Experimental Research On Shear Capacity Of UHPC Beams

Ultra-high Performance Concrete(UHPC)is considered the most innovative cementitious composite material of the past 30 years,exhibiting ultra-high strength,high toughness,and excellent durability.UHPC structures have been proven to overcome the shortcomings of ordinary concrete,such as susceptibility to cracking and poor durability.In the design of UHPC bridges,the calculation of shear capacity is a crucial aspect,requiring further study on the influence of steel fibers within the UHPC matrix on its shear performance.This paper investigates the shear performance of nine UHPC beams through a combination of experimental and numerical analysis methods.It also compares the applicability of different shear calculation methods for UHPC beams under various specifications.The main research findings and conclusions are as follows:(1)The paper analyzes different shear design theories and their inherent relationships.The ACI specification(USA)mainly adopts the semi-theoretical and semi-empirical formula based on the 45 o fixed-angle truss model to evaluate the shear of reinforced concrete components;Compared with the ACI specification,EN1992-1 and EN1992-2(Europe)believe that the calculation results of the fixed-angle truss model are conservative,and the angle range is set to 22.5o～45o,which further develops the variable angle truss model theory;The gray box calculation method based on variable angle truss model theory(data genetic learning calculation model)adjusts the contribution of experience coefficient through data genetic programming,and further improves the calculation accuracy of experimental results based on variable angle truss model theory calculation method;AASHTO LRFD-2007(USA)and CSAA23.3-94(Canada)based on the modified pressure field theory,considering the shear contribution of aggregate bite force and tensile stress in the matrix at the cracking of reinforced concrete components,and gives the calculation method of shear capacity of reinforced concrete components after cracking.This section also presents a comparative analysis of shear specifications for UHPC based on different theories.The Korean UHPC Shear Design Code(KCI-2012)and the French UHPC Shear Design Code(AFGC-2002)are based on the theoretical model of variable angle truss;The Australian UHPC Shear Design Code(DR AS5100-2014)is based on the modified pressure stress field theory,and considers the influence factor of coarse aggregate and the influence of longitudinal strain on the inclination angle in its matrix shear calculation formula.(2)The shear test of nine T-type UHPC beams was carried out,and the effects of different steel fiber content,stirrup ratio,and shear span ratio on the shear performance of UHPC beams were studied.The test results show that the failure modes of the shear test beam mainly include: oblique pressure failure,shear pressure failure,shear compression failure,and bending failure superimposed failure mode.When the steel fiber content and shear span are relatively small,and stirrups are not configured,the shear capacity of UHPC beam oblique section is low,and the beam body is shear and pressure failure.When the fiber distribution rate and shear span are relatively small,and when the stirrups are configured,the shear of the oblique section increases,the crack development is limited,and the beam body is damaged by oblique compression.With the increase of fiber distribution rate and shear span ratio,UHPC shear is improved,the bending moment at the loading point is increased,the shear distribution is more balanced,the development of bending cracks is more obvious,and the beam body is bent and shear failure.According to the analysis of test results,the recommended suitable fiber distribution rate is 2%～3% volumetric content.(3)The paper introduces the Concrete Plastic Damage(CPD)model into the UHPC constituents and establishes a refined finite element model of UHPC test beams with a shear span ratio(λ)of 1.59.Furthermore,the failure morphology of UHPC shear beams was analyzed and studied.The results show that the shear contribution of stirrups increases with the increase of steel fiber content under the condition that the shear span ratio of reinforced UHPC beams remains unchanged.(4)The results of UHPC shear tests in the existing literature were collated,and the UHPC shear test database was established.The effects of shear span ratio and steel fiber volume content on UHPC shear specifications were analyzed.Additionally,the calculated values of UHPC shear capacity based on different specifications were compared with the literature results,analyzing the mean,coefficient of variation,and non-conservative rate of the ratio of the standard calculation results to the literature results.The results show that when considering the influence of steel fiber content,the simplified modified pressure field theory(SMCFT)recommended by DR AS5100-2014 specification(Australia)has the highest calculation accuracy.When the UHPC shear beam has a small shear span ratio(λ ≤ 1.5),the calculation accuracy of the variable angle truss model in the KCI-2012 specification(Korea)is higher than that of other specifications.If the shear span ratio exceeds 1.5,the calculation accuracy based on the SMCFT model(Australian standard)is higher.