Experimental And Theoretical Study On Design Method Of UHPC Beams Based On Multiscale Analysis Of Interfacial Bond Behavior

Ultra-high performance concrete(UHPC)is a new class of super concrete with ultra-high compressive strength,considerable tensile strength and superior durability,in which superfine active admixture is added to improve the compactness and compressive strength and the steel fibers are dispersed to enhance the tensile strength.UHPC has been gradually applied from ancillary facilities,such as joint and shear connectors,to the main structural component,and it is even totally adopted in some structures.Researches in material engineering field mainly focus on optimizing mix proportion by using particle dense packing theory and adding nanoscale particles,investigating the effect of different aggregates and steel fibers on the interfacial bond behavior and material properties,hydrating and hardening mechanisms,rheological property and so on.In structure engineering field,researchers usually use the conventional method to investigate the structural behavior under the bending load,shear load,seismic load,impact load and blast load via testing,and to establish corresponding design methods at the macro level.It can be concluded that the material researches and structural studies are out of line.Especially,the nano-scale,micro-scale and meso-scale research achievements of material engineering cannot be utilized appropriately in conventional structural design theory.The core links between these two fields are the quantification of the contribution of each component on the mechanical properties of structural components from meso-scale level to macro-scale level and the interfacial bond behavior.The bond behavior between fibers and matrix is one of the critical problems.This dissertation presents an experimental investigation and theoretical study on the bond behavior between steel fibers and matrix(micro-scale to meso-scale),and the cross-scale(meso-scale to macro-scale)flexural analysis model and shear strength model of UHPC beams derived from multi-scale analysis.The proposed theories are verified by the experiment results.Details of the study are summarized as follows:(1)Experimental study on the bond behavior between steel fibers and UHPC matrix(micro-scale to meso-scale)90 pull-out specimens are tested to investigate the bond behavior between steel fibers and UHPC matrix with the parameters of fiber orientation,fiber type and maximum particle size of sand aggregate.The failure interface is observed using scanning electron microscope at a micro-scale.Two macro-scale failure modes,including fiber rupture and fiber being pulled out are observed and three micro-scale failure modes,incorporating interfacial transition zone failure,matrix failure and fiber surface scratching are identified.(2)Cross-scale flexural model of ultra-high performance fiber reinforced concrete(UHPFRC)beams based on a mesoscale constitutive model(meso-scale to macro-scale)The resistance mechanism of fibers on the residual tensile strength is investigated based on the mesomechanics analysis.A mesoscale constitutive model is proposed,taking into consideration uniform distribution,embedment length and orientation of fibers.The proposed model provides an alternative solution for the determination of UHPFRC residual tensile strength.A flexural strength model is subsequently derived on the basis of the proposed mesoscale constitutive model.The proposed model is verified by 9 bending test results and various machanisms on flexural strength are detailed discussed.(3)Experiment verification on the cross-scale flexural model of UHPFRC beamsAn experimental study is executed to verify the proposed cross-scale flexural model and to assess the flexural performance of UHPFRC beams with the test parameters of the fiber volume fraction,fiber type,longitudinal reinforcement ratio and concrete strength.The failure of the fiber bridging is one by one fiber and the cracks exibit more and fine distribution characteristics.A new ductility index,expressed in the form of dividing the ultimate deflection by flexural cracking deflection,is introduced to characterize the post-cracking ductility capacity.Calculations with the proposed model show good agreement with the test results and the complete loading response of the test beams can be predicted.(4)Cross-scale shear strength of UHPFRC beams:mesoscale fiber-matrix discrete model(meso-scale to macro-scale)To solve the key problem of determining the shear contribution of fibers,a new concept of an effective fiber distributed region(EDR)along the critical diagonal shear crack,where fibers are efficient at providing shear resistance,is proposed.Two methods to determine the width of EDR are proposed based on probability theory and the pullout load slip relationship.Combining the number of efficient fibers and the bond strength of a single fiber,the shear contribution of fibers is derived.A mesoscale fiber-matrix discrete shear strength model is proposed,which offers an alternative perspective in understanding the shear behavior of UHPFRC beams,especially the shear resistance of fibers at a mesoscale level.The proposed model takes into account the fiber orientation,embedment length and bond strength between the fibers and matrix and can reflect the shear force resistance mechanism of fibers.(5)Experiment verification on the cross-scale shear strength model of UHPFRC beamsEleven T-beams,reinforced with high strength steel,are tested to failure to verify the proposed cross-scale shear strength model and to investigate the effect of shear span to depth ratio,fiber ratio,fiber type,concrete strength and stirrup ratio on the shear behaviour,especially post-cracking shear strength and deformability,of UHPFRC beams.The shear failure of UHPFRC beams is firstly defined as semi-ductile,semi-brittle failure.To characterize the post-cracking shear behavior and stiffness degradation of UHPFRC beams,a new ductility index,which is expressed in the form of dividing ultimate deflection by shear cracking deflection,is introduced.The proposed shear strength model could accurately predict the shear strength of UHPFRC beams.(6)Evaluation of shear strength model based on the shear database for SFRC beamsThe shear test data of SFRC beams are widely collected from literatures,a shear database for SFRC beams(SFRC-SDB)is established with the total number of 941.Based on the analysis of the samples in the SFRC-SDB,the influence of main factors on shear behavior is investigated and the shear strength models are evaluated.