Concrete-filled Steel Tube Specimens Research Articles

Numerical study on interface debonding detection mechanisms with 2D spectral element method for concrete-filled steel tube using embedded PZT sensor

Interface debonding detection approaches based on stress wave measurement for concrete-filled steel tube (CFST) members have received a great deal of attention in civil and infrastructural engineering societies. The development of efficient computing approaches to investigate the mechanisms of the interface debonding detection approaches and the effect of interface debonding on wave propagation within CFST members is critical. In this study, the wave propagation within the cross section of CFST members with and without interface debonding under a single point excitation force is simulated with a two-dimensional (2D) spectral element method (SEM) at first and that of CFST models coupled with a surface-mounted piezoelectric lead zirconate titanate (PZT) actuator and an embedded PZT sensor in the concrete core under continuous and sweep sinusoidal signals is simulated with the 2D SEM and a piezoelectric spectral element. The debonding defect is modeled by removing concrete elements between the steel tube and the concrete core. Results show that interface debonding affects the wave propagation process, the output potentials of the embedded PZT sensors as well as the displacement responses at different locations of CFST specimens obviously. Rayleigh damping identified from experiments is introduced to the SEM models to reflect wave attenuation in the coupled models. The effect of interface debonding on the amplitude, the Fourier spectra and the wavelet packet energy of stress waves are analyzed. For comparison, another numerical model in which the PZT actuator is not located near the interface debonding defect is studied numerically and the effect of the debonding defect on the response of the embedded PZT sensor is illustrated. SEM provides an efficient way to simulate stress wave propagation and to understand the mechanisms of interfacial debonding defect detection for CFSTs with PZT technique.

Finite element analysis of square concrete-filled steel tube (CFST) columns under axial compressive load

The confinement provided in concrete-filled steel tube (CFST) columns can significantly increase the strength and ductility of the concrete columns. However, in non-circular CSFT, the confining stresses are non-uniform and anisotropic, making their incorporation in structural analysis and design fairly difficult. Herein, a novel finite element (FE) model, which takes into account the lateral expansion and triaxial behaviour of the confined concrete, plastic behaviour of the steel tube and interaction at the concrete-steel tube interface in the evaluation of the confining stress field, is developed. It is applied to analyse a total of 92 axially loaded square CFST specimens published in the literature with concrete cylinder strength ranging from 24 to 110 MPa, steel yield strength from 262 to 835 MPa and steel tube depth-to-thickness ratio from 18 to 102. Overall, the FE analysis yielded full-range load-strain curves in good agreement with the experimental results. Using the new FE model, parametric studies on the effects of corner radius have been conducted and it is found that increasing the corner radius would produce better confinement at post-peak stage and thus would improve the post-peak behaviour of square CFST columns.

08.57: The friction investigation of CFRP confined CFST specimens with modified concrete

ABSTRACTNowadays, concrete filled steel tube (CFST) composite members have in wide range application areas such as transverse beams, piers, slender columns, stub columns, wind turbines etc. around the world. The frictional behavior between concrete and steel tube has a vital role in selection of required size and materials during the laboratory experiments, in‐situ applications and modelling studies. In this study, 16 CFST members were subjected to push out tests within the aim of determination the effect of carbon fibres reinforced polymer (CFRP) confinement and modified concrete at the same time. Modified concrete indicates some mechanical additives are inside the concrete for instance steel fibres, polypropylene fibres and crumb rubber. Furthermore, external CFRP confinement was applied with three different arrangements on the outside circumference of the steel tube. Frictional stress created on the lateral interaction surface ranges between 0.18 MPa and 0.63 MPa due to maximum striping force. Maximum frictional strength was observed on the unconfined reference concrete. Although CFRP application seriously increased the axial strength of composite sections, this process decreases the frictional strength between concrete and steel tube, unexpectedly. Therefore, considering different frictional coefficient emerges as a foregone conclusion during the design procedure of CFRP confined CFST sections.

Finite element analysis of concrete-filled steel tube (CFST) columns with circular sections under eccentric load

The casting of concrete in concrete-filled steel tube (CFST) can, via the confinement effect of the steel tube, significantly increase the ductility of the concrete and, in the case of high-strength concrete (HSC), alleviate the shortfall in ductility of the HSC. This kind of structure is gaining popularity but its behaviour is quite complicated. In an axially loaded circular CFST column, the confinement is uniform and equi-biaxial (isotropic within the cross-section). But, when the circular CFST column is under eccentric load, the confinement becomes non-uniform and anisotropic. Such complicated confinement effect is not easy to analyse and for such analysis, a rigorous finite element (FE) method is generally needed. In this paper, a new FE model considering the lateral strain-axial strain relation of the confined concrete covering the full range from the initial elastic stage to the inelastic stage is developed for the analysis of circular CFST columns under eccentric load. The FE model is used to analyse a total of 95 CFST specimens tested by other researchers and the numerical results are compared to the published test results for verification.

Experimental study and calculation of laterally-prestressed confined concrete columns

In this paper, the effect of active confinement on the compressive behaviour of circular steel tube-confined concrete (STCC) and concrete-filled steel tube (CFST) columns is investigated. In STCC columns the axial load is only applied to the concrete core, while in CFST columns the load is carried by the whole composite section. A new method introduced to apply confining pressure on fresh concrete by laterally prestressing steel tubes. In order to achieve different prestressing levels, shortterm and long-term pressures are applied to the fresh concrete. Three groups of STCC and CFST specimens (passive, S-active and L-active groups) are tested under axial loads. The results including stress-strain relationships of composite column components, secant modulus of elasticity, and volumetric strain are presented and discussed. Based on the elastic-plastic theory, the behaviour of the steel tube is also analyzed during elastic, yielding, and strain hardening stages. The results show that using the proposed prestressing method can considerably improve the compressive behaviour of both STCC and CFST specimens, while increasing the prestressing level has insignificant effects. By applying prestressing, the linear range in the stress-strain curve of STCC specimens increases by almost twice as much, while the improvement is negligible in CFST specimens.

Shrinkage Behavior of CFST Members Stored in Natural Environments

To provide reasonable guidance for CFST practical structures in design and construction, shrinkage test of concrete-filled steel tube (CFST) members in natural environments was presented. In order to acquire some knowledge regarding the influence of the interaction between the steel tube and core concrete during concrete shrinkage, ordinary concrete (OC) specimens exposed to the atmosphere were also performed in this test. The experimental results have been compared with predicted shrinkage strains obtained from the CEB-FIP1990 model, B3 model, GL2000 model, ACI 209 model. The research results show that the measured shrinkage strains of CFST specimens presents characteristics of seasonal variation under the influence of natural environments. Although expansive additive is added in the concrete mix, there is still strain difference between the steel tube and core concrete. The influence of steel ratio on the shrinkage behavior of CFST is unclear. The value of CFST shrinkage strain is about 50% of the OC shrinkage strain. For the early-age shrinkage of CFST, the predicted result by the ACI209 model is better than the other three models. The CEB-FIP1990 model is recommended to be used to predict the long-term shrinkage behavior of CFST.

Behaviors of axially loaded square concrete-filled steel tube (CFST) Stub columns with notch in steel tube

Compressive tests of the CFST stub columns with artificial notches in the steel tubes were conducted in this paper to investigate the effects of the material imperfections of steel tubes on column performances. The load-strain responses, column strength and confining effects were discussed in detail. A parametric study, including the notch length, notch orientation, notch location and ductility, was also conducted. The results indicate that the notched CFST specimens have quite different failure modes from the intact CFST specimens. A notched CFST specimen may fail by bulging or closing of the notch, which was determined by the notch orientation. The results also show that the notched CFST specimens had lower mechanical performances than the intact CFST specimens because the notch steel tube could not offer sufficient confining effect on the concrete core inside it. Based on the experimental results, a practical calculation formula for the bearing capacity of notched CFST columns was proposed, which gave well agreement with the experimental results.

Axial and lateral stress–strain model for concrete-filled steel tubes

The confining stress acting on the concrete in a concrete-filled steel tube is dependent on the lateral strain of the confined concrete whilst the lateral strain of the confined concrete is dependent on the confining stress. Hence, the lateral strain and confining stress are inter-related. Up to now, it remains a difficult task to evaluate the lateral strain of concrete confined by a steel tube and the confining stress acting on the concrete. To resolve this problem, a theoretical model for evaluating the lateral strain and confining stress in a concrete-filled steel tube system at various stages of loading is developed. The theoretical model is first applied to analyze concrete-filled steel tube specimens tested by other researchers to verify its accuracy and then used to work out the required thickness-to-diameter ratios of steel tubes for achieving different levels of ductility.

Experimental studies on the behavior of concrete-filled steel tubes incorporating crumb rubber

This study summarizes the results of experimental push-out tests conducted on concrete-filled steel tubes (CFST) incorporating rubberized concrete with the main variables being the recycled rubber replacement ratio, certain sizes of steel tubes having circular and square sections and ages of testing (at 28days and one year). In addition, 150mm-cubes were prepared using rubberized concrete mixtures to investigate the impact of shredded rubber contents on the compressive strength of concrete. Four concrete mixtures were designed at a constant water-to-cement ratio of 0.5 and with a cement content of 400kg/m3. The mixtures were produced by replacing the fine aggregate with crumb-shredded tires at designated replacement levels of zero, 10%, 20% and 30% by total fine aggregate volume. Test results indicated that the use of rubber crumb increased the fresh-state workability and had an adverse influence on the compressive strength of concrete. The details of bond stress and the interface core slip in CFST specimens were estimated and the developed bond mechanisms were explained. The push-out testing of CFST specimens displayed a reduction in the bond strength and the ductility with increasing concrete age.

Behaviour of concrete filled-steel tubes under axial load

In order to study the mechanical mechanism of concrete-filled steel tubes (CFSTs) under axial compression, a theoretical model for analysing the behaviour of axially loaded CFST columns is presented in this paper. The model is derived from the theory of elasto-plasticity, non-linear elastic mechanics and materials properties. Based on a related numerical program, analytical results were compared with experimental data. The complete load–deformation relationship of specimens, steel tube stress, performance of the confined core concrete and the interaction between the outer tube and the core concrete were investigated. The analytical results indicated that this model is able to capture the mechanical behaviour of concrete-filled circular steel tubes. It was found that the axial and circumferential stresses of the steel tubes behaved non-linearly during the loading history. It was concluded that the behaviour of the confined core concrete was significantly influenced by the confining pressure. The constraining factor had an important effect on the improvement of the core concrete strength. The analytical results also indicated that the axial stress–strain relationship of concrete under constant confinement was not appropriate for the core concrete in CFST specimens. Finally, a comparison between the confining pressure and the confined concrete response is presented.

Bond behavior in concrete-filled steel tubes

In this paper, the bond behavior between the steel tube and the concrete in concrete-filled steel tubes (CFST) is investigated. A series of push-out tests on circular and square CFST specimens were conducted, and the main parameters considered in the test program were: (a) cross-sectional dimension (120–600mm); (b) steel type (carbon and stainless steels); (c) concrete type (normal, recycled aggregate and expansive concretes); (d) concrete age (31–1176days); and (e) interface type (normal interface, interface with shear studs and interface with an internal ring). The experimental results indicate that stainless steel CFST columns have lower bond strength compared with carbon steel counterparts, and the bond strength decreases remarkably with increasing cross-sectional dimension and concrete age. To enhance the bond strength, welding internal ring(s) onto the inner surface of the steel tube is the most effective method, followed by the methods of welding shear studs and using expansive concrete.

Failure mechanism of full-size concrete filled steel circle and square tubes under uniaxial compression

In this paper, the failure mechanisms of full-size concrete filled steel tubes (CFST) under uniaxial compression were investigated with nonlinear finite element method. Existing experimental results were employed to verify the validity of the finite element models of CFST specimens. Then, the numerical analysis was further conducted to study the mechanical behaviors of full-size CFST columns with circular and square cross sections under uniaxial compression. The simulation results indicate that the distribution of the contact pressure between circular steel tube and core concrete is much more uniform than that between square steel tube and concrete, resulting in much higher confinement and more efficient interaction between steel tube and core concrete in circular CFST columns, as well as ultimate load capacity and ultimate displacement. Extensive parametric analysis was also conducted to examine the effect of various parameters on the uniaxial compression behaviors of circular and square CFST columns.

Performance of circular concrete-filled fiber-reinforced polymer-steel composite tube columns under axial compression

A concrete-filled fiber-reinforced polymer-steel composite tube column consisting of inner concrete and an exterior fiber-reinforced polymer-steel tube was studied in this paper. A total of 22 specimens were tested under axial compression to investigate the performance of the circular concrete-filled fiber-reinforced polymer-steel composite tube columns. The main experimental parameters were the thickness of the steel tube, the number of fiber-reinforced polymer layers, the type of fiber-reinforced polymer, and the hybridization of fiber-reinforced polymer, in which basalt-fiber-reinforced polymer was used to manufacture concrete-filled fiber-reinforced polymer-steel composite tube columns and was compared with carbon fiber-reinforced polymer. The test results showed that local buckling of the steel tube can be suppressed and even prevented effectively with fiber-reinforced polymer strengthening, and the compressive strength of the concrete-filled fiber-reinforced polymer-steel composite tube specimens is enhanced by the external fiber-reinforced polymer confinement compared with that of the concrete-filled steel tube specimens. With the increase in the number of fiber-reinforced polymer layers or with a larger ultimate tensile strain in the fiber-reinforced polymer, the peak axial strain corresponding to fiber-reinforced polymer fracture increases accordingly. A new model that considers composite action was developed for strength prediction of concrete-filled fiber-reinforced polymer-steel composite tube columns, and the analytical results agree well with the experimental results.

Structural behavior of concrete filled steel tubular sections (CFT/CFSt) under axial compression

In this study, compressive strength, modulus of elasticity and steel tensile coupon tests are performed to determine material properties. Sixteen hollow cold formed steel tubes and 48 concrete filled steel tube specimens are used for axial compression tests. The effects of width/thickness ratio (b/t), the compressive strength of concrete and geometrical shape of cross section parameters on ultimate loads, axial stress, ductility and buckling behavior are investigated. Circular, hexagonal, rectangular and square sections, 18.75, 30.00, 50.00, 100.00 b/t ratio values and 13, 26, 35MPa concrete compressive strength values are chosen for the experimental procedure. Analytical models of specimens are developed using a finite element program (ABAQUS) and the results are compared. Circular specimens are the most effective samples according to both axial stress and ductility values. The concrete in tubes has experienced considerable amount of deformations which is not expected from such a brittle material in certain cases. The results provide an innovative perspective on using cold formed steel and concrete together as a composite material.

Experimental behaviour of box concrete-encased CFST eccentrically loaded column

Box concrete-encased, concrete-filled steel tube (CFST) piers have been used in bridges in high-earthquake-intensity regions in China in recent years. They are expected to have higher strength and better ductility than normal box reinforced concrete members owing to the contribution of CFSTs in the corners of the composite box section. However, studies on this type of composite piers are limited. This paper presents an experimental study on the behaviour of box concrete-encased CFST columns under eccentric compression. A total of 14 specimens, including 12 box concrete-encased CFST specimens and two reference box reinforced concrete specimens are tested. The main parameters in the test are height-to-width ratio and load eccentricity. A simplified method is proposed to predict the load capacity of the box concrete-encased CFST columns.

Creep analysis of CFT columns subjected to eccentric compression loads

By considering the creep characteristics of concrete core under eccentric compression, a creep model of concrete filled steel tubes (CFT) columns under eccentric compressive loads is proposed based on the concrete creep model B3. In this proposed model, a discrete element method is introduced to transform the eccentric loading into axial loading. The validity of the model is verified by comparing the predicting results with the published creep experiments results on CFT specimens under compressive loading, together with the predicting values based on other concrete creep models, such as ACI209, CEB90, GL2000 and elastic continuation and plastic flow theory. By using the proposed model, a parameters study is carried out to analysis the effects of practical design parameters, such as concrete mix (e.g. water to cement ratio, aggregate to cement ratio), steel ratio and eccentricity ratio, on the creep of CFT columns under eccentric compressive loading.

Experimental and analytical investigation of high-strength concrete-filled steel tube square columns subjected to flexural loading

The concrete-filled steel tube (CFT) columns have several benefits of high load-bearing capacity, inherent ductility and toughness because of the confinement effect of the steel tube on concrete and the restraining effect of the concrete on local buckling of steel tube. However, the experimental research into the behavior of square CFT columns consisting of high-strength steel and high-strength concrete is limited. Six full scale CFT specimens were tested under flexural moment. The CFT columns consisted of high-strength steel tubes (<TEX>$f_y$</TEX> = 325 MPa, 555 MPa, 900 MPa) and high-strength concrete (<TEX>$f_{ck}$</TEX> = 80 MPa and 120 MPa). The ultimate capacity of high strength square CFT columns was compared with AISC-LRFD design code. Also, this study was focused on investigating the effect of high-strength materials on the structural behavior and the mathematical models of the steel tube and concrete. Nonlinear fiber element analyses were conducted based on the material model considering the cyclic bending behavior of high-strength CFT members. The results obtained from the numerical analyses were compared with the experimental results. It was found that the numerical analysis results agree well with the experimental results.

Full-Scale Tests of Slender Concrete-Filled Tubes: Axial Behavior

AbstractComposite steel-concrete columns and beam-columns have been shown to provide superior performance when the intrinsic synergistic characteristics of concrete and steel are properly exploited. However, limited test data are available to justify the structural system response factors and comprehensive design equations in current design specifications. This research, through the testing of 18 full-scale, slender concrete-filled steel tube (CFT) beam-columns, addresses these needs by providing comprehensive data to calibrate advanced computational models and assess design equations. The CFT specimens were subjected to complex load protocols that included pure compression, uniaxial and biaxial bending combined with compression, pure torsion, and torsion combined with compression. The results for the pure compression tests reported in this paper indicate that current AISC design provisions provide an accurate estimation of column capacity for both strength and stiffness. The experimental loading response...

Experimental and numerical investigations of the compressive behavior of concrete filled steel tubes (CFSTs)

This paper presents an experimental study to investigate the compressive behavior of circular concrete filled steel tubes (CFSTs) when subjected to pure axial loading at a low rate of 0.6kN/s. CFSTs of three different diameter-to-thickness (Dt) ratios of 54, 32, and 20 are considered in this study filled with two concrete's compressive strengths of 44MPa and 60MPa. The measured compressive axial capacities are compared to their corresponding theoretical values predicted by four different international codes and standards: the American Institute of Steel Construction (AISC), the American Concrete Institute (ACI 318), the Australian Standard (AS), and Eurocode 4. Result comparisons also included some suggested equations found in the literature. It was found that the effect of (Dt) ratio on the compressive behavior of the CFST specimens is greater than the effect of the other factors. The underestimation of the axial capacities calculated by most of these codes reduces as the Dt ratio increases as verified by the experimental results. A nonlinear finite element (FE) numerical model using the commercial software package ABAQUS is also developed and verified using the presented experimental results.

Frost Resistance of Steel Fiber Reinforced Micro-Expansive Concrete Filled Steel Tube

Abstract. This investigation was done based on the concrete filled steel tube (CFST) winter construction of Ganhaizi Bridge. Steel fiber reinforced micro-expansive concrete (SREC) with high frost-resistance ability, excellent fluidity and good self-compacting ability was prepared. Strength development mode under low temperature was illustrated. Scanning electron microscope (SEM) was used to analyze the morphology features of hydration products at the age of 28 days. It was confirmed that when the content of sodium nitrite was 0.4%, SREC did not frost under -5°C. In addition, SEM showed that ettringite and CSH gels were in good growth state, and concrete structure was very dense. Furthermore, 6 CFST short columns were poured in the winter construction site of Ganhaizi Bridge, and another 27 short columns were cut from emitting slurry pipe filling with concrete emitted from main girder. The bearing capacity of all the CFST specimens was tested when the environment temperature rose to about 25°C. The experimental results indicated that the mechanical performances of CFST specimens with anti-freezing admixture did not exhibit degradation when cast in site under a low temperature and tested at 24.3°C. The concrete presented in this research substantially met the need of Ganhaizi Bridge CFST construction in winter.