# Behavior of Hollow Thin Welded Tubes Filled with Sand Slag Concrete

Authors: Noureddine Ferhoune; Meriem Senani; Abdelhamid Guettala
Journal: Advances in Materials Science and Engineering
 (2016)

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## Abstract

This paper presents the axial bearing capacity of thin welded rectangular steel stubs filled with concrete sand. A series of tests was conducted to study the behavior of short composite columns under axial compressive load; the cross section dimensions were 100 × 70 × 2 mm. A total of 20 stubs have been tested, as follows: 4 hollow thin welded tubes were tested to axial and eccentric load compression, 4 were filled with ordinary concrete appointed by BO columns, 6 were filled with concrete whose natural sand was completely substituted by a crystallized sand slag designated in this paper by BSI, and 6 were tucked in concrete whose natural sand was partially replaced by a crystallized sand slag called BSII. The main parameters studied are the height of the specimen (300 mm–500 mm), eccentricity of load and type of filling concrete. Based on test results obtained, it is confirmed that the length of the tubes has a considerable effect on the bearing capacity and the failure mode. In all test tubes, fracture occurred by the convex local buckling of steel section due to the outward thrust of the concrete; it was observed that the sand concrete improves the bearing capacity of tubes compounds compared to those filled with ordinary concrete.


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## Body

## 1. Introduction

Concrete-filled steel tube columns have many advantages in terms of their high strength, high ductility, high stiffness, and full usage of construction materials [1–3]. The problem of the behavior of these columns was the subject of several studies in different countries and attracted the attention of many researchers. From all the research conducted, it was found that the type of filling concrete directly affects the capacity and behavior of composite columns [4–8]. Very few experiments are done on built up cold-formed welded steel sections filled with concrete or recycled materials such as slag stone concrete designated here by SSC. The slag stone concrete has been tested under direct compression and was used as a filling material to overcome the undesired effects of imperfections of built up cold-formed sections. The gain in strength was found to reach a value of up to 2 and decreased linearly with the stubs height [9]. The characterization of slag sand concrete has been the subject of some studies; the research works approved the possibility to use the crystallized slag as sand in the composition concrete which may have a competitive performance to conventional concrete [10–12]. The use of slag sand concrete as stuffing materials in steel tubes can improve this behavior, and the use of blast furnace waste as substitute material helps to save a significant share of natural resources and protect the environment. The present work is a contribution to understanding the behavior of hollow thin welded tubes filled with sand slag concrete subjected to axial and eccentric compression.


## 2. Experimental Program

A series of tests was conducted to study the behavior of short composite columns under axial compressive load; the cross section dimensions were 100 × 70 × 2 mm, the detail of the section is shown in Figure1. A total of 20 stubs were tested, as follows: 4 rectangular empty tubes were tested to axial and eccentric load compression, 4 were filled with ordinary concrete, 6 were filled with a concrete substitute natural sand that was completely crystallized by a sand slag, and 6 others were filled with sand slag concrete or natural sand that was partially replaced by a crystallized sand slag. The main objectives of these tests were to clarify the mechanical performance of steel section filled by concrete sand compared to those filled with ordinary concrete. The main parameters studied are the height of the specimen (300 mm–500 mm), type of the applied load (axial or eccentric), and type of concrete filling; the composition of the filling concretes is presented in Table 1.Table 1
Concrete composition used for filling rectangular steel profiles.
Compositions
Unit
BO
BSI
BSII
Cement CPJ 42.5
Kg/m3
350
300
300
Water/cement
Calculated
—
0.5
0.81
0.61
Real
0.55
0.88
0.88
Sand dune 0/2.5
Kg/m3
725.59
—
500
Sand slag granulated 0/3.15
Kg/m3
—
1540
1275
Gravel 5/12.5
Kg/m3
1070.67
—
—Figure 1
Thin cold-formed and welded steel cross section filled with concrete details.


### 2.1. Materials Fabrication

The properties of material fabrication of concrete core are shown in Table1. The steel used for the manufacture of the section is U-profiles of cold rolled thickness 2 mm whose mechanical characteristics are as follows:(i)
Young modulus Ea = 205000 MPa.
(ii)
Yield stressσ
e
=
300 MPa.The rectangular welded steel columns filled with three types of concrete were cast vertically in three layers, each compacted on a vibrating table for 2 to 3 minutes to achieve more even concrete along the length of the specimen. All composite specimens were left in the curing room at an ambient temperature (T
=
20
–
25
C
° and humidity rate = 60–70%) for a period of 28 days. Both top and bottom faces of composite stubs were mechanically treated to remove surface irregularities and ensure that both steel and concrete are loaded during test. The 28 days’ mean stress of ordinary concrete is 20 MPa, 22.4 Mpa for BSI’ and 19 Mpa for BSII concrete mix.


### 2.2. Physical Properties of Crystallized Sand Slag

The crystallized slag sand use in the composition of concrete is from blast furnace of the El Hadjar steel complex, Algeria. The chemical compositions and the physical properties of the sands and gravel are presented in Tables2 and 3.Table 2
Physical properties of sands.
Sand type
Apparent density(kg/m3)
Specific density (kg/m3)
Porosity in percentage
Fines modulus (MF)
Water content in percentage
Sand equivalent in percentage
Dune sand (0/2.5 mm)
1610
2650
40
2.21
90.52
Crystallized sand slag (0/3.15 mm)
1570
2630
41
3.1
0.35
88.52Table 3
Physical properties of gravel.
Gravel type
Apparent density (kg/m3)
Specific density (kg/m3)
Porosity in percentage
Water content in percentage
Cleanness value
Absorption coefficientin percentage
Gravel (5/12.5 mm)
1300
2500
48
0.79
2
2.55


### 2.3. Test Rig and Procedure

All specimens were tested in a 500 kN compressive machine. Special attention was given to verifying the correct position of the stubs before any loading. The load was applied on the composite section (concrete and steel). Figure1 shows the detail of columns tested in this work. Both top and bottom faces of composite stubs were mechanically treated to remove surface irregularities. The top and bottom surfaces of the concrete-filled cold-formed steel tube columns were restrained against all degrees of freedom, except for the displacement at the loaded end, which is the top surface, in the direction of the applied load.


## 2.1. Materials Fabrication

The properties of material fabrication of concrete core are shown in Table1. The steel used for the manufacture of the section is U-profiles of cold rolled thickness 2 mm whose mechanical characteristics are as follows:(i)
Young modulus Ea = 205000 MPa.
(ii)
Yield stressσ
e
=
300 MPa.The rectangular welded steel columns filled with three types of concrete were cast vertically in three layers, each compacted on a vibrating table for 2 to 3 minutes to achieve more even concrete along the length of the specimen. All composite specimens were left in the curing room at an ambient temperature (T
=
20
–
25
C
° and humidity rate = 60–70%) for a period of 28 days. Both top and bottom faces of composite stubs were mechanically treated to remove surface irregularities and ensure that both steel and concrete are loaded during test. The 28 days’ mean stress of ordinary concrete is 20 MPa, 22.4 Mpa for BSI’ and 19 Mpa for BSII concrete mix.


## 2.2. Physical Properties of Crystallized Sand Slag

The crystallized slag sand use in the composition of concrete is from blast furnace of the El Hadjar steel complex, Algeria. The chemical compositions and the physical properties of the sands and gravel are presented in Tables2 and 3.Table 2
Physical properties of sands.
Sand type
Apparent density(kg/m3)
Specific density (kg/m3)
Porosity in percentage
Fines modulus (MF)
Water content in percentage
Sand equivalent in percentage
Dune sand (0/2.5 mm)
1610
2650
40
2.21
90.52
Crystallized sand slag (0/3.15 mm)
1570
2630
41
3.1
0.35
88.52Table 3
Physical properties of gravel.
Gravel type
Apparent density (kg/m3)
Specific density (kg/m3)
Porosity in percentage
Water content in percentage
Cleanness value
Absorption coefficientin percentage
Gravel (5/12.5 mm)
1300
2500
48
0.79
2
2.55


## 2.3. Test Rig and Procedure

All specimens were tested in a 500 kN compressive machine. Special attention was given to verifying the correct position of the stubs before any loading. The load was applied on the composite section (concrete and steel). Figure1 shows the detail of columns tested in this work. Both top and bottom faces of composite stubs were mechanically treated to remove surface irregularities. The top and bottom surfaces of the concrete-filled cold-formed steel tube columns were restrained against all degrees of freedom, except for the displacement at the loaded end, which is the top surface, in the direction of the applied load.


## 3. Results of Rectangular Empty Stubs Tests

The load carrying capacities of tested empty steel tubes V1 and V3 tested under concentric load are, respectively, varied from 146 and 138 kN as shown in Table4; after test, we noted that the large sides of section buckled outwards in contrary to small side. In the case of hollow thin walled tubes V2 and V4 tested to eccentric load, the load carrying capacity decreases by a rate of 9.58% and 10.14%, respectively, compared to V1 and V3 tubes. The phenomenon observed of the two tested tubes is always local buckling as shown in Figure 2.Table 4
Experimental results of rectangular empty stubs.
Concrete type
Specimen
Height (mm)
H
×
B
×
t (mm)
Eccentricity (mm)
Nue (kN)
Empty steel tubes
V1
300
101.30 × 69.80 × 2.30
0
146
V2
500
100.10 × 69.70 × 2.20
0
138
V3
300
99.80 × 70.10 × 2.30
20
132
V4
500
100.10 × 69.40 × 2.20
20
124Figure 2
Failure mode of empty tubes.


## 4. Results of Rectangular Filled Stubs Tests

The test results on the tubes filled with concrete granulated slag (BSI and BSII), made 28 days after the concrete casting date, showed the advantage of these types of concrete as tamper member with respect to ordinary concrete (BO). All test results are presented in Table5. The load carrying capacity varied from 315 kN to 385 kN; the test results of the columns subject to eccentric load confirm the diminution of load carrying capacity when the eccentricity load and stubs height increase.Table 5
Experimental results of composite columns.
Concrete type
Specimen
Height (mm)
H
×
B
×
t (mm)
Eccentricity (mm)
Nue (kN)
BO
PA1-1
300
100.20 × 70.10 × 2.30
0
341
PB1-2
300
99.90 × 68.75 × 2.20
20
320
PA1-3
500
99.50 × 69.10 × 2.20
0
330
PB1-4
500
99.55 × 68.75 × 2.20
20
315
BSI
PA2-1
300
99.75 × 69.50 × 2.20
0
385
PB2-2
300
99.62 × 68.06 × 2.20
20
359
PB2-3
300
99.62 × 69.55 × 2.25
20
363
PA2-4
500
99.91 × 69.13 × 2.30
0
368
PB2-5
500
98.58 × 69.75 × 2.15
20
353
PB2-6
500
99.37 × 69.10 × 2.20
20
357
BSII
PA3-1
300
99.72 × 68.50 × 2.30
0
374
PB3-2
300
99.82 × 69.55 × 2.20
20
351
PB3-3
300
98.37 × 69.10 × 2.20
20
349
PA3-4
500
100.50 × 68.55 × 2.30
0
365
PB3-5
500
100.63 × 68.75 × 2.30
20
344
PB3-6
500
98.60 × 69.85 × 2.30
20
341In Figure3, we see that the tubes subjected to an axial load with 300 mm height, filled by sand slag concrete and BSI BSII, give an ultimate load greater than that of columns (BO). The percentage of increase in load is, respectively, 11.43% and 08.82%. About 500 mm tubes filled with granulated slag concrete BSI and BSII have increased ultimate load of respectively, 10.33% and 9.59%, relative to the tube filled with normal concrete (BO).Figure 3
Influence of concrete core type on the bearing capacity under axial load.These test results on the laminated steel tubes welded and cold and thin walls under axial load showed the advantage of the use of slag sand concrete as packing member. Also the height of the tubes has a considerable effect on the bearing capacity; we note that the bearing capacity decreases with increasing the height of the tubes subjected to an axial load whatever the type of filling concrete.Figure4 shows the experimental ultimate load tubing made under eccentric loading (e
=
20 mm). The ultimate load of the stubs height 300 mm filled by sand BSI concretes and BSII increases, respectively, by the order of 11.36% and 8.57%, compared to load carrying capacity of column BO and to the tubes with 500 mm height increase of approximately 11.36% for BSI and 08.03% for BSII column compared to BO tube. After comparing the results of different bearing capacity obtained experimentally, we see that the rate of eccentric load has an important influence on the bearing capacity and local buckling forming position. It may be noted that when the eccentricity and stubs height increase the load decreases, as shown in Figures 5 and 6.Figure 4
Influence of concrete core type on the bearing capacity under eccentric load.Figure 5
Influence of the eccentricity of load on the bearing capacity (case of the stubs height 300 mm).Figure 6
Influence of the eccentricity of load on the bearing capacity (case of the stubs height 500 mm).Figure7 shows clearly that the EC4 provides superior load capacity of 1
%
÷
2
% for ordinary concrete-filled tubes under axial loading for stubs height 300 mm and 500 mm, respectively, and provides a capacity of 0.7% and 0.9%, respectively, for the tubes of height 300 mm filled with the granulated slag concrete BSI and BSII. The Euro code 4 gives a good agreement for hollow tubes filled in the case three concrete type.Figure 7
Experimental load Nue and EC4 load Nuc for the tubes height 500 mm.The comparison of load carrying capacity between filled and empty steel tubes clarifies the benefit of filling hollow thin welded tubes by sand slag concrete as shown in Figure8. The rate of increase load is more than twice of empty steel tubes load, which proves that the filling concrete contributes significantly to the increases of bearing capacity and is delaying the problem of instability of these columns.Figure 8
Comparison of load carrying capacity between empty and filled stubs.


## 5. Plastic Resistance of Composite Columns according to Euro Code 4

EC3 and EC4 are the most recently completed international standards in steel construction and composite construction, respectively. EC4 provided a simplified method of design for composite columns, which was based on the European buckling curves for the influence of instability and on cross section interaction curves determining the column section’s resistance. EC4 covers concrete encased and partially encased steel sections and concrete-filled sections with or without reinforcement. Design procedure of the composite columns considers the second-order effects including imperfections and ensures that under the most unfavorable combinations of actions at the ultimate limit state, instability does not occur. The plastic resistance to compressionN
p
l
,
R
d of a composite cross section was calculated by adding the plastic resistance of its components:(1)
N
p
l
,
R
d
=
A
a
f
y
γ
M
a
+
A
c
0.85
f
c
k
γ
c
+
A
s
f
s
k
γ
s
,where A
a, A
c, and A
s are the cross-sectional areas of structural steel, concrete, and reinforcement, respectively; f
y, f
c
k, and f
s
k are, respectively, the yield stress of steel cross section, the strengths of concrete, and the yield stress of steel reinforcement; γ
M
a, γ
c, and γ
s are partial safety factors at the ultimate limit states. The results of load carrying capacity according EC4 are presented in Table 6.Table 6
Load of composite columns according to Euro code 4 compared to experimental load.
Concrete type
Specimen
Stubs height (mm)
Nuc (EC4) (kN)
Nuc/Nue
BO
PA1-1
300
338.23
0.99
PB1-2
300
310.1
0.96
PA1-3
500
323.79
0.98
PB1-4
500
312.30
0.99
BSI
PA2-1
300
360.05
0.93
PB2-2
300
353.50
0.98
PB2-3
300
360.30
0.99
PA2-4
500
367.07
0.99
PB2-5
500
342.80
0.97
PB2-6
500
351.66
0.98
BSII
PA3-1
300
343.10
0.91
PB3-2
300
353.40
1.01
PB3-3
300
351.73
1.01
PA3-4
500
345.30
1.00
PB3-5
500
333.85
0.97
PB3-6
500
314.80
0.92


## 6. Design Method Proposed by Vrcelj and Uy [13]

Vrcelj and Uy [13] proposed a design method for the strength calculation of a slender concrete-filled steel box column loaded in compression, when the steel portion of the composite column is affected by local buckling. The following relationship is proposed by Vrcelj and Uy [13]:(2)
N
c
l
b
=
α
1
b
N
c
,where N
c
l
b is the slender column buckling load which incorporates local buckling, N
c is the column buckling load, and α
1
b is the interaction coefficient to account for local buckling and is in the range 0
≤
α
1
b
≤
1.0.α
1
b is calculated as(3)
α
1
b
=
100
-
ρ
r
100
,where the percentage reduction ρ
r is given as(4)
ρ
r
=
N
c
-
N
c
l
b
N
c
×
100
.The results of estimated load carrying capacity of different columns by this above design method are presented in Table7.Table 7
Load of composite columns according to Vrcelj method.
Concrete type
Specimen
Stubs height (mm)
Nu (Vrcelj) (kN)
Nu (Vrcelj)/Nue
BO
PA1-1
300
343.33
1.01
PB1-2
300
318.41
0.99
PA1-3
500
328.22
0.99
PB1-4
500
298.43
0.94
BSI
PA2-1
300
367.03
0.95
PB2-2
300
353.82
0.98
PB2-3
300
352.64
0.97
PA2-4
500
354.56
0.96
PB2-5
500
344.75
0.97
PB2-6
500
345.62
0.96
BSII
PA3-1
300
354.14
0.94
PB3-2
300
319.86
0.91
PB3-3
300
332.78
0.95
PA3-4
500
354.23
0.97
PB3-5
500
321.10
0.93
PB3-6
500
314.64
0.92


## 7. Breakage Mechanism of Filled Tubes

The failure mode of rectangular hollow tubes filled with three types of concrete study in this work is the local buckling of steel section; the steel section is deformed in a convex manner, as shown in Figures7 and 8. The local buckling took place in all samples with a small attenuation for longer steel stubs. Both large and small sides buckled outwards. We also noted a breakdown of some welding that occurred at the local buckling formation region has reached the maximum load as shown in Figures 9 and 10.Figure 9
Failure mode of BO and BSI columns.
(a)
BO column, eccentricitye
=
0
(b)
BO column, eccentricitye
=
20 mm
(c)
BSI column, eccentricitye
=
0
(d)
BSI column, eccentricitye
=
20 mmFigure 10
Failure mode of BSII column.
(a)
BSII column, eccentricitye
=
0
(b)
BSI column, eccentricitye
=
20 mm


## 8. Discussion

To investigate the behavior and design of axially loaded concrete sand filled with cold-formed steel tube columns, we present in this work the details of an experimental study of the compressive behavior and axial bearing capacity of thin welded rectangular steel stub columns filled with concrete sand. A comparison between experimental column load strength, load carrying capacity calculated according Euro code 4 designs [14], and design method proposed by Vrcelj and Uy [13] of different columns is exposed. Results of load carrying capacity calculated by the two methods cited above show that both methods under-estimate the load carrying capacity compared with experimental results for almost all columns tested in this work. This underestimation of load imposes an acceptable safe level for estimating the bearing capacity of columns, which allows us to be on the safe side. The local buckling forming position varies from column to another but is generally around 1/3 of column height counted from top surface in the case of columns tested to axial load. In the case of columns tested to eccentric load, the position of local buckling is at 1/4 of column height in the direction of eccentric load, and at around 1/6 of stubs height in the opposite direction. Welding opening signs and a concrete breakdown were seen in the local buckling formation areas.


## 9. Conclusion

This study has highlighted the axial and eccentric behavior of thin steel tubes welded and filled with three types of concrete: regular concrete appointed by BO, sand slag concrete made from 100% crystal sand slag designated by BSI, and concrete sand slag used in the concrete composition partially designated by BSII. After analyzing the results, we can highlight the following. Results show that the column strengths increase as the stubs height decreases. We see in all columns tested in this work the sign of local buckling developed when the ultimate bearing capacity is reached causing the instability of columns. The load carrying capacity iscalculated according Eurocode 4 prediction [14] and design method developed by Vrcelj and Uy [13](i)
Filling the hollow tubes by various types of sand slag concretes (BSI or BSII) improved behavior and increased bearing capacity relative to the empty steel tubes and tubes filled with BO concrete.
(ii)
The bearing capacity decreases when slenderness increases.
(iii)
The bearing capacity of composite stubs is strongly influenced by the load eccentricity in the case of three types of BO concrete, BSI, and BSII.
(iv)
The posts of the failure mode in the case of three types of concrete used are a way of convex local buckling accompanied by a burst of weld welding buckling training level.
(v)
The bearing capacity calculated by the Euro code 4 and design Vrcelj method are an acceptable approach compared to that recorded experimentally.



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