Kenaf Fiber Reinforced Polymer Composites forStrengthening RC BeamsN. A. K. Hafizah, M. A. R.Bhutta M. Y.Jamaludi, n, M. H. Warid

Journal of Advanced Concrete Technology, volume ( ), pp.12 2014 167-177

M. Ismail, M. S. Rahman, I. Yunus and M. Azman,

Flexural Behaviour of Reinforced Lightweight Concrete Beams Made with Oil Palm Shell (OPS)Md. Abdul Mannan,Delsye C.Teo John V. Kuriam,Journal of Advanced Concrete Technology, volume ( ), pp.4 2006 459-468

Behavior of Beams Strengthened with Steel Fiber RC OverlaysMohamed M. ZiaraJournal of Advanced Concrete Technology, volume ( ), pp.7 2009 111-121

Improving the Engineering and Fluid Transport Properties of Ultra-High Strength Concrete UtilizingUltrafine Palm Oil Fuel AshAhmad N. Mohammed, Megat AamiMegat Johari Abdullah M. Zeyad, , Bassam A. Tayeh, , Moruf O. YusufJournal of Advanced Concrete Technology, volume ( ), pp.12 2014 127-137

Journal of Advanced Concrete Technology Vol. 12, 167-177, June 2014 / Copyright © 2014 Japan Concrete Institute  167 

 

Scientific paper

Kenaf Fiber Reinforced Polymer Composites for Strengthening RC Beams N. A. K. Hafizah1, M. A. R. Bhutta2, M. Y. Jamaludin2, M. H. Warid3, M. Ismail3, M. S. Rahman2, I. Yunus4 and M. Azman5

Received 7 September 2013, accepted 23 May 2014 doi:10.3151/jact.12.167

Abstract This paper presents the findings of an experimental study to investigate the structural behavior of reinforced concrete (RC) beams externally strengthened with various types of kenaf fiber reinforced polymer composite laminates with 50% fiber volume content. The experimental parameters including flexural strength, stress-strain relationship, deflection, tensile stress and young modulus of beams strengthened by different kenaf fiber reinforced polymer composites are dis-cussed. In order to test the flexural strength of the RC beams, a total of eight beams divided into two series were used. In each series, every two of the beams were strengthened with kenaf/epoxy, kenaf/polyester, and kenaf/vinyl ester com-posite laminates respectively. The remaining two were kept as control specimens. The experimental moment capacity of each beam was calculated using tensile strength and young modulus of the composites. Comparisons were made on the load-deflection, strain behavior and failure mode. Moreover, theoretical moment capacity of each beam was calculated using conventional formulation to verify experimental results. The research findings indicated that all strengthened beams improved structural performance where the maximum flexural strength increase by 40% and maximum deflec-tion reduced by 24%.

1. Introduction

There are many existing reinforced concrete structures that need to be strengthened due to a change in function, increase in loading and construction error. The strength-ening process should preferably facilitate ease in han-dling during erection and is able to minimize disruption to the structure and its occupants. Some conventional materials used as strengthening materials are, for in-stance, timber and steel. The main disadvantage of using timber is its lower strength and that the material can easily decay from insects or fungi attacks. Even though steel has satisfying tensile resistance, it corrodes easily particularly if utilized for outdoor applications. Though such corrosion does not profoundly reduce the lami-

nates’ strength, it destroys the bonding between the strengthening and adhesive materials. Moreover, steel proves to be expensive and heavy in most rehabilitation works, in which case it further complicates the handling process during erection. Other materials that can be used are from synthetic composites materials such as glass fiber reinforced polymer (GFRP). Such materials possess better engineering properties as compared to conventional materials. However, these fiber reinforced polymer (FRP) materials impose high costs during the manufacture process, installation and end-life services (Liew 2008). Moreover, such materials are less envi-ronmental friendly (Joshi et al. 2004) and can possibly cause adverse effects on human health, particularly problems related to skin and respiratory systems (Kar-nani et al. 1997). Additionally, current pressing envi-ronmental issues on global warming (increase of carbon dioxide) have triggered rapid and drastic development of advanced materials like the natural fiber reinforced polymer composites (NFRPC) as sustainable materials for structure strengthening purposes.

The use of NFRPC as strengthening materials has been gaining the interest of many researchers as these materials provide attractive strengthening solutions for existing structures. These materials are superior to other materials in terms of the strength to weight ratio, ease in handling, sustainability and local availability. In Malay-sia, government agencies like the National Kenaf and Tobacco Board are undertaking intensive research and development (R&D) endeavors on natural fibers, pro-foundly the kenaf plant, due to the plant’s potential in replacing local tobacco. The other objective is to pro-mote and develop the kenaf industry to include other

1Ph.D. Student, Department of Structures and Materials, Faculty of Civil Engineering, Universiti Teknologi Malaysia, Johor, Malaysia. 2Associate Professor, Construction Research Centre (UTM CRC), Faculty of Civil Engineering, Universiti Teknologi Malaysia, Johor, Malaysia. E-mail: aamer.bhutta@yahoo.com (A. M. R. Bhutta)3Professor, Construction Research Centre (UTM CRC), Faculty of Civil Engineering, Universiti Teknologi Malaysia, Johor, Malaysia. 4Senior Lecturer, Department of Structures and Materials, Faculty of Civil Engineering, Universiti Teknologi Malaysia, Johor, Malaysia. 5Lecturer, Razak School of Engineering and Advanced Technology, Universiti Teknologi Malaysia, Inter-national Campus, Kuala Lumpur, Malaysia.

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applications. Previous research from Liew (2008), Dweib et al. (2006), Cutter (2008), Mesbah et al. (2004), Chow et al. (1992) and Ngatijo et al. (1990) cited that natural fiber has potential in structural and non-structural applications.

Kenaf is scientifically known as Hibiscus cannabinus L. and is closely related to cotton and jute (Hongqin et al. 2007; Bismarck et al. 2005). This is because the crystalline cellulose content and microfibril angles of the two fibers are fairly similar. Kenaf is cultivated for its fiber and is well suited in Southeast Asia, parts of Africa, and in specific parts of southeast Europe. In-dia, Bangladesh. China has also actively developed this plant and is now one of the largest kenaf producers in the world (Hongqin et al. 2007). Recently, kenaf in Malaysia has been identified as one of the potential natural raw fibers to replace tobacco in manufacturing a multitude of products for the construction, local auto-motive, textile and advanced technology sectors.

Some researchers have previously investigated the use of natural fibers as reinforcement in polymeric strengthening materials. For instance, Liew (2008) and Chew (2009) investigated the potential of unidirectional oil palm fiber polymer composites as strengthening ma-terial. The flexural strength and stiffness of the compos-ite improved significantly when the fiber volume ratio increased. This evidence shows that oil palm fiber polymer composite is a strengthening material suitable for reinforced concrete (RC) beams. The researchers further revealed that oil palm fiber reinforced polymer composites increased the stiffness and ultimate load of ordinary RC beam. Current researches on natural short fiber are not exhaustive, and researches on continuous fibers are considerably lesser. Therefore, this study has been executed to demonstrate that continuous natural fibers are, indeed, a potential reinforcement component in polymeric materials for structural elements as well. The experimental parameters including flexural strength, stress-strain relationship, deflection, tensile strength and young modulus of beams strengthened by different ke-naf fiber reinforced polymer composites are discussed. The experimental moment capacity of each beam has been calculated using tensile strength and young

modulus of the composites. Comparisons are made on the load-deflection, strain behavior and failure mode. Moreover, theoretical moment capacity of each beam has been calculated using conventional formulation to verify experimental results. This paper focuses only on the application of kenaf fiber to strengthening of dete-riorated RC concrete structures and to replace commer-cial fibers, it does not investigate long-term durability.

2. Materials and methods

2.1 Kenaf fiber and resins The kenaf fiber, as shown in Fig. 1, was obtained from the Malaysian Agricultural Research and Development Institute (MARDI), Selangor, Malaysia. Presently, Ke-naf plants in Malaysia are under government control and MARDI is the only representative selling Kenaf fibers. The Kenaf fiber type V36 is only obtainable from MARDI. The fibers were extracted from the bast through a bacteria retting process and the mechanical properties of fiber were presented in Table 1. The maximum length of the customized kenaf fibers is 1.5 m and its composite preparation depends heavily on the mould’s size, which in turn is dependent on the fabrica-tion content of the composite (2.31 g for every 10% of fiber content). The strength of kenaf fiber was garnered using the universal testing machine with a capacity of

Core

Bast

(a) (b)

Fig. 1 Kenaf stem (a) and Kenaf fiber (b).

Table 1 Mechanical properties of kenaf fiber and resins.

Type of resin Tensile strength (MPa)

Young’s modulus (GPa) References

Kenaf fiber 129 12.50 ASTM D3379-75

Epoxy 19.13 5.82 Polyester 14.02 2.61 Vinyl Ester 7.00 1.63

ASTM D3039

Table 2 Specifications of resins.

Type of resin

Density (g/cm3) Blending ratio

Epoxy 1.11 10 resin :6 hardener Polyester 1.12 1% catalyst (ESTEROX MEKP)Vinyl Ester 1.1 1 % catalyst (ESTEROX MEKP)

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2.5 kN in accordance to ASTM D3379-75. The cross head rate in this test was 3 mm/min and the gauge length was fixed at 30 mm.

Three types of resins were used in this research, namely epoxy, polyester, and vinyl ester. Selection of these resins in this study was made due to their easy availability and common use as construction materials. The resins’ properties and specifications are given in Tables 1 and 2, respectively.

2.2 Kenaf composite materials The methods used to produce and test the tensile strength of each kenaf composite material strictly fol-lowed the ASTM D3039 with different types of thermo-set matrices - epoxy, polyester and vinyl ester resin. A total of 5 samples for every resin type were shaped ac-cording to the standard with dimension of 25 mm x 6 mm x 200 mm. All the procedures were designed for hand lay-up system and precautions were taken into account during the fabricating process. Moreover, only longitudinal arrangement of kenaf fibers were of interest and tested in this study, since such unidirectional ar-rangement would be more effective in enhancing the mechanical properties of the reinforced structure. Dif-ferent fiber volume fractions;10%, 20%, 30%, 40% and

50% were examined for their tensile with universal test-ing machine as shown in Fig. 2, and 50% fiber content was found suitable in terms of economy and manage-ability. The polymer composite strain gauge series with 1 meter of prewire was used to obtain the value of strain. Slipped and defected samples were rejected. The afore-mentioned machine and LVDT (linear variable differen-tial transformer) were connected to the data logger to obtain test results from the computerized control sys-tems. The stress-strain curves were then plotted from the results using Microsoft Excel to obtain the ultimate tensile strength and Young’s modulus. 2.3 Manufacturing of beams The RC beam and concrete design were based on BS8110 and the standard concrete mix design was in accordance with the Department of Environmental (DOE), British. The characteristic strength of the con-crete was designed to be 30 MPa and the expected con-crete slump was 30-60 mm. The proportions in concrete materials, i.e., cement, water and aggregates required to meet specific strength, workability, durability, and other requirements are as listed in BS 5328. Deformed rein-forcing (T8: 8mm; T10: 10mm) bars were used. The properties of reinforcement bars are given in Table 3. The concrete mix proportion is given in Table 4. Figure 3 shows the dimensions of beams. The shear resistance was provided by reinforcing bars having a diameter of 6 mm and spacing of 70 mm. Trial mix was carried out prior to the actual testing and involved a total of 18 con-crete cubes with the size of 100 mm x 100 mm x 100 mm prepared and cured for further cube crushing test. This test was specifically conducted to obtain the com-pressive strength of the cured concrete after 7 days, 14 days and 28 days. The actual compressive strength of concrete cube specimens is 31 MPa. The hardened con-crete properties at 28 days are shown in Table 5.

Fig. 2 Tensile strength test.

Table 3 Properties of reinforcing bars.

Bar shapeNominal diameter

(mm)

Nominal area

(mm2)

Yield strength (MPa)

References

Deformed bar (T10) 10 157 539 BS EN

10002-1:2001

Table 4 Concrete mix proportions.

Concrete materials Mix proportions (kg/m3)

Cement 427 Fine aggregate 810

Coarse aggregate 950 Water 213

Table 5 Properties of hardened concrete.

Compressive strength (MPa)

Young’s modulus (GPa)Hardened concrete at

28 days (Grade 30) 35 29.2  

100mm 100mm200mm600mm 600mmP P

Length of composite = 1400mm(ended inside the support)

130mm

100mmX–X cross section

X

X

2T10

2T10

Fig. 3 Beam dimension.

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2.4 Manufacturing of composite strengthening laminates A total of six kenaf fiber composite laminates with size of 100 mm x 6 mm x1400 mm, as shown in Fig. 4, were prepared according to the method used to prepare the composite samples. However, this process is more man-power demanding. Constraint also lies in the period of composites polymerization where precautions have to be seriously taken into consideration. The strengthening laminate was glued with epoxy adhesive onto the soffit of the RC beams after both contact surfaces were roughed down. 2.5 Test on beams The composite beam samples were prepared and tested according to BS (EN) 1504-4:2004. A total of six beams strengthened with composite laminates were tested for flexural under a four-point load system. Two beams were not strengthened to serve as control samples. Be-fore the test was conducted, three concrete strain gauges and two polymer composite strain gauges were glued respectively onto the top and side surfaces of every RC beam and the surface of strengthening composites lami-nates. The strengthening composites laminate was ended inside the support. Each beam’s symmetrical cross section (neutral axis) was taken as the reference line to determine the locations of strain gauges at the side of all beams using the common transformed-section method.

Three LVDTs were used and placed at selected posi-tions in order to measure the deflection of beams. Two LVDTs were placed directly under the applied force and a LVDT was placed at the middle span of the beams. The LVDT, 300 kN of load cell and all strain gauges were connected to the data logger. Subsequently, the four-point bending test was set up by arranging the beam at the correct position. The reading of strain gauge and displacement were recorded every 1 kN increment of load. All samples were tested until the subjected load started to reduce and maximum displacement at the middle span had been reached. The structural failure mode was observed visually and the cracks were marked at every load increment.

2.6 Theoretical and experimental moment ca-pacity

The theoretical moment capacity was calculated con-ventionally according to ACI 440.2R-02 guidelines. Initially, the neutral axis, NA (shown in Fig. 5), was fixed at the centroid of the transformed-section for the strengthened beam. The equation stipulated in the first moment theory derived from Eq. 1 is Eq. 2, while adopted values of compression concrete parameter can be obtained from Eqs. 3 to 5.

AhA

χ ∑=

∑ (1)

( ) ( )[ ][ ]

( ) (

( ) ( ) ( ) (sc sc sc cuc cuc cuc st st st ct ct ct

sc sc cuc cuc st st ct ct

n A h n A h n A h n A h

n A n A n A n Aχ

+ + +=

+ + + (2)

where:

0.9cucA bχ= (3)

0.9 0.33cucbh χ χ= = (4)

2(0.9 )(0.3 ) 0.27cuc cucA h b bχ χ χ= = (5)

Since the width of beam, b was 100 mm, therefore: 2(0.9(100) )(0.3 ) 27cu cuA h χ χ χ= = (6)

All the parameters that can be calculated include the material modula ratio, nsc, ncu, nst, and nct. The modula ratio is calculated against the Young’s modulus of the concrete, where:

A = Area of the materials (mm2) h = Distance between centroid of material’s particular

area and datum (mm) nsc = modula ratio for compression reinforcing bar

against the concrete Young’s modulus ncu = modula ratio for compression concrete against

the concrete Young’s modulus nst = modula ratio for tension reinforcing bar against

the concrete Young’s modulus

Fig. 4 Kenaf fiber composite laminates.

(a)

(b) Fig. 5 Stress diagram (a) Normal beam (b) Strengthened beam.

Hafizah, Bhutta, Jamaludin, Warid, Ismail, Rahman, Yunus and Azman / Journal of Advanced Concrete Technology Vol. 12, 167-177, 2014 171

 

nct = modula ratio for tension composites laminates against the concrete Young’s modulus

Asc, Acuc, Ast, Act are the areas of the materials (mm2) at tension and compression sections. The length of hsc, hcuc, hst, and hct (mm) are depicted in Fig. 6.

After the neutral axis, χ (NA) was found, the moment capacity was determined based on conventional theo-retical formulation of strengthened RC beams recom-mended in ACI 440.2R-02 guidelines. The experimental value was adopted into the theoretical formulation and the theoretical value was compared to experimental value. Several assumptions were considered before us-ing the theoretical formulation. The assumptions are as follows:

i The strain distribution in the height of RC beam is linear.

ii There is perfect bonding between concrete sub-strate and composites laminate.

iii The contribution of concrete tensile strength is negligible in the analytical calculation.

iv All safety factors and other coefficients are uniform.

The moment capacity of strengthened RC beams, M, was determined using the conventional theoretical for-mulation from ACI440.2R as shown in Eqs. 7 to 13.

( ) ( ) ( )st st cuc ct ct cuc sc cuc scM F h h F h h F h hχ= − + − + − (7)

Furthermore,

sc sc sc scF A E ε= (8)

st st st stF A E ε= (9)

ct ct ct ctF A E ε= (10)

Strain of the materials,

( )cuc scsc

hε χε

χ−

= (11)

( )cuc stst

hε χε

χ−

= (12)

( )cuc ctct

hε χε

χ−

= (13)

where, Ast = Area of tensile reinforcing bar (mm2) Act = Area of strengthening laminate (mm2) Asc = Area of compression reinforcing bar (mm2)

Est = Young’s modulus of tensile reinforcing bar (GPa)

Ect = Young’s modulus of strengthening laminate (GPa)

Esc = Young’s modulus of compression reinforcing bar (GPa)

εcuc = Compressive strain of concrete at the top of RC beam (µε)

εst = Strain of tensile reinforcing bar (µε) εct = Strain of strengthening laminate (µε) εsc = Strain of compression reinforcing bar (µε) In this formulation, mechanical properties of materi-

als can be obtained from Figs. 13 and 18, and Tables 4 and Table 5. Lastly, the adopted values used in the theo-retical formulation are summarized in Table 8.

3. Results and Discussion

3.1 Mechanical properties of composite materi-als Due to large-scale size, two specimens of each type were manufactured and tested. Figs. 7 to 9 represent the average of two specimens for each type; therefore, four curves are shown in each figure. These figures show the stress-strain curve at 50% fiber content for every type of kenaf composites. The pattern shows the linear relation-ship between stress and strain parameter and this pattern give insight that the composite was brittle composite material. The ultimate tensile strength of composites with different type of resins, viz. Kenaf/Epoxy compos-

Fig. 6 Stress and strain distribution of Strengthened RCbeam.

0

20

40

60

80

100

120

0 0.5 1 1.5 2

Stre

ss (M

Pa)

Strain (%)

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5

Fig. 7 Stress-strain behaviour of Kenaf/Epoxy with 50% kenaf content.

0

20

40

60

80

100

120

0 0.5 1 1.5 2

Stre

ss (M

Pa)

Strain (%)

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5

Fig. 8 Stress-strain behaviour of Kenaf/Polyester with 50% kenaf content.

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ites, Kenaf/Polyester composites and Kenaf/Vinyl Ester composites is presented in Fig. 10. Each value presented in the figure represents the average of ultimate tensile strength for five specimens of kenaf composites. The Kenaf/Epoxy composites had higher ultimate tensile strength than Kenaf/Polyester composites and Ke-naf/Vinyl Ester composites. However, all composites showed similar ultimate tensile strength at 50% fiber content; the Kenaf/Epoxy composites had an ultimate strength of 78 MPa, whereas those of Kenaf/Polyester composites and Kenaf/Vinyl Ester composites were re-corded at 77 MPa and 79 MPa, respectively. Overall results showed that Kenaf/Epoxy had the highest ulti-mate tensile strength. It is believed to be due to superior mechanical properties of epoxy resins as compared to others. Additionally, the results showed that the overall kenaf composites’ performances improved with increas-

ing fiber content. Similar findings have been observed by Ku et al. (2011) and Seki (2009).

The Young’s modulus of kenaf fiber reinforced poly-mer composites were measured at the initial slope of the stress-strain diagram and presented as shown in Fig. 11. Each value presented in the figure represents the aver-age of Young’s modulus for five specimens of kenaf composites. The Young’s modulus, E, of Kenaf/Epoxy composites, Kenaf/Polyester composites, and Ke-naf/Vinyl Ester composites exhibited consistent trends with consistent increments when the fiber contents in-creased. Kenaf/Epoxy composites at 0% to 50% fiber content showed higher Young’s modulus as compared to other composites. Meanwhile, the Young’s modulus of Kenaf/Polyester composites and Kenaf/Vinyl Ester composites had almost similar properties. These results corresponded to those of ultimate tensile strength (UTS) in Fig. 10. Based on Fig. 11, it can be said that the Young’s modulus increases with increasing fiber content for all the composites. The results were also signifi-cantly influenced by the effects of fibers arrangement and viscosity of resins since the increment was abrupt and obvious when the fiber content had not reached 50%. There were also pre-stresses when fibers were confined in a distress-narrow condition at maximum fiber volume of 50% of fiber content. Other than that, the highest young’s modulus of 36 GPa was achieved by Kenaf/Epoxy composites due to excellent mechanical and adhesive properties of epoxy resin compared to other resins. Meanwhile, the Young’s modulus for Ke-naf/Polyester and Kenaf/Vinyl Ester composites were 15 GPa and 13 GPa, respectively.

3.2 Load-deflection behavior of beam Figure 12 shows the average maximum load and aver-age maximum deflection for different types of strength-ened beams and control beams. All strengthened beams had improved maximum flexural strength and reduced maximum deflection of approximately 40% and 24%, respectively compared to control beams. Concurrently, this has proven that with up to 50% of fiber content, the performance of all composite materials resemble each other to a certain extent. This performance corresponded with the behavior of resin type used in composite mate-rials as shown in Fig. 10. Different performance was,

0

20

40

60

80

100

120

0 0.5 1 1.5 2

Stre

ss (M

Pa)

Strain (%)

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5

Fig. 9 Stress-strain behaviour of Kenaf/Vinyl Ester with 50% kenaf content.

Fig.10 Ultimate tensile strength of kenaf fiber reinforced polymer composites.

Fig.11 Young’s modulus of kenaf fiber reinforced polymer composites.

Fig. 12 Load - deflection behavior strengthened RC

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nevertheless, observed in control beams. These control beams exhibited ductile behavior after achieving maxi-mum load whereas other beams demonstrated brittle behavior; the beams failed abruptly at maximum load. It could also be inferred that all strengthened beams, in general, had been over-reinforced. Nonetheless, all strengthened beams had increased structural stiffness as compared to control beams. This shows that kenaf fiber reinforced polymer composites laminates have the po-tential to be used as strengthening materials where it can increase the bearing capacity and stiffness of RC beams. 3.3 Stress-strain behavior of composites strengthening laminates Figure 13 shows the stress-strain behavior of compos-ites strengthening laminates during flexural strength. Its failure mode further supported the fact that all compos-ites strengthening laminates were brittle. Kenaf/Epoxy composites laminate had higher strength if compared to the others. Meanwhile, Kenaf/Polyester composites laminate and Kenaf/Vinyl Ester composites laminate had approximately the same stress. The same character-istics were discovered during composites material de-velopment. 3.4 Strain behavior at different depth of beam Figures 14 to 17 show the strain behaviour and the strain intersection point. The neutral axis was deter-mined as the point where the top of the beam met the strain intersection point. The distance of the neutral axis of control beam, RC beam strengthened with Ke-naf/Epoxy composites laminate, RC beam strengthened with Kenaf/Polyester composites laminate, and RC beam strengthened with Kenaf/Vinyl Ester composites laminate were 65 mm, 70 mm, 60 mm, and 60 mm, re-spectively. The neutral axis for the control beams and the strengthened beams indicated similar behaviour where the strain increased with increasing load, but at different locations of neural axis. Comparison summary of strain behaviour at different depths of beam is pre-sented in Fig. 18.

The summary of strain blocks for all beams under scrutiny at 30 kN load. The observation was made at a

Fig 14 Strain behaviour for control beam.

Fig 15 Strain behaviour for Kenaf / Epoxy strengthened beam.

Fig 16 Strain behaviour for Kenaf / Polyester strengthened beam.

Fig 17 Strain behaviour for Kenaf / Vinly Ester strength-ened beam.

Fig.13 Stress-strain behavior of kenaf composites strengthening laminates.

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loading of 30 kN so that relevant comparison can be made for all beams. As seen in this figure that the loca-tions of neutral axis for the strengthened beams were different to those of control beams, but the strain in compression zone was approximately maintained. The RC beams strengthened with Kenaf/Epoxy composites laminate (1293 µε), Kenaf/Polyester composites lami-nate (1437 µε) and Kenaf/Vinyl Ester composites lami-nate (1621 µε) had lower strain values than control beams (3936 µε) with 67%, 63% and 59% of reduction, respectively. This situation indicated that the RC beams strengthened with composites laminate had improved more than 50% of structural stiffness. In addition, the strain for RC beam strengthened with Kenaf/Epoxy composites laminate was higher than that strengthened with Kenaf/Polyester and Kenaf/Vinyl Ester.

3.5 Failure Mode and First Crack Development Flexural failure occurs when the shear span/depth ratio

exceeds the value of 5 (av/d > 5), in accordance with BS8110. Firstly, vertical cracks were initiated at the middle of the beam span before shear cracks were formed. Then, ultimate failure took place. In this study, the effect of the strengthening with the composites laminate on the capacity of the beam was investigated by monitoring the first crack developments. Table 6 shows the average first crack load and average deflec-tion at the first crack load. The first crack load and their deflection were determined to observe the initial struc-ture failure during the test. Initially, the strengthened beams had improved flexural strength as compared to control beam with 2.8 times, 1.7 times and 1.3 times greater for Kenaf/Epoxy, Kenaf/Polyester and Ke-naf/Vinyl ester strengthened beam, respectively. This was due to composites laminates functioning well by contributed tensile capacity of the soffit of the beam. It can be noticed that the flexural strength of the RC beam strengthened with Kenaf/Epoxy composites laminate had the superior strength and great stiffness as com-pared to other composites laminate. The average deflec-tion at initial crack did not explain the deformation ac-curately because the displacement continued until ulti-mate failure.

The results as shown in Table 7 summarize that all composite beam failures occurred after the rupture of the composites laminates as demonstrated in Fig. 19. However, ultimate failure of RC was due to bending failure of the beams and at this stage, the load was in-creased to achieve ultimate strength before the strength-ening laminate ruptured. The functionality of beams was carefully assessed when the strengthening laminates were mobilized to resist the loading before structural failure occurred (David and Sami 2008). It means that, failure is only established when the composites rupture and the fibers fracture before the beams crash (Xiong et al. 2004). In this study, such situation had been noticed

Fig.18 Strain blocks.

Table 6 First crack load and deflection during flexural test.

Type of Beam Average first crack load (kN) Average deflection (mm) Control beam 6.00 1.62

RC beam + Kenaf / Epoxy Composites laminate 23.05 5.10 RC beam + Kenaf / Polyester Composites laminate 16.00 3.45

RC beam + Kenaf / Vinyl Ester Composites laminate 14.05 2.99

Table 7 Failure modes for control and strengthened beams.

Specimen Sequence of failure mode Remarks

Control Beam 1) Vertical cracking 2) Crushing of concrete at compression zone Normal failure mode of ordinary RC beam

RC Beam +

Kenaf / Epoxy Laminate

1) Vertical cracking 2) Rupture of composites laminates 3) Crushing of concrete at compression zone

Material failure

RC Beam +

Kenaf / Polyester Laminate

1) Vertical cracking 2) Rupture of composites laminates 3) Crushing of concrete at compression zone

Material failure

RC Beam +

Kenaf / Vinyl Ester Laminate

1) Vertical cracking 2) Rupture of composites laminates 3) Crushing of concrete at compression zone

Material failure

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at the middle span which had maximum moment during the flexural test. The peeling off as well as debonding of the concrete surface was overlooked to concentrate on the performance of composites laminate under flexure action. This is similar to that done in the study of Khaled and Amir (2009).

3.6 Theoretical and Experimental Moment Ca-pacity The theoretical moment capacity was calculated to ver-ify the experimental results (all adopted values used in theoretical part, see Table 8) and the summary of com-parison is shown in Table 9. The results showed that the theoretical moment capacity for strengthened beam was close to the experimental findings. This means that the

conventional theoretical formulation recommended by ACI 440.2R-02 guidelines is capable of verifying the strengthened RC beams’ experimental results. However, the value of control beam was slightly lower than ex-perimental value indicating a slight underestimation. The same trend was reported by Mahjoub (2013) that all the verification data can be accepted since the experi-mental results are 80% of theoretical values (Mahjoub, 2013). However, in strengthening laminates 50% kenaf fiber contributed more than resins. Even though the ex-perimental moment capacity for all strengthened beams was approximately the same, the Kenaf/Epoxy strength-ened beam showed the highest structural stiffness as compared to others as was theoretically proven. There-fore, the kenaf fiber reinforced polymer composites can

Table 8 Summary of adopted value to the theoretical formulation.

Type of beam Control Kenaf/Epoxy Kenaf/Polyester Kenaf/Vinyl Ester χ 65 60 57 57

Acuc 5850 5400 5130 5130 Asc/Ast 157 157 157 157

Act - 600 600 600 Esc/Est 220 220 220 220 Ecuc 29.2 29.2 29.2 29.2 Ect - 143 120 120 ncuc 1 1 1 1

nsc/ nst 7.5 7.5 7.5 7.5 nct - 4.9 4.1 4.1 εcuc 3450 1200 1181 1305 εsc 1539 480 435.1 480.8 εst 1539 680 766.6 847 εct 3609 1460 1575 1740

 Table 9 Theoretical and experimental moment capacity of beams.

Moment capacity, M (kN.m) Type of Beam Neutral Axis (NA), χ (mm) Theoretical Experimental

Control 65 7 9 Kenaf/Epoxy 60 16 15

Kenaf/Polyester 57 15 15 Kenaf/Vinyl Ester 57 16 15

Rupture

(a) (b)

Fig. 19 Structural failure mode (a) Maximum failure at the middle beam span (b) Rupture of composites laminate.

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potentially be used as strengthening materials for RC beams. 4. Conclusions

The properties of kenaf fiber reinforced polymer com-posites and their potential as strengthening materials are established as follows: (1) The strength of all kenaf fiber composites gradually

increased with increasing fiber volume fraction, particularly, Kenaf/Epoxy composites showed higher ultimate tensile strength compared to Ke-naf/Polyester composites and Kenaf/Vinyl Ester composites.

(2) Kenaf/Epoxy composites with up to 50% fiber vol-ume fraction exhibited the highest Young’s modulus. Meanwhile, the Young’s modulus of Ke-naf/Polyester composites and Kenaf/Vinyl Ester composites were approximately the same.

(3) The structural stiffness of all beams strengthened by kenaf fiber reinforced polymer composites in-creased compared to control beams. Their ultimate flexural strength and deflection improved to 40% and 24%, respectively. The beams strengthened by kenaf fiber reinforced polymer composites with 50% fiber volume fraction exhibited similar stiff-ness under flexure action.

(4) As a result, kenaf fiber reinforced polymer compos-ites can be used as strengthening materials for RC beams.

Acknowledgements The authors are grateful to the Ministry of Higher Edu-cation, Malaysia (MOHE) and Research Management Centre (RMC), Universiti Teknologi Malaysia (UTM) for sponsoring this project under the two university re-search grants (RUG) No. Q.J130000.7122.00H83 and Q.J130000.7122.03H35. The authors are also thankful to the staff of Structures & Materials Laboratory, Fac-ulty of Civil Engineering for the facilities and support for experimental work. References American Concrete Institute, (2002). “Guide for the

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