Construction and Building Materials 40 (2013) 1118–1127

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Construction and Building Materials

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Experimental study of flax FRP tube encased coir fibre reinforced concretecomposite column

Libo Yan ⇑, Nawawi ChouwDepartment of Civil and Environmental Engineering, The University of Auckland, Auckland Mail Centre, Private Bag 92019, Auckland 1142, New Zealand

h i g h l i g h t s

" Feasibility of a flax FRP tube encased coir fibre reinforced concrete system." Significant increase in ultimate compressive strength as axial structural members." Confined concrete strength was predicted and compared with experimental results." Significant increase in load capacity and deflection as flexural members." Coir fibre inclusion modifies the failure pattern of confined concrete to ductile.

a r t i c l e i n f o

Article history:Received 19 September 2012Received in revised form 19 November 2012Accepted 30 November 2012

Keywords:Flax fibreFibre reinforced polymerCoir fibreFibre reinforced concreteConfinementDuctilitySlippage

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.11.116

⇑ Corresponding author. Tel.: +64 9 3737599x8452E-mail address: lyan118@aucklanduni.ac.nz (L. Ya

a b s t r a c t

The use of natural fibres as building materials is benefit to achieve a sustainable construction. This paperreports on an experimental investigation of a composite column consisting of flax fibre reinforced poly-mer (FFRP) and coir fibre reinforced concrete (CFRC), i.e. FFRP tube encased CFRC (FFRP-CFRC). In thisFFRP-CFRC, coir fibre is the reinforcement of the concrete and FFRP tube as formwork provides confine-ment to the concrete. Uniaxial compression and third-point bending tests were conducted to assess thecompression and flexural performance of the composite column. A total of 36 specimens were tested. Thetest variables were FFRP tube thickness and coir fibre inclusion. The axial stress–strain response, confine-ment performance, lateral load–displacement response, bond behaviour and failure modes of the com-posite column were analysed. In addition, the confined concrete compressive strength was predictedusing existing strength equations/models and compared with the experimental results. Results indicatethat the FFRP-CFRC composite columns using natural fibres have the potential to be axial and flexuralstructural members.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Natural fibres are a renewable resource and are available allmost over the world. The use of natural fibres by the constructionindustry will help to achieve a sustainable consumption pattern ofbuilding materials. The European Union recently established thatin a medium term raw materials consumption must be reducedby 30% and that waste production must be cut down by 40% [1].Thus cost-effective natural fibres as reinforcement of concrete toreplace the expensive, highly energy consumed and non-renew-able reinforced steel rebar are a major step to achieve a sustainableconstruction [2]. In addition, recently, the use of natural fibres toreplace carbon/glass fibres as reinforcement in FRP compositesfor engineering applications has gained popularity due to anincreasing environmental concern and requirement for developing

ll rights reserved.

1; fax: +64 9 373 7462.n).

sustainable material [3]. Thus the use of cost-effective natural fi-bres in FRP composites as concrete confinement is another stepto achieve a more sustainable construction.

Therefore, the purpose of this study is to investigate the feasi-bility of natural flax fabric reinforced epoxy composite tube en-cased coir fibre reinforced concrete (FFRP-CFRC) compositecolumn as axial and flexural structural members. Specifically, thisstudy presents an experimental study on the use of coir fibres asreinforcement in concrete and flax fibres as reinforcement for fibrereinforced polymer composites as concrete confinement for struc-tural applications. The new FFRP-CFRC composite structure is ex-pected to have good performance as axial and flexural structuralmembers.

2. Background

Currently composite columns are widely used in high-risebuilding, offshore structures and bridge, particularly in regions of

L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127 1119

high seismic risk due to the high strength-to-weight ratio and in-creased deformability [4]. Concrete filled fibre reinforced polymertube (CFFT) is one of the most common composite columns re-ported in the literature.

In CFFT columns, the pre-fabricated tubes made of glass/carbonfibre reinforced polymer (G/CFRP) materials act as permanentformworks for fresh concrete and also provide confinement to con-crete. The advantages of G/CFRP materials are their high strengthand stiffness. The non-corrosive characteristic also provides FRPas an alternative to replace steel reinforcement in civil structuralapplications [5].

Behaviour of plain concrete filled FRP tube (CFFT) has been welldocumented [6–8]. Davol et al. [6] studied CFFT columns as bend-ing members. The external FRP shell replaced the functions of steelrebar in conventional reinforced concrete (RC) members, namely,tension carrying capacity and shear resistance, as well as confine-ment of concrete core. Fam and Rizkalla [7] investigated flexuralbehaviour of small and large-scale CFFTs with diameters rangingfrom 89 to 942 mm and spans ranging from 1.07 to 10.4 m. Theyconcluded that the flexural behaviour of CFFT was highly depen-dent on stiffness of FRP materials and ratio of diameter-to-FRP tubethickness. In flexure, slippage between FRP tube and concrete coremay compromise the load carrying capacity of the CFFT. To preventthe slippage between FRP tube and concrete core, a study by EIChabib et al. [8] indicated that the use of expansive cement in con-crete created a somewhat better tube and concrete interfacial con-tact, however, did not fully prevent the slippage. Mirmiran et al. [9]considered the application of shear connector ribs which placed onthe interior surface of the GFRP tube. It was found that the ribs sig-nificantly improved the axial load-carrying capacity of GFRP tubeconfined concrete. Li et al. [10,11] proposed a novel advanced gridstiffened (AGS) FRP tube which made of a lattice of interlaced FRPribs. Test results indicated that the lateral load carrying capacitywas improved due to the enhanced interfacial bonding strengthbetween the tube and the concrete through the mechanicalinterlocking.

In flexure, FRP tube confinement leads to the significantly in-crease in lateral load capacity and mid-span deflection of the con-crete core; however, it is commonly observed that unlike that inRC columns, the post-peak load–deflection responses of the CFFTcolumns exhibit a brittle manner as a result of the non-yieldingcharacteristic of FRP materials [e.g. 6, 7]. After tested, when re-moved the external tubes, the plain concrete cores developedexcessive larger flexural cracks at the mid-span of the columns oreven damaged to several blocks which distributed along the col-umns, as observed in previous study [7]. Therefore, when consider-ing CFFT columns used in a practical project, small amount of steelreinforcement were usually considered in order to avoid the brittlefailure, e.g. the use of CFFT piles in the construction of the Route 40highway bridge over the Nottoway River in the United States [12].

Currently a wider application of G/CFRP materials in civil infra-structure is limited by the high initial cost, the insufficiency of longterm performance data, the lack of standard manufacturing tech-niques and design standards, durability of glass fibres, risk of fireand the concern that the non-yielding characteristic of FRP materi-als could result in sudden failure of the structure without priorwarning [7,13–15]. Among these limitations, cost and concern ofbrittle failure of FRP materials are probably the most influentialfactors when assessing the merits of FRP as construction materials.

Recently, the use of natural fibres to replace carbon/glass fibresas reinforcement in FRP composites has gained popularity due toincreasing environmental concern. Natural fibres are low cost fi-bres with low density. These are biodegradable, non-abrasive, re-duced energy consumption, less health risk, renewability,recyclability and bio-degradability. In addition, they are readilyavailable and their specific mechanical properties are comparable

to those of glass fibres used as reinforcement [16]. Therefore, nat-ural fibres represent a highly ‘‘sustainable’’ material. The use ofnatural fibres in FRP composites as building materials will promotethe ‘‘sustainable’’ development for construction industry. Naturalfibres such as flax, hemp, jute, coir and sisal, are cost effective, havelow density with high specific strength and stiffness, and are read-ily available [17]. Dittenber and GangaRao [18] compared morethan 20 commonly used natural fibres (e.g. sisal, ramie, kenaf, jute,hemp, flax, coir, cotton, etc.) with glass fibres in specific modulus,cost per weight and cost per unit length (capable of resisting100 kN load). They concluded that among those natural fibres, flaxfibre offers the best potential combination of low cost, light weight,and high strength and stiffness for structural applications. Assararet al. [19] reported that the tensile strength of flax/epoxy compos-ites is 300 MPa – putting them close to GFRP composites. Theinvestigation showed encouraging mechanical properties of bio-composites. Therefore, it is possible to use the more economicalbio-composites (e.g. flax FRP) to replace synthetic glass FRP inengineering applications.

Previous research on fibre reinforced concrete has shown thatshort natural fibres, used in cementitious matrices, can modify ten-sile and flexural strength, toughness, impact resistance and frac-ture energy [20]. Pacheco-Torgal and Jalali reviewed themechanical properties of several vegetable fibres (i.e. sisal, hemp,coir, banana and sugar cane bagasse) as reinforcement of cementi-tious building materials [2]. Among those natural fibres, coir fibre,as reinforcement fibre in concrete, was investigated widely due toits highest toughness among natural fibres and the extremely lowcost, as well as availability [21]. Li et al. [22] stated that flexuraltoughness and flexural toughness index of cementitious compos-ites with coir fibre increased by more than 10 times due to coir fi-bre bridging effect. Reis [23] also reported that coir fibre increasedconcrete composite fracture toughness and the use of coir fibresshowed even better flexural properties than synthetic fibres (glassand carbon). Therefore, the inclusion of coir fibre might be usefulto increase the flexural performance of CFFT, particularly in chang-ing the brittle failure pattern of the concrete core. Consequently,the use of natural fibres in concrete is not only benefit to enhancethe mechanical properties of concrete, but also promotes thedevelopment of sustainable ‘‘green’’ concrete and thus saves thenatural resources [24].

3. Objectives

The objective of this work is to perform a study on the use ofnatural coir fibres as reinforcement of concrete and natural flax fi-bres in FRP composites as the concrete confinement, i.e. flax FRPtube encased coir fibre reinforced concrete (FFRP-CFRC) compositestructure. The use of these environmentally friendly natural fibresis benefit to the development of sustainable construction. Uniaxialcompression and third-point bending tests were conducted to as-sess the composite columns as axial and flexural structural mem-bers. The effects of FFRP tube thickness and coir fibre inclusionon the compressive and flexural performance of the composite col-umns were investigated. In addition, the ultimate compressivestrength of FFRP tube confined concrete was predicted using theexisting G/CFRP confined concrete strength equations/models andcompared with the experimental results.

4. Experimental procedures

4.1. Materials

4.1.1. Flax FRP tubeIn this study, commercial bidirectional woven flax fabric (550 g/m2) was ob-

tained from Libeco, Belgium. The Epoxy used was the SP High Modulus Ampreg22 resin and slow hardener. FFRP tubes were fabricated using the hand lay-up pro-

Table 2Average mechanical properties of coir fibre.

Properties Coir fibre

Average diameter 0.25 mmLength 50 mmDensity 1.20 g/cm3

Tensile modulus 2.74 GPaTensile stress at break 286 MPaTensile strain at break 20.8%Aspect ratio 200

1120 L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127

cess at the Centre for Advanced Composites Materials at the University of Auckland.The detail of fabrication process of FFRP tubes was similar as that described in [25].Fabric fibre orientation was at 90o from the axial direction of the tube. The structureof the flax fabric is given in the study [26]. Two layer arrangements of FFRP tubewere considered: two layers and four layers. Tensile and flexural properties of FFRPcomposites were determined by a flat coupon test on Instron 5567 machine accord-ing to ASTM D3039 [27] and ASTM D790 [28], respectively. The typical tensile andflexural stress–strain curves of flax fabric reinforced epoxy polymer (FFRP) compos-ites are displayed in Figs. 1 and 2, respectively. Fig. 1 indicates that the tensilestress-stain responses of 2-layer and 4-layer FFRP composites are quite consistent.Their curves are purely linear with the strains up to 0.3% and followed by a moder-ate softening and nonlinear response until failure without yielding. The physicaland mechanical properties of FFRP composites are listed in Table 1.

4.1.2. ConcreteAll specimens were constructed from two concrete batches. One batch was

plain concrete (PC) and the other one was coir fibre reinforced concrete (CFRC). Bothconcrete batches were designed as PC with a 28-day compressive strength of25 MPa. The concrete mix design followed the ACI Standard 211.1 [29]. The mix ra-tio by weight was 1:0.68:3.77:2.96 for cement:water:gravel:sand, respectively. Thecement used was CEM I 42.5 normal Portland cement with a general use type. Thecoarse aggregate was gravel having a density of 1850 kg/m3. The gravel has a max-ium size of 15 mm (passing through 15 mm sieve and retained at 10 mm sieve). Thenatural sand was used as a fine aggregate with a fineness modulus of 2.75. For CFRCbatch, coir fibre was added during mixing. The coir fibres were obtained from Indo-

0

20

40

60

80

100

120

140

0 0.01 0.02 0.03 0.04 0.05

Ten

sile

str

ess

(MP

a)

Tensile strain

4L FFRP

2L FFRP

Fig. 1. Typical tensile stress–strain curves of flax FRP composites.

0

20

40

60

80

100

120

140

160

0 0.01 0.02 0.03 0.04 0.05 0.06

Fle

xura

l str

ess

(MP

a)

Flexural strain

4L FFRP

2L FFRP

Fig. 2. Typical flexural stress–strain curves of flax FRP composites.

Table 1Physical and mechanical properties of flax FRP composite.

No. of flaxlayers

FRP thickness(mm)

Tensile strength(MPa)

Tensile modulus(GPa)

Tensilestrain (%)

2 3.25 106 8.7 3.74 6.50 134 9.5 4.3

nesia. The fibres had been treated and cut to a length of 50 mm. The considered coirfibre weight content was 1% of the mass of the cement. The average mechanicalproperties of the coir fibres used in this study are given in Table 2. The mechanicalproperties (ultimate tensile strength, failure strain and Young’s modulus) of singlecoir fibres were determined using a universal MTS-type tensile testing machineequipped with a 10 N capacity load cell. The considered gauge length was10 mm. Before testing, the fibre was glued on a paper frame and its diameter wasdetermined from the average of optical measurements in three different spots.Then, the frame was clamped on the MTS machine. The cross-head displacementapplied was 1 mm/min. The test was repeated 10 times and the average values werereported. For each confined cylinder, one end of the FFRP tube was capped with awooden plate to generate as a formwork for the fresh concrete. Then concretewas cast, poured, compacted and cured in a standard curing water tank for 28 days.Fig. 3 displays the FFRP tubes and a FFRP-CFRC specimen during casting.

4.2. Test specimens and instrumentation

A total of 36 cylindrical specimens were constructed and tested in this study.Eighteen short cylindrical specimens (with an inner diameter of 100 mm and lengthof 200 mm) were tested under uniaxial compression to investigate the compressivebehaviour of FFRP-CFRC. Eighteen long cylindrical specimens (with an inner diam-eter of 100 mm and length of 520 mm) were under third-point bending test toinvestigate the flexural behaviour of FFRP-CFRC. The test variables are FFRP tubethickness and coir fibre inclusion. Test matrix of the specimens for this study islisted in Table 3. In the following context, ‘‘FFRP-PC’’ indicates flax FRP tube encasedplain concrete and ‘‘FFRP-CFRC’’ indicates flax FRP tube encased coir fibre reinforcedconcrete, respectively.

For each short cylindrical specimen, two strain gauges were mounted at mid-height of a cylinder aligned along the hoop direction to measure hoop strain. Twolinear variable displacement transducers (LVDTs) were mounted at mid-height ofthe cylinder aligned along the axial direction to measure axial strain, as shown inFig. 4. The compression test was conducted on an Avery-Denison machine usingstress control with a constant rate of 0.20 MPa/s based on ASTM C39 [30]. Eachsample was axially compressed to failure. Readings of the load, strain gauges andLVDTs were taken using a data logging system and were stored in a computer.

For each long cylindrical specimen, six strain gauges and three LVDTs wereused. Three strain gauges (i.e. gauges H1, H2 and H3) mounted at the mid-spanof a cylinder aligned along the hoop direction and three strain gauges (i.e. gaugesA1, A2 and A3) at the axial direction to measure the hoop and axial strains, respec-tively. One LVDT was covered the lower boundary of the composite column at themid-span to measure the deflection of the column. The other two LVDTs were in-stalled at the end of the column to measure the slip between the concrete coreand the FFRP tube, as shown in Fig. 5. The third-point bending test was conductedon Instron testing machine according to ASTM C78 [31] standard. Readings of theload, strain gauges and LVDTs were taken using a data logging system and werestored in a computer.

5. Results and discussion

5.1. Axial compressive test

One objective of this study is to evaluate the compressive per-formance of the FFRP-CFRC composite as axial structural member.The effect of FFRP tube thickness and coir fibre inclusion on the ax-

Flexural strength(MPa)

Flexural modulus(GPa)

Flexuralstrain (%)

Fibre volumefraction (%)

109 6.0 4.7 54.2144 8.7 5.2 55.1

Fig. 3. Specimens: (a) flax FRP tubes and (b) FFRP-CFRC.

Table 3Test matrix of the specimens considered in this study.

Specimen group No. of specimens No. of fabric layers Core diameter D (mm) Length (mm) Tube thickness t (mm)

PC 3 – 100 200 –CFRC 3 – 100 200 –2-layer FFRP-PC 3 2 100 200 3.254-layer FFRP-PC 3 4 100 200 6.502-layer FFRP-CFRC 3 2 100 200 3.254-layer FFRP-CFRC 3 4 100 200 6.50PC 3 – 100 520 –CFRC 3 – 100 520 –2-layer FFRP-PC 3 2 100 520 3.254-layer FFRP-PC 3 4 100 520 6.502-layer FFRP-CFRC 3 2 100 520 3.254-layer FFRP-CFRC 3 4 100 520 6.50

Fig. 4. Axial compression test.

L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127 1121

ial compressive stress–strain responses, confinement performance,and ductility and failure modes of the short cylindrical specimens

were discussed. The experimental results of the confined concretecompressive strength were compared with the predictions ob-tained from the available strength equations and models in the lit-erature and the design codes.

5.1.1. Axial stress–strain relationshipThe axial compressive stress–strain curves of short FFRP-PC and

FFRP-CFRC specimens are displayed in Figs. 6 and 7. The responseof FFRP-PC and FFRP-CFRC are consistent. These curves can bedivided into three regions, two linear stages connected by a nonlin-ear transition stage. In the first purely linear region, the stress–strain behaviour of either FFRP-PC or FFRP-CFRC is similar to thecorresponding unconfined PC or CFRC. In this region the appliedaxial stress is low, lateral expansion of the confined PC or CFRC isinconsiderable and confinement of FFRP tube is not activated.When the applied stress approaches the peak strength of uncon-fined PC or CFRC, the curve enters the nonlinear transition regionwhere considerable micro-cracks are propagated in concrete andthe lateral expansion significantly increased. With the growth ofmicro-cracks, the tube starts to confine the concrete core. The thirdapproximately linear region is mainly dominated by the structuralbehaviour of FFRP composites where the tube is fully activated toconfine the core, leading to a considerable enhance in compressivestrength and ductility of concrete. When axial stress increases, thehoop tensile stress in the FFRP tube also increases. Once this hoopstress exceeds the ultimate tensile strength of FFRP tube obtainedfrom the flat coupon tensile test failure of the FFRP tube starts.

5.1.2. Confinement performanceTable 4 lists the average compressive properties of the short

specimens obtained from three identical specimens. f 0co is the peak

Fig. 5. Schematic view of third-point bending test setup.

Fig. 6. Axial stress–strain behaviour of FFRP-PC.

Fig. 7. Axial stress–strain behaviour of FFRP-CFRC.

Table 4Average test results of short cylindrical specimens under axial compression.

Specimentype

f 0co

(MPa)eco

(%)f 0cc

(MPa)ecc

(%)fl

(MPa)

f 0ccf 0co

ecceco

PC 25.8 0.20 – – – – –CFRC 28.2 0.54 – – – – –2L-FFRP-PC 25.8 0.20 37.0 1.72 7.08 1.43 8.604L-FFRP-PC 25.8 0.20 53.7 2.25 18.72 2.08 11.252L-FFRP-CFRC 28.2 0.54 38.8 1.89 7.08 1.38 3.504L-FFRP-CFRC 28.2 0.54 56.2 2.70 18.72 2.00 5.00

1122 L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127

compressive strength of the unconfined concrete, f 0cc is the peakcompressive strength of the confined concrete, f 0ccf 0co is the confine-ment effectiveness of FFRP tube. fl is the lateral pressure betweenFFRP tube and concrete. eco and ecc is the axial strain for unconfinedconcrete and confined concrete at the corresponding peak com-pressive strength f 0co and f 0cc; respectively. ecc/eco is the axial strainratio of FFRP tube encased concrete.

As shown in Table 4, coir fibre inclusion increases concrete peakcompressive strength f 0co to 9.3% from 25.8 to 28.2 MPa, and en-hances the corresponding eco from 0.20% to 0.54%. Considering bothPC and CFRC, FFRP tube offers a significant enhancement in con-crete axial compressive strength, indicating the effect of confine-ment. The average confinement effectiveness f 0cc=f 0co of 2-layer and4-layer FFRP-PC are 1.43 and 2.08, and are 1.38 and 2.00 for 2-layerand 4-layer FFRP-CFRC, respectively. This data indicates that a lar-ger tube thickness leads to a larger confinement effectiveness ofthe composite column. Considering FFRP-PC and FFRP-CFRC withthe same tube thickness, coir fibre had insignificant effect on the

confinement effectiveness but increased the ultimate compressivestrength.

With respect to ultimate axial strain, the values of both FFRP-PCand FFRP-CFRC increased with an increase in FFRP tube thickness.The ultimate axial strain ecc of FFRP-CFRC is larger than that ofFFRP- PC for specimens with the same tube thickness, i.e., the axialstrain at the peak strength of 4-layer FFRP- CFRC is 2.70% and is2.25% of 4-layer FFRP-PC. This data implies that coir fibre inclusionhas a distinct enhancement in ultimate axial strain, compared withthe confined PC specimens. This can be interpreted by coir fibrebridging effect, which exerted effectively on holding and reducingmacro-cracks in the concrete core, although the stress–strain curveentered the second linear region, as shown in Fig. 7. Unlike that inPC core, coir fibre reduced and/or delayed the further propagationof lateral expansion of the concrete core and thus the rupture ofthe FFRP tube was delayed. Consequently, the fibre bridging effectincreased the ultimate axial strain of the FFRP-CFRC.

5.1.3. DuctilityDuctility of G/CFRP confined concrete can be evaluated based on

the axial strain ratio of the confined concrete to that of the uncon-fined concrete. It is also considered in this study to evaluate theductility of FFRP tube encased concrete. As displayed in Table 4,the strain ratios of 2-layer and 4-layer FFRP-PC are 8.60 and11.25, and are 3.5 and 5.0 for 2-layer and 4-layer FFRP-CFRC,respectively. Therefore, FFRP tube confinement led to the signifi-cant increase in the ductility of the proposed composite membersunder pure axial compression. As expected, the ductility of thespecimen increased with an increase in tube thickness. It shouldbe mentioned here that in the case of FFRP-CFRC, the axial strainof unconfined CFRC (0.54%) was considered for the calculation, thisleads to a relatively lower value of the strain ratio. If the strain of

Fig. 9. Failure modes of PC and CFRC cores after removed FFRP tube.

Table 5Strength models for circular columns.

Reference Equations

ACI committee440.2R-08 [32]

f 0cc ¼ f 0co þ 3:135f l

CAN/CSA S6-06bridge code [33]

f 0cc ¼ f 0co þ 2f l

Youssef et al. [34] f 0cc=f 0co ¼ 1þ 2:25ðfl=f 0coÞ1:25

Kono et al. [35] f 0cc=f 0co ¼ 1þ 0:0572f l

Lam and Teng [36] f 0cc=f 0co ¼ 1þ 2:0ðfl=f 0coÞWu and Zhou [37] f 0cc=f 0co ¼ fl=f 0co þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið16:7=f 00:42

co � f 00:42co =16:7Þ � fl=f 0co þ 1

p

L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127 1123

the unconfined PC (0.2%) was considered, the strain ratios of 2-layer and 4-layer FFRP-CFRC will be 9.45 and 13.50 respectively.These ratios are 9.9% (9.45 vs. 8.60) and 20.0% (13.5 vs. 11.25) lar-ger than the corresponding 2-layer and 4-layer FFRP-PC specimens.Therefore, coir fibre inclusion further increased the ductility.

5.1.4. Failure modes in compressionFor all the short FFRP-PC and FFRP-CFRC specimens, the failure

under compression was initiated at the middle height of the tubeand progressed towards its top and bottom ends. In each of theconfined specimen, only a single crack was observed and this crackpropagated along the fibre direction in the tube (Fig. 8). Failuremodes of the concrete core were evaluated. It was found that thefailure pattern was quite different between the concrete core with-out and with coir fibre reinforcement. After removed the tube, itwas observed that the PC core completely crushed. The CFRC corewas damaged with macro-cracks but still held together by the coirfibres (Fig. 9). It is evident that coir fibre inclusion can restrict thepropagation of the cracks in the concrete core for FFRP tube en-cased concrete.

5.1.5. Prediction of confined concrete compressive strengthFor G/CFRP confined concrete design, the ultimate axial com-

pressive strength is one of the most significant parameters. There-fore, in this study the ultimate strengths of FFRP-PC and FFRP-CFRCwere predicted using the strength equations from the ACI Commit-tee 440 (ACI 440.2R-08) guidelines [32] and CAN/CSA S6-06 Bridgecode [33], as well as the strength models by Youssef et al. [34],Kono et al. [35], Lam and Teng [36] and Wu and Zhou [37] consid-ered for G/CFRP confined concrete. The strength equations andmodels for circular FRP confined concrete columns are displayedin Table 5.

Table 6 gives a comparison between the predicted and theexperimental results for the tested specimens. The strength equa-tions of ACI 440.2R-08 [32] and Wu and Zhou [37] overestimate thestrength increase for both 2-layer and 4-layer FFRP-PC and FFRP-CFRC remarkably. The ACI model adopted the confinement effec-

Fig. 8. Typical failure of FFRP-

tiveness coefficient of 3.3 according to the model by Lam and Teng[38]. This model was generated based on an interpretation of theexisting test data of CFRP and GFRP confined concrete. The consid-ered G/CFRP materials had tensile strengths from 363 to 4400 MPaand tensile moduli from 19.9 to 629.6 GPa, which are much larger

PC (a) and FFRP-CFRC (b).

Table 6Comparison of predicted compressive strength of FFRP tube confined PC and CFRC.

Models FFRP tube confined PC FFRP tube confined CFRC

2 Layer (MPa) % Diff. 4 Layer (MPa) % Diff. 2 Layer (MPa) % Diff. 4 Layer (MPa) % Diff.

Test results 37.0 – 53.7 – 38.8 – 56.2 –

ACI committee 440.2R-08 [32] 48.0 29.7 84.5 57.4 50.4 29.9 86.9 54.6CAN/CSA S6-06 bridge code [33] 39.9 7.8 63.2 17.7 42.4 9.3 65.6 16.7Youssef et al. [34] 37.3 0.8 64.7 20.5 39.5 1.8 66.2 17.8Kono et al. [35] 36.3 �1.9 53.4 �0.6 39.6 2.1 58.4 3.9Lam and Teng [36] 39.9 7.8 63.2 17.7 42.4 9.3 65.6 16.7Wu and Zhou [37] 47.1 27.3 69.8 30.0 46.7 20.4 71.9 27.9

Loa

d (k

N)

Mid-span deflection (mm)

Fig. 10. Load–deflection behaviour of PC and FFRP-PC.

Loa

d (k

N)

Mid-span deflection (mm)

Fig. 11. Load–deflection behaviour of CFRC and FFRP-CFRC.

1124 L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127

than those of FFRP composites considered in this study (Table 1).This difference in FRP material properties may lead to the overes-timation for FFRP confined concrete by the model. Wu and Zhou[37] developed their model based on the Hoek–Brown failure crite-rion from rock mechanism which considered the tensile strength ofconcrete. They considered material parameter m depends on theplain concrete strength and proposed a closed form relation form denoting also plain concrete strength dependence, for concretestrengths between 18 and 80 MPa.

CAN/CSA S6-06 Bridge code [33], Youssef et al. [34] and Lamand Teng [36] predict the strength gain for 2-layer FFRP-PC andFFRP-CFRC well but slightly overestimate the strength for 4-layerFFRP-PC and FFRP-CFRC with a difference around 20%. Both CAN/CSA S6-06 code and Lam and Teng [36] used the confinement effec-tiveness coefficient of 2.0, which is much lower than that consid-ered in ACI 440.2R-08. This leads to the close prediction to thetest results of FFRP tube encased concrete.

The strength model by Kono et al. [35] predicts the ultimatecompressive strength accurately for both 2-layer and 4-layerFFRP-PC and FFRP-CFRC specimens with the differences all below5%. Kono et al. related the confined strength to the confinementpressure times the plain concrete strength linearly.

5.2. Third-point bending test

Another objective of this study is to evaluate the feasibility ofthe FFRP-CFRC composite columns as flexural structural members.The average test results for long cylindrical specimens under flex-ure obtained from three identical specimens are summarised in Ta-ble 7. The effect of FFRP tube confinement and coir fibre inclusionon the peak load, maximum deflection, failure modes and bondbehaviour of the composite columns were evaluated. The neutralaxis depths of the composite columns have been determined basedon the distribution of the measured strains.

5.2.1. Effect of FFRP tube on peak loadFig. 10 shows the load–deflection curves for PC, 2-layer and 4-

layer FFRP-PC specimens and Fig. 11 shows the curves for CFRC,

Table 7Average test results of long cylindrical specimens under flexure.

Specimen type PeakLoad (kN)

Increase dueto tube (%)

Increase dueto coir (%)

Max.deflection (m

PC 7.4 – – 0.52L-FFRP-PC 27.2 268a – 8.44L-FFRP-PC 78.9 1066a – 14.3

CFRC 10.1 – 36.5b 1.22L-FFRP-CFRC 29.7 267a 9.2b 9.44L-FFRP-CFRC 84.7 946a 7.4b 16.8

a Indicates the increase due to tube confinement when comparing with unconfined Pb Indicates the increase due to coir fibre inclusion when comparing with the correspo

2-layer and 4-layer FFRP-CFRC specimens. It is clear that the PCcolumns have negligible lateral load carrying capacity and mid-span deflection as a result of un-reinforcement. In the case of con-fined PC, the 2-layer FFRP-PC experienced 268% and 1360%, and the

m)Increase dueto tube (%)

Increase dueto coir (%)

Ultimate moment(kN mm)

Slip (mm)

– – 555 –1580a – 2040 0.62760a – 5918 1.1

– 140b 758 –683a 11.9b 2228 0.41300a 17.5b 6353 1.4

C or CFRC.nding unconfined PC or confined PC specimens with the same tube thickness.

L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127 1125

4-layer FFRP-PC experienced 1066% and 2760% enhancement inultimate load and deflection, respectively, compared with theunconfined PC specimen. In comparison with the unconfined CFRC,the increase in load and deflection of 2-layer FFRP-CFRC are 267%and 683%, and are 946% and 1300% of 4-layer FFRP-CFRC, respec-tively. This data indicated that FFRP tube confinement enhancedthe load carrying capacity and deflection of both PC and CFRC col-umns remarkably. In flexure, the FFRP tube acted as reinforcementof the concrete core and the concrete core provided the internalresistance force in the compression zone and increased the stiff-ness of the composite structure.

The enhancement in load and deflection of the FFRP-PC andFFRP-CFRC specimens also increased with an increase in tubethickness. From 2-layer to 4-layer FFRP-PC, the increase in loadand deflection are 190.1% (from 27.2 to 78.9 kN) and 72.3% (from8.3 to 14.3 mm), respectively. For the CFRC, the increase in loadand deflection from 2-layer to 4-layer FFRP confinement are185.2% (from 29.7 to 84.7 kN) and 78.7% (from 9.4 to 16.8 mm),respectively.

Fig. 10 also displays that the load–deflection responses of 2-layer and 4-layer FFRP-PC are similar, which are dominated bythe strength and stiffness of the FFRP composite material. Thecurves are approximately linear at the beginning of the deflectionand then become nonlinear until failure as that of the typical ten-sile stress–strain curves of FFRP composites given in Fig. 1. Whenexceeded the maximum load, the curves stop without hardening,which implies a brittle failure of the composite column since bothPC and FFRP are brittle materials.

5.2.2. Effect of coir fibre on ductilityCompared with PC specimen, the column with coir fibre rein-

forcement had a larger ultimate load and deflection with an in-crease of 36.5% and 140%, respectively. In comparison with thebrittle response of PC (Fig. 10), the post-peak response of CFRCexhibited a ductile manner (Fig. 11). The difference in load, deflec-

Fig. 12. Typical failure modes: (a) 4-layer FFRP-PC, (b) 4-layer FFRP-CFR

tion and failure mode were attributed to the result of coir fibrebridging effect. The coir fibres bridged the macro-cracks of the con-crete and provided an effective secondary reinforcement for crackcontrol. The fibres also bridged the adjacent surfaces of existingmicro-crack, impeded crack development and limited crack propa-gation by reducing the crack tip opening displacement. In the caseof confined CFRC, the increase in peak load and deflection of 2-layer and 4-layer FFRP-CFRC are 9.2% and 11.9%, and 7.4% and17.5% respectively when comparing to the corresponding FFRP-PC specimens. Therefore, coir fibre inclusion increased the lateralload carrying capacity and the maximum deflection of the compos-ite columns as flexural structural members. Further, it should bepointed out that there is a distinct post-peak hardening of theload–deflection of the FFRP-CFRC specimens. Compared with thesudden failure of FFRP-PC (Fig. 10), the addition of coir fibre mod-ified the failure pattern to be ductile, as given in Fig. 11. More dis-cussion will be given in the following section.

5.2.3. Failure modes in flexureFailure modes of FFRP-PC and FFRP-CFRC specimens are dis-

played in Fig. 12. In flexure, the failure of all the FFRP-PC andFFRP-CFRC initiated by the tensile rupture of the FFRP tube in thezone between the two concentrated loads (largest bending mo-ment appears in this zone), as displayed in Fig. 12a and b. For allthe FFRP-PC and FFRP-CFRC specimens, in flexure, each specimenonly had one crack on the surface of the FFRP tube. The crack beganat the bottom section of the tube and progressed towards theupper compression zone resulting in the development of the majorcrack. The crack was almost perpendicular to the axis of the tube.In the case of FFRP-PC, the major crack went through the entiretube and the composite member was sudden broken into twohalves (Fig. 12a). This is in exact accordance with the load–deflec-tion response of the FFRP-PC column. However, for confined CFRC,the major crack terminated at the compression zone of the com-posite column (Fig. 12b). After the test, the outer FFRP tube was re-

C, (c) CFRC core and (d) PC core. L denotes the span of the column.

1126 L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127

moved to examine the failure patterns of the concrete core. For PCcore (Fig. 12d), it was observed that there were large amounts ofvertical cracks and diagonal cracks along the two halves of the con-crete. The vertical cracks were located in the constant bending mo-ment zone and were thought to be the result of pure bending. Thediagonal cracks in the shear span were pointed to the two loadpoints due to the shear-flexure forces. Regarding to CFRC, the corehad a major crack with some small cracks in the zone between thetwo concentrated loads (Fig. 12c). No diagonal cracks in the shearspan were observed. Obviously, the coir fibres bridged the adjacentsurfaces of the major crack. Therefore, the comparison in failuremodes of PC and CFRC cores gives credence to the statement thatcoir fibre bridging dominated the post-peak ductile response ofFFRP-CFRC column under flexure in Fig. 11.

5.2.4. Bond between FFRP tube and concrete coreIn flexure, slippage between FFRP tube and concrete core may

compromise the load carrying capacity of the composite structure.To evaluate the bond of the composite structure, the slip at theends of the specimens between the tube and the concrete corewas measured. The measured average slips are 0.6 and 0.4 mmfor 2-layer FFRP-PC and FFRP-CFRC, and are 1.1 and 1.4 mm for4-layer FFRP-PC and FFRP-CFRC, respectively. This data indicatesthat coir fibre inclusion has no effect on the prevention of slippagebetween FFRP tube and concrete core. Therefore, special arrange-ment should be considered to roughen the inner surface of theFFRP tubes to prevent slippage and may further increase the loadcarrying capacity of the proposed composite column, i.e. the in-crease in tube/concrete interfacial bond through the mechanicalinterlocking [9–11].

6. Conclusions

This study experimentally investigated the compressive andflexural behaviour of natural flax fabric reinforced polymer (FFRP)tube confined plain concrete (PC) and coir fibre reinforced concrete(CFRC) columns. The study reveals:

� In axial compression, FFRP tube confinement enhances the com-pressive strength and ductility of both PC and CFRC signifi-cantly. The enhancement in strength and ductility increaseswith an increase in tube thickness. Coir fibre inclusion furtherincreases the strength and ductility of the composite member.� Both FFRP-PC and FFRP-CFRC specimens under compression

behave bi-linear stress–strain manner with an ascendingbranch at the second stage. Coir fibre inclusion does not changethis pattern but changed the failure mode of the confined con-crete core.� The ACI 440.2R-08 [32] and CAN/CSA S6-06 [33] design codes

and the strength models by Youssef [34], Lam and Teng [36]and Wu and Zhou [37] overestimate the ultimate compressivestrength of FFRP tube confined PC and CFRC. The predictionsby the strength equation of Kono et al. [35] match the experi-mental ultimate strengths of all the confined PC and CFRC wellwith the errors below 5%.� In flexure, FFRP tube confinement increases the ultimate lateral

load and mid-span deflection of the PC and CFRC membersremarkably. However, FFRP-PC columns exhibit a brittle failuremode while FFRP-CFRC columns behave a ductile manner dueto coir fibre bridging effect. Therefore, coir fibre increases theductility and flax FRP contributes to the significant increase inthe peak load of the composite structure.� Slippage between FFRP tube and concrete core is commonly

observed for the tested specimens. Coir fibre inclusion has noeffect on the prevention of slippage. To prevent the slippagebetween FFRP tube and concrete, in the following study an

attempt will be considered to increase tube and concrete inter-facial bond, i.e. along the longitudinal axis of the flax FRP tube,the embedment of several small flax FRP rings onto the innersurface of the FFRP tube with the help of epoxy.

In general, the feasibility of natural flax and coir fibres as con-struction building materials has been evaluated in this study. Thetest results indicate that the proposed FFRP-CFRC has the potentialto be axial and flexural structural members. Natural fibre rein-forced polymer composites as concrete confinement can increasethe compressive and flexural properties of concrete. Coir fibre asreinforcement of concrete can modify the failure pattern of FFRPtube encased concrete.

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