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

1. Construction and Building Materials 40 (2013) 1118–1127
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Experimental study of flax FRP tube encased coir fibre reinforced concrete
composite column
Libo Yan ⇑, Nawawi Chouw
Department 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 a b s t r a c t
Article history: The use of natural fibres as building materials is benefit to achieve a sustainable construction. This paper
Received 19 September 2012 reports on an experimental investigation of a composite column consisting of flax fibre reinforced poly-
Received in revised form 19 November 2012 mer (FFRP) and coir fibre reinforced concrete (CFRC), i.e. FFRP tube encased CFRC (FFRP-CFRC). In this
Accepted 30 November 2012
FFRP-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 the
compression and flexural performance of the composite column. A total of 36 specimens were tested. The
Keywords:
test variables were FFRP tube thickness and coir fibre inclusion. The axial stress–strain response, confine-
Flax fibre
Fibre reinforced polymer
ment performance, lateral load–displacement response, bond behaviour and failure modes of the com-
Coir fibre posite column were analysed. In addition, the confined concrete compressive strength was predicted
Fibre reinforced concrete using existing strength equations/models and compared with the experimental results. Results indicate
Confinement that the FFRP-CFRC composite columns using natural fibres have the potential to be axial and flexural
Ductility structural members.
Slippage Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction sustainable material [3]. Thus the use of cost-effective natural fi-
bres in FRP composites as concrete confinement is another step
Natural fibres are a renewable resource and are available all to achieve a more sustainable construction.
most over the world. The use of natural fibres by the construction Therefore, the purpose of this study is to investigate the feasi-
industry will help to achieve a sustainable consumption pattern of bility of natural flax fabric reinforced epoxy composite tube en-
building materials. The European Union recently established that cased coir fibre reinforced concrete (FFRP-CFRC) composite
in a medium term raw materials consumption must be reduced column as axial and flexural structural members. Specifically, this
by 30% and that waste production must be cut down by 40% [1]. study presents an experimental study on the use of coir fibres as
Thus cost-effective natural fibres as reinforcement of concrete to reinforcement in concrete and flax fibres as reinforcement for fibre
replace the expensive, highly energy consumed and non-renew- reinforced polymer composites as concrete confinement for struc-
able reinforced steel rebar are a major step to achieve a sustainable tural applications. The new FFRP-CFRC composite structure is ex-
construction [2]. In addition, recently, the use of natural fibres to pected to have good performance as axial and flexural structural
replace carbon/glass fibres as reinforcement in FRP composites members.
for engineering applications has gained popularity due to an
increasing environmental concern and requirement for developing 2. Background
⇑ Corresponding author. Tel.: +64 9 3737599x84521; fax: +64 9 373 7462. Currently composite columns are widely used in high-rise
E-mail address: lyan118@aucklanduni.ac.nz (L. Yan). building, offshore structures and bridge, particularly in regions of
0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.conbuildmat.2012.11.116

2. 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- to those of glass fibres used as reinforcement [16]. Therefore, nat-
creased deformability [4]. Concrete filled fibre reinforced polymer ural fibres represent a highly ‘‘sustainable’’ material. The use of
tube (CFFT) is one of the most common composite columns re- natural fibres in FRP composites as building materials will promote
ported in the literature. the ‘‘sustainable’’ development for construction industry. Natural
In CFFT columns, the pre-fabricated tubes made of glass/carbon fibres such as flax, hemp, jute, coir and sisal, are cost effective, have
fibre reinforced polymer (G/CFRP) materials act as permanent low density with high specific strength and stiffness, and are read-
formworks for fresh concrete and also provide confinement to con- ily available [17]. Dittenber and GangaRao [18] compared more
crete. The advantages of G/CFRP materials are their high strength than 20 commonly used natural fibres (e.g. sisal, ramie, kenaf, jute,
and stiffness. The non-corrosive characteristic also provides FRP hemp, flax, coir, cotton, etc.) with glass fibres in specific modulus,
as an alternative to replace steel reinforcement in civil structural cost per weight and cost per unit length (capable of resisting
applications [5]. 100 kN load). They concluded that among those natural fibres, flax
Behaviour of plain concrete filled FRP tube (CFFT) has been well fibre offers the best potential combination of low cost, light weight,
documented [6–8]. Davol et al. [6] studied CFFT columns as bend- and high strength and stiffness for structural applications. Assarar
ing members. The external FRP shell replaced the functions of steel et al. [19] reported that the tensile strength of flax/epoxy compos-
rebar in conventional reinforced concrete (RC) members, namely, ites is 300 MPa – putting them close to GFRP composites. The
tension carrying capacity and shear resistance, as well as confine- investigation showed encouraging mechanical properties of bio-
ment of concrete core. Fam and Rizkalla [7] investigated flexural composites. Therefore, it is possible to use the more economical
behaviour of small and large-scale CFFTs with diameters ranging bio-composites (e.g. flax FRP) to replace synthetic glass FRP in
from 89 to 942 mm and spans ranging from 1.07 to 10.4 m. They engineering applications.
concluded that the flexural behaviour of CFFT was highly depen- Previous research on fibre reinforced concrete has shown that
dent on stiffness of FRP materials and ratio of diameter-to-FRP tube short natural fibres, used in cementitious matrices, can modify ten-
thickness. In flexure, slippage between FRP tube and concrete core sile and flexural strength, toughness, impact resistance and frac-
may compromise the load carrying capacity of the CFFT. To prevent ture energy [20]. Pacheco-Torgal and Jalali reviewed the
the slippage between FRP tube and concrete core, a study by EI mechanical properties of several vegetable fibres (i.e. sisal, hemp,
Chabib et al. [8] indicated that the use of expansive cement in con- coir, banana and sugar cane bagasse) as reinforcement of cementi-
crete created a somewhat better tube and concrete interfacial con- tious building materials [2]. Among those natural fibres, coir fibre,
tact, however, did not fully prevent the slippage. Mirmiran et al. [9] as reinforcement fibre in concrete, was investigated widely due to
considered the application of shear connector ribs which placed on its highest toughness among natural fibres and the extremely low
the interior surface of the GFRP tube. It was found that the ribs sig- cost, as well as availability [21]. Li et al. [22] stated that flexural
nificantly improved the axial load-carrying capacity of GFRP tube toughness and flexural toughness index of cementitious compos-
confined concrete. Li et al. [10,11] proposed a novel advanced grid ites with coir fibre increased by more than 10 times due to coir fi-
stiffened (AGS) FRP tube which made of a lattice of interlaced FRP bre bridging effect. Reis [23] also reported that coir fibre increased
ribs. Test results indicated that the lateral load carrying capacity concrete composite fracture toughness and the use of coir fibres
was improved due to the enhanced interfacial bonding strength showed even better flexural properties than synthetic fibres (glass
between the tube and the concrete through the mechanical and carbon). Therefore, the inclusion of coir fibre might be useful
interlocking. to increase the flexural performance of CFFT, particularly in chang-
In flexure, FRP tube confinement leads to the significantly in- ing the brittle failure pattern of the concrete core. Consequently,
crease in lateral load capacity and mid-span deflection of the con- the use of natural fibres in concrete is not only benefit to enhance
crete core; however, it is commonly observed that unlike that in the mechanical properties of concrete, but also promotes the
RC columns, the post-peak load–deflection responses of the CFFT development of sustainable ‘‘green’’ concrete and thus saves the
columns exhibit a brittle manner as a result of the non-yielding natural resources [24].
characteristic of FRP materials [e.g. 6, 7]. After tested, when re-
moved the external tubes, the plain concrete cores developed 3. Objectives
excessive larger flexural cracks at the mid-span of the columns or
even damaged to several blocks which distributed along the col- The objective of this work is to perform a study on the use of
umns, as observed in previous study [7]. Therefore, when consider- natural coir fibres as reinforcement of concrete and natural flax fi-
ing CFFT columns used in a practical project, small amount of steel bres in FRP composites as the concrete confinement, i.e. flax FRP
reinforcement were usually considered in order to avoid the brittle tube encased coir fibre reinforced concrete (FFRP-CFRC) composite
failure, e.g. the use of CFFT piles in the construction of the Route 40 structure. The use of these environmentally friendly natural fibres
highway bridge over the Nottoway River in the United States [12]. is benefit to the development of sustainable construction. Uniaxial
Currently a wider application of G/CFRP materials in civil infra- compression and third-point bending tests were conducted to as-
structure is limited by the high initial cost, the insufficiency of long sess the composite columns as axial and flexural structural mem-
term performance data, the lack of standard manufacturing tech- bers. The effects of FFRP tube thickness and coir fibre inclusion
niques and design standards, durability of glass fibres, risk of fire on the compressive and flexural performance of the composite col-
and the concern that the non-yielding characteristic of FRP materi- umns were investigated. In addition, the ultimate compressive
als could result in sudden failure of the structure without prior strength of FFRP tube confined concrete was predicted using the
warning [7,13–15]. Among these limitations, cost and concern of existing G/CFRP confined concrete strength equations/models and
brittle failure of FRP materials are probably the most influential compared with the experimental results.
factors when assessing the merits of FRP as construction materials.
Recently, the use of natural fibres to replace carbon/glass fibres 4. Experimental procedures
as reinforcement in FRP composites has gained popularity due to
increasing environmental concern. Natural fibres are low cost fi- 4.1. Materials
bres with low density. These are biodegradable, non-abrasive, re-
4.1.1. Flax FRP tube
duced energy consumption, less health risk, renewability, In this study, commercial bidirectional woven flax fabric (550 g/m2) was ob-
recyclability and bio-degradability. In addition, they are readily tained from Libeco, Belgium. The Epoxy used was the SP High Modulus Ampreg
available and their specific mechanical properties are comparable 22 resin and slow hardener. FFRP tubes were fabricated using the hand lay-up pro-

3. 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. Table 2
The detail of fabrication process of FFRP tubes was similar as that described in [25]. Average mechanical properties of coir fibre.
Fabric fibre orientation was at 90o from the axial direction of the tube. The structure
of the flax fabric is given in the study [26]. Two layer arrangements of FFRP tube Properties Coir fibre
were considered: two layers and four layers. Tensile and flexural properties of FFRP Average diameter 0.25 mm
composites were determined by a flat coupon test on Instron 5567 machine accord- Length 50 mm
ing to ASTM D3039 [27] and ASTM D790 [28], respectively. The typical tensile and Density 1.20 g/cm3
flexural stress–strain curves of flax fabric reinforced epoxy polymer (FFRP) compos- Tensile modulus 2.74 GPa
ites are displayed in Figs. 1 and 2, respectively. Fig. 1 indicates that the tensile Tensile stress at break 286 MPa
stress-stain responses of 2-layer and 4-layer FFRP composites are quite consistent. Tensile strain at break 20.8%
Their curves are purely linear with the strains up to 0.3% and followed by a moder- Aspect ratio 200
ate softening and nonlinear response until failure without yielding. The physical
and mechanical properties of FFRP composites are listed in Table 1.
4.1.2. Concrete nesia. The fibres had been treated and cut to a length of 50 mm. The considered coir
All specimens were constructed from two concrete batches. One batch was fibre weight content was 1% of the mass of the cement. The average mechanical
plain concrete (PC) and the other one was coir fibre reinforced concrete (CFRC). Both properties of the coir fibres used in this study are given in Table 2. The mechanical
concrete batches were designed as PC with a 28-day compressive strength of properties (ultimate tensile strength, failure strain and Young’s modulus) of single
25 MPa. The concrete mix design followed the ACI Standard 211.1 [29]. The mix ra- coir fibres were determined using a universal MTS-type tensile testing machine
tio by weight was 1:0.68:3.77:2.96 for cement:water:gravel:sand, respectively. The equipped with a 10 N capacity load cell. The considered gauge length was
cement used was CEM I 42.5 normal Portland cement with a general use type. The 10 mm. Before testing, the fibre was glued on a paper frame and its diameter was
coarse aggregate was gravel having a density of 1850 kg/m3. The gravel has a max- determined from the average of optical measurements in three different spots.
ium size of 15 mm (passing through 15 mm sieve and retained at 10 mm sieve). The Then, the frame was clamped on the MTS machine. The cross-head displacement
natural sand was used as a fine aggregate with a fineness modulus of 2.75. For CFRC applied was 1 mm/min. The test was repeated 10 times and the average values were
batch, coir fibre was added during mixing. The coir fibres were obtained from Indo- reported. For each confined cylinder, one end of the FFRP tube was capped with a
wooden plate to generate as a formwork for the fresh concrete. Then concrete
was 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.
140
120 4.2. Test specimens and instrumentation
4L FFRP
Tensile stress (MPa)
2L FFRP A total of 36 cylindrical specimens were constructed and tested in this study.
100
Eighteen short cylindrical specimens (with an inner diameter of 100 mm and length
of 200 mm) were tested under uniaxial compression to investigate the compressive
80
behaviour 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 to
60
investigate the flexural behaviour of FFRP-CFRC. The test variables are FFRP tube
thickness and coir fibre inclusion. Test matrix of the specimens for this study is
40
listed in Table 3. In the following context, ‘‘FFRP-PC’’ indicates flax FRP tube encased
plain concrete and ‘‘FFRP-CFRC’’ indicates flax FRP tube encased coir fibre reinforced
20
concrete, respectively.
For each short cylindrical specimen, two strain gauges were mounted at mid-
0
height of a cylinder aligned along the hoop direction to measure hoop strain. Two
0 0.01 0.02 0.03 0.04 0.05
linear variable displacement transducers (LVDTs) were mounted at mid-height of
Tensile strain the cylinder aligned along the axial direction to measure axial strain, as shown in
Fig. 4. The compression test was conducted on an Avery-Denison machine using
Fig. 1. Typical tensile stress–strain curves of flax FRP composites. stress control with a constant rate of 0.20 MPa/s based on ASTM C39 [30]. Each
sample was axially compressed to failure. Readings of the load, strain gauges and
LVDTs were taken using a data logging system and were stored in a computer.
For each long cylindrical specimen, six strain gauges and three LVDTs were
160 used. Three strain gauges (i.e. gauges H1, H2 and H3) mounted at the mid-span
4L FFRP of a cylinder aligned along the hoop direction and three strain gauges (i.e. gauges
140 A1, A2 and A3) at the axial direction to measure the hoop and axial strains, respec-
2L FFRP tively. One LVDT was covered the lower boundary of the composite column at the
Flexural stress (MPa)
120
mid-span to measure the deflection of the column. The other two LVDTs were in-
100 stalled at the end of the column to measure the slip between the concrete core
and the FFRP tube, as shown in Fig. 5. The third-point bending test was conducted
80 on Instron testing machine according to ASTM C78 [31] standard. Readings of the
load, strain gauges and LVDTs were taken using a data logging system and were
60 stored in a computer.
40
5. Results and discussion
20
0 5.1. Axial compressive test
0 0.01 0.02 0.03 0.04 0.05 0.06
Flexural strain One objective of this study is to evaluate the compressive per-
formance of the FFRP-CFRC composite as axial structural member.
Fig. 2. Typical flexural stress–strain curves of flax FRP composites.
The effect of FFRP tube thickness and coir fibre inclusion on the ax-
Table 1
Physical and mechanical properties of flax FRP composite.
No. of flax FRP thickness Tensile strength Tensile modulus Tensile Flexural strength Flexural modulus Flexural Fibre volume
layers (mm) (MPa) (GPa) strain (%) (MPa) (GPa) strain (%) fraction (%)
2 3.25 106 8.7 3.7 109 6.0 4.7 54.2
4 6.50 134 9.5 4.3 144 8.7 5.2 55.1

4. L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127 1121
Fig. 3. Specimens: (a) flax FRP tubes and (b) FFRP-CFRC.
Table 3
Test 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 3.25
4-layer FFRP-PC 3 4 100 200 6.50
2-layer FFRP-CFRC 3 2 100 200 3.25
4-layer FFRP-CFRC 3 4 100 200 6.50
PC 3 – 100 520 –
CFRC 3 – 100 520 –
2-layer FFRP-PC 3 2 100 520 3.25
4-layer FFRP-PC 3 4 100 520 6.50
2-layer FFRP-CFRC 3 2 100 520 3.25
4-layer FFRP-CFRC 3 4 100 520 6.50
were discussed. The experimental results of the confined concrete
compressive 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 relationship
The axial compressive stress–strain curves of short FFRP-PC and
FFRP-CFRC specimens are displayed in Figs. 6 and 7. The response
of FFRP-PC and FFRP-CFRC are consistent. These curves can be
divided 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 the
corresponding unconfined PC or CFRC. In this region the applied
axial stress is low, lateral expansion of the confined PC or CFRC is
inconsiderable 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 region
where considerable micro-cracks are propagated in concrete and
the lateral expansion significantly increased. With the growth of
micro-cracks, the tube starts to confine the concrete core. The third
approximately linear region is mainly dominated by the structural
behaviour of FFRP composites where the tube is fully activated to
confine the core, leading to a considerable enhance in compressive
strength and ductility of concrete. When axial stress increases, the
hoop tensile stress in the FFRP tube also increases. Once this hoop
stress exceeds the ultimate tensile strength of FFRP tube obtained
Fig. 4. Axial compression test. from the flat coupon tensile test failure of the FFRP tube starts.
5.1.2. Confinement performance
ial compressive stress–strain responses, confinement performance, Table 4 lists the average compressive properties of the short
0
and ductility and failure modes of the short cylindrical specimens specimens obtained from three identical specimens. fco is the peak

5. 1122 L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127
Fig. 5. Schematic view of third-point bending test setup.
confinement effectiveness but increased the ultimate compressive
strength.
With respect to ultimate axial strain, the values of both FFRP-PC
and FFRP-CFRC increased with an increase in FFRP tube thickness.
The ultimate axial strain ecc of FFRP-CFRC is larger than that of
FFRP- PC for specimens with the same tube thickness, i.e., the axial
strain at the peak strength of 4-layer FFRP- CFRC is 2.70% and is
2.25% of 4-layer FFRP-PC. This data implies that coir fibre inclusion
has a distinct enhancement in ultimate axial strain, compared with
the confined PC specimens. This can be interpreted by coir fibre
bridging effect, which exerted effectively on holding and reducing
macro-cracks in the concrete core, although the stress–strain curve
entered the second linear region, as shown in Fig. 7. Unlike that in
PC core, coir fibre reduced and/or delayed the further propagation
Fig. 6. Axial stress–strain behaviour of FFRP-PC. of lateral expansion of the concrete core and thus the rupture of
the FFRP tube was delayed. Consequently, the fibre bridging effect
increased the ultimate axial strain of the FFRP-CFRC.
Table 4 5.1.3. Ductility
Average test results of short cylindrical specimens under axial compression. Ductility of G/CFRP confined concrete can be evaluated based on
Specimen 0
fco eco 0
fcc ecc fl 0
fcc ecc the axial strain ratio of the confined concrete to that of the uncon-
0 eco
type (MPa) (%) (MPa) (%) (MPa)
fco
fined concrete. It is also considered in this study to evaluate the
ductility of FFRP tube encased concrete. As displayed in Table 4,
PC 25.8 0.20 – – – – –
CFRC 28.2 0.54 – – – – – the strain ratios of 2-layer and 4-layer FFRP-PC are 8.60 and
2L-FFRP-PC 25.8 0.20 37.0 1.72 7.08 1.43 8.60 11.25, and are 3.5 and 5.0 for 2-layer and 4-layer FFRP-CFRC,
4L-FFRP-PC 25.8 0.20 53.7 2.25 18.72 2.08 11.25 respectively. Therefore, FFRP tube confinement led to the signifi-
2L-FFRP-CFRC 28.2 0.54 38.8 1.89 7.08 1.38 3.50 cant increase in the ductility of the proposed composite members
4L-FFRP-CFRC 28.2 0.54 56.2 2.70 18.72 2.00 5.00
under pure axial compression. As expected, the ductility of the
specimen increased with an increase in tube thickness. It should
be mentioned here that in the case of FFRP-CFRC, the axial strain
of unconfined CFRC (0.54%) was considered for the calculation, this
0
compressive strength of the unconfined concrete, fcc is the peak leads to a relatively lower value of the strain ratio. If the strain of
0 0
compressive strength of the confined concrete, fcc fco is the confine-
ment effectiveness of FFRP tube. fl is the lateral pressure between
FFRP tube and concrete. eco and ecc is the axial strain for unconfined
concrete and confined concrete at the corresponding peak com-
0 0
pressive strength fco and fcc ; respectively. ecc/eco is the axial strain
ratio of FFRP tube encased concrete.
As shown in Table 4, coir fibre inclusion increases concrete peak
0
compressive strength fco to 9.3% from 25.8 to 28.2 MPa, and en-
hances the corresponding eco from 0.20% to 0.54%. Considering both
PC and CFRC, FFRP tube offers a significant enhancement in con-
crete axial compressive strength, indicating the effect of confine-
0 0
ment. The average confinement effectiveness fcc =fco of 2-layer and
4-layer FFRP-PC are 1.43 and 2.08, and are 1.38 and 2.00 for 2-layer
and 4-layer FFRP-CFRC, respectively. This data indicates that a lar-
ger tube thickness leads to a larger confinement effectiveness of
the composite column. Considering FFRP-PC and FFRP-CFRC with
the same tube thickness, coir fibre had insignificant effect on the Fig. 7. Axial stress–strain behaviour of FFRP-CFRC.

6. 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 compression
For all the short FFRP-PC and FFRP-CFRC specimens, the failure
under compression was initiated at the middle height of the tube
and progressed towards its top and bottom ends. In each of the
confined specimen, only a single crack was observed and this crack
propagated along the fibre direction in the tube (Fig. 8). Failure
modes of the concrete core were evaluated. It was found that the
failure pattern was quite different between the concrete core with-
out and with coir fibre reinforcement. After removed the tube, it
was observed that the PC core completely crushed. The CFRC core
was damaged with macro-cracks but still held together by the coir
Fig. 9. Failure modes of PC and CFRC cores after removed FFRP tube.
fibres (Fig. 9). It is evident that coir fibre inclusion can restrict the
propagation of the cracks in the concrete core for FFRP tube en-
cased concrete.
Table 5
Strength models for circular columns.
5.1.5. Prediction of confined concrete compressive strength
Reference Equations
For G/CFRP confined concrete design, the ultimate axial com-
ACI committee 0 0
pressive strength is one of the most significant parameters. There- fcc ¼ fco þ 3:135f l
440.2R-08 [32]
fore, in this study the ultimate strengths of FFRP-PC and FFRP-CFRC 0 0
CAN/CSA S6-06 fcc ¼ fco þ 2f l
were predicted using the strength equations from the ACI Commit- bridge code [33]
tee 440 (ACI 440.2R-08) guidelines [32] and CAN/CSA S6-06 Bridge Youssef et al. [34] fcc =fco ¼ 1 þ 2:25ðfl =fco Þ1:25
0 0 0
code [33], as well as the strength models by Youssef et al. [34], Kono et al. [35] 0 0
fcc =fco ¼ 1 þ 0:0572f l
0 0 0
Kono et al. [35], Lam and Teng [36] and Wu and Zhou [37] consid- Lam and Teng [36] fcc =fco ¼ 1 þ 2:0ðfl =fco Þ
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ered for G/CFRP confined concrete. The strength equations and Wu and Zhou [37] 0 0 0 00:42 00:42
fcc =fco ¼ fl =fco þ ð16:7=fco À fco =16:7Þ Â fl =fco þ 1 0
models for circular FRP confined concrete columns are displayed
in Table 5.
Table 6 gives a comparison between the predicted and the tiveness coefficient of 3.3 according to the model by Lam and Teng
experimental results for the tested specimens. The strength equa- [38]. This model was generated based on an interpretation of the
tions of ACI 440.2R-08 [32] and Wu and Zhou [37] overestimate the existing test data of CFRP and GFRP confined concrete. The consid-
strength increase for both 2-layer and 4-layer FFRP-PC and FFRP- ered G/CFRP materials had tensile strengths from 363 to 4400 MPa
CFRC remarkably. The ACI model adopted the confinement effec- and tensile moduli from 19.9 to 629.6 GPa, which are much larger
Fig. 8. Typical failure of FFRP-PC (a) and FFRP-CFRC (b).

7. 1124 L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127
Table 6
Comparison 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 54.6
CAN/CSA S6-06 bridge code [33] 39.9 7.8 63.2 17.7 42.4 9.3 65.6 16.7
Youssef et al. [34] 37.3 0.8 64.7 20.5 39.5 1.8 66.2 17.8
Kono et al. [35] 36.3 À1.9 53.4 À0.6 39.6 2.1 58.4 3.9
Lam and Teng [36] 39.9 7.8 63.2 17.7 42.4 9.3 65.6 16.7
Wu and Zhou [37] 47.1 27.3 69.8 30.0 46.7 20.4 71.9 27.9
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 of
Load (kN)
concrete. They considered material parameter m depends on the
plain concrete strength and proposed a closed form relation for
m denoting also plain concrete strength dependence, for concrete
strengths between 18 and 80 MPa.
CAN/CSA S6-06 Bridge code [33], Youssef et al. [34] and Lam
and Teng [36] predict the strength gain for 2-layer FFRP-PC and
FFRP-CFRC well but slightly overestimate the strength for 4-layer
FFRP-PC and FFRP-CFRC with a difference around 20%. Both CAN/
CSA S6-06 code and Lam and Teng [36] used the confinement effec- Mid-span deflection (mm)
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 the Fig. 10. Load–deflection behaviour of PC and FFRP-PC.
test results of FFRP tube encased concrete.
The strength model by Kono et al. [35] predicts the ultimate
compressive strength accurately for both 2-layer and 4-layer
FFRP-PC and FFRP-CFRC specimens with the differences all below
5%. Kono et al. related the confined strength to the confinement
pressure times the plain concrete strength linearly.
Load (kN)
5.2. Third-point bending test
Another objective of this study is to evaluate the feasibility of
the 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 inclusion
on the peak load, maximum deflection, failure modes and bond
Mid-span deflection (mm)
behaviour of the composite columns were evaluated. The neutral
axis depths of the composite columns have been determined based Fig. 11. Load–deflection behaviour of CFRC and FFRP-CFRC.
on the distribution of the measured strains.
2-layer and 4-layer FFRP-CFRC specimens. It is clear that the PC
5.2.1. Effect of FFRP tube on peak load columns have negligible lateral load carrying capacity and mid-
Fig. 10 shows the load–deflection curves for PC, 2-layer and 4- span deflection as a result of un-reinforcement. In the case of con-
layer FFRP-PC specimens and Fig. 11 shows the curves for CFRC, fined PC, the 2-layer FFRP-PC experienced 268% and 1360%, and the
Table 7
Average test results of long cylindrical specimens under flexure.
Specimen type Peak Increase due Increase due Max. Increase due Increase due Ultimate moment Slip (mm)
Load (kN) to tube (%) to coir (%) deflection (mm) to tube (%) to coir (%) (kN mm)
PC 7.4 – – 0.5 – – 555 –
2L-FFRP-PC 27.2 268a – 8.4 1580a – 2040 0.6
4L-FFRP-PC 78.9 1066a – 14.3 2760a – 5918 1.1
CFRC 10.1 – 36.5b 1.2 – 140b 758 –
2L-FFRP-CFRC 29.7 267a 9.2b 9.4 683a 11.9b 2228 0.4
4L-FFRP-CFRC 84.7 946a 7.4b 16.8 1300a 17.5b 6353 1.4
a
Indicates the increase due to tube confinement when comparing with unconfined PC or CFRC.
b
Indicates the increase due to coir fibre inclusion when comparing with the corresponding unconfined PC or confined PC specimens with the same tube thickness.

8. L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127 1125
4-layer FFRP-PC experienced 1066% and 2760% enhancement in tion and failure mode were attributed to the result of coir fibre
ultimate load and deflection, respectively, compared with the bridging effect. The coir fibres bridged the macro-cracks of the con-
unconfined PC specimen. In comparison with the unconfined CFRC, crete and provided an effective secondary reinforcement for crack
the increase in load and deflection of 2-layer FFRP-CFRC are 267% control. The fibres also bridged the adjacent surfaces of existing
and 683%, and are 946% and 1300% of 4-layer FFRP-CFRC, respec- micro-crack, impeded crack development and limited crack propa-
tively. This data indicated that FFRP tube confinement enhanced gation by reducing the crack tip opening displacement. In the case
the load carrying capacity and deflection of both PC and CFRC col- of confined CFRC, the increase in peak load and deflection of 2-
umns remarkably. In flexure, the FFRP tube acted as reinforcement layer and 4-layer FFRP-CFRC are 9.2% and 11.9%, and 7.4% and
of the concrete core and the concrete core provided the internal 17.5% respectively when comparing to the corresponding FFRP-
resistance force in the compression zone and increased the stiff- PC specimens. Therefore, coir fibre inclusion increased the lateral
ness of the composite structure. load carrying capacity and the maximum deflection of the compos-
The enhancement in load and deflection of the FFRP-PC and ite columns as flexural structural members. Further, it should be
FFRP-CFRC specimens also increased with an increase in tube pointed out that there is a distinct post-peak hardening of the
thickness. From 2-layer to 4-layer FFRP-PC, the increase in load load–deflection of the FFRP-CFRC specimens. Compared with the
and deflection are 190.1% (from 27.2 to 78.9 kN) and 72.3% (from sudden failure of FFRP-PC (Fig. 10), the addition of coir fibre mod-
8.3 to 14.3 mm), respectively. For the CFRC, the increase in load ified the failure pattern to be ductile, as given in Fig. 11. More dis-
and deflection from 2-layer to 4-layer FFRP confinement are cussion will be given in the following section.
185.2% (from 29.7 to 84.7 kN) and 78.7% (from 9.4 to 16.8 mm),
respectively. 5.2.3. Failure modes in flexure
Fig. 10 also displays that the load–deflection responses of 2- Failure modes of FFRP-PC and FFRP-CFRC specimens are dis-
layer and 4-layer FFRP-PC are similar, which are dominated by played in Fig. 12. In flexure, the failure of all the FFRP-PC and
the strength and stiffness of the FFRP composite material. The FFRP-CFRC initiated by the tensile rupture of the FFRP tube in the
curves are approximately linear at the beginning of the deflection zone between the two concentrated loads (largest bending mo-
and then become nonlinear until failure as that of the typical ten- ment appears in this zone), as displayed in Fig. 12a and b. For all
sile stress–strain curves of FFRP composites given in Fig. 1. When the FFRP-PC and FFRP-CFRC specimens, in flexure, each specimen
exceeded the maximum load, the curves stop without hardening, only had one crack on the surface of the FFRP tube. The crack began
which implies a brittle failure of the composite column since both at the bottom section of the tube and progressed towards the
PC and FFRP are brittle materials. upper compression zone resulting in the development of the major
crack. The crack was almost perpendicular to the axis of the tube.
5.2.2. Effect of coir fibre on ductility In the case of FFRP-PC, the major crack went through the entire
Compared with PC specimen, the column with coir fibre rein- tube and the composite member was sudden broken into two
forcement had a larger ultimate load and deflection with an in- halves (Fig. 12a). This is in exact accordance with the load–deflec-
crease of 36.5% and 140%, respectively. In comparison with the tion response of the FFRP-PC column. However, for confined CFRC,
brittle response of PC (Fig. 10), the post-peak response of CFRC the major crack terminated at the compression zone of the com-
exhibited a ductile manner (Fig. 11). The difference in load, deflec- posite column (Fig. 12b). After the test, the outer FFRP tube was re-
Fig. 12. Typical failure modes: (a) 4-layer FFRP-PC, (b) 4-layer FFRP-CFRC, (c) CFRC core and (d) PC core. L denotes the span of the column.

9. 1126 L. Yan, N. Chouw / Construction and Building Materials 40 (2013) 1118–1127
moved to examine the failure patterns of the concrete core. For PC attempt will be considered to increase tube and concrete inter-
core (Fig. 12d), it was observed that there were large amounts of facial bond, i.e. along the longitudinal axis of the flax FRP tube,
vertical cracks and diagonal cracks along the two halves of the con- the embedment of several small flax FRP rings onto the inner
crete. The vertical cracks were located in the constant bending mo- surface of the FFRP tube with the help of epoxy.
ment zone and were thought to be the result of pure bending. The
diagonal cracks in the shear span were pointed to the two load In general, the feasibility of natural flax and coir fibres as con-
points due to the shear-flexure forces. Regarding to CFRC, the core struction building materials has been evaluated in this study. The
had a major crack with some small cracks in the zone between the test results indicate that the proposed FFRP-CFRC has the potential
two concentrated loads (Fig. 12c). No diagonal cracks in the shear to be axial and flexural structural members. Natural fibre rein-
span were observed. Obviously, the coir fibres bridged the adjacent forced polymer composites as concrete confinement can increase
surfaces of the major crack. Therefore, the comparison in failure the compressive and flexural properties of concrete. Coir fibre as
modes of PC and CFRC cores gives credence to the statement that reinforcement of concrete can modify the failure pattern of FFRP
coir fibre bridging dominated the post-peak ductile response of tube encased concrete.
FFRP-CFRC column under flexure in Fig. 11.
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