Study on Shear Strengthening of RC Continuous Beams with Different CFRP Wrapping Schemes

Transcription

1 Stuy on Shear Strengthening o RC Continuous Beams with Dierent Wrapping Schemes *N. Ali 1, A.A. Abul Sama 2, J. Jayaprakash 3, W.M. IESA 4 an M.B.S Alerjani 5 1,2,4,5 Faculty o Civil an Environmental Engineering, UTHM 3 School o Civil Engineering, Universiti Sains Malaysia, Malaysia *Corresponing wira@uthm.eu.my Abstract This paper presents the results o an experimental investigation or enhancing the shear capacity o reinorce concrete (RC) continuous beams using ierent wrapping schemes. A total o ive concrete beams were teste an various sheet conigurations an layouts were stuie to etermine their eects on ultimate shear strength an shear capacity o the beams. One beam was kept as control beams, while other beams were strengthene with externally bone strips with our or three sies boning an one or two layers o strips. From the test results, it was oun that all schemes were oun to be eective in enhancing the shear strength o RC beams. It was observe that the strength increases with the number o sheet layers an our sies wrap provie the most eective strengthening or RC continuous beam. Beam strengthene using this scheme showe 54% increase in shear capacity as compare to the control beam. Two preiction moels available in literature were use or computing the contribution o strips an compare with the experimental results. Keywors:, Continuous Beam, Shear Strengthening

2 1. INTRODUCTION Deterioration o concrete structures is one o the major problems o the construction inustry toay. Moreover, a large number o structures constructe in the past using the oler esign coes in ierent parts o the worl are structurally unsae accoring to toay esign coes [1]. Shear ailure o RC beams, cause by their brittle nature, has been ientiie as the most isastrous ailure moe, it occurre with no avance warning o istress. Shear eiciency may occur ue to many actors such as insuicient shear reinorcement or reuction in steel area ue to corrosion, increase service loa an construction errors [2]. Boning plates to the external surace o existing reinorce concrete elements have prove to be an eective an practical means o increasing strength an stiness [3]. The use o externally bone iber reinorce polymer (FRP) reinorcement to strength reinorce concrete structures is becoming an increasingly popular retroit technique. FRP is a composite material generally consisting o carbon, arami or glass ibers in polymeric matrix [4]. This research ocuse on using Carbon Fiber Reinorce Polymer () systems consisting o lexible sheets. The objective o this stuy were to investigate the eectiveness o using externally bone strips in repair an strengthen o reinorce concrete continuous beams. 2. SHEAR STRENGTH OF RC BEAM STRENGTHENED WITH FRP SHEET The nominal shear strength o RC beams strengthene with externally bone FRP sheets can be compute by equation (1): V V V V (1) n c s To compute the nominal shear strength as given in equation (1), it is important to quantiy the contribution o reinorcement to the shear capacityv. This stuy presents two moels use to obtainv. 2.1 Khalia Moel [5] The contribution o externally bone FRP sheets to the shear capacity o an RC beam may be calculate rom the equation 2. A e sin cos 2 ' c bw V Vs s 3 (2) Because linearly elastic until ailure, the eective stress may be compute as ollows: e R u (3) 2.1.1Reuction Coeicient Base on Sheet Fracture Failure The reuction coeicient was establishe as a unction o an expresse in equation (4). E R E 1.22 E (4) 2.1.2Reuction Coeicient Base on Deboning Failure Ater the shear cracks evelops, only that portion o the with o extening past the crack by the eective bone length is assume to be capable o carrying shear. The eective with w base on the shear crack angle o 45 an the wrapping scheme is expresse in equations (5-a) an (5-b). w (5-a) e I the sheets is wrappe aroun the beam entirely w e L (5-b) e I the sheets is in the orm o U-wrap In etermining the reuction coeicient or bon, the eective bon length L, has to be etermine. The e eective bon length L is a unction o the thickness e o the FRP sheet an the elastic moulus o the FRP. As the stiness o the sheet increases the eective bon length ecreases. L e e ln t E (6) The inal expression or the reuction coeicient R, or the moe o ailure controlle by eboning is expresse in Eq (7). ' 2 3 c w e 6 (7) R ( ( t E )) 10 u The above equation is applicable or axial rigiityt E, ranging rom 20 to 90 Gpa (kn/mm) Upper Limit o the Reuction Coeicient In orer to control the shear cracks with an loss o aggregate interlock, an upper limit o reuction coeicient R was suggeste R u (8) The inal reuction coeicient or the system is taken as the lowest value etermine rom the two possible moes o ailure an upper limit. Note, that i the sheet is wrappe entirely aroun the beam or an eective anchor is use, the ailure moe o eboning is not being consiere. The reuction coeicient is only controlle by racture an upper limit. 2.2 ACI 440 Moel [6] The shear strength provie by FRP reinorcement can be etermine by calculating the orce resulting rom the tensile stress in the FRP across the assume e

3 crack. The shear contribution o the FRP shear reinorcement is given by the equation: Av e (sin cos ) v V s (9) The tensile stress in the FRP shear reinorcement at nominal strength is irectly proportional to the level o strain that can be evelope in the FRP shear reinorcement at nominal strength. e e E (10) The eective strain is the maximum strain that can be achieve in the FRP system at the nominal strength an is governe by the ailure moe o FRP system an o the reinorce concrete member. The subsequent equation provie guiance on etermining the eective strain or ierent coniguration o FRP laminates use or shear strengthening o reinorce concrete members Completely Wrappe Members For completely wrappe reinorcement concrete column an beam members by FRP, loss o aggregate interlock o the concrete has been observe to occur at iber strain less than the ultimate iber. To preclue this moe o ailure, the maximum strain use or esign shoul be limite to 0.4% or completely wrappe applications. e u (11) 2.2.2Bone U-wraps or Bone Face Piles FRP systems that o not enclose the entire section (two an three sie wraps) have been observe to elaminate rom the concrete beore the loss o aggregate interlock o the section. The eective strain is calculate using a bon reuction coeicient k applicable to shear. v e k v u (12) The bon reuction coeicient is a unction o the concrete strength, the type o wrapping scheme use an the stiness o the laminate. The bon reuction coeicient can be compute as ollows: k1k 2Le kv ,900 u (13) The active bon length Le is the length over which the majority o the bon stress is maintaine. This length is given by the ollowing equation: 23,300 Le 0.58 ( nt E ) (14) The bon reuction coeicient also relies on two moiication actors k1 an k 2, that account or the concrete strength an the type o wrapping scheme use. Expressions or these moiication actors are given as ollows: k 1 ' c 27 2 / 3 v (15) v L e k 2 (or U-wraps) (16) k 2 v 2 L e (or two sies bone) (17) v The nominal shear capacity o the strengthene beam can be calculate by using the equation: Vn ( Vc Vs V ) (18) An aitional reuction actor ψ is applie to the shear contribution o the FRP reinorcement. The reuction actor o 0.85 is recommene or three sies FRP U-wrap or two opposite sies strengthening an 0.95 or ully-wrappe members. 3. EXPERIMENTAL PROGRAM 3.1 Test Specimens an Materials The experimental program consiste o testing ive ull-scale RC continuous beams uner our-point loaing. All specimens were esign accoring to BS 8110: Part 1: 1997 with ientical size o 150x350x5800 mm. All beams have an ientical reinorcement etails incluing longituinal reinorcement in the orm o 20 mm an stirrups reinorcement o 6 mm size at 200mm spacing center to center. Fig. 1 below shows specimen etails. A-A B-B Fig. 1: Reinorcement an cross section etails

4 All beams were cast using reay mix concrete with compressive strength o 30N/mm². Three bars o main reinorcement with length o 600 mm were teste uner uniaxial tension using Universal Testing Machine (UTM) to etermine the yiel strength (see Table 1). For this stuy, the FRP use was biirectional woven carbon iber abric. Mechanical properties o the are shown in Table 2. The type o ahesive use was Sikaur-330, a two part epoxy impregnating resin A an B. Table 3 shows the mechanical properties o the epoxy. Table 1: Material properties o main reinorcement Type High yiel steel Table 2: Mechanical properties o [7] Density Diameter o bar (mm) 20 Tensile strength Tensile E-moulus Elongation at break Yiel strength (N/mm²) g/cm³ 3'800 N/mm2 (nominal) N/mm2 (nominal) 1.5% (nominal). Average strength was not strengthene an was consiere as a reerence beam an our beams were strengthene using externally bone strips with ierent schemes. The test set-up as well as strengthening schemes are shown in Fig. 2. Each specimen has ierent characteristic where or C2.5-C, it was teste with no wrapping an loae to ailure. For C2.5-UA-V, it was wrappe with one layer o at our sies o the beam with orientation o 0/90º, while or C2.5-U-V, it was wrappe with one layer at three sie o the beam with orientation o 0/90. For C2.5-UA-V2, it was wrappe with two layers o at our sies o the beam with orientation o 0/90º an also or C2.5-U-V2, it was wrappe with two layers o but at three sies o the beam with orientation o 0/90. Table 4 shows the specimens esignation. No. Specimen Orientation ( ) Table 4: Specimens esignation Wrapping Schemes Loaing & Strengthening Conition 1 C2.5-C C2.5-UA- V 3 C2.5-U-V 4 5 C2.5-UA- V2 C2.5-U- V2 0/90 4 sies 0/90 0/90 0/90 3 sies 4 sies 3 sies Initially Strengthene (1 layer) Initially Strengthene (1 layer) Initially Strengthene (2 layer) Initially Strengthene (2 layer) Table 3: Mechanical properties o Sikaur-330[8] P/4 P/4 P/4 P/4 Density Tensile strength 1.3 Kg/L ± 0.1 Kg/L 30N/mm² Thermal resistance Continuous exposure + 45 C Elongation at break 0.9 % (a)control beam C2.5-C 3.2 Strengthening Scheme an Test Set-up The beams were teste as continuous beam with shear span to eective epth ratio o 2.5. One beam (b)beam C2.5-UA-V (ully-wrap)

5 (c)beam C2.5-U-V (U-wrap) ()Beam C2.5-UA-V2 (2 layers ully-wrap) (e)beam C2.5-U-V2 (2 layers U-wrap) LVDT Strain gauge Dimensions in mm Dimension in mm Fig. 2: Test set-up an strengthening schemes 4. EXPERIMENTAL RESULTS 4.1 Ultimate Loa an Moes o Failure All specimens aile in shear as expecte. For control beam, C2.5-C, lexural cracks were starte to orm at near the mi span at the bottom o the beam at a loa approximately 98kN. The shear cracks began to appear at a loa o approximately 102kN an as the loa increase, the shear crack wiene an propagate up to the inal ailure at a loa level o 286kN. The moe o ailure was shear crushing o the concrete. For specimens C2.5-UA-V which was ully-wrappe with strips, no cracks were visible on the sies o the beam until 121kN. A iagonal shear crack was observe near the mile o shear spans at a loa o 227kN. Finally, the beam aile at a total loa o 380kN. Test results shows that there was an increase o 33% in ultimate loa capacity compare to control beam C2.5-C. For specimens C2.5-U-V which wrappe three sies with, the irst crack was occurre at 115kN at the bottom mi span o the beam. This beam exhibite the irst crack at a higher loa than the control beam C2.5-C ue to the presence o the external bone system. The iagonal shear cracks were observe at 218kN an the corresponing ailure loa occurre at 356kN. The enhancement o the loa is 24.5% higher than the control beam. On the other han, the irst crack or beam C2.5- UA-V2 was evelope at a loa o 129kN. The iagonal shear cracks were observe at 248kN. As the loa increase, the iagonal cracks were extening. The total ultimate loa was recore at 439kN with 53.5% increase in loa capacity over the control beam C2.5-C. For specimens C2.5-U-V2 which was wrappe at three sies o the beam, the irst crack was occurre at 120kN. The iagonal shear cracks were observe at 236kN an the ailure o the specimen occurre when the total applie loa reaches 414kN. This was an increase o 45% in ultimate loa capacity compare to the control beam. Table 5 shows the irst cracks loa, ultimate loa, contribution o an the moes o ailure or all beams. Fig. 3 to 7 show cracking patterns an ailure moes o the beams. Table 5: Experimental results Specimen First crack loa (kn) Ultimate loa (kn) Shear orce (kn) Shear Enhancement (%) Moe o ailure C2.5-C Shear C2.5-UA- V C2.5-U-V C2.5-UA- V C2.5-U-V Shear- rupture Shear- rupture & peeling Shear- rupture Shear- rupture & peeling

6 Fig. 3: Cracking an ailure pattern o beam C2.5-C Fig. 7: Cracking an ailure pattern o beam C2.5-U-V2 Fig. 4: Cracking an ailure pattern o beam C2.5-UA-V 4.2 Loa-Displacement Behavior Fig. 8 shows the total applie loa versus mi-span election relationship or all teste specimens. All beams showe very similar stiness tren to each other. The smallest election was observe or beam C2.5-C. It was also observe that the stiness o the beam strengthene with one layer o (C2.5-UA- V) was less than that o the beam strengthene with two layer o (C2.5-UA-V2). Apart rom that, it was also observe that beams wrappe our sies with strips were stier than beams wrappe three sies with strips. Fig. 5: Cracking an ailure pattern o beam C2.5-U-V Fig. 8: Ultimate loa versus mi-span isplacement relationship Fig. 6: Cracking an ailure pattern o beam C2.5-UA-V2 4.3 Surace Strain in Strips an Concrete Surace The applie loa versus strain in strips an concrete surace o the specimens C2.5-C, C2.5-UA- V, C2.5-U-V, C2.5-UA-V2 an C2.5-U-V2 are shown in Fig. 9,10,11,12 an 13 respectively. For control beam C2.5-C, the maximum strain in concrete at ailure was 378με. In specimens C2.5-UA-V2, the strain gauge showe an abrupt increase in concrete strain at an applie loa o P, 65kN. This suen

7 increase in the strain gauge maybe ue to the wiening o the iagonal shear crack. The recore maximum strain in was aroun 7500με in specimen C2.5- UA-V2. Specimen C2.5-UA-V2 ha a similar orientation o an position the strain gauges as in specimen C2.5-UA-V but the ierent is the number o layers. There was a suen increase in concrete strain probably because o the propagation o iagonal crack unerneath the strips. From the graphs, we can see that the wrapping scheme o the strip aecte the composite strain where beams with ully wrappe get higher strain than the u- wrappe beams. Apart rom that, the strips showe an elongation o the strips inicating the eectiveness o the strip resisting shear. Fig. 11: Graph o Applie Loa vs Concrete Surace an Strain or C2.5-U-V Fig. 9: Graph o Applie Loa vs Concrete Surace an Strain or C2.5-C Fig. 12: Graph o Applie Loa vs Concrete Surace an Strain or C2.5-UA-V2 Fig. 10: Graph o Applie Loa vs Concrete Surace an Strain or C2.5-UA-V Fig. 13: Graph o Applie Loa vs Concrete Surace an Strain or C2.5-U-V2

8 5. CONCLUSION The test results inicate that strengthening o RC continuous beams using externally bone strips can be use to enhance the shear capacity o continuous beams. For beams teste in the experimental program, the shear capacity increase at a range rom 24.5% to 53.5 %. It was also observe that increasing amount o may not result in signiicant increase o the shear capacity. Apart rom that, beams which wrappe our sies with ha a higher capacity than those which wrappe three sies with. ACKNOWLEDGMENT The authors grateully acknowlege the inancial supports rom Funamental Research Grant Scheme (FRGS) une by Ministry o Higher Eucation, Malaysia. NOTATION a v - shear span A -area o shear reinorcement = 2 t w b w -with o the web o beam cross section (ACI ormat) -epth rom the top o the section to the tension steel reinorcement centroi -eective epth o the shear reinorcement (usually equal to or rectangular sections an -ts or T-sections) E -elastic moulus o FRP (GPa) ' c -nominal concrete compressive strength in MPa (ACI ormat) u -ultimate tensile strength o the FRP sheet in the irection o the principal ibers y -yiel strength o steel reinorcement L e -eective bon length (mm) R -reuction coeicient (ratio o eective average stress or strain in the FRP sheet to its ultimate strength or elongation) s -spacing o steel stirrups s -pacing o FRP strips t -thickness o the FRP sheet on one sie o the beam (mm) V c - nominal shear strength provie by concrete V - nominal shear strength provie by FRP shear reinorcement (ACI ormat) V n -nominal shear strength (ACI ormat) V s - nominal shear strength provie by steel shear reinorcement (ACI ormat) V u -actore shear orce at section (ACI ormat) w e -eective with o FRP sheet (mm) α- angle between incline stirrups an longituinal axis o member β -angle between the principal iber orientation an the longituinal axis o the beam ε e -eective strain o FRP ε u -ultimate tensile elongation o the iber material in the FRP composite φ -strength reuction actor (ACI ormat) actor or concrete (Eurocoe ormat), γc =1.5 REFERENCES [1] B.B. Ahikary an H. Mutsuyoshi (2004). Behavior o Concrete Beams Strengthene in Shear with Carbon Fiber Sheets, Journal o Composite or Construction ASCE, volume 8, 2004, page number [2] A.Khalia, A.Belarbi an A.Nanni, Shear perormance o RC members strengthening with externally bone FRP wraps. 12 th worl conerence on earthquake engineering,2000, paper 305,10 pp. [3] S.A. El-Raaie, A.F. Ashour an S.W. Garrity, Strengthening o reinorce concrete continuous beams with composites. Structural Engineering, Mechanics an Computation, 2001,volume 2, page [4] A.Khalia, W.J. Jol, A. Nanni an M.I. Abel Aziz, Contribution o externally bone FRP to shear capacity o RC lexural members. J.Compos.,volume 2, 1998, page number [5] A.Khalia an A.Nanni, Rehabilitation o rectangular simply supporte RC beams with shear eiciencies using composites. Construction an builing materials,2002, page [6] ACI Committee 440 (2008). Guie or the Design an Construction o Externally Bone FRP Systems or Strengthening Concrete Structures. [7] Sika Manuacturer s Prouct Data Sheet, Switzerlan, (Supplier: Sika Kimia Sn. Bh), Sika Wrap BI-C/15. Woven carbon iber abric or structural strengthening. Eition 11/09/2007. [8] Sika Manuacturer s Prouct Data Sheet, Switzerlan, (Supplier: Sika Kimia Sn. Bh.), Sika Wrap party epoxy impregnation resin. Eition 0209/1.