# Mechanical Behavior of Silica Fume Concrete Filled with Steel Tubular Composite Column

Authors: Maganti Sandeep Kauthsa Sharma; S. Umadevi; Yerra Sai Sampath; K. Vasugi; K. J. N. Sai Nitesh; V. Swamy Nadh; L. Natrayan
Journal: Advances in Materials Science and Engineering
 (2021)

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## Abstract

Recycling or utilization of industrial waste is becoming more popular as people become more environmentally conscious. Silica fume is a by-product of the smelting process in the silicon and ferrosilicon industries. This study examines the mechanical behavior of steel tubular composite column filled with conventional concrete and silica fume concrete experimentally under axial compressive loading. For the study, variability in steel tube thickness and column height with a constant diameter are considered. To explore the influence of silica fume in concrete, microstructural analyses are carried out by SEM, XRD, and FTIR. The experimental results reveal that the use of silica fume as a replacement of cement is feasible; the silica fume concrete-filled steel tubular (SCFST) column has marginal enhancement strength capacity compared to CFST column as thickness increases.


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## Body

## 1. Introduction

The concrete-filled steel tubular (CFST) composite columns are been used in the modern engineering systems. Detailed experimental and computational studies have been carried out in the past [1–3]. Main advantages of CFST columns are no reinforced cage and no formwork as the steel tube can extremely be used as formwork and they are fire resistant. Based on the various research works carried out, it can be said that circular columns should be preferred over square-shaped CFSTs [4, 5], in order to further enhance structural performance and meet various design requirements, the focus of tubular columns [6, 7], concrete-filled double-skin tubular columns [8], stub columns with carbon fiber reinforcement polymer (CFRP) wrap material [9–12], and concrete-filled aluminium columns [13, 14]. Such experiments were mainly aimed at using new alloys or modifying standard CFST column configurations to enhance the structural efficiency of composite columns. Yogeshwaran et al. [15] investigated 42 concretes with varying diameter-thickness ratio and three different concrete strengths. The strength, load-axial shortening, load-axial strain, and failure patterns of columns were studied and compared with American and Australian/New Zealand design approaches. Ren et al. [16] conducted an experimental and analytical study on 44 specimens with different shapes. The results of distinct CFST columns performed the ductile manner. Outward buckling is observed at the middle of the member.Infilled concrete makes a significant role in CFST columns; many researchers have attempted to build composite columns using different types of concrete other than traditional one. Wang et al. [17] had conducted an experimental investigation on twenty composite columns filled with the reactive powder concrete (RPC) under axial compression. Paranthaman et al. [18] had investigated RAC with filled in stainless steel composite specimens with square and circular shapes, with a replacement ratio of 0, 25%, 50%, and 75%. Also, results were compared with six different codes. Geng et al. [19] studied the time-depended behavior recycled aggregate concrete by 0, 50, and 100% compared with the numerical model.To examine the performance infilled concrete in the CFST stub column under axial compressive loading, various research works carried out the experiment with different mixes with a constant cement content or with industrial by-products andw/c ratio, which were mixed with industrial by-products such as steel slag, fly ash, silica fume, and lightweight aggregate [20–24]. They are examined with the FE model and Euro code 4. The results explain that despite the use of various types of aggregates, all CFST columns behavior was reasonably well. Zhu et al. [25] investigated the analytical investigation on three different larger diameter concrete-filled higher strength steel stub columns (CFHSST) under compression and compared with Euro code 4 predictions. It is found that load-displacement graphs of three concrete infilled stub columns were like one another. Load-strain curves and failure patterns are observed. Abed et al. [26] studied member stiffness. Abed et al. [26] conducted a test on the 20 concrete-filled steel tubular (CFST) stub columns with dune sand and 4 hollow columns under the axial compression. Results showed that the CFST stub columns with dune sand, identical to traditional CFST stub columns, acted in exhibited a high compressive strength outward buckling failure pattern. The use of waste materials/by-products in building products has been increasingly emphasized in the latest years. The use of by-products in concrete can bring important financial and environmental advantage in concrete making. In this study, for a sustainable ecofriendly environment, silica fume partially replaced with cement in conventional concrete as a core material and hollow steel pipe with a constant diameter, height variation, and thickness variation are the two constituents used.


## 2. Materials

### 2.1. Infill Material

In this experimental program, a nominal strength of 25 MPa with 0.4 water cement ratio was employed with various concrete combinations. To determine the compressive strength, cube specimens were prepared as per Indian Standards IS: 10086-1982. Silica fume was substituted by 0, 5, 10, and 15% with cement, manufacturing sand is completely replaced with fine aggregate in each mix, and superplasticizers up to 2% of the binder is used. A machine mixer was employed to prepare the mixes. Three specimens were tested to failure under the compressive testing machine of axial capacity 2000 kN. The optimum percent replacement of the silica fume is carried out for 7 and 28 days of curing of cube specimen. Mix proportions for each mix are given in Table1. Figure 1 shows the materials used for the concrete mix. The average compressive strength concrete cubes are given in Table 2.Table 1
Mix proportions for cube specimens.
MixCement (kg/m3)Silica fume (kg/m3)Water (kg/m3)Coarse aggregate (kg/m3)Manufacturing sand (kg/m3)1350014011408962365.7519.251401117.04877.683346.538.51401112.44874.064327.2557.751401109871.64Figure 1
Concrete constituents (cement, sand, silica fume, fly ash, aggregate, and superplasticizer).Table 2
Compressive strength behavior of cubes.
MixPercentage replacement of silica fume (%)7 days strength (MPa)28 days strength (MPa)1024.1233.282525.4835.1431026.8937.6141525.0835.87


## 2.1. Infill Material

In this experimental program, a nominal strength of 25 MPa with 0.4 water cement ratio was employed with various concrete combinations. To determine the compressive strength, cube specimens were prepared as per Indian Standards IS: 10086-1982. Silica fume was substituted by 0, 5, 10, and 15% with cement, manufacturing sand is completely replaced with fine aggregate in each mix, and superplasticizers up to 2% of the binder is used. A machine mixer was employed to prepare the mixes. Three specimens were tested to failure under the compressive testing machine of axial capacity 2000 kN. The optimum percent replacement of the silica fume is carried out for 7 and 28 days of curing of cube specimen. Mix proportions for each mix are given in Table1. Figure 1 shows the materials used for the concrete mix. The average compressive strength concrete cubes are given in Table 2.Table 1
Mix proportions for cube specimens.
MixCement (kg/m3)Silica fume (kg/m3)Water (kg/m3)Coarse aggregate (kg/m3)Manufacturing sand (kg/m3)1350014011408962365.7519.251401117.04877.683346.538.51401112.44874.064327.2557.751401109871.64Figure 1
Concrete constituents (cement, sand, silica fume, fly ash, aggregate, and superplasticizer).Table 2
Compressive strength behavior of cubes.
MixPercentage replacement of silica fume (%)7 days strength (MPa)28 days strength (MPa)1024.1233.282525.4835.1431026.8937.6141525.0835.87


## 3. Experimental Procedure

### 3.1. Steel Tubular Columns

In this test program, all the hollow steel tubes (HSS) of circular cross-section with a constant diameter of 75 mm are provided from the local supplier with a thickness of 1.5 mm and 2 mm, with heights of 300 mm and 500 mm. The experimental test setup for composite column under loading is shown in Figure2. The SCFST columns are for determination of physical properties of HSS section, the coupon test is followed according to the American Standard Testing Materials (ASTM), the hollow section of the both thickness (2 specimens for each thickness) should be cut accordingly, and it should be inserted into the tensile compressive testing machine.Figure 2
Coupon test specimen dimensions as per ASTM.


### 3.2. Infilled Composite Column

Totally, 24 infilled circular composite columns are cast with two different heights 300 mm and 500 mm and with a constant diameter of 75 mm were tested, which includes both the thickness of 1.5 mm and 2 mm. Conventional concrete mix and silica fume concrete matrix mix were used as the infilled material. The specimens were chosen based on height and thickness; concrete was placed in the corresponding hollow steel tubular columns in three layers, where the one end is closed with a thin film sheet placed on the flat base plate and the other end is open. Figure3 shows the experimental test setup with two dial gauges to observe the lateral deflection of the specimen. Mix proportions for infilled concrete column values are given in Table 3.Figure 3
Test setup for the composite column.Table 3
Mix proportions for infilled concrete column.
MixCement (kg/m3)Silica fume (kg/m3)Water (kg/m3)Coarse aggregate (kg/m3)Manufacturing sand (kg/m3)Superplasticizer (kg/m3)1391.350156.54808.231140.027.822352.2239.13156.548021135.187.82


## 3.1. Steel Tubular Columns

In this test program, all the hollow steel tubes (HSS) of circular cross-section with a constant diameter of 75 mm are provided from the local supplier with a thickness of 1.5 mm and 2 mm, with heights of 300 mm and 500 mm. The experimental test setup for composite column under loading is shown in Figure2. The SCFST columns are for determination of physical properties of HSS section, the coupon test is followed according to the American Standard Testing Materials (ASTM), the hollow section of the both thickness (2 specimens for each thickness) should be cut accordingly, and it should be inserted into the tensile compressive testing machine.Figure 2
Coupon test specimen dimensions as per ASTM.


## 3.2. Infilled Composite Column

Totally, 24 infilled circular composite columns are cast with two different heights 300 mm and 500 mm and with a constant diameter of 75 mm were tested, which includes both the thickness of 1.5 mm and 2 mm. Conventional concrete mix and silica fume concrete matrix mix were used as the infilled material. The specimens were chosen based on height and thickness; concrete was placed in the corresponding hollow steel tubular columns in three layers, where the one end is closed with a thin film sheet placed on the flat base plate and the other end is open. Figure3 shows the experimental test setup with two dial gauges to observe the lateral deflection of the specimen. Mix proportions for infilled concrete column values are given in Table 3.Figure 3
Test setup for the composite column.Table 3
Mix proportions for infilled concrete column.
MixCement (kg/m3)Silica fume (kg/m3)Water (kg/m3)Coarse aggregate (kg/m3)Manufacturing sand (kg/m3)Superplasticizer (kg/m3)1391.350156.54808.231140.027.822352.2239.13156.548021135.187.82


## 4. Results and Discussion

### 4.1. Microstructural Studies

The material’s mechanical properties depend only on its microstructure, which is observed using scanning electron microscopy (SEM). SEM analyses are used to determine the material’s surface area. Scanning electron microscopy not only provides topographical but also the compositional study of the material. The raw powder of silica fume, harden conventional concrete powder, and silica fume concrete powder are viewed after 28 days under SEM analysis. Figure4 shows the SEM microstructural images of the silica fume powder sample in various magnifications such as 50 μm, 20 μm, 10 μm, 5 μm, and 2 μm. The silica fume particles observed are spheroid shapes, and the particle sizes are too small that varies from 0.18 μm to 0.26 μm as shown in Figure 4. Also, noticed that in the SEM images, the raw material of silica fume is closely packed with no pores [22]. Figure 5 shows SEM images of conventional concrete after 28 days of curing in various magnifications such as 50 μm, 20 μm, 10 μm, 5 μm, and 2 μm. From the images, the formation of denser concrete was observed. This shows the formation of calcium silicate hydrate (CHS), portlandite (CH), and pores found. Particle sizes for conventional concrete vary from 0.19 μm to 0.26 μm. Formation a highly dense microstructure of CSH-formed concrete with a small amount of CH and less porous spaces are observed for silica fume concrete after 28 days of curing as shown in Figure 6, a much denser and more compacted material compared to conventional concrete. The scale of particles varies from 0.18 to 0.32 μm, noticed that the influence of silica fume rises in compressive strength [27].Figure 4
SEM microstructure of the silica fume powder sample.Figure 5
SEM microstructure images of the conventional concrete.Figure 6
SEM microstructure images of the silica fume concrete.


### 4.2. XRD Analysis

The qualitative materials are analyzed using X-ray diffraction (XRD). Mostly, this method is used to define the crystalline material structure. The graph is drawn between intensity and a frequency of 2θ. The raw powder of silica fume, harden conventional concrete powder, and silica fume concrete powder after 28 days of curing are tested under XRD analysis. The silica fume powder XRD pattern is illustrated in Figure 7. It observed the quartz presence from silica fume powder XRD graph. The first point is 2θ = 21.02, the second peak is 2θ = 26.5, and the third peak is 2θ = 28.14.Figure 7
XRD image of conventional concrete after 28 days of curing.XRD results for the conventional concrete after 28 days of curing are shown in Figure7. The result shows the presence of quartz (Q), silicate calcium hydrate (CSH), silicate calcium (CS), calcite (C), and portlandite (P) peaks. The first peak is measured at 2θ = 21°, the presence of a quartz compound, the second peak at 2θ = 24°, the presence of a calcite compound, and the peaks are at 2θ between 26° and 27.5°, indicating the presence of quartz compound reveals that crystalline phases in concrete. Figure 8 shows the XRD analysis of the sample of silica fume concrete. It shows the formation of calcite component significantly increased in silica fume concrete when the peak angle 2θ = 25° to 27° compared to plain concrete. This reflects the presence of amorphous material in compounds called calcite (C), quartz (Q), and calcium silicate (CS). It also reflects calcium silicate hydrate (CSH) formation at an angle 2θ = 42.92 [27].Figure 8
XRD image of silica fume concrete after 28 days of curing.


### 4.3. FTIR Analysis

Figure9 shows the FTIR graph of the conventional concrete after 28 days of the curing period. The primary characteristic peaks of C-S-H may be found between 1081 cm−1 and 981 cm−1 (Si-O asymmetric stretching vibration). The peaks below 3500 cm−1 are caused by stretching vibrations of free molecular water or adsorbed water on the surface of the control concrete. The presence of portlandite (CH) is easily recognized by FTIR spectroscopy, which is well detected by a strong distinct peak at 3640 cm−1. The S-O stretching vibration of (SO4)2− is also identified at 1150–1100 cm−1 [28].Figure 9
FTIR analysis graph of conventional concrete after 28 days of curing.Figure10 shows the silica fume powder picture from FTIR. The strong bands at 681 cm−1, 724 cm−1, 871 cm−1, 981 cm−1, 1081 cm−1, 1100 cm−1, and 1200 cm−1 attribute the basic Si-O vibrations. They are identical in terms of silica modifications. The peaks between 1130 cm−1 and 1200 cm−1 are characteristics of Si-O-Si bonds (symmetric and asymmetric stretch). The presence of sodium alginate bands ranging from 800 cm−1 to 470 cm−1 are Si-O vibration absorption bands. The-OH stretching in silanol occurs generally as peaks above 3500 cm−1. The peak bands between 3441 cm−1 and 3647 cm−1 indicate the presence of silica, while monohydrogen bond occurs between the bandwidths 3600 cm−1 and –3400 cm−1). The closeness of these peaks generally results in spectra that are heavily overlapped. The peak 981 cm−1 indicates the occurrence on calcium silicate hydrate [29, 30].Figure 10
FTIR analysis graph of silica fume concrete after 28 days of curing.


### 4.4. Composite Column Test

The measured axial load capacity for the 24 concrete composite columns of both lengths and both thicknesses with a constant diameter are presented in Table4, as an average of 3 specimens for each mix. From the experimental results, it was clear that the replacement of silica fume with cement in concrete enhances the strength of concrete specimen after 28 days water curing. It was that the axial loading capacity increases with an increase in the thickness.Table 4
Composite column specimens and test results.
S. noPercentage replacement of silica fume (%)Thickness (mm)Height (mm)Diameter (mm)Load carrying capacity (kN)101.530075335.52101.530075360.38301.550075301.554101.550075332.4250230075386.2561023007541670250075382.66810250075403.18


### 4.5. Mode of Failure

Figure11 shows the mode of failure for the coupon specimens after the tensile test; it was observed that failure occurred at the web portion of the specimen. Also, it was observed that there was a change in the area of the specimen after the test. Figure 12 shows the failure of the composite columns of 1.5 mm thickness and failure of the composite columns of 2 mm thickness. In both the cases, it observed the outward buckling at the middle portion of the column for 300 mm height specimens (both thicknesses). As the height of the column increases, it observed a shear failure at top and bottom with outward buckling at middle for 1.5 mm thick columns. The thickness increases the shear failure at the one end of the column.Figure 11
Mode of failure for coupon specimens 1.5 mm and 2 mm thick.
(a)(b)Figure 12
Mode of failure for composite columns of 1.5 mm and 2 mm steel tube thickness.
(a)(b)


## 4.1. Microstructural Studies

The material’s mechanical properties depend only on its microstructure, which is observed using scanning electron microscopy (SEM). SEM analyses are used to determine the material’s surface area. Scanning electron microscopy not only provides topographical but also the compositional study of the material. The raw powder of silica fume, harden conventional concrete powder, and silica fume concrete powder are viewed after 28 days under SEM analysis. Figure4 shows the SEM microstructural images of the silica fume powder sample in various magnifications such as 50 μm, 20 μm, 10 μm, 5 μm, and 2 μm. The silica fume particles observed are spheroid shapes, and the particle sizes are too small that varies from 0.18 μm to 0.26 μm as shown in Figure 4. Also, noticed that in the SEM images, the raw material of silica fume is closely packed with no pores [22]. Figure 5 shows SEM images of conventional concrete after 28 days of curing in various magnifications such as 50 μm, 20 μm, 10 μm, 5 μm, and 2 μm. From the images, the formation of denser concrete was observed. This shows the formation of calcium silicate hydrate (CHS), portlandite (CH), and pores found. Particle sizes for conventional concrete vary from 0.19 μm to 0.26 μm. Formation a highly dense microstructure of CSH-formed concrete with a small amount of CH and less porous spaces are observed for silica fume concrete after 28 days of curing as shown in Figure 6, a much denser and more compacted material compared to conventional concrete. The scale of particles varies from 0.18 to 0.32 μm, noticed that the influence of silica fume rises in compressive strength [27].Figure 4
SEM microstructure of the silica fume powder sample.Figure 5
SEM microstructure images of the conventional concrete.Figure 6
SEM microstructure images of the silica fume concrete.


## 4.2. XRD Analysis

The qualitative materials are analyzed using X-ray diffraction (XRD). Mostly, this method is used to define the crystalline material structure. The graph is drawn between intensity and a frequency of 2θ. The raw powder of silica fume, harden conventional concrete powder, and silica fume concrete powder after 28 days of curing are tested under XRD analysis. The silica fume powder XRD pattern is illustrated in Figure 7. It observed the quartz presence from silica fume powder XRD graph. The first point is 2θ = 21.02, the second peak is 2θ = 26.5, and the third peak is 2θ = 28.14.Figure 7
XRD image of conventional concrete after 28 days of curing.XRD results for the conventional concrete after 28 days of curing are shown in Figure7. The result shows the presence of quartz (Q), silicate calcium hydrate (CSH), silicate calcium (CS), calcite (C), and portlandite (P) peaks. The first peak is measured at 2θ = 21°, the presence of a quartz compound, the second peak at 2θ = 24°, the presence of a calcite compound, and the peaks are at 2θ between 26° and 27.5°, indicating the presence of quartz compound reveals that crystalline phases in concrete. Figure 8 shows the XRD analysis of the sample of silica fume concrete. It shows the formation of calcite component significantly increased in silica fume concrete when the peak angle 2θ = 25° to 27° compared to plain concrete. This reflects the presence of amorphous material in compounds called calcite (C), quartz (Q), and calcium silicate (CS). It also reflects calcium silicate hydrate (CSH) formation at an angle 2θ = 42.92 [27].Figure 8
XRD image of silica fume concrete after 28 days of curing.


## 4.3. FTIR Analysis

Figure9 shows the FTIR graph of the conventional concrete after 28 days of the curing period. The primary characteristic peaks of C-S-H may be found between 1081 cm−1 and 981 cm−1 (Si-O asymmetric stretching vibration). The peaks below 3500 cm−1 are caused by stretching vibrations of free molecular water or adsorbed water on the surface of the control concrete. The presence of portlandite (CH) is easily recognized by FTIR spectroscopy, which is well detected by a strong distinct peak at 3640 cm−1. The S-O stretching vibration of (SO4)2− is also identified at 1150–1100 cm−1 [28].Figure 9
FTIR analysis graph of conventional concrete after 28 days of curing.Figure10 shows the silica fume powder picture from FTIR. The strong bands at 681 cm−1, 724 cm−1, 871 cm−1, 981 cm−1, 1081 cm−1, 1100 cm−1, and 1200 cm−1 attribute the basic Si-O vibrations. They are identical in terms of silica modifications. The peaks between 1130 cm−1 and 1200 cm−1 are characteristics of Si-O-Si bonds (symmetric and asymmetric stretch). The presence of sodium alginate bands ranging from 800 cm−1 to 470 cm−1 are Si-O vibration absorption bands. The-OH stretching in silanol occurs generally as peaks above 3500 cm−1. The peak bands between 3441 cm−1 and 3647 cm−1 indicate the presence of silica, while monohydrogen bond occurs between the bandwidths 3600 cm−1 and –3400 cm−1). The closeness of these peaks generally results in spectra that are heavily overlapped. The peak 981 cm−1 indicates the occurrence on calcium silicate hydrate [29, 30].Figure 10
FTIR analysis graph of silica fume concrete after 28 days of curing.


## 4.4. Composite Column Test

The measured axial load capacity for the 24 concrete composite columns of both lengths and both thicknesses with a constant diameter are presented in Table4, as an average of 3 specimens for each mix. From the experimental results, it was clear that the replacement of silica fume with cement in concrete enhances the strength of concrete specimen after 28 days water curing. It was that the axial loading capacity increases with an increase in the thickness.Table 4
Composite column specimens and test results.
S. noPercentage replacement of silica fume (%)Thickness (mm)Height (mm)Diameter (mm)Load carrying capacity (kN)101.530075335.52101.530075360.38301.550075301.554101.550075332.4250230075386.2561023007541670250075382.66810250075403.18


## 4.5. Mode of Failure

Figure11 shows the mode of failure for the coupon specimens after the tensile test; it was observed that failure occurred at the web portion of the specimen. Also, it was observed that there was a change in the area of the specimen after the test. Figure 12 shows the failure of the composite columns of 1.5 mm thickness and failure of the composite columns of 2 mm thickness. In both the cases, it observed the outward buckling at the middle portion of the column for 300 mm height specimens (both thicknesses). As the height of the column increases, it observed a shear failure at top and bottom with outward buckling at middle for 1.5 mm thick columns. The thickness increases the shear failure at the one end of the column.Figure 11
Mode of failure for coupon specimens 1.5 mm and 2 mm thick.
(a)(b)Figure 12
Mode of failure for composite columns of 1.5 mm and 2 mm steel tube thickness.
(a)(b)


## 5. Conclusions

(i)
Optimum partial replacement is 10%. Compressive strength varies by 11.51% from normal mix to silica fume that replaced concrete after 28 days curing.(ii)
XRD results confirm the presence of quartz compound which helps in strengthening of concrete mix and forms a crystalline phase.(iii)
SEM analysis, it was observed that the microstructure gives the disintegration of Si and Al compounds. FTIR confirms strengthening of concrete mix.(iv)
Peak load of the normal concrete infilled column and silica fume concrete column with height 300 mm and 500 mm varies with 17.88% and 34.77%, respectively.(v)
Peak load of the hollow steel column with height 300 mm, 1.5 mm thickness, and 2 mm thickness varies with 8.04% and hollow steel column with height 500 mm, 1.5 mm thickness, and 2 mm thickness varies with 29.58%, respectively.(vi)
SCFST column strength capacity of 1.5 mm thick, 300 mm height varies with CFST column by 7.5%. For 1.5 mm thick, 500 mm height SCFST column varies with the CFST column by 9.3%. The strength capacity is 2 mm thick, and 300 mm height SCFST column varies with CFST column by 7.15%. For 2 mm thick, 500 mm height SCFST column varies with the CFST column by 5%.(vii)
Shear failure and outward buckling mode of failures are observed for long and short composite columns, respectively.



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*Source: 3632991-2021-07-31.xml*