STRENGTH AND DUCTILITY OF REINFORCED CONCRETE BEAM COLUMN JOINT STRENGTHENING BY HYBRID FRP AND GFRP SHEETS

N. Attari1*, S. Amziane2 and M. Chemrouk3 1*ENSA, Ecole Nationale Supérieure d’Architecture, Algiers, Algeria.

e-mail:attari_n@yahoo.fr 2Université Blaise Pascal, Polytechnique Clermont Ferrand, Département Génie Civil, France.

e-mail:Sofiane.Amziane@polytech.univ-bpclermont.fr 3Université des Sciences et de la Technologie Houari Boumediene, Algiers, Algeria.

e-mail:mchemrouk@yahoo.fr ABSTRACT: A large number of old buildings have been identified as having potentially critical detailing to resist earthquakes. The m ain r einforcement o f lap-spliced c olumns j ust a bove t he j oint r egion, di scontinuous bot tom be am reinforcement, and little or no joint transverse reinforcement are the most critical details of interior beam column joints in such buildings. This structural type constitutes a large share of the building stock, both in developed and developing countries, and h ence r epresents a substantial e xposure. Direct o bservation o f d amaged s tructures, f ollowing t he Algiers 2003 earthquake, has shown that damage occurs usually a t the beam-column joints, with failure in bending or shear, depending on geometry and reinforcement distribution and type. While substantial literature exist for the design of concrete frame joints to withstand this type of failure, after the earthquake many structures were classified as slightly damaged and, being uneconomic to replace them, at least in the short term, suitable means of repairs of the beam column joint area are being studied. Furthermore there exist a large number of buildings that need retrofitting of the joints before the next earthquake. The p aper reports t he r esults o f the experimental programme, constituted of three beam-column r einforced concrete joints at a scale of one to three (1/3) tested under the effect of a prestressing axial load acting over the column. T he b eams were subjected at t heir en ds to a n alternate cyclic loading under displacement control to simulate a seismic action. Strain and cracking fields were monitored with the help a digital recording camera. Following the analysis of the results, a comparison can be made between the performances in terms of ductility, strength and mode of failure of the different strengthening solution considered.

Key words: Fibre reinforced polymers; Joints; reinforced Concrete; Beam columns. 1. INTRODUCTION: Various authors (Bakoss et al., 1999, Elsanadedy and Mosallam, 2000, Ghobarah and Said, 2001, Granata and Parvin, 2001) have conducted tests on di fferent layout of FRP fabric and sheets bonded to R.C. beam-column connections. The tests all concord on the effectiveness of the strengthening procedure to increase stiffness and ductility while increases in shear and flexural strength and in energy dissipation are highly dependent on proper confinement of the concrete and anchorage of the wrapping. Carbon fibre epoxy reinforced polymer (CFRP)material was used to retrofit an external beam-column joint in shear ((Pantelides et al .,2000).the retrofitted specimen was wrapped with multiple layers of CFRP laminates. the joint cap acity was i ncreased b y 2 5% a nd t he specimens r eached 5 % d rift . significant improvements i n t he strength ,stiffness and ductility of the retrofitted joints were also achieved. Glass fibre (GFRP) material was used to retrofit external beam co lumn joints ( Ghobarah and Said ,2001 and 2002) .two joints were tested as control specimens with different column axial load (10% and 20% of the column axial capacity) . the control specimens were tested and t hen repaired an d r e-tested .two other specimens were rehabilitated and tested .the behaviour of the rehabilitated specimens was significantly improved over the as built specimens; the brittle shear failure of the beam column joints was eliminated and instead ductile flexural hinging

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of the beam occurred. the joints tested in this research program were designed with deficient shear strength but with adequate positive reinforcement anchoring in the joint . External application of FRP material provides a practical solution to improve the overall performance of an R.C. frame structure without the necessity of a radical alteration to the original structure. Externally bonded FRP may be used in a r epair capacity for structures t hat h ave u ndergone moderate earthquakes d amage o r t o reinforce structures that are considered to be vulnerable or substandard. The use of FRP offers several advantages, related to its h igh strength-to-weight ratio, resistance to corrosion, fast and relatively s imple ap plication (El Amoury, 2002). However, FRP is to date still rather expensive so its use must be optimised to minimise material wastage. One disadvantage of FRP is its dependence on bond to the concrete it is to strengthen, which is a function of the tensile capacity of the concrete and the type of surface preparation used. 2. EXPERIMENTAL PROGRAM 2.1. Tested specimens One t hird s cale r einforced co ncrete i nterior b eam-column j oint s pecimens were p repared i n t his s tudy. A schematic sketch of the specimen used is shown in Fig.1.and the reinforcement details are demonstrated in Fig.2. two 8 mm steel bars were used at the bottom and two 8mm were used on the top, steel stirrups of 6mm spaced every 100mm were used for shear. The beams and the column had a rectangular cross section with 100 mm by 150 mm.

300

400

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600 600

Axial Load

P/2 P/2

FRP

4T8

Stirrups Ø6 @100mm A A

B

B

4T8

A-A

B-B

Fig .1. : Strengthening and test set up All dimensions in (mm)

Fig.2. Specimen reinforcement All dimensions in (mm)

2.2. Materiel properties FRP fabrics The main ch aracteristics o f F RP s trengthening materials used i n t he e xperiments, as s upplied b y t he manufacturer, are given in Table 1§2. The HFRP is a hybrid bidirectional fabric braided of one type of glass and one type of carbon in two directions (0° a nd 90° ) with 21% c arbon a nd 29% glass i n t he warp a nd t he weft di rections , its n ominal weight 274 g/m²,and thickness of 0.13 mm. Table 1: Mechanical Properties of fibres Strengthening FRP Materials a

a SIKA Manufacturer Data (properties of the fibres)

Material

Modulus of

Elasticity GPa

Tensile Strength

MPa

Fibres Orientation

Thickness mm

Elongation at Failure

Surface mass g/m2

Fibre CFRP SikaWrap230C 238 3650 Unidirectional 0.15 1.7% 225

Epoxy Sika330 3.8 30 - 1 0.9% 500g

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FRP Composite The name composite is given to the assembly of matrixed fabric in the synthetic resin. The composite, obtained by fabrication on site, consists of approximately 60% fabric and 40% resin epoxy; these proportions may vary according to the conditions of use on the site. The mechanical p roperties o f F RP co mposite d esigned a nd u sed i n t his work were tested; t he t ests were performed in accordance with the (NF.T57-101), an equivalent of the (ASTM D638). The results are given in Table 2; these results should be taken with caution due to the high sensitivity of the experimental conditions and particularly, the thickness of the composite. Table 2: Mechanical Properties of Composite (fabric and epoxy) Materials.

Material Modulus of Elasticity

GPa

Tensile Strength

MPa

Fibers Orientation

Thickness mm

Elongation at Failure

% CFRP 43.5 403 Unidirectional 1.6 0.95 Hybrid

Fabric HFRP 27 218 Bi-directional 2 0.85

Concrete and steel The average concrete strength in compression was 39 Mpa. The yield strength of the steel bars used as tensile, shear and compressed reinforcement was determined by standard tensile tests giving an average value of 500 Mpa. 3. STRENGTHENING AND INSTRUMENTATION The s pecimens were s trengthened b y using car bon an d hybrid fabric FRP materials ( Table2). P rior to th e application o f the FRP, the concrete substrate was smoothed by grinding and cleaning. The cement paste was removed from the surface and the coarse aggregates were exposed. The corners of all the members were ground to create a flare. All the beams were subjected to a reversed cyclic load (Fig.2) up to failure using a hydraulic machine of ±250kN capacity. The supports were made from hardened steel plates, cut and formed with a suitable thickness to sustain the applied load without any deformation that may affect the test results. The bottom of the column was attached on the machine through a slab pad with special bolts. The column was subject to a constant prestressed axial load of 100kN which is about 25% of the ultimate load carrying capacity of the column. The d eflection o f t he b eam specimens i s measured at t he t ip o f t he b eam with t he help o f a d isplacement transducers (LVDT) placed respectively on the beam specimens and on the loading arm of the testing machine. The test beams were equipped with stain gauges and camera for deformation measurements and monitoring. The l oad cy cle was p redefined as s hown i n F ig.6; t he d isplacement s tarted f rom t he n eutral p osition an d oscillated h armonically a bout t hat p osition until failure o f t he b eam. It i ncreased at a u niform r ate 0.25mm/Cycle; each cycle consisting of five full waves of the same amplitude with a frequency of 0.3Hz.

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-5

0

5

10

15

0 50 100 150 200 250 300 350 400 450 500

Time(S)

Dis

plac

emen

t(mm

)

-1,5

-1

-0,5

0

0,5

1

1,5

0 10 20 30 40 50 60 70 80

Time(S)

Dis

plac

emen

t(mm

)

'

Fig.3: Cyclic-loading history used in this study. Specimen details: For Specimen Bj1 FRP sheets were applied in L shape to upgrade the joints. FRP has been applied in several layers. In step 1 FRP has been applied on the top and bottom surfaces concrete surfaces. The fibres were along the axes of the members (Fig.4). Subsequently, FRP wraps were provided over the inner layers (Fig.5).The direction of fibres in wraps was perpendicular to the axis of the members.

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Both the column and the beam are then wrapped by bidirectional hybrid fibres; same configuration is repeated for specimen Bj2 using carbon fibre fabric using 1 layer of overlays and single wrap overlap. Both

2L HFRP

2 HFRP

Fig. 4: Strengthening step1 Fig. 5: Strengthening step2 Fig. Test Set Up

4. DISCUSSIONS For the control specimen, the first crack was observed at a l oad of 8 kN as shown in Fig.6. The beams have failed at the joint through the formation of hinges. The hinges have formed between the two shear links of the beam. The concrete has spalled-off in such a way that vertical failure planes were created. This has resulted in free r otation of t he beam with no t ransfer of bending moment to the column. The control specimen failed in shear at the joint, it reached a maximum load of 25 kN and an ultimate deflection of 8.5 mm. The hysteresis b ehaviour o f t he c ontrol s pecimen s howed c onsiderable p inching with s evere s trength deterioration and stiffness degradation.

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0

10

20

30

-15 -10 -5 0 5 10 15

Deflection (mm)

Load

(kN

)

Fig .6: Beam tip Load Displacement for Control Specimen Bjc

The beam tip load–displacement characteristics for the specimens are discussed here. The displacement levels of the first few cycles did not generate any nonlinear deformation in the structure and the loops followed a straight line with its slope a s in itial s tiffness. T he o nset o f s tiffness d egradation was id entified b y s imultaneous appearance of tension cracks at the root of the cantilever beam. The calculations show that at this point the steel started to yield and it was not capable of taking any further load. The a dditional lo ad f rom t his p oint was c arried b y t he F RP. A t t his p oint l inearity o f t he a scending a nd the descending paths was lost and loops between the two paths appeared. We term this phenomenon yield. The post yield be haviour i s signified by monotonic de gradation of s tiffness. T o e nable c omparison a mong di fferent systems s tudied h ere t he ti p lo ad–displacement en velopes ar e p lotted (Fig 11) by j oining t he p eaks o f consecutive loops. These plots have better clarity. The rate of stiffness degradation can also be found out from these plots. From the graphs it can be seen that the load at yield was considerably higher in the FRP reinforced specimens than the control specimen.

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Deflection (mm)

Load

(kN

)

Fig 7: Beam tip Load Displacement for specimen Bj1

The Bj2 (Wrap carbon) exhibited the highest increase in the yield load followed by the Bj1, and Bjc specimens. It may be noted that the forces at the tensile face of the beam are shared by the steel and FRP in proportion of their relative stiffness. The stiffness of carbon is similar than that of hybrid; the FRP reinforced specimens had larger areas under the envelopes t han t he co ntrol s pecimen. T he B j2 s pecimen h ad t he l argest en velope ar ea f ollowed b y t he Bj1specimen Fig (7§10).

Fig.8 Failure mode of specimen Bj1 Fig. 9Failure mode of specimen Bj2

The control specimen has the lower initial stiffness, and when comparing the peak-to-peak stiffness of the tested joints, the stiffness degradation of the control specimen joint was higher than the specimens Bj1 and Bj2. The degradation of the stiffness with lateral movement is less in both the CFRP and the HFRP strengthened joints compared t o t hat i n t he c orresponding unstrengthened c ontrol s pecimen. This is a d esirable p roperty in earthquake s ituations. I t was o bserved i n t he p ast e arthquakes, t hat most o f t he RC s tructures f ailed ( or collapsed) due to a sudden loss of stiffness of structural joints with increasing lateral movement of the structure.

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Deflection (mm)

Load

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)

Fig 10: Beam tip Load Displacement for specimen Bj2

Both specimens the failure mode was in shear Fig (8§9) at the joint, the failure plane was approximately vertical.

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Fig.11 Hysteresis loop envelopes of the test specimens

CONCLUSIONS: The tests performed in this study demonstrated that externally bonded FRP reinforcement is a viable solution towards enhancing the strength, energy dissipation, and stiffness characteristics of poorly detailed(in shear) RC joints subjected to simulated seismic loads. Specimen s trengthened u sing H FRP s how stiffer behaviour than CFRP s trengthened s pecimens. E nergy dissipation capacity can be increased with the use of small amount of composites. Acknowledgements The authors are gratefully acknowledge the generous assistance of Sika France (particularly, M. Yvon Gicquel and M JJacques Béquignon) for the supplies of the reinforcement materials used in this study. REFERENCES: Bakoss SL, Li J, Samali B, Ye L. Reinforcement of concrete beam-column connections with hybrid FRP sheet.

Compos Struct 2000; 47:805–12. El-Amoury, T . a nd G hobarah, A. ( 2002). “Seismic r ehabilitation o f b eam-column j oint u sing G FRP s heets”.

Engineering Structures, 24, 1397-1407. Gergely , J ., P antelides, C . P . and R eaveley, L. D . ( 2000). “Shear s trengthening of R C T-joints using CFRP

composites”. Journal of Composites for Construction, 4:2, 56-64. Ghobarah, A., and Said, A.(2002). “Shear strengthening of beam-column joints”. Engineering Structures, 24, 81-

888. Ghobarah, A., and Said, A.(2001). “Seismic rehabilitation of beam-column joints using FRP laminates”. Journal

of Earthquake Engineering, 5:1, 113-129. Granata JP, Parvin A. “An experimental s tudy on Kevlar strengthening of beam column connection”. Co mpos

Struct 2001; 53:163–71. Mosallam SA. “Strength and ductility of reinforced concrete moment frame connections strengthened with quasi-

isotropic laminates”. Composites: Part B 2000; 31:481–97. Pantelides CP, Gergely J, Reaveley LD, Volnyy VA. Retrofit of PC bridge pier with CFRP advance composites;

J Struct Eng; ASCE 1999; 125(10):1094–9.

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