Experimental And Theoretical Study On Shear Performance Of Simply Supported T-shaped UHPC Beams

Ultra High Performance Concrete(UHPC)has been greatly developed in China in recent years because of its high strength,crack resistance,frost resistance,impermeability and durability,which is obviously superior to ordinary concrete.It has gradually shown a broad application prospect.The superior performance of UHPC materials can effectively reduce the structural size,reduce the weight of components,reduce the amount of steel bars,and meet the development needs of large-span,light and assembled civil engineering structures.The beam is one of the important stress components of the structure and the main component of the bridge.The shear performance of the beam is affected by many factors such as shear span ratio,material properties,reinforcement ratio,section form and so on.It has always been an important research direction in the field of structure.On the other hand,the traditional design theories and methods based on the properties of ordinary concrete materials are not fully applicable to UHPC beams.Therefore,the shear problem of UHPC beams is particularly prominent,which is one of the main problems that must be solved when UHPC materials further expand their engineering applications.In view of this,this paper takes UHPC simply supported T-beam as the object.Firstly,the main factors affecting the shear performance of UHPC beams are comprehensively investigated and summarized.On this basis,the shear performance test of UHPC simply supported T-beams was carried out,and the influence of shear span ratio and longitudinal reinforcement ratio on the shear performance of UHPC beams was compared and studied.Finally,by widely collecting the test data of UHPC beams at home and abroad,the shear performance database of UHPC beams was established.Combined with the test results of this paper,the shear capacity prediction model suitable for UHPC beams was fitted and verified.The main research contents and conclusions of this paper are as follows :(1)The influencing factors and influencing mechanism of shear performance of UHPC beams are comprehensively summarized and analyzed,and the typical models of shear capacity calculation of UHPC beams in domestic and foreign codes and research literatures are sorted out.Based on the existing experimental data,the above influencing factors and calculation models are statistically analyzed.The results show that the compressive strength,tensile strength,shear span ratio,longitudinal reinforcement ratio,stirrup ratio and steel fiber content have important influence on the shear capacity of UHPC beams,which must be considered in the shear design.The existing theoretical models lack a clear boundary for the critical shear span ratio of different shear failure modes of UHPC beams,and the boundary points of different shear span ratios have an important influence on the calculation of shear capacity.Some research models do not consider the steel fiber content rigorously,and there is a contradiction that the steel fiber content is 0 and the shear contribution of steel fiber in the shear capacity prediction model is not 0.Under the same parameters,there are great differences between the existing shear capacity prediction models.The main reason is that the calculation of the shear capacity contribution provided by steel fiber is quite different,and the theoretical research on the shear contribution of steel fiber needs further study.(2)The influence of shear span ratio and longitudinal reinforcement ratio on the crack development state,failure process and failure mode of UHPC simply supported Tbeam was compared and studied.The shear performance test of seven UHPC simply supported T-beams was carried out.The test results show that the shear span ratio has a great influence on the ultimate bearing capacity of UHPC simply supported T-beams.When the shear span ratio is 1.2,the ultimate bearing capacity of the test beam is the largest,reaching 418 k N.When the shear span ratio is 2.7,the ultimate bearing capacity of the test beam is the smallest,only 164 k N,a decrease of 60.8%;the longitudinal reinforcement ratio also has an important influence on the bearing capacity of UHPC simply supported T-beam.With the increase of longitudinal reinforcement ratio,the ultimate bearing capacity of the test beam increases by 68.8 %.With the decrease of the shear span ratio,the height of the compression zone in the mid-span section decreases obviously.When the shear span ratio is 1.2,the whole section of the test beam is subjected to tension.(3)The shear performance test data of 506 UHPC beams at home and abroad were collected,including cross-section characteristics,material properties,loading methods,failure modes,shear capacity and other detailed information,and the shear performance database of UHPC beams was established.Based on the database.Combined with the test results of this paper,the statistical analysis method was used to obtain the prediction model of shear capacity of UHPC beams according to the multivariate nonlinear regression method.The fitting model was comprehensively compared and analyzed by means of experimental data verification,accuracy verification of fitting formula and comparison with literature model.The results show that the predicted value of the model is good for the beam with web reinforcement.Compared with other models,the variance of the model is the smallest,which is 0.204.For beams without web reinforcement,the mean value of the model is about 0.98,the standard deviation is 0.57,and the coefficient of variation is 0.58,which is better than other existing models.