Stress-strain relationship of recycled self-compacting concrete filled steel tubular column subjected to eccentric compression

Abstract

As a typical compression member, the concrete-filled steel tube has been widely used in civil engineering structures. However, little research on recycled self-compacting concrete filled circular steel tubular (RSCCFCST) columns subjected to eccentric load was reported. In this study, 21 specimens were designed and experimental studies on the stress-strain relationship of were carried out to study the mechanical behaviors. Recycled coarse aggregate replacement ratio, concrete strength grade, length to diameter ratio and eccentric distance of specimens were considered as the main experimental parameters to carry out eccentric compression tests. The corresponding stress-strain relationship curves were used to analyze the influence of concerned parameters on eccentric load-bearing capacity of RSCCFCST columns. The experimental results show that the strain of the eccentric compression stress-strain curves increase with the increase of recycled coarse aggregate replacement ratio and concrete strength grade. With increase of eccentric distance, the ductility of specimens increases while the bearing capacity decreases. Moreover, a phenomenological model of RSCCFCST columns is proposed, which exhibits versatile ability to capture the process during loading. The present study is expected to further understanding the behaviors and to provide guidance of RSCCFCST columns in design and engineering applications.

Keywords

concrete filled circular steel tubular columns / recycled self-compacting concrete / eccentric compression / recycled coarse aggregate replacement ratio / stress-strain relationship

Introduction

At present, with development of urbanization, demolition and relocation of engineering construction have produced hundreds of millions of tons of construction waste every year. Consequently, the reasonable treatment of the construction wastes has attracted widespread attentions [¹–⁴]. For normal concrete, large consumption of natural aggregate would definitely pose a threat to the resources and environmental matters. In such case, an available method is to recycle the natural aggregate from construction waste to reuse the aggregate in recycled concrete. Extensive researches on recycled concrete have been carried out and results show that the mechanical properties of recycled concrete are not worse than normal concrete [⁵–¹¹]. That is, in engineering application, the recycled concrete is feasible. Therefore, how to further understand recycled concrete is of great significance to the sustainable development in economy and society.

Concrete filled steel tube has the advantages of high bearing capacity and good deformation performance, which has been widely used in civil engineering applications [¹²–¹⁶]. Concrete filled steel tube structure is a kind of new composite structure that is formed by filling concrete into steel tube. This new composite structure takes the advantages of mechanical behaviors of steel tube and concrete materials in the process of loading. During compression, the restraining effect of the steel tube can partly enhance the load carrying capacity of the core concrete. Meanwhile, the premature local buckling of steel tube can be delayed or even avoided for the presence of the core concrete. The interaction of the two components in concrete filled steel tube effectively improves the overall performance of the composite structure.

Compared with normal concrete structure, the strength of recycled concrete structure is somewhat weaker due to the existence of old cement mortar, weak interface transition zone and initial damage defects. Fortunately, recycled concrete filled steel tube seems to be an approach to effectively improve the mechanical performance of recycled concrete [¹⁷–²⁴]. The axial compression properties of steel tubular constrained recycled concrete cylindrical specimens was studied by Xiao et al. [²⁵] and the failure characteristics, load-axial strain relationship and lateral deformation coefficient of the specimen were analyzed. Experimental investigations of compression performance of recycled concrete filled steel tubular columns indicated that the failure process of recycled concrete filled steel tubular columns occur faster than normal concrete filled steel tubular columns [²⁶]. The ultimate bearing capacity and stiffness of recycled concrete filled steel tubular columns reduced in an inconspicuous range. However, other research has reported that under the same steel ratio, the ultimate bearing capacity and ductility of recycled concrete filled steel tubular columns are better than normal concrete filled steel tube specimens [²⁷]. A test on the bias performance of recycled concrete filled steel tubular columns was conducted by Yang and Han [²⁸]. The results revealed that the ultimate bearing capacity, elastic modulus and stiffness of the specimens decreased with increase of the recycled aggregate replacement ratio. The current research results have laid a good foundation for further research on the mechanical properties and design theory of recycled concrete filled steel tubular structures.

Recycled concrete filled steel tube can fully utilize the respective mechanical advantages of steel tube and recycled concrete, which provides an important way to reuse construction waste. However, in actual engineering applications, due to the quality of construction and pouring, the compactness of recycled concrete in steel tube is difficult to guarantee. The self-compacting concrete filled steel tube can achieve good self-compacting effect under its own gravity or slight vibration. Thus, massive researches on self-compacting concrete filled steel tube have been carried out. A set of self-compacting high-strength concrete filled steel tube was carried out by Yu et al. [²⁹]. The result indicates that the ductility of normal concrete is lower than that of self-compacting concrete filled steel tube. An experimental study on the mechanical behavior of 16 specimens for normal concrete and self-compacting concrete filled steel tube was conducted by Hossain and Lachemi [³⁰]. The test results represent that normal concrete shows better durability with the increase of slenderness ratio and the strain development of normal concrete filled steel tube is basically the same as that of self-compacting concrete. The difference between the performance of normal concrete composed of different concrete strength and steel tube strength and self-compacting concrete filled steel tube was studied by Khayat and Paultre [³¹]. The test results and analysis also implicate that the difference in mechanical properties between steel tube filling normal concrete and self-compacting concrete is small. The axial compression performance of self-compacting concrete filled steel tubular column was considered by Mahgub et al. [³²]. The results prove that elliptical self-compacting concrete filled steel tubular column with large length to diameter ratio has an overall buckling failure.

To make full use of the properties of steel tube, recycled concrete and self-compacting concrete, an experimental study on the mechanical properties of the recycled self-compacting concrete filled circular steel tubular (RSCCFCST) columns under eccentric compression was carried out in this study. The influence of recycled coarse aggregate replacement ratio, concrete strength grade, length to diameter ratio and eccentric distance on the mechanical behavior of RSCCFCST column is analyzed. Finally, a calculation model for RSCCFCST column is established.

Test overview

Test materials

The cement used in this test is P.O42.5 grade Portland cement. The fineness modulus of sand is 2.90, belonging to Zhongsha II zone. The water content is 4.2%, in accordance with “Construction Sand” (GB14684-2011 of China) Standard Class I requirements. Based on the “Fly Ash for Cement and Concrete” (GBT1596-2005 of China), a certain proportion of Class II fly ash is blended with apparent density of 2.2 g/cm³. Following the “Concrete admixture: Application Technical Specification (GB50119-2013 of China)”, polycarboxylate superplasticizer is used and the ratio of water reduction rate is about 30%. The recycled aggregate is the waste concrete test block with strength of C20–C60, which is broken by the jaw crusher. The recycled aggregate is divided into three sizes of 5 to 15 mm, 15 to 25 mm, and 25 to 31.5 mm by vibrating sieve machine. The mixing ratio of the cement, stone and sand in the mixture is taken as 1:2.5:1.5. The mixed recycled coarse aggregate belongs to a continuous grading of 5–31.5 mm, and the natural aggregate used in the test is also measured as a continuous grading of 5–31.5 mm. The gradation of the recycled coarse aggregate particles after mixing is listed in Table 1. The particle gradation of the sample was judged to be a continuous gradation of 5–31.5 mm according to the cumulative screen residual percentage of each screen.

Mechanical performance analysis

The self-compacting properties and mechanical performances of recycled self-compacting concrete are analyzed by carrying out the orthogonal test. The slump drop test and J-ring drop test of concrete are adopted to determine the self-compacting properties, as shown in Fig. 1. Then, a relatively optimal mix ratio design is selected to be used as the core concrete of RSCCFCST columns. The mix ratio of recycled self-compacting concrete used in this study is listed in Table 2. Correspondingly, the standard test cubes with volume of 150 mm × 150 mm × 150 mm were prepared by following the mix ratios that were listed in Table 2. Correspondingly, the mechanical properties of the self-compacting concretes were tested, as summarized in Table 3. It should be noticed that the compressive strength is generally used to compare the strength grade and quality of concrete under the same standard conditions (easy to manufacture and test), but it can hardly represent the real stress state of concrete members of civil engineering structures. Approximately, the axial compressive strength, which can be acquired by multiplying a reduction factor by compressive strength, is adopted to design concrete structure.

From the analysis of compressive strength of recycled self-compacting concrete, it can be seen that with increase of water-binder ratio (w/b ratio), the compressive strength decreases obviously, which can be explained as the increased porosity and cracks after hardening and reduced cement content for the proportional increase in water consumption. Moreover, for given w/b ratio, the compressive strength of recycled self-compacting concrete decreases slightly with the increase of recycled coarse aggregate replacement. This phenomenon can be attributed to the expanded bond interfaces between recycled concrete aggregate and the old mortar in recycled self-compacting concrete as the percentage of recycled aggregate replacement increases [³³].

Mechanical properties of steel tubes

The steel tube used in this test is Q235 seamless circular steel tube with an outer diameter of 140 mm and a wall thickness of 3.63 mm. Yield strength and tensile strength of steel are determined by carrying out the tensile test according to “Metal material tensile test- room temperature test method” (GB/T228.1-2010 of China). The standard tensile specimen that was used to approximately represent the tensile properties of tubes was prepared by cutting from the complete tube (part of the complete tube). That is, the tensile test of a part of the steel tube is used instead of the tensile test of the entire steel tube. Tensile specimen of steel tube and its failure mode are shown in Fig. 2. Clearly, the fracture dominates the failure of tensile specimen for the development and propagation of accumulated cracks [³⁴,³⁵]. An axial compression test was performed on an empty steel tube with a length of 500 mm, an outer diameter of 140 mm, and a wall thickness of 3.63 mm to determine the elastic modulus and Poisson’s ratio of the steel tube. The failure mode of the steel tube is revealed in Fig. 3(a). The corresponding stress-strain relationship of the steel tube (ε_sa was the longitudinal strain of steel tube, ε_s1 was the circumferential strain of steel tube) is shown in Fig. 3(b). It can be seen from the figure that at the initial stage of loading, the strain increases linearly. When the compressive stress reaches 200 MPa, the steel tube enters the elastoplastic stage. As the load continues to increase, the middle part of the steel tube is drummed and large plastic deformation and destruction is raised in the drumming area. The mechanical properties of steel tube are shown in Table 4.

Sample preparation

The outer diameter of the steel tube is 140 mm, and the wall thickness of the steel tube is 3.63 mm. The lengths of the steel tubes are 500, 1000, and 1500 mm, respectively. The recycled aggregates replacement ratios are 0%, 50%, and 100% respectively. The eccentric distances are 0, 20, 40, and 60 mm, respectively. The eccentricity rates are 0, 0.30, 0.60, and 0.90, respectively. The length-to-diameter ratios of specimens are 3.57, 7.14, 10.71, and the concrete strength grades are C30, C50, and C60, respectively. The specific parameters of the specimens are demonstrated in Table 5.

Loading scheme

The test was carried out on the 500t hydraulic compression testing machine, with 21 eccentric compression specimens and 7 axial compression specimens. The device diagram and setting drawing loaded in this test are illustrated in Fig. 4. The grading loading system is adopted by the specimens and the load of each stage is set to be 10% of the ultimate bearing capacity of the specimens is estimated. After reaching 80% of the ultimate bearing capacity of the specimens, the load of each stage is about 5% of the ultimate bearing capacity of the specimens. The instrument readings were recorded after each stage of loading, and the failure process of the RSCCFCST columns under eccentric compression was observed. The voltage stabilizing time for each load was 2 min. When the ultimate bearing capacity of the specimen begins to decline, slow continuous loading was adopted. If the ultimate bearing capacity does not decrease significantly the load will be stopped after the lateral deflection of the short columns reach 10 mm while the loading stopped after 20 mm for the medium long columns.

Experimental results

Failure modes

The failure modes of the RSCCFCST short column and the medium long column under eccentric compression are demonstrated in Figs. 5 and 6, respectively. It is found that the bending and buckling dominate the failure of RSCCFCST short columns. For the medium long column of RSCCFCST columns, a similar deformation was presented in the process of loading. Particularly, the additional moment effect is more obvious than that in the long column. The continued lateral deflection during loading finally causes the instability and failure of the load carrying capacity of specimen. Therefore, the failure mode of the medium long column is bending-dominated failure.

Stress-strain relation analysis

The effects of recycled coarse aggregate replacement ratio and concrete strength grade on the stress-strain relationship of the specimens with different eccentric distance are demonstrated in Figs. 7 and 8, respectively. At the initial stage of loading, the specimen is in the elastic stage and the recycled coarse aggregate replacement ratio and the concrete strength grade have no effect on the stress-strain curve of the specimen. As the load continues to increase, the longitudinal compressive strain develops quickly compared with the longitudinal tensile strain. The circumferential tensile strain develops faster than the circumferential compression strain. The appearance of an inflection point indicates that the specimen enters the elastoplastic stage. Investigations of the stress-strain relationships of columns under eccentric compression also demonstrate that the stress of the specimen is reduced while the strain growth rate is accelerated with increase of recycled coarse aggregate replacement ratio. Essentially, a high recycled coarse aggregate replacement ratio would enlarge the relatively interfacial transition zones and also the initial cracks of recycled coarse aggregate can be introduced during the crushing operation for recycled coarse aggregate. Accordingly, the strength of recycled coarse aggregate concrete member is slightly reduced while the ductility can be partly enhanced. An inverse trend is observed for the effect of concrete strength grade that the specimen stress value increases while the strain growth rate slows down with increase of the concrete strength grade. In the failure stage, the strain of specimen develops rapidly with nearly constant value of the stress. Besides, with increase of recycled coarse aggregate replacement ratio and concrete strength grade, the strain extension increases, which indicates that the specimen possess a good performance in ductility.

The effects of length to diameter ratio and eccentric distance on the stress-strain relationship of the specimens are analyzed in Figs. 9 and 10, respectively. The slope of the curve at the early stage of loading increases with increase of length to diameter ratio and eccentric distance increase. When the load is increased continually, specimen enters the elastoplastic stage with appearance of an inflection point. The stress of the specimen decreases and the strain growth speed increases with increase of length to diameter ratio and eccentric distance. For short columns, the flexural capacity of involved members is higher than that of long columns and the interaction between core concrete and the steel tube can be greatly enhanced. Compared with the medium long columns, the ultimate longitudinal strain and the circumferential strain in tension zone of the short column are both larger. This is because the strains of specimens with larger length to diameter ratio do not yield under tension or compression.

For specimens with smaller eccentric distance, the section is fully under compression at the initial stage of loading, and all strains on the section are compressive strain. When the load reaches 80% of the ultimate bearing capacity, the longitudinal strain is converted to tension, which is consistent with the bending direction of the specimen. For specimens with larger eccentric distance, which corresponds to the relatively larger initial bending moment, the longitudinal strain is tensile strain at the initial stage of loading. The tensile strain develops rapidly with increase of eccentric distance. When the load is close to the ultimate bearing capacity, the specimen enters the failure stage. And the stress of specimen decreases while the strain develops rapidly with increase of length to diameter ratio and eccentric distance increase. A high eccentric distance can induce the local stress concentration and the local bulge would be raised. Once the bulge phenomenon occurs, the confinement effect of steel tube in the critical region is released, which further decrease the flexural capacity of specimen. Then, the subsequent global buckling derogates the strength of columns.

The influence of the stress-strain relationship of the medium long RSCCFCST column under eccentric compression with 1/4, 1/2, and 3/4 height of specimen is revealed in Fig. 11. It is found that the curves at different heights are basically coincident. The stress-strain at both ends of the medium long columns with a height of 1000 mm are basically symmetric with respect to the central part, while the development trend of the circumferential tensile strain of the medium long columns with a height of 1500 mm is far greater than that of the circumferential compressive strain. This is because the second order effect is more obvious in the specimens with larger length to diameter ratio. Before the steel tube on the side far from the axial force yields, the concrete on the side close to the axial force is crushed. Then, specimen is damaged prematurely due to instability.

A phenomenological model of RSCCFCST columns subjected to eccentric compression

Under axial compression, the stress-strain relationship of the core concrete in concrete-filled steel tubes, which was widely used to numerically simulate the mechanical behavior of core concrete, was proposed by Han et al. [³⁶] on the basis of mount of experimental data. For RSCCFCST columns, the plastic behavior of core concrete partly dominated by the confined steel tube. Quantitatively, the confinement factor ξ was introduced to evaluate the effect, which has expression of

(1)\xi ={\displaystyle \frac{f_{\mathrm{y}}{A}_{\mathrm{s}}}{f_{\mathrm{c}}{A}_{\mathrm{c}}}},

where A_s and A_c are the cross-sectional areas of the steel and concrete, respectively, f_y the yield strength of the steel, f_c the compression strength of the concrete. For eccentric compression, considering the non-uniform distribution of stress within steel tube and core concrete, a revised confinement factor ξ′ is adopted, written as

(2){\xi }^{\text{'}}=k\xi .

The coefficient is expressed as

(3)\mathrm{k}=1-\alpha \left({\displaystyle \frac{e}{D}}\right),

where e is eccentric distance and α is adjusted gradient parameter, D the outer diameter of RSCCFCST columns. Then, the stress-strain relation of core concrete proposed by Han et al. [36] can be revised as

(4)\frac{\sigma }{{\sigma }_{\mathrm{c}}}=\{\begin{array}{cc}2{\displaystyle \frac{\epsilon }{{\epsilon }_{\mathrm{c}}}}-{\left({\displaystyle \frac{\epsilon }{{\epsilon }_{\mathrm{c}}}}\right)}^2,\hfill & \epsilon \le {\epsilon }_{\mathrm{c}}\hfill \\ {\displaystyle \frac{\epsilon }{{\epsilon }_{\mathrm{c}}}}{\displaystyle \frac{1}{{\beta }^{\prime }{({\displaystyle \frac{\epsilon }{{\epsilon }_{\mathrm{c}}}}-1)}^2+{\displaystyle \frac{\epsilon }{{\epsilon }_{\mathrm{c}}}}}},\hfill & \epsilon >{\epsilon }_{\mathrm{c}}\hfill \end{array}

in which

{\sigma }_{\mathrm{c}}=f_{\mathrm{c}\mathrm{k}},

{\epsilon }_{\mathrm{c}}={\epsilon }_{\mathrm{0}}+\mathrm{0.80}{\xi }^{\text{'}}{}^{\mathrm{0.2}},

{\epsilon }_{\mathrm{0}}=\mathrm{1.30}+\mathrm{0.0125}{f}_{\mathrm{c}\mathrm{k}},

{\beta }^{\text{'}}\mathrm{=(2}\mathrm{.36}\times {\mathrm{10}}^{-\mathrm{5}}{)}^{[\mathrm{0.25}+{({\xi }^{\text{'}}-\mathrm{0.5})}^{\mathrm{7.0}}]}\times \mathrm{0.5}{f_{\mathrm{c}\mathrm{k}}}^{\mathrm{0.5}}\times {\mathrm{10}}^{-\mathrm{4}},

where f_ck is the characteristic strength of concrete column.

Since the internal force of the eccentrically loaded RSCCFCST column is complex in the process of loading, the sectional analysis should be conducted to analyze the behavior of specimen, as shown in Fig. 12. The procedure is as follows: first, the initial curvature and the extreme boundary strain of the compressive zone should be assumed. Then, by dividing section with finite width, the stress of steel tube and core concrete can be calculated associating with the pre-assumed stress-strain of materials. Then, the axial force and moment can be obtained by integrating the internal forces on the divided strips. If the axial force and moment are equal to these that caused by the exerted force, the assumed sectional strain is valid otherwise the strain should be reset and the numerical analysis would be restarted until all the determined values are consistent with specific conditions.

Clearly, the accuracy of the theoretical mechanical behavior of RSCCFCST columns depends on the proposed model of core concrete and the elements of cross-sections. Besides, the complex procedure greatly restricts the practical application. Therefore, in order to evaluate the mechanical properties and to guide civil engineering application of the RSCCFCST columns conveniently, an empirical formula was proposed in this study. This formula can be adopted to directly capture the mechanical behavior of RSCCFCST columns under eccentric loading, which is expressed as

(5)\sigma =(a{\epsilon }^{\mathrm{2}}+b\epsilon )+\mathrm{c}\ln (d\epsilon +\mathrm{1})\mathrm{,}

where a, b, c, and d are the fitting parameters. For RSCSE-9, the fitting parameters are a = 4.158 MPa, b = –1.449 MPa, c = 1302.1MPa, and d = 0.1337, respectively. Then, by applying the proposed model expressed in Eq. (5), the stress-strain relationship curves of tested specimens are fitted, as shown in Fig. 13. It can be seen that the stress-strain relationship model of the specimens established in this study agrees well with the experimental values. Hence, the proposed theoretical calculation model provides accessible and valid way to investigate mechanical behavior of RSCCFCST columns under eccentric compression.

It is worth noticing that the proposed model is a purely phenomenological model, though it exhibits versatile ability to fit the stress-strain relations of eccentrically loaded RSCCFCST columns. The possible drawback of such model is that the physical meaning of introduced quantities may be obscure, which results in the unclear understanding of the deformation mechanism. For instance, the development and propagation of cracks raised in the core concrete in RSCCFCST columns during load cannot be revealed. Also, the interaction mechanism of the core concrete and the outer steel tube in RSCCFCST column is unable to be well explained. Such issues need to be further studied by developing mechanism-based model, like the phase field model [³⁷–³⁹] which can be adopted to investigate the weak interface transition zone and damage characteristics in the RSCCFCST columns. Alternatively, by studying the dependence of the coefficients of Eq. (5) on structural parameters (i.e., steel ratio, concrete grade, replacement ratio and eccentricity) individually, an improved phenomenological model that is suitable for RSCCFCST columns could be achieved by accounting for the parameters effect quantitatively. Then, the engineering guidance can be provided partly for the practical application of RSCCFCST columns.

Conclusions

Experimental investigations of 21 RSCCFCST columns under eccentric compression were carried out. The influence of recycled coarse aggregate replacement ratio, concrete strength grade, length to diameter ratio and eccentric distance on the stress-strain relation of specimens was analyzed. Based on the results, the following conclusions can be drawn.

1) For a certain water-to-blinder ratio, the compressive strength of recycled self-compacting concrete made with using recycled coarse aggregates to partly or totally substitute the natural aggregates exhibits nearly undiminished though the existence of relatively weak interfacial transition zones between recycled concrete aggregate and old cement mortar.

2) The bending and buckling behaviors dominate the failure of short column in form of recycled self-compacting concrete filled steel tube under eccentric compression loading while the middle long column exhibits failure in form of elastoplastic instability.

3) Three stages, i.e., elasticity stage, elastoplasticity stage, and failure stage, are basically exhibited on the stress-strain relationship curve. The stress-strain relations of eccentrically loaded RSCCFCST columns at different heights are basically coincident. The longitudinal stress-strains at both ends are nearly symmetric with respect to the central part, especially for the long columns.

4) A phenomenological model with four parameters was proposed to characterize the stress-strain relation of RSCCFCST column under eccentric compression. This mathematically continuous model exhibits versatile ability to fit the experimental results, which can be conveniently adopted to guide the civil engineering application of RSCCFCST columns.

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