Shear capacity of precracked and non-precracked reinforced concrete shear beams with externally bonded bi-directional CFRP strips

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Construction and Building

MATERIALS

Construction and Building Materials 22 (2008) 1148–1165

www.elsevier.com/locate/conbuildmat

Shear capacity of precracked and non-precracked reinforced concrete shear beams with externally bonded bi-directional CFRP strips J. Jayaprakash

a,*,1

a

, Abdul Aziz Abdul Samad b, Ashrabov Anvar Abbasovich c, Abang Abdullah Abang Ali d

Department of Civil Engineering, Universiti Putra Malaysia, S46/222, Malaysia b Universiti Tun Hussein Onn Malaysia, Malaysia c Tashkent Automobile and Road Construction Institute, Tashkent, Uzbekistan d Housing Research Centre, Universiti Putra Malaysia, Malaysia

Received 28 November 2005; received in revised form 20 February 2007; accepted 22 February 2007 Available online 20 April 2007

Abstract This paper exemplifies both the shear strengthening capacity and modes of failure of Reinforced Concrete (RC) rectangular shear beams bonded externally with Bi-Directional Carbon Fibre Reinforced Polymer (CFRP) Composites. In total, 16 beams were cast without any internal shear reinforcement. Out of this, four beams were preserved as control specimens, six specimens were precracked and repaired with CFRP strips (i.e. precracked/repaired beam) and the following six specimens were strengthened without the application of preloading or precracks (i.e. initially strengthened specimen). The variables examined in the experimental investigation were the longitudinal tensile reinforcement ratio, shear span to effective depth ratio, spacing of CFRP strips, and amount and orientation of CFRP strips. All beams were tested under simply supported condition. Tests result shows that the effectiveness and shear capacity of the CFRP strengthened specimens. The shear enhancement of the CFRP strengthened beams varied between 11% and 139% over the control beam. This study confirms that the CFRP strip technique significantly enhances the shear capacity of reinforced concrete shear beams. The experimental results of the shear-CFRP strengthened beam were compared with the theoretical results. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Shear; CFRP; Strips; Reinforced concrete

1. Introduction Strengthening of Reinforced Concrete (RC) structural members using externally bonded Fibre Reinforced Polymer (FRP) fabrics have been attracted by many researchers [1–21]. The demand to use the FRP fabrics or sheets is due to its better characteristics than other conventional materials. The major characteristics include high strength to weight ratio, high stiffness, light weight, flexibility and resistance to corrosion [3]. Moreover, there are several *

1

Corresponding author. E-mail address: [email protected] (J. Jayaprakash). Former PhD Graduate.

0950-0618/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.02.008

other advantages attributed to their use including ease of bonding to curved or any irregular surfaces, easy to install on site without any special equipments, minimal traffic interruption, and less time consumption [4–6]. In recent years, the exploitation of Fibre Reinforced Polymer composites, as an external reinforcement is an evergreen technique of improving the structural performance of reinforced concrete structures. Literature review reported that the flexural strengthening behaviour of reinforced concrete beams has been abundantly addressed [7–13]. In fact the flexural strengthening mechanism of reinforced concrete beams was studied well but not complicated like shear mechanism. Shear failure of reinforced concrete beam is catastrophic and could

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Nomenclature Af bw df Ef fc0 ffe kv k1 k2 Le

2ntfwf width of beam effective depth of FRP fabric equal to the effective depth of cross section tensile modulus of FRP compressive strength of concrete cylinder tensile stress in FRP at ultimate bond reduction coefficient bond reduction coefficient for concrete strength bond reduction coefficient for wrapping scheme active bond length

occur suddenly without any advance forewarning. Many of the existing RC beams have been found to be deficient in shear strength and need to be strengthened [5]. Several factors need to be considered in shear deficient structures such as lack of shear reinforcement or reduction in steel area due to corrosion, increased service load than the design, construction faults and old design codes. Most of the past research [2,4,5,14–18] have dealt with shear strengthening of reinforced concrete beams using uni-directional fibre reinforced polymers sheets but the bi-directional CFRP fabric has not been fully addressed [19,20]. Specifically, all investigations have been focusing much on continuous wrapping as an external reinforcement. Results also proved that the continuous wrapping along the web of the beam increases the shear capacity of the strengthened beams. However, limited work has been done on the application of bi-directional CFRP sheet in the form of strips [21]. The CFRP strip technique is more economical compared to the continuous wrapping system. 2. Objectives of this study The overall objective of this investigation was to study both the shear strengthening capacity and modes of failure of reinforced concrete rectangular shear beams bonded externally with bi-directional Carbon Fibre Reinforced Polymer (CFRP) composites. Specific objectives are as follows:  Investigating the effectiveness of the bi-directional CFRP strip technique in strengthening full-scale reinforced concrete rectangular beams without any internal shear reinforcement (i.e. no steel stirrups).  Investigating how the factors such as longitudinal tensile reinforcement ratio, shear span to effective depth ratio, spacing of CFRP strips, and amount and orientation of CFRP strips influence the shear capacity of the strengthened reinforced concrete beams.  Comparing the shear capacity of CFRP strengthened beams with the theoretical results.

n tf sf Vc Vs Vf efe efu b

number of FRP plies or layers thickness of FRP spacing of FRP fabric strips nominal shear strength contributed by concrete nominal shear strength contributed by steel nominal shear strength contributed by FRP effective FRP strain ultimate FRP strain angle between the principle fibre orientation and the longitudinal axis of the beam

To accomplish the above objectives, an experimental program has been carefully designed and accordingly performed at the Structural Engineering Laboratory of the Universiti Putra Malaysia. A total of 16 rectangular beams were fabricated without any internal shear reinforcement. These beams have been tested for symmetrical and concentrated loading up to failure. 3. Experimental program 3.1. Fabrication of beams Sixteen rectangular beams were of 2980 mm total span with a cross section of 120 mm width and 340 mm depth. These specimens were cast in wooden frame at the Structural Engineering Laboratory of the Universiti Putra Malaysia. Two batching of ready mix concrete were used to fabricate the beams and standard cylinders. Both the beams and cylinders were cured for at least 28 days before testing. Total 16 specimens were classified into two categories: namely BT and BS. Each category had eight beams. In group BT, two 20 mm bars were used as longitudinal tensile reinforcement (q = 1.69%) along the soffit of the beam. Specimens in the group BS were reinforced with two numbers of 16 mm bars as tensile reinforcement (q = 1.08%). All specimens were designed based on BS 8110 and the tensile reinforcement ratio has been taken below 2.0% since the earlier investigations [17,22,23] studied with the longitudinal tensile reinforcement ratio of 2.0–4.0%. No internal shear reinforcements were provided in any of the rectangular specimens. The tensile rebars were curtailed at the ends without the provision of any anchorages. Fig. 1 portrays the reinforcement details of rectangular beams. The strength of FRP material changes with direction of fibres (i.e. anisotropic nature) therefore the specimens were bonded externally with two different orientations namely vertical (0/90 deg) and inclined (45/135 deg) to study the shear strength of the strengthened beam. In the vertically orientated reinforcement system, the CFRP strips were placed inclined to the diagonal shear crack (i.e. similar to

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20 or 16mm Dia

310mm

340mm

120mm

2980mm

20/16mm Fig. 1. Reinforcement details of group BT/BS.

steel stirrups) however, in case of inclined orientation, the CFRP strips were oriented perpendicular to the diagonal shear crack. 3.2. Material properties Concrete: Specimens in subgroups BT2 and BS2 were fabricated using first batch ready mix concrete of grade 30 N/mm2. In the second batching, specimens in subgroups BT1 and BS1 were also cast with same concrete grade of

30 N/mm2. The results of average compressive strength of concrete cylinder are shown in Table 1. Internal tensile reinforcement: The steel bars 16 mm and 20 mm were subjected to tensile test and the measured tensile strengths were of 311.22 MPa and 554.17 MPa respectively. CFRP reinforcement: Bi-Directional CFRP fabric was used as external shear reinforcement. Table 2 illustrates the properties of bi-directional CFRP fabrics based on the Sika Manufacturer’s manual sheet [24].

Table 1 Summary of experimental results Specimen description

av/ d

fc (MPa)

Tensile Rebars

BT1aa BT1-1 (precracked– repaired) BT1-1I (initially strengthened) BT1-2I (initially strengthened)

2.5 2.5

27.38 27.38

2Nos 20 mm bars

2.5

27.38

2.5

27.38

BS1a BS1-1 (precracked– repaired) BS1-2 (precracked– repaired)

2.5 2.5

27.38 27.38

2.5

27.38

BT2a BT2-1 (precracked– repaired) BT2-2 (precracked– repaired) BT2-2I (initially strengthened)

4.0 4.0

16.73 16.73

4.0

16.73

4.0

16.73

BS2a BS2-1 (precracked– repaired) BS2-2 (precracked– repaired) BS2-2I (initially strengthened) BS2-1I (initially strengthened)

4.0 4.0

16.73 16.73

4.0

16.73

4.0

16.73

4.0

16.73

2Nos 16 mm bars

2Nos 20 mm bars

2Nos 16 mm bars

Orientation

Max deflection at failure load (mm)

Failure load (kN)

Failure mode

Width and spacing

CFRP reinforcement

– U-strip 80 mm @ 150 mm c/c U-strip 80 mm @ 150 mm c/c U-strip 80 mm @ 200 mm c/c

– 0/90 deg

7.77 12.86

98.14 134.73

0/90 deg

15.54

174.64

0/90 deg

25.48

134.73

Shear Shear-CFRP fracture Shear-CFRP fracture Shear-CFRP fracture

– U-strip 80 mm @ 150 mm c/c U-strip 80 mm @ 200 mm c/c

– 0/90 deg

8.89 14.85

74.86 121.42

0/90 deg

11.35

101.46

– U-strip 80 mm @ 150 mm c/c L-strip 80 mm @ 150 mm c/c L-strip 80 mm @ 150 mm c/c

– 0/90 deg

6.21 23.57

64.88 134.73

45/135 deg

14.76

121.42

45/135 deg

15.18

154.68

– U-strip 80 mm @ 200 mm c/c L-strip 80 mm @ 150 mm c/c L-strip 80 mm @ 150 mm c/c U-strip 80 mm @ 150 mm c/c

– 0/90 deg

15.40 14.12

61.56 108.19

Flexural Flexural

45/135 deg

11.42

81.51

Flexural

45/135 deg

12.09

88.16

Flexural

0/90 deg

17.69

68.21

Flexural

Shear Shear-CFRP fracture Shear-CFRP fracture Shear Shear-CFRP fracture Shear-CFRP fracture Shear-CFRP fracture

J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165 Table 2 Material properties of carbon fabrics and epoxy resin (based on Sika Manufacturer’s manual [24])

Fibre orientation Thickness (mm) Tensile strength (MPa) Modulus of elasticity (MPa) Adhesive strength (MPa)

CFRP fabrics

Epoxy resin (Sikadur330)

0/90 (bidirectional) 0.09 3800 230,000

– – 30 3800

–

4

Adhesive: Sikadur 330, epoxy was used for bonding CFRP fabric strips with the concrete stratum. The material properties of Sikadur 330 [24] are shown in Table 2. 3.3. Specimens description Fig. 2 depicts the description of specimens in the experimental program. Specimens consisted of two groups were designated as BT and BS which were differentiated with respect to the amount of tensile reinforcement ratio. Group BT was subdivided into two subgroups based on the shear span to effective depth ratio namely subgroup BT1 and subgroup BT2 representing the shear span to effective depth ratio of 2.5 and 4.0 respectively. Similarly the group BS was segmented into subgroups namely BS1 and BS2. Each subgroup had one reference or control beam. Specimens BT1-1, BS1-1 & BS1-2, BT2-1 & BT2-2 and BS1-1 & BS2-2 in subgroups BT1, BS1, BT2 and BS2 were precracked and repaired with CFRP strips which are termed as precracked/repaired specimens. Whereas the specimens BT1-1I, BT1-2I, BT2-2I, BS2-1I and BS2-2I were strengthened without any precracks or preloading named as initially strengthened specimens. 3.4. Application of CFRP reinforcement The surface preparation is the foremost preliminary step in the installation of CFRP fabrics on the concrete surface.

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The debonding failure may occur in the strengthened beam due to improper surface preparation. It could be prevented by paying much attention to the surface preparation before embedding the fibre fabrics on the concrete surface. The surface of specimens was grinded uniformly without any undulations or irregularities by using mechanical grinder to expose the aggregate particles. Besides, the structural sharp edges or corners were rounded to a radius of at least 10 mm. The concrete surface must be free from any oil or greasy substance. The embedded dust particles and dust disintegrated materials were removed from the surface by blowing with air and also with wire brushing to expose the aggregate. The beam surface must be dried properly prior to application of CFRP sheets. Initially the pores appearing on the prepared concrete surface must be filled with the pre-processed epoxy by using scrapper. Upon completion of the initial treatment, a layer of epoxy was applied at an appropriate thickness of about 1 mm either with brush or with rollers. The function of the saturated resin is to impregnate the dry fibres; maintain the fibres in their intended orientation; distribute stress to the fibres; and prevent the fibres from abrasion and environmental effects. The CFRP sheets were measured and cut to the desired shape and dimensions. The strips were placed on the concrete surface and gently pressed onto the coated epoxy resin. When CFRP fabrics were applied in the form of strips, care must be taken to prevent damages to the fabrics. Subsequently, the sheet or fabrics was rolled by using ribbed roller in both vertical and horizontal direction (e.g. 90/0 deg) to squeeze out the excess of epoxy. Eventually the final layer of epoxy was applied on the surface of the fabrics or sheets. 3.5. Instrumentation and experimental setup Fig. 3 portrays the experimental setup of the reinforced concrete beams. All specimens were tested under simply supported condition. One Linear Variable Displacement Transducer was placed at the mid span of the specimen to monitor the mid deflection at each increment of load.

Rectangular Beams

Group BT

Subgroup BT1

Control: BT1aa Precracked/repaired: BT1-1 Initially strengthened : BT1-1I & BT1-2I

BT1-1: 80mm vertical U-strips @ 150mm c/c BT1-1I: 80mm vertical U-strips @ 150mm c/c BT1-2: 80mm vertical U-strips @ 200mm c/c

Group BS

Subgroup BT2

Control: BT2a Precracked/repaired: BT2-1 & BT2-2 Initially strengthened : BT2-2I

BT2-2: 80mm inclined L-strips @ 150 c/c BT2-2I: 80mm inclined L-strips @ 150 c/c BT2-1: 80mm vertical U-strips@ 150 c/c

Subgroup BS1

Control: BS1a Precracked/repaired: BS1-1 & BS1-2

BS1-1: 80mm vertical U-strips@ 150 c/c BS1-2: 80mm vertical U-strips@ 200 c/c

Fig. 2. Description of specimens in the experimental program.

Subgroup BS2

Control: BS2a Precracked/repaired: BS2-1 & BS2-2 Initially strengthened : BS2-1I & BS2-2I

BS2-1: 80mm vertical U-strips @ 200 c/c BS2-2: 80mm inclined L-strips @ 150 c/c BS2-2I: 80mm inclined L-strips @ 150 c/c BS2-1I: 80mm vertical U-strips @ 150 c/c

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P a v =775mm 120 mm wide strip

4

340mm 2 1

8 7

6 5

3

M U-Strip

80mm

150mm 775mm

250mm

120mm

930mm

775mm

250mm

2980mm (a) Specimens BT1-1, BT1-1I and BS1-1.

P a v =775mm 120 mm wide strip

8

340mm

3 1

6

5 4

7

M 2 U-Strip

80mm

200mm 250mm

120mm

775mm

930mm

775mm

250mm

2980mm (b) Specimens BT1-2I and BS1-2.

P a v =1240mm

120 mm wide strip

120mm

8 6 340mm 1

3 2

7

5

4

M 80mm

150mm 250mm

1240mm

1240mm

U-Strip

250mm

2980mm (c) Specimens BT2-1 and BS2-1I. Fig. 3. Experimental set up and location of strain gauges on the surface of CFRP strip and concrete surface.

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P a v =1240mm A

120 mm wide strip

120mm

8 7 340mm

5 M B

4

2

3

1 L-Strip

150mm

80mm

1240mm

250mm

6

1240mm

250mm

2980mm (d) Specimen BT2-2.

P a v =1240mm A 6 340mm 3 1

7 5

4 M

2 B

L-Strip

150mm

80mm

1240mm

250mm

120mm

120 mm wide strip

8

1240mm

250mm

2980mm (e) Specimens BS2-2, BS2-2I and BT2-2I.

P a v =1240mm

120 mm wide strip

120mm

C8

340mm

C3 F1

C2

F6 F5 F4

F7

M

200mm 250mm

80mm

1240mm

1240mm 2980mm (f) Specimen BS2-1. Fig. 3 (continued)

U-Strip

250mm

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The control specimen BT1aa was loaded to 54.90 kN followed by unloading to zero load, then reloaded to failure. The recorded mid deflection in the first cycle of loading was 2.33 mm. During the first cycle of loading, the specimen BT1aa observed a diagonal shear crack in the vicinity of the middle of shear spans at a load of 48 kN and in the second cycle of loading the beam attained shear failure in the right shear span at a peak load of 98.14 kN (see Fig. 4a). The beam BT1-1 was preloaded to 68.21 kN in the first cycle of loading then reloaded same as first cycle to widen the precracks. At precracking phase, the shear

cracks appeared similar to that of the control beam. At the end of second cycle of loading, the recorded maximum deflection at mid span was 3.54 mm. After completing the precracking phase, the beam was repaired with 80 mm wide CFRP vertical U-strips had a spacing of 150 mm centre to centre along the shear spans and a 120 mm wide CFRP strip applied on the flexural zone of the beam. The bottom CFRP strip was applied before applying the U-strips. The orientation of the CFRP vertical U-strips was 0/90 deg to the longitudinal axis of the beam. After repaired with CFRP strips, the diagonal crack was observed at a load approximately of 108 kN. Failure of the beam occurred in the left shear span at a total applied load of approximately 135 kN due to the rupture of CFRP strip along the shear crack. Tests result shows that there was an increase of 37% in ultimate load capacity compared to the control specimen BT1aa. The shear with CFRP fracture mode of failure of the specimen BT1-1 is shown in Fig. 4b. Beams BT1-1I and BT1-2I were strengthened without the application of preloading phase (i.e. no precracks) and were wrapped with 80 mm vertical CFRP U-strip spaced at 150 mm and 200 mm centre to centre respectively. The soffit of the beam (length 2480 mm) was bonded with 120 mm wide CFRP strip similar to specimen BT1-1. For the beams BT1-1I and BT1-2I, diagonal shear cracks were exhibited at 95 kN and 61 kN and their corresponding failure loads occurred at 175 kN and 135 kN respectively. These beams observed shear with CFRP fracture failure similar to the precracked/repaired specimen BT1-1 but the failure occurred in the right shear span. Fig. 4c shows the shear-CFRP rupture failure of the initially strengthened beam BT1-2I. At the ultimate failure load, the shear crack in both the precracked/repaired and initially strengthened beams propagated along the longitudinal reinforcement towards the end support. This bond failure was due to the absence of anchorage by the tensile reinforcement. The percentage of shear enhancement of specimens BT1-21 and BT1-1I were 37% and 78% higher than the control beam BT1aa. The ultimate failure load of initially strengthened beam BT1-1I was 1.3 times greater than the precracked/repaired beam BT1-1. The shear enhance-

Fig. 4a. Shear failure pattern for control specimen BT1aa.

Fig. 4b. Shear-CFRP fracture failure pattern for precracked/repaired specimen BT1-1.

The strain gauges were placed in the concrete surface and CFRP strips to measure the surface strains. Figs. 3a, b, c, d, e, and f depict the position of strain gauges of CFRP strengthened specimens BT1-1/BT1-1I/BS1-1, BT1-2I/ BS1-2, BT2-1/BS2-1I, BT2-2, BS2-2/BS2-2I/BT2-2I, and BS2-1 respectively. The load was applied using the hydraulic jack and measured by pressure gauge. Specimens in subgroups BT1 and BS1 were subjected to four point bending system whereas the specimens in subgroups BT2 and BS2 were loaded by acting point load at mid span. The control specimen was loaded at equal increments to develop precracks followed by unloading to zero load, then reloaded to failure. The test procedure of the precracked/repaired specimen consisted of two phases. In the first phase, the beam was loaded for two cycles in order to develop precracks and the second phase of loading was conducted after the specimen has been repaired with CFRP strips. But the initially strengthened specimens were strengthened with CFRP strip without any preloading or precracks. After strengthening, the beams were loaded at equal intervals till the failure. During the application of loading, the modes of failure such as debonding or peeling of fibre fabrics/sheets from the concrete surface or rupture of fabric strips, flexural and crushing of concrete were observed. In addition, the crack, pattern was clearly traced. 4. Shear strength and failure pattern 4.1. Subgroups BT1 and BS1

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Fig. 4c. Shear-CFRP fracture failure for initially strengthened specimen BT1-2I.

ment of beam BT1-1I shows an increase of 30% over BT12I. This indicates that the spacing of CFRP strip has an influence on the shear enhancement such that an increase in spacing of CFRP strip decreases the shear capacity of the strengthened beam. In subgroup BS1, the control beam BS1a was subjected to a load of 61.55 kN and corresponding mid deflection of 5.55 mm. At first cycle, the diagonal crack emerged at a load of 48.25 kN in the left side of the shear span. In the reloading phase the diagonal crack undergoes propagation towards the load point due to increase of load and attained a shear failure in the left shear span at ultimate load of 74.86 kN. The precracked/repaired specimens BS1-1 and BS1-2 were preloaded for two cycles to a maximum load of 48.26 kN and 41.60 kN respectively. The maximum mid deflection observed at the end of two cycles of loading was 3.54 mm for specimen BS1-1 and 2.79 mm for specimen BS1-2. The initiation of diagonal crack in these beams was similar to the control beam. Similar to specimens BT1-1I and BT1-2I in the subgroup BT1, these specimens BS1-1 and BS1-2 were repaired with vertical CFRP vertical U-strips spaced at 150 mm and 200 mm centre to centre and a 120 mm wide flexural CFRP strip was also applied along the soffit of the beam. Failure of the repaired beams BS1-1 and BS1-2 were observed similar to the CFRP strengthened specimens in subgroup BT1 but attained a ultimate load of 121.42 kN and 101.47 kN respectively. As the applied load increases, the diagonal cracks were propagated and failed in shear with CFRP fracture like the CFRP strengthened beams in the subgroup BT1. The observed diagonal cracks in specimens BS1-1 and BS1-2 were at 82 kN and 75 kN respectively. The increased shear enhancement of the specimens BS1-1 and BS1-2 were approximately of 62% and 36% greater over the control beam. Specimens BT1-1 and BS1-1 had same amount of external shear reinforcement and loading pattern but the shear enhancement of specimen BT1-1 was 11% greater than the specimen BS1-1 due to the change in amount of internal tensile reinforcement ratio. It was observed that the shear capacity of

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the strengthened specimen influences with the amount of internal tensile reinforcement as well. Analogous to the initially strengthened specimens in the subgroup BT1, the precracked/repaired specimen BS1-1 was approximately 17% more than that of the specimen BS1-2. This was due to the increase of spacing of CFRP strips. It was also observed that the precracked/repaired or initially strengthened specimens in subgroups BT1 and BS1 had bond failure along the longitudinal reinforcement due to the absence of anchorage by the tensile reinforcement. Furthermore there was no debonding of CFRP strip from the concrete surface or peeling of CFRP in any of the precracked/repaired or initially strengthened specimens. The key observations of the CFRP strengthened specimens in subgroups BT1 and BS1 are listed below:  The shear capacity of the CFRP strengthened specimens in the subgroups BT1 and BS1 was varied ranging from 37% to 78% and 36% to 62% respectively.  Tests result shows that increasing the spacing of CFRP strip reinforcement decreases the shear capacity of the strengthened beam.  The results also showed that the ultimate shear strength of the strengthened beam BT1-1 increases to a value of 11% with the increase in longitudinal reinforcement ratio of 56%.  By increasing the spacing of CFRP strip, the distribution of crack in the initially strengthened specimen BT1-2I was more compared to the specimen BT1-1I. It shows that the spacing of CFRP strip not only influences the shear capacity but it also affects the crack distribution.  The distribution and propagation of cracks in the precracked/repaired specimen BT1-1 was lesser than the initially strengthened specimen BT1-1I even with same amount of internal longitudinal and external shear reinforcements. It indicates that the CFRP strip helps to prevent the widening and propagation of early-developed cracks.  No debonding of CFRP strip from the concrete surface or peeling of CFRP strip was observed.  The bond failure along the tensile reinforcement was observed in the precracked/repaired and initially strengthened specimens due to the absence of anchorage by the tensile reinforcement.

4.2. Subgroups BT2 and BS2 The specimens in the subgroup BT2 were subjected to point load at mid span representing a shear span to effective depth ratio of 4.0. The control beam BT2a was precracked to a load of 48.25 kN corresponding to a mid deflection of 3.59 mm and unloaded to zero. With the increase of load in the reloading phase, the diagonal shear crack appeared approximately at a load of 55 kN and failed cataclysmically at right shear span of the beam with

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Fig. 4d. Shear failure pattern for control specimen BT2a (back face).

Specimens BT2-2 and BT2-2I have same amount and distribution of CFRP strip inclined at 45/135 deg to the longitudinal axis of the beam. The width and spacing of CFRP strips was 80 mm and 150 mm centre to centre respectively. These CFRP strips were placed as ‘L’ at an inclination of 45/135 deg to the longitudinal axis of the beam. The inclined CFRP L-strips from front and rear sides of the beam were overlapped on the soffit of the beam. As in subgroups BT1 and BS1, CFRP strip of width 120 mm was provided along the effective span of the beam prior to application of CFRP L-strips. The specimen BT2-2 was precracked prior to strengthening but the beam BT2-2I was strengthened directly without performing any precracking. The beam BT2-2 was subjected to two cycles of loading to 61.55 kN with a maximum deflection of 3.73 mm. The crack propagation of beams BT2-2 and BT2-1 were similar in the precracked phase. After strengthening, the beam BT2-2 was loaded in a single stroke to the failure. The diagonal crack was observed between the CFRP strips (i.e. unwrapped portion) close to the mid span at a load of approximately 108 kN. With the increase of load, the beam failed in shear with CFRP fracture in the left shear span at a peak load of 121 kN. Fig. 4f depicts the shear with CFRP fracture or rupture failure of the precracked/repaired beam BT2-2. The mode of failure of specimen BT2-2I was similar to specimen BT2-2 but the shear failure occurred at right shear span at a load of 155 kN. The shear crack occurred at a load of 108 kN respectively. It was also observed that the diagonal cracks emerged between the CFRP strips near to the mid span but the propagation and distribution of cracks were dissimilar to vertical CFRP strengthened (i.e. U-strip) specimen BT21. The distribution of flexural and shear cracks of initially strengthened specimen BT2-2I were more than the repaired specimen BT2-2. This indicates that the crack propagation was prevented in the repaired specimen BT2-2. Also the crack proliferation and distribution of specimen BT2-2 was less compared to the BT2-1 specimen due to the change in orientation of CFRP strip. The shear enhancement of specimens BT2-2 and BT2-2I were 87% and

Fig. 4e. Shear-CFRP fracture failure for precracked/repaired specimen BT2-1 (back face).

Fig. 4f. Shear-CFRP fracture failure for precracked/repaired specimen BT2-2.

an ultimate load of 65 kN. At failure load, the shear cracks propagated along the longitudinal reinforcement very similar to control beams BT1aa and BS1a. Fig. 4d shows the shear failure pattern of the control beam BT2a. The specimens BT2-1 and BT2-2 were designated as precracked/repaired beams. Specimen BT2-1 was preloaded for two cycles to a maximum load of 55 kN and a corresponding mid deflection of 3.36 mm. Subsequently, beam BT2-1 was repaired with CFRP strip orientation of 0/ 90 deg (i.e. U-strips) to the longitudinal axis of the beam. A 120 mm wide CFRP strip of length 2480 mm was provided along the soffit of the beam. The vertical CFRP Ustrips were applied along the whole span of specimen with a spacing of 150 mm centre to centre. When subjected to loading diagonal cracks originated in between the CFRP strips (i.e. unwrapped portion). These diagonal cracks appeared nearer to the middle of the shear span at a load of 95 kN. Further increase of load initiated failure abruptly in the right shear span at an ultimate load of approximately 135 kN. Fig. 4e indicates the shear-CFRP rupture or fracture failure of the repaired beam BT2-1. Test result also showed that the shear capacity of the beam BT2-1 increased up to 107.64% compared to the control beam BT2a.

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139% greater than the control beam BT2a. The ultimate failure load of the beam BT2-2 was however less compared to BT2-1. The strengthened specimen BT2-2 was damaged by formation of cracks through its depth during the handling operation. This damage was the prime reason for the reduction in ultimate load of beam BT2-2 (45/ 135 deg) in comparison to BT2-1 (0/90 deg U-strip). It can also be seen from the test results that the initially strengthened specimen BT2-2I was 28% greater than the precracked/repaired specimen BT2-2. All specimens in subgroup BS2 were tested similar to the subgroup BT2. The control specimen BS2a was preloaded at 55 kN with a deflection of 9.46 mm and continued to reload towards failure. The initiation of shear crack was at 48 kN but less pronounced when compared to the flexural cracks. Eventually it failed in flexure at a failure load of 61.56 kN. As in subgroups BT1, BS1 and BT2, all specimens in this subgroup BS2 were bonded with a 120 mm wide CFRP strip along the soffit of an effective length of 2480 mm except the control beam BS2a. This bottom strip was applied before the application of vertical (U-strip) or inclined (L-strip) CFRP strips along the web of the beam. Specimen BS2-1 was repaired with vertical CFRP U-strips at a spacing of 200 mm centre to centre. Two cycles of loading was applied with a load of 28.30 kN (corresponding deflection of 2.15 mm) in order to develop cracks across the web of the beam. After strengthening, diagonal cracks appeared at 75 kN near the middle of the shear span were propagated up to eighty percent of the overall depth of the beam due to the increase of loading. The failure was initiated at 108.19 kN in flexural mode (see Fig. 4g). The shear enhancement was increased approximately of about 76% over the control specimen BS2a. Specimens BS2-2 and BS2-2I were wrapped with CFRP L-strips orientated at 45/135 deg to the longitudinal axis of the beam. The inclined L-strip was used similar to specimens BT2-2 and BT2-2I (from subgroup BT2). The specimen BS2-2 had undergone precracking stage similar to the specimen BS2-1, however BS2-2I was strengthened without precracks (i.e. no precracking phase). In specimen

BS2-2, the observed deflection in the precracking phase was 2.18 mm. The precracking pattern in the specimen BS2-2 was observed identical to the specimen BS2-1. In the second phase of loading, inclined cracks were emerged between the CFRP strip gaps (i.e. unwrapped portion) at 69 kN but the flexural cracks at the mid span were well pronounced and visible. There was no crack propagation near the middle of the shear span unlike specimen BS2-1 with vertical U-strips. This might be due to the change in inclined orientation of CFRP strips. It was also observed that the propagation of early-developed precracks was arrested due to the orientation of inclined CFRP strips. The beam BS2-2 finally failed in flexure at a peak load of 81.51 kN. The initially strengthened specimen BS2-2I attained a similar flexural mode of failure as in precracked/repaired specimen BS2-2 at an ultimate load of 88.16 kN. The flexural failure pattern of the initially strengthened beam BS22I is depicted in Fig. 4h. The shear crack was initiated in the unstrengthened portion (i.e. between CFRP strips) at a load 69 kN. The amount of cracks was significantly reduced in both beams (BS2-2 and BS2-2I) compared to the vertically wrapped BS2-1 specimen. This may be due to the orientation of the CFRP strip. The shear capacities of the beams BS2-2 and BS2-2I were increased respectively by 32.42% and 43.21% greater than the control specimen BS2a. Compared to the shear enhancement of the specimens BS2-2 and BS2-2I, it was found that the initially strengthened specimen BS2-2I obtained a shear enhancement of 8% greater than the precracked/repaired beam BS2-2. The propagation of diagonal cracks towards the compression zone was prevented in the beam wrapped with inclined strips (L-strips) compared to the vertically wrapped (U-strips) CFRP beams. Specimen BS2-1I was a initially strengthened specimen with a fibre orientation of 0/90 deg (i.e. U-strips) to the longitudinal axis of the beam. It was wrapped by vertical CFRP U-strips spaced at 150 mm centre to centre along the span of the beam. Owing to the increase of load, new cracks were originated along the unstrengthened

Fig. 4g. Flexural failure pattern for precracked/repaired specimen BS2-1.

Fig. 4h. Flexural failure pattern for initially strengthened specimen BS22I.

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J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165

location in the flexural zone. The diagonal crack was originated approximately at a load of 55 kN but was not as significant as the flexural cracks. The beam finally failed in flexure at a peak load of 68.21 kN (see Fig. 4i). The enhancement of this specimen was approximately of 11% greater than the control beam BS2a. Specimen BS2-1I with vertical U-strip had 23% less in comparison to the specimen BS2-2I strengthened with inclined Lstrips. It shows that the change in orientation of CFRP strip influences the shear capacity of the strengthened specimen. In control beam BS2a, the beam failed in flexure due to the influence of tensile strength of longitudinal rebars (i.e. fy = 311.22 MPa). Similarly the CFRP strengthened beams were also failed in flexural mode. This is probably due to the influence of excess of CFRP was used as shear reinforcement and strength of longitudinal tensile rebars. Some key observations of the CFRP strengthened specimens in subgroups BT2 and BS2 stated below:  Shear enhancement of the CFRP strengthened beams in subgroups BT2 and BS2 differs in between 87–139% and 11–76% over the control beam.  As in subgroups BT1, BS1, there was no debonding or peeling of CFRP strip in the precracked/repaired or initially strengthened specimens in subgroups BT2. At failure, the shear cracks propagated along the tensile reinforcement towards the support similar to that of the CFRP strengthened specimens in the subgroup BT1 and BS1. This bond failure was attributed to the absence of anchorage by the tensile reinforcement.  Specimens with inclined L-CFRP strips (45/135 deg) attained a shear enhancement of 42% greater in comparison of specimen with vertical U-Strips (0/90 deg). Similarly the CFRP shear contribution of the inclined specimen attained greater than the specimen with vertical CFRP reinforcement. It can be concluded that the change in orientation of CFRP strip influences the ultimate load carrying capacity and the CFRP shear contribution of the strengthened specimen.

 From the experimental observation, the distribution and propagation of cracks in inclined L-strips (45/135 deg) specimens were less in comparison to the specimens with U-strips (0/90 deg) because the inclined L-strips were placed perpendicular to the diagonal crack. It shows that the inclined orientation of CFRP strip affects the propagation and distribution of cracks.  When comparing the specimens BT1-1 (subgroup BT1) and BT2-1 (subgroup BT2), the CFRP shear contribution of the specimen BT2-1 with shear span to effective depth ratio 4.0 was 83% more than the specimen BT11 with av/d ratio 2.5. Similarly specimen BS2-1 with av/d = 4.0 attained a shear contribution of 76% greater in comparison to the specimen BS1-2 with shear span to effective depth ratio of 2.5. From the results, it was found that the shear contribution of the specimen with av/d ratio of 4.0 achieved a maximum value of 83% greater than the specimen with shear span to effective depth ratio 2.5.  The initially strengthened specimen BS2-1I with CFRP spacing 150 mm c/c had greater amount of external shear reinforcement than the specimen BS2-1 strengthened with CFRP strip spacing of 200 mm c/c but the ultimate load carrying capacity of specimen BS2-1I was 37% less compared to BS2-1 specimen. It indicates that the increasing the amount of external shear reinforcement does not shows significant shear enhancement. It was also observed that the amount of crack in specimen BS2-1 was more in comparison to specimen BS2-1I due to the increase in spacing of CFRP strips. The spacing of CFRP strips not only influences the load carrying capacity but also affects the distribution of cracks.  All specimens in subgroup BS2 had ductile mode of failure but the subgroups BT1, BS1 and BT2 were failed suddenly in shear with CFRP fracture. The ductility of the CFRP strengthened specimens in this subgroup BS2 were well pronounced compared to specimens in subgroups BT1, BS1 and BT2.  Shear enhancement of specimen BT2-2I was 76% greater than specimen BS2-2I due to the increase of longitudinal reinforcement ratio. 5. Load–displacement profile

Fig. 4i. Flexural failure pattern for initially strengthened specimen BS21I.

The applied load versus deflection profile of reinforced concrete CFRP strengthened specimens in subgroups BT1, BS1, BT2 and BS2 are shown in Fig. 5a, b, c, and d respectively. Test results showed that the precracked/ repaired and initially strengthened specimens attained relatively higher failure load compared to the control specimens. In Fig. 5a, the stiffness of the repaired phase of the beam BT1-1 was similar to the precracked phase due to the presence of external CFRP reinforcement. After repaired with CFRP strips, the stiffness of control beam BT1aa and the precracked/repaired beam BT1-1 exhibited similar behaviour until the load 94.82 kN. Also specimens BT1-1I and BT1-2I have the same trend up to a load of

J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165 BT1aa - Control

BT1-1 - Precracked

BS1a - Control

BS1-1 - Precracked

BT1-1 - Repaired

BT1-1I - Initially Strengthened

BS1-1 - Repaired

BS1-2 - Precracked

BS1-2 - Repaired

200

200

150

150

A pp lie d L o ad (kN)

A p p l i ed L o a d (k N )

BT1-2I - Initially Strengthened

100

50

100

50

0

0 0

5

10 15 20 Mid Deflection (mm)

25

30

0

5

(a) Subgroup BT1.

10 15 20 Mid Deflection (mm)

25

30

(b) Subgroup BS1.

BT2a - Control

BT2-2 - Precracked

BT2-2 - Repaired

BT2-1 - Precracked

BT2-1 - Repaired

BT2-2I - Initially Strengthened

BS2a - Control

BS2-2I - Initially Strengthened

BS2-2 - Precracked

BS2-2 - Repaired

BS2-1 - Precracked

BS2-1 - Repaired

BS2-1I - Initially Strengthened

200

200 Applied Load (kN)

A p pl i e d L o a d ( kN )

1159

150 100

50

150 100

50 0

0 0

5

10 15 20 Mid Deflection (mm)

25

30

(c) Subgroup BT2.

0

10

20 30 Mid Deflection (mm)

40

50

(d) Subgroup BS2.

Fig. 5. Load deflection curves for subgroups BT1, BS1, BT2, and BS2.

48.26 kN. As applied load increased beyond 48.26 kN, the initially strengthened member BT1-1I exhibited slightly higher stiffness than member BT1-2I. The difference in stiffness was observed due to the increase in spacing of CFRP strips along the transversal direction. The mid span deflection of the strengthened specimens BT1-1I and BT1-2I were respectively 1.2 and 2.02 times the deflection of the control beam BT1a. But the deflection of these beams had smaller value than the control beam for the same load. The load–deflection curve of the precracked/repaired beam BT1-1 was stiffer compared to initially strengthened specimens BT1-1I and BT1-2I. The load–deflection profile shows that the precracked/repaired or initially strengthened beam failed suddenly without any yielding plateau. Fig. 5b illustrates the comparison of applied load versus mid deflection for subgroup BS1. Specimens BS1-1 and BS1-2 have similar stiffness up to an applied load of 41 kN. Beyond this load, there was a reduction in stiffness in the beam BS1-2. This behaviour was also observed in subgroup BT1 as well due to the increase of spacing of

CFRP strips. Specimens BS1-1 and BS1-2 had observed a maximum deflection of 14.85 mm and 11.35 mm at failure load which is greater than the control beam BS1a. The load–deflection relationship of specimens BS1-1 and BS12 indicates that there was a sudden increase in deflection at the peak load due to the catastrophic shear along with CFRP fracture failure. The trend also indicates that specimens BS1-1 and BS1-2 have similar stiffness in the precracked and strengthened phases as in specimen BT1-1. Similar to the subgroups BT1, the mid deflection of the CFRP strengthened specimens in subgroup BS1 had smaller value than the control beam for the same load. At failure load, these CFRP strengthened specimens had greater deflection value over the control beam. Fig. 5c portrays the curves of the applied load versus mid deflection of the specimens in subgroup BT2. The stiffness of the specimen BT2-1 was similar in both the precracked and repaired phases due to the presence of external CFRP reinforcement. However, in the specimen BT2-2, the deflection curve of the repaired phase shows

J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165 C3

F6

F7

C8

180 150 A p p li e d L oa d (k N )

no similarity with the precracked phase due to the damage occurred during handling operation. In this subgroup, the members BT2-1, BT2-2 and BT2-2I have exhibited similar stiffness. There was a proportional increase in deflection with the increase in load before the failure load. However the deflection was increased abruptly at ultimate failure load. The mid deflection of the precracked/repaired (BT2-1 and BT2-2) and initially strengthened beam (BT22I) were almost identical before the failure load occurred. Similar to subgroups BT1 and BS1, the CFRP strengthened specimens in subgroup BT2 had smaller deflection value than the control beam for the same load. But the deflection at failure load of the strengthened beams was greater in comparison to the control specimen. The load–deflection profile of the specimens in the subgroup BS2 is shown in Fig. 5d. Similar to the subgroups BT1, BS1, and BT2 the precracked/repaired beams BS2-1 and BS2-2 had similar stiffness in the precracked and repaired phases. In the initially strengthened specimen BS1-1I, the trend of stiffness remained similar to the specimen BS2-2I before the failure load. No significant change in stiffness was observed in any of CFRP wrapped beams. These CFRP wrapped beams attained less shear capacity due to the premature flexural mode of failure. It also shows that this subgroup attained significant ductile behaviour compared to other subgroups BT1, BS1 and BT2. The tested specimens attained a yielding plateau at failure load in this subgroup BS2. The maximum deflection of the members BS2-1I, BS2-2 and BS2-2I were less than the control specimen BS2a. However the beam BS2-1 has attained greater deflection compared to the other specimens.

120 90 C8

60 C4

30

F2

0 -300

-200

-100

150mm

0

100 200 Strain (10-6)

300

400

F1

C2

F6

C8

C3

F4

F5

180 150 120 90 60

C8 F5 C3

30 F1

C3

C4

F6

F7

C8

200mm

0 100 Strain (10-6)

C3

C4

180

180

150

150

90

C8

60

C4 F2

30

F1

F6 F5

F7

200

F5

C8

120 90 C8

60 C4

C3

F2

30 F1

F6 F5

150

300

450

600

750

-6

Strain (10 ) Fig. 6a. Load versus surface strain in CFRP fabric strips and concrete surface for precracked/repaired specimen BT1-1.

-300

-200

-100

F7

C3 150mm

0

0

300

F6

150mm

0 -150

-100

F4

Fig. 6c. Load versus surface strain in CFRP fabric strips and concrete surface for initially strengthened specimen BT1-2I.

F1

120

-300

-200

Ap plied Load ( kN)

A p p l ie d Lo ad ( k N )

F2

F5

-300

F6 C7

C2

0

F1

500

Fig. 6b. Load versus surface strain in CFRP fabric strips and concrete surface for initially strengthened specimen BT1-1I.

6. Surface strain in concrete surface and CFRP strips Figs. 6a–6k illustrate the strain distribution in CFRP strips and concrete surface for beams BT1-1, BT1-1I, BT1-2I, BS1-1, BS1-2, BT2-1, BT2-2, BS2-1, BS2-2, BS2-

F7

F6 F5

C3

F1

A ppl ied Load (kN )

1160

0

100

200

300

-6

Strain (10 ) Fig. 6d. Load versus surface strain in CFRP fabric strips and concrete surface for precracked/repaired specimen BS1-1.

J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165 F1

C2

C3

F5

F6

C8

F1

F4

1161

C2

F5

C8

180

180 150 F1

Appl ied Lo ad (kN)

Appli ed Lo ad (k N)

150 120 90 C8

60

F6 C7

F5 C3

30

F1

-200

-100

200mm

90 60

F4

30 0

100

200

-300

300

0

300

F4

900

1200

1500

Strain (10 )

Fig. 6e. Load versus surface strain in CFRP fabric strips and concrete surface for precracked/repaired specimen BS1-2. C2

600 -6

Strain (10-6)

F1

C8

C2

120

200mm

0

F7

C2

0 -300

F6 F5 F4

C3

F5

F7

Fig. 6h. Load versus surface strain in CFRP fabric strips and concrete surface for precracked/repaired specimen BS2-1.

C8

F1

C2

180

F4

F5

C8

180

C8 F7

A p plied L oad (kN )

150

F6 F5 C3 F4

150

C2

90 60

C8 F6

30

C3 F1

-200

-100

90 60 30

150mm

0

100

200

0

300

-300

C2

-200

-100

100

200

F7

Fig. 6i. Load versus surface strain in CFRP fabric strips and concrete surface for precracked/repaired specimen BS2-2.

C8 F1

180 F7 F5

150

C2

C3

F5

F7

F6 F4

180

C3

C2 150mm

F1

150

80mm

Ap p lied Load (kN )

120 90 60 30

120 90 C8

60

F7 F6 F5 C3 F4

30

F1

C2 150mm

-200

-100

80mm

0

0 -300

0

300

Strain (10 )

F5

C8

0 -6

Fig. 6f. Load versus surface strain in CFRP fabric strips and concrete surface for precracked/repaired specimen BT2-1.

Applied Load (kN)

80mm

120

F5

Strain (10-6)

F1

150mm

C2

0 -300

F4

F7

Appl i ed Load (k N)

F1

120

100

200

300

-6

Strain (10 ) Fig. 6g. Load versus surface strain in surface strain in CFRP fabric strips and concrete surface for precracked/repaired specimen BT2-2.

0

50

100

150

200

250

300

-6

Strain (10 ) Fig. 6j. Load versus surface strain in CFRP fabric strips and concrete surface for initially strengthened specimen BS2-2I.

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J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165 F1

C2

specimens with vertical CFRP U-strips, these beams had less strain in fibres prior to the initiation of diagonal cracks. The change in orientation not only increases the shear capacity but also prevents the formation of diagonal cracks when compared to specimens strengthened with U-strips. The secondary fibre (45 deg) strain gauges which oriented perpendicular to principal fibres were failed to measure any strain readings in specimens BT2-2, BS2-2 and BS2-2I. The concrete compressive strains in the initially strengthened and precracked/repaired specimens were less than the concrete strain limit.

F5

180

Ap plied L oad (kN)

150 120 90 C8

60 F6 C3

30

F1

F4

F7

F5

C2 150mm

0 0

100

200

300

400

500

600

700

Strain (10-6) Fig. 6k. Load versus surface strain in CFRP fabric strips and concrete surface for initially strengthened specimen BS2-1I.

2I and BS2-1I respectively. In specimens BT1-1, BT1-1I, BT1-2I, BS1-1, BS1-2, BT2-1, BS2-1 and BS2-1I the strain gauges were placed 90 deg and 0 deg (i.e. in fibre direction) to the longitudinal axis of the beam. But the strain gauges in the specimens BT2-2, BT2-2I, BS2-2 and BS2-2I were oriented 45 deg and 135 deg to the longitudinal axis of the beam. The strains in CFRP fabric strips were measured with respect to the directions of the fibres. The principal or vertical CFRP fibres (90 deg) strain value in the CFRP strengthened specimens BT1-1 (F5), BS1-2 (F4 and F6), BT2-2 (F4), BT2-1 (F4), and BS2-1I (F5) were very low prior to the diagonal crack formation which then increases rapidly until failure. It shows that CFRP strip stressed prior to failure, indicating the effectiveness of the CFRP strip resisting in shear. In specimens BT1-1I, BT1-2I, BS1-1, and BS2-1 observed less strain in CFRP strips but the concrete strain increases suddenly due to the appearance of diagonal crack near the location of strain gauges. It can also be seen from Fig. 6a through 5e that the secondary or horizontal (0 deg) CFRP strains in both the initially strengthened and precracked/repaired specimens were relatively less with the vertical strain in the CFRP strips. It indicates that the horizontal fibres in CFRP strip were not carrying any load like vertical CFRP fibres. But these fibres were probably acting as restraint to prevent the debonding of CFRP strip from the concrete surface. In the initially strengthened beams, the concrete surface between the CFRP strips starts to crack due to the increase of load but the CFRP strip became active after the formation of crack in the concrete surface. However in the case of precracked/repaired beam, the composite fabric begins to be stressed from the beginning of load. For the specimens BT2-2, BS2-2 and BS2-2I strengthened with inclined L-strips had less CFRP fibre (135 deg) strains in specimens wrapped with vertical U-strips. It shows that the inclined CFRP orientation prevents the formation of diagonal crack since the CFRP strips were placed perpendicular to diagonal crack. Similar to the

7. Evaluation of the shear capacity of the CFRP strengthened beam In customary shear design approach, the shear strength of a reinforced concrete section may be computed by the adding the contribution of shear strength of the concrete and steel reinforcement [25]. But in the case of beam with externally bonded FRP sheets, the nominal shear strength may be formulated by the addition of a third component to account for the contribution of FRP sheet to the shear strength. For RC beam strengthened with externally bonded composite material, the nominal shear strength of the externally strengthened concrete section is expressed as follows: Vn ¼VcþVsþVf

ð1Þ

In 2003, the ACI committee 440 [26] proposed FRP contribution to shear strength of the FRP bonded beams. The shear contribution of the FRP shear reinforcement can be computed by the following equation: Vf ¼

Af f fe ðsin b þ cos bÞd f sf

ð2Þ

In this equation, the component tensile stress in the FRP reinforcement at ultimate is replaced by effective strain times the tensile modulus of FRP Ef ffe ¼ efe Ef

ð3Þ

The effective strain efe is the maximum strain that can be achieved in the FRP system at the ultimate load and is governed by the failure pattern of FRP strengthened beam. The subsequent equations provide direction to determine the effective strain efe for different configuration of FRP laminates used for shear strengthening of reinforced concrete members efe ¼ 0:004 6 0:75efu ðfor completely wrapped membersÞ efe ¼ k v efu 6 0:004 ðfor bonded U-wraps or bonded face pliesÞ

ð4Þ ð5Þ

The bond reduction coefficient kv is a function of the concrete strength, the type of wrapping scheme used, and the stiffness of the laminate. The bond reduction coefficient can be computed as follows:

J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165

k 1 k 2 Le 6 0:75 ð6Þ 11; 900efu The active bond length Le is the length over which the majority of the bond stress is maintained. The length is given by equation 23; 300 Le ¼ ð7Þ
0:58 ntf Ef

1163

uted by the external CFRP reinforcement was estimated by subtracting the shear strength of the reference beam from the CFRP strengthened beam. The strip technique had proved that the shear capacities of the precracked/ repaired and initially strengthened rectangular beams without internal stirrups were ranging from 11% to 139% over the control beam. The shear capacity of these rectangular beams was theoretically computed using ACI 440 Format [26]. Fig. 7 compares the experimental results with theoretical values of both the precracked/repaired and initially strengthened rectangular beams. The shear strength of the strengthened beams is computed by adding the contribution of shear strength of the concrete ‘Vc’ and external CFRP reinforcement ‘Vf’ since these beams were tested in the absence of internal shear reinforcement. It can be seen that the experimental values of the strengthened beams BS1-1, BS1-2, BT1-1, and BT2-2 had good agreement with the theoretical values but the specimens BT1-1I, BT1-2I, BT2-1, BT2-2I, and BS2-1 were relatively greater in comparison to the predicted values. The computed shear capacity of the specimens BS2-1I, BS2-2, and BS2-2I were less than the experimental results. From the overall discussion, it can be concluded that the predicted theoretical results of the rectangular beams without internal shear

kv ¼

The bond reduction coefficient also relies on two modification factors, k1 and k2, that accounts for the concrete strength and the type of wrapping scheme used respectively. Expressions for these modification factors are given as follows.  0 2=3 fc k1 ¼ ð8aÞ 27 d f  Le k2 ¼ ðfor U-wrapsÞ ð8bÞ df d f  2Le k2 ¼ ðfor two sides bondedÞ ð8cÞ df The comparison of experimental and theoretical results of the control, precracked/repaired and initially strengthened beams are shown in Table 3. The shear capacity contrib-

Table 3 Summary of comparison of experimental and theoretical results Specimen description

Experimental results

Theoretical results (ACI 440 format)

Ultimate Load (kN)

Contribution of CFRP (kN)

Shear enhancement (%)

Shear force (kN)

Vc (kN)

Vf (kN)

Vn = Vc + Vf (kN)

BT1aa BT1-1 (precracked– repaired) BT1-1I (initially strengthened) BT1-2I (initially strengthened)

98.14 134.73

– 36.59

– 37.28

49.07 67.365

38.45 38.45

– 27.38

38.45 65.84

174.64

76.50

77.95

87.32

38.45

27.38

65.84

134.73

36.59

37.28

67.365

38.45

20.53

58.99

BS1a BS1-1 (precracked– repaired) BS1-2 (precracked– repaired)

74.86 121.42

– 46.56

62.19

37.43 60.71

33.12 33.12

– 27.38

33.12 60.50

101.46

26.61

35.55

50.73

33.12

20.53

53.66

BT2a BT2-1 (precracked– repaired) BT2-2 (precracked– repaired) BT2-2I (initially strengthened)

64.88 134.73

– 69.85

– 107.66

32.44 67.365

32.63 32.63

– 23.36

32.63 55.99

121.42

56.54

87.15

60.71

32.63

33.04

65.66

154.68

89.80

138.41

77.34

32.63

33.04

65.66

BS2a BS2-1 (precracked– repaired) BS2-2 (precracked– repaired) BS2-2I (initially strengthened) BS2-1I (initially strengthened)

61.56 108.19

– 46.64

– >75.79

30.78 54.095

28.10 28.10

– 17.52

28.10 45.62

81.51

19.95

>32.44

40.755

28.10

33.04

61.14

88.16

26.60

>43.23

44.08

28.10

33.04

61.14

68.21

6.65

>10.82

34.105

28.10

23.36

51.46

–

1164

J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165

Exp

The

BS2-2I

BS2-2

BS2-1I

BT2-2I

BT2-2

BT2-1

BT1-2I

BT1-1

BT1-1I

BS2-1

60

BS1-2

Shear Force (kN)

80

BS1-1

100

40

20

0

Specimen Fig. 7. Comparison of experimental results with theoretical values of precracked/repaired and initially strengthened rectangular beams.

reinforcement shows reasonable accuracy with the experimental results. 8. Conclusions The experimental results demonstrate that the shear strengthening capacity of the rectangular shear beams using externally bonded Bi-Directional Carbon Fibre Reinforced Polymer strip approach significantly enhanced the shear capacity. The overall shear capacity of the CFRP strengthened beams varies between 11% and 139% over the control beam. From the experimental investigation, it was found that this CFRP strip approach not only increased the strength of the precracked/repaired or initially strengthened beams but also controls the debonding of strip from the concrete stratum. The shear capacity and modes of failure of the strengthened beams were examined with different test variables. The following conclusions are drawn based on the experimental and theoretical investigation of RC rectangular shear beams.  The study points out that the bi-directional CFRP strip technique is more attractive and economical for repairing or upgrading the reinforced concrete beams. This technique is a reliable one to reduce the material quantity and cost.  This study confirms that the bi-directional CFRP strip strengthening technique contributes shear capacity to RC rectangular shear beams. It shows that the external CFRP strip acts as shear reinforcement similar to the internal steel stirrups.  Experiments indicate that the RC strengthened beams had two types of failure mode at ultimate; flexural and shear with CFRP rupture. However, no debonding of CFRP strips from any of the strengthened specimens. This may probably due to the presence of secondary fibres (i.e. 0 and 135 deg) acting as restraint.

 The shear strength of the strengthened beam was increased up to a maximum value of 76% with the increase in longitudinal tensile reinforcement ratio by 56%. It shows that by increasing the amount of longitudinal tensile reinforcement ratio affects the shear capacity of the CFRP wrapped beams.  The spacing of CFRP strips also affects the shear capacity of the precracked/repaired or initially strengthened specimens.  It was found that the specimens with inclined CFRP orientation of 45/135 deg (L-strips) had observed lesser crack distribution and propagation than the specimens with CFRP orientation of 0/90 deg (U-strips) to the longitudinal axis. The shear capacity of specimens with 45/ 135 deg CFRP orientation attained a maximum percentage of 42% higher than those with the CFRP orientation of 0/90 deg to the longitudinal axis. It can be concluded that the change in orientation of CFRP strip influences the ultimate load carrying capacity and the CFRP shear contribution of the strengthened specimen. The result also indicates that the orientation of CFRP strips not only affects the cracking pattern but also affects the shear capacity.  This study also found that the CFRP shear contribution of the strengthened specimen with av/d ratio of 4.0 achieved a maximum value of 83% greater than the specimen with shear span to effective depth ratio 2.5.  Application of the external CFRP shear reinforcement strip arrests the propagation and widening of earlydeveloped precracks during the precracking phase of the precracked/repaired beams.  The stiffness of the CFRP strengthened specimens increases with the decrease of spacing of CFRP strips. However there was no significant difference in stiffness of the strengthened beams with the change in orientation of CFRP strips.  The strain on the unwrapped concrete surface (i.e. between CFRP strips) is greater than the surface strain of the CFRP strips.  Specimens in subgroups BT1, BS1 and BT2 were failed along the longitudinal reinforcement with shear-CFRP fracture due to the inadequate anchorage by the tensile steel reinforcement.  The shear capacity of the external bonded CFRP rectangular beams without internal shear reinforcement shows reasonable accuracy with the results of the ACI 440 format [26]. With the decrease of tensile reinforcement ratio, the obtained shear force of the strengthened beams was less than theoretical value. This may probably due to the influence of the attained tensile strength of longitudinal tensile rebars (i.e. fy = 311.22 MPa) which was relatively lesser than the nominal value (i.e. 460 MPa).  The effectiveness of the FRP can be achieved by carefully preparing the surface, selecting the epoxy and placing the fibre strips or sheets.

J. Jayaprakash et al. / Construction and Building Materials 22 (2008) 1148–1165

8.1. Applications of CFRP strip technique Recommendations for the application of the bi-directional CFRP technique are as follows:  It recommends not to use low width CFRP strip (i.e. <80 mm) in order to attain greater strength. Furthermore it is having practical difficulties including the placing of strip on the concrete stratum.  The bi-directional CFRP U-strips with 0/90 deg orientation and L-strips inclined at 45/135 deg to the longitudinal axis of the beam are recommended for the shear strengthening of the reinforced concrete beams.

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