Concrete-filled Steel Tube Structures Research Articles

Post-fire bond-slip performance of concrete-filled stainless steel tube columns

Concrete-filled stainless steel tube (CFSST) structures exhibit superior strength, durability, and fire resistance. These attributes, coupled with their aesthetic appeal and corrosion resistance, make them highly suitable for a broad range of civil engineering applications, ensuring long-term performance and safety across diverse environments. This study investigates the post-fire bond-slip performance of CFSST structures to facilitate the strengthening and retrofitting of fire-damaged CFSST structures. The paper reviews the bond-slip performance and fire resistance of CFSST. A series of fire tests and push-out tests were conducted on 99 circular CFSST stub columns, considering variables such as fire duration, wall thickness of the stainless steel tube, and concrete strength. The analysis covers the slip mechanism, load-slip relationship, sliding load, ultimate displacement, and ultimate bond strength of specimens post-fire. The bonding force between interfaces transitions from cementation to mechanical interlock and ultimately to friction, with some overlap among these mechanisms. A positive correlation exists between sliding load and wall thickness when fire duration is short. For shorter fire durations, fire exposure reduces the ultimate displacement while increasing bond-slip stiffness. Finally, a formula is proposed to predict the ultimate bond strength of CFSST stub columns following exposure to ISO 834 standard fire conditions.

Mechanical properties of geopolymer recycled concrete infilled steel tubes with three sectional types

The steel tubes with geopolymer recycled concrete (GRC) infill are actively seeking environmentally friendly and low-carbon building materials as alternatives in the process of promoting sustainable development, maximizing the use of waste materials and resources, and reducing consumption of natural resources and environmental pollution. In this paper, three types of steel tube specimens with GRC infill were proposed, i.e., the geopolymer recycled concrete-filled steel tube (GRCFST), geopolymer recycled concrete-filled double skin steel tube (GRCFDST), and geopolymer recycled concrete-filled double section steel tubes (GRCFDSST). These GRC-steel tube structures were tested under axial compressive loading, and the influence of some crucial factors on bearing capacity, ductility and strain distribution of specimens was considered, such as the slag content, and the types of steel tube section and the replacement rate of recycled aggregate. The experimental results show that the steel tube can provide effective confinement for geopolymer recycled concrete and thus compensate for the adverse effect induced by recycled aggregate. The slag content has less effect on the bearing capacities of the GRCFDSST specimen compared with the GRCFST and GRCFDST specimens due to the stronger confinement effect of the former. The ductility of GRCFDSST specimens is better than the other steel tube specimens under the same parameter conditions. This study, building on existing literature, provides new insights and design recommendations for the mechanical performance of concrete-filled steel tube structures by using geopolymer recycled concrete as a filling material.

A Concrete Core Void Imaging Approach and Parameter Analysis of Concrete-Filled Steel Tube Members Using Travel Time Tomography: Multi-Physics Simulations and Experimental Studies.

Concrete-filled steel tube (CFST) members have been widely used in civil engineering due to their advanced mechanical properties. However, internal defects such as the concrete core voids and interface debonding in CFST structures are likely to weaken their load-carrying capacity and stiffness, which affects the safety and serviceability. Visualizing the inner defects of the concrete cores in CFST members is a critical requirement and a challenging task due to the obvious difference in the material mechanical parameters of the concrete core and steel tube in CFST members. In this study, a curved ray theory-based travel time tomography (TTT) with a least square iterative linear inversion algorithm is first introduced to quantitatively identify and visualize the sizes and positions of the concrete core voids in CFST members. Secondly, a numerical investigation of the influence of different parameters on the inversion algorithm for the defect imaging of CFST members, including the effects of the model weighting matrix, weighting factor and grid size on the void's imaging quality and accuracy, is carried out. Finally, an experimental study on six CFST specimens with mimicked concrete core void defects is performed in a laboratory and the mimicked defects are visualized. The results demonstrate that TTT can identify the sizes and positions of the concrete core void defects in CFST members efficiently with the use of optimal parameters.

Seismic Vulnerability Analysis of Concrete-Filled Steel Tube Structure under Main–Aftershock Earthquake Sequences

Earthquakes are often followed by higher-intensity aftershocks, which tend to aggravate the accumulated and more severe damage to building structures. The seismic vulnerability of concrete-filled steel tube (CFST) structures under major aftershocks is more complex. In this paper, a CFST frame and a frame with buckling-restrained braces (BRBs) are studied, and the finite element analysis software Midas 2022 is used to analyze the seismic vulnerability of the two types of structures under main shock and main–aftershock. The results show that the structural vulnerability of the two structures is significantly higher under the main–aftershock sequences than under the main shock alone. Compared with the CFST structure, the structure with BRBs can effectively reduce the structural displacement and the hysteretic energy, decrease the plastic deformation risk of the structural components, and improve the seismic performance. The structure with BRBs can significantly reduce the probability of structural collapse under the main–aftershock sequence and can provide a reliable guarantee of the stability of the building.

Experimental study on axial compression properties of concrete filled steel tubular stub columns in varying ambient temperature

To better understand the mechanical property issues of in-service concrete-filled steel tube (CFST) bridges susceptible to natural environmental temperatures, 13 CFST stub columns were assessed under axial compression with different ambient temperatures and steel ratios, with the strength of the core concrete as the main parameters. The influence of ambient temperature on the peak strain and joint elastic modulus of the CFST stubs was investigated. The influence mechanism of steel content and core concrete strength on the residual toughness of the CFST at different ambient temperatures was revealed. In this study, we proposed a modified constitutive equation for CFST, taking into account the ambient temperature. We demonstrated that the peak strain, residual toughness ratio, and elastic modulus of the CFST stubs significantly decreased when the temperature decreased. The calculated values of the modified constitutive equation for CFST, taking into account the ambient temperature, were in good agreement with the test results and could be used to predict the trends in stress properties of the CFST stubs at different temperatures. The results could serve as a critical reference for the design and calculation of CFST structures in very cold regions.

The influence of basalt fiber on the mechanical performance of concrete-filled steel tube short columns under axial compression

With rapid economic and social development, both concrete-filled steel tube (CFST) composite structures and basalt fiber (BF) have been widely applied in the field of civil engineering. To investigate the laws and characteristics of the influence of chopped BF on the mechanical properties of CFST columns and further promote the application of BF in CFST structures, the axial compressive bearing capacity test of 18 CFST short columns was carried out, and the influence of BF of different lengths on their structural mechanical properties was analyzed. The test results were compared with the theoretical calculation results and the finite element analysis results to verify the reasonableness of the test results. The results reveal that the axial compressive bearing capacity of the CFST short column after adding BF is significantly improved compared to the ordinary CFST short column, in which the bearing capacity and the ductility coefficient are increased by approximately 8.1% and 31.6%, respectively, on average. In addition, changing the length of BF has less effect on the bearing capacity of CFST short columns, the rate of increase in bearing capacity decreases with an increase in the steel ratio of CFST, and the coefficient of ductility increases with the increase in the steel ratio.

Compression behavior of CFST columns under combined influence of gap and freeze-thaw

Owing to factors related to construction quality and environment, concrete-filled steel tube (CFST) structures that have been in use for many years in regions experiencing seasonal freeze–thaw cycles are susceptible to the combined effects of a spherical-cap gap and freeze–thaw cycling. Therefore, in this study, a combination of experiments and the finite element method is used to investigate the effects of these two factors on the axial compression behavior of CFST stub columns. The experimental results show that the main failure mode of the specimen is local buckling at the end or the middle and inward depression of the steel tube on the existing gap side. The degradation of mechanical parameters, such as the ultimate bearing capacity and ductility of the specimens, gradually increased with increasing gap ratio and number of freeze–thaw cycles. In the numerical simulation section, the change in the stress response to freeze–thaw cycling on the gap side is analysed. The impact of the constraint effect coefficient on the ultimate bearing capacity degradation of the specimens is also investigated. Finally, a formula for calculating the ultimate capacity of CFST stub columns affected by the two factors is proposed, and the formula for calculating the ultimate bearing capacity in the code is improved. The results of this study can provide a reference for durability prediction and structural safety evaluation of CFST structures with a spherical-cap gap in cold regions.

Experimental study and design method of embedded concrete-filled rectangular steel tube column base under cyclic load

Embedded column base has been widely used in concrete-filled steel tube (CFST) structures due to its high rigidity and good seismic performance. However, the strength analysis of the embedded CFST column base was not yet clear since there was little experimental research on it. In this paper, the experimental investigations of six embedded concrete-filled rectangular steel tube (CFRST) column bases under constant axial load and cyclic load were carried out to study the effects of embedded depth, improvement measures and axial compression ratios on the seismic performance of the column bases. The experimental results indicated that setting stiffeners, increasing the embedded depth or axial compression ratios could improve the seismic performance of the column base. In this experiment, the critical embedded depths of the columns with or without stiffeners were 0.7 and 1.5 times the column width B respectively. When the embedded depth was 0.7B, the contribution of anchor bolts to the moment resistance capacity of the column bases could reach up to 14.2%. Based on the analysis of the force transmission mechanism between the column and the concrete foundation in the embedded CFRST column base, a design method considering the contribution of different components to the moment resistance capacity of the CFRST column bases is proposed. The prediction results are in good agreement with the tested results and collected data, within ± 15% error. The parameter analysis based on the design method has revealed the influence patterns of various parameters on the moment resistance capacity of the column bases.

Performance of Prefabricated Hollow Concrete-Filled Steel Tube Bracings on Transverse Bending: Experimental and Numerical Analyses

Due to the complex hydrogeological conditions in coastal regions, the use of internal bracing systems is necessary for supporting coastal foundation pits. This paper introduces a novel prefabricated foundation pit bracing system based on Hollow Concrete-Filled Steel Tube (H-CFST) structures that can be reused, offering significant economic and societal benefits. However, there is a severe lack of research on the application of H-CFST bracing systems. Through model tests and finite element simulations, the load-displacement characteristics and failure modes of prefabricated H-CFST bracing under transverse bending were investigated. The study revealed that when a wall thickness of 1.5 d was chosen, the self-designed hoop effectively mitigated strength and stiffness reduction at the bracing connection point. When the load reached 150 kN, the outer steel tube of the H-CFST components experienced localized yielding, and when the load was increased to 300 kN, the end supports exhibited cracking. Finite element analysis provided a more accurate prediction of bracing failure at 147.18 kN, and it offered valuable insights for optimizing the bracing design. Based on the above research, theoretical methods for calculating the bearing capacity of each bracing component under transverse bending conditions have been proposed and validated against experimental results.

Behavior of CFRP strengthened concrete-filled steel tubes with spherical-cap gap under axial compression

Carbon fiber-reinforced polymer (CFRP) strengthening can be considered as an effect approach to compensate the unfavorable influence caused by the spherical-cap gap on concrete-filled steel tube (CFST) structures. In the present study, the influence of CFRP wrapped on the mechanical behavior of CFST with spherical-cap gap (CFST-G) is examined. Sixteen CFST stub columns are experimentally tested under axial compression loads, considering the gap ratio and the number of CFRP layers (nCFRP) as the main parameters for analysis. The test results show that the bearing capacity decreases with increasing gap ratio, and the wrapped CFRP effectively improves the axial bearing capacity of CFST-G. Notably, the strengthening effect increases as nCFRP increases. Meanwhile, finite element (FE) models are established and verified against the test results. The proposed FE model is then used to conduct behavior analysis, and the working mechanism of CFST-G with CFRP strengthened is revealed. Parametric studies are also carried out by the FE model. The results show that, under the same gap ratio, the strength index decreases with increasing steel strength, steel ratio, and nCFRP, whereas it increases as the concrete strength increases. Based on the test and numerical analysis results, a simplified formula to calculate the bearing capacity of CFRP-strengthened CFST-G is proposed, providing a basis and reference for practical engineering and design codes.

Axial compressive behaviour of ultra-high performance concrete filled steel tube stub columns at elevated temperatures

Ultra-high performance concrete (UHPC) has many advantages such as high strength, good plasticity and toughness, and excellent durability. However, it is more likely to experience explosive spalling at high temperatures than ordinary concrete due to its low porosity. The ultra-high performance concrete-filled steel tubes (UHPCFSTs), where steel tubes are placed outside of UHPC to reduce its high-temperature spalling, could be an effective structural type to make full use of the advantages of both steel and UHPC. Most of the existing fire protection design codes for concrete-filled steel tube structures are based on ordinary concrete, and the performance and failure mechanisms of UHPCFSTs under high temperatures are still unclear. On this basis, an experimental study on the axial compressive performance of UHPC filled circular steel tube stub columns at constant elevated temperatures was carried out. The effects of temperature, steel tube thickness and coarse aggregate content on the compressive properties and failure modes of UHPCFST members were analysed. The results have shown that the UHPCFST mainly suffered from the overall pier coarse type and drum type failure modes. The failure characteristics were highly influenced by temperature magnitude, and 400℃ was the critical temperature where the structural performance change significantly beyond this point. The constraint effect of the steel tube on UHPC increased with the increase of thickness of steel tube, but it would be weakened with the increase of temperature or the coarse aggregate content. Finally, based on the average temperature method, a formula for calculating the ultimate load bearing capacity and equivalent stress–strain relationship of an UHPCTST stub column under a constant high temperature was proposed.

Impact acoustic inspection of interfacial debonding defects in concrete-filled steel tubes

Concrete-filled steel tube (CFST) structures are commonly used in the construction of high-rise buildings and bridges. However, the presence of debonding defects at the steel–concrete interface of CFST structures due to poor construction quality and concrete shrinkage can significantly reduce their load-bearing capacity. In this paper, an impact acoustic method based on the flexural vibration mode is proposed to detect interfacial debonding in CFST structures. A portable instrument comprising a hammer and a data recording and analysis module is developed for real-time inspection. A vibration energy ratio (VER) criterion, which compares the energy distribution between flexural vibration and thickness stretch modes, is proposed to evaluate the degree of debonding. The effectiveness of the proposed method is verified using a CFST column specimen with debonding defects. The laboratory experimental results demonstrate that the proposed method can detect and roughly pinpoint the debonding area. Additionally, the proposed method is applied to evaluate debonding defects in the CFST columns of the Shenzhen SEG Building, which experienced abnormal shaking on May 18–20, 2021. Impact acoustic measurements were conducted along 27 circumferential rings of nine CFST columns, revealing serious debonding damage to the building with a comprehensive debonding rate of 50.4%. It is concluded that that the proposed impact acoustic method can detect and locate debonding defects in CFST structures, and has a broad application prospect for rapid inspection in civil engineering due to its high efficiency.

Interfacial debonding detection for CFST structures using an ultrasonic phased array: Application to the Shenzhen SEG building

Concrete filled steel tube (CFST) structures have been widely applied to the construction of high-rise buildings and bridges. However, interfacial debonding defects generally occur due to deficient construction quality, concrete shrinkage, and other reasons. In this paper, an imaging-based approach using a designed ultrasonic phased array is proposed for the semi-quantitative characterization of interfacial debonding defects in CFST columns. A total focusing method (TFM) is employed for high-resolution image reconstruction from the full matrix capture (FMC) dataset acquired by the phased array. Based on the reflection coefficient of the steel–concrete interface in the imaging result, a criterion is proposed to evaluate the degree of debonding damage. The proposed approach is used to evaluate the debonding defects in the 16 CFST columns of the Shenzhen SEG Building, which experienced abnormal shaking on May 18–20, 2021. The accuracy of the proposed method is fully validated with 15 boreholes on the CFST columns. Then, ultrasonic phase array measurements were conducted along 34 circumferential rings on nine of the CFST columns. The results show that the comprehensive debonding rate is as high as 46.6%. It is concluded that the proposed ultrasonic phased array method can accurately detect and locate the debonding defects in CFST structures, and this method has a broad application prospect for non-destructive inspection in civil engineering.

Effect of Different Expansive Agents on the Deformation Properties of Core Concrete in a Steel Tube with a Harsh Temperature History

The shrinkage of core concrete during construction is the key reason for the separation of steel pipes and core concrete. Utilizing expansive agents during cement hydration is one of the main techniques to prevent voids between steel pipes and core concrete and increase the structural stability of concrete-filled steel tubes. The expansion and hydration properties of CaO, MgO, and CaO + MgO composite expansive agents in C60 concrete under variable temperature conditions were investigated. The effects of the calcium-magnesium ratio and magnesium oxide activity on deformation are the main parameters to consider when designing composite expansive agents. The results showed that the expansion effect of CaO expansive agents was predominant in the heating stage (from 20.0 °C to 72.0 °C at 3 °C/h), while there was no expansion in the cooling stage (from 72.0 °C to 30.0 °C at 3 °C/d, and then to 20.0 °C at 0.7 °C/h); the expansion deformation in the cooling stage was mainly caused by the MgO expansive agent. With the increase in the active reaction time of MgO, the hydration of MgO in the heating stage of concrete decreased, and the expansion of MgO in the cooling stage increased. During the cooling stage, 120 s MgO and 220 s MgO resulted in continuous expansion, and the expansion curve did not converge, while 65 s MgO reacted with water to form brucite in large amounts, leading to its lower expansion deformation during the later cooling process. In summary, the CaO and 220 s MgO composite expansive agent in the appropriate dosage is suitable for compensating for the shrinkage of concrete in the case of a fast high-temperature rise and slow cooling rate. This work will guide the application of different types of CaO-MgO composite expansive agents in concrete-filled steel tube structures under harsh environmental conditions.

Study on Shearing Behavior of Circular Concrete-Filled CFRP (Carbon Fiber-Reinforced Plastics)-Steel Tube.

Concrete-filled CFRP steel tube (CF-CFRP-ST) structures often suffer from shear loading in practical engineering, such as joints with diagonal braces. To study the shear properties of CF-CFRP-ST, we take the concrete strength and the longitudinal CFRP layers as the main parameters, and static shear tests of overall 9 circular concrete-filled CFRP-steel tube (C-CF-CFRP-ST) and 3 circular concrete-filled steel tube (C-CFST) are carried out. The research is carried out from two aspects: experiment and finite element. The experimental results show that the shear loading-displacement curves of the specimens can be divided into elastic stage, strengthening stage, and softening stage. The increases of the strength of the concrete and the layers of the transverse CFRP can both enhance the shearing load carrying capacity of the specimen. With the increase of concrete strength, there is no obvious change in the shape of the shear stress-shear strain curves of the specimens, and the shear stress and the stiffness of the curve in the elastic stage of the specimen are slightly increased. The shear loading-displacement curves of the specimens are simulated by using finite element software ABAQUS and it is found that the predicted results agree reasonably well with the test results. Then, the whole process of loading and the parameters of the main influencing factors are analyzed. Finally, the calculation equation of CFRP concrete-filled steel tubular shear capacity is established.

Experimental study on dynamic compressive and splitting tensile properties of high strength expansive concrete

Concrete shrinkage usually results in the decrease in bearing capacity, durability and impact resistance of Concrete-Filled steel tube (CFST) structures during its service life. High strength expansive concrete (HSEC) is recently developed to deal with the shrinkage cracking in CFST structures. In this study, dynamic compressive tests and dynamic splitting tensile tests on the developed grade C60, C70 and C80 HSEC were performed using a split Hopkinson pressure bar device. Test results show that the expansive concrete is a typical rate-sensitive material, and its dynamic compressive strength and dynamic splitting tensile strength both increase with the strain rate. The compressive strength dynamic increase factor (DIFc) of HSEC is smaller than that of the ordinary concrete under the same strain rate, whereas the splitting tensile dynamic increase factor (DIFt) is larger than that of the ordinary concrete. All the test data were classified to establish calculation models of DIFc, peak toughness ( Rp), specific energy absorption (SEA), and DIFt, which provide a theoretical basis for the design and application of HSEC and CFST in engineering.

Experimental investigations of FRP-interlayer-steel confined concrete columns under axial compression

A fibre-reinforced polymer (FRP)-interlayer-steel confined concrete (FISC) column is proposed in this study to improve the corrosion resistance and bearing capacity of concrete-filled steel tube (CFST) structures. In FISC columns, grouting material is used to bond the FRP tube and steel tube to produce a dual-confined composite skin for the core concrete. To establish the performance levels of FISC columns, 35 specimens in two batches of four types of columns, including FISC columns, FRP-confined concrete-filled steel tubular columns, concrete-filled FRP tubular columns, and CFST structures were used. The specimens were tested under axial compression, and the FRP types (glass- and carbon-fibre-reinforced polymer), FRP thickness, and steel thickness were the main parameters examined in the tests. Failure patterns, load–axial shortening curves, and strain development were observed, and the dual confining effect of FRP tube and steel tube on core concrete was discussed. The test results showed that the FRP tube and grouting material suppressed the outward buckling of a steel tube, and the load-bearing capacity of the core CFST was substantially enhanced. In addition, owing to the existence of an interlayer, a strain lag was observed on the strain transfer from the steel tube to the FRP tube, effectively delaying the cracking of the resin on the FRP tube. Furthermore, the bearing capacity and ultimate axial deformation of FISC columns increased linearly with the addition of FRP thickness but had little changes when the thickness of the steel tube differed.

Experimental Studies on Seismic Performance of UHPSFRC-Filled Square Steel Tubular Columns

The excellent seismic performance of concrete-filled steel tube (CFST) structures has been widely recognized, but there is a paucity of research on composite columns using UHPC with added steel fibers. This paper presents the experimental studies and numerical analyses with OpenSees on seismic performance of ultra-high performance steel fiber-reinforced concrete (UHPSFRC)-filled square steel tubular columns. Five half-scaled specimens of UHPSFRC-filled square steel tubular columns were tested under a combination of constant axial compression and cyclic horizontal load, with parameters of width-to-thickness ratio (28.5, 19.9 and 14.7) and axial compression ratio (0.133, 0.266 and 0.399) of the steel tube. With the decrease in width-to-thickness ratio, the maximum bending moment capacity increased by 33.5% and 15.3%, and the energy dissipation capacity and ductility increased, while the strength degradation and stiffness degradation reduced. With the increased axial compression ratio, the loading capacity increased from 55.3 to 70.2 kNm (26.94%). The results indicate that UHPSFRC-filled square steel tubular columns improve seismic performance by decreasing the axial compression ratio and by increasing the width-to-thickness ratio. When the width-to-thickness ratio was reduced, the steel tubular was able to provide higher lateral restraint to the internal UHPC; thus, seismic performance was improved. With the increase in the axial compression ratio, the second-order damage effect of the members was greatly affected, and it accelerated the plastic damage. A modified UHPSFRC model considering steel tubular constraints was adopted, and the nonlinear dynamic modeling of the column response using OpenSees led to good agreement with the tested response of the column under cyclic motion. The theoretical calculation model can better predict the bending capacity of the UHPSFRC-filled square steel tubular columns. However, the calculation formulas of initial stiffness and yield bending moment need further research.

A Simple Formula for Prediction of Compressive Strength of Square CFT Columns

Concrete filled steel tube structures are becoming very popular in the modern civil engineering projects. Studying composite structures is useful, since it is an innovative and contemporary way to build structures. This type of structure has the ability to use respective strength of both steel and concrete due to confinement. Prefabrication of steel tube section is beneficial, and allows rapid installation into main structure. It also reduces the assembly cost and construction time. This paper will present the simple equation to predict the compressive strength of square concrete filled steel tube by using Finite Element Analysis (FEA)based software ABAQUs. In this study, 3D non-linear finite element models of short square composite columns were prepared using ABAQUS. The results were compared with published experimental tests of a concrete filled steel tube short columns. After getting the good agreement with the experimental results, a simple equation for the prediction of compressive strength is presented by considering the width to thickness ratio of steel tube. Results are validated with experimental results. The equation can predict the compressive strength only for the given material strengths and in future, the simple equation can be improved by considering different parameters e.g. material strength, slenderness ratio and end conditions.

Experimental study on the steel–concrete bond behaviour of circular concrete-filled stainless steel tubes

The paper reports an experimental investigation on the bond stress–slip relationship in concrete-filled stainless steel tube (CFSST) structures. Push-out tests were conducted on twelve specimens covering two stainless steel grades, two concrete grades, two cross-sectional dimensions and two internal ring conditions (with and without the internal ring). The effects of these factors on the bond stress–slip relationship in CFSST members were analysed and discussed. A theoretical model, based on the assumption that no restraining force from the steel tube was applied to the concrete core, was developed to predict the bond stress–slip response of CFSSTs and was evaluated against the test results. An empirical model was also proposed based on regression analysis to replicate the experimental bond stress–slip relationships, which can be used as input data for analytical and numerical simulations in future studies of CFSST structures.