Seismic performance of reinforced concrete beam–column joint strengthening by frp sheets

Structures 20 (2019) 353–364

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Seismic performance of reinforced concrete beam–column joint strengthening by frp sheets

T

⁎

Nassereddine Attaria, , Youcef Si Youcefa, Sofiane Amzianeb a b

Ecole Polytechnique d'Architecture et d'Urbanisme (EPAU), Laboratoire Ville Architecture et Patrimoine (LVAP), BP177, El-Harrach, Algiers, Algeria Université Blaise Pascal, Polytech Clermont Ferrand, Département Génie Civil, 63174 Aubière, France

A R T I C LE I N FO

A B S T R A C T

Keywords: Beam column-joint Strengthening Seismic performance Hybrid FRP Fibre reinforced polymer

Reinforced concrete beam-column joints are generally recognised as critical regions in frame structures which experience severe inelastic deformations during earthquakes. This paper presents the results of an experimental campaign to investigate the optimal seismic performance of a method of strengthening reinforced concrete beam-column joints. Ten reinforced concrete beam-column joints, at a scale of one to three (1/3), were subjected to reverse cyclic loading under constant axial load to simulate an earthquake, and tested with the deflection being monitored. The joints were strengthened with fibre-reinforced polymer (FRP) systems, made up of a combination of carbon and fibreglass fabric, and a hybrid braided FRP fabric. Of these, four samples were strengthened with different FRP systems, while two were used as benchmarks. Once damaged, four specimens were repaired with FRP and re-tested. Strain and cracking fields were monitored using an Aramis digital video camera. The test results provided useful information on the strengthening configurations adopted, in relation to strength, ductility and energy dissipation capacity. The results establish the efficiency of glass fibre GFRP in upgrading deficient beam-column joints in an equivalent proportion than carbon fibre CFRP. It was observed that using of a hybrid sheet (Glass-Carbon) improves the ductility and dissipation energy of the RC joints to a great extent, with a highly competitive cost.

1. Introduction Many old buildings made of reinforced concrete column-beam structures, and built according to older regulations, are potentially vulnerable to seismic action. Therefore, in the event of an earthquake, the energy dissipation and ductility capacities of these structures are heavily dependent on the stability of the beam-column joints. Several non-conventional methods have been used to improve the seismic performance of RC beam-column joints, including injection of epoxy glue or special grouting [1–3]; use of profile bars or steel plates with a cage system [4–10]; and the use of reinforced concrete jackets [11–13]. With the emergence of composite materials, fibre-reinforced polymers (FRPs) have become the option of choice for strengthening and retrofitting RC structural elements, due to their high strength-to-weight and stiffness-to-weight ratios [14–23]. An interesting property of FRPs is their flexibility for use with different forms. As such, they are well suited for use with beam column joints, which are characterised by their relatively complex geometric

⁎

forms. On the other hand, composite materials have certain drawbacks, such as vulnerability to delamination and brittle failure. It is therefore important to gain a better understanding of how FRPs can be used for strengthening and/or repairing concrete structures in order to take full advantage of their benefits. Prota [24] used a combination of carbon rods and a carbon-fibre wrap to retrofit 11 beam-column joints, and demonstrated that the combined action of internal and external retrofitting improves the ductility of the joint. Le Trung [25] examined eight joints retrofitted with different configurations of a carbon-fibre fabric wrap. The study also included specimens that did not comply with seismic resistance requirements. Ductility was found to increase significantly and the failure mode changed to bending failure at the beam. They found a 31% increase in strength and ductility, which is a fivefold greater increase than with the non-conforming specimen. No tangible improvements were found with L-shaped reinforcements. Lee Chiou and Shih [26] tested three reinforced concrete beamcolumn joints at a scale of 1/3, retrofitted externally with a carbon-fibre wrap and anchored with plate steel but with no transverse

Corresponding author. E-mail address: [email protected] (N. Attari).

https://doi.org/10.1016/j.istruc.2019.04.007 Received 31 December 2018; Received in revised form 5 April 2019; Accepted 9 April 2019 2352-0124/ © 2019 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.

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detailed in Fig. 1, except for the control specimen NC2, which was fitted with steel stirrups at the joint. Four longitudinal steel bars of 8 mm in diameter were used for both the beam and the column. Steel stirrups, 6 mm in diameter, spaced at 100 mm intervals, were used as shear reinforcement. The beams and the column had a rectangular cross section of 100 × 150 mm.

reinforcement in the joints. They observed that the energy dissipated by the joint was 90% higher than the control. Stiffness and resistance also increased. The behaviour of the unanchored joint was similar to that of the control, which speaks to the importance of anchoring. Several authors have used glass-fibre reinforced polymers (GFRPs) to retrofit beam-column joints [27–29], demonstrating that GFRP anchors increase shear strength and ductility in reinforced concrete. The mode of failure changed from shearing to bending, and energy dissipation was higher than the control. Antonopoulos and Triantaffillou [30] tested 18 joints retrofitted with CFRP (Carbon-Fibre Reinforced Polymer), GFRP wrap and plate. Several parameters were studied, including the effect of axial loading. The authors found that a 150% increase in axial load increased the load capacity by 65 to 85%. They also found that the initial damage had a considerable impact on the response of the damaged specimen. In terms of strength, fibreglass and carbon fibre produce similar results. On the other hand, energy dissipation is 10–20% greater with fibreglass, and adequate anchoring is required. Other authors have studied hybrid strengthening [31–35], and have confirmed that the use of a hybrid composite with a low elastic modulus contributes to good adhesion and prevents de-bonding of the retrofit. Unfortunately, there are some drawbacks associated with using FRP systems for strengthening, such as brittle failure and linear stress-strain behaviour to failure, without a discernible yield plateau. Considering that fibreglass has better deformation properties and exhibits much greater energy dissipation, the objective of this article is to present a hybrid glass and carbon FRP strengthening system designed to mitigate these drawbacks. The paper also reviews some important findings of an experimental study to assess the effectiveness of this system in strengthening reinforced concrete beam-column joints.

3.2. Material properties 3.2.1. FRP fabric The fibres used for the experiments were:

• A unidirectional carbon fibre fabric wrap. • A unidirectional glass fibre fabric wrap. • A bidirectional hybrid fibre fabric wrap (Warp: 21% carbon, 29% glass; Weft: 21% carbon, 29% glass)

The main characteristics of the manufactured FRP strengthening materials used for the tests are shown below in Table 1. 3.2.2. FRP composite The term “composite” refers to the structure combining the matrix fabric and a synthetic resin. The composite, which was prepared on site, comprises approximately 60% fabric and 40% epoxy resin. These proportions may vary depending on the conditions of use on site (Fig. 2). The mechanical properties of the test FRP composite were determined and are shown in Table 2. The tests were performed in accordance with the NF T57–101 standard, which is equivalent to the ASTM D638 standard. However, due to the high sensitivity of the experimental conditions, the results should be viewed with a grain of scepticism, especially as regards composite thickness.

2. Research objectives 3.2.3. Concrete and steel The mean compressive strength of the concrete cylinders was 39 MPa. The yield strength of the steel bars used as tensile, shear and compressed reinforcement was determined using standard tensile test methods. We obtained a mean value of 500 MPa.

The main objectives of this work are to assess the effectiveness of FRP strengthening or repair on reinforced concrete beam-column joints. We investigated the choice of hybrid fibres and strengthening configurations in order to optimise the behaviour of the reinforced element and improve its deformability. Indeed, this structural property is of critical importance in areas of significant seismic activity.

3.3. Test set-up and instrumentation

3. Experimental

The specimens were strengthened using the FRP materials shown in Table 2. Prior to applying the FRP, the concrete substrate was smoothed by grinding and cleaned. The cement paste was removed from the surface and the coarse aggregates were exposed. The corners of all elements were ground to create a flared shape. All the beams were subjected to reverse cyclic loading to failure

3.1. Tested specimens 1/3-scale reinforced concrete interior beam-column joint specimens were prepared for this study. The specimens and reinforcement used are

100

A-A 4T8 A Stirrups Ø6 @100mm

A B

B 150

4T8

PVC reservation sleeve for threaded rods

B-B 100

Fig. 1. Specimen reinforcement 354

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Table 1 Mechanical properties of FRP materialsa. Material

Modulus of Elasticity GPa

Tensile Strength MPa

Fibre orientation

Thickness mm

Elongation at failure

Area density g/m2

CFRP/fibre SikaWrap®230C GFRP/fibre SikaWrap®430G Hybrid fabric HFRP(Hexcel) Sıka Carbodur Epoxy Sika330 Epoxy Sika 30

238 76 – 165 3.8 12.8

3650 2200 – 2800 30 30

Unidirectional Unidirectional Bidirectional Unidirectional – –

0.13 0.17 – 1.2 1 1

1.5% 2.8% – 1.7% 0.9% –

225 430 274 – 500 g –

a

Manufacturer's data from SIKA and HEXCEL (fibre properties).

of the beam (Fig. 6).

using a ± 250 kN capacity hydraulic machine (Swick) (Fig. 4). The supports were made from hardened steel plates that were cut and formed to a suitable thickness to sustain the applied load without any deformation that could affect the test results. The bottom of the column was attached to the machine through a slab pad with special bolts. The column was subjected to a constant prestressed axial load of 100 kN, which represents about 25% of the ultimate load-bearing capacity of the column. Deflection of the beam specimens was measured at the tip of the beam using displacement transducers (Linear Variable Differential Transformers – LVDTs) placed respectively on the beam specimens and on the loading arm of the testing machine. The test beams were fitted with strain gauges and a camera for monitoring and measuring the deformation. The load cycle was pre-defined, as shown in Fig. 3. The displacement started from the neutral position and oscillated harmonically about that position until the beam ultimately failed. It increased at a uniform rate of 0.25 mm/cycle, with each cycle consisting of five full waves of the same amplitude at a frequency of 0.3 Hz.

3.4.2. Repair of beam column joint Four joints were repaired. The first two joints – NC1R and NC2R – were the control specimens NC1 and NC2, which were previously damaged and then repaired. The other two joints had been previously damaged (pre-cracked) and then repaired. The purpose of these tests was to study the impact of composite reinforcement on damaged joints and compare their behaviour with the control. 3.4.2.1. Repair Joint NC1R. The damage to the NC1 control specimen resulted in severe failure of the right beam, with a rupture of the steel bars in the upper part (Fig. 11.a). This led us to carry out a three-step repair procedure using three types of epoxy glue. The first step consisted of levelling the beam and thoroughly cleaning the damaged area before closing the cracks by injecting an epoxy glue (SikaDur 52), which is less viscous and penetrates into the smallest fissures. The second step consisted of closing and filling the holes with an epoxy glue (SikaDur 30), as shown in Fig. 7.a. The third step consisted of applying the FRP reinforcements. We began by introducing four pultruded carbon elements (Sika CarboDur) into each of the anchor grooves created in the column, in the same way. We then applied a second L-shaped GFRP fabric reinforcement and a Ushaped GFRP fabric reinforcement to the bottom of the beams. The repairs are shown in Fig. 7b and c.

3.4. Strengthening and repair configurations 3.4.1. A and B series The NA2 and NA3 specimens were reinforced respectively using a carbon fabric wrap and a bidirectional hybrid fabric. L-shaped reinforcements were set up at the four corners of the joint (Table 3). The fibres were arranged parallel to the axis of the beams. We then confined and maintained the L-reinforcements with a fabric wrap so that the fibres were perpendicular to the axis. This was done to ensure the beam was properly anchored to the column and to obtain optimal fibre performance. It should be noted that the centre of the joint was left without reinforcement so it could be used with an orthogonal beam (Fig. 5), as it would be in a floor, for example. For joints NB4 and NB5, we considered the case of a floor with a down-stand reinforced beam. Firstly, we applied an adhesive L-shaped fabric reinforcement to each corner of the joint such that the fibres were parallel to the horizontal axis of the beam. We then applied a U-shaped reinforcement to the lower part of the beam such that the fibres were parallel to the axis

Twin layer GFRP+CFRP

3.4.2.2. Repair specimen NC2R. The control specimen NC2 was severely damaged during the tests, with a total rupture of two steel bars on the upper side of the beam and one on the lower side (Fig. 11.b). The repair process was identical to that used for NC1R (Fig. 8.a). The strengthening configuration used was a hybrid combination of GFRP and CFRP, as shown in Figs. 8.b and 8.c). 3.4.2.3. Repair specimen NR1 and NR2. The repair test for these specimens was performed in two steps. The first step entailed precracking the beam-column interface by applying cyclic loading. The pre-cracking damage, with a crack width of about 0.2 mm, was

HFRP, hybrid fabric Glass/Carbon (Hexcel)

Fig. 2. Tested composite materials. 355

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Table 2 Mechanical properties of the composite (fabric and epoxy) materials. Material

Modulus of Elasticity GPa

Tensile strength MPa

Ultimate Load kN/mm

Fibre Orientation

Thickness mm

Elongation at Failure %

CFRP GFRP Hybrid(Twin) CFRP + GFRP Hybrid fabric HFRP

43.5 19.2 28 27

403 325 400 218

0.644 0,650 0.640 –

Unidirectional Unidirectional Unidirectional Bidirectional

1.5 2 1.6 2

0.95 1.7 2.1 0.85

15

5

Displacement (mm)

Displacement (mm)

10

'

0

-5

-10

'

-15 0

100

200

300

400

500

0

20

40

Time(S)

60 Time (S)

80

100

Fig. 3. Graph of the cyclic loading employed in this study.

Hydraulic cylinder

Preloading

Study Area

Fixing the specimen

Camera

Prestressing rod

Fig. 4. View of the test set up.

Confinement Etape2

200

300 Etape 1

L Fabric Wrap 400

FabricWrap

200

600

Step 1

600

Step 2

Fig. 5. Strengthening of specimen NA2 et NA3.

The repair process for the NR1 and NR2 joints was substantially the same as for the previously repaired NC1R and NC2R joints. Before bonding, reinforcement glue (SikaDur epoxy 52) was injected into the cracks, allowing better penetration of the adhesive into the sealing cracks.

determined by assessing the steel strain to ensure that it did not exceed its yield strength, in compliance with Eurocode 2, Section 7.3.4 [36]. Using the camera, we were able to closely monitor the damage process. The test was halted when the width of the cracks located in the corner of the joint reached 0.2 mm (Fig. 9). 356

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Confinement Etape2

200

300 Etape 1

400

200

600 600 Fig. 6. Strengthening specimen NB4 et NB5.

Confinement CFRP

200

300 GFRP

400

200

600 600

a. Repairing step1

c. NC1 Repairing configuration

b. NC1 Repaired step2

Fig. 7. a Repairing step1, b NC1 Repaired step2, c NC1 Repairing configuration GFRP CFRP

200

300

400

200

600 600

a. before strengthening

b. strengthening step 1and 2

c. strengthening step3

Fig. 8. a: before strengthening, b: strengthening steps 1 and 2, c: strengthening step 3.

longitudinal axes.

For the NR1 joint (Fig. 10a), we began by introducing four pultruded carbon elements (Sika CarboDur) into the anchor grooves created in the column. Next, two layers of L-shaped GFRP sheeting were applied on the top and the bottom of the beams at each joint. The fibres ran along the longitudinal axes of the beams. To repair specimen NR2, only GFRP fabric was used. The joint was assumed to have no orthogonal beams. In addition to the L-shaped GFRP sheeting, we applied two layers of fibreglass fabric wrap across the beam joints. The fibres were arranged parallel to the axis of the beams (Fig. 10b). It should be noted that in all strengthening or repair configurations, the columns were confined with GFRP fibres perpendicular to their

4. Results and discussion 4.1. Behaviour of NC1 and NC2 joints The curves in Fig. 11a and b show that the behaviour of control specimens NC1 and NC2 is largely identical save for a slight difference where joint NC2 is concerned. For the control specimen, the first crack was observed at a load of 8 kN (Fig. 11a). The beams failed at the joint, with hinge-type structures being formed. The hinges appeared between the two shear links of the 357

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1.The specimen was mechanically damaged

2.Crack was controlled with the help of the digital camera

3. Pre-Cracking specimen

Fig. 9. Pre-Cracked specimen NR1 & NR2 and measurement of the crack width. GFRP GFRP

200

300

400

200

600 600

a. Specimen NR1 repair

b. Strengthening scheme of the specimen NR2

Fig. 10. a. Specimen NR1 repair, b. Strengthening scheme of the specimen NR2.

with the ductility ratios of the strengthened beams to those of the reference specimen (without external strengthening).

beam. The concrete spelled off in such a way that vertical failure planes were created, resulting in free rotation of the beam with no transfer of bending moment to the column. Under shear stress, the control specimen failed at the joint (Fig. 11a) and attained a maximum load of 11.20 kN and an ultimate deflection of 7.64 mm. The hysteresis behaviour of the control specimen showed considerable pinching with severe deterioration in strength and stiffness.

4.2. Behaviour of retrofitted joints 4.2.1. A series The NA2 specimen is strengthened with a carbon fibre fabric wrap. First-cycle deflection levels (Fig. 12.a) did not induce any nonlinear deformations in the structure, and the loops followed a straight line whose slope indicates initial stiffness. The onset of loss of stiffness was identified, as was the point at which the steel yielded. Residual deformation occurs and the additional load is absorbed by the composite reinforcement. The failure occurred at the interface, with the appearance of shear stress cracks. There was a vertical crack at the joint (Fig. 12.b). The peak load reached was 14.3 kN, with a maximum deflection of 8.9 mm. The reinforcement was identical to that in the A series, except that a bi-directional hybrid fibre fabric was used. Behaviour similar to that exhibited by the NA2 joint was observed. The failure occurred at the column-beam interface (Fig. 13.b), with a shear stress crack that tended to follow the bending of steel rods in the column. The overall behaviour is shown in Fig. 13.a, where we notice a clear increase in initial stiffness. However, the final deflection is limited to 8.36 mm (Table 4) at a peak load of 13.2 kN. Envelope curves (Fig. 14) were plotted in order to compare the effectiveness of the reinforcement against the unreinforced NC1 specimen. The improvement in failure load and stiffness in the reinforced specimens is plain to see: the strength gain was 13% and 5% respectively.

4.1.1. Local measures and ductility Strain measurements, as recorded by the Gom-Aramis camera (Fig. 11a), were taken at the beam-column interface crack. Greater deformation was observed in the steel rods in specimen NC2 due to the stirrups in the joint, which afforded better deformability. The change in the cyclic tensile properties of the steel beams shows that, before 2% deformation, the interface crack is opened and closed (Fig. 11c), so the joint can be considered safe. However, beyond this level of deformation, the steel begins to yield. The whole crack is opened, leaving residual deformation, and the steel-concrete bond is affected. Ductility is an important parameter for earthquake-resistant structures. The ductility factor is computed as the ratio of ultimate deflection to the deflection at initial yield of the internal steel (Table 4). For strengthened beams, Henrik Thomson et al. [37] have used the energy-based definition of ductility (Fig. 11d). The energy absorbed by the specimen during loading is quantified as the area under the ‘load vs. deflection’ curves. For the energy model, ductility (μE) is defined as the ratio between the energy of the system at failure Eu and the energy at first yield of the steel Ey. The computed ductility indexes are shown in Table 4, along 358

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20

Load (kN)

10

-10

0

-5

0

5

10

-10

-20 Deflection (mm)

a 20

Load (kN)

10

-10

0

-5

0

5

10

-10 -20 Deflection (mm)

b

20

20

0 -0.001 -10

0.009

Load (kN)

Load (kN)

NC2 bottom

Strain NC1 Top beam

10

0.019

10 0 -0.001

0.009

0.019

-10 Strain (mm/mm)

-20

Strain (mm/mm)

-20

Force

Force

c

Ey ∆y Displacement

Eu Displacement ∆u

d Fig. 11. a: Behaviour and failure mode specimen NC1. b: Behaviour and failure mode specimen NC2. c: Concrete strain NC1 and NC2. d: Definition of Ductility Index.

The hysteresis curve for the specimen (Fig. 15.a) illustrates this finding. The joint was reinforced by applying four L-shaped layers (2CFRP + 2GFRP) on the top of the beam, and 2 continuous GFRP bending reinforcements on the bottom. From the hysteresis curve (Fig. 15.a), it can be seen that the behaviour is better in the ascending portion than in the descending branch. The maximum load reached in both parts was 15 kN (Table 4). In the bottom part, the behaviour was very ductile, due primarily to the flexural reinforcement, with a deflection in excess of 13.5 mm. In the descending part, which corresponds to the upper part of the beam, there was a severe drop in load capacity. Behaviour was less ductile, with a deflection of 9 mm. In the failure mode for this specimen, cracks appeared in critical

It should be noted that the two configurations used for the reinforced specimens were approximately the same in terms of deflection or dissipated energy (see Table 4), which leads us to conclude that hybrid reinforcement, as opposed to carbon reinforcement alone, can have a similar strengthening effect. 4.2.2. B series The aim of this series was to study the influence of two hybrid strengthening systems on the behaviour of the beam-column joint, placed on the down stand. The asymmetry of the composite bending reinforcement on the beams (Fig. 6) resulted in behaviour which was asymmetrical between the top and bottom of the beam. 359

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20

Load (kN)

10 0

-15

-5

5

15

-10 -20 Deflection (mm)

a : Hysteresis curve spécimen NA2

b: Failure mode specimen NA2

Fig. 12. a: Hysteresis curve spécimen NA2, b: Failure mode specimen NA2.

20

Load (kN)

10

0

-15

-10

-5

0

5

10

15

-10

-20 Deflection (mm)

b: failure mode specimen NA3

a: Hhysteresis curve specimen NA3

Fig. 13. a: Hhysteresis curve specimen NA3, b: failure mode specimen NA3.

appearance of shear stress cracking (Fig. 16.b). A more pronounced vertical crack appeared on the right side, with the steel rods at the top rupturing. The peak load reached was 15.4 kN, with a maximum deflection of 13.8 mm (see Table 4) in the ascending part and 9 mm in the descending part. The envelope curves in Fig. 17 offer an overview of the effect of hybrid fibre reinforcement on the behaviour of the joint. In this way, a comparison can be drawn between the different strengthening systems. The graph shows that the behaviours of specimens NB4 and NB5 are extremely similar: in terms of strength, both joints attain a peak load of 15 kN. On the other hand, specimen NB4 has better ductility (mainly due to the GFRP reinforcement against bending), as evidenced by the substantial post-peak deflection of the bottom reinforcement of the beam, which corresponds to the ascending part of the graph. This results in significant energy dissipation, with a gain of 78% (Table 4). This energy is represented by the area below the curve for the specimen NB4.

20 env NA2 env NA3

10

Load (kN)

env NC1

-15

0 -10

-5

0

5

10

15

-10

-20 Deflection (mm) Fig. 14. Envelop of the hysteresis loops NA2, NA3.

areas (Fig. 15b). Vertical cracks appeared at the interfaces due to tensile forces on the concrete, with hinges appearing prior to ultimate failure. This failure occurred very rapidly on the top part due to the weak anchoring of the reinforcement, compared to the bottom part. At the end of the test, we found the steel rods at the top right of the beam had ruptured. For the joint NB5, we used the same strengthening configuration design as the previous sample (NB4), but with a hybrid fabric composite reinforcement (HFRP). The behaviour observed was very similar to that of the NB4 joint (Fig. 16.a). Over the first few cycles of loading, the fibres on the lower part of the beam stiffened. On the other hand; when the reinforcement was not sufficiently anchored and the beam was not reinforced against bending, there was considerable deformation and a rapid loss of stiffness. The failure occurred at the interface, with the

4.3. Response of rehabilitated specimens 4.3.1. Behaviour and failure mode of NC1R The hysteresis curve representing overall load versus deflection behaviour for specimen NC1R (Fig. 18.a) is asymmetrical. The descending part of the graph shows a sudden drop in stiffness and strength of the beam. This is due to the severity of the damage before repair: mainly to steel bars that had previously been ruptured in the upper part of the beam. Hence, it can be concluded that the rupture of the steel bars caused irreversible failure of the beam. At this stage, the contribution of external reinforcement is very limited, and the original performance level cannot be regained by repair. The failure mode was shear 360

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20

Load (kN)

NB4

-20

10 0

-10

0

10

20

-10 -20 Deflection (mm)

a: Hysteresis curve specimen NB4

b: Failure mode specimen NB4

Fig. 15. a: Hysteresis curve specimen NB4, b: Failure mode specimen NB4.

20

Load (kN)

NB5

-20

-10

10 0

0

10

20

-10 -20 Deflection (mm)

a: Hysteresis curve specimen NB5

b: failure mode specimen NB5

Fig. 16. a: Hysteresis curve specimen NB5, b: failure mode specimen NB5.

(Fig. 18.b). The ascending part of the graph shows improvement, which corresponds to the reinforcement of the lower part of the beam. The steel bars in this part did not rupture, so the reinforcement provides greater strength, and the repair resulted in better behaviour and greater energy dissipation than with control specimen NC1. When the envelope curve of control joint NC1 before repair (Fig. 18.a) is compared with joint NC1R after repair, the difference is found to be more than significant, showing that, with the failure of steel bars, the repair does not deliver the expected results unless the reinforcement is sufficiently anchored to replace the steel bars.

20 env NB5 env NC1

-20

10

0 -10

0

10

20

-10

-20 Deflection (mm)

4.3.2. Behaviour and failure mode of NC2R The deflection-load curve for this specimen (Fig. 19a) shows that the repair did not produce the expected results. The joint was restored but its behaviour did not even begin to match that at the initial stage. Initially there was a rapid increase in stiffness, but this dropped

Fig. 17. Envelope of the hysteresis loops for B series

20 NC1R env NC1

Load (kN)

Load (kN)

env NB4

-20

10

0 -10

0

10

20

-10

-20 Deflection (mm)

a: Hysteresis curve of specimen NC1R

b: Failure mode specimen NC1R

Fig. 18. a: Hysteresis curve of specimen NC1R, b: Failure mode specimen NC1R. 361

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20 NC2R

10

Load (kN)

env NC2

0

-20

-10

0

10

20

-10

-20 Deflection (mm)

a: Hysterisis curve of specimen NC2R

b : Failure mode specimen NC2R

Fig. 19. a: Hysteresis curve of specimen NC2R, b: Failure mode specimen NC2R.

25 NR1

15

Load (kN)

5

-15

-10

-5 0

-5

5

10

15

-15 -25 Deflection (mm)

a: Hysteresis curve of specimen NR1

b: Failure mode specimen NR1

Fig. 20. a: Hysteresis curve of specimen NR1, b Failure mode specimen NR1.

25 NR2

15

Load (kN)

env NC1

5

-15

-10

-5

-5 0

5

10

15

-15 -25 Deflection(mm)

a: Hysteresis curve of specimen NR2

b : Failure mode specimen NR2

Fig. 21. a: Hysteresis curve of specimen NR2, b: Failure mode specimen NR2.

3.5 30

3.0

20

2.5

10

2.0

Env NR2 Env NR1

Load (kN)

env NC1

-15

3.32

Deflecon duclity Energy duclity 2.35 1.87

3.29 2.60

1.99

1.85

1.70

1.5 0 -10

-5

0

5

10

1.0

15

-10

0.5

-20

0.0 NC1

-30 Deflection (mm)

NC2

NA2

NA3

NB4

NB5

Fig. 23. Ductility index of specimens.

Fig. 22. Envelop of the hysteresis loops specimens NR1 et NR2.

362

NR1

NR2

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Table 3 Tested beam-column joints strengthening and repairing schemes. No

state

Specimen

Details

1 2 3 4 5 6 7 8 9 10

Control

NC1 NC2 NA2 NA3 NB4 NB5 NC1R NC2R NR1 NR2

Without stirrups in the joint With stirrups in the joint 4 × 1 L CFRP fabric wrap 0° + 1 CFRP fabric wrap 90° (See Fig. 5) 4 × 2 L HFRP fabric wrap 0° + 2 Hexcel-HFRP fabric wraps 90° (See Fig. 5) 4 × 1 L CFRP fabric wrap 0° + 2 GFRP (on the bottom of the beam) (See Fig. 6) 4 × 1 L HFRP fabric wrap 0° + 2 Hexcel-HFRP fabric wraps (on the bottom of the beam) (See Fig. 6) 4 Pultruded Carbon + 4 L GFRP +2 U GFRP fabric 90° (See Fig. 7) 4 × 1 L CFRP+GFRP fabric 0° +2 U GFRP 90° (See Fig. 8) 4 Pultruded Carbon + 4 × 1 L GFRP fabric wrap 4 × 2 L GFRP fabric wrap + 2 GFRP (on the faces of the beam) (See Fig. 10)

Strengthening

Series A Series B

Repairing

Note: NC1 and NC2 represent the control specimens without any strengthening. Table 4 Ductility and tested specimen data. At Steel Yield

NC1 NC2 NA2 NA3 NB4 NB5 NC1R NC2R NR1 NR2

At Failure

Ductility

Ductility ratios

At peak

Strength gain %

Load

Def. Δy

Energy Eu

Def. Δu

Load

Deflection μΔ

Energy μE

Deflection μΔ

Energy μE

Δpeak

Load

11.20 11.71 13.66 12.73 14.11 14.50 15.20 6.13 17.66 19.45

4.77 5.27 4.92 4.30 4.70 5.64 3.82 2.01 4.43 4.51

65.23 67.27 94.60 84.10 172.37 172.50 – – 94.54 235.72

7.64 8.22 8.90 8.36 13.53 13.79 – 2.01 7.53 13.53

12.11 12.49 12.29 7.41 13.50 7.70 10.60 6.13 7.56 20.67

1.60 1.56 1.81 1.94 2.88 2.45 – 1.00 1.70 3.00

1.87 1.85 2.35 1.99 3.32 2.60 – – 1.70 3.29

1.00 0.97 1.13 1.22 1.80 1.53 – – 1.06 1.88

1.00 0.99 1.26 1.06 1.78 1.39 – – 0.91 1.76

7.02 7.25 6.90 5.55 8.75 6.53 6.77 2.01 5.03 12.02

12.57 12.83 14.30 13.20 15.14 15.38 – 6.13 18.41 23.82

0.00 2.07 13.76 5.01 20.45 22.35 – – 46.46 89.50

as the reinforcement was no longer able to serve its purpose. Only the steel remained until its ultimate deformation (Fig. 20.b). The load versus deflection curves in Fig. 21a show that the retrofitted specimen NR2 had a larger area than the reference specimens, representing a high energy dissipation capacity. This indicated the ability of the structural element to resist fracture when subjected to static or dynamic loading. This result was essentially to be expected, given the reinforcement of the central joint, which can be likened to a local confinement of the core. Initially, there is a slight increase in rigidity compared to NC1. Once the elastic limit of the steel bars is reached, a plastic bearing is obtained, with both reinforcements being activated at the same time. This results in a deflection of 13.6 mm, which represents a 350% increase in energy dissipation compared to the control (Table 4), and a maximum strength of 23 kN, representing an 89% increase over the control. The delamination of the composite indicates the sudden drop in resistance (Fig. 21.b). It can be concluded that the composite action of the reinforcement fully achieved its purpose in this particular case. The reinforcement of the beam faces acted as additional longitudinal reinforcement, but this could have been rendered more effective by placing an anchor perpendicular to the beam. Fig. 21.b shows the delamination of the fibreglass GFRP reinforcement without vertical rupture, but with rupture of the steel bars in the upper part of the beam and disintegration of the concrete, giving rise to an unstable mechanism. The final failure occurred with the detachment of the reinforcement at the joint faces and the bursting of the concrete in the interface area. The image from the Aramis camera shows the composite breaking away from the faces. The envelope curves in Fig. 22 show that specimen NR2 – which was repaired using GFRP reinforcement – exhibits good behaviour, with a high level of energy dissipation and a ductility ratio of 1.8 (Table 4) compared to the control specimen.

abruptly when the load reached 7 kN. The maximum deflection recorded was 7.5 mm. The poor behaviour of this repaired specimen was mainly due to the severe damage in the pre-repair phase, when several steel bars were ruptured. This was compounded by the very significant plastic deformation of the steel bars. By comparing the envelope curve of the NC2 specimen before repair with that of the NC2R joint after repair, we see that the initial behaviour was not restored. Accordingly, it can be concluded that external reinforcement cannot remedy the failure of the steel bars unless it is sufficiently anchored and connected to the core of the joint. The failure occurred at the interfaces where cracks appeared; these were the most fragile areas (Fig. 19.b). 4.3.3. Behaviour and failure mode of NR1 and NR2 From the overall behaviour curve (Fig. 19a), it can be seen that the repair and reinforcement of this node had a positive impact. The first part of the graph shows a significant increase in stiffness and resistance. Due to the high elastic modulus of pultruded carbon, the resistance achieved was 18.4 kN. This demonstrates that the composite action of the reinforcement was perfect up until the point when the steel bars yielded. However, when the final phase was reached, there was a sudden drop in resistance due to the loss of the composite action, with the strengthening peeling off at the anchors. The graph is largely symmetrical – i.e. it demonstrates perfect symmetry between reinforcement and behaviour – except for the ascending part, where there was a premature failure of the carbon sheet. Nevertheless, energy dissipation was better in the retrofitted joint compared to that of the control specimen (Table 4), with a deflection of 7.5 mm. As can be seen in the shot taken with the Aramis camera, shear failure occurred at the interfaces and at the same locations as the precracking. Two large shear cracks appeared on either side of the beam. As the fissure opened more widely, the carbon sheets became detached, 363

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Thus, if the steel bars have not been damaged and the configuration of the joint is such that the central core can be strengthened, then the composite reinforcement will be fully effective.

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5. Conclusions The primary objective of this research was to collect data that would enable us to better understand the behaviour of glass and hybrid strengthening systems, which are relatively inexpensive compared to carbon fibre systems. A secondary objective was to investigate the deformability of the specimens after reinforcement, with a view to enhancing the seismic performance of RC beam-column joints, given that ductility is an important factor, especially in areas of significant seismic activity. Based on the results of the study, the following conclusions can be drawn:

• According to Table 4, a comparison between two specimens (NA2

and NA3), reinforced with a carbon fibre-based system (CFRP) and a glass fibre-based system (GFRP) respectively, produced similar results in terms of load capacity and energy dissipation. The use of unidirectional glass‑carbon hybrid fibre reinforcement has been demonstrated to be effective, resulting in a significant improvement in loading capacity and ductility in RC beam-column joints (Fig. 23). Based on the results, it can be stated that, with adequate anchoring, the use of fibreglass alone, or with a hybrid glass‑carbon composite, is an excellent solution to strengthen reinforced concrete structures, in terms of both structural behaviour and cost. The RC beam-column joint with inadequate anchorage of steel bars in the bottom part of the beam showed brittle shear failure of the joint, accompanied by slippage of the bars in the bottom part of the beam. The effectiveness of the reinforcement was confirmed for all configurations (see Fig. 23), but much less so for the un-strengthened interior joint or central zone. However, in the case of an exterior joint, the reinforcement was fully effective, transforming the behaviour from brittle fracture to ductile fracture. Using a U-shaped GFRP jacket maintained the integrity of the concrete by confinement, and significantly improved the ductility and load-bearing capacity of the repaired joint. In specimens NB4 and NB5, there was an improvement in ductility – with ratios of 1.8 and 1.5 respectively – and a 22% increase in load-bearing capacity. These specimens also dissipated three times more energy than the control specimens. ▪ The effectiveness of a fibreglass composite reinforcement on an external joint was confirmed for repairs on RC beam-column joints. The results of the NR2 specimens are edifying: their load-bearing capacity was 90% higher and they dissipated approximately four times more energy than the control. ▪ Fig. 23 confirms that fibreglass has tremendous potential for use in beam-column joints in earthquake-prone zones to enhance ductile behaviour.

• • • • •

Acknowledgements The authors are truly thankful to the Algerian Government (MESRS) for funding research. They also gratefully acknowledge the generous assistance of Sika France (particularly, Yvon Gicquel and Jacques Béquignon) for supplying the reinforcement materials used in this study. References [1] Issa Camille A, Debs Pauls. Experimental study of epoxy repairing of cracks in concrete. Construct Build Mater 2007;21(1):157–63. January.

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