Introduction and literature review

Concrete and steel are the foremost extensively used and consumed materials within the construction of building structures. The fashionable developments within the construction sector prove further inclination towards the development of RCC multi-story structures everywhere in the planet. The planning of such high-rise structures demands huge components, particularly columns resulting in the introduction of huge axial forces and moments. Due to continuous efforts of researchers, the CFT column is a highly developed system in the building of structures. However, CFT column sections like circular, rectangular, square, octagonal, polygonal and elliptical are widely used in the engineering structures. But there is more local buckling in square or rectangular CFT columns due to less confinement by steel tube and concrete when compared to circular CFT column [1,2,3,4,5,6,7,8,9]. Many researchers have studied the close behaviour of CFT column and suggested the design guidelines for analysis [4, 6, 7]. The concrete-filled tube columns also are called as composite columns which are extensive important parts in composite structures. A lot of research has been performed on the experimental and numerical investigation with the purpose to review the performance of concrete-filled tube specimens with different grades of steel and variable thicknesses like mild steel, aluminium, high strength steel and many more. A numerical study has been conducted to estimate the buckling failure causing the major failure mode found in concrete-filled tube columns and suggests a design rule to prevent this failure [4,5,6,7,8]. An experimental investigation has been conducted to estimate the load-carrying capacity of CFT columns by using different types of sand like quarry dust, concrete debris, sea sand and demolished concrete blocks [10,11,12,13]. Many researchers have observed that the artificial sand improves the strength of concrete [14, 15]. Also, a lot of study has been conducted on the load-carrying capacity of CFT columns with recycled aggregate, and from that study, it is observed that the recycled aggregates improve the strength of CFT columns [16,17,18,19,20]. Till date, extensive study has been performed to review the performance of concrete-filled tube specimens subjected to concentric and eccentric loadings [21,22,23,24]. A few researchers have developed FE model and simulated the performance of CFT columns by using finite element software [18, 25]. A lot of efforts have been made by the various researchers in the development of technologies in steel and composite structures. The castellated and cellular beams are the examples of such technologies. The researchers have thoroughly investigated the behaviour of such beams in terms of failure modes, optimization of shape and size of openings [26,27,28,29,30,31].

Even though there is a broad study on the performance of CFT columns, very few studies have been performed on artificial sand CFST column. Presently, the design procedures for the CFT columns are given in American code (AISC 360-10) [32] and Eurocode (EC-4) [33]. Generally, the number of researchers has studied the natural sand CFT columns. Hence, there is need to study artificial sand CFT columns. Also, results of natural sand and artificial sand CFT column have been validated in ANSYS [38]. Furthermore, the construction of CFT column in India is advanced technology. There is no codal provision of the design rules for CFT columns in Indian code [34]. Therefore, the study has been performed to examine the performance of artificial sand CFT columns constructed with locally available sand collected from vertical shaft impact (VSI) stone crusher. A series of specimens having varying D/t ratios were tested under concentric loading. The primary aim of this study is to test the CFT specimen for variable tube thickness under concentric loading. The secondary aim is to examine the effect of artificial sand concrete in compressive strength and the behaviour of tested columns and thirdly to study the existing design codes (EC-4 and AISC 360-10) for the natural and artificial sand CFT columns.

The conclusion of this study will be benefited for the construction and design of composite column structures. The compressive strength and behaviour of artificial sand CFT specimens are higher than the corresponding natural sand CFT specimens.

Experimental set-up and investigation

The investigation was administered on circular hollow steel column specimens filled with natural and artificial sand concrete of grade M25 having a height of the column of 600 mm with variable wall thickness of 3.2 mm, 4.0 mm and 4.8 mm as shown in Fig. 1 having the modulus of rigidity 76,900 MPa, density 7850 kg/mm3 and Poisson’s ratio 0.3. The Young’s modulus of hollow steel tube is 2 × 105 N/mm2. The ratio of area of steel to concrete is defined as sectional area ratio (α = As /Ac). The steel ratios considered in this study are 0.15, 0.21 and 0.26. The hollow circular tube diameter (D) of 88.70 mm and three variable walls’ thickness of 3.2 mm, 4.0 mm and 4.8 mm are chosen as per IS: 1239:2004, clauses 8.1 and 10.1.1.1 [35].

Fig. 1
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Plan and side view of specimens with dimensions

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The properties of sand as obtained are presented in Table 1. The artificial sand artificially produced in vertical shaft impact (VSI) crusher was collected from VSI crusher at Vasunde, Pune, Maharashtra, India. A uniformly natural sand and artificial sand (zone- II) as shown in Fig. 2 are used. The ingredient material utilized in this study was collected from local sources. Ordinary Portland Cement of 53 grades was used. Potable water was used during the preparation of concrete mix. Six cubes of grade M25 [37] having the size of 150 mm × 150 mm × 150 mm concrete has been casted, and therefore, the cubes were tested to gauge the strength of concrete. After 28-day curing of specimens, the average compressive strength (fcu) of natural sand and artificial sand concrete was found to be 32.2 MPa and 33.8 Mpa, respectively.

Table 1 Properties of concrete materials
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Fig. 2
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Details of sand

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In order to determine the material properties of steel, three coupon specimens of thickness of 3.2 mm, 4.0 mm and 4.8 mm were cut from the steel tube and prepared according to ASTM E8 [36]. The coupon test is shown in Fig. 3. All three specimens were tested for axial tensile test, and obtained results of tensile coupons test for steel tubes are given in Table 2.

Fig. 3
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Steel tube coupon test

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Table 2 Steel tube coupon test results
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The parameters were investigated on a complete twelve specimens of CFT columns under axial compression. The six specimens consists of natural sand (NS) concrete of variable wall thickness 3.2 mm (C3-RS1 and C3-RS2), 4.0 mm (C4-RS1 and C4-RS2) and 4.8 mm (C5-RS1 and C5-RS2), and the remaining six specimens are of artificial sand (AS) concrete of variable wall thickness 3.2 mm (C3-MS1 and C3-MS2), 4.0 mm (C4-MS1 and C4-MS2) and 4.8 mm (C5-MS1 and C5-MS2). The design mix of M25 grade concrete is given in Table 3 [37]. The specimens were cured for twenty-eight days in curing tank for proper bonding. Figure 4 shows the natural and artificial sand CFT specimens.

Table 3 Design mix for normal grade concrete [37]
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Fig. 4
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Details of CFT specimens

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The capacity of 1000 kN Universal Testing Machine (UTM) was utilized, and all the CFT columns were tested under axial compressive force. The strain gauge was connected on the outside surface at mid height of the specimen to measure the longitudinal deflection, and the lateral deflection was measured by using dial gauge attached to universal testing machine. Both the ends of specimens were hinged, and then after that the application of axial compressive loading was started. The most important intention of this study is to find out the result of natural sand and artificial sand concrete in CFT column with variable tube thickness. The diagrammatic and laboratory test set-up for testing of CFT specimens is as shown in Figs. 5 and 6, respectively. The 5 kN initial load was given to hold the specimen vertically straight. The deflections were recorded at each load increment. The load was applied up to the failure of all CFT specimens. The axial load and deflection were measured during the testing of specimens. The CFT column sections were analysed from the behaviour of load–deflection curves.

Fig. 5
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Diagrammatic test setup

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Fig. 6
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Laboratory experiment setup

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Numerical investigation

The maximum load-carrying capacities of natural and artificial sand M25 grade CFT columns are calculated for different D/t ratios by using Eurocode 4 [33] and AISC 360-10 code [32].

Eurocode-4

The maximum load-carrying capacities of CFT columns is calculated by using two coefficients η1 and η2 as specified below.

$$ P_{{\text{p}}}\, { = }\,A_{{\text{a}}} \eta_{{2}} P_{{\text{y}}} + \, A_{{\text{c}}} p_{{{\text{ck}}}} [{1} + \, \eta_{{1}} ] \, + {\text{ As }}p_{{{\text{sk}}}} $$

where t is the circular tubular section thickness. η1and η2 are the two coefficients given by

$$ \eta_{{1}} = \eta_{{{1}0}} \left[ {{1} + \frac{10e}{d}} \right]\;\;\;\;\;\eta_{{2}} = \eta_{{2}} + \, \left( {{1} - \eta_{{{2}0}} } \right)\frac{10e}{d} $$

The Fck value was taken as:

$$ F_{{{\text{ck}}}} = \, 0.{83 }F_{{{\text{cu}}}} $$
$$ {\text{As an alternative of}}\;\;\,\;\;F_{{{\text{ck}}}} = \, 0.{67 }F_{{{\text{cu}}}} $$

where Fcu = Concrete cube strength after 28 days.

AISC 360-10 code

The limit state of flexural buckling design is used to determine the maximum load-carrying capacities (ɸPn) and allowable load-carrying capacities (Pn/Ω) for CFT columns as given below:

$$ \, \Phi = \, 0.{75 }\left( {{\text{LRFD}}} \right);\;\;\,\;\Omega = { 2}.00 \, \left( {{\text{ASD}}} \right) $$

(a) When \(Pe \ge \, 0.{44}P_{o}\)

$$ P_{{\text{n}}} = \, P_{{\text{o}}} \left[ {0.658^{{\left( {\frac{{P_{o} }}{{P_{e} }}} \right)}} 0.658^{{\left( {\frac{{P_{o} }}{{P_{e} }}} \right)}} } \right] $$

(b) When Pe < 0.44Po;

$$ P_{{\text{n}}} = \, 0.{877}P_{e} $$
$$ P_{{\text{o}}} = \, A_{{\text{s}}} F_{{\text{y}}} + \, A_{{{\text{sr}}}} F_{{{\text{yr}}}} + {\text{ C2 Acf'}} {\text{c}} $$

C2 = 0.95 for circular and 0.85 for rectangular section.

$$ P_{e} = \, \pi^{{2}} \left( {{\text{EI}}_{{{\text{eff}}}} } \right)/\left( {{\text{KL}}} \right)^{{2}} $$
$$ {\text{EI}}_{{{\text{eff}}}} = \, E_{{\text{s}}} I_{{\text{s}}} + \, C_{{3}} E_{{\text{c}}} I_{{\text{c}}} $$
$$ {\text{C}}_{{3}} = \, 0.{6 } + { 2}\left( {\frac{{A_{s} }}{{A_{c } + A_{s} }}} \right) \le 0.{9} $$

where Leff is the effective length of CFT column. The parameters As, Fy, Es and Is are the area, yield stress, young’s modulus and moment of inertia of the steel section respectively and Ac, Fc, Ec and Ic are the area, specified compressive strength, young’s modulus and moment of inertia for the concrete respectively.

Analytical investigation

Finite element modelling is usually performed to understand the response of structure for the different type of loading conditions. The modelling precision for specimen is very important; therefore, three-dimensional concrete-filled tube column specimens are formed by providing its height, outer diameter and wall thickness in SolidWorks software and then exported in ANSYS [38] as shown in Fig. 7. To achieve the consistent stress distribution, the concrete tube is modelled as brick elements with three degrees of freedom. The steel tube is modelled as 3D shell element with four-node quadrilateral shell and all node having three translational and three rotational degrees of freedom. In order to calculate the correct results from the finite element model, the three-dimensional hexagonal meshing was discretized in same mesh size to every element in the model to make sure that both the materials share the same node. The compatibility condition for every linking element has been ensured. The property of material given in Table 4 such as Young’s modulus, Poisson’s ratio, compressive strength, yield stress, ultimate stress and density is assigned to model. The different types of models were developed for different tube thicknesses. The deflection of different model is observed after the experimental load applied to those models. Figure 7 shows the created models of CFT column specimens using ANSYS 14.5 [38]. The compressive force was applied uniformly spread above the top face of column nodes. The force was given in Z-direction and was consent to move in Z-direction but not allowed move in X- and Y-direction. The static structural analysis was performed, and the deflection was checked for variable thickness of natural and artificial sand concrete specimens for compressive force.

Fig. 7
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Modelling and loading of CFT specimen

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Table 4 Material properties (Elastic)
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Result and discussion

CFT specimen’s failure modes

The circular CFT specimens have taken maximum axial compressive load, and it is observed that many specimens have clear view of local buckling over the mid-height. As the function of load increased, the steel has been shows very good ductile deformation and even after severe buckling behaviour, no rupture was observed in the tested CFT specimens. Due to the confinement of CFT specimens, no inward buckling was observed as the internal support provided by core concrete. The most of the specimens were buckled outward, and bulging is also observed in some of the specimen. The buckling behaviour of artificial sand CFT specimens shows better result as compared to natural sand CFT specimens. The tested CFT specimen failure patterns are as shown in Fig. 8.

Fig. 8
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Experimental failure modes under compression

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The deformed shape of the studied artificial and natural sand CFT specimens at failure is as shown in Fig. 9. The artificial and natural sand CFT specimen fails with global lateral buckling because the column is vulnerable to global buckling and due to the restraint from the in filled concrete. The significant local buckling is also observed in the compressive zone above the mid-height of the artificial and natural sand CFT specimen even as slight local buckling is observed at mid-height of the CFT specimen as shown in Fig. 9.

Fig. 9
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Typical failure mode of CFT columns: a significant local buckling of artificial sand specimen, b global buckling of artificial sand specimen, c significant local buckling of natural sand specimen, d global buckling of natural sand specimen

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Load–deflection of CFT specimens

The experimental results obtained from natural sand and artificial sand M25 grade CFT column specimens are compared. Also, the experimental outcomes are compared with the analytical outcomes. As per the graphical representations of axial load–deflection performance, the participation of CFT specimens for strain is activated with adopting the use of artificial sand concrete as well as an incremental thickness of wall tube. With also natural or artificial sand concrete infill, the experimentally tested CFT specimens indicate ductile behaviour with no rapid fall of the force. It is obvious that the load–deflection curves exhibited elastic pattern during the primary loading phase. Then, the steel tube buckled, and therefore, the elastic–plastic phase was reached. The load–deflection curve, peak deflection and peak load for the various types of CFT specimens are given below.

From Figs. 10, 11 and 12, it is observed that the experimental compressive strength of C3-RS, C4-RS and C5-RS is 400 kN, 485 kN and 540 kN, respectively, and the corresponding deflections are 6.4 mm, 4.6 mm and 3.4 mm. Furthermore, from Figs. 13, 14 and 15, the analytical deflection observed for C3-RS, C4-RS and C5-RS is 6.3 mm, 4.5 mm and 3.3 mm, respectively, and the corresponding compressive load is 400 kN, 485 kN and 540 kN.

Fig. 10
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Load- deflection curve for 3.2 mm thick tube natural sand CFT specimens

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Fig. 11
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Load- deflection curve for 4.0 mm thick tube natural sand CFT specimens

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Fig. 12
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Load- deflection curve for 4.8 mm thick tube natural sand CFT specimens

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Fig. 13
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Deflection under compressive force of 3.2 mm thick tube natural sand CFT specimen

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Fig. 14
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Deflection under compressive force of 4.0 mm thick tube natural sand CFT specimen

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Fig. 15
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Deflection under compressive force of 4.8 mm thick tube natural sand CFT specimen

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Similarly, from Figs. 16, 17 and 18, it is observed that the experimental compressive strength of C3-MS, C4-MS and C5-MS is 423 kN, 493 kN and 560 kN, respectively, and the corresponding deflections are 5.9 mm, 4.2 mm and 2.9 mm. Furthermore, from Figs. 19, 20 and 21, the analytical deflection observed for C3-MS, C4-MS and C5-MS is 5.7 mm, 3.98 mm and 2.7 mm, respectively, and the corresponding compressive load is 423 kN, 493 kN and 560 kN. Also, it is observed that the artificial sand CFT specimens give the maximum result as compared to natural sand CFT specimens. The artificial sand CFT specimens take an additional compressive force, and its deformation at that compressive force was minimum as compared to natural sand CFT specimens. The analytical and experimental results are 99% similar within the compressive loading investigation.

Fig. 16
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Load- deflection curve for 3.2 mm thick tube artificial sand CFT specimens

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Fig. 17
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Load- deflection curve for 4.0 mm thick tube artificial sand CFT specimens

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Fig. 18
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Load- deflection curve for 4.8 mm thick tube artificial sand CFT specimens

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Fig. 19
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Deflection under compressive force of 3.2 mm thick tube artificial sand CFT specimen

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Fig. 20
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Deflection under compressive force of 4.0 mm thick tube artificial sand CFT specimen

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Fig. 21
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Deflection under compressive force of 4.8 mm thick tube artificial sand CFT specimen

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Comparison of results with design codes

The maximum load-carrying capacities of the tested columns were predicted by using the design philosophies as per the EC4 [33] and AISC 360-10 [32] code. The ultimate load-carrying capacities of D/t ratios 27.72, 22.18 and 18.48 are calculated by using EC-4 [33] and AISC 360-10 [32]. The designed values are consequently compared with the collapse loads gained from the experimental results shown in Table 5 and Fig. 22.

Table 5 Comparison of CFT specimen’s load-carrying capacity
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Fig. 22
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Comparison of CFT specimen’s load-carrying capacity

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Eurocode 4

Eurocode 4 [33] provides an excellent prediction of the steel tube 4.8-mm-thick specimens (smaller D/t ratio 18.48) when compared to steel tube 3.2-mm and 4.0-mm-thick specimens. For the natural sand CFT columns with 3.2 mm, 4.0 mm and 4.8 mm tube thickness, the major variation among experimental and Eurocode 4 [33] is 22%, 21% and 20%, correspondingly. Meanwhile, for the artificial sand CFT columns with 3.2 mm, 4.0 mm and 4.8 mm tube thickness, the major variation between experimental and Eurocode 4 is 24%, 22% and 22%, respectively. The average Nue/NEC4 of the natural sand and the artificial sand CFT columns with 3.2 mm, 4.0 mm and 4.8 mm tube thickness is 1.26 and 1.32, 1.28 and 1.28, 1.25 and 1.29, respectively.

AISC 360-10 code

The load-carrying capacity of experimental results compared with AISC 360-10 [32], and from that study, the results of AISC 360-10 [32] have been found safe for the steel tube thickness of 3.2-mm, 4.0-mm and 4.8-mm specimens. For the natural sand CFT columns with 3.2 mm, 4.0 mm and 4.8 mm tube thickness, the major variation among experimental and Eurocode 4 is 29%, 28% and 26%, correspondingly. Meanwhile, for the artificial sand CFT columns with 3.2 mm, 4.0 mm and 4.8 mm tube thickness, the major variation between experimental and Eurocode 4 is 30%, 29% and 27%, respectively. The average Nue/NEC4 of the natural sand and the artificial sand CFT columns with 3.2 mm, 4.0 mm and 4.8 mm tube thickness is 1.41 and 1.44, 1.40 and 1.41, 1.36 and 1.38, correspondingly.

Conclusions

The subsequent conclusions are drawn from the experimental, numerical and analytical investigations carried out on variable tube thickness M25 grade CFT specimens under compressive loading:

From the load–deflection curves for the artificial sand and the natural sand CFT specimens with similar steel tube thickness, it is observed that there is a variation in the load-carrying capacity. However, the thinner steel tube columns have higher bonding capacity compared to thicker steel tube columns.

The load-carrying capacity of artificial sand CFT specimens is higher than the natural sand CFT specimens with steel tube thickness 3.2 mm, 4.0 mm and 4.8 mm. The load-carrying capacity of artificial sand CFT specimens is increased due to the confinement of the steel tube and concrete. Therefore, to achieve a better economy, the natural sand could be replaced with artificial sand for CFT columns. Furthermore, the thinner steel tube columns have very less load-carrying capacity compared to thicker steel tube columns as bonding effect is not observed to a large extent because of a significant amount of local buckling of the thinner steel tube columns.

The Eurocode 4 [33] and AISC 360-10 [32] codes underestimate the load-carrying capacity for the artificial sand and the natural sand CFT columns. The thinner steel tube columns (D/t is 27.72) show lesser load-carrying capacity because of the steel tube local buckling due to the smaller confinement, and the thicker steel tube columns (D/t is 18.48) show more load-carrying capacity because of the effective confinement by the steel tube to the concrete core. The major variation between experimental results and design codes is 24% and 30% for the Eurocode 4 [33] and AISC 360-10 [32] correspondingly.

Also, it is concluded that artificial sand CFT columns indicated the excellent ductile deformation behaviour during the axial compressive loading. As the thickness of tube increases, axial load-carrying capacity increases and lateral deformation decreases for both natural and artificial sand CFT columns. Also, the failure modes of specimens were observed outward buckling above the mid-height of CFT columns. The experimental and analytical results like load and deflection behaviour generally agreed with one another.