Nonlinear flexural behaviour of flax FRP double tube confined coconut fibre reinforced concrete

    Nonlinear flexural behaviour of flax FRP double tube confined coconut fibre reinforced concrete Jiaxin Chen, Nawawi Chouw PII: DOI: Reference:

S0264-1275(15)30925-4 doi: 10.1016/j.matdes.2015.12.069 JMADE 1093

To appear in: Received date: Revised date: Accepted date:

16 July 2015 6 December 2015 14 December 2015

Please cite this article as: Jiaxin Chen, Nawawi Chouw, Nonlinear flexural behaviour of flax FRP double tube confined coconut fibre reinforced concrete, (2015), doi: 10.1016/j.matdes.2015.12.069

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Jiaxin Chen*, Nawawi Chouw

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Nonlinear flexural behaviour of flax FRP double tube confined coconut fibre reinforced concrete

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Department of Civil and Environment Engineering, the University of Auckland, Auckland, New Zealand

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*Corresponding author. Address: Department of Civil and Environmental Engineering, the University of Auckland, Auckland Mail Centre, Private Bag 92019, Auckland 1142, Tel.: +64 22 1626134; fax: +64 93737462.

Abstract:

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E-mail address: [email protected] (J. Chen).

The flexural performance of coconut fibre reinforced concrete composites, confined with flax fibre reinforced polymer tubes was investigated. Six 520 mm long cylindrical specimens were tested under four-point bending. Two kinds of specimens were considered. The single tube type consists of coconut fibre reinforced concrete confined by a single, outer, flax fibre reinforced polymer tube, whereas the double tube type involves both the outer polymer tube, and a polymer tube running along the centre line of the concrete cylinder. The ultimate load, flexural deformation, and outer tube strains in the longitudinal and hoop directions were experimentally measured. Additionally, the cracking moments and neutral axis depths of both the double tube and single tube confined specimens, were compared. It was found that the double tube specimens possessed excellent flexural characteristics, particularly in comparison to the single tube specimens.

ACCEPTED MANUSCRIPT Keywords: Flax fibre, Fibre reinforced polymer, Coconut fibre, Fibre reinforced concrete, Double flax fibre reinforced polymer tube confinement

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1. INTRODUCTION

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In recent years, research and engineering interest has been shifting from synthetic fibre-reinforced

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polymeric materials to natural fibre-reinforced polymeric materials. Synthetic materials such as aramid, carbon and glass fibre reinforced plastics, have now been tested and developed to the stage where they

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can now be used with confidence in the aerospace, automotive and construction industries. However, because these materials consist of glass/carbon fibres, they have the drawbacks of being high-cost, non-

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renewable with a low potential for recycling, and are non-biodegradable. In contrast, natural fibres have excellent potential as construction and building materials, and can be used to promote the development of

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sustainable construction. Research into the mechanical properties and physical performance of various

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concrete composite materials have been carried out. These studies include the use of coconut outshell

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[1,2], flax [3-6], sisal, banana [7,8], sugar cane bagasse, bamboo, and jute fibres [9-13]. The results have been mainly encouraging and it has been shown that natural fibre reinforced polymer has a number of useful attributes, i.e. improved tensile strengths and flexural modulus. In the case of natural fibre reinforced concrete post cracking resistance, energy absorbing capability, and fatigue strengths can be enhanced over that associate with the plain concrete. The natural lignocellulosic fibres allow for a concrete mix that is lightweight, environmentally friendly, and relatively cost effective. This is especially advantageous in many developing countries, where such natural fibres are often available in abundance. More recently, Yan and Chouw [14,15] proposed coconut fibre reinforced concrete (CFRC) structure confined by a flax fibre reinforced polymer (FFRP) tube. This FFRP-CFRC composite structural member combines the advantages of both FFRP and CFRC composites. Flax fabric is used as the reinforcement of the FFRP tube, and confines the concrete, while coir in the cementitious matrix increases the fracture resistant properties of CFRC. Yan and Chouw’s studies revealed a number of features of FFRP-CFRC

ACCEPTED MANUSCRIPT composites. With regard to flexural behaviour, FFRP tubes have the function of providing tensile and shear strength. The stiffness of the FFRP tube strongly influences the flexural behaviour of FFRP tube

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confined CFRC [16]. However, their work focused only on the confinement effects of the outer FFRP

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tube.

In recent decades, some researchers and engineers have realised that dual confined concrete structures

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have outstanding performance in the bending test. The flexural behaviour of concrete-filled double skin

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steel tube (CFDST) members have been studied by several researchers, including Tao and Han [17,18], Idriss and Ozbakkaloglu [19], Uenaka and Kitoh [20], Fam et al. [21], Li et al. [22], Zhao and Grzebieta

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[23]. The inner and outer steel tubes can be circular hollow sections, square hollow sections or rectangular hollow sections. Several possible combinations of CFDST members were tested. All results showed that

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double skin structures display very good flexural characteristics, especially in regard to their strength and

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ductility. This is because the compressive concrete is confined by the steel tube, and these tubes are capable of simultaneously enhancing the compressive strength, while also providing ductile behaviour. It

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is noted that these studies of double sandwich structural composites, focused only on the steel confinement. Today, however, steel is typically expensive, and in modern day construction, an average of 200 kg of steel reinforcement (about 8% of the overall weight) is used for each cubic meter of concrete poured [22]. Furthermore, it is only a question of time before steel will corrode. Therefore, there are obvious advantages in promoting the use of cementitious building materials reinforced with natural fibres. Research in this field has promise of contributing significantly to more sustainable construction. In this work, the influence of an inner FFRP tube on the flexural response of a new double FFRP tube reinforced CFRC (DFFRP-CFRC) long cylindrical composite beam is investigated for the first time. This composite consists of double concentric FFRP tubes of the same length. The double FFRP tubes are infilled by CFRC. No conventional reinforcement was used in any of the beams.

ACCEPTED MANUSCRIPT 2. Experimental work 2.1. Test specimens and materials

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Table 1 gives the test matrix of the specimens. It consists of six long cylindrical specimens with a length of 520 mm and a CFRC core diameter of 100 mm. Three of the specimens are of single FFRP tube

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confined coconut fibre reinforced concrete (FFRP-CFRC), while the other three are double FFRP tubes

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confined CFRC (DFFRP-CFRC) composite specimens (see Fig. 3). For both FFRP tube, single confined CFRC and double confined CFRC specimens, the outer tube consisted of four layers of flax fabric. The

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average thickness of the outer tube is 5.3 mm. The inner tube of DFFRP-CFRC was designed using three

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layers of flax fabric, and this tube had an average wall thickness of 3.05 mm. The four-point bending test

Table 1

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Specimen configuration.

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was performed until failure, according to ASTM C78 [25].

The material of the FFRP composites used the commercial bidirectional woven flax fabric (500 g/m2), which has a plain woven structure with a count of 7.4 threads/cm in both the warp and weft directions [4]. The flax fabric was obtained from Libeco, Belgium. The epoxy used was the SP High Modulus Ampreg 22 resin and slow hardener; the mix ratio is 100:26, respectively. Table 2 lists the mechanical properties of flax fibre and epoxy resin. More details of the physical properties of flax, epoxy and the mechanical properties of FFRP composite are provided by Yan [3].

Table 2 Mechanical properties of flax fibre and epoxy resin [16].

ACCEPTED MANUSCRIPT The FFRP tubes were fabricated using the hand lay-up process. This was carried out at the Centre of Advanced Composites Materials (CACM) at the University of Auckland. For the details of the fabrication

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process of the FFRP tubes, the reader is referred to a previous study [16]. An additional 20 mm length of buffer was provided to the designed cylinder height, with 10 mm at each end in order to ensure that each

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FFRP tube was of 200 mm length with two cleanly cut ends. The fabric fibre orientation was at 90° from

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the axial direction of the aluminium tube.

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For CFRC, Portland cement, gravel, natural sand, and clean water were used for each specimen. The mix ratio by weight was 1: 0.58: 3.72: 2.37 for cement: water: gravel: and sand, respectively. This mix design

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follows the ACI Standard 211 [26], and is expected to achieve a 28-day compressive strength of 25 MPa. The coir (extracted from the outer shell of coconuts) was obtained from Bali, Indonesia. The fibres were

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loosened and separated from one another, and the coir dust removed. The fibres were then manually

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straightened, combed, and cut into the required lengths (50 mm). The coconut fibres are then added into the concrete during mixing. The mass of the fibres equates to 1% of the overall mass of the cement. The

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clustering of coconut fibres during the whole casting process was avoided. The coconut fibres were evenly distributed throughout the mix by introducing them to the mix gradually, and also by occasionally stopping the concrete mixer and manually separating the fibres. After cutting clean both ends of the FFRP tube, one end of the tube was sealed by silicon on a wooden plate as a means of water-proof treatment. A wooden clamp was developed to ensure that the inner FFRP tube is aligned with the longitudinal centre-line of the specimen (Fig. 1(b)). The concrete was then poured, compacted and cured in a standard curing water tank for 28 days. Prior to the experiments, both end surfaces of all FFRP-CFRC cylinders were prepared with plaster to provide both ends with a smooth and uniform surface.

Fig. 1 (a) Casting FFRP-CFRC and (b) DFFRP-CFRC specimens.

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2.2. Instrumentation and test set-up

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For each of the four-point bending tests on the cylindrical specimens, six strain gauges and three LVDTs

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were used (Figs. 2 and 3). Three strain gauges (i.e. gauges H1, H2 and H3) were mounted at the mid-span of each specimen, and aligned along the hoop direction in order to monitor the lateral strain. Three other

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strain gauges (i.e. gauges A1, A2 and A3) were placed in the longitudinal direction of the tube in order to

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measure the axial strains. One LVDT was placed under the lower boundary of the specimen at mid-span, in order to measure the deflection, as shown in Fig. 4. Two LVDTs were installed at the end of the

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specimen in order to measure the relative displacement between the outer FFRP tube and the adjacent concrete (see Figs. 2 and 3). The four-point bending test was conducted using an Instron testing machine

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in accordance with ASTM C78 [25].

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Fig.2 Schematic view of four-point bending test set-up.

Fig. 3 Cross-section view of four-point bending test set-up.

Fig. 4 Set-up of four-point bending test.

3. Results and discussion

3.1. Test results

The test results (averaged values obtained from three identical specimens) for two different configurations of cylindrical specimens under flexural loading are listed in Table 3. For both

ACCEPTED MANUSCRIPT configurations of confined concrete specimens, the ability to carry the lateral loading was confirmed. The peak loads of FFRP-CFRC and DFFRP-CFRC specimens are 68.09 kN and 77.71 kN, respectively.

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The CFRC core confined by double FFRP tubes not only increased the load carrying capacity by 14% in

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comparison with single FFRP tube confinement, but also exhibited remarkable tensile strength, as well as ductility. In comparison with FFRP-CFRC structures, the maximum deflection of DFFRP-CFRC

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structures was slightly reduced by 0.7 mm (3.5%). At the same time, the weight of DFFRP-CFRC

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structures compared with that of FFRP-CFRC structures was reduced by 8.55% (this being due to the inner void). In comparison with a single FFRP tube completely filled with CFRC, the inner void within

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the DFFRP-CFRC core reduces the mass of the composite. Even though less concrete is used, the inner tube is able to compensate for the contribution of the concrete at the centre part of the FFRP-CFRC

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composite, and also able to enhance the bending stiffness of the DFRC-CFRC composite. Together with

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coir in concrete, the FFRP confinement contributes to a delay in the development of cracks in the beam. All test results indicate that compared with FFRP-CFRC, DFFRP-CFRC composites allow larger

Table 3

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deflections with enhanced load bearing capacity.

Test results of specimens under flexure.

3.2. Effect on load–deflection response The relationship between load and mid-span deflection of the specimens are displayed in Fig. 5. It is clear that the coir reinforced concrete confined by both single FFRP tube and double tubes exhibits a nonlinear load-deflection relationship before reaching the peak load. In a previous study by Yan et al. [16], the load-deflection relationship of plain concrete (PC) and CFRC (Fig. 6) presented essentially linear behaviour prior to brittle failure. Similar to PC, FFRP-CFRC and DFFRP-CFRC specimens also

ACCEPTED MANUSCRIPT demonstrate brittle characteristics just beyond the ultimate deflection, with sudden failure occurring (this is often accompanied by a loud noise). The results clearly show that a high load bearing capacity and

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large deflection of the confined concrete specimen is caused by the strength and bending stiffness of the

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FFRP tube and bridging effects of coconut fibres.

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Fig. 5 A comparison of load-deflection relationship. (a) Influence of inner tube, between (b) FFRP-CFRC and (c) DFFRP-CFRC specimens.

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Another interesting point should be noted that all the load-deflection curves of single tube confined concrete specimens (FFRP-PC, FFRP-CFRC) have sudden declines in load just prior to the ultimate load bearing capacity as shown in Figs. 5(a) and (b) and 6. The previous conclusion made by Yan [27] can be

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confirmed, i.e. during loading, each decline of load bearing capacity reflects slippage at the interface

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between the FFRP tube and CFRC core. However, the load-deflection relationship of DFFRP-CFRC

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specimens (see Figs. 5(a) and (c)) shows a smoothly increasing line, with no slippage happening during the bending test. This is because the bond between both inner and outer FFRP tubes and CFRC core can transfer greater shear forces at the DFFRP-CFRC interface. Hence, if slippage can be prevented, an uppermost development of load bearing capacity can be achieved. This result indicates that DFFRPCFRC specimens could have significant capability to impede slippages due to the stronger total bond between inner and outer FFRP tubes and the CFRC core.

Fig. 6 Load–deflection relationship for PC, CFRC, FFRP-PC and FFRP-CFRC [16].

3.3. Effect of double tube confinement on the load-axial strain relationship

ACCEPTED MANUSCRIPT The longitudinal strains of the FFRP-CFRC and DFFPR-CFRC specimens were obtained from three strain gauges (see A1, A2 and A3 in Fig. 2). In Fig. 7 the longitudinal strain of DFFRP-CFRC specimens

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at the three locations shows larger maximum values than those of the FFRP-CFRC specimens. Up to the point of failure of the FFRP-CFRC specimens, in the compression zone and the middle of the beam, the

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strain development of both configurations of specimens matches well. With increasing load, the DFFRP-

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CFRC strain continues to increase, because the double confined CFRC composite can sustain larger loads (see Table 3). However, in the tensile zone, the strain at the bottom mid-span (measured by A3) of the

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DFFRP-CFRC specimen shows a delay in its response, in comparison with that of the FFRP-CFRC

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specimen. This discrepancy in load-strain development is presumably due to the fact that the inner tube not only provides support to the top part of the specimen, but also provides additional tensile strength to the bottom part of the beam. While the upper part of the DFFRP-CFRC specimen performs as well as the

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FFRP-CFRC specimen, when compared with that of the FFRP-CFRC specimens, damage at the lower part of the DFFRP-CFRC specimen is less significant due to reinforcement of the inner tube. This

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performance indicates that the DFFRP-CFRC specimen has a greater load bearing capacity than that of a comparable FFRP-CFRC specimen.

Fig. 7 Relationship between load and longitudinal strain at mid-span of the FFRP outer tube.

4. Cracking moment of FFRP-CFRC and DFFRP-CFRC beams The cracking moment Mcr is the moment when the actual tension exceeds the tensile strength that leads to crack development. The cracking moment capacity of FFRP-CFRC and DFFRP-CFRC beams can be estimated using the following equation, which is derived from elastic theory and the cross-section properties.

ACCEPTED MANUSCRIPT Mcr =

(1)

where fr is the modulus of rupture of concrete (cracking strength), Ig is moment of inertia of cross-section,

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and yt is the distance from the centre of the gravity of the beam to the extreme fibre of the tension side.

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The moment of inertia, for DFFRP-CFRC beams, is calculated using the following equations：

Ig = Iconcrete + ηo-tube Io-tube + ηi-tube Ii-tube

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–

(3)

and Ii-tube=

(4)

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Io-tube =

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Iconcrete =

(2)

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and ηi-tube = Ei-tube/Econcete; where Young’s modulus of concrete can be determined using Econcrete

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tube/Econcete

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where ηo-tube and ηi-tube are respectively the modular ratio of outer and inner FFRP tube, ηo-tube = Eo-

(MPa) [27] and the Young’s modulus Ei-tube and Eo-tube of FFRP tube is given in Table 4. is the specified compressive strength at 28 days. Dic is the outer concrete diameter (100 mm), Dit is

the inner diameter of the inner-tube, and Ti and To are the thickness of the inner and outer FFRP tube, respectively (See Table 1). Fig. 8 gives a sketch of DFFRP-CFRC’s cross-section to clearly define the above parameters.

Table 4. Young’s modulus of FFRP tube.

Fig. 8 Schematic cross-section view of DFFRP-CFRC.

ACCEPTED MANUSCRIPT In the previous study by Yan [27] on the flexural behaviour of FFRP-CFRC beams, the cracking strength of concrete was predicted using Eq. (5) (where k =0.62 and 0.4):

co λ = 1.0

(5)

for normal-weight concrete

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fr = k λ

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According to the study by Fam [28], the flexural behaviour of plain concrete filled glass FRP tube

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(CFFT) can be predicted well when k =1.

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By comparing the predicted cracking moment of the four layer FFRP-CFRC specimen with that of conventional steel reinforced concrete beams and CFFTs, performed by Fam [28], Yan and Chouw [27]

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confirmed that the cracking strength of FFRP-CFRC beams is larger than that of steel reinforced concrete beams, but lower than that of glass fibre reinforced concrete beams.

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However, because the DFFRP-CFRC beam has a void; it is not easy to find a suitable coefficient k for

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calculating its modulus of rupture. Since FFRP-CFRC and DFFRP-CFRC beams were constructed in a

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similar way, the following formula, adopted from ASTM C78-02 [26], is used to calculate the cracking strength of FFRP-CFRC and DFFRP-CFRC beams under a four-point bending test.

fr =

(6)

where P = Peak load under flexural, N; L = Length of the span between two support point, mm; b = Width of the beam, mm; d = Depth of the beam, mm. The cracking moments Mcr of FFRP-CFRC and DFFRP-CFRC beams are compared in Table 5. This result is not the actual bending moment, because the Young’s modulus of the three layer inner tube is an assumed value. It is only used to compare these two configurations of FFRP confined composite beams

ACCEPTED MANUSCRIPT and to provide a simple theoretical estimate. Table 5 indicates that due to the inner FFRP tube, the

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DFFRP-CFRC beams have larger cracking strengths than those of the FFRP-CFRC.

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5. Neutral axis depth

The approach proposed by Yan and Chouw [27] is used to determine the depth of the neutral axes of the

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FFRP-CFRC and DFRP-CFRC beams. The axial strains measured at the extreme compression and tension zone of the beams (obtained from the strain gauges A1 and A3, respectively) are used to plot

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strain distribution along the depth of the beams. The axial strains at six levels of the ultimate load are considered, i.e. 0%, 20%, 40%, 60%, 80% and 100%. Fig. 9 shows that the depth of the neutral axes of 4-

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layer FFRP-CFRC and DFFRP-CFRC beams are 0.4 D and 0.43 D, respectively. This result of a deeper

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capacity under a bending load.

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neutral axis confirms a stronger contribution of the inner tube in the tensile zone to the load bearing

Fig. 9 Neutral axis depths for the four layer (a) FFRP-CFRC and (b) DFFRP-CFRC beams.

6. Conclusions

The research addresses for the first time the nonlinear flexural behaviour of flax fibre and coconut fibre reinforced polymer-concrete composite beams. The influence of double tube confinement on the results obtained from four-point bending tests of DFFRP-CFRC beams is discussed. Based on the test results and the comparisons with theoretical predictions, the following conclusions can be drawn: (1) In comparison with the unconfined CFRC beam both DFFRP-CFRC and FFRP-CFRC beams possess a significant ductility. The confinement of concrete by the outer FFRP tube provides additional shear resistance.

ACCEPTED MANUSCRIPT (2) The flexural behaviour of DFFRP-CFRC beams, in terms of the flexural stiffness, cracking strength, and the ultimate load bearing capacity, is improved due to an additional longitudinal reinforcement

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provided by the inner FFRP tube.

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(3) The inner FFRP tube is observed to eliminate the slippage between the CFRC core and FFRP tubes

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and prevents sudden declines in the flexural capacity.

(4) Double FFRP tube confined CFRC members reduced the weight of the beam without adverse effects

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on the bending stiffness.

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Acknowledgements

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The authors would like to thank the University of Auckland for supporting this research under the FRDF Award 3702507. The authors would also like to thank the reviewers for their valuable comments, which helped in improving the quality of the article.

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Figure captions: Fig. 1 (a) Casting FFRP-CFRC and (b) DFFRP-CFRC specimens. Fig. 2 Schematic view of four-point bending test set-up. Fig. 3 Cross-section view of four-point bending test set-up. Fig. 4 Set-up of four-point bending test.

ACCEPTED MANUSCRIPT Fig. 5 A comparison of load-deflection relationship. (a) Influence of inner tube, between (b) FFRP-CFRC and (c) DFFRP-CFRC specimens. Fig. 6 Load–deflection relationship for PC, CFRC, FFRP-PC and FFRP-CFRC [16].

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Fig. 7 Relationship between load and longitudinal strain at mid-span of the FFRP outer tube.

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Fig. 8 Schematic cross-section view of DFFRP-CFRC.

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Fig. 9 Neutral axis depths for the four layer (a) FFRP-CFRC and (b) DFFRP-CFRC beams.

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Figures:

Fig. 1 (a) Casting FFRP-CFRC and (b) DFFRP-CFRC specimens.

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Fig. 2 Schematic view of four-point bending test set-up.

Fig. 3 Cross-section view of four-point bending test set-up.

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Fig. 4 Set-up of four-point bending test.

Fig. 5 A comparison of load-deflection relationship. (a) Influence of inner tube, between (b) FFRP-CFRC and (c) DFFRP-CFRC specimens.

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Fig. 6 Load–deflection relationship for PC, CFRC, FFRP-PC and FFRP-CFRC [16].

Fig. 7 Relationship between load and longitudinal strain at mid-span of the FFRP outer tube.

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Fig. 8 Schematic cross-section view of DFFRP-CFRC.

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Fig. 9 Neutral axis depths for the four layer (a) FFRP-CFRC and (b) DFFRP-CFRC beams.

ACCEPTED MANUSCRIPT Table Captions: Table 1. Specimen configuration.

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Table 2. Mechanical properties of flax fibre and epoxy [16]. Table 3. Test results of specimens under flexure.

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Table 4. Young’s modulus of FFRP tube.

Table 1 Specimen configuration. No. of specimens

No. of fabric layers * Li and Lo

FFRP-CFRC DFFRP-CFRC

3 3

-- ; 4 3;4

Thickness (mm) * Ti and To

Fibre volume fraction (%) inner and outer tube

-- ; 5.3 3.05 ; 5.3

-- ; 54 52.5; 54

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Configuration

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SC

Table 5. Cracking moments of four layer FFRP-CFRC and DFFRP-CFRC.

TE

D

* Li and Lo indicates the number of fabric layers of inner tube and outer tube, respectively; * Ti and To corresponds to the thickness of inner tube and outer tube.

Material Flax fibre Epoxy

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Table 2 Mechanical properties of flax fibre and epoxy [16]. Diameter (mm)

Density (g/cm3)

0.708 N/A

1.43 1.09

Elastic modulus (GPa) 16.4 3.6

Tensile strength (MPa) 3.2 4.5

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Table 3 Test results of specimens under flexure.

S1

65.5

S2

64.5

S3

74.3

22.03

S4

73.08

21.19

S5

80.86

S6

79.17

77.71

17.08

19.99

19.9

19.2

16.43

D

Table 4 Young’s modulus of FFRP tube. Number of fabric layers Young’s modulus Etube (GPa) 2 8.7 [15] * 9.1 9.5 [15]

TE

3 4

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* The Etube of 3-layer FFRP tube is the assumption average value from two and four layers modulus.

Table 5 Cracking moments of four layer FFRP-CFRC and DFFRP-CFRC. Mcr (kN mm) Eqs. DFFRP-CFRC FFRP-CFRC (1) and (6)

1550

1851

Weight (kg)

Average (kg)

Wreduction (%)

12.36

--

11.3

8.55

12.38

RI

68.09

Average (mm)

PT

Max deflection (mm) 20.6

MA

DFFRP-CFRC

Average (kN)

NU

FFRP-CFRC

Peak load (kN)

SC

Specimen Configuration Number

12.42 12.28 11.37 11.28 11.26

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RI

PT

Graphical Abstract

Four point bending test

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TE

D

DFFRP-CFRC beam

Ultimate load

flexural deformation

strains

ACCEPTED MANUSCRIPT Highlights: Double flax fibre reinforced polymer confined coir concrete composite was studied.

PT

An additional inner confinement increases lateral load bearing capacity.

RI

Inner confinement eliminates slippages at polymer-concrete interface.

Centre concrete void does not adversely affect the composite flexural strength.

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TE

D

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SC

Compared to single confinement double confined composite has higher cracking strength