International Journal of Mining Science and Technology 24 (2014) 311–316

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International Journal of Mining Science and Technology

journal homepage: www.elsevier .com/locate / i jmst

Novel hybrid FRP tubular columns for sustainable mining infrastructure:Recent research at University of Wollongong

http://dx.doi.org/10.1016/j.ijmst.2014.03.0042095-2686/� 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

⇑ Corresponding author. Tel.: +61 2 4221 3786.E-mail address: taoy@uow.edu.au (T. Yu).

Yu Tao ⇑, Remennikov Alex M.Faculty of Engineering and Information Sciences, University of Wollongong, NSW 2522, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 October 2013Received in revised form 15 November 2013Accepted 8 December 2013Available online 29 April 2014

Keywords:Hybrid columnTubular columnFibre reinforced polymerSustainable mining infrastructure

This paper introduces, for applications in the mining industry, an innovative hybrid column form whichconsists of an inner steel tube, an outer fibre-reinforced polymer (FRP) tube and an annular concrete infillbetween them. The two tubes may be concentrically placed to produce a section form more suitable forcolumns, or eccentrically placed to produce a section form more suitable for beams. The FRP is combinedwith steel and concrete in these hybrid structural members in such a way that the advantages of FRP areappropriately exploited while its disadvantages are minimized. As a result, these hybrid members pos-sess excellent corrosion resistance as well as excellent ductility and seismic resistance. This paper sum-marizes existing research on this new form of structural members, and discusses their potentialapplications in mining infrastructure before presenting a summary of the recent and current studies atUniversity of Wollongong (UOW) on their structural behaviour and design.

� 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction

In recent years, fibre-reinforced polymer (FRP) composites havefound increasingly wide applications in civil engineering, both inthe retrofit of existing structures and in new construction. FRPcomposites possess several advantages over steel, including theirhigh strength-to-weight ratio and good corrosion resistance. As aresult, the use of FRP composites as externally bonded reinforce-ment for the retrofit of structures has become very popular inrecent years [1]. These same advantages can also be exploited innew construction, and indeed a large amount of research aroundthe world is currently under way examining the performance ofvarious forms of structures made of FRP composites alone (i.e.,all FRP structures) or FRP composites in combination with othermaterials (i.e., hybrid structures) [2,3]. Examples include FRPbridge decks, concrete filled FRP tubes as columns and piles, andFRP cables.

Compared with the two primary traditional structural materials,namely steel and concrete, FRP composites also have some disadvan-tages. These include their relatively high cost, linear–elastic–brittlestress–strain behavior, low elastic modulus-to-strength ratio, andpoor fire resistance. In retrofit applications, cost savings arise froma number of aspects that offset the higher material cost, but this isharder to achieve in new construction at present. The low elastic

modulus-to-strength ratio is not critical in retrofit applications asthe FRP is generally used to resist tension. The poor fire perfor-mance is also not an acute problem in retrofit applications eitherbecause the structure is in the open space (e.g., bridges) or becausethe FRP is not required to make any contribution to structural resis-tance during a fire. When FRP composites are deployed in new con-struction, the consequences of their weaknesses need to beminimized as in retrofit applications. Based on these consider-ations, it may be concluded that the successful application of FRPcomposites in new construction requires the following three crite-ria to be met [2,3]: (a) cost effectiveness at least in terms of a life-cycle cost assessment; (b) FRP to be used in areas subject to tensionas much as possible; (c) fire resistance to be non-critical. It shouldbe noted that the third criterion is easily met for bridge structuresand other outdoor structures, while the first two requirements veryoften mean that FRP composites should be used in combinationwith other materials to form hybrid structures. It is apparent thatthe area of hybrid structures should be a major research focus inthe use of FRP composites in new construction. Within the area ofhybrid structures, the aim shall be to optimally combine FRP withtraditional structural materials such as steel and concrete to createinnovative structural forms that are cost-effective and high-perfor-mance [2,3]. To this end, simple duplications of existing structuralsystems are often inadequate.

This paper introduces, for applications in the mining industry, anew type of FRP–concrete–steel structural member developed atThe Hong Kong Polytechnic University, in which the three

312 T. Yu, A.M. Remennikov / International Journal of Mining Science and Technology 24 (2014) 311–316

constituent materials are optimally combined to achieve severaladvantages not available with existing columns. The rationale andadvantages of this new form of structural members are explained,and their potential applications in mining infrastructure arediscussed before the recent and current studies at University ofWollongong (UOW) on their structural behavior and design aresummarized.

2. Section forms and advantages

This new type of structural members (Fig. 1), proposed by Prof. JGTeng at The Hong Kong Polytechnic University, is referred to ashybrid FRP–concrete–steel double-skin tubular members (DSTMs)or hybrid DSTMs for brevity [2,3]. A hybrid DSTM consists of an outertube made of fibre-reinforced polymer (FRP) and an inner tube madeof steel, with the space between filled with concrete. The two tubesmay be concentrically placed (Fig. 1a and b) to produce a sectionform more suitable for columns, or eccentrically placed to producea section form more suitable for beams (Fig. 1c and d). In hybridDSTMs, the FRP tube offers mechanical resistance primarily in thehoop direction to confine the concrete and to enhance the shearresistance of the member. Hybrid DSTMs may be constructedin situ or precast, with the two tubes acting as the stay-in-placeform. The sections of the two tubes may be both circular (Fig. 1aand c), rectangular (Fig. 1d), or in another shape; they may also haveshapes different from each other (Fig. 1b). Shear connectors need tobe provided between the steel tube and the concrete, particularly inbeams, but are generally not needed for the FRP tube which isnormally designed to have only a small longitudinal stiffness.

The most important advantage of hybrid DSTMs is their excellentcorrosion resistance, as the FRP tube is highly resistant to corrosionwhile the steel tube is protected by the FRP tube and the concrete.The other main advantages of hybrid DSTMs include: (1) excellentductility, as the concrete is well confined by the two tubes and out-ward local buckling of the steel tube is constrained by the concrete;(2) a high strength/stiffness-to-weight ratio as the inner void largelyeliminates the redundant concrete; (3) ease of construction, as thetwo tubes act as a permanent form for casting concrete, and the pres-ence of the inner steel tube and the concrete allows easy connectionto other members. Teng et al. further discussed the rationale andadvantages of hybrid DSTMs [3].

3. Comparison with existing column forms

3.1. Comparison with hollow RC columns

Among the existing forms of columns which may be replaced byhybrid DSTMs, hollow RC columns are the most cost-effective. Hol-low RC columns have been widely used because of their high bend-ing resistance coupled with reduced weight. A detailed cost

FRP tube

Steel tube

Concrete

FRP tube

Concrete

Steel tube

(b)(a)

(c) (d)

Fig. 1. Typical sections of hybrid DSTMs.

comparison between a hybrid DSTM and a hollow RC columnwas undertaken in 2006 and Teng et al. reported in detail [4]. Thiscomparison indicates that the construction costs of hybrid DSTMsand hollow RC columns are similar, but the hybrid DSTM has a lar-ger section capacity than the hollow RC column when the axialforce is reasonably high but the two sections have similar sectioncapacities when bending dominates the behavior. Besides the loadcapacity, it should also be noted that hybrid DSTMs possess twoimportant advantages over hollow RC columns: excellent corrosionresistance and excellent seismic resistance.

3.2. Comparison with other column forms

Steel–concrete DSTMs with both skins made of steel have beenused in construction and have been intensively researched. Suchcolumns have a higher initial construction cost than hollow RC col-umns for the same structural performance, similar to the well-known fact that concrete-filled steel tubes are more expensivethan RC columns. FRP–concrete DSTMs with two FRP tubes havealso been explored but the use of FRP instead of steel for the innertube does not lead to any significant advantage but a number ofsignificant disadvantages (e.g., higher cost, reduced stiffness toconfine concrete, brittle failure in tension).

Intensive recent research has been conducted on concrete-filledFRP tubes (CFFTs) (i.e., with a solid concrete core) as columns andpiles. There have also been some field applications of CFFTs. If excel-lent durability is the overriding criterion, CFFTs are a possiblechoice but they are significantly more expensive than RC columnsbecause the FRP tube needs to be thick and to be provided with bothlongitudinal and hoop fibres. If the FRP tube is not sufficiently thick,premature buckling under compression will considerably compro-mise its confinement of the concrete core. Even if buckling does notoccur, adverse interaction exists between axial compression andhoop tension in such a tube. A hybrid DSTM may be seen as a CFFTwith its FRP tube split into an outer FRP tube containing the hoopfibres and an inner FRP tube containing the axial fibres which isthen replaced by a much stiffer and much more ductile steel tube.The inner void can also be used for the passing of service ducts. Itneeds to be emphasized that the FRP tube in a DSTM should gener-ally be quite thin, as its main purpose is to enhance the ductility ofthe column. Hybrid DSTMs offer many advantages over CFFTs: (a)better confinement of concrete by the FRP tube which containsfibres predominately oriented in the hoop direction; such a tubeis mainly subjected to hoop tension and does not buckle under axialstraining as has been observed in numerous existing tests of FRP-confined concrete columns; (b) ductile failure in bending as thesteel inner tube acts as the longitudinal reinforcement; and (c) sav-ings in cost as the FRP tube is used as a confining device for ductilecolumn response and does not need to be thick.

4. Potential practical application

Because of their excellent corrosion resistance, hybrid DSTMsare most suitable for use in structures which are likely to beexposed to a harsh environment (e.g., coastal structures and under-ground structures). Hybrid DSTMs can be used as compressionmembers, such as piles, various towers (e.g., wind turbine towersand electricity transmission towers) and other similar structures.In longwall mining, hybrid DSTMs can be used in a roof supportsystem for maingates and/or tailgates. The excellent structural per-formance of hybrid DSTMs makes them a promising alternative toexisting systems (e.g., Fig. 2) [5,6]. The presence of an inner void inhybrid DSTMs is also an important advantage which can beexploited in mining applications. The inner void can be used forthe passing of service ducts or for ventilation, so that hybrid DSTMs

Fig. 2. Roof support systems [7].

Pre-embedded

FRP barsBeam/deck connectors

FRP tube

Concrete

tube

Concrete

reinforcement

Steel

Fig. 4. Hybrid DSTM/slab units [8].

T. Yu, A.M. Remennikov / International Journal of Mining Science and Technology 24 (2014) 311–316 313

serve not only as structural components, but also as functionalcomponents.

In practical applications, when the length of DSTMs becomesvery large, they can be constructed using a segmental method,which involves the segmental construction of the outer FRP tubeand the inner steel tube and the use of the two tubes as the perma-nent formwork for the casting of concrete, as schematically illus-trated in Fig. 3 [8]. The flanges of the steel tubular segments andthe FRP profiles inside the FRP tubular segments serve not onlyas longitudinal connectors between the segments, but also as (1)shear connectors between the concrete and the tubes; and (2) stiff-eners to both tubes, thus improving the structural performance ofhybrid DSTMs.

Hybrid DSTMs can also be used as flexural members in struc-tures exposed to a harsh environment (e.g., underground struc-tures). Hybrid DSTMs can be used alone or be integrated into aconcrete slab reinforced with FRP bars to form a durable floor sys-tem (Fig. 4) [8]. In such cases, pre-embedded steel (stainless steelmay be used here) reinforcement in the DSTM can be spliced withthe bottom layer of FRP bars in the deck using mechanical couplersfor beam-slab connection (Fig. 4). Additional shear connectors inthe form of U-shaped dowels (or other appropriate forms) passingthrough the FRP tube may also be used to ensure the longitudinalcomposite action between the beam and the deck (Fig. 4). As thefibres in the FRP tube are close to the hoop direction, the passingof steel bars through the tube is expected to have little effect onits overall performance.

5. Existing research

A large amount of research has been conducted on hybridDSTMs by the first author and his colleagues. Most of the existingwork has been on the static structural behaviour of circular hybrid

Concrete orreinforced concrete

Upper steel tube

Elange (steel)

Weld

Shim

Steel bolt, nutand washers

Lower steel tube

Upper FRP shell

Adhesive

FRP profileShim

SealantSealant

FRP profileLower FRP shell

Dowel bar

Fig. 3. Segmental construction of tall DSTCs [8].

DSTMs (Fig. 1a and c). Teng et al. also concluded the results fromthese existing studies [3,9–14].

Teng et al. explained in detail the rationale for the new memberform together with its expected advantages, and presented preli-minary experimental results to demonstrate some of the advanta-ges of this new member form, such as excellent ductility and shearresistance [3]. Fig. 5 shows the typical axial load-axial strain curvesof hybrid DSTMs with different FRP tubes (i.e., one-ply, two-plyand three-ply FRP tubes respectively). Yu et al. presented theresults of a systematic experimental study on the flexural behaviorof hybrid DSTMs as well as results from a corresponding theoreti-cal model based on the fibre element approach [9]. Fig. 6 shows atypical beam specimen after test, having experienced large deflec-tions (exceeding 1/15 span) without a significant reduction (lessthan 10%) in the load capacity. Yu et al. also showed that the flex-ural response of hybrid DSTMs, including their flexural stiffness,cracking load and ultimate load, can be substantially improvedby shifting the inner steel tube towards the tension zone or by pro-viding FRP bars as additional longitudinal reinforcement [9]. Wonget al. presented a systematic experimental study on the compres-sive behavior of hybrid DSTMs and compared the performance ofhybrid DSTMs with that of FRP-confined solid cylinder/column(FCSC) specimens and FRP-confined hollow cylinder/column(FCHC) specimens; a good understanding of the behavior of con-crete in hybrid DSTMs resulted from this study [10]. Yu et al. devel-oped a new plastic-damage model for FRP-confined concrete basedon a critical review of the previous D-P type plasticity models[11,12]. A finite element model incorporating the new plastic-dam-age model was shown to provide close predictions of the testresults of hybrid DSTMs [12]. Yu et al. proposed a design-orientedstress–strain model for the confined concrete in hybrid DSTMssubjected to axial compression [13]. Yu et al. presented experimen-tal results on the behavior of hybrid DSTMs subjected to eccentriccompression as well as a so-called ‘‘variable confinement model’’for the confined concrete to account for the effect of strain gradienton confinement effectiveness [14]. These studies have led to a

200

400

600

800

1000

1200

1400

0 0.005 0.010 0.015 0.020 0.025Axial strain

Axi

al lo

ad (k

N)

One-ply specimenTwo-ply specimenThree-ply specimen

Fig. 5. Axial load–strain behavior.

Fig. 6. Beam specimen after test [3].

Steel tube

FRP tube

Concrete

Fig. 8. Rectangular hybrid DSTMs.

314 T. Yu, A.M. Remennikov / International Journal of Mining Science and Technology 24 (2014) 311–316

simple design approach for the static behavior hybrid DSTMs ascolumns, and this design approach has recently been adopted bythe Chinese national standard ‘‘Technical Code for InfrastructureApplication of FRP Composites’’ [15].

Besides the work on the static behavior of circular DSTMs, Yuand Teng presented results from two series of axial compressiontests on square DSTMs with a cross-section shown in Fig. 1b, dem-onstrating the excellent ductility of such hybrid DSTMs [16]. Zhanget al. presented results from a series of large-scale tests where cir-cular hybrid DSTMs filled with high strength concrete (HSC) withan unconfined strength of up to 117 MPa were tested under com-bined axial and cyclic lateral loads [16,17]. Typical results shownin Fig. 7 from a test of a hybrid DSTM filled with HSC demonstratedthat the column possesses excellent seismic resistance despite theuse of brittle HSC [17].

6. Recent research at UOW

Intensive research on the behavior and design of hybrid DSTMsis continuing at UOW, in collaboration with The Hong Kong Poly-technic University. In the remainder of the paper, the latestadvances of recent research at UOW are briefly presented, cover-ing: (1) behavior of rectangular hybrid DSTMs; (2) lateral impacttests on hybrid DSTMs.

6.1. Behavior of rectangular hybrid DSTMs

The existing studies discussed above were limited to the sectionform, as shown in Fig. 1a–c. For some practical applications, rect-angular hybrid DSTMs may be preferred (e.g., when the columnis subjected to different load levels in the two horizontal direc-tions, rectangular columns are preferred instead of circular orsquare ones). Therefore, a combined experimental and theoreticalstudy on rectangular hybrid DSTMs with a section form shown inFig. 8 are currently ongoing at UOW. In such rectangular hybridDSTMs, circular inner tubes are used to make the constraint tothe concrete more efficient.

-150 -100 -50 0 50 100 150- 200

- 150

- 100

- 50

0

50

100

150

200

Lateral displacement (mm)

Lat

eral

load

(kN

)

Fig. 7. Lateral load–displacement curve.

The experimental program included axial compression tests ona total of eight specimens, covering three different section forms(i.e., hybrid DSTMs; FRP-confined solid concrete columns; FRP-confined hollow concrete columns with two inner voids but nosteel tubes), two types of steel tubes, two values of corner radius,and two types of FRP tubes. All the specimens had a cross-sectionof 185 mm � 105 mm and a column height of 370 mm. In prepara-tion of the specimens, FRP tubes were prefabricated via a wet-layup process, and were used as the form for casting concrete.The tests were conducted at the High Bay Lab of UOW, using theDenison 5000 kN compression testing machine with a displace-ment control rate of 0.3 mm/min. A number of strain gauges wereattached on the outer FRP tubes and the inner tubes to measure thestrains in the axial and the hoop directions. Two LVDTs were usedto measure the axial shortening of the specimens. All test data,including the strains, loads, and displacements, were recordedsimultaneously by a data logger. Fig. 9 shows the experimentalset-up.

The axial load–strain curve of a typical rectangular hybridDSTM specimen is shown in Fig. 10. The curve is close to anapproximately elastic-perfectly-plastic curve with a long plasticplateau until an axial strain of around 0.03. At this strain level,the specimen failed by the hoop rupture of the FRP tube nearone of the corners. The results shown in Fig. 10 suggest that rect-angular hybrid DSTMs are very ductile compared with reinforcedconcrete columns for which an ultimate axial strain of 0.0035 isnormally expected.

The theoretical part of this study involves the development of atraditional section analysis of the so-called fibre element approachfor rectangular hybrid DSTMs based on the plane section assump-tion. The analytical procedure involves the determination of theposition of the neural axis for a given strain of the extremecompression fibre by force equilibrium and the evaluation of thebending moment by integrating the contributions of stresses overthe section.

6.2. Lateral impact tests

6.2.1. Tests on circular hybrid DSTMsTwo series of tests were conducted to investigate the behavior

of circular hybrid DSTMs subjected to lateral impact loading. Eachseries included four specimens among which one was tested under

Fig. 9. Experimental set-up.

Axial strain

0.01 0.02 0.03 0.04 0.05 0.060

100200300400500600700800900

1000

Axi

al lo

ad (

kN)

Fig. 10. Axial load–strain curve of hybrid DSTM. Fig. 12. A specimen after test.

175

150

125

100

75

50

25

20 40 60 80 100 120 140 160 180 2000

Time (ms)

Impa

ct lo

ad (

kN)

Fig. 13. Typical impact load–time curve.

0.05

160

140

120

100

80

60

40

20

0 0.10 0.15 0.20 0.25 0.30

Time (s)

Mid

-spa

n de

flec

tion

(mm

)

Fig. 14. Typical mid-span deflection–time curve.

T. Yu, A.M. Remennikov / International Journal of Mining Science and Technology 24 (2014) 311–316 315

static three-point bending as the control specimen, while the otherthree were tested under lateral impact loading. The main differ-ence between the two series of specimens was the void ratio (being0.5 and 0.7 respectively for the two series), which is the ratio of thediameter of the inner steel tube to that of the outer FRP tube. Othertest variables included the thickness of the FRP tube and the steeltube, and the end constraint of the beam.

These tests were performed using an instrumented drop ham-mer facility at the High Bay Lab of UOW (Fig. 11). In the tests, a592 kg mass was released from a predetermined height to directlyimpact the specimens at the mid-span. The heights were deter-mined using the results from the static three-point bending testconducted in the same series. A dynamic load cell was mountedon the drop hammer to measure the contact force between thespecimen and the drop hammer; a high-speed camera was usedto capture the deflection during the impact process. Fig. 12 showsa specimen after test.

A typical impact load-time curve is shown in Fig. 13, while atypical mid-span deflection-time curve is shown in Fig. 14. Figs. 13and 14 are for a specimen with an outer diameter of 152.4 mm, aclear span of 1300 mm and a void ratio of 0.5. The test resultsshowed that hybrid DSTMs possess excellent ductility, and are ableto sustain very large inelastic rotation without significant reduc-tion in the load capacity. The maximum end rotation was foundto be over 10�, which is significantly higher than normallyexpected for reinforced concrete flexural members (i.e., 4–5�).Hybrid DSTMs therefore have very good potential for resistinglarge blast and impact loads.

6.2.2. Tests on square hybrid DSTMsA series of tests on the lateral impact behavior of square hybrid

DSTMs are ongoing. A total of four specimens with a cross-sectionshown in Fig. 1b were prepared, including one specimen to betested under static three-point bending as the control specimen,and the other three to be tested under lateral impact loading. Allthe specimens have a cross-section of 200 mm � 200 mm, a clear

Fig. 11. Instrumented drop hammer facility.

span of 2000 mm and a steel tube with an outer diameter of139.7 mm. The test variables included the thickness of the FRP tubeand the steel tube. Prefabricated FRP tubes were used as the formfor casting concrete, as shown in Fig. 15.

Fig. 15. Preparation of square hybrid DSTMs.

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7. Conclusions

This paper has discussed the rationale and advantages of hybridFRP–concrete–steel double-skin tubular members (i.e., hybridDSTMs), discussed their potential applications in mining infra-structure, provided a brief summary of existing and ongoingresearch on hybrid DSTMs. Hybrid DSTMs have a great potentialfor use in a roof support system for maingates and/or tailgates inlongwall mining. The presence of an inner void in hybrid DSTMsis also an important advantage which can be exploited in miningapplications. While existing and current research is mainly con-cerned with structural behavior of hybrid DSTMs, exciting oppor-tunities exist for the exploration of real practical applications ofsuch novel structural members in mining infrastructure.

Acknowledgments

The authors are grateful for the financial support received fromthe University of Wollongong through the 2013 URC Small GrantsScheme. The authors are grateful to Prof. JG Teng of The Hong KongPolytechnic University who is the inventor of the novel columns andprovided many helpful suggestions during this project. The authorsalso wish to thank their colleagues Drs. Ting Ren and Jan Nemcik fortheir valuable discussions in the preparation of this paper.

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