Research On Mechanical Properties Of 3D Printed Concrete Beam Members Strengthened By BFRP Bars

The application of 3D printing technology in the field of construction engineering is a brand new engineering construction concept.At present,systematic achievements have been made in the development of 3D printed concrete materials,the research of working performance and mechanical properties.However,as a new construction technology,the compatibility research between 3D printing technology and the traditional concrete reinforcement method is rarely involved.Both traditional concrete and 3D printed concrete have the characteristics of weak pressure and brittle failure,which makes it difficult to construct the structure independently.FRP bars are a new type of structural reinforcement materials,among which basalt fiber bars(BFRP)are made of natural materials,with low cost,light weight,high strength,excellent corrosion resistance and fatigue resistance.It is expected to be combined with 3D printed concrete to become a new type of assembled reinforced concrete structure and realize mechanized intelligent construction.In view of the spatial anisotropy of 3D printed concrete caused by printing extrusion process,which is different from the mechanical performance of traditional concrete,based on the analysis of the collaborative working performance of BFRP bars and 3D printed concrete,the mechanical properties of 3D printed concrete beam members strengthened by BFRP bars were studied in this paper.The following work was mainly carried out:(1)According to the bond performance of BFRP bars and 3D printed concrete,the pull-out experiment of 24 specimens in 8 groups was carried out by taking the surface morphology of BFRP bars and the specimen forming mode(3D printing and formwork casting)as variables,and the bond performance and influencing factors between BFRP bars and 3D printed concrete were analyzed.The research shows that the molding method and the surface morphology of BFRP bars have significant effects on the bond performance,and the bond strength of the drawing specimens formed by template casting is better than that of the drawing specimens formed by 3D printing.Under the same condition,the bonding force between the BFRP bar with sand on the surface and the 3D printed concrete is 3%-12%higher than that between the BFRP bar with smooth surface and the 3D printed concrete.The formula for calculating the bond strength of 3D-printed specimens is derived based on the influence of the defects between the layers of 3d-printed concrete on the bond section area,and the calculated results are in good agreement with the experimental values.(2)For the printing method,the influence of longitudinal reinforcement ratio on the bearing capacity of 3D-printed concrete beam members strengthened by BFRP bars was designed and 10 flexural beam members were designed and made.The results showed that:under the same reinforcement ratio,the mechanical difference between the parallel printing method and the circular printing method was not significant.Research shows that the flat section assumption is still applicable for 3D printed concrete beams.The cracking load of 3D-printed concrete beams reinforced with BFRP bars did not change significantly with the increase of reinforcement ratio,but the ultimate load gradually increased.According to the different modes of compressive failure and tensile failure,the formulas of flexural load capacity are derived respectively.In order to prevent the failure of oblique section,the "embedded"stirrup was designed to enhance the shear capacity.This shows that the shear structure of BFRP reinforced concrete beams is the key point in the design of beam members.(3)Aiming at 3D printing increased BFRP reinforcement concrete beams to carry out the numerical simulation of concrete with CT scan,3D printing information to determine the printing substrate,pore distribution in the interlayer between the defects and the defect of geometry size and porosity,according to the size of the porosity reduction of mechanical performance,respectively on the substrate,interlayer between the defects and the defect gives different material properties.Numerical simulation of beam member load deflection curve,longitudinal bar strain curve and failure mode are in good agreement with experimental beam data.The maximum error of cracking load is-10.38%and the maximum error of ultimate load is-5.9%.BFRP bar nominal yield strength of the desirable for the ultimate strength of 75%-85%,when the concrete is concrete beam crushed and BFRP bar reaches nominal yield strength,can be thought of beam optimum muscle damage occurs,using finite element simulation under different reinforcement ratio are given reasonable optimum muscle damage occurs concrete strength value,provide reference for the follow-up test and engineering application.(4)Because BFRP bars are linear elastic materials,and the shear failure of concrete beam members is characterized by obvious brittleness,the shear design of 3D printed concrete strengthened by BFRP bars has become a key technology in structural design.In view of the stirrup ratio,and stirrup stirrup form material,conducted 11 reinforced with BFRP bar 3D printing concrete beam shear performance test,the test shows that the 3D printing of shear bearing capacity of concrete beam is provided by the concrete and the shear stirrups,but due to the test in the process of local bond-slip reinforcement,stirrups strength utilization degree is low,BFRP stirrup strength utilization rate is about 7%,while the HRB400 stirrup utilization rate is about 30%.With the increase of the hoop ratio,the deviation between the shear bearing capacity and the theoretical calculation value increases.The shear bearing capacity of beams with HRB400 stirrup is increased by 39.56%-104.32%,and that of beams with BFRP stirrup is increased by 33.91%-86.18%.