The Influence Of Prestress Level And Material Properties On The Mechanical Properties Of Concrete-filled Steel Tube

Concrete-filled steel tubular structures become an important engineering structure in engineering practice because of their superior bearing capacity,plasticity and toughness,convenient construction operations,and high economic benefits.Ordinary concrete-filled steel tube structure can better play the advantages of two kinds of steel and concrete materials,but the constraint effect of the steel tube can only be exerted when the core concrete is under pressure,lateral deformation is obvious,and vertical cracking occurs.The existence of initial pre-compressive stress of core concrete can make the steel tube participate in lateral restraint sooner,so that the whole process of core concrete is under three-way compression,which can delay its cracking and effectively extend the elastic state of concrete,so the core concrete pre-compression stress the application methods and the performance of prestressed structures are also gaining increasing attention.In this paper,through experiments and theoretical analysis,several methods are proposed to apply pre-compressive stress on the core concrete.The main research is to add aluminum powder to the core concrete and use the aluminum foam to produce volume expansion to achieve the application of pre-compression stress on the concrete.Pre-compression stress is generated before permanent setting and is retained forever,so as to improve the mechanical properties of the concrete-filled steel tube structure.The main contents of this paper are as follows:(1)A screw concrete preloading method was designed based on screw precession method.Through the threaded hole on the surface of the steel pipe,the screw is screwed into the steel pipe,and the pre-stressed concrete is extruded to achieve the pre-stress;the water is used instead of the concrete to pour into the steel pipe,and the feasibility study of the screw pressurizing the pre-stressed component is carried out.The comparison between the results and the theoretical analysis verified the feasibility and reliability of the screw pressurizing prestressed concrete-filled steel tube(CFST)complete set.However,this method of prestressing has higher requirements on the sealing performance of the device.(2)The process of aluminum-powdered aerated concrete was investigated,and the properties of the aerated concrete were experimentally studied,including parameters such as strength and expansion rate.The appropriate water-cement ratio and aluminum powder content were proposed through experimental analysis.Underthe conditions of satisfying the expansion rate and material strength,the use of 2%aluminum powder(in the amount of cement)can ensure that the concrete has a 20%expansion rate and the strength reduction is controlled within 24%.(3)Conducted four self-stress tests on aluminum-clad aerated steel tubular concrete columns.The self-stress value showed an increase in the first 40 minutes of the reaction of aluminum powder.After the initial setting of the concrete,the prestressing force was applied,and the maximum self stress value was 4.23 MPa.The prestress loss of the self-stressing steel tube concrete was further studied.The results showed that :Under normal curing conditions,the prestress value applied is reliable and stable,and the long-term loss of prestress is controlled within 8%,which verifies the feasibility of the entire installation and process flow.(4)Axial compressive comparative tests of 4 aluminum powder reinforced aerated steel tube concrete and 2 ordinary steel tube concrete columns were carried out to study the mechanical properties of concrete filled steel tube short columns under different prestressing levels.The results show that the existence of initial prestress can improve the axial compression performance of CFST.Compared with ordinary CFST columns,the maximum load in the elastic stage of prestressed specimens is up to 88% of the ultimate bearing capacity,and the ultimate bearing capacity is increased by about 15%.(5)The advantages and disadvantages of several prestressed processes were compared,and the applicability of different prestressing methods was obtained.The volume expansion of the concrete produced by the screw precession method and the refilling of aluminum powder,and the loss of prestress caused by the two kinds of prestressed methods are small.The external high-pressure pump application method can be combined with the modern construction process,and the operability is strong.(6)The numerical simulation software was used to establish the axial compression test model of prestressed concrete-filled steel tubular columns.The whole process of prestressing and axial compression of the specimens was simulated.The software analysis results were in good agreement with the experimental results,and revealed the prestressing in depth.Concrete axial working mechanism.