change of the bearing capacity of the components is quantified by adjusting the constitutive relationship of the UHPC.