Study On Shear Behavior Of Pretensioned Prestressed UHPC Beams With Large Diameter Strand

Compared with the ordinary concrete structure,the pretensioned concrete structure has the characteristics of small section,light weight,high quality,material saving and expanding the degree of structure assembly.This structure can give full play to the material performance of high strength steel bar and high performance concrete,and improve the crack resistance and durability of the components.With the use of high strength materials such as large diameter strand and ultra-high performance concrete(UHPC),the research on pretensioned concrete structures needs to be supplemented and improved.This thesis combines experimental research,theoretical analysis and numerical simulation to study the prestress transfer length and shear behavior of pretensioned concrete beams made of large diameter strand and ultra-high performance concrete.The main work is as follows:(1)The experimental study on the prestress transfer length of pretensioned concrete beams is carried out.Using large diameter strand and ultra-high performance concrete,4 pretensioned concrete beams were designed and manufactured,and a pretensioned concrete beam was made of ordinary concrete for comparison.The strain of concrete surface on the same horizontal plane as the strand is measured before and after the strand is tensioned.The influences of stirrup spacing,concrete strength and measurement time on the transfer length were studied.The test results show that with the increase of stirrup ratio and concrete strength,the transfer length decreases obviously.The transfer length increased with the extension of the measurement time after the release,but the trend slowed down and reached the basic stability at 21 days.(2)The shear test of pretensioned concrete beams made of large diameter strand and ultrahigh performance concrete is completed.Under the condition of shear span ratio of 2.5,twopoint symmetrical concentrated static loading tests were carried out on four pretensioned UHPC beams.The overall deformation(deflection,curvature,crack development,relative slip,etc.)and local strain(concrete,steel strand,longitudinal bar,stirrup,etc.)of the specimens were measured and recorded during loading.The influence of stirrup spacing on the ultimate bearing capacity and failure mode of the specimen was studied.The test results show that the pre-tensioned prestressed UHPC beams have no bond failure and their failure mode is shear failure with certain ductility when the shear-span ratio is 2.5.The actual bearing capacity is about 50% larger than the calculated bearing capacity,and the bearing capacity of the specimens decreases sharply by about 8% after reaching the ultimate load,but it does not fail completely,which indicates that the pretensioned concrete beam made of large diameter strand and ultra-high performance concrete has high bearing capacity and good ductility.(3)Based on the above experimental research and the characteristics of pretensioned prestressed UHPC structures,the practical calculation formula of shear bearing capacity is deduced through the analysis and improvement of some parameters of truss-arch theory.Comparing the theoretical calculation results with the experimental results,it is found that the improved truss-arch model fits well,which shows that it can provide a reference for actual engineering design and construction.(4)Based on the above measured results,the whole process of releasing and loading of four pretensioned prestressed UHPC specimens is simulated by ABAQUS.The plastic damage model is adopted for concrete.The interface between strand and concrete is simulated by bond contact.The prestress is applied by the initial stress method.The results of finite element calculation are in good agreement with the experimental results,which indicates that it can be used as a powerful supplement to the experimental research.The above research results can provide references for engineering application of pretensioned concrete beams made of large diameter strand and ultra-high performance concrete.