Study On The Axial Tensile Performance And Conversion Relationship Between Flexural And Tensile Properties Of UHPC

With its superior mechanical property and durability,ultra-high performance concrete(UHPC)promotes the development of civil engineering structures towards light,thin and long span.Compared with normal concrete,one of the outstanding advantages of UHPC is its excellent tensile performance.From the perspective of structure,the tensile performance of UHPC is affected by the size of specimen,and from the perspective of material,the tensile performance of UHPC is affected by the properties of matrix material and steel fiber.In order to explore the axial tensile performance,flexural performance and their conversion relationships,this paper mainly completes the following work:(1)From the perspective of structure,direct tensile tests were carried out on three types of dog bone shape specimens with different thicknesses to study the relationship between tensile stress,strain and crack of UHPC.The results show that the tensile strength of UHPC increases slightly,the strain capacity decreases significantly and the strain softening rate increases as the specimen thickness decreases from 100 mm to 30 mm.The thinner UHPC specimens have stronger ability to limit the development of cracks,and the average number of cracks is more.(2)Taking the length of steel fiber as a variable,the influence of steel fiber characteristics on axial tensile performance of UHPC was studied.The results show that the tensile strength of UHPC decreases slightly,and the strain capacity increases significantly with the increase of the length of steel fiber when the thickness,content and diameter of steel fiber are the same.The long fiber is more effective in limiting the development of macroscopic cracks than the short fiber.(3)From the perspective of material,non destructive test,damage test and image analysis were carried out to study the distribution and orientation of steel fiber at different positions of the plate specimen and its influence on the flexural performance.The results show that the flow of UHPC induces fiber orientation effect,and the steel fiber tends to be distributed in the horizontal flow direction of UHPC,the distribution tends to be uniform,but the increase of flow distance will lead to a slight decrease in local steel fiber content.The distribution and orientation of steel fibers affect the cracking and post cracking behavior of the specimens.The flexural performance of the specimens cut at 0° superior to other angles,and the phenomenon of multiple cracking is obvious,the number of cracks is more.The bending strength of the specimens cut at 45° and 90° decreased by more than 25% and 50% respectively.The fiber orientation coefficient is introduced to reduce the bending strength of the molded specimens to accurately reflect the bending strength of different regions of the plate specimen.The global and local fiber orientation coefficients are 1.18 and 2.51 respectively.The existence of fiber orientation effect is verified by image analysis.The sections of the specimens cut at 0° have the largest number of fibers,which are densely distributed and the average inclination is the smallest.(4)On the basis of the tests,the results of the four-point bending tests and the three-point bending tests were converted into the stress-strain relationships and the stress-crack width relationships of UHPC by the point-by-point back analysis method,which were in good agreement with the results of the direct tensile tests,verifying the feasibility of the conversion relationship.The damage test results of plate specimen were converted into stress-strain relationships of UHPC,and the relative relationships basically conformed to the relationships of global fiber orientation coefficient at different angles.