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Behavior of hybrid GFRP–perforated-steel tube-encased concrete column under uniaxial compression
Composite Structures 142 (2016) 313–324
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Behavior of hybrid GFRP–perforated-steel tube-encased concrete column under uniaxial compression Liang Huang a,⇑, Peng Yin b,⇑, Libo Yan c,d,⇑, Bohumil Kasal c,d,⇑ a
College of Civil Engineering, Hunan University, Changsha 410082, China The Henry Samueli School of Engineering, University of California, Irvine, Irvine, CA 92697, United States c Centre for Light and Environmentally-Friendly Structures, Fraunhofer Wilhelm-Klauditz-Institut WKI, Bienroder Weg 54E, 38108 Braunschweig, Germany d Department of Organic and Wood-Based Construction Materials, Technical University of Braunschweig, Hopfengarten 20, 38102 Braunschweig, Germany b
a r t i c l e
i n f o
Article history: Available online 8 February 2016 Keywords: GFRP Perforated-steel Uniaxial compression Model
a b s t r a c t This study presents an experimental investigation on concrete confined by glass fiber reinforced polymer (GFRP) and perforated-steel tube subjected to uniaxial compression. The compressive behavior of this hybrid structure is compared with GFRP tube encased concrete and steel tube encased concrete. The three key experimental parameters were investigated: the layers of GFRP tube (1, 2, 3 and 4), the perforation diameter of porous steel tube (5 mm and 10 mm), and the grid form of perforated steel tube (axial type and helical type). The effects of these parameters on the failure mode, stress–strain relationship and ductility of the specimens were discussed. A simplified design model was proposed for the specimens and the predictions were in good agreement with the experimental results. In addition, the interactive function between steel and GFRP on the confined concrete was analyzed which considered an influence factor provided by these two confinement materials. This study showed that the hybrid GFRP–perforated steel tube exerts the full potential of these two materials, i.e. GFRP with its high tension strength enhanced the loading carrying capacity and the perforated-steel with its high elastoplasticity enhanced the deformation performance in hardening segment, softening branch and residual strength plateau of the concrete, respectively. This study therefore demonstrated that the combination of GFRP and perforated steel tubes is a new effective method to confine concrete. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Confinement systems can significantly improve the load capacity and deformation performance of concrete structural element. Over the decades, steel tube and FRP tube-encased concrete columns have been widely used in infrastructure such as in highrise building, off-shore structure and bridges [1–4]. Steel tube-encased concrete columns possess excellent properties related to structural performance such as high ductility, and fatigue and impact-resistance. In addition, steel tube itself can serve as formwork of concrete which reduces the cost of traditional wood formwork and labor to remove it after concrete is cured [5– 7]. However, the potential corrosion of steel main limit its usage. In addition, the core concrete shrinks during the cement setting and
⇑ Address: Department of Organic and Wood-Based Construction Materials, Technical University of Braunschweig, Hopfengarten 20, 38102 Braunschweig, Germany (L. Yan, B. Kasal). E-mail addresses: [email protected] (L. Huang), [email protected] (P. Yin), [email protected] (L. Yan), [email protected] (B. Kasal). http://dx.doi.org/10.1016/j.compstruct.2016.02.016 0263-8223/Ó 2016 Elsevier Ltd. All rights reserved.
hardening, which results in a gap between the steel tube and the concrete core. Consequently, the interfacial slip due to insufficient bonding between steel and concrete can result in ineffective stress transfer between them. These possible disadvantages would compromise the structural performance of steel tube-encased concrete columns. FRP composites have been successfully used in the field of civil engineering in the last two decades because of their high strengthto-weight ratio, good corrosion resistance, and electromagnetic neutrality. In FRP tube encased concrete, the FRP tube prevents the carbonization of the core concrete and also serves as formwork of the concrete during construction [8–14]. However, because of a brittle failure mode and non-ductile behavior, FRP materials will fully lose its loading capacity once the failure of the FRP starts, which leads to the failure of FRP tube encased concrete showing a brittle failure without any warning prior in the rupture of FRP. Although both steel tube and FRP tube-encased concrete columns demonstrate advantages over traditional reinforcement concrete columns, there is a need for further improvements and innovations due to the aforementioned limitations. Therefore, the
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hybrid structure combining FRP and steel tube-encased column has been proposed by Feng et al. [15], Zhang et al. [16], Wang et al. [17], Teng et al. [18], Ganesh Prabhu and Sundarraja [19], Fakharifar et al. [20] and Ghanbari Ghazijahani et al. [21]. Such combination delays the onset of plastic hinge at the end of steel tube-encased concrete column by wrapping external FRP material. Some researchers [22] assigned the effects of different void area ratio by using serval types steel tube and carbon FRP (CFPR) on the structural performance on steel–FRP tube encased concrete. It indicated that CFRP tube can delay the local buckling of steel tube significantly and increase the load capacity and ductility of concrete column. In addition, the corrosion of the steel can be eliminated by applying FRP jacket outside the steel tube. Compared with CFRP materials, GFRP has lower ultimate tension capacity but a higher ultimate elongation, In addition, GFRP is an electrical insulating material which inhibits the electrochemical corrosion with steel. CFRP is a conductive material and the electrochemical corrosion of steel would occur if the resin layer separating the steel and CFRP is compromised. Steel is an isotropic material, which provides less confinement at optimal transverse direction compared with an anisotropic material with the same amount. Some researches drilled the steel tube to form grids, this would increase the confinement efficiency and the interfacial bonding strength [23]. A new hybrid GFRP–perforated-steel tube was proposed in this study that will overcome the above deficiencies. The hybrid tube is fabricated with perforated steel tube and then externally wrapped with GFRP skin, which is used as formwork for fresh concrete. This hybrid GFRP–perforated-steel tube addresses the following issues: (1) the interlocking mechanism is generated between the steel grids and the concrete. As the perforations in the steel tube are filled with concrete, an effective mechanical interlocking is achieved; (2) the perforated-steel tube increases the transverse confinement efficiency compared to a regular steel tube with the same amount of steel; (3) GFRP material has compatible deformation with steel tube, resulting in an inspiring ductility for the concrete column compared with concrete confined with CFRP tube only; (4) the hybrid confinement concrete column possesses high corrosion resistance and relatively low cost. This study focuses on the mechanism behavior of this hybrid column subjected to axial compression load. The effects of GFRP layers, diameter of perforations on steel tube, grid form of steel tube were investigated. Stress–strain behavior, failure mode, ultimate condition, compressive ductility and other properties were studied and analyzed. In addition, a stress–strain model was proposed for the future hybrid GFRP–perforated-steel tube-encased concrete column design.
2. Experiment 2.1. Test matrix The total of 30 cylindrical specimens with a diameter of 150 mm and a height of 300 mm were fabricated and tested under axial compression. The hybrid GFRP–perforated-steel tube-encased concrete specimens were designed on the basis of the following parameters: (1) the number of GFRP wrap layers; (2) the diameter of perforation on steel tube; (3) grid form of steel tube. Each parameter is classified into several variables: (1) the number GFRP layers is divided into one, two, three and four layers, which noted as G1, G2, G3, and G4, respectively; (2) the diameter of perforation on steel tube is divided into 10 mm and 5 mm, which noted as D10 and D5, respectively; (3) grid form of steel tube is divided into helical and axial style, which is denoted as ‘H’ and ‘A’, respectively. Additionally, control group were set as follows: (1) plain concrete cylinder, which is denoted as PC; (2) GFRP tube-encased concrete
cylinder with two layers of GFRP, which noted as G2; (3) perforated-steel tube-encased concrete cylinder with perforation diameter of 10 mm and helical style, which is denoted as D10-H. The compressive behavior of GFRP tube encased concrete and steel tube encased concrete is used to compare with that of the hybrid structure. There are three identical specimens were fabricated and tested for each groups. The details of test matrix is summarized in Table 1. 2.2. Materials properties 2.2.1. Concrete The components of concrete are Type I Portland cement, gravel, natural sand, water, and superplasticizer. The mix design by weight was cement:water:coarse aggregate:fine aggregate:admixture = 1:0.43:3.51:1.80:0.0075. Three plain concrete cylinders were tested to determine the average maximum strength of unconfined concrete, f co , and its corresponding strain, eco . The 28th day cylinder compressive strength is 27.61 MPa. 2.2.2. Perforated-steel tubes The perforated-steel tubes are shown in Fig. 1. The tension test coupons were cut from perforated-steel tube and three identical coupons were tested to determine the mechanical properties of the steel tubes. The average thickness of the coupon was 0.7 mm. The details of stress–strain relationship are shown in the Table 2. It shows that there was no obvious yield stage in stress–strain relationship and that the material went into hardening stage gradually after the elastic stage. The stress–strain relationships of the specimens are shown in Fig. 2. 2.2.3. GFRP Tensile test was conducted on GFRP laminate according to ASTM specification D3039 to obtain its material mechanical properties. Six identical tensile coupons were obtained from the GFRP material used to wrap the steel tube. Aluminum flat plates were glued at both ends of coupons to prevent the initial failure at grips. The tensile strength was 419 ± 24 MPa; the tensile elastic modulus was 38 ± 2.5 GPa; the average tensile ultimate stain was 0.0239 ± 0.0015. 2.3. Specimen preparations The fabrication procedural was divided into two parts. The first part was the fabrication of perforated-steel tubes. The thickness of steel tube is 0.7 mm, each tube has 300 mm height and 148 mm inner diameter. Once the steel tubes were ready, they were drilled to form the perforations. There were two types of grid form: one was helical form creating a spiral pattern of perforations; the other was axial form creating a column pattern of perforations. Both grids were manufactured in two alternatives: 5 mm and 10 mm
Table 1 Test matrix of specimens. Group
Specimen
GFRP layer
Diameter of perforation (mm)
Grid form
1 2 3 4 5 6 7 8 9 10
PC D10-H G2 G1–D10-H G2–D10-H G3–D10-H G4–D10-H G2–D10-A G2–D5-H G2–D5-A
— — 2 1 2 3 4 2 2 2
— 10 — 10 10 10 10 10 5 5
— Helical — Helical Helical Helical Helical Axial Helical Axial
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(a)
(b)
(c)
(d)
Fig. 1. Perforated-steel tubes (a) D10-A (b) D5-A (c) D10-H (d) D5-H.
(a) Table 2 Material properties for perforated-steel coupons. Specimen
Yield strength (MPa)
Ultimate strength (MPa)
Elastic modulus (GPa)
D10-A D5-A D10-H D5-H
378 315 296 235
435 394 362 309
195 259 162 185
500 400
(b)
Fig. 3. GFRP tube combined with perforated-steel tube. (a) GFRP with perforatedsteel tube of 10 mm diameter hole. (b) GFRP with perforated-steel tube of 5 mm diameter hole.
were installed on the middle portion of the outer surface of GFRP tubes at every 120° along the circle direction. Every units consisted of two strain gauges which measured the strains in transverse and longitudinal directions. The bottom end of tubes were capped with a plastic plate before casting concrete. The concrete was cast, vibrated, and cured for 28 days under standard wet curing procedure. Both concrete ends were sanded before testing. 2.4. Instrumentation and loading
300
The 10,000 kN standard displacement-controlled test machine was used in this experiment, as showed in Fig. 4. The following data was recorded:
200 D10-A D5-A D10-H D5-H
100 0 0
10000 20000 30000 40000 50000 60000 70000 -6
Strain με /10
Fig. 2. Tensile stress–strain relationship of perforated-steel coupons.
diameter perforations. The control group of perforated-steel tube was covered a thin plastic film to avoid leakage form the perforations after filling the concrete. The steel tube described in this paper had the same inner diameter, height, thickness, and mechanical properties. Three strain gauges were installed on the middle portion of inner surface of steel tube for every 120° along the circle direction. Every units consisted of two strain gauges which measured strains in transverse and longitudinal directions. The second part was to fabricate hybrid GFRP perforated-steel tube. Initially, the GFRP sheet was cut and trimmed into appropriate length with an overlap length of 150 mm. The GFRP was saturated with epoxy and then wrapped around the perforated-steel tube. Two extra GFRP strips with the width of 40 mm were wrapped around the top and bottom ends of cylinders in order to avoid end failure. The excess epoxy and air bubble were squeezed out of the surface of GFRP tube to make a uniform epoxy saturated surface. The hybrid GFRP perforated-steel tubes were air-dried for 7 days, as showed in Fig. 3. The control group of GFRP tube was fabricated following the same procedure. A thin plastic film was initially wrapped around a PVC tube with a diameter of 150 mm, and then the GFRP sheet saturated with epoxy was warped around the PVC tube. The GFRP tube was pulled out from the PVC tube after 3 h before it became complete hardening and shrinkage. The GFRP tubes were also air-dried for 7 days. Three strain gauges
(1) Axial load and displacement of specimens. (2) Axial and transversal strain on the middle portion of outside GFRP tube. (3) Axial and transversal strain on the middle portion of inside perforated-steel tube. The testing procedure was divided into two parts: alignment pre-testing and formal testing. The specimens were placed at the center of a compression plate at the beginning of the alignment pre-testing. Both ends of specimens were flattened with quartz sand, and spherical hinge with the capacity of 200 tons was installed between specimens and compression plate to ensure the axial compression loading. The application of initial load was estimated to be 10% of the maximum loading capacity and maintained the rate of 0.1 mm/min to ensure the specimens at the elastic range during alignment pre-testing. The tests were displacement controlled with the rate of 0.3 mm/min. Compression loading were applied continuously until the failure of specimens. 3. Results and discussion 3.1. Failure mode Perforated-steel tube-encased concrete cylinder under axial compression loading was performed first. In order to ensure the failure occurred at the middle portion of tubes, both ends of cylinder were confined with extra GFRP. The failure started with initial wrinkling followed by buckling of the steel tube. As the load increased, the grid became fractured with an obvious bulging phenomenon at the final load stage. The failure modes of perforatedsteel tube-encased concrete cylinder are indicated in Fig. 5.
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Fig. 4. Test setup.
Then, axial compression of GFRP tube-encased cylinders was conducted. During the testing, cracking sound of resin was heard before the ultimate compression capacity was achieved. Fibers in GFRP tube tore out with blasting sound when ultimate compression capacity was achieved. The cracking fiber extended to both ends of cylinder. The GFRP tube fractured into bands in the final loading stage. The core concrete were observed to crack into pieces at the same time. The failure mode of GFRP tube-encased concrete cylinder is shown in Fig. 6. Finally, GFRP–perforated-steel tube-encased concrete cylinders under axial compression were tested. Although the seven categories of specimens have different parameters such as perforation diameter, grid form and GFRP layer, the failure mode for all these specimens was similar. In the initial elastic loading stage, the sur-
Fig. 5. Failure mode of porous-steel tube-encased concrete cylinder. (a) Before tests (b) after tests.
face deformation of the specimens did not show significant change. The intermittent blasting sound could be heard from GFRP tube at increasing loading. The typical failure modes for GFRP–perforatedsteel tube-encased concrete cylinder are illustrated in Fig. 7. In addition, plain concrete cylinder under axial compression was conducted as control group. 3.2. Stress–strain behavior The stress–strain behavior of specimens are illustrated in Fig. 8. The complete stress–strain relation of GFRP–perforated-steel tube encased concrete cylinder was firstly discussed which includes elastic segment, hardening period, softening branch and residual strength plateau. In the initial linear elasticity period, the confinement provided by GFRP and perforated-steel tube had very limit effect on core concrete. The core concrete was subjected to a higher confinement while loading increased and entered the non-linear hardening stage. Compared with stress–strain relationship of GFRP tube-encased concrete cylinder under axial compression, the GFRP–perforatedtube encased concrete cylinder had a similar behavior in hardening period, which indicated that the GFRP tube contributed significantly to confinement. When maximum loading capacity was reached, GFRP ruptured and the core concrete was subjected to an attenuated confinement force which resulted in a descending branch of stress–strain curves. Compared with stress–strain relationship of perforated-steel tube-encased concrete cylinder, the GFRP–perforated-steel tubeencased concrete cylinder had a similar behavior in softening branch and residual strength plateau stage. The descending branch possessed a relative long characteristics which maintained a residual loading and deformation capacity. It indicated that perforatedsteel tube developed the majority confinement effect, making a well performance in final stage and avoiding brittle failure. Therefore, the combination of GFRP and perforated-steel produced the best possible advantages of these two materials, which possess advanced post-peak ductile and loading capacity behavior in comparison with GFRP–concrete and steel–concrete. 3.3. Interaction between and steel and GFRP
Fig. 6. Failure mode of GFRP tube-encased concrete cylinder. (a) Before tests (b) after tests.
In GFRP–perforated-steel tube encased concrete, the core concrete and perforated-steel tube bear the applied loads directly under axial compression, GFRP tube does not carry the axial loads but contributes the confinement pressure in radial direction. In the initial loading, the Poisson’s ratio of the core concrete was lower than that of the perforated-steel tube, which means that the deformation in transverse direction of the core concrete was smaller than that of the perforated-steel tube. The interactive squeeze
L. Huang et al. / Composite Structures 142 (2016) 313–324
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Fig. 7. Failure mode of hybrid GFRP–perforated-steel tube-encased concrete cylinder. (a) Before tests (b) after tests (c) local bulging (d) core concrete crushed.
function between concrete and the steel was too small to be observed. The axial strain of the concrete increased with an increase in applied load which resulted in the generation of micro-crack of the core concrete. Then, the radial deformation of the core concrete exceeded that of the porous-steel tube gradually, which enhanced the interactive squeeze function between each other and generated the circular strain in the perforated-steel tube. The core concrete entered a plastic stage subsequently, the Poisson’s ratio and transversal deformation of the concrete also increased. The interactive squeeze function of GFRP and perforated-steel tube appeared because of the increased radial deformation of the perforated-steel tube. The tension strain in circular direction was then generated in the GFRP tube. At this stage, the core concrete was subjected to three dimensional compressive stress status, the perforated-steel tube was under axial compression, radial compression and circular tension stress status, while the GFRP tube was in hoop tension stress status. When perforated-steel tube was subjected to the three dimensional stress state within an elastic limit, the volume change for GFRP–perforated-steel tube encased concrete cylinder could be ignored because of the confinement effect provided by the GFRP. The cracks in the core concrete continued to expand with the increasing loading and the volume of core concrete also increased. The interactive squeeze function among core concrete, perforatedsteel tube and the GFRP tube increased constantly as well as the hoop strain in the perforated-steel and GFRP tube. Meanwhile, the axial compression stress redistribution occurred between the core concrete and the perforated-steel tube. The ratio for axial compression stress of perforated-steel tube decreased, the stress status transformed from axial compression to radial tension mainly. In addition, the core concrete obtained higher compression capacity when confined by both perforated-steel and GFRP tube, the ratio for axial compression stress of core concrete increased. The perforated-steel tube yielded afterwards and generated the plastic flow, GFRP tube obtained an increasing circular tension stress and fractured when ultimate tensional capacity was reached. The maximum compression loading and capacity for GFRP– perforated-steel tube encased concrete cylinder was achieved at this moment. The rate of axial compression stress and hoop tension stress of perforated-steel tube was observed to expedite after the fracture of GFRP tube, the buckle and rupture were observed when the ultimate stress was reached. Perforated-steel tube remained a more effective three dimension stress status due to the lateral confining pressure provided by the outer GFRP tube, it delayed the buckle of steel tube and gained less radial deformation under the same applied loading. It also prevented the brittle failure of the GFRP tube because of the plastic property of the perforated-steel tube. The perforated-steel tube transferred from plastic stage into secondary hardening stage when GFRP tube ruptured, which covered the shortage of
inadequate circular confinement and improved the performance of deformation in the final stage. 3.4. Influence factor provided by steel and GFRP 3.4.1. GFRP layers The stress–strain relationship curve of different layers of GFRP are shown in Fig. 9. At the initial loading, all the curves represented a linear behavior and adjoined each other. It indicated that the layers of GFRP had no significant effect at this elastic range of the concrete. Compared with specimen without GFRP confinement, the GFRP–perforated-steel tube encased concrete cylinder exhibited a hardening stage after the linear elasticity stage. The specimens with no layer of GFRP, D10-H, reached the maximum stress of 30.12 MPa, specimens with one, two, three and four layers of GFRP, G1–D10-H, G2–D10-H, G3–D10-H and G4–D10-H, had the maximum stress of 32.98, 46.72, 54.87 and 71.58 MPa, respectively. It explained that with more layers of GFRP, the hardening stage could be expanded. The increased number of GFRP layers enhanced the loading capacity and plastic deformation performance. The maximum compressive stress and strain for different layers of GFRP specimens are shown in Table 3. 3.4.2. Perforation diameter of perforated-steel The stress–strain relationship curves of two layers GFRP– perforated-steel tube-encased concrete cylinder with different steel perforation diameters are illustrated in Fig. 10. In the initial elastic stage, the curves of specimens G2–D5-H and G2–D10-H behaved in a linear manner. At the stress hardening stage, these curves were parallel to each other. The specimens G2–D5-H and G2–D10-H had a long and slow process compared with the specimen G2 at the softening branch and residual strength plateau stage. The material performance test showed that D10-H perforated-steel possessed the average yield strength and ultimate strength of 296 MPa and 362 MPa, respectively. While the D5-H perforated-steel possessed the average yield strength and ultimate strength of 235 MPa and 309 MPa, respectively. It expected that the loading carrying capacity of the D10-H perforated-steel tube was larger than that of the D5-H perforated-steel tube. As verified in Fig. 10, the specimen G2– D10-H had a higher loading capacity at the end of elastic and the hardening stages compared with the specimen G2–D5-H at the corresponding stages. Thus, it can be concluded that the specimen with a larger perforation diameter of perforated-steel tube had a higher loading capacity in axial compression. 3.4.3. Grid form of perforated-steel The stress–strain relationship curves of perforated-steel tube with different grid forms are showed in Fig. 10. The comparison items are specimens G2–D5-A versus G2–D5-H and G2–D10-A ver-
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G1-D10-H
G2-D10-H 80
80 70
specimen1 specimen2 specimen3
60
60 2
Axial stress (N/mm )
2
Axial stress (N/mm )
specimen1 specimen2 specimen3
70
50 40 30 20 10 0 0.00
50 40 30 20 10
0.01
0.02
0.03
0.04
0.05
0.06
0 0.00
0.07
0.01
0.02
Norminal axial strain
0.03
G3-D10-H
0.07
0.05
0.06
0.07
specimen1 specimen2 specimen3
70
2
Axial stress (N/mm )
60
2
Axial stress (N/mm )
0.06
80
specimen1 specimen2 specimen3
60 50 40 30
50 40 30
20
20
10
10
0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0 0.00
0.07
0.01
0.02
Norminal axial strain
0.03
G2-D5-H
G2-D10-A
70
0.04
Norminal axial strain
80
80
specimen1 specimen2 specimen3
70
specimen1 specimen2 specimen3
60
2
2
Axial stress (N/mm )
60
Axial stress (N/mm )
0.05
G4-D10-H
80 70
0.04
Norminal axial strain
50 40 30
50 40 30
20
20
10
10
0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 0.00
0.01
Norminal axial strain
0.02
0.03
0.04
0.05
0.06
0.07
Norminal axial strain Fig. 8. Stress–strain curves of specimens.
sus G2–D10-H. All these specimens exhibited the parallel curves at the hardening stage indicating the similar tendency in bearing capacity improvement. The material test showed that the D10-A perforated-steel possessed the average yield strength and ultimate strength of 378 MPa and 435 MPa, respectively, which were higher than those of D5-H. As shown in Fig. 10, the specimen G2–D5-A had a higher loading capacity at the end of the elastic and the hardening stages than that of the specimen G2–D5-H. The same conclusion can be drawn for D10-A and D10-H, which is also illustrated in
Fig. 10. The loading carrying capacity performance of specimen with the axial type of the steel tube is superior to that with a helical type. 3.5. Ductility The ductility index could be obtained from confinement ratio, confinement effectiveness and fracture energy, indicated in Eqs. (1) and (2). Fig. 11 shows schematic of calculation ductility index.
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G2-D5-A
G2 80
80
specimen1 specimen2 specimen3
70
70 60
2
2
Axial stress (N/mm )
60
Axial stress (N/mm )
specimen1 specimen2
50 40 30
50 40 30
20
20
10
10
0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0 0.00
0.07
0.01
0.02
Norminal axial strain
0.03
0.04
D10-H
0.06
0.07
0.05
0.06
0.07
PC
80
80
specimen1 specimen2 specimen3
70
70
specimen1 specimen2 specimen3
60 2
2
Axial stress (N/mm )
60
Axial stress (N/mm )
0.05
Norminal axial strain
50 40 30
50 40 30
20
20
10
10
0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0 0.00
0.07
Norminal axial strain
0.01
0.02
0.03
0.04
Norminal axial strain Fig. 8 (continued)
80
60 2
Axial stress (N/mm )
D, which denoted as ecu . The ultimate strain and the corresponding strain of maximum stress of plain concrete is indicated as eccu and ecco , respectively. Table 4 showed the average values of ductility index in each specimens group. Confinement ratio is defined as ultimate or maximum compressive axial strain of confined concrete to that of plain concrete. Confinement effectiveness is defined as maximum compressive stress of confined concrete to that of plain concrete. Fracture energy is the area under the stress–strain curve, which indicates the energy dissipated by specimens. The average values of ductility index in each specimens group were obtained, which shown in Table 4.
PC D10-H G1-D10-H G2-D10-H G3-D10-H G4-D10-H
70
50 40 30 20
Confinement ratio ¼ ecc =ecco
10 0 0.00
or
ecu =ecuo
Confinement effectiveness ¼ f cc =f co 0.01
0.02
0.03
0.04
0.05
0.06
0.07
Norminal axial strain Fig. 9. The stress–strain relationship of different layers of GFRP.
The curve OAC stands for a general stress–strain relationship of specimens. The specimens achieve the maximum loading capacity at point A and failure at point C. The maximum stress is at point E, indicated as f cc , the corresponding strain is at point B, indicated as ecc . The ultimate compressive axial strain of specimens is at point
ð1Þ ð2Þ
It could be concluded that confinement ratio, confinement effectiveness and fracture energy increased while adding the layer of GFRP. It is noted that the fracture energy for G2–D10-H is larger than that of summation of D10-H and G2, which indicated that the combination of these two materials could enhance the energy dissipation effects compared with the summation of two separate materials. The specimens did not fail immediately after the GFRP fractured; the steel and some parts of GFRP absorbed the residual energy. It also obtained that the 10 mm diameter perforation with axial style achieved the maximum ductility index with the same layer of GFRP group.
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Table 3 The comparison of maximum loading capacity and related strain for different layers of GFRP specimens. Specimens
PC
D10-H
G1–D10-H
G2–D10-H
G3–D10-H
G4–D10-H
Max stress (MPa) Related strain Stress increased ratio Strain increased ratio
27.61 0.004 — —
30.12 0.006 1.09 1.57
32.98 0.018 1.19 4.80
46.72 0.021 1.69 5.50
54.87 0.025 1.99 6.64
71.58 0.040 2.59 10.47
the analysis-oriented models are generated by incremental numerical procedure. This study adopted a design-oriented model due to large volume of experiment results, the regression analysis was utilized and stress–strain design model of hybrid GFRP– perforated-steel tube-encased concrete column under axial compression was proposed.
50
G2-D5-H G2-D10-A G2-D5-A G2-D10-H
2
Axial stress (N/mm )
40
30
20
10
0 0.00
0.01
0.02
0.03
0.04
0.05
Norminal axial strain
3.6.1. Derivation process According to the analysis of stress–strain relationships, GFRP provided mainly the confinement effect in the stress hardening stage before approaching the peak stress, while the perforatedsteel provided mainly the confinement effect in the rapid stress drop and strain softening stage after post-peak response. This study took into account Lam and Teng’s design model [24] for uniaxial compression behavior of FRP confined concrete. It also considered the effect of perforated-steel-confined concrete by using regression analysis by O’shea and Bridge’s [25]. Lam and Teng proposed stress–strain curves as Eqs. (3) and (4)
Fig. 10. The stress–strain relationship of different perforation diameter and grid form of perforated-steel tube.
rc ¼ Ec ec
ðEc E2 Þ2 2 ec 4f co
ð0 6 ec 6 et Þ
ð3Þ
rc ¼ f co þ E2 ec ðet 6 ec 6 ecc Þ
ð4Þ
co co where et ¼ E2f , E2 ¼ f ccef . Remaining variables in the equations are c E2 cc
defined above. According to Richard [26], the strength of confined concrete f cc and corresponding strain ecc is proportional to the confinement force, see Eqs. (5) and (6).
Fig. 11. Schematic of calculation ductility index.
3.6. Confinement model Design-oriented and analysis-oriented are two major types of models that utilized in analyzing constitutive relationship. The design-oriented models are based on experimental results while
f cc f ¼ 1 þ k1 l f co f co
ð5Þ
ecc f ¼ 1 þ k2 l eco f co
ð6Þ
where f co and eco are stress and related strain of plain concrete without confinement, respectively. f l is lateral confinement force. k1 and k2 are constants to be determined by experiments. This is the weakness of the above approach since the constants are mere correction factors allowing the fitting of experiments.
Table 4 The average values of ductility index in each specimens group. Specimens
ecc
ecu
f cc (MPa)
f cc /f co
ecc /ecco
ecu /ecuo
Fracture energy
PC D10-H G2 G1–D10-H G2–D5-H G2–D5-A G2–D10-H G2–D10-A G3–D10-H G4–D10-H
0.004 0.006 0.026 0.018 0.024 0.016 0.021 0.026 0.025 0.040
0.006 0.027 0.026 0.035 0.045 0.045 0.046 0.044 0.044 0.059
27.61 30.12 42.43 32.98 42.64 43.30 46.72 48.58 54.87 71.58
1.00 1.09 1.54 1.19 1.54 1.57 1.69 1.76 1.99 2.59
1.00 1.57 6.78 4.80 6.19 4.27 5.50 6.80 6.64 10.47
1.00 4.47 4.32 5.81 7.42 7.42 7.65 7.41 7.25 9.83
0.078 0.375 0.791 0.663 1.211 1.209 1.246 1.283 1.576 2.891
321
L. Huang et al. / Composite Structures 142 (2016) 313–324
3.6.2. Lateral confinement force The lateral confinement force f l were provided by GFRP (f lf ) and perforated-steel (f ls ), which could be calculated as f l ¼ f lf þ f ls , where f ls is written in Eq. (7).
f ls ¼ 2
f hs ts Dc
ð7Þ
where f hs is hoop stress of perforated-steel, t s is the thickness of perforated-steel tube, Dc is diameter of core concrete. For the perforated-steel tube, Von-Mises yield criterion can be used, therefore, the hoop stress f hs may not equal to the yield stress f y when perforated-steel tube yielded. It is suggested that the equation f hs ¼ bf y can be adopted in this situation. O’shea and Bridge [25] studied different strength of thin steel confined concrete and obtained Eq. (8) by regression analysis.
(
b ¼ 1:0
0 6 c 6 0:4
ð8Þ
b ¼ 10ð0:7cÞ c P 0:4 3 where c ¼
qffiffiffiffiffi
f co . fy
Therefore, Eq. (7) can be written as f ls ¼ 2
bf y t s . Dc
The effective lateral confinement force is calculated in Eq. (9).
f lf ¼ 2
f hf tf Ef ef t f ¼2 d d
ð9Þ
where f hf is the hoop stress of GFRP, tf is the thickness of GFRP, Ef is the elastic modulus of GFRP, ef is the actual fracture strain of GFRP, d is the diameter of confined concrete cylinder. 3.6.3. Peak stress and strain The peak stress and strain of GFRP–perforated-steel tubeencased concrete cylinder were defined as the rupture stress and strain of GFRP. The core concrete was subjected to three dimensional stress condition after confinement was activated, the stress level was directly affected by the lateral confinement force. Equation (5) can be rewritten as
f cc f co
f lf þ f ls ¼ 1 þ k1 f co
ð10Þ
The coefficient k1 can be obtained by linear regression analysis of the experiment results shown in Fig. 12(a). The square value of correlation coefficient for fitting straight line and experimental
data points was 0.9946, which demonstrated a good agreement of each other. The calculation formula of peak stress of GFRP– perforated-steel tube-encased concrete cylinder is shown in Eq. (11).
f lf þ f ls f cc ¼ 1 þ 2:58 f co f co
ð11Þ
The peak strain of core concrete enhanced significantly by the lateral confinement provided by GFRP and perforated-steel tube. The peak and ultimate strain of unconfined concrete can be adopted by 0.002 and 0.004, respectively. In this situation, Eq. (12) is given below with a frequently used form for existing strain models for FRP confined concrete.
ecc f ¼ 2 þ k2 l eco f co
ð12Þ
The coefficient k2 can be obtained by linear regression analysis of experiment results, which is indicated in Fig. 12(b). The square value of correlation coefficient for fitting straight line and experimental data points was 0.9766, which demonstrated a good agreement of each other. The calculation formula of peak stress of GFRP– perforated-steel tube-encased concrete cylinder is shown in Eq. (13).
f lf þ f ls ecc ¼ 2 þ 46:12 eco f co
ð13Þ
3.6.4. Comparison of model and experiment results The simplified design model of GFRP–perforated-steel tubeencased concrete cylinder was proposed in this study based on Eqs. (3), (4), (11) and (13). Fig. 13 shows the comparison between the predicted results by the model and their experimental data, which demonstrated a good agreement. 4. Comparison with existing confined concrete structures The confinement performance (with respect to confinement effectiveness and confinement ratio) of GFRP–perforated-steel tube encased concrete column is compared with that of existing confined concrete structure, i.e. (1) FRP tube confined plain concrete or FRP wrapped plain concrete and (2) FRP tube confined reinforced concrete or FRP wrapped confined reinforced concrete. A group of test results of existing confined concrete structures
2.4
28 26
2.2
24 22
2.0
20 18
1.8
2
fcc/fco
R =0.9946
εcc/εco
1.6
2
16
R =0.9766
14 12 10
1.4
8 6
1.2
4 1.0 0.0
0.1
0.2
0.3
0.4
0.5
2 0.0
0.1
0.2
0.3
(flf+fls)/fco
(flf+fls)/fco
(a)
(b)
0.4
0.5
Fig. 12. Linear fit of the equations to the experimental data obtained from the cylinder test. (a) Linear fit of the peak stress (b) linear fit of the peak strain.
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L. Huang et al. / Composite Structures 142 (2016) 313–324
80
G1-D10-H
80
Test
70
Model
60
50
50
Stress/MPa
60 40 30 20
G2-D10-H
Test Model
40 30 20
10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
80 G3-D10-H 70
80 G4-D10-H 70
Test Model
60
50
50
Stress/MPa
60 40 30
Test Model
40 30
20
20
10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
80 G2-D5-A 70
80 G2-D5-H 70
Test Model
60
60
50
50
Stress/MPa
Stress/MPa
Stress/MPa
Stress/MPa
70
40 30 20 10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
80 G2-D10-A 70
Test Model
40 30 20 10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
Test Model
Stress/MPa
60 50 40 30 20 10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain Fig. 13. Comparison of model and experiment results.
was collected from the literature and is listed in Table 5. To make the results comparable, the collected specimens have similar specimen size, unconfined concrete strength, thickness of FRP and/or steel reinforcement ratio as these used for GFRP–perforated-steel tube encased concrete in this study. 4.1. FRP tube confined plain concrete or FRP wrapped confined plain concrete In this section, the confinement performance of GFRP– perforated-steel tube encased concrete column is compared with CFRP wrapped or CFRP tube confined plain concrete reported in [27,28]. In Ref. [27], the tensile strength and modulus of CFRP used for FRP tube confined plain concrete was 3550 MPa and 235 GPa, respectively. In Ref. [28], the tensile strength and modulus of the CFRP used for FRP wrapped plain concrete was 1577 MPa and 105 GPa, respectively. As can be seen in Table 5, the values of
confinement effectiveness of GFRP–perforated-steel tube encased concrete column are comparable to those of CFRP tube or CFRP wrapped concrete reported by Shehata et al. [27], Xiao and Wu [28], even the tensile modulus and tensile strength of the CFRP are significantly larger than those of glass FRP (i.e. tensile strength and modulus of 419 MPa and 38 GPa respectively) used in this study. Regarding failure mode, it was reported that FRP tube encased plain concrete or FRP wrapped plain concrete exhibited brittle and sudden failure of the structure without prior warning due to the non-yielding characteristic of FRP material and the brittle behavior of plain concrete. In addition, the concrete core was fully crushed after the rupture of the outer FRP and lose all the load bearing capacity [28,29]. While for GFRP–perforated-steel tube encased concrete column, the failure of the structure was relatively ductile and the concrete core can carry residual load, as shown in failure mode of GFRP–perforated-steel tube encased concrete column in Fig. 7 and the post-peak descending branch in compressive
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L. Huang et al. / Composite Structures 142 (2016) 313–324 Table 5 Test results of FRP confined concrete specimens. Data sources
D (mm)
L (mm)
t (mm)
Fiber
qs (%)
f co (MPa)
f cc (MPa)
ecc
erup
f l;a (MPa)
Shehata [27]
150
300
0.165 0.330
Carbon
–
29.8
57.0 72.1
0.0123 0.0174
0.0123 0.0119
6.36 17.99
Xiao and Wu [28]
152
305
0.380 0.380 0.380 0.760 0.760 0.760
Carbon
–
33.7
47.9 49.7 49.4 64.6 75.2 71.8
0.0120 0.0140 0.0124 0.0165 0.0225 0.0216
0.0084 0.0115 0.0087 0.0091 0.0100 0.0100
4.41 6.04 4.57 9.56 10.50 10.50
Lee et al. [30]
150
300
0.11 0.22 0.33 0.44
Carbon
1.31 1.31 1.31 1.31
36.2
41.7 57.8 69.1 85.4
0.0024 0.0024 0.0024 0.0024
0.010 0.015 0.020 0.027
6.61 13.22 19.83 26.44
This study
150
300
0.3 0.6 0.6 0.6 0.6 0.9 1.2
Glass
1.70 1.75 1.75 1.70 1.70 1.70 1.70
27.61
32.98 42.64 43.30 46.72 48.58 54.87 71.58
0.018 0.024 0.016 0.021 0.026 0.025 0.040
0.008 0.016 0.016 0.018 0.015 0.018 0.014
3.31 4.69 6.35 7.45 8.84 9.66 14.36
stress–strain curves of GFRP–perforated-steel tube encased concrete column in Fig. 8. Thus, the GFRP–perforated-steel tube encased concrete column exhibits comparable load carrying capacity to that of CFRP tube confined plain concrete or CFRP wrapped plain concrete where the tensile properties of carbon FRP are significantly larger. However, the failure mode of GFRP–perforatedsteel tube encased concrete was ductile but that of FRP confined plain concrete was brittle. 4.2. FRP tube confined reinforced concrete or FRP wrapped confined reinforced concrete In this section, the confinement performance of GFRP– perforated-steel tube encased concrete column is compared with FRP wrapped or FRP tube confined reinforced concrete reported in [30]. Lee et al. [30] investigated compressive behavior of CFRP confined spiral steel reinforced concrete columns, the spiral steel reinforcement ratio was 1.31%, which is relatively smaller than that of 1.70% for perforated-steel tube. The yield strength of the spiral reinforcement used by Lee et al. was 1200 MPa, which is lar-
ecc eco
f l;a f co
f cc f co
5.857 8.286
0.213 0.604
1.91 2.42
0.923 1.077 0.954 1.269 1.731 1.662
0.131 0.179 0.136 0.284 0.312 0.312
1.42 1.47 1.46 1.92 2.23 2.13
4.17 6.25 8.33 11.25
0.183 0.365 0.548 0.730
1.15 1.60 1.91 2.36
4.80 6.19 4.27 5.50 6.80 6.64 10.47
0.12 0.17 0.23 0.27 0.32 0.35 0.52
1.19 1.54 1.57 1.69 1.76 1.99 2.59
ger than that of the perforated-steel (235–378 MPa) used in this study. The tensile strength and modulus of the CFRP used was 4510 MPa and 250 GPa, respectively, which are significantly larger than those of GFRP used in this study. The reported confinement effectiveness for the spiral steel reinforced concrete column with 1, 2, 3, and 4-layer CFRP was 1.15, 1.60, 1.91 and 2.36, respectively, which is close to the confinement effectiveness of GFRP– perforated-steel tube encased concrete, ranging from 1.19 to 2.59 for the specimens with 1, 2, 3 and 4-layer GFRP. The reported confinement ratio by Lee et al. [30] for specimens with CFRP from 1 to 4 layers ranged from 0.183 to 0.730, which are similar to those of GFRP–perforated-steel tube encased concrete, between 0.12 and 0.52. The typical failure mode of CFRP confined spiral steel reinforced concrete is given in Fig. 14 [3]. Therefore, the GFRP– perforated-steel tube encased concrete has comparable structural performance to that of CFRP confined spiral steel reinforced concrete. Overall, it can be concluded that the GFRP–perforated-steel tube encased concrete has the potential to axial structural member with higher load carrying capacity and ductility. 5. Conclusion This paper presented the behavior of hybrid GFRP–perforated steel tube-encased concrete column under axial compression. The main conclusions are as follows.
Fig. 14. Failure mode of CFRP confined reinforced concrete after compression test [31].
(1) The failure mode for GFRP–perforated steel tube-encased concrete column was ductile. The failure process were GFRP tube fractured firstly, perforated-steel tube yielded followed by bulging and crushing of the concrete core. The entire specimens received a ‘drum’ shape in the final stage. The loading capacity was reached when the GFRP fractured; the specimens became completely damaged when the perforated-steel tube bulged and fractured. (2) Stress–strain curves of GFRP–perforated-steel tube-encased concrete column could be divided into three parts: (1) linear elastic, (2) non-linear hardening, softening branch and (3) residual strength plateau. In the non-linear hardening stage, the stress–strain curves were similar to the ones of GFRP confined concrete because GFRP provided mainly confinement. In the softening and residual strength plateau stages, the stress–strain curves were similar to those of
324
(3)
(4)
(5)
(6)
(7)
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perforated-steel tube confined concrete because the plastic deformation and confinement force were mainly depended on perforated-steel tube. Ductility index, including confinement ratio, confinement effectiveness and fracture energy, could be controlled by the layers of GFRP, diameter of perforation and form of perforation. More layers of GFRP could result in better ductility performance. The combination of axial arrangement and larger diameter perforations resulted in better ductility ratios. The load capacity and plastic deformation of GFRP– perforated-steel tube-encased concrete column was enhanced when the number of layers of the GFRP increased. The perforation diameter of 10 mm of D10-H type possessed higher yield and ultimate strength than that of perforation diameter of 5 mm, which demonstrated 10 mm diameter could obtain better enhancement of loading capacity. The yield and ultimate strength of axial type is higher to that of helical type, thus the compression capacity performance of axial type is superior to helical type. GFRP–perforated-steel tube-encased concrete column behaved bi-linear manner under uniaxial loading. The stress–strain curves of elasticity and hardening stage could be expressed as parabola and straight line. The simplified design model could be obtained by regression analysis according to Lam and Teng’s model. The calculation results were in good agreement of experiment results. The combination of GFRP and perforated-steel is the new effective method to confine concrete; it utilized the advantages of these two materials. GFRP with its high tension strength enhanced the loading capacity and deformation performance in hardening stages while perforated-steel with its high elasto-plasticity improved loading capacity and deformation properties in softening branch and residual strength plateau stage. The comparison in axial compressive behavior of GFRP– perforated-steel tube-encased concrete with other existing confined concrete indicates that this GFRP–perforated-steel tube-encased concrete has the potential to be axial structural member with higher load carrying capacity and ductility.
Acknowledgements The authors gratefully acknowledge the financial support provided by Hunan Provincial Natural Science Foundation of China (No. 2015JJ1004). The authors also wish to thank Dongsheng He for his valuable contribution to the experimental work. References [1] Zaghi AE, Saiidi MS, Mirmiran A. Shake table response and analysis of a concrete-filled FRP tube bridge column. Compos Struct 2012;94:1564–74. [2] Wan B. Using fiber-reinforced polymer (FRP) composites in bridge construction and monitoring their performance: an overview. In: Kim YJ, editor. Advanced composites in bridge construction and repair. Woodhead Publishing; 2014. p. 3–29.
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Behavior of hybrid GFRP–perforated-steel tube-encased concrete column under uniaxial compression Liang Huang a,⇑, Peng Yin b,⇑, Libo Yan c,d,⇑, Bohumil Kasal c,d,⇑ a
College of Civil Engineering, Hunan University, Changsha 410082, China The Henry Samueli School of Engineering, University of California, Irvine, Irvine, CA 92697, United States c Centre for Light and Environmentally-Friendly Structures, Fraunhofer Wilhelm-Klauditz-Institut WKI, Bienroder Weg 54E, 38108 Braunschweig, Germany d Department of Organic and Wood-Based Construction Materials, Technical University of Braunschweig, Hopfengarten 20, 38102 Braunschweig, Germany b
a r t i c l e
i n f o
Article history: Available online 8 February 2016 Keywords: GFRP Perforated-steel Uniaxial compression Model
a b s t r a c t This study presents an experimental investigation on concrete confined by glass fiber reinforced polymer (GFRP) and perforated-steel tube subjected to uniaxial compression. The compressive behavior of this hybrid structure is compared with GFRP tube encased concrete and steel tube encased concrete. The three key experimental parameters were investigated: the layers of GFRP tube (1, 2, 3 and 4), the perforation diameter of porous steel tube (5 mm and 10 mm), and the grid form of perforated steel tube (axial type and helical type). The effects of these parameters on the failure mode, stress–strain relationship and ductility of the specimens were discussed. A simplified design model was proposed for the specimens and the predictions were in good agreement with the experimental results. In addition, the interactive function between steel and GFRP on the confined concrete was analyzed which considered an influence factor provided by these two confinement materials. This study showed that the hybrid GFRP–perforated steel tube exerts the full potential of these two materials, i.e. GFRP with its high tension strength enhanced the loading carrying capacity and the perforated-steel with its high elastoplasticity enhanced the deformation performance in hardening segment, softening branch and residual strength plateau of the concrete, respectively. This study therefore demonstrated that the combination of GFRP and perforated steel tubes is a new effective method to confine concrete. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Confinement systems can significantly improve the load capacity and deformation performance of concrete structural element. Over the decades, steel tube and FRP tube-encased concrete columns have been widely used in infrastructure such as in highrise building, off-shore structure and bridges [1–4]. Steel tube-encased concrete columns possess excellent properties related to structural performance such as high ductility, and fatigue and impact-resistance. In addition, steel tube itself can serve as formwork of concrete which reduces the cost of traditional wood formwork and labor to remove it after concrete is cured [5– 7]. However, the potential corrosion of steel main limit its usage. In addition, the core concrete shrinks during the cement setting and
⇑ Address: Department of Organic and Wood-Based Construction Materials, Technical University of Braunschweig, Hopfengarten 20, 38102 Braunschweig, Germany (L. Yan, B. Kasal). E-mail addresses: [email protected] (L. Huang), [email protected] (P. Yin), [email protected] (L. Yan), [email protected] (B. Kasal). http://dx.doi.org/10.1016/j.compstruct.2016.02.016 0263-8223/Ó 2016 Elsevier Ltd. All rights reserved.
hardening, which results in a gap between the steel tube and the concrete core. Consequently, the interfacial slip due to insufficient bonding between steel and concrete can result in ineffective stress transfer between them. These possible disadvantages would compromise the structural performance of steel tube-encased concrete columns. FRP composites have been successfully used in the field of civil engineering in the last two decades because of their high strengthto-weight ratio, good corrosion resistance, and electromagnetic neutrality. In FRP tube encased concrete, the FRP tube prevents the carbonization of the core concrete and also serves as formwork of the concrete during construction [8–14]. However, because of a brittle failure mode and non-ductile behavior, FRP materials will fully lose its loading capacity once the failure of the FRP starts, which leads to the failure of FRP tube encased concrete showing a brittle failure without any warning prior in the rupture of FRP. Although both steel tube and FRP tube-encased concrete columns demonstrate advantages over traditional reinforcement concrete columns, there is a need for further improvements and innovations due to the aforementioned limitations. Therefore, the
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hybrid structure combining FRP and steel tube-encased column has been proposed by Feng et al. [15], Zhang et al. [16], Wang et al. [17], Teng et al. [18], Ganesh Prabhu and Sundarraja [19], Fakharifar et al. [20] and Ghanbari Ghazijahani et al. [21]. Such combination delays the onset of plastic hinge at the end of steel tube-encased concrete column by wrapping external FRP material. Some researchers [22] assigned the effects of different void area ratio by using serval types steel tube and carbon FRP (CFPR) on the structural performance on steel–FRP tube encased concrete. It indicated that CFRP tube can delay the local buckling of steel tube significantly and increase the load capacity and ductility of concrete column. In addition, the corrosion of the steel can be eliminated by applying FRP jacket outside the steel tube. Compared with CFRP materials, GFRP has lower ultimate tension capacity but a higher ultimate elongation, In addition, GFRP is an electrical insulating material which inhibits the electrochemical corrosion with steel. CFRP is a conductive material and the electrochemical corrosion of steel would occur if the resin layer separating the steel and CFRP is compromised. Steel is an isotropic material, which provides less confinement at optimal transverse direction compared with an anisotropic material with the same amount. Some researches drilled the steel tube to form grids, this would increase the confinement efficiency and the interfacial bonding strength [23]. A new hybrid GFRP–perforated-steel tube was proposed in this study that will overcome the above deficiencies. The hybrid tube is fabricated with perforated steel tube and then externally wrapped with GFRP skin, which is used as formwork for fresh concrete. This hybrid GFRP–perforated-steel tube addresses the following issues: (1) the interlocking mechanism is generated between the steel grids and the concrete. As the perforations in the steel tube are filled with concrete, an effective mechanical interlocking is achieved; (2) the perforated-steel tube increases the transverse confinement efficiency compared to a regular steel tube with the same amount of steel; (3) GFRP material has compatible deformation with steel tube, resulting in an inspiring ductility for the concrete column compared with concrete confined with CFRP tube only; (4) the hybrid confinement concrete column possesses high corrosion resistance and relatively low cost. This study focuses on the mechanism behavior of this hybrid column subjected to axial compression load. The effects of GFRP layers, diameter of perforations on steel tube, grid form of steel tube were investigated. Stress–strain behavior, failure mode, ultimate condition, compressive ductility and other properties were studied and analyzed. In addition, a stress–strain model was proposed for the future hybrid GFRP–perforated-steel tube-encased concrete column design.
2. Experiment 2.1. Test matrix The total of 30 cylindrical specimens with a diameter of 150 mm and a height of 300 mm were fabricated and tested under axial compression. The hybrid GFRP–perforated-steel tube-encased concrete specimens were designed on the basis of the following parameters: (1) the number of GFRP wrap layers; (2) the diameter of perforation on steel tube; (3) grid form of steel tube. Each parameter is classified into several variables: (1) the number GFRP layers is divided into one, two, three and four layers, which noted as G1, G2, G3, and G4, respectively; (2) the diameter of perforation on steel tube is divided into 10 mm and 5 mm, which noted as D10 and D5, respectively; (3) grid form of steel tube is divided into helical and axial style, which is denoted as ‘H’ and ‘A’, respectively. Additionally, control group were set as follows: (1) plain concrete cylinder, which is denoted as PC; (2) GFRP tube-encased concrete
cylinder with two layers of GFRP, which noted as G2; (3) perforated-steel tube-encased concrete cylinder with perforation diameter of 10 mm and helical style, which is denoted as D10-H. The compressive behavior of GFRP tube encased concrete and steel tube encased concrete is used to compare with that of the hybrid structure. There are three identical specimens were fabricated and tested for each groups. The details of test matrix is summarized in Table 1. 2.2. Materials properties 2.2.1. Concrete The components of concrete are Type I Portland cement, gravel, natural sand, water, and superplasticizer. The mix design by weight was cement:water:coarse aggregate:fine aggregate:admixture = 1:0.43:3.51:1.80:0.0075. Three plain concrete cylinders were tested to determine the average maximum strength of unconfined concrete, f co , and its corresponding strain, eco . The 28th day cylinder compressive strength is 27.61 MPa. 2.2.2. Perforated-steel tubes The perforated-steel tubes are shown in Fig. 1. The tension test coupons were cut from perforated-steel tube and three identical coupons were tested to determine the mechanical properties of the steel tubes. The average thickness of the coupon was 0.7 mm. The details of stress–strain relationship are shown in the Table 2. It shows that there was no obvious yield stage in stress–strain relationship and that the material went into hardening stage gradually after the elastic stage. The stress–strain relationships of the specimens are shown in Fig. 2. 2.2.3. GFRP Tensile test was conducted on GFRP laminate according to ASTM specification D3039 to obtain its material mechanical properties. Six identical tensile coupons were obtained from the GFRP material used to wrap the steel tube. Aluminum flat plates were glued at both ends of coupons to prevent the initial failure at grips. The tensile strength was 419 ± 24 MPa; the tensile elastic modulus was 38 ± 2.5 GPa; the average tensile ultimate stain was 0.0239 ± 0.0015. 2.3. Specimen preparations The fabrication procedural was divided into two parts. The first part was the fabrication of perforated-steel tubes. The thickness of steel tube is 0.7 mm, each tube has 300 mm height and 148 mm inner diameter. Once the steel tubes were ready, they were drilled to form the perforations. There were two types of grid form: one was helical form creating a spiral pattern of perforations; the other was axial form creating a column pattern of perforations. Both grids were manufactured in two alternatives: 5 mm and 10 mm
Table 1 Test matrix of specimens. Group
Specimen
GFRP layer
Diameter of perforation (mm)
Grid form
1 2 3 4 5 6 7 8 9 10
PC D10-H G2 G1–D10-H G2–D10-H G3–D10-H G4–D10-H G2–D10-A G2–D5-H G2–D5-A
— — 2 1 2 3 4 2 2 2
— 10 — 10 10 10 10 10 5 5
— Helical — Helical Helical Helical Helical Axial Helical Axial
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(a)
(b)
(c)
(d)
Fig. 1. Perforated-steel tubes (a) D10-A (b) D5-A (c) D10-H (d) D5-H.
(a) Table 2 Material properties for perforated-steel coupons. Specimen
Yield strength (MPa)
Ultimate strength (MPa)
Elastic modulus (GPa)
D10-A D5-A D10-H D5-H
378 315 296 235
435 394 362 309
195 259 162 185
500 400
(b)
Fig. 3. GFRP tube combined with perforated-steel tube. (a) GFRP with perforatedsteel tube of 10 mm diameter hole. (b) GFRP with perforated-steel tube of 5 mm diameter hole.
were installed on the middle portion of the outer surface of GFRP tubes at every 120° along the circle direction. Every units consisted of two strain gauges which measured the strains in transverse and longitudinal directions. The bottom end of tubes were capped with a plastic plate before casting concrete. The concrete was cast, vibrated, and cured for 28 days under standard wet curing procedure. Both concrete ends were sanded before testing. 2.4. Instrumentation and loading
300
The 10,000 kN standard displacement-controlled test machine was used in this experiment, as showed in Fig. 4. The following data was recorded:
200 D10-A D5-A D10-H D5-H
100 0 0
10000 20000 30000 40000 50000 60000 70000 -6
Strain με /10
Fig. 2. Tensile stress–strain relationship of perforated-steel coupons.
diameter perforations. The control group of perforated-steel tube was covered a thin plastic film to avoid leakage form the perforations after filling the concrete. The steel tube described in this paper had the same inner diameter, height, thickness, and mechanical properties. Three strain gauges were installed on the middle portion of inner surface of steel tube for every 120° along the circle direction. Every units consisted of two strain gauges which measured strains in transverse and longitudinal directions. The second part was to fabricate hybrid GFRP perforated-steel tube. Initially, the GFRP sheet was cut and trimmed into appropriate length with an overlap length of 150 mm. The GFRP was saturated with epoxy and then wrapped around the perforated-steel tube. Two extra GFRP strips with the width of 40 mm were wrapped around the top and bottom ends of cylinders in order to avoid end failure. The excess epoxy and air bubble were squeezed out of the surface of GFRP tube to make a uniform epoxy saturated surface. The hybrid GFRP perforated-steel tubes were air-dried for 7 days, as showed in Fig. 3. The control group of GFRP tube was fabricated following the same procedure. A thin plastic film was initially wrapped around a PVC tube with a diameter of 150 mm, and then the GFRP sheet saturated with epoxy was warped around the PVC tube. The GFRP tube was pulled out from the PVC tube after 3 h before it became complete hardening and shrinkage. The GFRP tubes were also air-dried for 7 days. Three strain gauges
(1) Axial load and displacement of specimens. (2) Axial and transversal strain on the middle portion of outside GFRP tube. (3) Axial and transversal strain on the middle portion of inside perforated-steel tube. The testing procedure was divided into two parts: alignment pre-testing and formal testing. The specimens were placed at the center of a compression plate at the beginning of the alignment pre-testing. Both ends of specimens were flattened with quartz sand, and spherical hinge with the capacity of 200 tons was installed between specimens and compression plate to ensure the axial compression loading. The application of initial load was estimated to be 10% of the maximum loading capacity and maintained the rate of 0.1 mm/min to ensure the specimens at the elastic range during alignment pre-testing. The tests were displacement controlled with the rate of 0.3 mm/min. Compression loading were applied continuously until the failure of specimens. 3. Results and discussion 3.1. Failure mode Perforated-steel tube-encased concrete cylinder under axial compression loading was performed first. In order to ensure the failure occurred at the middle portion of tubes, both ends of cylinder were confined with extra GFRP. The failure started with initial wrinkling followed by buckling of the steel tube. As the load increased, the grid became fractured with an obvious bulging phenomenon at the final load stage. The failure modes of perforatedsteel tube-encased concrete cylinder are indicated in Fig. 5.
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Fig. 4. Test setup.
Then, axial compression of GFRP tube-encased cylinders was conducted. During the testing, cracking sound of resin was heard before the ultimate compression capacity was achieved. Fibers in GFRP tube tore out with blasting sound when ultimate compression capacity was achieved. The cracking fiber extended to both ends of cylinder. The GFRP tube fractured into bands in the final loading stage. The core concrete were observed to crack into pieces at the same time. The failure mode of GFRP tube-encased concrete cylinder is shown in Fig. 6. Finally, GFRP–perforated-steel tube-encased concrete cylinders under axial compression were tested. Although the seven categories of specimens have different parameters such as perforation diameter, grid form and GFRP layer, the failure mode for all these specimens was similar. In the initial elastic loading stage, the sur-
Fig. 5. Failure mode of porous-steel tube-encased concrete cylinder. (a) Before tests (b) after tests.
face deformation of the specimens did not show significant change. The intermittent blasting sound could be heard from GFRP tube at increasing loading. The typical failure modes for GFRP–perforatedsteel tube-encased concrete cylinder are illustrated in Fig. 7. In addition, plain concrete cylinder under axial compression was conducted as control group. 3.2. Stress–strain behavior The stress–strain behavior of specimens are illustrated in Fig. 8. The complete stress–strain relation of GFRP–perforated-steel tube encased concrete cylinder was firstly discussed which includes elastic segment, hardening period, softening branch and residual strength plateau. In the initial linear elasticity period, the confinement provided by GFRP and perforated-steel tube had very limit effect on core concrete. The core concrete was subjected to a higher confinement while loading increased and entered the non-linear hardening stage. Compared with stress–strain relationship of GFRP tube-encased concrete cylinder under axial compression, the GFRP–perforatedtube encased concrete cylinder had a similar behavior in hardening period, which indicated that the GFRP tube contributed significantly to confinement. When maximum loading capacity was reached, GFRP ruptured and the core concrete was subjected to an attenuated confinement force which resulted in a descending branch of stress–strain curves. Compared with stress–strain relationship of perforated-steel tube-encased concrete cylinder, the GFRP–perforated-steel tubeencased concrete cylinder had a similar behavior in softening branch and residual strength plateau stage. The descending branch possessed a relative long characteristics which maintained a residual loading and deformation capacity. It indicated that perforatedsteel tube developed the majority confinement effect, making a well performance in final stage and avoiding brittle failure. Therefore, the combination of GFRP and perforated-steel produced the best possible advantages of these two materials, which possess advanced post-peak ductile and loading capacity behavior in comparison with GFRP–concrete and steel–concrete. 3.3. Interaction between and steel and GFRP
Fig. 6. Failure mode of GFRP tube-encased concrete cylinder. (a) Before tests (b) after tests.
In GFRP–perforated-steel tube encased concrete, the core concrete and perforated-steel tube bear the applied loads directly under axial compression, GFRP tube does not carry the axial loads but contributes the confinement pressure in radial direction. In the initial loading, the Poisson’s ratio of the core concrete was lower than that of the perforated-steel tube, which means that the deformation in transverse direction of the core concrete was smaller than that of the perforated-steel tube. The interactive squeeze
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Fig. 7. Failure mode of hybrid GFRP–perforated-steel tube-encased concrete cylinder. (a) Before tests (b) after tests (c) local bulging (d) core concrete crushed.
function between concrete and the steel was too small to be observed. The axial strain of the concrete increased with an increase in applied load which resulted in the generation of micro-crack of the core concrete. Then, the radial deformation of the core concrete exceeded that of the porous-steel tube gradually, which enhanced the interactive squeeze function between each other and generated the circular strain in the perforated-steel tube. The core concrete entered a plastic stage subsequently, the Poisson’s ratio and transversal deformation of the concrete also increased. The interactive squeeze function of GFRP and perforated-steel tube appeared because of the increased radial deformation of the perforated-steel tube. The tension strain in circular direction was then generated in the GFRP tube. At this stage, the core concrete was subjected to three dimensional compressive stress status, the perforated-steel tube was under axial compression, radial compression and circular tension stress status, while the GFRP tube was in hoop tension stress status. When perforated-steel tube was subjected to the three dimensional stress state within an elastic limit, the volume change for GFRP–perforated-steel tube encased concrete cylinder could be ignored because of the confinement effect provided by the GFRP. The cracks in the core concrete continued to expand with the increasing loading and the volume of core concrete also increased. The interactive squeeze function among core concrete, perforatedsteel tube and the GFRP tube increased constantly as well as the hoop strain in the perforated-steel and GFRP tube. Meanwhile, the axial compression stress redistribution occurred between the core concrete and the perforated-steel tube. The ratio for axial compression stress of perforated-steel tube decreased, the stress status transformed from axial compression to radial tension mainly. In addition, the core concrete obtained higher compression capacity when confined by both perforated-steel and GFRP tube, the ratio for axial compression stress of core concrete increased. The perforated-steel tube yielded afterwards and generated the plastic flow, GFRP tube obtained an increasing circular tension stress and fractured when ultimate tensional capacity was reached. The maximum compression loading and capacity for GFRP– perforated-steel tube encased concrete cylinder was achieved at this moment. The rate of axial compression stress and hoop tension stress of perforated-steel tube was observed to expedite after the fracture of GFRP tube, the buckle and rupture were observed when the ultimate stress was reached. Perforated-steel tube remained a more effective three dimension stress status due to the lateral confining pressure provided by the outer GFRP tube, it delayed the buckle of steel tube and gained less radial deformation under the same applied loading. It also prevented the brittle failure of the GFRP tube because of the plastic property of the perforated-steel tube. The perforated-steel tube transferred from plastic stage into secondary hardening stage when GFRP tube ruptured, which covered the shortage of
inadequate circular confinement and improved the performance of deformation in the final stage. 3.4. Influence factor provided by steel and GFRP 3.4.1. GFRP layers The stress–strain relationship curve of different layers of GFRP are shown in Fig. 9. At the initial loading, all the curves represented a linear behavior and adjoined each other. It indicated that the layers of GFRP had no significant effect at this elastic range of the concrete. Compared with specimen without GFRP confinement, the GFRP–perforated-steel tube encased concrete cylinder exhibited a hardening stage after the linear elasticity stage. The specimens with no layer of GFRP, D10-H, reached the maximum stress of 30.12 MPa, specimens with one, two, three and four layers of GFRP, G1–D10-H, G2–D10-H, G3–D10-H and G4–D10-H, had the maximum stress of 32.98, 46.72, 54.87 and 71.58 MPa, respectively. It explained that with more layers of GFRP, the hardening stage could be expanded. The increased number of GFRP layers enhanced the loading capacity and plastic deformation performance. The maximum compressive stress and strain for different layers of GFRP specimens are shown in Table 3. 3.4.2. Perforation diameter of perforated-steel The stress–strain relationship curves of two layers GFRP– perforated-steel tube-encased concrete cylinder with different steel perforation diameters are illustrated in Fig. 10. In the initial elastic stage, the curves of specimens G2–D5-H and G2–D10-H behaved in a linear manner. At the stress hardening stage, these curves were parallel to each other. The specimens G2–D5-H and G2–D10-H had a long and slow process compared with the specimen G2 at the softening branch and residual strength plateau stage. The material performance test showed that D10-H perforated-steel possessed the average yield strength and ultimate strength of 296 MPa and 362 MPa, respectively. While the D5-H perforated-steel possessed the average yield strength and ultimate strength of 235 MPa and 309 MPa, respectively. It expected that the loading carrying capacity of the D10-H perforated-steel tube was larger than that of the D5-H perforated-steel tube. As verified in Fig. 10, the specimen G2– D10-H had a higher loading capacity at the end of elastic and the hardening stages compared with the specimen G2–D5-H at the corresponding stages. Thus, it can be concluded that the specimen with a larger perforation diameter of perforated-steel tube had a higher loading capacity in axial compression. 3.4.3. Grid form of perforated-steel The stress–strain relationship curves of perforated-steel tube with different grid forms are showed in Fig. 10. The comparison items are specimens G2–D5-A versus G2–D5-H and G2–D10-A ver-
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G1-D10-H
G2-D10-H 80
80 70
specimen1 specimen2 specimen3
60
60 2
Axial stress (N/mm )
2
Axial stress (N/mm )
specimen1 specimen2 specimen3
70
50 40 30 20 10 0 0.00
50 40 30 20 10
0.01
0.02
0.03
0.04
0.05
0.06
0 0.00
0.07
0.01
0.02
Norminal axial strain
0.03
G3-D10-H
0.07
0.05
0.06
0.07
specimen1 specimen2 specimen3
70
2
Axial stress (N/mm )
60
2
Axial stress (N/mm )
0.06
80
specimen1 specimen2 specimen3
60 50 40 30
50 40 30
20
20
10
10
0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0 0.00
0.07
0.01
0.02
Norminal axial strain
0.03
G2-D5-H
G2-D10-A
70
0.04
Norminal axial strain
80
80
specimen1 specimen2 specimen3
70
specimen1 specimen2 specimen3
60
2
2
Axial stress (N/mm )
60
Axial stress (N/mm )
0.05
G4-D10-H
80 70
0.04
Norminal axial strain
50 40 30
50 40 30
20
20
10
10
0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 0.00
0.01
Norminal axial strain
0.02
0.03
0.04
0.05
0.06
0.07
Norminal axial strain Fig. 8. Stress–strain curves of specimens.
sus G2–D10-H. All these specimens exhibited the parallel curves at the hardening stage indicating the similar tendency in bearing capacity improvement. The material test showed that the D10-A perforated-steel possessed the average yield strength and ultimate strength of 378 MPa and 435 MPa, respectively, which were higher than those of D5-H. As shown in Fig. 10, the specimen G2–D5-A had a higher loading capacity at the end of the elastic and the hardening stages than that of the specimen G2–D5-H. The same conclusion can be drawn for D10-A and D10-H, which is also illustrated in
Fig. 10. The loading carrying capacity performance of specimen with the axial type of the steel tube is superior to that with a helical type. 3.5. Ductility The ductility index could be obtained from confinement ratio, confinement effectiveness and fracture energy, indicated in Eqs. (1) and (2). Fig. 11 shows schematic of calculation ductility index.
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G2-D5-A
G2 80
80
specimen1 specimen2 specimen3
70
70 60
2
2
Axial stress (N/mm )
60
Axial stress (N/mm )
specimen1 specimen2
50 40 30
50 40 30
20
20
10
10
0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0 0.00
0.07
0.01
0.02
Norminal axial strain
0.03
0.04
D10-H
0.06
0.07
0.05
0.06
0.07
PC
80
80
specimen1 specimen2 specimen3
70
70
specimen1 specimen2 specimen3
60 2
2
Axial stress (N/mm )
60
Axial stress (N/mm )
0.05
Norminal axial strain
50 40 30
50 40 30
20
20
10
10
0 0.00
0.01
0.02
0.03
0.04
0.05
0.06
0 0.00
0.07
Norminal axial strain
0.01
0.02
0.03
0.04
Norminal axial strain Fig. 8 (continued)
80
60 2
Axial stress (N/mm )
D, which denoted as ecu . The ultimate strain and the corresponding strain of maximum stress of plain concrete is indicated as eccu and ecco , respectively. Table 4 showed the average values of ductility index in each specimens group. Confinement ratio is defined as ultimate or maximum compressive axial strain of confined concrete to that of plain concrete. Confinement effectiveness is defined as maximum compressive stress of confined concrete to that of plain concrete. Fracture energy is the area under the stress–strain curve, which indicates the energy dissipated by specimens. The average values of ductility index in each specimens group were obtained, which shown in Table 4.
PC D10-H G1-D10-H G2-D10-H G3-D10-H G4-D10-H
70
50 40 30 20
Confinement ratio ¼ ecc =ecco
10 0 0.00
or
ecu =ecuo
Confinement effectiveness ¼ f cc =f co 0.01
0.02
0.03
0.04
0.05
0.06
0.07
Norminal axial strain Fig. 9. The stress–strain relationship of different layers of GFRP.
The curve OAC stands for a general stress–strain relationship of specimens. The specimens achieve the maximum loading capacity at point A and failure at point C. The maximum stress is at point E, indicated as f cc , the corresponding strain is at point B, indicated as ecc . The ultimate compressive axial strain of specimens is at point
ð1Þ ð2Þ
It could be concluded that confinement ratio, confinement effectiveness and fracture energy increased while adding the layer of GFRP. It is noted that the fracture energy for G2–D10-H is larger than that of summation of D10-H and G2, which indicated that the combination of these two materials could enhance the energy dissipation effects compared with the summation of two separate materials. The specimens did not fail immediately after the GFRP fractured; the steel and some parts of GFRP absorbed the residual energy. It also obtained that the 10 mm diameter perforation with axial style achieved the maximum ductility index with the same layer of GFRP group.
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Table 3 The comparison of maximum loading capacity and related strain for different layers of GFRP specimens. Specimens
PC
D10-H
G1–D10-H
G2–D10-H
G3–D10-H
G4–D10-H
Max stress (MPa) Related strain Stress increased ratio Strain increased ratio
27.61 0.004 — —
30.12 0.006 1.09 1.57
32.98 0.018 1.19 4.80
46.72 0.021 1.69 5.50
54.87 0.025 1.99 6.64
71.58 0.040 2.59 10.47
the analysis-oriented models are generated by incremental numerical procedure. This study adopted a design-oriented model due to large volume of experiment results, the regression analysis was utilized and stress–strain design model of hybrid GFRP– perforated-steel tube-encased concrete column under axial compression was proposed.
50
G2-D5-H G2-D10-A G2-D5-A G2-D10-H
2
Axial stress (N/mm )
40
30
20
10
0 0.00
0.01
0.02
0.03
0.04
0.05
Norminal axial strain
3.6.1. Derivation process According to the analysis of stress–strain relationships, GFRP provided mainly the confinement effect in the stress hardening stage before approaching the peak stress, while the perforatedsteel provided mainly the confinement effect in the rapid stress drop and strain softening stage after post-peak response. This study took into account Lam and Teng’s design model [24] for uniaxial compression behavior of FRP confined concrete. It also considered the effect of perforated-steel-confined concrete by using regression analysis by O’shea and Bridge’s [25]. Lam and Teng proposed stress–strain curves as Eqs. (3) and (4)
Fig. 10. The stress–strain relationship of different perforation diameter and grid form of perforated-steel tube.
rc ¼ Ec ec
ðEc E2 Þ2 2 ec 4f co
ð0 6 ec 6 et Þ
ð3Þ
rc ¼ f co þ E2 ec ðet 6 ec 6 ecc Þ
ð4Þ
co co where et ¼ E2f , E2 ¼ f ccef . Remaining variables in the equations are c E2 cc
defined above. According to Richard [26], the strength of confined concrete f cc and corresponding strain ecc is proportional to the confinement force, see Eqs. (5) and (6).
Fig. 11. Schematic of calculation ductility index.
3.6. Confinement model Design-oriented and analysis-oriented are two major types of models that utilized in analyzing constitutive relationship. The design-oriented models are based on experimental results while
f cc f ¼ 1 þ k1 l f co f co
ð5Þ
ecc f ¼ 1 þ k2 l eco f co
ð6Þ
where f co and eco are stress and related strain of plain concrete without confinement, respectively. f l is lateral confinement force. k1 and k2 are constants to be determined by experiments. This is the weakness of the above approach since the constants are mere correction factors allowing the fitting of experiments.
Table 4 The average values of ductility index in each specimens group. Specimens
ecc
ecu
f cc (MPa)
f cc /f co
ecc /ecco
ecu /ecuo
Fracture energy
PC D10-H G2 G1–D10-H G2–D5-H G2–D5-A G2–D10-H G2–D10-A G3–D10-H G4–D10-H
0.004 0.006 0.026 0.018 0.024 0.016 0.021 0.026 0.025 0.040
0.006 0.027 0.026 0.035 0.045 0.045 0.046 0.044 0.044 0.059
27.61 30.12 42.43 32.98 42.64 43.30 46.72 48.58 54.87 71.58
1.00 1.09 1.54 1.19 1.54 1.57 1.69 1.76 1.99 2.59
1.00 1.57 6.78 4.80 6.19 4.27 5.50 6.80 6.64 10.47
1.00 4.47 4.32 5.81 7.42 7.42 7.65 7.41 7.25 9.83
0.078 0.375 0.791 0.663 1.211 1.209 1.246 1.283 1.576 2.891
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3.6.2. Lateral confinement force The lateral confinement force f l were provided by GFRP (f lf ) and perforated-steel (f ls ), which could be calculated as f l ¼ f lf þ f ls , where f ls is written in Eq. (7).
f ls ¼ 2
f hs ts Dc
ð7Þ
where f hs is hoop stress of perforated-steel, t s is the thickness of perforated-steel tube, Dc is diameter of core concrete. For the perforated-steel tube, Von-Mises yield criterion can be used, therefore, the hoop stress f hs may not equal to the yield stress f y when perforated-steel tube yielded. It is suggested that the equation f hs ¼ bf y can be adopted in this situation. O’shea and Bridge [25] studied different strength of thin steel confined concrete and obtained Eq. (8) by regression analysis.
(
b ¼ 1:0
0 6 c 6 0:4
ð8Þ
b ¼ 10ð0:7cÞ c P 0:4 3 where c ¼
qffiffiffiffiffi
f co . fy
Therefore, Eq. (7) can be written as f ls ¼ 2
bf y t s . Dc
The effective lateral confinement force is calculated in Eq. (9).
f lf ¼ 2
f hf tf Ef ef t f ¼2 d d
ð9Þ
where f hf is the hoop stress of GFRP, tf is the thickness of GFRP, Ef is the elastic modulus of GFRP, ef is the actual fracture strain of GFRP, d is the diameter of confined concrete cylinder. 3.6.3. Peak stress and strain The peak stress and strain of GFRP–perforated-steel tubeencased concrete cylinder were defined as the rupture stress and strain of GFRP. The core concrete was subjected to three dimensional stress condition after confinement was activated, the stress level was directly affected by the lateral confinement force. Equation (5) can be rewritten as
f cc f co
f lf þ f ls ¼ 1 þ k1 f co
ð10Þ
The coefficient k1 can be obtained by linear regression analysis of the experiment results shown in Fig. 12(a). The square value of correlation coefficient for fitting straight line and experimental
data points was 0.9946, which demonstrated a good agreement of each other. The calculation formula of peak stress of GFRP– perforated-steel tube-encased concrete cylinder is shown in Eq. (11).
f lf þ f ls f cc ¼ 1 þ 2:58 f co f co
ð11Þ
The peak strain of core concrete enhanced significantly by the lateral confinement provided by GFRP and perforated-steel tube. The peak and ultimate strain of unconfined concrete can be adopted by 0.002 and 0.004, respectively. In this situation, Eq. (12) is given below with a frequently used form for existing strain models for FRP confined concrete.
ecc f ¼ 2 þ k2 l eco f co
ð12Þ
The coefficient k2 can be obtained by linear regression analysis of experiment results, which is indicated in Fig. 12(b). The square value of correlation coefficient for fitting straight line and experimental data points was 0.9766, which demonstrated a good agreement of each other. The calculation formula of peak stress of GFRP– perforated-steel tube-encased concrete cylinder is shown in Eq. (13).
f lf þ f ls ecc ¼ 2 þ 46:12 eco f co
ð13Þ
3.6.4. Comparison of model and experiment results The simplified design model of GFRP–perforated-steel tubeencased concrete cylinder was proposed in this study based on Eqs. (3), (4), (11) and (13). Fig. 13 shows the comparison between the predicted results by the model and their experimental data, which demonstrated a good agreement. 4. Comparison with existing confined concrete structures The confinement performance (with respect to confinement effectiveness and confinement ratio) of GFRP–perforated-steel tube encased concrete column is compared with that of existing confined concrete structure, i.e. (1) FRP tube confined plain concrete or FRP wrapped plain concrete and (2) FRP tube confined reinforced concrete or FRP wrapped confined reinforced concrete. A group of test results of existing confined concrete structures
2.4
28 26
2.2
24 22
2.0
20 18
1.8
2
fcc/fco
R =0.9946
εcc/εco
1.6
2
16
R =0.9766
14 12 10
1.4
8 6
1.2
4 1.0 0.0
0.1
0.2
0.3
0.4
0.5
2 0.0
0.1
0.2
0.3
(flf+fls)/fco
(flf+fls)/fco
(a)
(b)
0.4
0.5
Fig. 12. Linear fit of the equations to the experimental data obtained from the cylinder test. (a) Linear fit of the peak stress (b) linear fit of the peak strain.
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80
G1-D10-H
80
Test
70
Model
60
50
50
Stress/MPa
60 40 30 20
G2-D10-H
Test Model
40 30 20
10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
80 G3-D10-H 70
80 G4-D10-H 70
Test Model
60
50
50
Stress/MPa
60 40 30
Test Model
40 30
20
20
10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
80 G2-D5-A 70
80 G2-D5-H 70
Test Model
60
60
50
50
Stress/MPa
Stress/MPa
Stress/MPa
Stress/MPa
70
40 30 20 10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
80 G2-D10-A 70
Test Model
40 30 20 10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain
Test Model
Stress/MPa
60 50 40 30 20 10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Axial strain Fig. 13. Comparison of model and experiment results.
was collected from the literature and is listed in Table 5. To make the results comparable, the collected specimens have similar specimen size, unconfined concrete strength, thickness of FRP and/or steel reinforcement ratio as these used for GFRP–perforated-steel tube encased concrete in this study. 4.1. FRP tube confined plain concrete or FRP wrapped confined plain concrete In this section, the confinement performance of GFRP– perforated-steel tube encased concrete column is compared with CFRP wrapped or CFRP tube confined plain concrete reported in [27,28]. In Ref. [27], the tensile strength and modulus of CFRP used for FRP tube confined plain concrete was 3550 MPa and 235 GPa, respectively. In Ref. [28], the tensile strength and modulus of the CFRP used for FRP wrapped plain concrete was 1577 MPa and 105 GPa, respectively. As can be seen in Table 5, the values of
confinement effectiveness of GFRP–perforated-steel tube encased concrete column are comparable to those of CFRP tube or CFRP wrapped concrete reported by Shehata et al. [27], Xiao and Wu [28], even the tensile modulus and tensile strength of the CFRP are significantly larger than those of glass FRP (i.e. tensile strength and modulus of 419 MPa and 38 GPa respectively) used in this study. Regarding failure mode, it was reported that FRP tube encased plain concrete or FRP wrapped plain concrete exhibited brittle and sudden failure of the structure without prior warning due to the non-yielding characteristic of FRP material and the brittle behavior of plain concrete. In addition, the concrete core was fully crushed after the rupture of the outer FRP and lose all the load bearing capacity [28,29]. While for GFRP–perforated-steel tube encased concrete column, the failure of the structure was relatively ductile and the concrete core can carry residual load, as shown in failure mode of GFRP–perforated-steel tube encased concrete column in Fig. 7 and the post-peak descending branch in compressive
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L. Huang et al. / Composite Structures 142 (2016) 313–324 Table 5 Test results of FRP confined concrete specimens. Data sources
D (mm)
L (mm)
t (mm)
Fiber
qs (%)
f co (MPa)
f cc (MPa)
ecc
erup
f l;a (MPa)
Shehata [27]
150
300
0.165 0.330
Carbon
–
29.8
57.0 72.1
0.0123 0.0174
0.0123 0.0119
6.36 17.99
Xiao and Wu [28]
152
305
0.380 0.380 0.380 0.760 0.760 0.760
Carbon
–
33.7
47.9 49.7 49.4 64.6 75.2 71.8
0.0120 0.0140 0.0124 0.0165 0.0225 0.0216
0.0084 0.0115 0.0087 0.0091 0.0100 0.0100
4.41 6.04 4.57 9.56 10.50 10.50
Lee et al. [30]
150
300
0.11 0.22 0.33 0.44
Carbon
1.31 1.31 1.31 1.31
36.2
41.7 57.8 69.1 85.4
0.0024 0.0024 0.0024 0.0024
0.010 0.015 0.020 0.027
6.61 13.22 19.83 26.44
This study
150
300
0.3 0.6 0.6 0.6 0.6 0.9 1.2
Glass
1.70 1.75 1.75 1.70 1.70 1.70 1.70
27.61
32.98 42.64 43.30 46.72 48.58 54.87 71.58
0.018 0.024 0.016 0.021 0.026 0.025 0.040
0.008 0.016 0.016 0.018 0.015 0.018 0.014
3.31 4.69 6.35 7.45 8.84 9.66 14.36
stress–strain curves of GFRP–perforated-steel tube encased concrete column in Fig. 8. Thus, the GFRP–perforated-steel tube encased concrete column exhibits comparable load carrying capacity to that of CFRP tube confined plain concrete or CFRP wrapped plain concrete where the tensile properties of carbon FRP are significantly larger. However, the failure mode of GFRP–perforatedsteel tube encased concrete was ductile but that of FRP confined plain concrete was brittle. 4.2. FRP tube confined reinforced concrete or FRP wrapped confined reinforced concrete In this section, the confinement performance of GFRP– perforated-steel tube encased concrete column is compared with FRP wrapped or FRP tube confined reinforced concrete reported in [30]. Lee et al. [30] investigated compressive behavior of CFRP confined spiral steel reinforced concrete columns, the spiral steel reinforcement ratio was 1.31%, which is relatively smaller than that of 1.70% for perforated-steel tube. The yield strength of the spiral reinforcement used by Lee et al. was 1200 MPa, which is lar-
ecc eco
f l;a f co
f cc f co
5.857 8.286
0.213 0.604
1.91 2.42
0.923 1.077 0.954 1.269 1.731 1.662
0.131 0.179 0.136 0.284 0.312 0.312
1.42 1.47 1.46 1.92 2.23 2.13
4.17 6.25 8.33 11.25
0.183 0.365 0.548 0.730
1.15 1.60 1.91 2.36
4.80 6.19 4.27 5.50 6.80 6.64 10.47
0.12 0.17 0.23 0.27 0.32 0.35 0.52
1.19 1.54 1.57 1.69 1.76 1.99 2.59
ger than that of the perforated-steel (235–378 MPa) used in this study. The tensile strength and modulus of the CFRP used was 4510 MPa and 250 GPa, respectively, which are significantly larger than those of GFRP used in this study. The reported confinement effectiveness for the spiral steel reinforced concrete column with 1, 2, 3, and 4-layer CFRP was 1.15, 1.60, 1.91 and 2.36, respectively, which is close to the confinement effectiveness of GFRP– perforated-steel tube encased concrete, ranging from 1.19 to 2.59 for the specimens with 1, 2, 3 and 4-layer GFRP. The reported confinement ratio by Lee et al. [30] for specimens with CFRP from 1 to 4 layers ranged from 0.183 to 0.730, which are similar to those of GFRP–perforated-steel tube encased concrete, between 0.12 and 0.52. The typical failure mode of CFRP confined spiral steel reinforced concrete is given in Fig. 14 [3]. Therefore, the GFRP– perforated-steel tube encased concrete has comparable structural performance to that of CFRP confined spiral steel reinforced concrete. Overall, it can be concluded that the GFRP–perforated-steel tube encased concrete has the potential to axial structural member with higher load carrying capacity and ductility. 5. Conclusion This paper presented the behavior of hybrid GFRP–perforated steel tube-encased concrete column under axial compression. The main conclusions are as follows.
Fig. 14. Failure mode of CFRP confined reinforced concrete after compression test [31].
(1) The failure mode for GFRP–perforated steel tube-encased concrete column was ductile. The failure process were GFRP tube fractured firstly, perforated-steel tube yielded followed by bulging and crushing of the concrete core. The entire specimens received a ‘drum’ shape in the final stage. The loading capacity was reached when the GFRP fractured; the specimens became completely damaged when the perforated-steel tube bulged and fractured. (2) Stress–strain curves of GFRP–perforated-steel tube-encased concrete column could be divided into three parts: (1) linear elastic, (2) non-linear hardening, softening branch and (3) residual strength plateau. In the non-linear hardening stage, the stress–strain curves were similar to the ones of GFRP confined concrete because GFRP provided mainly confinement. In the softening and residual strength plateau stages, the stress–strain curves were similar to those of
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(3)
(4)
(5)
(6)
(7)
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perforated-steel tube confined concrete because the plastic deformation and confinement force were mainly depended on perforated-steel tube. Ductility index, including confinement ratio, confinement effectiveness and fracture energy, could be controlled by the layers of GFRP, diameter of perforation and form of perforation. More layers of GFRP could result in better ductility performance. The combination of axial arrangement and larger diameter perforations resulted in better ductility ratios. The load capacity and plastic deformation of GFRP– perforated-steel tube-encased concrete column was enhanced when the number of layers of the GFRP increased. The perforation diameter of 10 mm of D10-H type possessed higher yield and ultimate strength than that of perforation diameter of 5 mm, which demonstrated 10 mm diameter could obtain better enhancement of loading capacity. The yield and ultimate strength of axial type is higher to that of helical type, thus the compression capacity performance of axial type is superior to helical type. GFRP–perforated-steel tube-encased concrete column behaved bi-linear manner under uniaxial loading. The stress–strain curves of elasticity and hardening stage could be expressed as parabola and straight line. The simplified design model could be obtained by regression analysis according to Lam and Teng’s model. The calculation results were in good agreement of experiment results. The combination of GFRP and perforated-steel is the new effective method to confine concrete; it utilized the advantages of these two materials. GFRP with its high tension strength enhanced the loading capacity and deformation performance in hardening stages while perforated-steel with its high elasto-plasticity improved loading capacity and deformation properties in softening branch and residual strength plateau stage. The comparison in axial compressive behavior of GFRP– perforated-steel tube-encased concrete with other existing confined concrete indicates that this GFRP–perforated-steel tube-encased concrete has the potential to be axial structural member with higher load carrying capacity and ductility.
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