Copyright. Regan Mechelle Bramblett

Transcription

1 Copyright by Regan Mechelle Bramblett 2000

2 Flexural Strengthening of Reinforced Concrete Beam Uing Carbon Fiber Reinforced Compoite by Regan Mechelle Bramblett, B.S.Arch.E. Thei Preented to the Faculty of the Graduate School of The Univerity of Texa at Autin in Partial Fulfillment of the Requirement for the Degree of Mater of Science in Engineering The Univerity of Texa at Autin Augut 2000

3 Flexural Strengthening of Reinforced Concrete Beam Uing Carbon Fiber Reinforced Compoite Approved by Superviing Committee:

4 Dedication To my family.

5 Acknowledgement I would like to thank my uperviing profeor, Dr. Michael E. Kreger and Dr. Sharon L. Wood, who patiently upported me throughout the year of teting and writing. I am alo very grateful to my friend, Sergio Breña, who taught me how to pour concrete and deign concrete beam trengthened with compoite. It wa an honor to learn from their experience. I would alo like to thank Janna Renfro, Nicole Garcia, and the lab technical taff for helping with all the laboriou tak involved with large cale teting. The project wa ponored the Texa Department of Tranportation and wa overeen by Richard Wilkion and Mark Steve. A pecial thank goe to TxDot for ponoring my graduate education. I alo greatly appreciate the generoity of Mater Builder, Mitubihi, Fyfe, and Sika for the donation of material for teting. Mot of all, I thank my family for upporting my endeavor through the year. Without their love and encouragement, I would not have accomplihed o much. Augut 18, 2000 v

6 Abtract Flexural Strengthening of Reinforced Concrete Beam Uing Carbon Fiber Reinforced Compoite Regan Mechelle Bramblett, M.S.E. The Univerity of Texa at Autin, 2000 Supervior: Michael E. Kreger The Texa Department of Tranportation ha etimated that 18,000 bridge in ue in the tate were deigned for load le than today tandard. Some heavier truck are retricted from paing over thee bridge in order to prevent damage from overloading. One olution to thi problem i to trengthen the bridge uing carbon fiber compoite. The objective of thi thei i to develop a trengthening method by invetigating the behavior of reinforced concrete beam with externally applied carbon fiber compoite material. Twenty beam were teted monotonically to failure, which wa characterized by compoite debonding or compoite rupture. The reult were ued for the deign and teting of fullcale bridge ection that were part of a econd phae of the reearch project. Reult may eventually be implemented on bridge throughout Texa. vi

7 Table of Content Lit of Table... x Lit of Figure... x Chapter 1: Introduction Reearch Motivation Jutification for Studying Compoite Material Reearch Objective Scope... 7 Chapter 2: Analytical Model ued to Etimate the Flexural Capacity of Strengthened Cro Section Objective Aumption Idealized Material Behavior Compoite Material Reinforcing Steel Concrete Procedure ued to calculate the Flexural Capacity of Strengthened Beam Calculation of Compoite Material Area Correponding with Balanced-Strain Condition Calculation of Flexural Capacity for Concrete-Controlled Failure Calculation of Flexural Capacity for Compoite-Controlled Failure Chapter 3: Experimental Program Introduction Specimen Detail Decription of Beam Section Compoite Strengthening Scheme vii

8 Bottom Application Bottom Application with Tranvere Strap Side Application Side Application with Tranvere Strap Overview of Tet Program Tet Setup Loading Program Material Propertie Concrete Steel Reinforcement Compoite Material Type & Application Method Material Propertie Intrumentation and Data Acquiition Chapter 4: Preentation and Evaluation of Tet Reult Objective Quantitie Conidered in Evaluation of Specimen Behavior General Obervation Oberved Failure Mode Comparion of Meaured and Computed Capacity Strain Ratio Evaluation of Specimen Behavior Bottom Application Bond Length Equivalent Material from Different Manufacturer Equivalent Bond Stre Influence of Surface Preparation Summary Bottom Application with Tranvere Strap viii

9 B Sytem C Sytem Summary Side Application B Sytem C Sytem D Sytem Summary Side Application with Tranvere Strap Recommended Application Method Chapter 5: Summary and Concluion Overview of Tet Program Summary of Tet Bottom Application of Compoite Material Side Application of Compoite Material Addition of Tranvere Strap Recommendation Appendix A: Cracking Pattern Appendix B: Strain Profile at Critical Section Reference Vita ix

10 Lit of Table Table 3.1: Summary of Tet Program Table 3.2: Cylinder Data for Concrete Batch I Table 3.3: Cylinder Data for Concrete Batch II Table 3.4: Cylinder Data for Concrete Batch III Table 3.5: Average Concrete Compreive Strength on Day of Tet Table 4.1: Summary of Reult for All Specimen x

11 Lit of Figure Figure 1.1: Loading Criteria for Standard HS Truck (AASHTO, 1996)... 2 Figure 1.2: Loading Criteria for Standard H Truck (AASHTO, 1996)... 2 Figure 1.3: Method for Increaing the Capacity of Exiting Concrete Beam... 4 Figure 1.4: CFRP Application (Mater Builder, 1998)... 6 Figure 1.5: CFRP Flexural and Shear Strengthening (Sika, 1999)... 6 Figure 2.1: Idealized Stre-Strain Relationhip for Compoite Material... 9 Figure 2.2: Idealized Stre-Strain Relationhip for Steel Reinforcement... 9 Figure 2.3: Idealized Stre-Strain Relationhip for Concrete in Compreion Figure 2.4: Stre and Strain Diagram for Balanced-Strain Condition Figure 2.5: Stre and Strain Diagram for Concrete-Controlled Failure Figure 2.6: Stre and Strain Diagram for Compoite-Controlled Failure Figure 3.1: Detail for the Reinforced Concrete Beam Figure 3.2: Schematic of Bottom Application Figure 3.3: Schematic of Bottom Application with Tranvere Strap Figure 3.4: Schematic of Side Application Figure 3.5: Schematic of Side Application with Tranvere Strap Figure 3.6: Elevation of Tet Setup Figure 3.7: End View of Tet Setup Figure 3.8: Strength - Age Curve Figure 3.9: Stre-Strain Relationhip for Concrete in Compreion Figure 3.10: Stre Strain Repone for No. 5 Bar from Heat xi

12 Figure 3.11: Stre Strain Repone for No. 5 Bar from Heat Figure 3.12: Compoite Matrix Figure 3.13: Schematic of Wet Lay-up Compoite Application Component Figure 3.14: Tow Sheet Flexible Fiber Type Figure 3.15: Fabric Weave Flexible Fiber Type Figure 3.16: Surface Preparation Figure 3.17: Application of Primer Figure 3.18: Fiber Preparation Figure 3.19: Wet Lay-up Method 1 Epoxy Application Figure 3.20: Wet Lay-up Method 1 Fiber Application Figure 3.21: Wet Lay-up Method 2 Epoxy Application Figure 3.22: Wet Lay-up Method 2 Fiber Saturation Figure 3.23: Wet Lay-up Method 2 Fiber Application Figure 3.24: Schematic of Pultruded Compoite Application Component Figure 3.25: Pultruion Proce Figure 3.26: Cleaning Compoite Figure 3.27: Epoxy Application to Compoite Figure 3.28: Epoxy Application to Concrete Surface Figure 3.29: Compoite Application Figure 3.30: Rolling Compoite to Remove Exce Epoxy and Air Void Figure 3.31: Strain Gage Location Figure 4.1: Typical Load-Deflection Repone Figure 4.2: Initial Cracking in Epoxy xii

13 Figure 4.3: Whitening and Fanning of Crack Figure 4.4: Initial Evidence of Debonding - Crack Propagating Parallel to Compoite Figure 4.5: Debonding Failure Figure 4.6: Typical Cracking Pattern and Crack Width Prior to Failure Figure 4.7: Photograph of Relative Diplacement Figure 4.8: Photograph of Debonding Initiated by Prying Action Figure 4.9: Free Body Diagram of Crack in Contant Moment Region and in Shear Span Figure 4.10: Debonding Between Tranvere Strap Figure 4.11: Rupture of Compoite at Critical Section Figure 4.12: Strength Expreed a a Percentage of the Computed Capacity for Each Strengthened Specimen Figure 4.13: Strain Ratio for All Specimen Figure 4.14: Load-Deflection Repone for Different Bond Length Figure 4.15: Load-Deflection Repone for Equivalent Material Figure 4.16: Load-Deflection Repone for Equivalent Bond Stre Figure 4.17: Load-Deflection Repone for C Sytem Surface Preparation Figure 4.18: Evidence of Poor Compoite Bond Figure 4.19: Load-Deflection Repone for D Sytem Surface Preparation Figure 4.20: Evidence of Good Compoite Bond Figure 4.21: Debonding of Strap xiii

14 Figure 4.22: Load-Deflection Repone for B Sytem Beam with Tranvere Strap Figure 4.23: Load-Deflection Repone for C Sytem with Tranvere Strap Figure 4.24: Strain Profile at Yield and Ultimate for Specimen B2 and C Figure 4.25: Typical Cracking Aociated with Side Application Figure 4.26: Load-Deflection Repone for B Sytem Side Application Figure 4.27: Specimen B3 at Failure Figure 4.28: Load-Deflection Repone for C Sytem Side Application Figure 4.29: Specimen D3 at Failure Figure 4.30: Load-Deflection Repone for D Sytem Side Application Figure 4.31: Debonding Failure with Strap Rupture for Specimen with Side Application with Strap Figure 4.32: Load-Deflection Repone for D Sytem Side Application with Tranvere Strap Figure 4.33: Average CFRP Strain at Rupture for the B Compoite Sytem and Bottom Application with Tranvere Strap Figure 4.34: Average CFRP Strain for the D Compoite Sytem and Side Application with Tranvere Strap xiv

15 Chapter 1: Introduction 1.1 RESEARCH MOTIVATION Current AASHTO Standard Specification for Highway Bridge require that new bridge tructure be deigned to upport at leat an HS-20 truck load like that illutrated in Figure 1.1 (AASHTO, 1996). Many flat lab, pan joit, and T- beam concrete bridge were contructed in Texa between the 1930 and 1950 for H-10, H-15 and H-20 loading (Figure 1.2). The Texa Department of Tranportation, herein referred to a TxDOT, ha etimated that 18,000 bridge in ue in the tate were deigned for le than HS-20 loading criteria (BRINSAP, Dec 1998). Becaue tandard deign live load have increaed over the year, many bridge in the tate have retriction on the amount of live load, or truck ize, that can pa over the bridge. Bridge with live load retriction are poted for ue only by truck that meet the acceptable live load criteria. Load poting make it difficult to route truck acro the tate, o trengthening deficient bridge may be an attractive alternative to load poting. In addition, road uage ha increaed over the year, which ha created a demand for widening ome bridge. According to TxDOT tandard, a bridge mut atify HS-20 loading criteria in order to be a candidate for widening. 1

16 to 30-0 To Produce Maximum Loading 6 K 8 K 24 K HS K 32 K HS K Figure 1.1: Loading Criteria for Standard HS Truck (AASHTO, 1996) K H K 6 K H K 8 K H K Figure 1.2: Loading Criteria for Standard H Truck (AASHTO, 1996) 2

17 A mentioned previouly, many bridge are currently load poted to protect againt overload. However, it i common knowledge in the bridge deign profeion that thee bridge are often ubjected to overload from the agricultural trucking indutry and concrete indutry (BRINSAP, 1997). For example, TxDOT bridge engineer are aware of logging truck weighing a much a 140 kip uing bridge poted for H-15 loading which permit a total two-axle load of 30 kip. It i likely that thee violation are accelerating bridge deterioration, and clearly TxDOT engineer are in need of a olution to thi problem. Becaue load poting will not prevent overloaded vehicle from cauing damage to bridge with inufficient trength, trengthening may be the bet long-term olution. 1.2 JUSTIFICATION FOR STUDYING COMPOSITE MATERIALS There are many option available for increaing the capacity of exiting bridge, uch a attaching teel plate to exiting girder, adding tructural concrete, or completely replacing the bridge. The different method available for increaing the capacity of hort-pan, off-ytem (non-intertate highway) bridge, where flexural trength i deficient, are illutrated chematically in Figure 1.3 for rectangular reinforced concrete beam like thoe ued in thi tudy. Compoite fiber ytem were choen in lieu of more traditional method for the reaon that follow. 3

18 Reinforced Concrete Beam Strengthened with FRP Reinforced Concrete Beam Strengthened with Steel Plate Reinforced Concrete Beam Strengthened with Added Section New Reinforced Concrete Beam Section Figure 1.3: Method for Increaing the Capacity of Exiting Concrete Beam In the pat, bridge have been renovated with teel plate anchored to the extreme tenion fiber of concrete beam. Becaue a large amount of material may be required to achieve the appropriate trength level, the teel plate will likely be too heavy to lift without the aitance of a crane or other heavy equipment. In addition, teel plate will add extra dead load to an exiting bridge. Becaue the bridge may cro over a roadway, thi contruction procedure could reult in the roadway being out of ervice during the trengthening proce. There i alo the concern that the teel plate will corrode when expoed to weather or deicing chemical, leading to reduction in plate ection, or corroion will occur between the teel plate and concrete urface (Meier, 1995), reulting in expanion force between the concrete and teel. Another method for trengthening exiting girder i the addition of teel reinforcement and concrete to the extreme tenion fiber of girder. Thi method alo add dead load and reduce clearance beneath bridge. 4

19 There i alway the option to replace an exiting bridge with a new, tronger tructure. In ome cae, thi may be the mot economical olution if labor cot are low. For ome cae thi may not be an option becaue of the hitorical ignificance of the tructure. Compoite ytem are lightweight, thin, eay to intall, and quite trong. Subtantially le material i required to develop the ame trength a adding teel plate or reinforced concrete to a ection. Material cot for a given required trength are ignificantly higher than for material ued in traditional trengthening method, but ufficient time can be aved during contruction. The aving in labor cot can offet additional material cot aociated with the compoite material (Baett, 1997). The major drawback to uing compoite for trengthening i the limited amount of reearch that ha been performed related to attaching compoite to reinforced concrete member. Deign engineer generally do not have acce to guideline for deign and uage of compoite material. Specialized companie with an engineering taff typically have been reponible for the deign and application of compoite for trengthening of tructure. Figure 1.4 and 1.5 how example of recently trengthened tructure uing CFRP for flexural and hear trength enhancement. 5

20 Figure 1.4: CFRP Application (Mater Builder, 1998) Figure 1.5: CFRP Flexural and Shear Strengthening (Sika, 1999) 6

21 1.3 RESEARCH OBJECTIVE The objective of thi thei wa to develop method for reliably attaching carbon fiber compoite to reinforced concrete beam for the purpoe of increaing flexural trength, and to develop recommendation for etimating the capacity of the trengthened beam. The reult were ued in the deign of trengthened full-cale beam tet, and eventually may be implemented for ue in deigning CFRP trengthening cheme for exiting bridge. Phae I of the overall reearch program, decribed herein, invetigated the behavior of reinforced concrete beam with four type of commercially available carbon fiber compoite material applied uing different configuration. Recommendation for attaching CFRP to the beam and for calculating the capacity of the trengthened member were developed from the tet reult. 1.4 SCOPE Chapter 2 decribe the analytical model developed to etimate the flexural capacity of rectangular reinforced concrete beam ection trengthened with carbon fiber compoite. Chapter 3 preent a decription of the experimental program, and Chapter 4 evaluate the tet reult. A ummary of the teting program and recommendation are preented in Chapter 5. 7

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23 Chapter 2: Analytical Model ued to Etimate the Flexural Capacity of Strengthened Cro Section 2.1 OBJECTIVE The purpoe of thi chapter i to preent the analytical model ued to etimate the capacity of the reinforced concrete pecimen teted during thi phae of the reearch project. However, thi procedure i not intended to be ued for deign becaue important iue related to bond and anchorage of the compoite material to the concrete are not included explicitly in the model. 2.2 ASSUMPTIONS In order to develop a model for etimating flexural capacity of rectangular, reinforced concrete beam trengthened uing CFRP compoite, the following implifying aumption were made: 1) The compoite i bonded perfectly to the concrete urface, 2) Plane ection remain plane, and 3) Load carried by the reinforced concrete member at the time that the CFRP compoite are intalled are ignificantly lower than the yield load; however, the reinforced concrete member may be cracked. 2.3 IDEALIZED MATERIAL BEHAVIOR Compoite Material The compoite i idealized a a linear-elatic material (Figure 2.1). Rupture train reported by the manufacturer typically range from 0.7% to 2.2% (Cape Compoite Incorporated, 2000). The manufacturer upplie critical 9

24 compoite parameter, uch a rupture train, that are needed for deign of trengthened beam. fpu Ep εpu Figure 2.1: Idealized Stre-Strain Relationhip for Compoite Material Reinforcing Steel The tre-train relationhip for teel reinforcement i idealized by a trilinear relationhip a hown in Figure 2.2. The tri-linear model account for yielding and train hardening. Ultimate tre and train were not pecified becaue rupture of the tenile teel reinforcement wa not anticipated during teting. fy Eh E εy εh Figure 2.2: Idealized Stre-Strain Relationhip for Steel Reinforcement 10

25 2.3.3 Concrete The modified Hognetad model (1951) hown in Figure 2.3 wa choen to decribe the tre-train behavior of concrete in compreion. The rectangular tre block typically ued in deign and recommended by ACI Committee 318 (1999) for ue in flexural trength calculation cannot be ued to determine the capacity of a trengthened cro ection. The rectangular tre block i only valid when the extreme compreion fiber reache the cruhing train. For ome of the trengthened beam, the extreme compreion fiber train will be le than the cruhing train when the compoite material rupture. Therefore, the nonlinear relationhip between tre and train in the concrete mut be conidered explicitly in thi procedure. The tenile trength of the concrete wa ignored. f c ε 0 ε cu = ' ε ε ε ε cu c c f c f = ' ε ε ε ε c c c f c f f c ε 0 ε cu = ' ε ε ε ε cu c c f c f = ' ε ε ε ε c c c f c f Figure 2.3: Idealized Stre-Strain Relationhip for Concrete in Compreion 11

26 2.4 PROCEDURE USED TO CALCULATE THE FLEXURAL CAPACITY OF STRENGTHENED BEAMS The failure condition for a reinforced concrete beam trengthened uing CFRP compoite can be defined by cruhing of the concrete or rupture of the compoite. The actual failure mode i determined by the area of compoite attached to the cro ection and the hear tre at which the compoite debond from the urface of the concrete. A indicated in Section 2.2, perfect bond between the compoite and the concrete wa aumed in the analytical model. Therefore, the area of the compoite will determine the mode of failure in thi analyi. An approach imilar to that ued for reinforced concrete member will be ued to determine the mode of failure in the trengthened cro ection. The area of compoite correponding to imultaneou cruhing of the concrete and rupture of the compoite will be defined a the balanced train condition. When a maller area of compoite i attached to the cro ection, failure will be controlled by rupture of the compoite. If a larger area of compoite i ued, failure will be controlled by cruhing of the concrete. The area of compoite material correponding to balanced train condition i decribed in Section Analyi procedure for cro ection controlled by cruhing of the concrete are decribed in Section 2.4.2, and procedure for cro ection controlled by rupture of the compoite are decribed in Section

27 2.4.1 Calculation of Compoite Material Area Correponding with Balanced- Strain Condition Ditribution of train and tre correponding to balanced train condition are hown in Figure 2.4. The following procedure may be ued to determine the correponding area of compoite material. b εcu A d c ε x Cc C h d N.A. tp dp=h+tp/2 A bp CFRP Strain Diagram ε εpu Stre Diagram T Tp Figure 2.4: Stre and Strain Diagram for Balanced-Strain Condition f pu 1) Aume ε c = ε cu, ε p = ε pu =. E 2) From train compatibility, olve for c, ε and ε. p ε cud p c = ( 2-1) ε + ε ) ( cu pu pu ( d c) ε = ε ( 2-2) ( d c) p ' c ( c d') ε = ε ( 2-3) c 3) Calculate teel tree f and f. 13

28 If ε ε, then f ' y ' y = ε E h h If ε ε, then f f = ε E If ε < ε ε, then f = f y = f y + ( ε ε ) * E h h ( 2-4) 4) Calculate the reultant concrete compreion force. Ue modified Hognetad model hown in Figure 2.3 for the concrete tre-train relationhip. C c c = f cbdy ( 2-5) 0 5) Compute tenion and compreion force in teel. C T = A ' f ' = A f ( 2-6) 6) From equilibrium, olve for T p. T p = C + C T ( 2-7) c 7) Calculate area of compoite required for balanced train condition. Tp A p, b = ( 2-8) f pu 8) Determine which material control failure. If If A A p p A > A p, b p, b, then compoite rupture control computed capacity. ( 2-9), then concrete cruhing control computed capacity. 14

29 2.4.2 Calculation of Flexural Capacity for Concrete-Controlled Failure Ditribution of train and tre correponding to concrete-controlled failure condition are hown in Figure 2.5. The following procedure may be ued to determine the correponding flexural capacity of the trengthened croection. b εcu A d c ε x Cc C h d N.A. tp dp=h+tp/2 A bp CFRP ε T Tp εp Strain Diagram Stre Diagram Figure 2.5: Stre and Strain Diagram for Concrete-Controlled Failure 1) c ε cu ε = 2) Aume c, then from train compatibility compute ε p, ε and ε. cu ( d p c) ε p = ε ( 2-10) c cu ( d c) ε = ε ( 2-11) c ' cu ( c d') ε = ε ( 2-12) c 3) Calculate teel and compoite tree. 15

30 If ε ε, then f ' y ' y = ε E h h If ε ε, then f f = ε E If ε < ε ε, then f = f y = f y + ( ε ε ) * E h h ( 2-13) f p = ε E ( 2-14) p p 4) Calculate the reultant concrete compreion force and location of the reultant concrete compreion force, x, meaured from the neutral axi. Ue the modified Hognetad model hown in Figure 2.3 for the concrete tre-train relationhip. C c c = f cbdy ( 2-15) 0 x c 0 = c 0 f c f bydy c bdy ( 2-16) 5) Compute the tenion and compreion force in the longitudinal teel and compoite. C T T p = A ' f ' = A = A p f f p ( 2-17) 6) Check equilibrium. I C + C = T + T p? ( 2-18) c 16

31 7) Repeat tep 2 through 6 until T + T p = C + C c. 8) Compute the reultant location for the tenile force from top of the ection. g T = ( T * d T + T p + T p * d p ) ( 2-19) 9) Compute the reultant location for the compreion force from top of the ection. g C ( C = * d' C + C c + C * ( c x)) c ( 2-20) 10) Compute flexural capacity of the trengthened cro ection. M ult = T + T ) *( g g ) ( 2-21) ( p T C Calculation of Flexural Capacity for Compoite-Controlled Failure Ditribution of train and tre correponding to compoite-controlled failure condition are hown in Figure 2.6. The following procedure may be ued to determine the correponding flexural capacity of the trengthened croection. 17

32 b εc A d c ε x Cc C h d N.A. tp dp=h+tp/2 A bp CFRP Strain Diagram ε εpu Stre Diagram T Tp Figure 2.6: Stre and Strain Diagram for Compoite-Controlled Failure 1) ε p = ε pu 2) Aume c, then from train compatibility compute ε c, ε and ε. puc ε c = ε ( 2-22) ( d c) p pu ( d c) ε = ε ( 2-23) ( d c) p ' c ( c d') ε = ε ( 2-24) c 3) Calculate teel tree. If ε ε, then f ' y ' y = ε E h h If ε ε, then f f = ε E If ε < ε ε, then f = f y = f y + ( ε ε ) * E h h ( 2-25) 18

33 4) Calculate the reultant concrete compreion force and location of the reultant concrete compreion force, x, meaured from the neutral axi. Ue the modified Hognetad model hown in Figure 2.3 for the concrete tre-train relationhip. C c c = f cbdy ( 2-26) 0 x c 0 = c 0 f c f bydy c bdy ( 2-27) 5) Compute the tenion and compreion force in the longitudinal teel and compoite. C T T p = A ' f ' = A = A p f f pu ( 2-28) 6) Check equilibrium. I C + C = T + Tp? ( 2-29) c 7) Repeat tep 2 through 6 until T + T p = C + C c. 8) Compute the reultant location for tenile force from top of the ection. g T = ( T * d T + T p + T p * d p ) ( 2-30) 19

34 9) Compute the reultant location for the compreion force from top of the ection. g C ( C = * d' C + C c + C * ( c x)) c ( 2-31) 10) Compute flexural capacity of the trengthened cro ection. M ult = T + T ) *( g g ) ( 2-32) ( p T C 20

35 Chapter 3: Experimental Program 3.1 INTRODUCTION Thi chapter preent a detailed decription of the experimental program. Twenty rectangular, reinforced concrete beam were fabricated in Ferguon Structural Engineering Laboratory. Eighteen of thee beam were trengthened with carbon fiber compoite to enhance flexural capacity before they were loaded to failure. 3.2 SPECIMEN DETAILS Decription of Beam Section Becaue of difference in trength of the carbon fiber compoite elected for trengthening the rectangular reinforced concrete beam, two ize of beam were ued in the experimental program: 8"x14" by 9-6 and 8 x16 by Each beam wa reinforced with two #5 bottom bar, two #3 top bar and No. 6 gage wire tirrup paced 4 in. on center. See Figure 3.1 for the reinforcement detail. In order to create a baeline for comparion of reult, the firt crack wa forced to occur at a point of maximum moment, defined in ection Thi crack i referred to a the crack initiator and wa created by placing a mall piece of heet metal, approximately in. thick and extending 0.25 in. deep, acro the full width of the beam. The maximum train in the beam were meaured at thi location. 21

36 2 #3 2 #5 No. 6 Gage Wire 5" 11" 14" 4" 4" = 3'-4" 2'-2" 4" = 3'-4" 4" 9'-6" 8" 2 #3 2 #5 No.6 Gage Wire 5" 13" 16" 4" 4" = 4'-0" 1'-10" 4" = 4'-0" 4" 8" 10'-6" Figure 3.1: Detail for the Reinforced Concrete Beam Compoite Strengthening Scheme Becaue the primary objective of thi portion of the reearch tudy wa to identify effective method for attaching carbon fiber compoite to implyupported reinforced concrete beam in order to increae their flexural trength, carbon fiber wa firt bonded to the bottom urface of the beam in order to ue the material mot efficiently. However, all of the early tet pecimen failed with the compoite delaminated from the bottom urface of the concrete. A teting progreed, it became apparent that bonding material to the ide, rather than the bottom of beam, might delay bond failure at the concrete/compoite interface. Furthermore, the ue of tranvere compoite trap wa invetigated for improving anchorage of longitudinal compoite trip. 22

37 In all, four different configuration of the compoite were invetigated: bottom application, bottom application with tranvere trap, ide application, and ide application with tranvere trap (Figure 3.2 through 3.5). Each configuration i dicued in the following ubection Bottom Application The experimental parameter in thi phae of the tudy were elected to invetigate the effect of bonded length of compoite, relative to a critical ection, on anchorage of the carbon fiber compoite for flexural trengthening. Carbon fiber compoite wa bonded to the extreme tenion fiber and i referred to herein a "bottom application". Compoite material wa bonded almot the full length of the hear pan on one end of each beam. The other end had the compoite bonded a pecified ditance from a crack initiator that wa located beneath the cloet load point. The ditance from the crack initiator to the end of the compoite wa defined a the bonded length (Figure 3.2). The objective of the tet with the CFRP applied to the bottom urface of the cro ection were to compare trength obtained from beam with different bonded length and identify the bonded length neceary to develop the trength of the carbon fiber compoite. The tet were intended to be analogou to tet conducted to define development length of reinforcing bar (Ferguon, 1962). 23

38 P Crack initiator Bonded Length FRP Figure 3.2: Schematic of Bottom Application Bottom Application with Tranvere Strap Tet pecimen with bottom application of the compoite tended to fail prematurely becaue vertical movement along crack caued the compoite to debond from the bottom urface of the cro ection. Becaue pecimen with bottom application of the compoite did not develop the trength of the compoite, even when the material wa bonded over nearly the entire hear pan, an enhancement wa needed to prevent the compoite from debonding prematurely. Several publication have uggeted uing bolt to help anchor the compoite to the concrete cro ection (Spadea, 1998). Thi option wa dimied becaue the compoite fiber, which are unidirectional, would likely plit and pull out around the bolt. Other recommendation include clamping the compoite with plate and bolt or clamping the compoite with tranvere compoite trap. 24

39 The option choen wa tranvere compoite trap (Figure 3.3). The trap were imilar to thoe teted previouly to enhance hear trength (Triantafillou, 1998). Narrow, 2-in. wide compoite trip were placed over the longitudinal compoite and up the ide face of each beam 3 in. from the top urface of the beam. The trap, which were paced at h/2 on center, were intended to permit ome debonding between trap while keeping the compoite anchored, and to retrain hear crack in order to prevent vertical offet from occurring along the bottom face of beam. P Figure 3.3: Schematic of Bottom Application with Tranvere Strap Side Application Becaue bottom application of the compoite (without tranvere trap) tended to reult in udden debonding of the material due to a vertical offet at ome crack location on the bottom face of beam, ide application of the compoite (Figure 3.4) wa tried to determine if thi application would be le affected by the relative diplacement at crack location. 25

40 P Figure 3.4: Schematic of Side Application Side Application with Tranvere Strap Beam with compoite material applied to ide face generally failed becaue of debonding at the compoite/concrete interface and/or tenile failure of the concrete adjacent to the compoite. Tranvere trap, imilar to thoe ued with the bottom application, were attached to provide additional anchorage for the longitudinal carbon fiber and to ditribute interface tree to more concrete. A for the bottom application cae, trap were 2 inche wide and were paced h/2 on center. P Figure 3.5: Schematic of Side Application with Tranvere Strap 26

41 3.2.3 Overview of Tet Program A erie of twenty pecimen wa teted. Two of the pecimen were untrengthened to provide baeline trength and deformation data to be ued in evaluating the repone of trengthened pecimen. Tet variable included application type and method, compoite type and manufacturer, number of plie of compoite material applied, bonded compoite length, number of tranvere trap applied, and effect of urface preparation. Table 3.1 ummarize the tet program. 27

42 Table 3.1: Summary of Tet Program Specimen Concrete Batch # Compoite Type # of Plie Approximate Width of Plie Bonded Length (in) # of Tranvere Strap Along Shear Span Application Type Concrete Surface Preparation 28 Control A&B I A1 I A 2 2" 10 0 Bottom Grind A2 I A 2 2" 14 0 Bottom Grind A3 I A 2 2" 30 0 Bottom Grind A4 II A 1 4" 15 0 Bottom Grind B1 II B 2 3" 35 0 Bottom Grind B2 II B 2 2" 35 7 Bottom w/ Strap Grind B3 II B 2 2" 35 0 Side Grind B4 III B 2 2" 35 4 Bottom w/ Strap Grind B5 III B 2 2" 24 4 Bottom w/ Strap Grind Control C&D I C1 I C 2 2" 45 0 Bottom Grind C2 I C 2 2" 45 0 Bottom Sand-blat C3 I C 2 2" 45 7 Bottom w/ Strap Grind C4 I C 2 2" 45 0 Side Grind D1 II D 1 2" 45 0 Bottom Grind D2 II D 1 2" 45 0 Bottom Sand-blat D3 II D 2 2" 45 0 Side Grind D4 III D 2 2" 45 7 Side w/ Strap Grind D5 III D 2 2" 30 4 Side w/ Strap Grind Note: Specimen trengthened with A and B compoite were 8" x 14" x 9'-6". Specimen trengthened with C and D compoite were 8" x 16" x 10'-6". Specimen from concrete batch I and II ued teel from Heat 1. Specimen from concrete batch III ued teel from Heat 2.

43 3.3 TEST SETUP The tet etup wa deigned to apply a two-point loading to beam trengthened with carbon fiber compoite. Load wa applied uing a 140-kip capacity Enerpac hydraulic ram in conjunction with a tiffened teel wide flange ection placed on neted roller on top of the beam (Figure 3.6 and 3.7). Beam were upported by 3 x7 x2 neoprene bearing pad that were placed on concrete pedetal capable of accommodating both the 9-6 and 10-6 pan. LOADING FRAME HYDRAULIC RAM CONCRETE SPECIMENS LOAD CELL SPHERICAL BEARING LOADING BEAM CONCRETE PEDESTAL 2 LINEAR POTENTIOMETER 1'-2" or 1'-4" 2" 3' -0" 4" 3'- 6" or 4'-0" 1'-10" 3'-6" or 4'-0" 4" 9'- 6" or 10'- 6" Figure 3.6: Elevation of Tet Setup 29

44 LOADING FRAME HYDRAULIC RAM SPHERICAL BEARING LOADING BEAM CONCRETE SPECIMENS 2 LINEAR POTENTIOMETER CONCRETE PEDESTAL 4'-0" Figure 3.7: End View of Tet Setup 3.4 LOADING PROGRAM Each beam wa teted by applying monotonically-increaing load to failure. Load wa applied at two point paced 22 in. apart and centered along the pan in approximately 1500-lb increment (3000-lb total) until yield. Beyond yield, midpan diplacement of beam wa increaed in approximately 0.15-in. increment until failure occurred. Between load or diplacement increment, beam were inpected for crack and for damage to the compoite trengthening ytem. Crack pattern were recorded, critical crack width were meaured, and damage wa photographed. Becaue of leakage in the hydraulic ytem, load on 30

45 beam dropped during the inpection proce. Thi load wa reapplied before proceeding to the next load or diplacement increment. 3.5 MATERIAL PROPERTIES Concrete A total of twenty pecimen were contructed uing three batche of concrete. The trength-age curve for each batch of concrete i hown in Figure 3.8. For each batch, cylinder were typically teted after 3, 7, 14, and 28 day at a rate of 50 kip/minute to meaure the concrete compreive trength (Table 3.2, 3.3, and 3.4). Intead of teting cylinder the day of every beam tet, cylinder were teted the firt day of beam teting (econd to lat entry in Table 3.2, 3.3, and 3.4) and the lat day of beam teting (the lat entry in Table 3.2, 3.3, and 3.4) for each concrete batch. Thee trength typically correpond to different age depending on the batch becaue of the flexible teting chedule. Thee average cylinder trength on the firt and lat day of beam teting were ued to calculate the average cylinder trength for each batch of concrete. Becaue cylinder trength did not vary ignificantly, an average of the tet day trength wa computed (Table 3.5) and ued for f c ' in the calculation of pecimen capacitie. 31

46 Batch I Batch II Batch III Time (day) Figure 3.8: Strength - Age Curve Table 3.2: Cylinder Data for Concrete Batch I No. Day No. of Cylinder Average f'c (pi) Standard Deviation (pi)

47 Table 3.3: Cylinder Data for Concrete Batch II No. Day No. of Cylinder Average f'c (pi) Standard Deviation (pi) Table 3.4: Cylinder Data for Concrete Batch III No. Day No. of Cylinder Average f'c (pi) Standard Deviation (pi) Table 3.5: Average Concrete Compreive Strength on Day of Tet Batch # Average f'c (pi) I 5040 II 5290 III 5020 Average 5120 Two cylinder were teted from Concrete Batch III to obtain a repreentative tre-train relationhip for the concrete. The value for ε 0 wa 33

48 taken from thee curve a and ε cu wa choen a for ue in the modified Hognetad model (Hognetad, 1951) during the calculation of pecimen capacitie. The curve are hown in Figure Modified Hognetad Model Typical Cylinder Strain Figure 3.9: Stre-Strain Relationhip for Concrete in Compreion Steel Reinforcement Reinforcement had a nominal yield tre of 60 ki. To determine the actual tre-train relationhip for the teel reinforcement, tenion coupon were teted at a maximum train rate of /minute. Two #5 bar were teted from the firt batch of teel that wa received, Heat 1, and three were teted from Heat 2. For beam capacity calculation, an idealized tri-linear relationhip wa ued to repreent the tre-train repone for the teel reinforcement. The model developed for each heat of teel i hown in Figure 3.10 and 3.11 along with the meaured tre-train repone for the bar teted. 34

49 The yield tre of compreion reinforcement, #3 bar, wa determined from teting to be 60 ki for Heat 1 and 66 ki for Heat 2. Unlike the tenile reinforcement, the compreion reinforcement did not yield during teting. Therefore, the complete tre-train relationhip wa not meaured for the #3 bar Strain Figure 3.10: Stre Strain Repone for No. 5 Bar from Heat 1 35

50 Strain Figure 3.11: Stre Strain Repone for No. 5 Bar from Heat Compoite Material Type & Application Method Compoite fiber ytem are available in many form. Fiber can be made of gla, aramid, carbon, or graphite. Unidirectional, carbon fiber were choen for teting becaue of the higher modulu and tenile trength aociated with the material. A a reult, le material wa needed compared with other compoite material to obtain the ame capacity for the trengthened beam. Carbon fiber alo i lightweight and ha excellent fatigue reitance compared with other compoite (Inoue, 1995). Carbon fiber compoite alo have better long-term performance becaue they are not uceptible to creep, which can control the behavior of gla compoite. Thee fiber can be manufactured and applied in everal way. 36

51 Carbon fiber compoite are made of two material: rein and carbon fiber (Figure 3.12). The rein provide for tranfer of force between individual fiber. It alo provide ome compreive trength and chemical protection. The fiber provide tiffne and tenile trength to the ytem. Gla Polyeter Fiber Material Aramid (Kevlar) Carbon Polymer Matrix Epoxy Vinyleter Function: Provide tiffne Tenile trength Function: Force tranfer to fiber Compreive trength Chemical protection Figure 3.12: Compoite Matrix a) Wet Lay-up One type of compoite application i the wet lay-up ytem illutrated in Figure Thi ytem arrive on ite a a roll of flexible, unidirectional fiber and everal container of epoxy component. Thee flexible heet can conform to nearly any concrete urface. Epoxy for thi ytem i very fluid and i intended to act not only a the bonding agent but alo form the compoite matrix. Fabric 37

52 fiber are produced in everal form a hown in Figure 3.14 and Two form were ued for thee tet: a tow heet and a weave. The tow heet i a layer of unidirectional fiber attached to a heet of paper. The paper i ued only for application purpoe. The fabric weave i bundle of unidirectional fiber woven with tranvere fiber. The tranvere fiber are a mall fraction of the longitudinal fiber and are intended only to maintain integrity of the main fiber during application. Unidirectional Carbon Fiber Epoxy Epoxy Primer Ground Concrete Surface Figure 3.13: Schematic of Wet Lay-up Compoite Application Component 38

53 Figure 3.14: Tow Sheet Flexible Fiber Type Figure 3.15: Fabric Weave Flexible Fiber Type 39

54 The application proce begin by preparing the concrete urface. It i important to grind the concrete urface to mooth any irregularitie, remove cale, and roughen the urface to enhance bond between the fiber compoite and concrete (Figure 3.16). Once the urface ha been properly ground, all dut mut be removed from the urface by cleaning with a cotton towel and acetone to achieve good bond. After the urface i prepared, it i recommended by the manufacturer to ue an epoxy primer to eal the concrete urface (Figure 3.17). Epoxie ued in the wet lay-up ytem have low vicoity and are eaily aborbed by the concrete. The primer i applied firt to enure the concrete doe not aborb the epoxy needed to impregnate the compoite material and form the compoite matrix. Figure 3.16: Surface Preparation 40

55 Figure 3.17: Application of Primer The next tep in the proce i cutting the fabric to the deired ize (Figure 3.18). Then the epoxy i mixed according to manufacturer pecification and applied to the prepared urface of the concrete. At thi point, the proce varie from product to product. The objective of the next tep, which can be done in everal way, i to impregnate the fiber with epoxy to form the compoite matrix. Two different method are decribed below. 41

56 Figure 3.18: Fiber Preparation Method 1 begin by applying the epoxy to the urface of the concrete and then applying the fiber over thi layer of epoxy (Figure 3.19 and 3.20). Once the fiber are in place, it i recommended by the manufacturer to wait approximately 20 minute to allow the epoxy to impregnate the fiber before applying the final coat of epoxy. Finally, the compoite i allowed to cure at room temperature for even day. 42

57 Figure 3.19: Wet Lay-up Method 1 Epoxy Application 43

58 Figure 3.20: Wet Lay-up Method 1 Fiber Application Method 2 begin by applying the epoxy to the prepared urface of the beam (the particular application hown in Figure 3.21 doe not have a primer applied to the urface). Then the fiber mut be immered in epoxy (Figure 3.22) before applying the fiber to the concrete urface (Figure 3.23). Then a final coat of epoxy i applied to the fiber once they are in place. Finally, the compoite i allowed to cure at room temperature for even day. 44

59 Figure 3.21: Wet Lay-up Method 2 Epoxy Application Figure 3.22: Wet Lay-up Method 2 Fiber Saturation 45

60 Figure 3.23: Wet Lay-up Method 2 Fiber Application Variability in compoite behavior i introduced becaue of the many tep required in the wet lay-up proce. It i important to follow manufacturer procedure carefully in order to control the quality of the compoite matrix. b) Pultruded Another type of compoite ytem i the pultruded ytem illutrated in Figure Thi ytem arrive on ite in the form of tiff, thin plate that can be applied only to flat urface. The plate are quite tiff becaue the fiber have been pre-impregnated in a fluid epoxy at the manufacturing plant. 46

61 Epoxy (Bonding agent) Pultruded Carbon Fiber Reinforced Polymer Plate Ground or Sandblated Concrete Surface Figure 3.24: Schematic of Pultruded Compoite Application Component The pultruion proce begin with a roving of fiber bundle that are pulled through an epoxy or rein bath. The epoxy bath impregnate the fiber to form the compoite. The impregnated fiber are then pulled through a pre-former where they are given a general cro-ection hape. Then they are pulled through the forming and curing die where the compoite i given it final hape and i cured. Finally, the compoite i cut to the deired length requeted by the purchaer. The proce i ummarized in Figure

62 Pre-former Hydraulic Puller Fiber Roving Rein Bath Forming and Curing Die Cutting Saw Figure 3.25: Pultruion Proce The pultruded application proce begin with urface preparation by grinding or andblating the urface. With pultruded ytem, no primer i needed. There i no fear of the concrete aborbing epoxy needed to form the compoite matrix becaue the fiber have been pre-impregnated at the manufacturing plant. The next tep in the proce i cleaning the compoite with a clean towel and acetone (Figure 3.26). Then a thin layer of epoxy (1/16 to 1/8 ) i applied to the compoite urface and concrete urface (Figure 3.27 and 3.28). If the epoxy i too thick, bond will likely be compromied becaue the epoxy will be required to reit force for which it wa not deigned. The epoxy for thi ytem i vicou and act only a a bonding agent between the fiber compoite and concrete urface. Next, the plate are preed into place againt the epoxy and a roller i ued to pre all exce epoxy and air pocket out from under the plate (Figure 3.29 and 3.30). Exce epoxy i removed and the ytem i allowed to cure for even day. 48

63 Figure 3.26: Cleaning Compoite Figure 3.27: Epoxy Application to Compoite 49

64 Figure 3.28: Epoxy Application to Concrete Surface 50

65 Figure 3.29: Compoite Application Figure 3.30: Rolling Compoite to Remove Exce Epoxy and Air Void 51

66 Pultruded compoite offer better quality control in the formation of the compoite matrix becaue they are fabricated in a controlled environment. However, ue for the material are limited becaue the ytem i quite tiff and cannot conform to all urface Material Propertie Five different carbon fiber compoite ytem were ued in thi tudy. Material propertie varied from product to product even though the main ingredient in the compoite wa carbon fiber. Each manufacturer ha it own tandard for producing and teting their product. Material propertie are tabulated in Table 3.6. Compoite thickne i the deign thickne and i defined a the carbon fiber thickne for ue in calculating force in the compoite for one ply of material. The manufacturer publihe thi value. Compoite trength i the tenile trength of the compoite matrix. The compoite modulu i the modulu of elaticity for the compoite matrix and i defined a the lope of the tretrain repone for the material. Thi property i alo provided by the manufacturer. The product lited in Table 3.6 were provided by Fyfe, Mater Builder, Mitubihi/Replark, and Sika, and the product propertie were obtained from the repective product pecification. 52

67 Table 3.6: Carbon Fiber Compoite Material Propertie a Publihed by Manufacturer A B C D E Compoite Thickne (in) Compoite Strength (ki) Compoite Modulu (ki) 33,000 33,400 9,000 22,500 10,600 Elongation at Failure (%) * 1.33 Compoite Type Wet Lay-up Wet Lay-up Wet Lay-up Pultruded Wet Lay-up Manufacturer Mater Builder Mitubihi Fyfe Sika Sika *Strain a publihed by manufacturer. f pu /E p = INSTRUMENTATION AND DATA ACQUISITION Strain gage were bonded to teel reinforcing bar, ide face of the beam in the concrete compreion zone, and on carbon fiber compoite typically at the critical ection (Figure 3.31). The crack initiator wa located directly under one load point and wa ued to define the critical ection, which alo defined the edge of the contant moment region of the beam. A ection wa alo intrumented at a location halfway between the critical ection and the end of the compoite in pecimen from batch II and III. The concrete gage, CG1 and CG2, had a 60- mm gage length. The teel gage, SG1, SG2, and SG3, and the CFRP gage, CFRP1, had either a 5-mm or 6-mm gage length depending on availability of gage. 53

68 1.5 in CG1 SG1 CG2 SG3 SG2 CFRP1 Figure 3.31: Strain Gage Location Five 2-in. linear diplacement potentiometer were ued to meaure diplacement at or near midpan and at the upport (Figure 3.6). Diplacement potentiometer at the upport location on oppoite ide of the beam were ued to meaure compreion of the bearing pad and to check for poible torional effect from mialignment of the loading ram. Total applied load wa meaured with a 50-kip capacity Strainene load cell placed between the pherical bearing and the teel wide-flange preader beam ued to apply load at two point on the beam. Electronic reading from the intrument were collected every 2 econd during loading uing a Hewlett Packard canner and were tored in preadheet format on a peronal computer. 54

69 Chapter 4: Preentation and Evaluation of Tet Reult 4.1 OBJECTIVE Thi chapter preent the reult of monotonic load tet of twenty beam pecimen, and evaluate the effectivene of variou carbon-fiber compoite application cheme for flexural trengthening of reinforced concrete beam. Detail of the teting program were preented in Chapter 3. General obervation for the entire group of pecimen are preented firt, then pecimen behavior i examined in four group aociated with the placement of the compoite: bottom application, bottom application with tranvere trap, ide application, and ide application with tranvere trap. 4.2 QUANTITIES CONSIDERED IN EVALUATION OF SPECIMEN BEHAVIOR Yield load and diplacement, ultimate load and diplacement, diplacement ductility, maximum meaured train, and computed capacity are ummarized in Table 4.1 for all pecimen. Yield load and the correponding deflection were taken a the point on the load-deflection repone curve when tiffne of the beam changed dramatically (See Figure 4.1). Yield load wa not identified from train meaurement, becaue meaured train correponding with yield train at the critical ection tended to lag behind yielding at other location between the point of load application. 55

70 Ultimate load and deflection (Figure 4.1) were eaier to identify for the pecimen becaue the maximum load occurred when the compoite material debonded, the compoite ruptured, or concrete cruhed Yield Ultimate 5 Cracking Deflection (in) Figure 4.1: Typical Load-Deflection Repone One quantity ued to compare the effectivene of the carbon fiber trengthening cheme wa the ratio of meaured to computed capacity. The computed flexural capacity wa determined uing the procedure outlined in Chapter 2. The load correponding to the computed flexural capacity wa calculated uing the pecimen detail decribed in Chapter 3 and tatic. Diplacement ductility ratio wa alo ued to compare pecimen repone becaue ductility ratio i a relative indicator of pecimen deformation capacity. The diplacement ductility ratio wa calculated a the ultimate deflection divided by the yield deflection. 56

71 CFRP train were meaured at the critical ection in order to compare maximum meaured train with ultimate train value publihed by the variou compoite manufacturer. To better compare the maximum meaured train from all pecimen, meaured train were normalized with repect to the appropriate value of publihed ultimate train. Thi ratio of meaured train to nominal train capacity will hereafter be referred to a the train ratio. 57

72 Table 4.1: Summary of Reult for All Specimen Application Scheme Failure Yield Deflection (in) Yield Load (kip) Meaured Maximum Deflection (in) Maximum Load (kip) Computed Capacity (kip) % of Computed Capacity Achieved Control A & B None Cruhing % 6.0 Ductility Ratio μ=δu/δy Maximum Meaured CFRP Strain Publihed CFRP Strain Strain Ratio 58 A1 Bottom Debonding % A2 Bottom Debonding % A3 Bottom Debonding % A4 Bottom Debonding % B1 Bottom Debonding % B2 Bottom w/ Strap Fiber Rupture % B3 Side Fiber Rupture % B4 Bottom w/ Strap Fiber Rupture % B5 Bottom w/ Strap Fiber Rupture % Control C & D None Cruhing % 7.3 C1 Bottom Debonding % C2 Bottom Debonding % C3 Bottom w/ Strap Fiber Rupture % C4 Side Debonding % 2.8 Unavailable D1 Bottom Debonding % D2 Bottom Debonding % D3 Side Debonding % D4 Side w/ Strap Debonding % D5 Side w/ Strap Debonding %

73 4.3 GENERAL OBSERVATIONS Comment about the failure mode, percent of computed load achieved, and train ratio are dicued for all trengthened pecimen Oberved Failure Mode Only two of the three type of failure controlled the ultimate repone of the pecimen: debonding of the compoite and rupture of the compoite. Concrete cruhing did not occur a expected in ome pecimen becaue The compoite material debonded prematurely. All pecimen behaved imilarly before yield. Each pecimen exhibited an initial tiffne prior to cracking that wa conitent with gro-ection propertie. Following cracking, tiffne wa reduced by 8 to 28 percent. There wa little evidence of ditre in the compoite before yield unle the bond length wa extremely hort. After yield, epoxy in the wet lay-up ytem howed initial ign of ditre through cracking and whitening in the vicinity of crack, indicative of the early ign of local debonding of the CFRP (Figure 4.2). A load progreed, crack in the epoxy adjacent to the compoite material fanned out toward the compoite a hown in Figure 4.3. The firt evidence of debonding of the compoite material occurred when thee fan-haped crack turned parallel to the edge of the compoite (Figure 4.4). A load wa increaed in pecimen with no tranvere trap, ultimate load wa achieved by compoite debonding (Figure 4.5). The debonding failure wa udden, and beam that experienced thi type of failure exhibited limited ductility. 59

74 Figure 4.2: Initial Cracking in Epoxy Figure 4.3: Whitening and Fanning of Crack 60

75 Figure 4.4: Initial Evidence of Debonding - Crack Propagating Parallel to Compoite Figure 4.5: Debonding Failure 61

76 A alluded to previouly, the initiation of debonding failure occurred at dicrete crack location. An example of a typical crack pattern for a beam on the verge of debonding failure i hown in Figure 4.6. Additional crack pattern at ultimate for other pecimen are provided in Appendix A. A crack developed along the hear pan, a mall, vertical, differential diplacement occurred acro the critical crack (Figure 4.7). Thi relative diplacement of 1/16 to 1/8 in. worked to pry the compoite from the bottom urface (Figure 4.8). Deign equation for computing the required bonded length of compoite fiber to be attached to concrete are baed on average hear tree between the concrete urface and compoite. An offet at a flexure-hear crack reult in tenile tree between the compoite and concrete on one ide of the crack (Figure 4.9). Deign equation publihed by the compoite material manufacturer do not conider tenile tree reulting from thi prying action at crack location. Critical Crack 0.003" 0.007" 0.009" 0.040" 0.007" 0.010" 0.010" 0.009" 0.009" 0.005" Figure 4.6: Typical Cracking Pattern and Crack Width Prior to Failure 62

77 Figure 4.7: Photograph of Relative Diplacement Figure 4.8: Photograph of Debonding Initiated by Prying Action 63

78 M M M V M V Figure 4.9: Free Body Diagram of Crack in Contant Moment Region and in Shear Span In order to achieve the econd failure mode, compoite rupture, tranvere compoite trap were provided to prevent debonding failure of the longitudinal compoite fiber. Tranvere trap allowed the longitudinal compoite material to debond locally between trap (Figure 4.10), but maintained ufficient bonded length over the hear pan to prevent the debonding failure dicued earlier. Becaue debonding wa prevented at tranvere trap location, the compoite wa able to reach it rupture trength (Figure 4.11). However, if an inufficient number of tranvere trap i provided, the longitudinal compoite will till debond and rupture the tranvere trap. 64

79 Figure 4.10: Debonding Between Tranvere Strap Figure 4.11: Rupture of Compoite at Critical Section 65

80 4.3.2 Comparion of Meaured and Computed Capacity The ratio of meaured to computed capacity, expreed a a percentage, i preented in Figure 4.12 for all trengthened beam. Thi comparion wa made to facilitate comparion between pecimen trengthened with different compoite cheme, and to aid in identifying reliable method for ue in deign. Figure 4.12 indicate that only two of the pecimen reached the calculated capacity, pecimen B2 and B3. However, it mut be noted that ome of the initial tet pecimen had bonded compoite length that realitically could not be expected to develop the trength of the compoite. No obviou trend i apparent in Figure Trend are more apparent when comparing imilar product and application method. Figure 4.12 will be referenced throughout the chapter in order to make concluion about the capacitie for particular group of pecimen. 66

81 110% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% A1 A2 A3 A4 B1 B2 B3 B4 B5 C1 C2 C3 C4 D1 D2 D3 D4 D5 Figure 4.12: Strength Expreed a a Percentage of the Computed Capacity for Each Strengthened Specimen Strain Ratio Figure 4.13 compare the train ratio for all pecimen. The train ratio, which i the ratio of maximum meaured train in the compoite to manufacturerpublihed train capacity, wa calculated to identify value that might be ued in deign calculation, and to invetigate the uefulne of the manufacturerpublihed train capacitie. There appear to be no general trend among the pecimen, aide from the fact that maximum train in the compoite never reached the nominal train capacity for any compoite material ued. However, once again, it mut be noted that the bond length ued in ome pecimen (A1 and A2) made it impoible to develop the trength of the compoite. Strain ratio for pecimen that experienced compoite rupture (thee are denoted by hatching in Figure 4.12) ranged from 0.63 to Maximum 67

82 meaured compoite train were unpredictable. Specimen that achieved higher load ometime had lower maximum meaured train value in the compoite material than pecimen that achieved lower ultimate load. It appear that the publihed rupture train overetimate the ueful train in the compoite by at leat 12% Debonding Compoite Rupture A1 A2 A3 A4 B1 B2 B3 B4 B5 C1 C2 C3 C4 D1 D2 D3 D4 D5 Figure 4.13: Strain Ratio for All Specimen 4.4 EVALUATION OF SPECIMEN BEHAVIOR Tet reult from group of pecimen are compared and evaluated in thi ection. Specific comment about failure mode, percent of computed capacity achieved, diplacement ductility factor, and train ratio are dicued Bottom Application Several different variable were teted with thi application to invetigate the affect of bonded length, equivalent material from different manufacturer, 68

83 nominal bond tre, and urface preparation on the ultimate capacity of the trengthened beam Bond Length The firt pecimen teted, A1, had a bond length of 10 inche (defined in Chapter 3). Thi pecimen performed a expected by debonding of the CFRP a a reult of the hort bond length. It achieved only 86% of the computed capacity of the trengthened beam becaue the compoite debonded before it could rupture (Table 4.1). Bond length wa ucceively increaed after each tet that experienced a debonding failure. Specimen A2 had a bond length of 14 inche, and it alo debonded at a lightly higher percent (90%) of the computed capacity. The final pecimen in thi group, A3, had a bond length that extended 30 inche, and it too failed by debonding reaching an even higher load, 99% of the computed capacity. The load deflection repone curve verify that the compoite bond length did not affect yield load, a expected. A bond length increaed, ultimate load and deflection alo increaed (Figure 4.14). A bond length increaed, ductility of the trengthened pecimen alo increaed from 1.4 to 1.9 to 3.3 for A1, A2, and A3 repectively (Table 4.1). The trengthened beam had reduced ductility compared to the untrengthened beam (Control A&B). Addition of compoite material to the beam ha the ame effect a adding additional tenion reinforcement; ductility of the beam i reduced compared to the control beam. 69

84 35 A3 30 A1 A2 Load (kip) Control Computed Capacity Deflection (in) Figure 4.14: Load-Deflection Repone for Different Bond Length CFRP train were meaured at the critical ection. One would expect that a ultimate load increaed, CFRP train would alo increae. However, thi wa not the cae. The train ratio demontrate no apparent trend when comparing Specimen A1, A2, and A3 (Figure 4.12). The catter in Figure 4.13 can be attributed to the unpredictability of the brittle debonding failure. It can alo be attributed to the location of the critical crack leading to debonding, which did not alway occur at the crack initiator (location of CFRP train gage) Equivalent Material from Different Manufacturer Baed on the reult of the initial bond length tet, the teting program wa revied to examine everal other factor that could affect the required bond 70

85 length. Firt the A and B ytem were compared becaue the material were imilar. The pecified modulu, ultimate trength, and compoite thickne of material A were within 97% of the pecified propertie for material B. Specimen A3 had a bond length of 30 inche and failed by debonding, a mentioned previouly. It attained 99% of the computed capacity. Specimen B1 had a bond length of 35 inche and alo failed by debonding; it achieved 89% of the computed capacity (Table 4.1). Specimen A3 reached a higher ultimate load than B1, but B1 exhibited a higher yield load and had greater tiffne after yield than A3. Thi difference can be attributed to a difference in compoite width, which wa a contruction error. The width of the compoite ued to trengthen B1 and A3 were approximately 3 inche and 2 inche, repectively. The area of the compoite increaed the tiffne and yield load and reduced ductility of pecimen B1 a anticipated. 71

86 35 A3 30 B1 Load (kip) Control 10 5 Computed Capacity for A3 Computed Capacity for B Deflection (in) Figure 4.15: Load-Deflection Repone for Equivalent Material The train ratio for thee two pecimen again demontrate an unexpected trend. There wa a ubtantial difference in the maximum meaured train for the two pecimen, even though the bond length wa longer for B1. B1 achieved 48% of the publihed ultimate train and A3 achieved 80% of the publihed ultimate train. Thi large range in train ratio can again be attributed to the unpredictability of the debonding failure, which can be a function of the local urface condition, weakne of the concrete cover, and quality of compoite. The failure of B1 at a train ratio of 48% i all-the-more urpriing conidering the bonded length wa 5 inche longer and the width of the compoite wa 50% greater than for A3. 72

87 Equivalent Bond Stre It wa alo important to tet the manufacturer-recommended bond length equation by varying the width and bond length of the compoite while maintaining a contant area of bonded compoite. Equation 4-1 wa ued to determine the bond length required for one ply and two plie of compoite. f put pn l dp = (Mater Builder, 1998) ( 4-1) ' f l t dp f p pu f ' 3 c = Bond Length (in) = Compoite Rupture Stre (pi) = CFRP Deign Thickne (in) n = Number of Plie c = Concrete Compreive Strength (pi) Becaue the bonded urface area and nominal cro-ectional area of the compoite were the ame for both pecimen, A3 and A4, the nominal interface hear tre between the concrete and compoite and the nominal tenile force in the compoite were equivalent for each pecimen. Specimen A3 had a two-ply, 2-inch wide, 30 inch bond length, and it debonded a noted previouly at 99% of the computed capacity. Specimen A4 had a one-ply, 4-inch wide 15 inch bond length, and debonded at 93% of the computed capacity (Table 4.1). Thee pecimen did not perform a anticipated. A hown in Figure 4.16 the load-deflection repone were not equivalent. They not only differed in maximum load achieved, but alo ductility. Specimen A3 achieved a higher ductility ratio than A4 (Table 4.1). The reult indicate that the bond tre 73

88 equation recommended by one manufacturer i not ufficient to decribe the behavior of CFRP bonded to the urface of reinforced concrete beam A4 A3 Load (kip) Control Computed Capacity Deflection (in) Figure 4.16: Load-Deflection Repone for Equivalent Bond Stre The train ratio for A4 (52%) wa ubtantially le than the ratio for A3 (80%). The horter bond length ued for pecimen A4 might have been more uceptible to prying action than the longer length ued in pecimen A Influence of Surface Preparation The larger beam (both longer pan and deeper cro-ection) in thi group trengthened with ytem C and D were teted firt with the maximum bond length poible, 45 inche. Specimen C1 and D1 alo failed by debonding at 95% and 72% of the computed capacitie. Becaue the pecimen debonded 74

89 while invetigating the maximum bond length, the influence of urface preparation wa choen a the next variable to invetigate. Previou pecimen were all prepared by grinding the concrete urface, o C2 and D2 were prepared by andblating. The effect of urface preparation were difficult to interpret becaue andblating decreaed the capacity of the C ytem and lightly increaed the capacity of the D ytem (Figure 4.17 and 4.19). Difference might be attributed to the compoite ytem ued: wet lay-up for ytem C, veru pultruded for ytem D. Specimen C2 achieved 84% of the computed capacity, and pecimen D2 achieved 75% of the computed capacity (Table 4.1) C2 C1 Computed Capacity Load (kip) Control Deflection (in) Figure 4.17: Load-Deflection Repone for C Sytem Surface Preparation 75

90 Reduction in bond capacity (and pecimen capacity) for the wet layup ytem when the urface of the concrete wa prepared by andblating wa evident in the appearance of the concrete urface after failure. A typical pecimen with poor compoite bond i hown in Figure 4.18, the concrete urface wa mooth. Specimen with thi failure urface were indicative of inadequate bond between the compoite and concrete. Figure 4.18: Evidence of Poor Compoite Bond Unlike the C ytem, the D ytem wa a pultruded compoite, and andblating lightly improved the behavior of thi ytem (Figure 4.19). Specimen D1 achieved 72% of the computed capacity while D2 achieved 75% of the computed capacity (Table 4.1). The reult indicate no clear improvement in bond due to andblating. The percent of computed capacity achieved for thi 76

91 ytem wa low compared to the other ytem dicued thu far. Once again, failure by debonding wa udden and without warning Load (kip) D1 D2 Control Computed Capacity Deflection (in) Figure 4.19: Load-Deflection Repone for D Sytem Surface Preparation For the D ytem the maximum meaured train were very low compared with the publihed rupture train. D1 reached only 19% of the publihed train and D2 reached 25% of the publihed train (Table 4.1). The light increae in trength wa alo evident in the appearance of the concrete and compoite urface after failure. A hown in Figure 4.20 for a typical pecimen with good compoite bond, the concrete urface wa rough. Thi failure urface ugget good bond between the compoite and concrete and i ignificantly different than that hown in Figure 4.18 for poor compoite bond. 77

92 Figure 4.20: Evidence of Good Compoite Bond There are many other factor that affect the effectivene of the material more ignificantly than urface preparation, uch a, high hear tre in the concrete, high interface hear tre between the concrete and compoite, inadequate anchorage, and weakne of concrete cover. The remainder of the pecimen were prepared by grinding the urface of the concrete Summary All of the pecimen with CFRP applied to the bottom urface of the concrete failed by debonding, which lead one to believe there i no development length for the beam length teted that will fully engage the trength of the compoite fiber. The debonding failure wa very udden and brittle. It wa initiated by the relative vertical diplacement that occurred at crack location. A the bond length increaed, the load and diplacement alo increaed 78

93 a expected. However, the failure wa unpredictable and did not correpond with the idealized behavior. Debonding failure prevented the trength of the compoite material from being developed Bottom Application with Tranvere Strap Thi application required ue of a wet lay-up ytem for the tranvere trap. Two wet lay-up compoite ytem, B and C, were teted with thi application. The variable teted in thi group were the bond length of the longitudinal compoite and the number of trap provided along each hear pan. The tet performed indicated that a the bond length increaed, capacity and ultimate deflection increaed; and the capacity increaed a the number of trap increaed for a given length of longitudinal CRFP material. Thi et of tet alo demontrated improved ductility. A peak load wa approached, the compoite exhibited evidence of ditre through cracking and popping ound, whitening of epoxy in the vicinity of crack, debonding between tranvere trap, and debonding of tranvere trap (Figure 4.21). Thee were all ign that the ytem wa reaching it limit. Once ultimate load wa attained, failure wa very udden and violent when the longitudinal compoite ruptured. 79

94 Figure 4.21: Debonding of Strap B Sytem Two pecimen were ued for comparion in thi et of tet: the untrengthened pecimen and pecimen B1 with the compoite bonded to the bottom of the beam with a 35 inch bond length. Three pecimen were teted with tranvere trap: B2, B4, and B5. B2 had the compoite bonded the maximum length of 35 inche and had trap paced at h/2 along the full length of the hear pan (even trap). B4 had the ame layout a B2, except trap were only placed along half the hear pan (four trap). Specimen B5 had a bond length of 24 inche and trap paced at h/2 along half the hear pan (four trap). B2 exhibited the bet performance of the three pecimen with tranvere trap (Figure 4.22). It achieved 102% of the computed capacity and a diplacement ductility of 3.3 (Table 4.1). Specimen B4 had the next bet 80

95 performance reaching a imilar ductility of 3.4 but a lower ultimate load (98% of the computed capacity) than B2. Specimen B5 wa inferior to both B2 and B4 with a capacity that wa 96% of the computed capacity and a ductility ratio of 2.9. Specimen B2, B4, and B5 with tranvere trap achieved higher ultimate load that were cloer to the computed capacity than the pecimen that failed becaue of compoite debonding, uch a B1 (Table 4.1) B5 B1 B2 Load (kip) B4 Control 10 Computed Capacity for B1 Computed Capacity for B2 5 Computed Capacity for B4 & B Deflection (in) Figure 4.22: Load-Deflection Repone for B Sytem Beam with Tranvere Strap Strain ratio value ranged from 75% to 88% of the publihed ultimate train (Figure 4.13). The catter aociated with thee train ratio wa ubtantially le than for pecimen where compoite debonding occurred. 81

96 C Sytem For the C ytem, only one pecimen, C3, wa teted with the tranvere trap, and it repone wa compared to the untrengthened pecimen and pecimen C1 with no tranvere trap. Specimen C3 had a 45 inch bond length and tranvere trap at h/2 pacing along the full hear pan (even trap). It reached 99% of the computed capacity, and failure wa governed by compoite rupture (Table 4.1). C3 alo achieved a lightly higher diplacement ductility than C1 (2.9 veru 2.7) C1 C3 30 Computed Capacity Load (kip) Control Deflection (in) Figure 4.23: Load-Deflection Repone for C Sytem with Tranvere Strap 82

97 Strain ratio were unexpectedly the ame for thee two pecimen (Figure 4.13). It wa expected that C3 would have achieved a lightly higher maximum train than C1 becaue of the higher load achieved by C Summary Thi group of tet demontrated that fracture of longitudinal compoite material wa poible with the addition of tranvere trap. The compoite ruptured in every pecimen at the critical ection. Not only wa maximum load increaed, but diplacement ductility wa alo enhanced in thee pecimen. There wa alo ignificant warning leading up to failure. Popping and cracking ound, debonding between tranvere trap, and debonding of portion of ome of the tranvere trap, preceded failure. Rupture of the compoite did not guarantee achieving the publihed ultimate compoite train. For thee pecimen the highet train ratio wa Strain meaurement made at the critical ection at yield and at ultimate for the two bet-behaved pecimen in thi group (B2 and C3) are hown in Figure The train plot indicate that compoite train at ultimate are ubtantially le than would be expected if plane ection remained plane up to failure. Thi i likely due to debonding of the compoite between tranvere trap reulting in meaured train that reflect the total deformation between two trap divided by the ditance between thoe two trap. 83

98 B2 C Yield Ultimate Figure 4.24: Strain Profile at Yield and Ultimate for Specimen B2 and C Side Application Thi application exhibited imilar failure characteritic a for earlier bottom-application pecimen. One notable imilarity wa the manner in which crack propagated parallel to the compoite a debonding initiated after yield and prior to failure (Figure 4.25). Flexural crack divided into everal crack in the vicinity of the compoite. 84

99 Figure 4.25: Typical Cracking Aociated with Side Application Thi application wa teted uing both wet lay-up and pultruded compoite. Three ytem were teted: B, C, and D. The only variable conidered in thi group wa the location of the primary compoite, bottom of beam veru ide of beam. The depth of the compoite wa approximately 1 ½ in. le for the ide application than for the bottom application. All compoite were bonded the maximum length that could be accommodated by the pan. The load capacitie achieved by beam with ide application were lightly lower than beam with bottom application becaue the compoite wa not bonded to the extreme tenion fiber of the beam. The reduced moment arm reulted in reduced capacity. However, thi did not affect the deformation capacity of the trengthened beam. 85

100 B Sytem Specimen B3 had a bond length of 35 inche on the ide of the beam. Repone of B3 i compared to the beam B1 with the compoite bonded to the bottom of the beam and with a 35 inch bond length, and B2 with the compoite bonded the maximum length of 35 inche and tranvere trap paced at h/2 along the full length of the hear pan. The load-deformation repone of B3 i imilar to that for B2 (Figure 4.26). Specimen B3 achieved 100% of the computed capacity and failed by compoite rupture (Table 4.1). Maximum meaured compoite train wa 71% of the publihed train (Table 4.1) B1 B2 Load (kip) B3 Control 10 5 Computed Capacity for B1 Computed Capacity for B2 Computed Capacity for B Deflection (in) Figure 4.26: Load-Deflection Repone for B Sytem Side Application 86

101 Figure 4.27 illutrate the failure of thi pecimen. Not only did the compoite rupture, but concrete cover wa alo pulled off during failure. Figure 4.27: Specimen B3 at Failure C Sytem Specimen C4 had a bond length of 45 inche on the ide of the beam, which wa the maximum poible length. Repone i compared to the repone of beam C1, with the compoite bonded to the bottom of the beam with a 45 inch bond length, and the repone of C3, with a bond length of 45 inche and tranvere trap paced at h/2 along the full length of the hear pan. Specimen C4 achieved a maller load (89% of computed capacity) than both C1 and C3 (Figure 4.28). Deformation capacity wa imilar to that for C1 and C3. C4 failed by debonding (Figure 4.12). However, the capacity and ductility for C4 were reaonably cloe to the ame capacity and ductility a for 87

102 C3, which failed by rupture of the compoite material. Thi ugget that the compoite in C4 may have been very cloe to rupture. Strain in the compoite i not known for C4 becaue of an electronic malfunction in the data acquiition equipment C1 C3 Load (kip) C4 Control Computed Capacity for C4 Computed Capacity for C1 & C Deflection (in) Figure 4.28: Load-Deflection Repone for C Sytem Side Application D Sytem Specimen D3 alo had a bonded length of 45 inche on the ide of the beam. It repone i compared to the repone of the untrengthened beam, Contol C &D, and the trengthened beam D1 with the compoite bonded to the bottom of the beam with a 45 inch bond length. The pultruded plate ued to trengthen D1 and D3 were only available in certain width and thicknee, o 88

103 the area of compoite wa not equal for thee pecimen, unlike the other ideapplication pecimen. D3 had twice a much compoite bonded a D1 becaue it had two plie bonded to the ide (one on each ide) intead of one bonded to the bottom of the beam. Specimen D3 performed well compared to D1, although it alo failed by debonding (Figure 4.28). It had a much higher yield and ultimate load than D1. The yield load and pot-yield tiffne were greatly increaed for pecimen D3 becaue it had twice the amount of compoite bonded that D1 had (Figure 4.30). Specimen D3 achieved 77% of the computed capacity, which wa lightly better than D1 which attained 72% of the computed capacity (Figure 4.12). The train ratio wa again low at 23% of the publihed train value. Thi ytem did not make efficient ue of the compoite, which wa controlled by debonding failure. Figure 4.29 illutrate the failure. Not only did the compoite debond, but alo concrete cover palled off during failure. Thi indicate that debonding wa a function of the tenile capacity of the concrete intead of the tenile trength of the epoxy. 89

104 Figure 4.29: Specimen D3 at Failure Load (kip) D1 D3 Control Computed Capacity for D1 Computed Capacity for D Deflection (in) Figure 4.30: Load-Deflection Repone for D Sytem Side Application 90

105 Summary Side application of the compoite generally produced behavior that wa uperior to comparable pecimen with bottom application of the compoite. In one cae (B3) the compoite ruptured, and in other the compoite debonded from the beam. Even pecimen C4 that failed by debonding wa very cloe to reaching the ame load achieved by the bottom application with tranvere trap becaue it wa not affected by the vertical offet at the critical crack location. The reult from the pecimen with B and C ytem were acceptable becaue the percent of computed capacity wa quite high and train ratio approached the publihed value. The performance of the D ytem till wa quite inefficient becaue the ytem wa debonding prematurely. Not enough pecimen were teted to recognize trend in bond length, urface preparation, etc. for thi application method. However, it could be inferred that it i poible to attain the capacity of the wet lay-up ytem, and performance i generally better than for bottom-application pecimen. The pultruded ytem did not benefit ubtantially from the ide application alone Side Application with Tranvere Strap One more application cheme, ide application with trap, wa attempted becaue the D ytem (utilizing pultruded plate) did not reach the computed capacity for any of the tet decribed earlier. Thi application wa only teted for the D ytem. In the previou group of tet, the D ytem pulled off large piece of concrete cover and expoed teel reinforcement when it debonded. Thi application wa imilar to the previou tranvere trap application except an 91

106 attempt wa made to not only anchor the compoite but alo the concrete becaue debonding appeared to be a function of the concrete tenile trength. The two pecimen with tranvere trap, D4 and D5, did not achieve the computed capacity becaue the compoite debonded after tranvere trap ruptured. However, the reult were improved from thoe for the pecimen without tranvere trap. The trap, made of wet lay-up compoite E, were not ufficiently trong to prevent the compoite from debonding (Figure 4.31). The pecimen with the bet performance, D4, had a maximum bond length for the compoite of 45 inche and had tranvere trap paced at h/2 along the full length of each hear pan (Figure 4.32 and Table 4.1). It achieved 92% of the computed capacity, and a diplacement ductility of 2.6. Specimen D5 diplayed a lightly lower ductility and ultimate load (89% of the computed capacity) than D4. D5 had a bond length of 30 inche, and the tranvere trap were paced at h/2 along half of each hear pan. Even though thee pecimen failed by debonding, they achieved higher load that were cloer to the computed capacity than pecimen that did not have tranvere trap to enhance anchorage (Figure 4.12). 92

107 Figure 4.31: Debonding Failure with Strap Rupture for Specimen with Side Application with Strap 93