Meso-scale Numerical Lattice Simulation And Shear Theory Of UHPC-NC Composite Beams

Ultra-high performance concrete(UHPC)has ultra-high strength,good durability and construction performance.It can be used in new construction and strengtheningt projects to form ultra-high performance concrete-normal strength concrete(UHPCNC)composite beam.UHPC-NC composite beam can not only improve the service life of the structure,but also achieve good social benefits at a small environmental and economic cost during the service period.As a new type of structure,the UHPCNC composite beam has few researches on its shear behavior and its failure mechanism is very complicated.Under the combined bending and shear stress,the concrete in the interfacial area will be damaged prior to the structure,and the overall cracking failure process is discontinuous.From the perspective of material failure,the steel fibers in UHPC will have a complex "bridging" effect after the matrix is cracked.The existing macroscopic numerical model can neither analyze the discontinuous failure of the composite beam nor simulate the "bridging" of steel fibers.Therefore,the failure of composite beams needs to be studied from a finer scale.At the same time,there are few related shear calculation theories and methods for composite beams,which also hinders its application.Based on the above background,this thesis studies the shear behavior of composite beams from the aspects of mechanical performance and refined numerical simulation,and proposes a refined numerical model analysis method,shear calculation model and simplified calculation formula suitable for composite beams.The main contents of this thesis are as follows:1)Based on the numerical lattice model,the mesoscopic modeling method of UHPC-NC composite beam is built.In this thesis,the nodes and elements of the lattice model are firstly studied,and a method is proposed to use the node degrees of freedom to limit the distribution range of lattice nodes,and use beam elements to connect adjacent nodes to form a lattice model.Then,based on the mesostructure of concrete and UHPC materials,concrete is divided into coarse aggregate,matrix and interface transition zone,and UHPC is divided into matrix and steel fiber.The mesostructure models of the two materials use the random coarse aggregate method and the random steel fiber method to determine the phase gradation and corresponding positions,and project the mesostructure model onto the lattice model,and assign different lattice elements with corresponding material properties to simulate the mesostructure of materials.Finally,based on the modeling methods of concrete and UHPC models,this thesis proposes a modeling method of combining the meso-lattice models of the two materials to form a composite beam meso-lattice analysis model by using interface elements..2)Based on the mechanical properties test of the UHPC-NC interface,a multi-parameter neural network strength prediction model was established.In this thesis,the mechanical test of the UHPC-NC interfacial zone is firstly carried out.The test includes shear test,flexural-tensile test(modulus of failure)and slant shear test.During the test,the mechanical behavior,crack form and failure pattern of the specimen were recorded in detail for strength analysis.Then,due to the diversity of material strength and interface processing methods,in order to obtain the interface strength prediction model suitable for different working conditions,this thesis uses the artificial neural network model to establish multi-parameter shear models of UHPC-NC interfacial zone based on experimental and literature data.Finally,based on the flexural-tensile strength prediction model,the strength of interfacial element of the lattice model is estimated,and the flexural-tensile and direct-shear lattice models are used to verify its accuracy.The influence relationship of strength is established,and the formula for calculating shear strength of the interfacial zone is established.3)Based on the UHPC-NC composite beam shear test and numerical model,the shear failure mechanism of the composite beam is studied.In order to study the shear failure mechanism of UHPC-NC composite beams,the shear test of composite beams is firstly carried out in this paper.The shear test includes beams with and without stirrups,which consider the effects of UHPC layer,longitudinal reinforcement ratio and shear span-to-depth ratio on the failure mechanism of composite beams.During the test,this thesis records the mechanical behavior,crack development and failure mode of the specimen in detail,and focuses on observing the failure mode of the interfacial zone and the UHPC layer.Then,on the basis of the composite beam,the mesoscopic lattice numerical simulation of the composite beam is carried out.The lattice numerical simulation results show that in terms of crack development,the lattice model can completely describe the crack development of the UHPC-NC composite beam,the main crack inclination angle and other key factors that determine the failure mechanism.In terms of ultimate load and mechanical behavior,the lattice model also shows high accuracy.Finally,based on the test and numerical simulation results,this thesis summarizes and studies the crack development process,the overall stress behavior,the failure mode of the interfacial zone and the UHPC failure mode of the composite beam.4)Based on the shear failure mechanism of the UHPC-NC composite beam and the truss-arch model,the shear calculation model and simplified calculation formula are established.In this thesis,based on the composite beam shear test and numerical simulation results,combined with the existing shear calculation model,the UHPC-NC composite beam shear calculation model is established using the truss-arch model.The calculation model takes into account the shear contributions of stirrups,concrete and UHPC layers separately,and is validated against the experimental data presented in this thesis and in the literature.The results show that the shear model can not only effectively predict the shear resistance of the composite beam,but also is sensitive to the size effect,the UHPC layer and the longitudinal reinforcement,and can accurately predict the shear resistance caused by the changes of various factors.Then,in order to obtain a simplified calculation formula suitable for engineering design and construction reference,according to the test and parametric analysis results,this thesis determines the simplified calculation parameters of the lower limit and obtains a simplified calculation formula with a certain degree of safety.