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Experimental behaviour of steel fiber high strength reinforced concrete and composite columns
Journal of Constructional Steel Research 74 (2012) 98–107
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Journal of Constructional Steel Research
Experimental behaviour of steel fiber high strength reinforced concrete and composite columns Serkan Tokgoz a,⁎, Cengiz Dundar b, A. Kamil Tanrikulu b a b
Civil Engineering, Mersin University, 33340 Mersin, Turkey Civil Engineering, Cukurova University, 01330 Adana, Turkey
a r t i c l e
i n f o
Article history: Received 16 March 2011 Accepted 28 February 2012 Available online 27 March 2012 Keywords: Reinforced concrete column Concrete-encased composite column Steel fiber Ultimate strength
a b s t r a c t This paper presents experimental behaviour of eccentrically loaded plain and steel fiber high strength reinforced concrete and concrete-encased composite columns. In the experimental study, a total of 32 square section both reinforced concrete and composite column specimens were fabricated at 0, 0.5, 0.75 and 1.0% volume fractions of steel fiber contents to examine the effects of steel fibers on column behaviour. Besides this, the composite columns were constructed and tested using almost the same conditions with reinforced concrete columns to investigate the column experimental behaviour. The complete load−deflection behaviour and strength of column specimens were obtained and the results were discussed in the study. In addition, the column specimens were analysed based on a theoretical method considering the nonlinear behaviour of the materials. The presented experimental study indicates that the inclusion of steel fibers in the range 0.75 to 1.0% volume fraction improves confinement and ductility features of high strength reinforced concrete and composite columns significantly. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction High strength concrete has been increasingly used in the construction of structures, such as high-rise buildings, bridges, piles etc. High strength concrete offers many significant benefits in terms of strength, durability, and modulus of elasticity. However, it is widely believed that high strength concrete exhibits brittle behaviour under compression. The inclusion of steel fibers into high strength concrete definitely improves confinement, ductility and deformability of concrete. Several experimental and analytical studies were carried out to describe the mechanical behaviour of steel fiber high strength concrete. Fanella and Naaman [1] studied on the stress–strain properties of fiber reinforced concrete and an analytical relationship was proposed to predict the complete stress–strain curve of fiber reinforced mortar in compression. Ezeldin and Balaguru [2] presented experimental stress–strain behaviour of fiber reinforced concrete with compressive strength ranging from 35 MPa to 85 MPa. An analytical expression was proposed to represent the complete stress–strain curve of steel fiber reinforced concrete. Hsu and Hsu [3] conducted an experimental research to determine the complete stress–strain relationship of steel fiber high strength reinforced concrete under compression and empirical stress–strain equations were proposed in the study. Taerwe and Van Gysel [4] presented
⁎ Corresponding author at: Civil Engineering, Mersin University, Engineering Faculty, Civil Engineering Department, 33340, Mersin, Turkey. Tel.: + 90 324 361 00 33; fax: + 90 324 361 00 32. E-mail addresses: [email protected] (S. Tokgoz), [email protected] (C. Dundar), [email protected] (A.K. Tanrikulu). 0143-974X/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2012.02.017
experimental and analytical researches to describe the realistic stress– strain curve for high strength fiber concrete. Maalej and Lok [5] examined the flexural behaviour of steel fiber concrete. Nataraja et al. [6] proposed a simple analytical model to generate both ascending and descending portions of the stress–strain curve of steel fiber reinforced concrete. Ramesh et al. [7] tested prism specimens to study the behaviour of confined steel fiber reinforced concrete and an analytical model was suggested to predict the stress–strain behaviour of confined fiber reinforced concrete. Lim and Nawy [8] investigated the mechanical characteristics of plain and steel fiber reinforced high strength concrete under uniaxial and biaxial loading conditions. Thomas and Ramaswamy [9] reported an experimental program and analytical assessments of the influence of addition of fibers on the mechanical properties of concrete. Empirical relationships were developed to assess the strength properties of steel fiber reinforced concrete. Bencardino et al. [10] researched the stress–strain behaviour of steel fiber reinforced concrete in compression and the validity of the models proposed in literature in defining the post peak behaviour of steel fiber concrete was examined. Ductility and confinement are very important features for high strength concrete column members especially in the seismically active regions. Therefore, using steel fibers into high strength concrete columns has become popular. It is significant to describe the behaviour of such members for analysis and design. Ganesan and Ramana Murthy [11] conducted experimental research to describe the behaviour of steel fiber reinforced concrete columns under axial load. Hsu et al. [12] and Foster and Attard [13] tested square section steel fiber high strength reinforced concrete columns to investigate the effects of steel fibers on the strength and ductility of columns. Foster
S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
125 mm
125 mm
37.5 50
25
25
Plain concrete column
y
37.5
8 mm 37.5
5
125 mm PC
25
Longitudinal reinforcement
99
50
x
5
37.5
25 25
25
125 mm
25 25 (SCC−0, CC1−0, CC2−0 and CC3−0)
(SC−0, C1−0, C2−0 and C3−0)
s L
Column section
A
A
125 mm 125 mm
Lateral reinforcement
37.5 50 25
25
Steel fiber column
y
125 mm
25
PC
37.5
37.5
5 5
50
x
25
25
(SC−I,II,III, C1−I,II,III, C2−I,II,III and C3−I,II,III)
125 mm
37.5
25 25
8 mm
25
(SCC−I,II,III, CC1−I,II,III, CC2−I,II,III and CC3−I,II,III)
Section A−A Fig. 1. Details of column specimens. Table 1 Concrete composition of the column specimens. Specimen Gravel no. (kg/m3)
Sand (kg/m3)
Cement (kg/m3)
Water (kg/m3)
Plasticizer (kg/m3)
Steel fiber (kg/m3)
SC-0 SC-I SC-II SC-III SCC-0 SCC-I SCC-II SCC-III C1-0 C2-0 C3-0 C1-I C2-I C3-I C1-II C2-II C3-II C1-III C2-III C3-III CC1-0 CC2-0 CC3-0 CC1-I CC2-I CC3-I CC1-II CC2-II CC3-II CC1-III CC2-III CC3-III
650 640 630 620 650 640 630 620 720 700 650 710 690 640 720 700 630 720 690 620 720 700 650 710 690 640 720 700 630 720 690 630
420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420
125 125 125 125 125 125 125 125 145 135 125 145 135 125 145 135 125 145 135 125 145 135 125 145 135 125 145 135 125 145 135 125
15 15 15 15 15 15 15 15 5 8 15 5 8 15 5 8 15 5 8 15 5 8 15 5 8 15 5 8 15 5 8 15
− 39.25 58.88 78.50 − 39.25 58.88 78.50 − − − 39.25 39.25 39.25 58.88 58.88 58.88 78.50 78.50 78.50 − − − 39.25 39.25 39.25 58.88 58.88 58.88 78.50 78.50 78.50
1100 1100 1100 1100 1100 1100 1100 1100 1120 1120 1100 1120 1120 1100 1100 1100 1100 1100 1100 1100 1120 1120 1100 1120 1120 1100 1100 1100 1100 1100 1100 1100
[14] proposed a model to determine the quantity of effective fibers to ensure a good level of ductility for high strength reinforced concrete columns. Tokgoz [15] conducted experimental study to describe the biaxially loaded steel fiber high strength reinforced concrete columns. In addition to reinforced concrete columns, it is known that composite columns with symmetrical and unsymmetrical cross sections have been employed in high rise buildings, bridges and earthquake resistant structures. Composite columns provide effective stiffness and load carrying capacity and also prevent local buckling effect due to the contribution of structural steel material. Many experimental and theoretical studies were carried out to determine the behaviour of composite columns in the past years. Research outcomes of the most of these works were notified in the previous studies [16,17]. Additional theoretical researches to determine the behaviour of plain concrete composite columns using finite element method were also performed by Liang [18], Ellobody et al. [19], Ellobody and Young [20] and Young and Ellobody [21]. In these studies, the suggested theoretical methods were verified with design rules [22–24]. Apart from the highlighted studies, Tokgoz and Dundar [25] examined the influence of steel fibers on the experimental behaviour of L-shaped section high strength reinforced concrete and composite columns. Very limited experimental data mainly on steel fiber high strength composite columns under eccentric compression is available. Thus, further experimental studies are necessary to better describe the behaviour of steel fiber high strength composite columns for rational design of concrete structures. This research study focuses primarily on the determination of the structural behaviour of steel fiber high strength concrete-encased composite columns. Therefore, a total of 32 square section plain and
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Stress-Strain Relations
Stress (MPa)
80 60 40 20
CC3-0
CC3-I
CC3-II
CC3-III
0 0.001
0
0.002
0.003
0.004
0.005
0.006
Strain Fig. 2. The typical stress−strain diagrams of plain and steel fiber concrete.
2. Experimental program 2.1. Test specimens The experimental work includes a total of 32 square section high strength reinforced concrete and composite columns. The specimens were fabricated using different portions of concrete mixtures at 0, 0.5, 0.75 and 1.0% volume fractions of steel fibers to examine the influence of steel fibers on both types of columns. Eight specimens Table 2 Experimental properties of the column specimens. Specimen no.
fc (MPa)
L/r (r = 0.3B)
ex (mm)
ey (mm)
SC-0 SC-I SC-II SC-III SCC-0 SCC-I SCC-II SCC-III C1-0 C2-0 C3-0 C1-I C2-I C3-I C1-II C2-II C3-II C1-III C2-III C3-III CC1-0 CC2-0 CC3-0 CC1-I CC2-I CC3-I CC1-II CC2-II CC3-II CC1-III CC2-III CC3-III
73.42 76.87 72.53 69.71 65.94 74.72 76.52 73.75 53.82 58.46 69.28 56.32 59.03 71.86 56.74 61.20 72.24 50.48 58.81 67.43 52.28 58.19 68.42 55.67 61.13 71.27 55.72 60.64 66.21 52.30 59.04 66.24
22.67 22.67 22.67 22.67 22.67 22.67 22.67 22.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67
45 45 45 45 45 45 45 45 40 45 50 40 45 50 40 45 50 40 45 50 40 45 50 40 45 50 40 45 50 40 45 50
45 45 45 45 44.25 44.25 44.25 44.25 40 45 50 40 45 50 40 45 50 40 45 50 39.25 44.25 49.25 39.25 44.25 49.25 39.25 44.25 49.25 39.25 44.25 49.25
were short reinforced concrete columns (SC-0,I,II,III) and short composite columns (SCC-0,I,II,III). The other specimens were designed as slender reinforced concrete columns (C1-0,I,II,III, C2-0,I,II,III, C3-0, I,II,III) and concrete-encased composite columns (CC1-0,I,II,III, CC20,I,II,III, CC3-0,I,II,III). The short and slender column specimens were 850 mm and 1300 mm in length, respectively. The cross section details of the column specimens are presented in Fig. 1. According to the ACI Standard 318-08 [24], it shall be permitted to neglect the effects of slenderness when the ratio of column effective length to the radius of gyration of cross section of compression member (L/r) is less than 22. Here, the radius of gyration of cross section is approximately taken as 0.3B based on gross section. Therefore, the value of the L/r ratio of 22.67 for the series of SC and SCC columns almost represents the lower limit for slenderness effect and these specimens have been considered as short columns in this study. The reinforced concrete and composite column specimens had consisted of four 8 mm in diameter longitudinal deformed bars located at each corner of the section. The composite columns were fabricated with considering T-shaped structural steel material (Fig. 1). The ratio of structural steel section area to the gross concrete area of the cross section was 0.0304. The yield strength of the longitudinal bar and the structural steel were 550 MPa and 235 MPa, respectively. The lateral reinforcements were designed at 80 mm and 100 mm
N
Steel plate
e
Transducer
L
steel fiber high strength reinforced concrete and similar composite column specimens were constructed with using different steel fiber contents. The specimens were tested under biaxial bending and shortterm axial compression in order to investigate the column strength capacities and to examine the effects of steel fibers on the experimental behaviour of columns. In addition, the column specimens were analysed based on a theoretical method [15,17] to predict the ultimate strength capacities and to attain complete load–deflection curves. A good degree of correlation has been achieved between the test and the theoretical results in the study.
Column specimen
N Fig. 3. Test setup and instrumentation.
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101
(a)
(b)
Fig. 4. (a,b) The typical failure mode of the column specimens.
spacing for short and slender column specimens, respectively. The lateral reinforcements were bent into 135° hooks at the ends. The heavily reinforced brackets were designed at both ends of the column specimens to prevent any local failures of the end zones. The materials of the column specimens consisted of Portland CEM I 42.5 R type cement content, maximum size of 16 mm well dry and clean aggregate, tap water and super plasticizer to maintain good workability. The end hooked RC 65/35 BN-type steel fibers were used and randomly distributed in high strength concrete mixture. The fibers had a length of 35 mm, diameter of 0.55 mm, aspect ratio of 64, and density of 7850 kg/m 3. Concrete composition of the reinforced concrete and concrete-encased composite column specimens are given in Table 1. All the column specimens were cast horizontally inside a steel formwork in the Structural Laboratory at Cukurova University in
Adana, Turkey. The specimens were compacted using hand-held mechanical vibrator to vibrate and compact the concrete material. Three control concrete cylinder specimens (150 mm in diameter and 300 mm in length) were cast from each concrete mixture. The cylinder specimens were cured under the same condition of the column specimens. The control cylinder specimens were tested in axial compression on the day of column test to determine the main concrete compressive strength and to attain complete stress−strain relationship. In addition, the mechanical behaviour of plain and steel fiber high strength concrete has been examined. The typical experimental concrete stress −strain diagrams containing various steel fiber contents are shown in Fig. 2. The column specimen features of the concrete compressive strength (fc), approximate slenderness ratio (L/r) and the load eccentricity (ex,ey) are given in Table 2. The average concrete compressive
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strength of reinforced concrete and composite column specimens varied from 50.48 to 76.87 MPa. The eccentricities for composite columns have been given with respect to the plastic center of the cross section [16]. The reinforced concrete and composite column specimens (e.g., C1-0,I,II,III and CC1-0,I,II,III) were constructed and tested using almost the same experimental conditions in order to compare their experimental results (Tables 1 and 2). The stress−strain relations indicate that the inclusion of steel fibers has significantly effect on mechanical behaviour of high strength concrete material. Considerable increase in strain at peak stress has been observed. Beneficial effect is noticed especially in the descending branch of the stress–strain curve (Fig. 2). Higher values of ultimate fiber strain have been achieved by increasing steel fiber content. But, the stress−strain curves are exposed that steel fibers have no considerable effect on the concrete compressive strength and modulus of elasticity of concrete [25]. 2.2. Test setup and instrumentation The column specimens were tested with pinned conditions at both ends under short-term axial load and biaxial bending. The columns were loaded vertically using universal testing machine in the Structural Laboratory at Cukurova University. A data acquisition system was used to collect the digital measurements during the tests. Linear variable differential transducers were instrumented to the column specimens to measure the lateral deflections in both principal directions and axial deformations at the most heavily compressed region. Besides this, a 500 kN capacity load cell was used in order to measure the applied axial load. The transducers and the load cell were calibrated before they were used in this test study. The typical test setup and instrumentation are illustrated in Fig. 3. The column specimens were tested in a vertical position subjected to monotonically increasing axial load at a rate of 1 kN/s. The axial load was applied to the column specimens with different eccentricity values. During the tests, the lateral deflections in both principal directions were recorded at each load increment by using the data acquisition system. The column specimens were loaded from zero load up to failure. 2.3. Test results and discussion The plain and steel fiber column specimens initially behaved in a nearly similar manner. First cracks generally appeared at almost close to midheight on the tension region of columns. As the load increased, it was seen that the existing cracks propagated and new flexural cracks appeared before reaching the ultimate load. These tensile cracks occurred along the column length. Cover spalling started at the compressed region of the section especially for plain high strength concrete columns. After that, for plain concrete columns, a sudden loss of strength was measured, concrete cover spalled off and these columns failed suddenly and explosively manner owing to the brittleness behaviour of high strength concrete material. Besides this, buckling of longitudinal reinforcement was observed. On the other hand, it was seen from the tested steel fiber columns that cover spalling was prevented by addition of steel fibers and significant deformability was observed. The crack length for both steel fiber high strength reinforced concrete and composite columns occurred shorter than plain concrete columns. The column specimens including 0.5% volume fraction of steel fibers exhibited moderate ductile behaviour. On the other hand, the remaining columns had 0.75 and 1.0% volume fractions of steel fibers behaved significantly more ductile. These columns exhibited sufficient degree of deformation beyond the peak load. Therefore, the explosive type of failure was prohibited by adding steel fibers and the load carrying capacity dropped more slowly for both reinforced concrete and composite column specimens. The typical failure mode of
the column specimens is illustrated in Fig. 4(a,b). The ultimate load capacity of composite column specimens is significantly influenced by load eccentricity, concrete compressive strength, structural steel and slenderness effect as well as high strength reinforced concrete columns. The ultimate strength results of columns were recorded higher at low eccentricity. In addition, the strength capacity of composite columns were significantly higher than that of reinforced concrete columns resulted from the contribution of structural steel (Tables 2 and 3). Moreover, the composite columns behaved more resist and less buckled than that of reinforced concrete columns. It was seen that no significant lateral deformation was recorded for short columns, but the lateral displacements were considerable for slender columns. The slender columns started to buckle at a load level less than failure load due to the slenderness effect. The typical column experimental load−deflection diagrams for x axis (Exp-X Axis) and y axis (Exp-Y Axis) are presented in Fig. 5(a,b). The diagrams indicate that the inclusion of steel fibers into high strength concrete improves the ductility and deformability of reinforced concrete and composite columns. No significant distinction from zero load to peak load has been obtained for plain and steel fiber columns. It is concluded that steel fiber does not significantly affect the column ultimate load capacity (Fig. 5(a,b)). The experimental study indicates that the addition of 0.75 and 1.0% volume fractions of steel fibers has provided significant confinement, ductility and deformability for high strength reinforced concrete and composite columns. However, loss of workability of concrete has started to appear at 1.0% volume fraction of steel fibers especially for steel fiber high strength composite columns. Thus, it is concluded that the inclusion of steel fibers in the range 0.75 to 1.0% volume fraction has given reasonable effect on the behaviour of high strength both reinforced concrete and composite columns. Photographs of all the short and slender column specimens tested in this study are shown in Fig. 6(a,b). The modes of failure exhibited that Table 3 Ultimate strength results of the column specimens. Specimen no.
Ntest (kN)
SC-0 249 SC-I 256 SC-II 264 SC-III 243 SCC-0 236 SCC-I 262 SCC-II 253 SCC-III 265 C1-0 211 C2-0 194 C3-0 159 C1-I 215 C2-I 192 C3-I 167 C1-II 212 C2-II 205 C3-II 173 C1-III 219 C2-III 201 C3-III 176 CC1-0 219 CC2-0 183 CC3-0 167 CC1-I 234 CC2-I 213 CC3-I 198 CC1-II 236 CC2-II 194 CC3-II 188 CC1-III 238 CC2-III 214 CC3-III 196 Mean ratio Standard deviation
Nu (kN)
Mux (kN.cm)
Muy (kN.cm)
Nu/Ntest
227.29 234.65 222.58 216.84 234.71 244.14 247.55 242.72 191.39 170.44 150.47 194.85 172.93 160.34 194.38 173.95 163.17 189.97 180.64 163.19 207.84 181.42 170.24 226.15 206.12 194.59 226.86 206.06 189.17 219.89 203.35 188.98
1022.74 1096.43 1002.42 976.53 1039.96 1081.84 1096.85 1075.46 859.61 844.23 827.64 884.77 863.84 886.02 882.96 872.46 895.12 855.75 891.11 898.48 867.51 846.53 878.44 947.57 968.52 1011.48 950.72 967.49 981.54 918.91 954.65 980.46
1022.74 1096.43 1002.42 976.53 1056.49 1098.93 1114.27 1092.54 859.61 844.23 827.64 884.77 863.84 886.02 882.96 872.46 895.12 855.75 891.11 898.48 886.65 861.01 890.03 968.51 985.19 1024.91 971.74 984.15 996.64 941.67 971.15 995.54
0.913 0.917 0.843 0.892 0.995 0.932 0.978 0.916 0.907 0.879 0.946 0.906 0.901 0.960 0.917 0.849 0.943 0.867 0.899 0.927 0.949 0.991 1.019 0.966 0.968 0.983 0.961 1.062 1.006 0.924 0.950 0.964 0.938 0.0491
S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
103
Load-Deflection Curve (C2)
(a) 250
Load (kN)
200 150 100 C2-0 C2-II
50
C2-I C2-III
0 0
2
4
6
8
10
12
14
16
18
Deflection (mm)
(b)
Load-Deflection Curve (CC3)
250
Load (kN)
200 150 100 CC3-0 CC3-II
50
CC3-I CC3-III
0 0
2
4
6
8
10
12
14
16
18
20
Deflection (mm) Fig. 5. (a,b) The typical experimental load−deflection curves of the column specimens.
most of the tested columns crushed near the midheight of the specimen indicating a typical compression failure of the column in compression. More critical local buckling and crushing were observed especially for
slender plain high strength reinforced concrete and also composite columns than those of short columns. The failure modes designate that this type of failure has been reduced by inclusion of steel fibers.
(a)
(b)
Fig. 6. (a,b) Column specimens after failure.
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Additionally, the results of the effects of steel fibers on the experimental behaviour of plain and steel fiber high strength reinforced concrete columns and also L-shaped section reinforced concrete and composite columns have been reported in the previous works [15,25].
The basic equations of equilibrium for the axial load N, and the bending moments Mx and My are determined in terms of the stress resultants as follows: N ¼
X
Ac σ c þ
X
At σ t þ
X
As σ s
ð1Þ
3. Analysis method The analysis of eccentrically loaded plain and steel fiber reinforced concrete and composite columns has been previously studied by Tokgoz [15] and Tokgoz and Dundar [17]. In the proposed method, the nonlinear stress−strain relation can be used for the materials and slenderness effect is taken into account. The proposed analysis method is based on the following assumptions: (i) Plane sections remain plane before and after bending. (ii) Empirical expressions to represent the stress−strain relations of plain and steel fiber high strength concrete suggested by Hsu and Hsu [3] were used for the compression zone of the column member. (iii) Elastic−perfectly plastic relation was assumed for the steel materials. (iv) Perfect bond exists between concrete and steel. (v) The effects of creep and any tensile stresses due to shrinkage are ignored. (vi) Axial and shear deformation effects are neglected. A biaxially loaded composite column cross section and the typical stress and strain distribution are shown in Fig. 7. The proposed analysis procedure has taken into account the stress–strain relationships of the materials. Thus, the compression zone of the concrete section and entire section of structural steel are divided into segmental subdivisions parallel to the neutral axis. The stress resultants of the concrete and structural steel material have been calculated in the centre of each segment [16]. The centroidal strain value at each segment of the cross section can be expressed as a linear strain relationship based on the assumption that plane section remains plane during bending. Then, the stress resultants of the members can be obtained by using the assumed stress–strain relationships of the materials [15,17].
Mx ¼
My ¼
X
X
Ac σ c yc þ
Ac σ c xc þ
X
X
At σ t yt þ
At σ t xt þ
X
X
As σ s ys
ð2Þ
As σ s xs
ð3Þ
where Ac, At and As are the elemental area of concrete segment, elemental area of structural steel segment and area of reinforcing steel bar, respectively; σc, σt and σs are the concrete stress, structural steel stress and reinforcing steel stress, respectively; (xc, yc), (xt, yt) and (xs, ys) indicate the distance between, respectively, the centre of elemental area of concrete, the centre of elemental area of structural steel and reinforcing steel bar, and the plastic centre in x–y plane. The slenderness effect is taken into account by using the Moment Magnification Method recommended by ACI 318-08 Building Code [24]. In the essence of analysis, the primary moments are magnified with the moment magnification factor (δ). The precise analysis algorithm to determine the ultimate strength and load−deformation behaviour of reinforced concrete and composite columns can be attained from References [15,17]. In the study, a computer program has been developed to perform the analysis procedure of biaxially loaded both short and slender plain and steel fiber high strength reinforced concrete and composite columns. 4. Comparison of experimental and theoretical results The short and slender reinforced concrete and composite column specimens with and without steel fibers were analysed based on the proposed theoretical method [15,17] for the prediction of ultimate strength capacities and load−deflection curves. The nonlinear stress–strain relations were considered for plain and steel fiber high strength concrete materials [3,12]. Besides this, elastic–perfectly
Stress σc
Strain εc
NA h a x ypc
c PC
εs
ey
N As
xpc
ex
Structural steel
y Fig. 7. Composite column section under axial load and biaxial bending.
S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
plastic behaviour was assumed for reinforcing steel bars and structural steel in the analysis. The experimental results, computed theoretical strength capacities and comparative results of the predicted load to test load are shown in Table 3. A good degree of correlation has been achieved between the test and the analysis results for most of the column specimens (Table 3). The values of predicted load to test load ratio (Nu/Ntest) have been computed below 0.90 for a few specimens. This is due to gain a little higher experimental ultimate strength than expected. In the study, the average value of predicted load to test load has been obtained 0.938. By comparing the experimental results of columns constructed and tested using the same experimental conditions, the addition of various amounts of steel fibers has no significant effect on ultimate strength capacity of both reinforced concrete and composite column specimens (Table 3). The theoretical load−deflection curves for x axis (Theo-X Axis) and y axis (Theo-Y Axis) have been obtained using the developed computer program. The typical experimental and theoretical load −deflection curves of plain and steel fiber high strength reinforced concrete and composite columns are presented in Fig. 8(a−f). The diagrams have been found to be in good agreement. It is shown in the diagrams that the addition of steel fibers has considerable effect
(a)
105
on the deformability and ductility features of column specimens. In addition, the typical experimental and theoretical load−axial strain curves are shown in Fig. 9. The comparative results indicate that the use of the stress−strain relation has given reasonable accuracy to determine the load−deflection behaviour of plain and steel fiber high strength reinforced concrete and composite columns.
5. Conclusions A total of 32 both short and slender reinforced concrete and similar composite column specimens including different volume fraction of steel fibers have been tested in this study. The following conclusions can be drawn based on the experimental results of the study: (1) Adding steel fibers into high strength concrete definitely improves mechanical behaviour of composite columns. Crushing of concrete core, cover spalling, and buckling of the compressed reinforcing bars have been prevented by inclusion of steel fibers. (2) Local buckling is more extensive especially for slender plain high strength reinforced concrete columns than that of short columns. More structural stiffness has been provided due to the presence of shaped steel in composite columns. It is observed that buckling
Load-Deflection Curve (C1-I)
250
Load (kN)
200 150 100
Exp-X Axis Exp-Y Axis Theo- X&Y Axes
50 0 2
0
4
6
8
10
12
14
16
18
Deflection (mm)
(b)
Load-Deflection Curve (C2-II)
250
Load (kN)
200 150 100
Exp-X Axis Exp-Y Axis Theo- X&Y Axes
50 0 2
0
4
6
8
10
12
14
16
18
20
Deflection (mm)
(c)
Load-Deflection Curve (C3-III)
200
Load (kN)
150
100
Exp-X Axis Exp-Y Axis Theo- X&Y Axes
50
0 0
2
4
6
8
10
12
14
Deflection (mm) Fig. 8. (a–f) Experimental and theoretical load−deflection curves of the column specimens.
16
18
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(d)
Load-Deflection Curve (CC1-I)
250 Load (kN)
200 150
Exp-X Axis Exp-Y Axis Theo-X Axis Theo-Y Axis
100 50 0 0
2
4
6
8
10
12
14
16
18
20
Deflection (mm)
(e)
Load-Deflection Curve (CC2-II)
250 Load (kN)
200 150
Exp-X Axis Exp-Y Axis Theo- X Axis Theo- Y Axis
100 50 0 0
2
4
6
8
10
12
14
16
18
Deflection (mm)
(f)
Load-Deflection Curve (CC3-III)
250 Load (kN)
200 150 Exp-X Axis Exp-Y Axis Theo-X Axis Theo-Y Axis
100 50 0 0
2
4
6
8
10
12
14
16
Deflection (mm) Fig. 8 (continued)
Load-Axial Strain Relation (CC3-III)
Load (kN)
250 200 150 100
Experimental Theoretical
50 0 0
0.001
0.002
0.003
0.004
0.005
0.006
Axial Strain Fig. 9. The typical experimental and theoretical load−axial strain diagrams.
failure has been reduced by inclusion of steel fibers for both reinforced concrete and composite columns. (3) The slenderness and load eccentricity have significant effects on the load carrying capacity of reinforced concrete and composite columns. On the other hand, steel fibers have no considerable effect on modulus of elasticity of concrete and column ultimate strength capacity. (4) The experimental study reveals that the addition of steel fibers at a volume fraction in the range 0.75 to 1.0% significantly improves the confinement, ductility and deformability of high strength both reinforced concrete and composite columns. (5) The tested columns have been analyzed for the prediction of ultimate strength capacities and complete load−deflection curves. Good degree of accuracy has been obtained between the test and the theoretical results of plain and steel fiber high strength reinforced concrete and composite columns.
6. Notations
a Ac As At B c ex ey fc h
horizontal distance between the origin of the x–y axis system and the neutral axis; elemental area of concrete segment; area of reinforcing steel bar; elemental area of structural steel segment; dimension of section in the stability direction; vertical distance between the origin of the x–y axis system and the neutral axis; eccentricity of column in x direction; eccentricity of column in y direction; peak stress of concrete; distance from the maximum compressive fiber to the neutral axis;
S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
L Mux Muy Mx My N NA Ntest Nu PC r s x, y xpc, ypc xc, yc xs, ys xt, yt εc εs σc σt σs δ
column length; theoretical bending moment about x–axis; theoretical bending moment about y-axis; bending moment about x-axis; bending moment about y-axis; axial load; neutral axis; experimental axial load; theoretical ultimate axial load; plastic center; radius of gyration of cross section; lateral reinforcement spacing; coordinates of cross section point in x–y plane; plastic centre coordinates of cross section; distance between the centre of elemental area of concrete and the plastic centre in x–y plane; distance between reinforcing steel bar and the plastic centre in x–y plane; distance between the centre of elemental area of structural steel and the plastic centre in x–y plane concrete compressive strain; steel strain; concrete stress; structural steel stress; reinforcing steel stress; moment magnification factor.
Acknowledgements The research described in this paper was funded by the Scientific and Technological Research Council of Turkey (TUBITAK, Project No. 108M511). The authors would like to thank to BEKAERT Izmit Steel Cord Industry and Trade Co., and CIMSA Cement Industry and Trade Co. The assistance of Cukurova University laboratory staffs is also acknowledged. References [1] Fanella DA, Naaman AE. Stress−strain properties of fiber reinforced mortar in compression. ACI Journal 1985;82(4):475–83. [2] Ezeldin AS, Balaguru PN. Normal and high−strength fiber reinforced concrete under compression. J Mater Civil Eng 1992;4(4):415–29.
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[3] Hsu LSM, Hsu CTT. Stress–strain behavior of steel-fiber high-strength concrete under compression. ACI Struct J 1994;91(4):448–57. [4] Taerwe L, Van Gysel A. Influence of steel fibers on design stress–strain curve for high-strength concrete. J Eng Mech 1996;122(8):695–704. [5] Maalej M, Lok TS. Flexural behavior of steel fiber reinforced concrete. J Mater Civ Eng 1999;11(2):179–80. [6] Nataraja MC, Dhang N, Gupta AP. Stress–strain curves for steel fiber reinforced concrete under compression. Cem Concr Compos 1999;21:383–90. [7] Ramesh K, Seshu DR, Prabhakar M. Constitutive behaviour of confined fibre reinforced concrete under axial compression. Cem Concr Compos 2003;25:343–50. [8] Lim DH, Nawy EG. Behaviour of plain and steel-fibre-reinforced high-strength concrete under uniaxial and biaxial compression. Mag Concr Res 2005;57(10):603–10. [9] Thomas J, Ramaswamy A. Mechanical properties of steel fiber-reinforced concrete. J Mater Civ Eng 2007;19(5):385–92. [10] Bencardino F, Rizzuti L, Spadea G, Swamy RN. Stress−strain behavior of steel fiber-reinforced concrete in compression. J Mater Civ Eng 2008;20(3):255–63. [11] Ganesan N, Ramana Murthy JV. Strength and behavior of confined steel fiber reinforced concrete columns. ACI Mater J 1990;87(3):221–7. [12] Hsu CTT, Hsu LSM, Tsao WH. Biaxially loaded slender high-strength reinforced concrete columns with and without steel fibres. Mag Concr Res 1995;47(173): 299–310. [13] Foster SJ, Attard MM. Strength and ductility of fiber-reinforced high-strength concrete columns. J Struct Eng 2001;127(1):28–34. [14] Foster SJ. On behavior of high-strength concrete columns:cover spalling, steel fibers, and ductility. ACI Struct J 2001;98(4):583–9. [15] Tokgoz S. Effects of steel fiber addition on the behaviour of biaxially loaded high strength concrete columns. Mater Struct 2009;42(8):1125–38. [16] Dundar C, Tokgoz S, Tanrikulu AK, Baran T. Behaviour of reinforced and concreteencased composite columns subjected to biaxial bending and axial load. Build Environ 2008;43(6):1109–20. [17] Tokgoz S, Dundar C. Experimental tests on biaxially loaded concrete-encased composite columns. Steel Compos Struct 2008;8(5):423–38. [18] Liang QQ. Strength and ductility of high strength concrete-filled steel tubular beam-columns. J Constr Steel Res 2009;65(3):687–98. [19] Ellobody E, Young B, Lam D. Eccentrically loaded concrete encased steel composite columns. Thin-Walled Struct 2011;49(1):53–65. [20] Ellobody E, Young B. Numerical simulation of concrete encased steel composite columns. J Constr Steel Res 2011;67(2):211–22. [21] Young B, Ellobody E. Performance of axially restrained concrete encased steel composite columns at elevated temperatures. Eng Struct 2011;33(1):245–54. [22] AISC. Specification for structural steel buildings. American Institute for Steel Construction; 2005. ANSI/AISC 360-05, Reston, Chicago, Illinois, USA. [23] EC4. Eurocode 4: Design of composite steel and concrete structures−Part 1–1: General rules and rules for buildings. London, UK: British Standards Institution; 2004. BS EN 1994-1-1. [24] ACI. Building code requirements for structural concrete and commentary. American Concrete Institute; 2008 (ACI 318(08). [25] Tokgoz S, Dundar C. Tests of eccentrically loaded L-shaped section steel fiber high strength reinforced concrete and composite columns. Eng Struct 2012, doi: 10.1016/j.engstruct.2012.01.009.
Contents lists available at SciVerse ScienceDirect
Journal of Constructional Steel Research
Experimental behaviour of steel fiber high strength reinforced concrete and composite columns Serkan Tokgoz a,⁎, Cengiz Dundar b, A. Kamil Tanrikulu b a b
Civil Engineering, Mersin University, 33340 Mersin, Turkey Civil Engineering, Cukurova University, 01330 Adana, Turkey
a r t i c l e
i n f o
Article history: Received 16 March 2011 Accepted 28 February 2012 Available online 27 March 2012 Keywords: Reinforced concrete column Concrete-encased composite column Steel fiber Ultimate strength
a b s t r a c t This paper presents experimental behaviour of eccentrically loaded plain and steel fiber high strength reinforced concrete and concrete-encased composite columns. In the experimental study, a total of 32 square section both reinforced concrete and composite column specimens were fabricated at 0, 0.5, 0.75 and 1.0% volume fractions of steel fiber contents to examine the effects of steel fibers on column behaviour. Besides this, the composite columns were constructed and tested using almost the same conditions with reinforced concrete columns to investigate the column experimental behaviour. The complete load−deflection behaviour and strength of column specimens were obtained and the results were discussed in the study. In addition, the column specimens were analysed based on a theoretical method considering the nonlinear behaviour of the materials. The presented experimental study indicates that the inclusion of steel fibers in the range 0.75 to 1.0% volume fraction improves confinement and ductility features of high strength reinforced concrete and composite columns significantly. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction High strength concrete has been increasingly used in the construction of structures, such as high-rise buildings, bridges, piles etc. High strength concrete offers many significant benefits in terms of strength, durability, and modulus of elasticity. However, it is widely believed that high strength concrete exhibits brittle behaviour under compression. The inclusion of steel fibers into high strength concrete definitely improves confinement, ductility and deformability of concrete. Several experimental and analytical studies were carried out to describe the mechanical behaviour of steel fiber high strength concrete. Fanella and Naaman [1] studied on the stress–strain properties of fiber reinforced concrete and an analytical relationship was proposed to predict the complete stress–strain curve of fiber reinforced mortar in compression. Ezeldin and Balaguru [2] presented experimental stress–strain behaviour of fiber reinforced concrete with compressive strength ranging from 35 MPa to 85 MPa. An analytical expression was proposed to represent the complete stress–strain curve of steel fiber reinforced concrete. Hsu and Hsu [3] conducted an experimental research to determine the complete stress–strain relationship of steel fiber high strength reinforced concrete under compression and empirical stress–strain equations were proposed in the study. Taerwe and Van Gysel [4] presented
⁎ Corresponding author at: Civil Engineering, Mersin University, Engineering Faculty, Civil Engineering Department, 33340, Mersin, Turkey. Tel.: + 90 324 361 00 33; fax: + 90 324 361 00 32. E-mail addresses: [email protected] (S. Tokgoz), [email protected] (C. Dundar), [email protected] (A.K. Tanrikulu). 0143-974X/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2012.02.017
experimental and analytical researches to describe the realistic stress– strain curve for high strength fiber concrete. Maalej and Lok [5] examined the flexural behaviour of steel fiber concrete. Nataraja et al. [6] proposed a simple analytical model to generate both ascending and descending portions of the stress–strain curve of steel fiber reinforced concrete. Ramesh et al. [7] tested prism specimens to study the behaviour of confined steel fiber reinforced concrete and an analytical model was suggested to predict the stress–strain behaviour of confined fiber reinforced concrete. Lim and Nawy [8] investigated the mechanical characteristics of plain and steel fiber reinforced high strength concrete under uniaxial and biaxial loading conditions. Thomas and Ramaswamy [9] reported an experimental program and analytical assessments of the influence of addition of fibers on the mechanical properties of concrete. Empirical relationships were developed to assess the strength properties of steel fiber reinforced concrete. Bencardino et al. [10] researched the stress–strain behaviour of steel fiber reinforced concrete in compression and the validity of the models proposed in literature in defining the post peak behaviour of steel fiber concrete was examined. Ductility and confinement are very important features for high strength concrete column members especially in the seismically active regions. Therefore, using steel fibers into high strength concrete columns has become popular. It is significant to describe the behaviour of such members for analysis and design. Ganesan and Ramana Murthy [11] conducted experimental research to describe the behaviour of steel fiber reinforced concrete columns under axial load. Hsu et al. [12] and Foster and Attard [13] tested square section steel fiber high strength reinforced concrete columns to investigate the effects of steel fibers on the strength and ductility of columns. Foster
S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
125 mm
125 mm
37.5 50
25
25
Plain concrete column
y
37.5
8 mm 37.5
5
125 mm PC
25
Longitudinal reinforcement
99
50
x
5
37.5
25 25
25
125 mm
25 25 (SCC−0, CC1−0, CC2−0 and CC3−0)
(SC−0, C1−0, C2−0 and C3−0)
s L
Column section
A
A
125 mm 125 mm
Lateral reinforcement
37.5 50 25
25
Steel fiber column
y
125 mm
25
PC
37.5
37.5
5 5
50
x
25
25
(SC−I,II,III, C1−I,II,III, C2−I,II,III and C3−I,II,III)
125 mm
37.5
25 25
8 mm
25
(SCC−I,II,III, CC1−I,II,III, CC2−I,II,III and CC3−I,II,III)
Section A−A Fig. 1. Details of column specimens. Table 1 Concrete composition of the column specimens. Specimen Gravel no. (kg/m3)
Sand (kg/m3)
Cement (kg/m3)
Water (kg/m3)
Plasticizer (kg/m3)
Steel fiber (kg/m3)
SC-0 SC-I SC-II SC-III SCC-0 SCC-I SCC-II SCC-III C1-0 C2-0 C3-0 C1-I C2-I C3-I C1-II C2-II C3-II C1-III C2-III C3-III CC1-0 CC2-0 CC3-0 CC1-I CC2-I CC3-I CC1-II CC2-II CC3-II CC1-III CC2-III CC3-III
650 640 630 620 650 640 630 620 720 700 650 710 690 640 720 700 630 720 690 620 720 700 650 710 690 640 720 700 630 720 690 630
420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420 420
125 125 125 125 125 125 125 125 145 135 125 145 135 125 145 135 125 145 135 125 145 135 125 145 135 125 145 135 125 145 135 125
15 15 15 15 15 15 15 15 5 8 15 5 8 15 5 8 15 5 8 15 5 8 15 5 8 15 5 8 15 5 8 15
− 39.25 58.88 78.50 − 39.25 58.88 78.50 − − − 39.25 39.25 39.25 58.88 58.88 58.88 78.50 78.50 78.50 − − − 39.25 39.25 39.25 58.88 58.88 58.88 78.50 78.50 78.50
1100 1100 1100 1100 1100 1100 1100 1100 1120 1120 1100 1120 1120 1100 1100 1100 1100 1100 1100 1100 1120 1120 1100 1120 1120 1100 1100 1100 1100 1100 1100 1100
[14] proposed a model to determine the quantity of effective fibers to ensure a good level of ductility for high strength reinforced concrete columns. Tokgoz [15] conducted experimental study to describe the biaxially loaded steel fiber high strength reinforced concrete columns. In addition to reinforced concrete columns, it is known that composite columns with symmetrical and unsymmetrical cross sections have been employed in high rise buildings, bridges and earthquake resistant structures. Composite columns provide effective stiffness and load carrying capacity and also prevent local buckling effect due to the contribution of structural steel material. Many experimental and theoretical studies were carried out to determine the behaviour of composite columns in the past years. Research outcomes of the most of these works were notified in the previous studies [16,17]. Additional theoretical researches to determine the behaviour of plain concrete composite columns using finite element method were also performed by Liang [18], Ellobody et al. [19], Ellobody and Young [20] and Young and Ellobody [21]. In these studies, the suggested theoretical methods were verified with design rules [22–24]. Apart from the highlighted studies, Tokgoz and Dundar [25] examined the influence of steel fibers on the experimental behaviour of L-shaped section high strength reinforced concrete and composite columns. Very limited experimental data mainly on steel fiber high strength composite columns under eccentric compression is available. Thus, further experimental studies are necessary to better describe the behaviour of steel fiber high strength composite columns for rational design of concrete structures. This research study focuses primarily on the determination of the structural behaviour of steel fiber high strength concrete-encased composite columns. Therefore, a total of 32 square section plain and
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Stress-Strain Relations
Stress (MPa)
80 60 40 20
CC3-0
CC3-I
CC3-II
CC3-III
0 0.001
0
0.002
0.003
0.004
0.005
0.006
Strain Fig. 2. The typical stress−strain diagrams of plain and steel fiber concrete.
2. Experimental program 2.1. Test specimens The experimental work includes a total of 32 square section high strength reinforced concrete and composite columns. The specimens were fabricated using different portions of concrete mixtures at 0, 0.5, 0.75 and 1.0% volume fractions of steel fibers to examine the influence of steel fibers on both types of columns. Eight specimens Table 2 Experimental properties of the column specimens. Specimen no.
fc (MPa)
L/r (r = 0.3B)
ex (mm)
ey (mm)
SC-0 SC-I SC-II SC-III SCC-0 SCC-I SCC-II SCC-III C1-0 C2-0 C3-0 C1-I C2-I C3-I C1-II C2-II C3-II C1-III C2-III C3-III CC1-0 CC2-0 CC3-0 CC1-I CC2-I CC3-I CC1-II CC2-II CC3-II CC1-III CC2-III CC3-III
73.42 76.87 72.53 69.71 65.94 74.72 76.52 73.75 53.82 58.46 69.28 56.32 59.03 71.86 56.74 61.20 72.24 50.48 58.81 67.43 52.28 58.19 68.42 55.67 61.13 71.27 55.72 60.64 66.21 52.30 59.04 66.24
22.67 22.67 22.67 22.67 22.67 22.67 22.67 22.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67 34.67
45 45 45 45 45 45 45 45 40 45 50 40 45 50 40 45 50 40 45 50 40 45 50 40 45 50 40 45 50 40 45 50
45 45 45 45 44.25 44.25 44.25 44.25 40 45 50 40 45 50 40 45 50 40 45 50 39.25 44.25 49.25 39.25 44.25 49.25 39.25 44.25 49.25 39.25 44.25 49.25
were short reinforced concrete columns (SC-0,I,II,III) and short composite columns (SCC-0,I,II,III). The other specimens were designed as slender reinforced concrete columns (C1-0,I,II,III, C2-0,I,II,III, C3-0, I,II,III) and concrete-encased composite columns (CC1-0,I,II,III, CC20,I,II,III, CC3-0,I,II,III). The short and slender column specimens were 850 mm and 1300 mm in length, respectively. The cross section details of the column specimens are presented in Fig. 1. According to the ACI Standard 318-08 [24], it shall be permitted to neglect the effects of slenderness when the ratio of column effective length to the radius of gyration of cross section of compression member (L/r) is less than 22. Here, the radius of gyration of cross section is approximately taken as 0.3B based on gross section. Therefore, the value of the L/r ratio of 22.67 for the series of SC and SCC columns almost represents the lower limit for slenderness effect and these specimens have been considered as short columns in this study. The reinforced concrete and composite column specimens had consisted of four 8 mm in diameter longitudinal deformed bars located at each corner of the section. The composite columns were fabricated with considering T-shaped structural steel material (Fig. 1). The ratio of structural steel section area to the gross concrete area of the cross section was 0.0304. The yield strength of the longitudinal bar and the structural steel were 550 MPa and 235 MPa, respectively. The lateral reinforcements were designed at 80 mm and 100 mm
N
Steel plate
e
Transducer
L
steel fiber high strength reinforced concrete and similar composite column specimens were constructed with using different steel fiber contents. The specimens were tested under biaxial bending and shortterm axial compression in order to investigate the column strength capacities and to examine the effects of steel fibers on the experimental behaviour of columns. In addition, the column specimens were analysed based on a theoretical method [15,17] to predict the ultimate strength capacities and to attain complete load–deflection curves. A good degree of correlation has been achieved between the test and the theoretical results in the study.
Column specimen
N Fig. 3. Test setup and instrumentation.
S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
101
(a)
(b)
Fig. 4. (a,b) The typical failure mode of the column specimens.
spacing for short and slender column specimens, respectively. The lateral reinforcements were bent into 135° hooks at the ends. The heavily reinforced brackets were designed at both ends of the column specimens to prevent any local failures of the end zones. The materials of the column specimens consisted of Portland CEM I 42.5 R type cement content, maximum size of 16 mm well dry and clean aggregate, tap water and super plasticizer to maintain good workability. The end hooked RC 65/35 BN-type steel fibers were used and randomly distributed in high strength concrete mixture. The fibers had a length of 35 mm, diameter of 0.55 mm, aspect ratio of 64, and density of 7850 kg/m 3. Concrete composition of the reinforced concrete and concrete-encased composite column specimens are given in Table 1. All the column specimens were cast horizontally inside a steel formwork in the Structural Laboratory at Cukurova University in
Adana, Turkey. The specimens were compacted using hand-held mechanical vibrator to vibrate and compact the concrete material. Three control concrete cylinder specimens (150 mm in diameter and 300 mm in length) were cast from each concrete mixture. The cylinder specimens were cured under the same condition of the column specimens. The control cylinder specimens were tested in axial compression on the day of column test to determine the main concrete compressive strength and to attain complete stress−strain relationship. In addition, the mechanical behaviour of plain and steel fiber high strength concrete has been examined. The typical experimental concrete stress −strain diagrams containing various steel fiber contents are shown in Fig. 2. The column specimen features of the concrete compressive strength (fc), approximate slenderness ratio (L/r) and the load eccentricity (ex,ey) are given in Table 2. The average concrete compressive
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strength of reinforced concrete and composite column specimens varied from 50.48 to 76.87 MPa. The eccentricities for composite columns have been given with respect to the plastic center of the cross section [16]. The reinforced concrete and composite column specimens (e.g., C1-0,I,II,III and CC1-0,I,II,III) were constructed and tested using almost the same experimental conditions in order to compare their experimental results (Tables 1 and 2). The stress−strain relations indicate that the inclusion of steel fibers has significantly effect on mechanical behaviour of high strength concrete material. Considerable increase in strain at peak stress has been observed. Beneficial effect is noticed especially in the descending branch of the stress–strain curve (Fig. 2). Higher values of ultimate fiber strain have been achieved by increasing steel fiber content. But, the stress−strain curves are exposed that steel fibers have no considerable effect on the concrete compressive strength and modulus of elasticity of concrete [25]. 2.2. Test setup and instrumentation The column specimens were tested with pinned conditions at both ends under short-term axial load and biaxial bending. The columns were loaded vertically using universal testing machine in the Structural Laboratory at Cukurova University. A data acquisition system was used to collect the digital measurements during the tests. Linear variable differential transducers were instrumented to the column specimens to measure the lateral deflections in both principal directions and axial deformations at the most heavily compressed region. Besides this, a 500 kN capacity load cell was used in order to measure the applied axial load. The transducers and the load cell were calibrated before they were used in this test study. The typical test setup and instrumentation are illustrated in Fig. 3. The column specimens were tested in a vertical position subjected to monotonically increasing axial load at a rate of 1 kN/s. The axial load was applied to the column specimens with different eccentricity values. During the tests, the lateral deflections in both principal directions were recorded at each load increment by using the data acquisition system. The column specimens were loaded from zero load up to failure. 2.3. Test results and discussion The plain and steel fiber column specimens initially behaved in a nearly similar manner. First cracks generally appeared at almost close to midheight on the tension region of columns. As the load increased, it was seen that the existing cracks propagated and new flexural cracks appeared before reaching the ultimate load. These tensile cracks occurred along the column length. Cover spalling started at the compressed region of the section especially for plain high strength concrete columns. After that, for plain concrete columns, a sudden loss of strength was measured, concrete cover spalled off and these columns failed suddenly and explosively manner owing to the brittleness behaviour of high strength concrete material. Besides this, buckling of longitudinal reinforcement was observed. On the other hand, it was seen from the tested steel fiber columns that cover spalling was prevented by addition of steel fibers and significant deformability was observed. The crack length for both steel fiber high strength reinforced concrete and composite columns occurred shorter than plain concrete columns. The column specimens including 0.5% volume fraction of steel fibers exhibited moderate ductile behaviour. On the other hand, the remaining columns had 0.75 and 1.0% volume fractions of steel fibers behaved significantly more ductile. These columns exhibited sufficient degree of deformation beyond the peak load. Therefore, the explosive type of failure was prohibited by adding steel fibers and the load carrying capacity dropped more slowly for both reinforced concrete and composite column specimens. The typical failure mode of
the column specimens is illustrated in Fig. 4(a,b). The ultimate load capacity of composite column specimens is significantly influenced by load eccentricity, concrete compressive strength, structural steel and slenderness effect as well as high strength reinforced concrete columns. The ultimate strength results of columns were recorded higher at low eccentricity. In addition, the strength capacity of composite columns were significantly higher than that of reinforced concrete columns resulted from the contribution of structural steel (Tables 2 and 3). Moreover, the composite columns behaved more resist and less buckled than that of reinforced concrete columns. It was seen that no significant lateral deformation was recorded for short columns, but the lateral displacements were considerable for slender columns. The slender columns started to buckle at a load level less than failure load due to the slenderness effect. The typical column experimental load−deflection diagrams for x axis (Exp-X Axis) and y axis (Exp-Y Axis) are presented in Fig. 5(a,b). The diagrams indicate that the inclusion of steel fibers into high strength concrete improves the ductility and deformability of reinforced concrete and composite columns. No significant distinction from zero load to peak load has been obtained for plain and steel fiber columns. It is concluded that steel fiber does not significantly affect the column ultimate load capacity (Fig. 5(a,b)). The experimental study indicates that the addition of 0.75 and 1.0% volume fractions of steel fibers has provided significant confinement, ductility and deformability for high strength reinforced concrete and composite columns. However, loss of workability of concrete has started to appear at 1.0% volume fraction of steel fibers especially for steel fiber high strength composite columns. Thus, it is concluded that the inclusion of steel fibers in the range 0.75 to 1.0% volume fraction has given reasonable effect on the behaviour of high strength both reinforced concrete and composite columns. Photographs of all the short and slender column specimens tested in this study are shown in Fig. 6(a,b). The modes of failure exhibited that Table 3 Ultimate strength results of the column specimens. Specimen no.
Ntest (kN)
SC-0 249 SC-I 256 SC-II 264 SC-III 243 SCC-0 236 SCC-I 262 SCC-II 253 SCC-III 265 C1-0 211 C2-0 194 C3-0 159 C1-I 215 C2-I 192 C3-I 167 C1-II 212 C2-II 205 C3-II 173 C1-III 219 C2-III 201 C3-III 176 CC1-0 219 CC2-0 183 CC3-0 167 CC1-I 234 CC2-I 213 CC3-I 198 CC1-II 236 CC2-II 194 CC3-II 188 CC1-III 238 CC2-III 214 CC3-III 196 Mean ratio Standard deviation
Nu (kN)
Mux (kN.cm)
Muy (kN.cm)
Nu/Ntest
227.29 234.65 222.58 216.84 234.71 244.14 247.55 242.72 191.39 170.44 150.47 194.85 172.93 160.34 194.38 173.95 163.17 189.97 180.64 163.19 207.84 181.42 170.24 226.15 206.12 194.59 226.86 206.06 189.17 219.89 203.35 188.98
1022.74 1096.43 1002.42 976.53 1039.96 1081.84 1096.85 1075.46 859.61 844.23 827.64 884.77 863.84 886.02 882.96 872.46 895.12 855.75 891.11 898.48 867.51 846.53 878.44 947.57 968.52 1011.48 950.72 967.49 981.54 918.91 954.65 980.46
1022.74 1096.43 1002.42 976.53 1056.49 1098.93 1114.27 1092.54 859.61 844.23 827.64 884.77 863.84 886.02 882.96 872.46 895.12 855.75 891.11 898.48 886.65 861.01 890.03 968.51 985.19 1024.91 971.74 984.15 996.64 941.67 971.15 995.54
0.913 0.917 0.843 0.892 0.995 0.932 0.978 0.916 0.907 0.879 0.946 0.906 0.901 0.960 0.917 0.849 0.943 0.867 0.899 0.927 0.949 0.991 1.019 0.966 0.968 0.983 0.961 1.062 1.006 0.924 0.950 0.964 0.938 0.0491
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103
Load-Deflection Curve (C2)
(a) 250
Load (kN)
200 150 100 C2-0 C2-II
50
C2-I C2-III
0 0
2
4
6
8
10
12
14
16
18
Deflection (mm)
(b)
Load-Deflection Curve (CC3)
250
Load (kN)
200 150 100 CC3-0 CC3-II
50
CC3-I CC3-III
0 0
2
4
6
8
10
12
14
16
18
20
Deflection (mm) Fig. 5. (a,b) The typical experimental load−deflection curves of the column specimens.
most of the tested columns crushed near the midheight of the specimen indicating a typical compression failure of the column in compression. More critical local buckling and crushing were observed especially for
slender plain high strength reinforced concrete and also composite columns than those of short columns. The failure modes designate that this type of failure has been reduced by inclusion of steel fibers.
(a)
(b)
Fig. 6. (a,b) Column specimens after failure.
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Additionally, the results of the effects of steel fibers on the experimental behaviour of plain and steel fiber high strength reinforced concrete columns and also L-shaped section reinforced concrete and composite columns have been reported in the previous works [15,25].
The basic equations of equilibrium for the axial load N, and the bending moments Mx and My are determined in terms of the stress resultants as follows: N ¼
X
Ac σ c þ
X
At σ t þ
X
As σ s
ð1Þ
3. Analysis method The analysis of eccentrically loaded plain and steel fiber reinforced concrete and composite columns has been previously studied by Tokgoz [15] and Tokgoz and Dundar [17]. In the proposed method, the nonlinear stress−strain relation can be used for the materials and slenderness effect is taken into account. The proposed analysis method is based on the following assumptions: (i) Plane sections remain plane before and after bending. (ii) Empirical expressions to represent the stress−strain relations of plain and steel fiber high strength concrete suggested by Hsu and Hsu [3] were used for the compression zone of the column member. (iii) Elastic−perfectly plastic relation was assumed for the steel materials. (iv) Perfect bond exists between concrete and steel. (v) The effects of creep and any tensile stresses due to shrinkage are ignored. (vi) Axial and shear deformation effects are neglected. A biaxially loaded composite column cross section and the typical stress and strain distribution are shown in Fig. 7. The proposed analysis procedure has taken into account the stress–strain relationships of the materials. Thus, the compression zone of the concrete section and entire section of structural steel are divided into segmental subdivisions parallel to the neutral axis. The stress resultants of the concrete and structural steel material have been calculated in the centre of each segment [16]. The centroidal strain value at each segment of the cross section can be expressed as a linear strain relationship based on the assumption that plane section remains plane during bending. Then, the stress resultants of the members can be obtained by using the assumed stress–strain relationships of the materials [15,17].
Mx ¼
My ¼
X
X
Ac σ c yc þ
Ac σ c xc þ
X
X
At σ t yt þ
At σ t xt þ
X
X
As σ s ys
ð2Þ
As σ s xs
ð3Þ
where Ac, At and As are the elemental area of concrete segment, elemental area of structural steel segment and area of reinforcing steel bar, respectively; σc, σt and σs are the concrete stress, structural steel stress and reinforcing steel stress, respectively; (xc, yc), (xt, yt) and (xs, ys) indicate the distance between, respectively, the centre of elemental area of concrete, the centre of elemental area of structural steel and reinforcing steel bar, and the plastic centre in x–y plane. The slenderness effect is taken into account by using the Moment Magnification Method recommended by ACI 318-08 Building Code [24]. In the essence of analysis, the primary moments are magnified with the moment magnification factor (δ). The precise analysis algorithm to determine the ultimate strength and load−deformation behaviour of reinforced concrete and composite columns can be attained from References [15,17]. In the study, a computer program has been developed to perform the analysis procedure of biaxially loaded both short and slender plain and steel fiber high strength reinforced concrete and composite columns. 4. Comparison of experimental and theoretical results The short and slender reinforced concrete and composite column specimens with and without steel fibers were analysed based on the proposed theoretical method [15,17] for the prediction of ultimate strength capacities and load−deflection curves. The nonlinear stress–strain relations were considered for plain and steel fiber high strength concrete materials [3,12]. Besides this, elastic–perfectly
Stress σc
Strain εc
NA h a x ypc
c PC
εs
ey
N As
xpc
ex
Structural steel
y Fig. 7. Composite column section under axial load and biaxial bending.
S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
plastic behaviour was assumed for reinforcing steel bars and structural steel in the analysis. The experimental results, computed theoretical strength capacities and comparative results of the predicted load to test load are shown in Table 3. A good degree of correlation has been achieved between the test and the analysis results for most of the column specimens (Table 3). The values of predicted load to test load ratio (Nu/Ntest) have been computed below 0.90 for a few specimens. This is due to gain a little higher experimental ultimate strength than expected. In the study, the average value of predicted load to test load has been obtained 0.938. By comparing the experimental results of columns constructed and tested using the same experimental conditions, the addition of various amounts of steel fibers has no significant effect on ultimate strength capacity of both reinforced concrete and composite column specimens (Table 3). The theoretical load−deflection curves for x axis (Theo-X Axis) and y axis (Theo-Y Axis) have been obtained using the developed computer program. The typical experimental and theoretical load −deflection curves of plain and steel fiber high strength reinforced concrete and composite columns are presented in Fig. 8(a−f). The diagrams have been found to be in good agreement. It is shown in the diagrams that the addition of steel fibers has considerable effect
(a)
105
on the deformability and ductility features of column specimens. In addition, the typical experimental and theoretical load−axial strain curves are shown in Fig. 9. The comparative results indicate that the use of the stress−strain relation has given reasonable accuracy to determine the load−deflection behaviour of plain and steel fiber high strength reinforced concrete and composite columns.
5. Conclusions A total of 32 both short and slender reinforced concrete and similar composite column specimens including different volume fraction of steel fibers have been tested in this study. The following conclusions can be drawn based on the experimental results of the study: (1) Adding steel fibers into high strength concrete definitely improves mechanical behaviour of composite columns. Crushing of concrete core, cover spalling, and buckling of the compressed reinforcing bars have been prevented by inclusion of steel fibers. (2) Local buckling is more extensive especially for slender plain high strength reinforced concrete columns than that of short columns. More structural stiffness has been provided due to the presence of shaped steel in composite columns. It is observed that buckling
Load-Deflection Curve (C1-I)
250
Load (kN)
200 150 100
Exp-X Axis Exp-Y Axis Theo- X&Y Axes
50 0 2
0
4
6
8
10
12
14
16
18
Deflection (mm)
(b)
Load-Deflection Curve (C2-II)
250
Load (kN)
200 150 100
Exp-X Axis Exp-Y Axis Theo- X&Y Axes
50 0 2
0
4
6
8
10
12
14
16
18
20
Deflection (mm)
(c)
Load-Deflection Curve (C3-III)
200
Load (kN)
150
100
Exp-X Axis Exp-Y Axis Theo- X&Y Axes
50
0 0
2
4
6
8
10
12
14
Deflection (mm) Fig. 8. (a–f) Experimental and theoretical load−deflection curves of the column specimens.
16
18
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S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
(d)
Load-Deflection Curve (CC1-I)
250 Load (kN)
200 150
Exp-X Axis Exp-Y Axis Theo-X Axis Theo-Y Axis
100 50 0 0
2
4
6
8
10
12
14
16
18
20
Deflection (mm)
(e)
Load-Deflection Curve (CC2-II)
250 Load (kN)
200 150
Exp-X Axis Exp-Y Axis Theo- X Axis Theo- Y Axis
100 50 0 0
2
4
6
8
10
12
14
16
18
Deflection (mm)
(f)
Load-Deflection Curve (CC3-III)
250 Load (kN)
200 150 Exp-X Axis Exp-Y Axis Theo-X Axis Theo-Y Axis
100 50 0 0
2
4
6
8
10
12
14
16
Deflection (mm) Fig. 8 (continued)
Load-Axial Strain Relation (CC3-III)
Load (kN)
250 200 150 100
Experimental Theoretical
50 0 0
0.001
0.002
0.003
0.004
0.005
0.006
Axial Strain Fig. 9. The typical experimental and theoretical load−axial strain diagrams.
failure has been reduced by inclusion of steel fibers for both reinforced concrete and composite columns. (3) The slenderness and load eccentricity have significant effects on the load carrying capacity of reinforced concrete and composite columns. On the other hand, steel fibers have no considerable effect on modulus of elasticity of concrete and column ultimate strength capacity. (4) The experimental study reveals that the addition of steel fibers at a volume fraction in the range 0.75 to 1.0% significantly improves the confinement, ductility and deformability of high strength both reinforced concrete and composite columns. (5) The tested columns have been analyzed for the prediction of ultimate strength capacities and complete load−deflection curves. Good degree of accuracy has been obtained between the test and the theoretical results of plain and steel fiber high strength reinforced concrete and composite columns.
6. Notations
a Ac As At B c ex ey fc h
horizontal distance between the origin of the x–y axis system and the neutral axis; elemental area of concrete segment; area of reinforcing steel bar; elemental area of structural steel segment; dimension of section in the stability direction; vertical distance between the origin of the x–y axis system and the neutral axis; eccentricity of column in x direction; eccentricity of column in y direction; peak stress of concrete; distance from the maximum compressive fiber to the neutral axis;
S. Tokgoz et al. / Journal of Constructional Steel Research 74 (2012) 98–107
L Mux Muy Mx My N NA Ntest Nu PC r s x, y xpc, ypc xc, yc xs, ys xt, yt εc εs σc σt σs δ
column length; theoretical bending moment about x–axis; theoretical bending moment about y-axis; bending moment about x-axis; bending moment about y-axis; axial load; neutral axis; experimental axial load; theoretical ultimate axial load; plastic center; radius of gyration of cross section; lateral reinforcement spacing; coordinates of cross section point in x–y plane; plastic centre coordinates of cross section; distance between the centre of elemental area of concrete and the plastic centre in x–y plane; distance between reinforcing steel bar and the plastic centre in x–y plane; distance between the centre of elemental area of structural steel and the plastic centre in x–y plane concrete compressive strain; steel strain; concrete stress; structural steel stress; reinforcing steel stress; moment magnification factor.
Acknowledgements The research described in this paper was funded by the Scientific and Technological Research Council of Turkey (TUBITAK, Project No. 108M511). The authors would like to thank to BEKAERT Izmit Steel Cord Industry and Trade Co., and CIMSA Cement Industry and Trade Co. The assistance of Cukurova University laboratory staffs is also acknowledged. References [1] Fanella DA, Naaman AE. Stress−strain properties of fiber reinforced mortar in compression. ACI Journal 1985;82(4):475–83. [2] Ezeldin AS, Balaguru PN. Normal and high−strength fiber reinforced concrete under compression. J Mater Civil Eng 1992;4(4):415–29.
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