Aggregate Concrete-filled Steel Tube Research Articles

Global buckling and residual capacities of circular recycled aggregate concrete-filled stainless steel tube (RACFSST) columns after exposure to fire

The post-fire flexural buckling behaviour and capacities of circular recycled aggregate concrete-filled stainless steel tube (RACFSST) columns are studied in this paper, underpinned by testing and numerical modelling. A testing programme was firstly conducted on twelve column specimens designed with three recycled coarse aggregate replacement ratios (0%, 35% and 70%), including nine column specimens after exposure to the ISO 834 standard fire for 15 min, 30 min and 45 min and three reference column specimens at ambient temperature. The test failure modes, load–mid-height lateral deflection curves and failure loads were reported in detail. The influences of heating durations and recycled coarse aggregate replacement ratios on failure loads and mid-height lateral deflections at failure loads of circular RACFSST column specimens were discussed and their lateral deflection distributions were analysed. The testing programme was followed by a numerical modelling programme, where thermal and mechanical finite element models were developed to repeat the experimental results and afterwards used to carry out parametric studies to generate a numerical data pool over a wide range of cross-section dimensions and member lengths. Based on the test and numerical data, the relevant design rules for circular natural aggregate concrete-filled carbon steel tube columns at ambient temperature, as specified in the European code, Australian/New Zealand standard and American specification, were evaluated, using post-fire material properties, for their applicability to circular RACFSST columns after exposure to fire. It was revealed from the evaluation results that the three design codes led to overall acceptable levels of accuracy and consistency in predicting post-fire flexural buckling capacities of circular RACFSST columns, but with some unsafe predictions.

Eccentric compression behaviors of iron tailings and recycled aggregate concrete-filled steel tube columns

This paper presents a sustainable method for using iron ore tailings (IOTs) sand and recycled coarse aggregate (RCA) in concrete and concrete-filled steel tube (CFT) columns. The study first examines the impact of RCA replacement ratios (0 %, 50 %, 100 %) on concrete's mechanical properties, finding that both compressive and tensile strengths decrease with higher RCA content due to the negative effects of old interfacial transition zones and adhered cement in RCA. Then, twelve iron tailing and recycled aggregate concrete-filled steel tube (ITRAC-CFT) columns were tested under eccentric compression to assess the effects of varying RCA replacement ratios, eccentricity ratios (0.3, 0.5), and slenderness ratios (12.12, 19.05). Results indicate that RCA ratios have minimal impact on failure modes, ultimate bearing capacity, and stiffness, although ductility increases with higher RCA content. As slenderness ratios increase, the ultimate bearing capacity decreases by 5.3 % and 6.8 % for eccentricities of 0.3 and 0.5, respectively. The study further validates these findings through the finite element method (FEM), showing strong agreement between experimental and predicted results, average value, standard deviation, and coefficient of variation of Nu/NFE were 1.02 %, 0.04, and 3.92 %, respectively. Finally, a comparison with existing design codes (EC4, AISC 360, and GB 50396) reveals that AISC 360 provides the most accurate predictions of the ultimate bearing capacity of ITRAC-CFT columns. The research concludes that the comprehensive use of IOTs sand and RCA in CFT structural concrete is a highly viable and eco-friendly solution, offering potential benefits for sustainable construction practices.

Impact of steel tube wall thickness on axial compression behavior of reinforced and recycled aggregate concrete-filled square steel tube short columns

Tests and numerical simulations were conducted to investigate the impact of wall thickness of steel tube on the axial load performance of reinforced and recycled aggregate concrete (RAC)-filled square steel tube short columns (R-RACFST). The test groups consisted of R-RACFST, concrete-filled square steel tube (CFST), and RAC-filled square steel tube (RACFST). Each group had five different wall thicknesses of the steel tube, and the RAC infill was made of 100 % recycled coarse aggregate (RCA). Two replicate specimens were prepared for each group and wall thickness. The tests were used to analyze the effect of steel ratio on failure mode, loading performance, and ductility. The analyses clarified that the reinforcement effectively enhanced the confinement effect on RAC, compensating for the inadequate confinement of the square tubes and the brittleness of the RAC with 100 % RCA. To further understand the acting role of reinforcement, numerical parametric studies were conducted. A novel numerical model for R-RACFSTs was established and verified with the tests. A total of 81 models were then simulated, considering the coupling effects between the wall thickness of the steel tube, the amount of reinforcement, and the strength of RAC. Based on the simulations, the coupling relationship between the steel tube and reinforcement, as well as the optimal range of steel ratio, were determined. Additionally, strength prediction equations were studied and proposed. The findings indicate that it is feasible to fill RAC with 100 % RCA in R-RACFSTs; when combined with appropriate reinforcement, R-RACFSTs with thinner tubes can achieve the same or better performance compared to RACFSTs with thicker tubes; the recommended reasonable range for the steel ratio and corresponding confinement coefficient in R-RACFSTs are 6.4 % to 13.7 % and 1.00 to 2.04, respectively; the predictions proposed are accurate and reliable.

Post-fire flexural buckling and resistances of square recycled aggregate concrete-filled stainless steel tube (RACFSST) columns

The flexural buckling behaviour and residual resistances of square recycled aggregate concrete-filled stainless steel tube (RACFSST) columns after exposure to fire are studied in this paper, based on experiments and numerical modelling. Twelve column specimens, fabricated from concretes with three recycled coarse aggregate replacement ratios (0%, 35% and 70%), were tested after exposure to the ISO 834 standard fire for 0 min (i.e. at ambient temperature), 15 min, 30 min and 45 min. The experimental investigation included heating of specimens, cylinder tests as well as post-fire initial global geometric imperfection measurements, tensile coupon tests and pin-ended column tests. The test setups, procedures and results were fully reported and the ductility indices, lateral deflection distributions and longitudinal strains were discussed and analysed. A numerical investigation was afterwards carried out, where the test results were used to validate thermal and mechanical finite element models; upon validation, parametric studies were conducted to expand the test data bank over a wider range of cross-section dimensions and member lengths. On the basis of the test and numerical data, the relevant design rules for square natural aggregate concrete-filled carbon steel tube columns at ambient temperature, as set out in the European code, Australian/New Zealand standard and American specification, were assessed, using post-fire material properties, for their applicability to square RACFSST columns after exposure to fire. It was found from the assessment results that the three design codes resulted in overall accurate and consistent post-fire flexural buckling resistance predictions but some were unsafe.

Compressive axial load performance of GFRP–confined recycled aggregate concrete–filled steel tube stub columns

This study examined the axial compressive properties of glass–fiber reinforced plastic (GFRP)–confined recycled aggregate concrete–filled steel tube (RACFST) stub columns. The differences in the failure modes, load–displacement curves, and load–strain curves of the composite columns with different parameters were analyzed, including the number of GFRP layers, steel tube thickness, recycled aggregate substitution rate, and recycled aggregate concrete compressive strength. The results revealed a bulging failure mode in the middle of the GFRP–confined RACFST stub columns. With the increasing substitution rate of recycled aggregate, the bulging deformation of the specimen increased. This eventually resulted in a reduction in the stiffness and bearing capacity but an improvement in the ductility at a later stage. The early stiffness and ultimate bearing capacity of the specimen can be increased by thickening the steel tube or strengthening the recycled aggregate concrete. However, the ductility is greatly reduced in the later stage. Based on the axial compression test of GFRP–confined RACFST stub columns, the axial compression bearing capacity formula was derived. Compared with the existing formula, the formula presented in this paper has greater accuracy. In addition, a finite element (FE) model of the composite column was established to analyze the stress development during the compression process.

Axial compressive performance of thin-walled square steel tube concrete short column with built-in steel slag block

In the process of iron and steel production, steel slag is produced as waste. The steel slag processing method generally produces steel slag cement as a substitute for gravel and fine aggregates. Steel slag aggregates replace natural aggregate concrete used in concrete components. In addition, steel slag expansion can compensate for the shrinkage characteristics of concrete and steel pipes inside closed wet environments, and the outer steel tube hoop constraint can avoid late hydration expansion structure cracking. Through orthogonal testing, the wall thickness of the steel pipe (3 mm, 3.75 mm, 4.5 mm), content of the steel slag blocks (10 %, 15 %, 20 %) and particle size of the steel slag blocks (40–60 mm, 60–80 mm) were used as experimental variables to conduct axial compression tests on 9 groups of short columns made of slag aggregate concrete-filled thin-walled square steel tubes. Based on the analysis of the test process and displacement, strain, ductility and ultimate bearing capacity results, the failure pattern and mechanism of the short column specimens under axial compression were obtained, and a bearing capacity calculation formula was obtained according to the existing theoretical methods. The results indicate three failure patterns: local convexity, shearing and circumferential bulging. The factors influencing the bearing capacity, from most influential to least influential, are the particle size of the steel slag, the wall thickness of the steel pipe and the amount of steel slag. The Poisson's ratio of core concrete has an important effect on its passive binding force, which affects the longitudinal displacement and load-bearing capacity of the component. The deduced formula can be used to accurately calculate the ultimate bearing capacity of the square steel tube and steel slag concrete. This article transforms the drawbacks of steel slag concrete into advantages and, by combining the characteristics of steel tube concrete, proposes a new type of composite component that has a certain degree of innovation and application value.

Axial Compressive Behaviours of Coal Gangue Concrete-Filled Circular Steel Tubular Stub Columns after Chloride Salt Corrosion.

The axial compressive behaviours of coal gangue concrete-filled steel tube (GCFST) columns after chloride salt corrosion were investigated numerically. Numerical modelling was conducted through the static analysis method by finite element (FE) analysis. The failure mechanism, residual strength, and axial load-displacement curves were validated against tests of the coal gangue aggregate concrete-filled steel tube (GCFST) columns at room and natural aggregate concrete-filled steel tube (NCFST) columns after salt corrosion circumstance. According to the analysis on the stress distribution of the steel tube, the stress value of the steel tube decreased as the corrosion rate increased at the same characteristic point. A parametric analysis was carried out to determine the effect of crucial variation on residual strength. It indicated that material strength, the steel ratio, and the corrosion rate made a profound impact on the residual strength from the FE. The residual strength of the columns exposed to chloride salt was in negative correlation with the corrosion rate. The impact on the residual strength of the column was little, obvious by the replacement rate of the coal gangue. A simplified design formula for predicting the ultimate strength of GCFST columns after chloride salt corrosion exposure was proposed.

A Review of Research on Recycled Aggregate Concrete Filled Steel Tube

Recycled aggregate concrete filled steel tube as a new combined structural form, can effectively improve the inherent defects of recycled aggregate concrete. Existing studies are mainly theoretical analysis, experimental research and numerical simulation to investigate the mechanical properties and deformation capacity of recycled aggregate concrete filled steel tube columns. Based on the review of the current research status of recycled aggregate concrete filled steel tube columns in recent years, the basic mechanical properties, durability, seismic performance, fire and high temperature resistance of recycled aggregate concrete filled steel tube are described, and suggestions are made for future research directions.

Acoustic Emission Time-Space Evolution Characteristics of the Bond Behavior of Recycled Aggregate Concrete-Filled Steel Tubes

In order to effectively observe the damage behavior of the interface between steel tube and concrete, and to reveal the law of time-space evolution and the bond failure mechanism of the bond, the Acoustic Emission (AE) non-destructive monitoring technology was used to monitor the process of repeated push-out tests of concrete-filled carbon steel tube (CFST), recycled aggregate concretefilled carbon steel tube (RACFST), and recycled aggregate concrete filled stainless steel tube (RACFSST) specimens. F-S (force-slip) curve and strain distribution were obtained from the tested specimens, in addition to various acoustic emission (AE) parameters such as hit count, energy release rate, and time difference localization points. To establish a relationship between the acoustic emission characteristic parameters, the normalized characteristic parameters, and the F-S curve, we took into account the time-space evolution process of interface bonding failure and developed a constitutive model incorporating the concept of macro-interlocking. The results show that for peak bonding strength, CFST is slightly lower than the RACFST specimen in the first load cycle, while in the second to fourth load cycles, CFST > RACFST > RACFSST. The AE localization points show a continuous and linear distribution, and the time-space evolution characteristics of AE localization points in the first load cycle show a extension from the two ends-middle-lower middle. In the second and fourth load cycles, the positioning points are concentrated in the elastic stage and the upper intermediate stage. The macro-interlocking directly affects the bonding interface characteristics, and the macro-interlocking coefficient defined can quantitatively describe the macro-interlocking of the steel tube and predict the τ-S curve of the RACFST under repeated load cycles.

Axial compression behavior of coal gangue coarse aggregate concrete-filled steel tube stub columns

To investigate the load-strain curves and failure modes of spontaneous-combustion coal gangue coarse aggregate (SCGCA) concrete-filled steel tube (CFST) stub columns under axial compression, 36 SCGCA CFST specimens and 18 comparative SCGCA concrete specimens were firstly tested with various SCGCA replacement ratios, concrete strengths and steel tube thicknesses. The results reveal that the bearing capacity of SCGCA CFST significantly decreases with increasing SCGCA replacement ratio, while corresponding strain increases. The addition of SCGCA can activate confining stress offered by steel tube in advance. The hoop strain of SCGCA CFST specimens increases with increasing SCGCA replacement ratio under same axial strain, resulting in enhanced confining stress. An analysis-oriented model of SCGCA CFST stub columns was then established on basis of test results, and an extended parameter analysis was further conducted using the model. Finally, the feasibility of design methods in existing design codes for evaluating bearing capacity of SCGCA CFST stub columns under axial compression was discussed.

Circular recycled aggregate concrete-filled stainless steel tube stub columns after exposure to fire: Experiments, simulations, and design

This paper presents experimental and numerical investigations into the post-fire cross-section compressive behaviour and resistances of circular recycled aggregate concrete-filled stainless steel tube (RACFSST) stub columns. Twelve stub column specimens, fabricated from concretes with three recycled coarse aggregate replacement ratios (0%, 35% and 70%), were tested after exposure to the ISO-834 standard fire for 0 min (i.e. at ambient temperature), 15 min, 30 min and 45 min. The experimental investigation included heating and cooling of specimens, cylinder tests and post-fire tensile coupon tests and stub column tests. The test results, including load–end shortening curves, failure loads and failure modes, were fully reported and the initial compressive stiffnesses and confinement effect were discussed. The numerical investigation was subsequently conducted, where thermal and mechanical finite element models were developed and validated against the test results and then used to perform parametric studies to generate further numerical data. Given the absence of design codes for stainless steel–recycled aggregate concrete composite structures after exposure to fire, the relevant design rules for circular natural aggregate concrete-filled carbon steel tube stub columns at ambient temperature, as set out in the European code, Australia/New Zealand standard and American specification, were assessed, using post-fire material properties, for their applicability to circular RACFSST stub columns after exposure to fire, based on the test and numerical data. The assessment results generally revealed that the European code and the Australian/New Zealand standard led to an acceptable level of design accuracy and consistency, while the American specification resulted in rather conservative and scattered post-fire cross-section compression resistance predictions.

Mechanical and environmental performance of structural concrete with coal gangue fine aggregate

Using coal gangue sand as fine aggregate is a feasible solution for the sustainable development of construction engineering. The mechanical and environmental properties of the gangue fine aggregate concrete-filled steel tube (GFACFST) and reinforced gangue fine aggregate concrete (RGFAC) with different gangue fine aggregate replacement ratios (rGFA) were investigated experimentally and numerically. The results showed that the GFACFST and RGFAC with various rGFA showed similar failure modes. The stiffness of the stubs decreases with the increase in rGFA. The ultimate strength of the GFACFST and RGFAC decreases by 3.8 % and 12.4 % respectively when rGFA is 100 %. The ductility of GFACFST shows an ascending tendency with the increase in rGFA, resulting from the increase in the confining effect of the steel tube on the core concrete. The stress-strain constitutive model of gangue fine aggregate concrete is proposed for numerical modeling. A design method for the bearing capacity of GFACFST and RGFAC is proposed by considering various gangue fine aggregate replacement ratios of which results differ from the tested and simulated values in the range of ±5 % and ±11 %. The life cycle assessment analysis shows that the environmental impact of GFACFST is less than that of RGFAC at the same axial compressive strength. The improvement of the mechanical-environmental performance efficiency by increasing cross-section diameter is more significant than that by increasing steel ratio.

Post-fire behaviour and resistances of square recycled aggregate concrete-filled stainless steel tube stub columns

Experimental and numerical studies on the cross-section compressive behaviour and residual resistances of square recycled aggregate concrete-filled stainless steel tube (RACFSST) stub columns after exposure to fire are reported in this paper. An experimental programme was firstly carried out on twelve stub column specimens with three recycled coarse aggregate replacement ratios (0%, 35% and 70%) after exposure to the ISO-834 standard fire for 0 min (i.e. at ambient temperature), 15 min, 30 min and 45 min. The test results, including load–end shortening curves, failure loads and failure modes, were presented, with the initial compressive stiffness and confinement effect analysed. The experimental programme was followed by a numerical modelling programme, where thermal and mechanical finite element models were developed and validated against the test results and afterwards used to conduct parametric studies to generate additional numerical data over a wide range of cross-section dimensions. Based on the test and numerical data, the relevant design rules for square natural aggregate concrete-filled carbon steel tube stub columns at ambient temperature, as specified in the European code, Australia/New Zealand standard and American specification, were evaluated, using post-fire material properties, for their applicability to square RACFSST stub columns after exposure to fire. The evaluation results generally revealed that the European code and Australian/New Zealand standard led to a good level of design accuracy, while the American specification resulted in slightly conservative post-fire cross-section compression resistance predictions.

Eccentric load-carrying capacity calculation of L-Shaped Columns with recycled aggregate concrete-filled tubes

A composite L-shaped column made of square steel tubes filled with concrete is well-suited for residential steel structures because it effectively prevents the beams and columns from protruding from the walls, thus maximizing the usable space within a room. Recycled aggregate concrete (RAC), a sustainable material, can efficiently recycle construction waste and reduce energy consumption. However, compared to natural concrete, RAC has lower elastic modulus and compressive strength. When RAC is filled into steel tubes, it effectively restrains the RAC and allows it to support the internal buckling of the steel tubes. This study investigates L-shaped columns of recycled aggregate concrete-filled steel tubes (L-RACFST) through eccentric compression tests and numerical validation. By applying the superposition principle, the study evaluates the eccentric compression strength calculation method of L-RACFST columns and plots the N-M relationship curve. Comparing the results of the tests and numerical studies, it is evident that the derived formula is reliable for predicting the eccentric compression bearing capacity of L-RACFST columns. Furthermore, the N-M relationship curve drawn from the formula provides a more accurate prediction of safety. This paper delves into the stress mechanisms of L-RACFST columns under eccentric compression loads. The calculation formula can be used to determine the bearing capacity of L-RACFST columns under eccentric compression loads, and the N-M relationship curve can be employed to predict the safety performance of L-RACFST columns under such loads.

Theoretical model for spontaneous combustion gangue coarse aggregate concrete filled steel tube stub columns

Spontaneous combustion gangue coarse aggregate concrete filled steel tube (SCGCA-CFST) provides a possibility for the structural application of spontaneous combustion gangue coarse aggregate (SCGCA) concrete. To establish an accurate stress-strain model for SCGCA-CFST, 12 SCGCA-CFST specimens and 18 SCGCA concrete specimens were tested under monotonic axial compression with steel tube thickness and SCGCA replacement ratio (0%, 50%, 100%) as variables. The experimental results demonstrated that as the SCGCA replacement ratio increased, the elastic modulus of SCGCA concrete decreased by 6.65%–11.65%, the cylinder compressive strength decreased by 11%–21.95%, and the axial strain at peak axial stress increased by 7.1%–14.27%. Furthermore, the crack extension forms and dilation properties of the core concrete in SCGCA-CFST differed significantly from those of normal coarse aggregate concrete filled steel tube (NCA-CFST). The incorporation of SCGCA reduces the effectiveness of steel tube confinement, which causes the existing stress-strain models of NCA-CFST overestimate the enhancement effect of steel tube confinement on the strength and deformability of core SCGCA concrete. Based on this, the analysis-oriented stress-strain model for NCA-CFST was modified to be applicable for SCGCA-CFST by incorporating the influence of aggregate characteristics, and the accuracy of that was verified.

Axial compression performance of steel tube columns filled with slurry-wrapping recycled concrete after exposure to high temperatures

Due to frequent fire accidents and stone scarcity, fire protection design and recycled aggregates are issues that needed to be solved in the construction industry. Steel tubes and slurry-wrapping reinforcement treatment can effectively improve the load-bearing capacity of recycled concrete. This paper reports on the mechanical behavior of slurry-wrapping recycled aggregate concrete-filled steel tube (CFST) columns after exposure to high temperatures. Three series of 30 CFST columns were designed with the coarse aggregate type and exposure temperature as variation parameters: natural aggregate concrete-filled steel tube (NACFST) column, recycled aggregate concrete-filled steel tube (RACFST) column, and slurry-wrapping recycled aggregate concrete-filled steel tube (SRACFST) column. The results show that the failure pattern of the specimen is less affected by the exposure temperature and the coarse aggregate type. The compressive strength of the RACFST columns is significantly increased after the slurry-wrapping treatment. With the temperature increasing, the ultimate strength tends to increase initially and then drop. Since the recycled concrete has more pores, the compressive strength loss rate of the RACFST columns is lower than that of the NACFST and SRACFST columns at 800 °C. The lateral deformation factor is maintained until the exposure temperature reaches 400 °C and then begins to decrease as the temperature increase. It is not accurate to predict the strength of CFST columns after high temperatures by ultrasonic pulse velocity. The predicted strength of CFST column after high temperatures was compared with design codes, and an equation considering the material strength variation was proposed.

Axial compression behavior and stress–strain relationship of slurry-wrapping treatment recycled aggregate concrete-filled steel tube short columns

Abstract The utilization of recycled concrete can effectively solve the problems of large accumulations of construction waste and the constant consumption of natural aggregates. Slurry-wrapping modification and concrete-filled steel tubes (CFSTs) are effective approaches to enhance the utilization ratio of recycled concrete. The study involved 4 ordinary CFST columns and 16 slurry-wrapping recycled aggregate concrete-filled steel tube (SRACFST) columns. The variable parameters included the slurry-wrapping recycled aggregate replacement ratio and the section form. The result shows that the ultimate strength of the specimens reached the minimum and maximum at 25 and 100% aggregate replacement ratio. The favorable strength performance of the SRACFST columns is attributed to the dominant positive effect of the slurry-wrapping treatment in reducing matrix porosity over the negative impact of the recycled aggregate’s complex interface transition zone. Due to the hoop factor, the residual bearing ratio peaks at a 25% substitution ratio. Four design standards were used to predict the ultimate strength, with GB50936 and EC4 yielding more accurate estimations, while AISC360-10 and AIJ provided conservative estimations. This study presents the equations of the stress–strain curve for the whole axial compression process, which can be directly applied in the theoretical and numerical analysis of RACFST columns.

The Axial Compression Behavior of Basalt Fiber-Reinforced Recycled Aggregate Concrete-Filled Circular Steel-Tubular Column

Recycled aggregate concrete (RAC) technology has received a lot of attention as a green environmental protection technology. However, the unsatisfactory mechanical behavior of RAC restricts its application in engineering practice. The structure of basalt fiber-recycled aggregate concrete-filled circular steel tubes (C-BFRACFST) can dually improve the mechanical behavior of RAC. To observe the axial compression behavior of the C-BFRACFST column, seven specimens were designed with recycled aggregate replacement ratio (0%, 50%, 100%), basalt fiber (BF) content (0 kg/m3, 2 kg/m3, 4 kg/m3) and length–diameter (L/D, 5, 8, 11) as variable parameters for axial compression tests. The failure mode, load–displacement/strain curve, axial compression deformation, ultimate bearing capacity, energy dissipation, and ductility of specimens have been analyzed. The derived constitutive relation of core basalt fiber-reinforced recycled aggregate concrete (BFRAC) constrained by the circular steel tube and the 3D finite element model of C-BFRACFST column have been established to simulate the whole process of compression. It is observed that instability or shear failure occurs in specimens under axial compression load. When the recycled aggregate replacement ratio was increased from 50% to 100%, the change in the energy-dissipation capacity of the specimens was not significant but the ultimate bearing capacity and displacement ductility coefficient decreased by 3.45% and 8.91%, respectively. When the BF content was increased from 2 kg/m3 to 4kg/m3, the change in the ultimate bearing capacity of specimens was not significant; the energy-dissipation capacity at the later stage of bearing increased, and the displacement ductility coefficient was noted to increase by 13.34%. When the L/D was increased from 8 to 11, the energy-dissipation capacity of specimens was decreased, and the ultimate bearing capacity and displacement ductility coefficient declined by 1.37% and 43.52%, respectively. The finite element simulation results are in agreement with the test results.

Recycled aggregate concrete-filled steel tube columns with small-diameter circular FRP-confined sea-sand concrete cores: Conceptual and experimental investigations

Recycled aggregate concrete (RAC) prepared from waste concrete can achieve the green recycling of resources; however, its inferior material properties hinder its applications. Sea-sand concrete (SSC) can help alleviate the resource shortage of fresh water and river sands but is prone to steel corrosion. To address these challenges, this study developed a new RAC-filled steel tube column with a small-diameter circular fibre-reinforced polymer (FRP)-confined SSC core. This paper presents and discusses the results of the axial compression tests conducted on RAC-filled steel tube columns, with and without small-diameter circular FRP-confined SSC cores. The study investigated the effects of the configuration of the FRP-confined SSC core, diameter-to-thickness ratio of the FRP tubes, compressive strength of RAC and SSC, and cross-sectional shape of the column. The results revealed that using small-diameter circular FRP-confined SSC cores to partially replace RAC was effective in improving the compressive performance of RAC-filled steel tube columns. The increase in the number of FRP tubes, in-filled concrete strength, and confinement stiffness of the steel and FRP tubes contributed to improvements in the structural strength and ductility, resulting in progressive ladder-shaped declines in the post-peak curve. Finally, a reasonable load–capacity model is presented for the proposed columns.

Axial compression behavior of steel tube filled with steel-fiber-reinforced self-stressing recycled aggregate concrete with moderate slenderness

An innovative steel-fiber-reinforced self-stressing recycled aggregate concrete-filled steel tube (SSRCFST) is proposed for the widespread application of recycled coarse aggregates (RAs). This study aims to explore the axial compression behavior of SSRCFST columns with moderate slenderness. A total of 27 specimens were designed with various slenderness ratios, steel-tube diameter–thickness ratios, concrete strength grades, RA quality replacement rates, and steel-fiber contents. The failure modes, load–deflection relationship, strain response, ultimate bearing capacity, and deformation capacity of the specimens were investigated. The experimental results indicated that an increase in the slenderness ratio aggravated the sectional stress gradient, transforming the specimens from strength failure to buckling failure. Although self-stress improves the ultimate bearing capacity of the SSRCFST columns, the second-order effect becomes more evident, and faster strength degradation of the slender SSRCFST columns occurs. A refined finite element model of slender SSRCFST columns was proposed based on concrete constitutive law modifications and temperature–load applications. The numerical results show that self-stress contributes to the confinement effect in the initial elasticity phase. Moreover, a new prediction model that accurately captured the compressive properties of slender SSRCFST columns is proposed.