Recycled Aggregate Concrete-filled Steel Tube Research Articles

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.

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.

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.

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.

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.

Axial compression behaviour of round-ended recycled aggregate concrete-filled steel tube stub columns (RE-RACFST): Experiment, numerical modeling and design

Round-ended recycled aggregate concrete-filled steel tube (RE-RACFST) column is an innovative composite member, which holds the advantage of round-ended concrete-filled steel tube (RE-CFST) and expands the application of RAC in structural members. This paper presents the testing and numerical investigation on the axial compression behaviour of RE-RACFSTs. A total of 18 stub columns were tested to determine the failure mode, axial load versus strain response, vertical and circumferential strains of the round-ended steel tube, and the confinement effect from external steel tube in different positions. The effects of cross-sectional aspect ratio, coarse recycled aggregate (CRA) replacement and steel ratio were analyzed. In general, the ductile behaviour of the specimens reduced with increasing aspect ratio and decreasing steel ratio. The confinement effect during the entire compression process was investigated based on the vertical and circumferential strains. Finite element (FE) models were then developed, and a constitutive model suitable for core RAC in RE-RACFST columns was proposed. Parametric analysis was then carried out to evaluate the effects of the aspect ratio, steel ratio, CRA replacement level, and material strength on the axial compression response. Finally, several existing design provisions for rectangular CFST and RAC-FST members were assessed the feasibility of their application in RE-RACFSTs by using the equivalent method.

A restoring force model for frames with steel tube columns filled with recycled aggregate concrete

AbstractTo establish a restoring force model for recycled aggregate concrete‐filled steel tube (RACFST) frames, the quasistatic tests on two frames comprising two RACFST columns and a reinforced recycled aggregate concrete (RRAC) beam each was carried out. The recycled coarse aggregate replacement ratio was 100%. The failure mechanism, hysteretic curves, and strain response of the specimens were discussed. By analyzing the degradation of the load bearing capacity with increasing displacement amplitude and combining the hysteretic curves and stiffness degradation relationships, a restoring force model of load versus displacement adopting a trilinear approximation was proposed. The characteristic points of the model were the relative yield point, relative peak point, and relative failure point. Results showed that bending‐shear or bending failure occurred at the ends of specimen beams and plastic hinges formed there earlier than at the column ends. The hysteretic curves of load versus displacement were essentially symmetric in a full shuttle shape. The proposed restoring force model adopting trilinear approximation agreed well with the experimental hysteretic curves under low‐cycle loading and can thus be recommended to perform the elastoplastic analysis of seismic response for this new type of composite structures.

Behaviour of eccentrically loaded circular recycled aggregate concrete-filled stainless steel tube stub columns

This paper presents an experimental and numerical study of eccentrically loaded circular recycled aggregate concrete-filled stainless steel tube (RACFSST) stub columns. The testing programme was conducted on twelve specimens designed with different stainless steel tube section sizes and recycled coarse aggregate replacement ratios, and included tensile coupon tests, concrete cylinder tests and eccentric compression tests. The test results, including the failure loads and modes, load–mid-height lateral deflection curves and evolution of neutral axis, were reported and discussed. Following the testing programme, a numerical modelling programme was conducted, with finite element models developed and validated against the test results and then employed to perform parametric studies to generate further numerical data. The confinement effects were analysed based on the numerical data. The relevant design interaction curves, as set out in the European code, American specification and Australian/New Zealand standard, were assessed for their applicability to circular RACFSST stub columns under combined compression and bending. On the basis of the quantitative and graphical assessments, it has been found that (i) each set of codified design interaction curve leads to similar assessment results for eccentrically loaded circular RACFSST stub columns with different recycled coarse aggregate replacement ratios and (ii) the failure load predictions from the codified design interaction curves are conservative.

A restoring force model for CFRP seismic-damaged RACFST columns: Theoretical, experimental, and simulation analysis

The seismic capacity of frame columns, an important support system of buildings, is a key parameter affecting the collapse of buildings during earthquakes and is used in elastic–plastic seismic response analysis. A restoring force model based on a three-stage skeleton curve for carbon fiber-reinforced polymer (CFRP) and seismic damaged, rectangular recycled aggregate concrete-filled steel tubes (RACFST) frame columns was constructed. Dimensionless processing was used, and the model considered fixed points, the softening point, and the sudden drop in the bearing capacity at the displacement amplitude. The model’s accuracy was verified using experiments and OpenSees analysis, and a parametric analysis was conducted. The results show that the values obtained from the experiment, simulation, and calculation are in good agreement, demonstrating the high accuracy of the restoring force model and the OpenSees analysis results. The proposed theoretical restoring force model of the CFRP seismic-damaged RACFST columns accurately reflects the hysteresis load–displacement relationship in the yield phase, peak phase, and damage phase. Thus, it can be used to analyze the seismic capacity and plastic seismic response of the reinforced RACFST structures with seismic damage.

Dynamic response of square recycled aggregate concrete-filled steel tube columns subjected to close-range blast loads

To study the dynamic response of square recycled aggregate concrete-filled steel tube (RACFST) columns under explosion loads, explosion examinations and numerical analyses were performed. Based on the measured longitudinal strain and residual displacement of the columns, the effects of recycled aggregate replacement percentage, steel tube thickness, explosive charge weight, blast center distance, and explosion height on the dynamic response of square RACFST columns are analyzed, and the failure modes are investigated. Then, the established numerical model was verified through test results, and further explore the dynamic response of square RACFST columns. It indicates that square RACFST columns suffer localized damage after detonation, and no obvious global deformation occurs. The localized failures, including blast pit, steel tube bulge or rupture, and weld fracture, could be observed on these columns. The square RACFST columns strain and displacement values are positively correlated with TNT equivalent and RA content, while negatively correlated with steel tube thickness and blast center distance. Furthermore, the concrete absorbs most of the total energy, implying the important role of core concrete in the explosion resistance of RACFST columns.

Bond behavior of recycled aggregate concrete-filled steel tube after elevated temperatures

This paper experimentally investigated the interfacial bond behavior of recycled aggregate concrete-filled steel tubes (RACFST) after different temperatures (i.e., 20 °C, 200 °C, 400 °C, 600 °C). A total of 40 specimens (twenty circular columns and twenty square columns) were tested by means of push-out to study the effect of the following parameters: (a) experienced temperature (T), (b) mass replacement percentage of recycled coarse aggregate (γ) and (c) cross-section shape. The influences and mechanism of above parameters on bond behavior were discussed. The test results indicate that T and γ had different influences on bond behavior and interfacial damage resistance. The interfacial failure process of the square specimen was non-uniform compared with that of the circular specimen and its overall failure process was more brittle. The bond behavior and interfacial damage resistance of the circular specimen were generally stronger than those of the square specimen. Besides, applicability of several existing prediction formulas for bond strength was evaluated. Lastly, more accurate prediction formulas for bond strength and analytical models for bond stress-slip relationships were proposed.

Seismic Behaviors of Tailings and Recycled Aggregate Concrete-Filled Steel Tube Columns

Seismic Behaviors of Tailings and Recycled Aggregate Concrete-Filled Steel Tube Columns

Residual axial capacity of square recycled aggregate concrete-filled steel tube columns after blast loads

To study the residual axial capacity of square recycled aggregate concrete-filled steel tube (RACFST) columns after close-range explosion loads, the explosion tests and the axial compression tests were performed. Firstly, 18 square RACFST columns were fabricated, and 15 of them were subjected to explosion tests. Secondly, the axial loads were applied to 3 intact columns and 9 blast-damaged columns. The curves of the load-displacement, the load-strain, and the load-mid-span deflection of all columns are analyzed. After the tests, the failure modes of the columns and core concrete were analyzed. Furthermore, the theoretical axial capacity of the RACFST columns was calculated according to the Chinese standard GB50936-2014. Based on the residual load-bearing capacity, the damage index is adopted to assess the damage degree of 9 blast-damaged columns. It indicates that both the blast-damaged and intact columns show diagonal shear failure under axial compression; the range of bearing capacity loss of the blast-damaged columns is approximately 1.9–32.1%, of which 5 columns are slightly damaged and 4 columns are moderately damaged, indicating that the columns still maintain a strong axial capacity reserve after explosion loads.

Study on the hysteretic behavior of recycled aggregate concrete-filled steel tube columns containing ferronickel slag

To study the hysteretic behavior of recycled aggregate concrete-filled steel tube (RACFST) columns containing ferronickel slag (FNS), this paper designed and fabricated nine specimens with the recycled coarse aggregate replacement ratios (RCAs), diameter-thickness ratio, and axial load ratios as the design parameters, which were tested using low-cycle reciprocation loading tests. The damage patterns of all specimens were observed; the hysteresis and skeleton curves of each specimen were obtained. Comparative analyses of the ultimate strength, hysteretic behavior, ductility, stiffness degeneration, energy dissipation capacity, and other seismic parameters were performed. Additionally, a series of numerical models of RACFST columns were established and the relevant parametric studies were discussed. The test results showed that the damage position of the specimen was located in the plastic hinge area at the bottom of the column, which was mainly characterized by the bulging and fracturing of steel tubes. The hysteresis curves of all specimens were relatively full, and the RCAs had a negligible influence on the hysteretic behavior of the columns. With the increasing axial loading ratio, the degeneration rates of ultimate strength and stiffness were accelerated, the deformation capacity was inversely deteriorated, while the hysteretic energy dissipation was unexpectedly enhanced. As the diameter-thickness ratio declined, both the ultimate strength and ductility of the columns were improved accordingly. Furthermore, an applicable numerical model was established and compared with the test results. The numerical results were in good agreement with the test results and especially could reflect the hysteretic behaviors of columns. Moreover, the results obtained from the parametric analyses showed that the slenderness ratio, axial load ratio, and steel ratio all exhibited a relatively large effect on the ultimate strength and ductility of the specimens, while the effect of the RAC strength was relatively small.

Experimental study on the seismic behavior of CFRP-strengthened seismic-damaged recycled aggregate concrete-filled rectangular steel tube frame columns

The seismic performance of rectangular steel tube steel tube frame columns filled with recycled aggregated concrete and reinforced with carbon fiber cloth was evaluated. Four frame columns were designed and fabricated at a scale of 1:2, seismic damage was simulated, and low-cycle reciprocating load tests were conducted. The results showed that the damage process and damage pattern of the specimens were similar to those of concrete-filled steel pipe frame columns. The hysteresis curve of the reinforced specimens was fuller than that of the unreinforced specimen, and the shape of the hysteresis curve changed from a shuttle to a bow shape; The carbon fiber cloth improved the specimen's bearing capacity and stiffness, significantly improved the ductility, energy dissipation capacity and seismic performance. The reinforcement effect of the carbon fiber cloth was related to the pre-damage level of the specimens, and used of carbon fiber cloth improved the seismic performance of the specimens. The hysteresis curve and skeleton curve obtained from the test were used to establish the restoring force model of the specimens, and the displacement amplitude of the bearing capacity obtained from the model and experiment were compared. The restoring force model can be used for the elastic-plastic seismic analysis of the seismically damaged rectangular steel tube frames filled with recycled aggregate concrete.

Seismic behavior of full-scale square high-strength RACFST columns

The research purpose was to investigate the seismic behavior of full-scale square high-strength recycled aggregate concrete-filled steel tube (RACFST) columns. With the recycled coarse aggregate (RCA) replacement ratio, shear-span ratio and axial compression ratio as the variation parameters, seven column specimens were subjected to low-frequency cyclic loading tests. The test results showed that square high-strength RACFST columns had reliable seismic performance, their failure modes were similar to those of normal CFST columns, and the damage was mainly concentrated at the column roots. The bearing capacity and ductility of high-strength RACFST columns were less affected by the RCA replacement ratio, while the shear-span ratio and axial compression ratio had a greater impact on the peak bearing capacity, ductility, and other hysteretic behavior indexes. Meanwhile, a restoring force model established based on the test research and theoretical analysis agreed well with the test hysteresis curves. Finally, a fiber element model (FEM) using Opensees software was developed and a good agreement was achieved between the test and predicated results. The so-validated numerical model was then used to investigate the influence of the key parameters, such as the RCA replacement ratio, diameter-to-thickness ratio, axial compression ratio, shear-span ratio, and steel yield strength on the seismic performance of square high-strength RACFST columns.

Experimental investigation on recycled aggregate concrete filled steel tubular stub columns under axial compression

This paper presents an extensive experimental investigation of the circular recycled aggregate concrete filled steel tubes (RACFST) stub columns under to axial compression to assess their cross-section behaviour. Recycled aggregate concrete is a sustainable concrete produced with recycled coarse aggregate (RAC) from demolition wastes generally used to fabricate non-structural elements. When compared with natural coarse aggregate (NCA), the recycled coarse aggregate presents lower mechanical properties due to physical and chemical changes that occurred during the recycling process, and thus the resulting RAC possess lower strength than the conventional natural aggregate concrete. However, the confinement provided by the circular steel tube can increase the strength and ductility of RAC in recycled aggregate concrete filled steel tubes (RACFST) columns. The test programme presented in this paper comprised twenty-three composite columns with replacement ratios (the mass percentages of the NCA replaced by RCA in concrete) corresponding to 0% (nature coarse aggregate), 30 and 50% (recycled aggregate concrete), plus four steel columns used for comparison. The experiments have shown that, as anticipated, the concrete core enhances the composite section load-carrying capacity. However, the control over the contents of this concrete core influences the composite section's ductility. If the fill material presents a brittle behaviour, stress redistributions occur in both steel and concrete to compensate for this effect. Since the behaviour of natural aggregate concrete filled steel tubes (NACFST) and RACFST columns are similar, the present paper starts from a study based on a comparative assessment of the recommendations presented in ABNT NBR 8800, ABNT NBR 16239, AISC 360–16, Eurocode 4 and Australian/New Zealand AS/NZS 2327 design standards. These design recommendations proved to be consistent and in line with the performed experiments. Additionally, the results indicated that the cross-section slenderness ratio (D/t) and the confinement ratio directly influence the composite section response.

Axial behavior of carbon fiber-reinforced polymer–confined recycled aggregate concrete-filled steel tube slender square columns

The behavior of fiber-reinforced polymer (FRP)–confined recycled aggregate concrete-filled steel tube (RACFT) columns is barely studied. Especially, that of slender specimens has not been investigated so far. In this article, an experimental test of FRP-confined RACFT slender square columns was conducted to study the influences of recycled aggregate (RA) replacement ratios, FRP thicknesses, and wrapping schemes on their axial behavior. Results in this article suggest that the RA replacement ratio barely affects the initial stiffness of load-deflection curves of specimens. Moreover, the specimen with a higher RA replacement ratio has a lower axial stress but larger strain at the peak point. The external FRP jackets (either partial or full wrap) can effectively improve the performance of axially loaded RACFT columns, and the improvement of ductility due to the increase of the FRP thickness is more significant than that of axial compressive strength. Additionally, it was found that the axial strength and ultimate axial strain decrease with increasing slenderness ratios. Furthermore, the influences of slenderness ratios on the behavior of such columns are more significant for the column with a larger length-to-width ratio. Finally, a design model for FRP-confined RACFT slender square columns is developed, which can predict the results of the present test accurately.

Study on axial compressive stiffness of RAC-encased RACFST composite columns

Twenty four specimens were designed to investigate the axial compressive stiffness of the recycled aggregate concrete(RAC)-encased recycled aggregate concrete-filled steel tube (RACFST) composite columns. The working mechanism of the RAC-encased RACFST composite column was investigated and the full range load–displacement curves of the composite columns were obtained. The interaction of each component of the composite column, i.e. the effect of the outer RAC from the stirrup and the steel tube, and the effect of the core RAC from the stirrup and the steel tube, were analyzed. The constitutive models of the outer unconfined RAC, the outer confined RAC, the core RAC were discussed. Thereupon, simple formulas were proposed to calculate the axial compressive stiffness of composite columns. These formulas can assist engineers to predict the axial deformation of structure in high-rise buildings more accurately.