Novel hybrid composites based on glass and sisal fiber for retrofitting of reinforced concrete structures

Construction and Building Materials 133 (2017) 146–153

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Novel hybrid composites based on glass and sisal fiber for retrofitting of reinforced concrete structures Akhila Padanattil a, Jayanarayanan Karingamanna b, Mini K.M. a,⇑ a b

Department of Civil Engineering, Amrita School of Engineering, Coimbatore Amrita Vishwa Vidyapeetham, Amrita University, India Department of Chemical Engineering and Materials Science, Amrita School of Engineering, Coimbatore Amrita Vishwa Vidyapeetham, Amrita University, India

h i g h l i g h t s
Novel method for retrofitting of concrete structures by hybrid fiber reinforcements.
Increase in the axial load carrying capacity upon FRP wrapping.
Ductility of hybrid fiber wrapped concrete same as that of carbon fiber counterparts.
Hybridization of natural and synthetic fibers is a substitute for carbon fiber in retrofitting.

a r t i c l e

i n f o

Article history: Received 18 August 2016 Received in revised form 7 December 2016 Accepted 12 December 2016

Keywords: Hybrid fiber composites Confinement Axial compressive behaviour Ductility Durability

a b s t r a c t In this work an attempt has been made to assess the efficacy of hybrid composite system as a potential choice for the retrofitting of reinforced concrete structures. The combination of synthetic and natural fibers are used for the external confinement of concrete cylinders. A comparative performance analysis of hybrid sisal-glass fiber reinforced polymer (HSGFRP) confinement vis a vis carbon fiber reinforced polymer (CFRP), glass fiber reinforced polymer (GFRP) and sisal reinforced polymer (SFRP) individual confinement is carried out. The axial compressive behaviour, stress-strain response and energy absorption characteristics are studied. The inclusion of sisal fibers along with glass fiber is found to improve the energy absorption characteristics. For predicting the ultimate strength of HSGFRP confined concrete, a new equation was developed based on the lateral confining pressure which shows good agreement with the experimental results. Durability performance studies indicated that exposure to wet/dry conditions and temperature variations resulted an increase in strength for all FRP confined specimens and whereas it decreased for unconfined ones. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The deterioration of reinforced concrete structures can be attributed to several reasons viz., fatigue failure, insufficient reinforcement, exposure to aggressive surroundings leading to steel reinforcement corrosion etc. Hence, for maintaining the structural integrity, retrofitting techniques are to be developed, which ensures the safety and serviceability criteria of the structures. Earlier days prestressed or non prestressed steel was used for the repair but exposure to moisture, chloride penetration and temperature variations reduced the alkalinity of the concrete, leading to corrosion of steel. Such methods are found to be suitable in certain cases where intensity of corrosion was within acceptable limits. Consequently, the need for development of effective and econom⇑ Corresponding author. E-mail address: [email protected] (M. K.M.). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.045 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

ical strengthening strategies resulted in extensive utilization of fiber reinforced polymers (FRP) as reinforcing material in concrete. The salient properties of FRP’s like high tensile strength, high strength to weight ratio, corrosion resistance, ease of installation and high specific strength makes them the most popular material for structural strengthening [1]. Recent reports [2,3] indicate that confinement of concrete columns with FRP significantly improve the ultimate strength and ductile behaviour of concrete. The advantage of FRP confinement on structures is mainly the improvement in load carrying capacity and ductility without considerable increase in cross sectional area and weight. Carbon Fiber Reinforced Polymer (CFRP), Glass Fiber Reinforced Polymer (GFRP), Aramid Fiber Reinforced Polymer (AFRP) etc. are the most commonly used ones for the reinforcement of concrete structures. Several researches has been done in the past related to the use of FRPs in strengthening of concrete structures [4–8]. However, their high cost is a limiting factor and efforts are on for

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the development of composites from alternate materials. Natural fibers like sisal [9], jute [9], flax [10] etc. were tried out for retrofitting of concrete. In comparison with synthetic fibers, natural fibers exemplify low density, moderate tensile and flexural properties and are less costly. Studies have shown that natural fibers have great potential in reinforcing cement matrix [9–14]. As the production of synthetic fibers warrants higher energy, the application of natural fibers are preferred from the sustainability point of view [15]. Nevertheless, natural fibers also have disadvantages such as low durability and strength compared to synthetic fibers. Studies have therefore shown that hybridisation [16,17] of natural fibers with synthetic fibers will help in improving the properties of natural FRP composites [18–20]. Upon hybridization, there is a synergy of the positive features of both the fibers. Studies performed on hybrid composites like Sisal-Jute-GFRP, Sisal-GFRP [18], and Abaca-Jute-GFRP [19] indicated superior properties compared to the individual natural fiber reinforced system. The present work is oriented towards the performance analysis of plain concrete cylinders confined with hybrid Sisal–GFRP composite systems. Uniaxial compression test is performed on cylinders externally confined with FRP composite systems in order to determine the confinement effect on axial load carrying capacity. Ductility, stiffness, ultimate strength, and failure mode are also determined to find out the axial and hoop performance of the confined cylinder. The performance of the hybrid system is compared with different natural and artificial individual FRP composite systems such as CFRP, GFRP, and Sisal FRP etc. The peak strength obtained from experiment is compared with existing models proposed for FRP confined concrete systems. In order to evaluate the ultimate compressive strength of Sisal-GFRP confined hybrid system a mathematical model is made use of proposed in earlier reports [21–25]. For determining the behaviour of FRP confined specimen on a long term basis durability studies are performed. Unconfined specimens and FRP confined specimens are subjected to three different exposure conditions like temperature variation, wet/dry cycle and alkaline environment. The results of the tests performed is expected to give an indication for feasibility of FRPs as reinforcing material in structural applications.

GFRP (HSGFRP) composites obtained after hand lay up process is given in Table 1.

2.2. Wrapping of concrete cylinders with FRP The concrete cylinders after curing for 28 days are kept for drying and the surface defects are removed. The FRP sheets are cut according to required dimensions of the cylinder. Epoxy and hardener was mixed in the ratio 100:15 by weight. A coat of epoxy hardener mix was applied initially on concrete cylinder to fill the voids. Then the respective natural and synthetic sheets are placed on the outer surface of respective concrete cylinders and another coat of epoxy resin-hardener was again applied immediately on top of the sheet. This step was repeated till the required number of layers was achieved. All the FRP confined cylinders are kept for curing at room temperature before the axial compression test. Fig. 1 illustrates the different steps involved in the specimen preparation.

2.3. Mechanism of confinement FRP composite system provides passive confinement effect where the confinement action starts after concrete expands laterally. As the concrete axial stress increases, lateral strain increase and confinement mechanism develops a hoop tensile stress equalized by uniform radial pressure acting against concrete lateral expansion. For circular columns, the circumferential lateral confining pressure is assumed to be uniform. On application of axial compression, tensile stress acts on FRP in hoop direction. The confining pressure increases as the concrete expands laterally. Failure occurs when FRP reaches ultimate tensile strain due to maximum value of confining pressure. The Lateral confining pressure is calculated using the following equation [24]

fl ¼

2tffrp 2tEfrp eu qfrp f frp ¼ ¼ D D 2

2. Methodology 2.1. Materials 2.1.1. Concrete For concrete preparation Ordinary Portland Cement of grade 53 conforming to IS 12269-2013 [26] was used. The fine aggregates used was clean river sand with a fineness modulus of 2.88 and coarse aggregate having bulk density of 1.58 kg/l and maximum size 20 mm. The mix proportioning of concrete was carried out according to IS 10262-2009 [27] and characteristic compressive strength observed was 23 MPa. The mix proportion by weight of cement: sand: coarse aggregate was maintained as 1:2.61:3.60. Water cement ratio of 0.5 was selected. Fifty concrete cylinders with 100 mm diameter and 200 mm height were cast and cured for 28 days. 2.1.2. Fiber reinforced polymer (FRP) composite For the preparation of the composite epoxy resin araldite LY 556 and hardener HY 991 was used as matrix by taking a mix ratio of 100:15 by weight. Epoxy resin has a tensile strength of 85 MPa and modulus of 3.8 GPa. Sisal fiber was obtained in the form of bidirectional woven fabric of thickness 1.15 mm from M/S Dc Mills, Cherthala, Kerala and Glass fiber woven mat of 300 gsm and Carbon fiber fabric of 200 gsm was supplied by M/S Hindoostan Composite Solutions. The tensile strength test results of hybrid sisal-

qfrp ¼

ð1Þ

pDt 4t ¼ pD2 =4 D

ð2Þ

where Efrp the modulus of elasticity of FRP, f frp is the ultimate tensile strength of FRP, D is the diameter of concrete cylinder, t is the thickness of FRP, qfrp is the volumetric ratio of FRP.

3. Results and discussion Two different cases are analysed in the present study to evaluate the compression behaviour of FRP wrapped plain concrete cylinder. All the specimens are subjected to axial compressive loads in a testing machine of 2000 kN capacity. The various parameters like load, stress, displacement, strain etc. are measured and compared.

Table 1 Strength properties of HSGFRP composites. HSGFRP

Ultimate tensile strength (MPa)

Modulus of Elasticity (GPa)

Elongation at break (%)

1 Layer 2 Layer 3 Layer

233.2 368.4 441

11.81 12.42 13.36

1.60 1.94 2.26

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(a) Epoxy hardener mix applied on concrete cylinder

(c) Wrapping of concrete cylinder withGFRP sheet

(b) Wrapping of concrete cylinder with sisal sheet

(d) Concrete specimen wrapped with Hybrid Sisal-GFRP composites

Fig. 1. Steps involved in specimen preparation.

3.1. Confinement effect due to natural, artificial and hybrid system – case 1 3.1.1. Axial compressive behaviour The axial compression test results of concrete specimens confined with different FRP’slike CFRP, GFRP, natural sisal and hybrid

Table 2 Compression test results of FRP confined specimens. SI. No

Specimen

Compressive strength f0 coor f0 cc (MPa)

Axial compressive strain ecoor ecc (%)

1

Plain concrete Sisal GFRP CFRP HSGFRP

21.1

0.32

27.6 38.5 74.5 52.5

1.35 0.72 1.3 1.55

2 3 4 5

sisal-GFRP composite are presented in Table 2. The strength of unconfined concrete is represented by f0 co and the concrete strength of FRP confined specimen is represented by f0 cc. f0 cc/f0 co represent the confinement effectiveness of FRP strengthened concrete. ecc represents the axial strain for FRP strengthened specimen and eco for unconfined concrete. ecc/eco gives the confinement ratio of confined concrete. From the results it is evident that confinement of plain concrete cylinders with FRP significantly enhanced the load carrying capacity. From Table 2 it is clear that compared to plain concrete specimen FRP confined specimens has good efficiency in enhancing load carrying capacity. The highest increase in axial load carrying capacity was shown by carbon fiber (CFRP). CFRP confined specimens has shown an increase in axial load carrying capacity by 253%, followed by hybrid sisal-GFRP specimen which has shown an increase by 148%, and then GFRP where the axial load carrying capacity increased by 82.46% and then by sisal FRP which has shown an

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increase in axial load carrying capacity by 30.8%. The increase in confinement effectiveness for CFRP, HSGFRP, GFRP and sisal FRP are 3.53, 2.48, 1.82 and 1.30 respectively compared to plain concrete. The axial strain of FRP confined specimen has also shown significant increase by 4.06, 4.84, 2.25, 4.21 times for CFRP, hybrid sisal-GFRP, GFRP and Sisal FRP specimens respectively compared to unconfined specimen. The enhancement in axial load carrying capacity is mainly due to the confining pressure offered by the FRP around the concrete cylinder. Both CFRP and GFRP composite wrapping greatly enhanced the axial load carrying capacity of concrete cylinders. Sisal FRP confinement has also shown significant increase in axial load carrying capacity. The performance of HSGFRP composite wrapping was better than individual sisal and GFRP confinement. This is a testimony of the synergetic effect of both the fibers contributing to the improvement in load bearing capacity of the structure. All specimens exhibited highly ductile behaviour when sufficiently confined by either FRP wraps. The maximum value of ductility was shown by the hybrid specimen followed by sisal FRP specimen. Thus it can be inferred that the presence of natural fibers contribute to the improvement in ductility.

3.1.2. Stress-strain response The axial compressive stress-strain curves for control and FRP confined specimens is shown in Fig. 2. In all the cases the concrete specimens are confined with single layer FRP. The curves of the confined specimens exhibit three different regimes. In the initial stage, the confined specimens exhibit a linear trend similar to behaviour of unconfined concrete. In the second stage a transition occurs where the strength of concrete exceeds the unconfined concrete strength and concrete starts expanding laterally which initiates the confining action of FRP. The third stage again shows a linear trend with reduced slope. The confinement effect of FRP’s are manifested in the second and third stage of the curve. In the case of control specimen, the failure is occurring at the end of first stage itself. This is because the control specimens exhibits brittle failure whereas FRP strengthened specimens shows ductile mode of failure i.e. failure after yielding. Compared to synthetic fiber confinement the presence of natural fiber has shown reasonable improvement in ductility of the confined specimens. Hence, the

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possibility of catastrophic failure for FRP strengthened specimen is minimal, which makes it a suitable application for earthquake prone areas. Similar results was reported in an earlier study conducted by Tara Sen and Ashim Paul [9]. 3.1.3. Ductile behaviour and energy absorption In order to avert catastrophic failure, it is desirable for the structure to undergo considerable deformation. Ductility can be taken as a measure of the deformation capability of the structure. Members showing ductile behaviour are able to give sufficient warnings before failure as they have higher energy absorption rate. Increase in confinement ensures increased ductility leading to higher energy absorption rate without failure. In the present study ductility is represented by energy ductility index which can be defined as the ratio of fracture energy of FRP strengthened concrete to plain concrete. It is evident from Table 3 that FRP confinement improves ductility characteristics. The fracture energy of sisal FRP, GFRP, HSGFRP, CFRP wrapped specimens was 6.65, 3.75, 15.91, and 16.63 times the fracture energy of plain concrete. From the ductility index, it can be seen that the ductility of the specimens increased noticeably. The results are also in agreement with Libo Yan [10] where they reported that ductility concrete specimens increased by flax FRP wrapping. Overall, FRP wrapping increased the ultimate compressive strength, axial strain, fracture energy and ductility of the concrete specimens remarkably. The energy absorption capacity of HSGFRP confined cylinders is almost equal to CFRP confined specimens. 3.2. Confinement with varying number of layers of HSGFRP composite systems – case2 For detailed analysis on effectiveness of confinement with hybrid system, uniaxial compression load was applied on plain concrete cylinders confined with varying number of layers of HSGFRP composite systems. 3.2.1. Axial compressive behaviour The axial compression test results of concrete specimens confined with varying number of layers of HSGFRP is presented in Table 4. From the results it is evident that confinement of plain

Fig. 2. Axial stress-strain curves of specimens confined with FRP.

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Table 3 Energy ductility index of concrete strengthened with FRP. Specimen

Energy absorbed (Nm)

Energy ductility index

Plain concrete Sisal GFRP HSGFRP CFRP

64.86 431.81 243.85 1032.25 1079.06

1 6.65 3.75 15.91 16.63

Table 4 Compression test results of HSGFRP confined concrete specimens. Specimen

f0 coor f0 cc (MPa)

ecoor ecc (%)

fl (MPa)

Plain concrete HSGFRP-1 layer HSGFRP-2 layer HSGFRP-3 layer

21.1 52.5 61 70

0.32 1.55 1.70 1.86

– 14.3 18.24 22.53

concrete cylinders with HSGFRP has pronounced effect in improving the load carrying capacity. The increase in number of layers from 1 to 3 resulted in improving axial load carrying capacity by 148.8%, 189% and 231.75% respectively compared to that of plain concrete. Thus the load carrying capacity improved in proportion to increase in number of layers: from 1 to 3 is 2.48, 2.89 and 3.31 respectively compared to plain concrete specimen. There is also a tremendous increase in axial strain of HSGFRP confined specimen. The axial strain ratio of HSGFRP confined specimen increased by 4.84, 5.31 and 5.81 times for 1, 2 and 3 layer(s) respectively compared to unconfined concrete specimen. The enhancement in axial load carrying capacity and axial strain from 1 layer to 3 layers of HSGFRP can be attributed to the increased lateral confining pressure offered on application of compressive load. Failure of test specimens occurs when FRP reaches the ultimate tensile strain and at this stage the confining pressure attains a maximum value. As shown in Table 4, the ultimate tensile strain value of HSGFRP increases with increase in the number of layers. Consequently the confining action offered by 3 layer HSGFRP is highest followed by 2 layer and 1 layer resulting in greater axial load carrying capacity and ductility. In order to ensure adequate ductility for an FRP confined column it was reported that minimum confinement pressure developed should be equal to or greater than 4 MPa [2]. Hence the HSGFRP satisfy the required condition for FRP confined column.

3.2.2. Stress-strain response The axial compression stress-strain curves for control and HSGFRP confined specimens are shown in Fig. 3 and follows the same trend as reported in Section 3.1.2. Here the confinement effect increases with increased number of layers, whereas only a marginal increase in ductility is observed with increased number of layers. 3.2.3. Ductile behaviour and energy absorption The confinement of concrete cylinder with HSGFRP improves the energy ductility index which is a measure of ductility characteristics as reported in Table 5. The fracture energy of 1-layer, 2layer, and 3-layer sisal-GFRP was 16, 21.8 and 29 times the fracture energy of unconfined concrete. A similar performance is reported by Libo Yan [10] in flax FRP wrapped concrete cylinders. 3.3. Analytical model for compressive strength of concrete with frp confinement From the existing models [21–25], the generalised equations for confined concrete strength f0 cc for FRP confined circular sections is given by 0

0

f cc ¼ f co þ k1  f l

ð3Þ

k1 is the confinement effectiveness coefficient. It is reported that the value of k1 varied in each case depending on the type of confinement and test results. In the present work an equation is developed to predict the peak compressive strength of HSGFRP confined concrete. A graph is plotted between ultimate compressive strength and lateral confining pressure of HSGFRP confined concrete cylinder with various thicknesses like 1-layer, 2-layers and 3-layers and is shown in Fig. 4. It can be inferred that the strengthening effect is proportional to lateral confining pressure between sisal-GFRP and concrete. The equation of the test results is approximated as 0

f cc ¼ 20:45 þ 2:205f l

ð4Þ

From experimental results it is observed that strength of unconfined concrete (f0 co) obtained is 21.1 MPa which is near to the value 20.45 MPa obtained in Eq. (4), which can also be written as 0

f cc fl 0 ¼ 1 þ 2:21 0 f co f co

ð5Þ

0

where ff 0cc represents the confinement effectiveness or strengthening co

ratio. From Eq. (5) it is observed that the strengthening ratio is directly proportional to FRP strength and inversely proportional to the strength of unconfined concrete. By substituting the value of fl in Eq. (5), the strength of 1, 2, 3 layers of HSGFRP wrappings is predicted as 52.7, 61.4, 70.89 MPa respectively which is in good agreement with the results given in Table 4. The proposed equation matches with earlier research conducted in Flax fiber wrapped concrete specimen [10]. Hence it can be concluded that the proposed model can be suitably used for predicting the strength of HSGFRP confined concrete.

Table 5 Energy ductility index for concrete strengthened with HSGFRPlayers.

Fig. 3. Axial stress-strain curves of specimens confined with HSGFRP.

Specimen

Energy absorbed (Nm)

Energy ductility index

Plain concrete HSGFRP-1 layer HSGFRP-2 layer HSGFRP-3 layer

64.86 1032.25 1415.64 1886.28

1 15.91 21.82 29.08

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80

wet/dry exposure has marginal effect on the strength of all FRP confined specimen except for natural sisal FRP confinement. There was also an increase in axial strain of all FRP confined specimens. From the results it was observed that exposure to wet/dry cycling resulted an increase in strength and axial strain for control specimen, CFRP confined specimen, GFRP confined specimen as well as HSGFRP specimen. This may be mainly due to the curing effect provided when exposed to wet cycle. But in case of sisal FRP confined specimen although strength is decreased, the strain value increased considerably. The decrease in strength may be due to lack of confinement caused by the degradation of fiber.

y = 2.2058x + 20.454 R² = 0.8584

70

f’cc

(MPa)

60 50 40 30 20 10 0

0

5

10

15

20

25

fl (MPa) Fig. 4. Peak compressive strength Vs lateral confining pressure of sisal-GFRP confined concrete.

3.4. Durability study The durability and performance of concrete structures are largely affected by various environmental conditions. Many researchers [28–31] carried out studies on durability of FRP confined concrete compression members. In the present work durability studies on FRP confined concrete specimen is performed under three environmental exposure conditions, which civil infrastructure is generally subjected to, for a period of 45 days. In all the cases concrete cylinder is confined with 1 layer of GFRP, CFRP, Sisal and HSGFRP wrapping. The different environmental conditions to which the specimens are exposed in the present study are as follows: 1) Control Environment: +27 °C and 50% relative humidity 2) Temperature variations: +27 °C to +50 °C. Maximum temperature +50 °C maintained for 24 h and then decreased to room temperature +27 °C for 24 h. 3) Alkali Solutions: submersion in pH 12NaOH solution. The results of durability study for unconfined and FRP confined concrete specimens under the above mentioned conditions are reported in Table 6. 3.4.1. Effect of wet/dry exposure Durability of FRP confined specimen under wet/dry exposure is analysed by keeping the specimen in a chamber and subjecting to alternate wet and dry cycles in sea water condition. Seawater environment was simulated by mixing 35 g salt in a litre of water [6]. The results of axial compression test performed on FRP confined specimens exposed to wet and dry cycle along with the control specimens kept at room temperature (+27 °C and 50% relative humidity) is presented in Fig. 5. In case of unconfined specimens wet/dry cycling resulted in an increase in strength by 21.5%. The

3.4.2. Effect of temperature variation The effect of temperature variation is studied by subjecting the specimens to heating and cooling cycling (+27 °C to +50 °C). The maximum temperature in each cycle was +50 °C which is maintained for 24 h and then decreased to room temperature +27 °C for 24 h. For unconfined specimens temperature cycling resulted a slight decrease in strength by 10%. The results of axial compression test performed on FRP confined specimens exposed to temperature cycle along with the control specimens kept at room temperature (+27 °C and 50% relative humidity)is presented in Fig. 6. It is observed that exposure to temperature variation has significant effect on the strength of FRP wrapped specimen. The initial behaviour of heat/cool exposed specimen and unexposed specimens are similar. There was an increase in strength observed for all FRP confined specimens whereas strength decreased for unconfined specimen. The increase in strength can be due to the fact that exposure to high temperature caused increase in stiffness of FRP. As a result the confinement action improved resulting in increase in strength whereas for unconfined specimen exposure to heat/cool cycle resulted in micro cracking causing decrease in strength. Hence exposure to temperature cycle has positive effect on FRP confinement which makes the application suitable for tropical areas. There was also an increase in case of axial strain for the entire FRP confined specimen except for GFRP confinement. 3.4.3. Effect of alkaline solution In this method the specimens are kept immersed in alkaline NaOH solution of pH 12. Based on the test results it was found that exposure to alkaline environment resulted in decrease in strength by 25% for unconfined specimen except for GFRP confined specimen. The effect of alkaline solution on FRP confined specimen is reported in Fig. 7. There was a reduction in strength of HSGFRP confined specimen by 17% on subjecting to alkaline environment showing a drastic reduction in the energy absorption from 1032 Nm to 285 Nm. Similarly the strength decrease was 19% for sisal FRP confined specimen and 10% for CFRP confinement. In case of GFRP confined specimen alkali treatment has shown significant improvement in the axial load carrying capacity. Exposure to alkaline environment resulted in decrease in strength for control specimen, CFRP, HSGFRP and sisal FRP confined specimen. This decrease in strength is due to the degradation

Table 6 Durability test results of confined and unconfined specimens. Specimen

Control HSGFRP Sisal FRP GFRP CFRP

Room temperature

Wet/dry

Temperature variation

NaOH solution

f0 co or f0 cc (MPa)

eco or ecc (%)

f0 co or f0 cc (MPa)

eco or ecc (%)

f0 co or f0 cc (MPa)

eco or ecc (%)

f0 co or f0 cc (MPa)

eco or ecc (%)

21.1 52.5 27.6 38.5 74.5

0.32 1.55 1.35 0.72 1.3

25.5 55 25.6 39.7 85

0.38 1.9 1.85 0.95 2.0

19.1 60 35 43.56 90

0.26 1.65 1.9 0.6 1.6

15.7 44.8 23.1 43.5 67.5

0.2 0.65 0.62 0.79 0.75

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Fig. 5. Axial stress strain behaviour of FRP confined concrete between wet/dry exposure and room temperature.

Fig. 6. Axial stress strain behaviour FRP confined concrete between heat/cool cycle and room temperature.

Fig. 7. Axial stress strain behaviour of FRP confined specimen between alkaline exposure and room temperature.

caused to the FRP on alkaline exposure resulting in loss of strength. In case of GFRP confined specimen alkaline exposure resulted in increase in strength. Compared to other FRPs, GFRP has much more resistance to alkaline environment resulting in increase in strength.

4. Conclusion A novel technique for the retrofitting of concrete structures using hybrid sisal-glass fiber reinforced polymer was employed in the present study. The various performance parameters like axial load carrying capacity, ductility, energy absorption rate and

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durability of individual synthetic and natural FRP and hybrid FRP wrapped structures was compared. A remarkable increase in the axial load carrying capacity was observed upon FRP wrapping. The axial strain improved by 484% in the case of HSGFRP. The improvement in the axial strain improved the ductility of the structure which can be considered as a significant criterion for seismic resistant design of structures to avoid catastrophic failure. The energy ductility index of HSGRP wrapped concrete was almost same as that of CFRP wrapped ones. When the number of layers of HSGFRP wrapping was increased to three, its axial compressive strength performance was at par with that of single layer CFRP wrapped structure. The hybridization of natural and synthetic fibers can be considered as a viable option to replace costly synthetic fibers like carbon in retrofitting applications. A mathematical model was developed to predict the peak strength of HSGFRP confined concrete structures. The durability tests pointed towards the use of suitable surface treatment for sisal to further improve the axial strength and strain. References [1] L.C. Hollaway, J.G. Teng, Strengthening and Rehabilitation of Civil Infrastructures Using Fiber-Reinforced Polymer (FRP) Composites, Wood Head publishing limited, Cambridge, England, 2008. [2] Shamsher Bahadur Singh, Analysis and Design of FRP Reinforced Concrete Structures, second ed., Tata McGraw-Hill Publishing Company Limited, New Delhi, 2013. [3] Hee Sun Kim, Yeong Soo Shin, Flexural behavior of reinforced concrete (RC) beams retrofitted with hybrid fiber reinforced polymers (FRPs) under sustaining loads”, Compos. Struct. 93 (2013) 802–811. [4] Y. Xiao, H. Wu, Compressive behaviour of concrete confined by carbon fibre composite jackets, J. Mater. Civ. Eng. 12 (2) (2000) 139–146. [5] A.G. MacDiarmid, A.J. EPezhmanTaghia, Suhaimi Abu Bakar, Mechanical behaviour of confined reinforced concrete-CFRP short column-based on finite element analysis, World Appl. Sci. J. 24 (7) (2013) 960–970. [6] Houssam Toutanji, Yong Deng, Strength and durability performance of concrete axially loaded members confined with AFRP composite sheets, Composites B 33 (2002) 255–261. [7] Han-Liang Wu, Yuan-Feng Wang, Liu Yu, Xiao-Ran Li, Experimental and computational studies on high strength concrete columns confined by Aramid fiber reinforced polymer sheets, J. Compos. Constr. 13 (2) (2009) 125–134. [8] K.M. Mini, Rini John Alapatt, Anjana Elizabeth David, Aswathy Radhakrishnan, Minu Maria Cyriac, R. Ramakrishnan, Experimental study on strengthening of R.C beam using glass fibre reinforced composite, Struct. Eng. Mech. 50 (3) (2014) 275–286. [9] Tara Sen, Ashim Paul, Confining concrete with sisal and jute FRP as alternatives for CFRP and GFRP, Int. J. Sustainable Built Environ. 4 (2015) 248–264. [10] Libo Yan, Plain concrete cylinders and beams externally strengthened with natural flax fabric reinforced epoxy composites, Mater. Struct. 49 (6) (2016) 2083–2095. [11] Tara Sen, H.N. Jagannatha Reddy, Strengthening of RC beams in flexure using natural jute fiber textile reinforced composite system and its comparative study with CFRP and GFRP strengthening systems, Int. J. Sustainable Built Environ. 2 (2013) 41–55.

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