Compressive Stress-Strain Relationship Of Ultra-High Performance Concrete With Coarse Aggregate Confined By Square FRP Tube

Ultra high performance concrete(UHPC)has excellent properties such as ultrahigh strength,high toughness,high durability and strong micro crack self-healing ability,but it has high cost and poor ductility.Adding appropriate amount of coarse aggregate into UHPC can form super high performance concrete with coarse aggregate(UHPC-CA),which can effectively reduce the cost;the load-bearing and deformation capacity of UHPC-CA can also be significantly improved by injecting UHPC-CA into FRP tube.As the basis of material component research and structural nonlinear analysis,stress-strain relationship is an important reference for component performance analysis and engineering design calculation.At present,the research on the stress-strain relationship of FRP tube confined UHPC-CA mainly focuses on circular section column,but the research on square/rectangular section column which is more widely used in practical engineering is still insufficient.Therefore,based on the National Key Research and Development Plan: "research and application of high performance fiber reinforced composites and new structure key technologies"(No.2017YFC0703001),the compressive stress-strain relationship of UHPC-CA confined by square FRP tube is studied in this paper,the main work and research results are as follows:(1)Considering the four factors of tube thickness,chamfer radius,fiber arrangement angle and coarse aggregate volume content,22 groups of square single winding FRP tube confined UHPC-CA stub columns were designed and fabricated by selecting the most commonly used CFRP and GFRP materials in engineering.Through the axial compression test,the characteristics of the stress-strain curve of FRP tube confined UHPC-CA are analyzed,and the influence of four factors such as tube thickness on the stress-strain curve is investigated.The results show that with the increase of the confinement,the stress-strain curve of the specimens will transition from weak confinement to strong confinement,and the greater the thickness of the tube and the radius of the chamfer,the more obvious the confinement effect of the FRP tube is;when the fiber arrangement angle decreases,the tensile strength of the FRP tube decreases,the fracture strain increases,and the confinement effect of the FRP tube first increases and then decreases;When the coarse aggregate increases,the strength of UHPC-CA first increases and then decreases,which further affects the confinement strengthening effect of FRP confined UHPC-CA,but the influence degree is low.(2)Based on the analysis of the stress-strain curves of UHPC-CA confined by CFRP and GFRP tubes,18 groups of square composite wrapped FRP tube confined UHPC-CA stub columns were designed and fabricated in order to give full play to the advantages of the two materials,considering the three factors of internal and external arrangement,material proportion and chamfer radius.Through the axial compression test,the characteristics of stress-strain curve of UHPC-CA confined by composite winding tube are analyzed,and the influence of composite winding mode on the stressstrain curve is discussed.The results show that the curve shape of UHPC-CA confined by composite winding tube is similar to that of UHPC-CA confined by single winding tube;in terms of winding mode,the winding mode of CFRP inside and GFRP outside is more conducive to the tube to exert the restraining effect,and the higher the content of CFRP,the stronger the restraining effect of the tube.When the chamfer radius increases,the integrity of the composite wound tube is further improved,and the influence of the winding mode is gradually weakened.(3)Through the arrangement of strain gauges and pressure sensors,the strain data in the middle of the specimen and the interaction force of FRP inner wall were obtained.The distribution and development process of FRP tube binding force were analyzed,and the interaction law between FRP tube and UHPC-CA was revealed.The results show that under the square section,the distribution of confinement is uneven,and significant stress concentration appears at the chamfer.When the chamfer radius increases,the distribution of confinement is more uniform,and the stress concentration is weakened.In addition,there is a short excitation process from the beginning of the binding force to the full exertion of the binding effect of FRP tube,and the binding force of the inner FRP tube is slightly earlier than that of the outer FRP tube.(4)Based on the analysis of the characteristics of stress-strain curve and the influence law of parameters,and referring to the existing stress-strain relationship model of confined concrete,the stress-strain relationship models of square FRP tube confined UHPC-CA are proposed for the two cases of strong constraint and weak constraint,respectively.The predicted results of the proposed model are in good agreement with the experimental curves in this paper.Finally,on the basis of summarizing the analysis results of the full text,the prospect of the follow-up research on this topic is put forward.