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8 The Willamette River Bridge located near Newberg, Ore- The first group of models is those relying on an empirically gon, was found to have significant diagonal cracking dur- determined value of strain/stress associated with failure of the ing an inspection conducted by the Oregon Department member for which the shear contribution of the FRP is deter- of Transportation (ODOT) in late summer of 2001. CFRP mined; the principal equations of the analytical models in this strips of 12 inch width were applied vertically in a U-shape group are listed in Table 2.1. The second group of models is wrapping scheme (Williams and Higgins, 2008). those based on the determination of an effective FRP strain; the The Ebay Island Viaduct Bridge is a 21/4 mile long section corresponding principal equations are listed in Table 2.2. The of westbound Washington State Route 2 that crosses over third group of models focuses on the non-uniformity of environmentally sensitive wetlands near the outflow of the strain distribution in externally bonded FRP reinforce- the Snohomish River into Puget Sound near Everett, ments; the corresponding principal equations are listed in Washington. The bridge was built during the late 1960s. Table 2.3. The fourth group of models is mechanics-based In 1996, bridge condition inspectors noted that the bot- theoretical approaches that do not rely on experimental results toms of the existing precast concrete webs exhibited con- for regression or calibration; the principal equations of these siderable concrete spalling accompanied with primary steel models are listed in Table 2.4. reinforcement corrosion. In 1999, carbon fiber sheets were bonded to the deteriorated elements for flexural strength- 2.4 Experimental Investigations ening and to compensate for steel reinforcement loss due Reported in the Literature to corrosion. In addition, carbon fiber sheets were applied with a U-wrap configuration to compensate for the shear A review was conducted of experimental investigations capacity loss due to the cross-sectional loss of stirrups caused which included studies on (1) the behavior of concrete girders by corrosion. The carbon fiber repairs were inspected annu- strengthened in shear with externally bonded FRP, (2) bond ally after the completion of the repair project with no behavior of FRP-concrete interface, and (3) anchorage systems debonding or deterioration of the carbon fiber plies being to enhance the effectiveness of FRP strengthening systems. reported through spring 2007 (Dornsife, 2007). 2.4.1 Studies on the Behavior of Concrete 2.3 Existing Analytical Models Girders Strengthened in Shear with Externally Bonded FRP This section summarizes the analytical models previously developed for determining the shear resistance of reinforced The review included 49 experimental studies, encompass- concrete members strengthened with externally bonded FRP. ing more than 500 test specimens. The review provides infor- Seventeen models were found in the literature. These models mation on the objectives, the methodology, the experimental have been divided into four groups based on their approaches program, the test method, the FRP used and its orientation, and are presented in the same units as the original papers. as well as the strengthening scheme used (configuration). Table 2.1. Models based on experimentally determined limiting value of FRP shear strain/stress. Reference Equations Author (Year) ts hs 2 ave d 2 VP = (for shear strips) SP Al-Sulaimani et al. dhw (1994) VP = 2 ave (for shear wings) 2 dh j VP = 2 ult (for U-jackets) 2 V f = A f E f vcu d (for FRP oriented at 0/90 degree) Chajes et al. (1995) V f = Af E f vcu d 2 (for FRP oriented at 45/135 degree) * Terms are defined in notations section.

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9 Table 2.2. Models based on an effective FRP strain. Reference Equations Author (Year) 0.9 V frp , d = frp E frp frp , ebw d (1 + cot ) sin frp Triantafillou frp , e = 0.0119 - 0.0205 ( frp E frp ) + 0.0104 ( frp E frp ) when 0 frp E frp 1 GPa 2 (1998) frp ,e = -0.00065 ( frp E frp ) + 0.00245 when frp E frp > 1 GPa V f = 0.9 frp E frp fe bw d (1 + cot ) sin (Eurocode format) Af f fe ( sin + cos ) d f Vf = (ACI format) sf fe = R fu f fe = Rf fu Based on the effective FRP stress: R = 0.5622 ( frp E frp ) - 1.2188 ( frp E frp ) + 0.778 0.5 when frp E frp < 1.1 GPa 2 Based on bond mechanism: 0.0042 ( f c' ) 2/3 Khalifa et al. w fe (1998) R= (E ) 0.58 t frp f fu d f Effective width: w fe = d f (complete wrapping) w fe = d f - Le (U-wrap) w fe = d f - 2 Le (side bonded) d 2 f c' bw d s f ,max = w f + Vs + V f 4 3 Vn = Vc + Vse + V f ,max d f ( cot + cot f ) sin f V f ,max = f , ave E f 2nt f w f sf d / 2 + 0.5 ( d f - d / 2 ) Hutchinson and f ,ave = f max Rizkalla df (1999) f max = L feC ( 6.134 - 0.580ln t f E f ) L fe = e and C = constant strain rate of 110 10-6 mm-1 d cot se = f , ave sin f fs sy Vse = se E s Av where s (continued on next page)

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10 Table 2.2. (Continued). V f = 0.9 frp E frp fe bw d (1 + cot ) sin (Eurocode format) Af f fe ( sin + cos ) d f Vf = (ACI format) sf fe = R fu f fe = Rf fu Khalifa and Nanni R is the least of : R = 0.5622 ( frp E frp ) - 1.2188 ( frp E frp ) + 0.778 0.5 2 (2000) R= (f ) c ' 2/3 w fe 738.93 - 4.06(t f E frp ) 10-6 fu d f 0.006 R= fu fk ,e V fd = 0.9 E f f bw d (1 + cot ) sin (Eurocode format) f fk ,e = f ,e max = 0.005 = 0.8 (recommended) f V f = f f ,e , A E f f ( sin + cos ) bd (ACI format) f ,e, A = 0.9 f ,e max, A = 0.006 0.56 f 2/3 Triantafillou and f ,e = 0.65 c 10-3 (CFRP debonding failure mode) Antonopoulos Ef f (2000) 0.30 f c2 / 3 f ,e = 0.17 f ,u (shear failure combined with or followed by CFRP fracture) Ef f 0.47 f c2 / 3 f ,e = 0.048 f ,u (shear failure combined with or followed by AFRP fracture) Ef f 1/ 0.56 0.65 10-3 (E ) f f lim = max f c2 / 3 = 0.018 f c2/ 3 a Af Vf = f , tot E f eff d f d sf eff = 3 10-5 tot -0.6522 , tot = n f + s Chaallal et al. a 1 + 2a / d (2002) New deep beam coefficient: f , tot = + (1000 tot - 0.6 ) 1 d 12 but greater than 1 + 2 a / d 12

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11 Table 2.2. (Continued). V f = 0.9 frp E frp fe bw d (1 + cot ) sin (Eurocode format) Af f fe ( sin + cos ) d f Vf = (ACI format) sf fe = R fu f fe = Rf fu R is the least of : R = 0.5622 ( frp E frp ) - 1.2188 ( frp E frp ) + 0.778 0.5 2 Pellegrino and Modena (2002) 0.006 R= fu { R = R* 0.0042 ( f cm ) 2/3 w fe / (E t frp f ) 0.58 fu d } 0 R* = -0.53ln s , f + 0.29 1 s , f = E s Asw E f A f for continuous fiber sheet: Vf = wfet f f fe sin2 for FRP strips: Af f fe ( sin + cos ) d f Vf = sf f fe = Rf fu , fe = R fu Based on model calibration: Hsu et al. R = 1.4871 ( f E f / f c' ) - 0.7488 (2003) Based on bonding mechanism: max Le R= 1 2 f fu t f max = ( 5 10 -6 f c'2 ) - ( 2.73 10 -2 f c' ) + 925.3 ( English ) max = ( 7.64 10 f -4 c '2 ) - ( 2.73 10 -2 f c ' ) + 6.38 ( Metric ) * Terms are defined in notations section. Whenever necessary, the review provides comments or com- with U-wrap and side-bonding configurations as well as on parisons with other studies. The numerical data extracted from the use of mechanical anchorage systems to address the issue the experimental studies were assembled in a database. of debonding. Also, few tests have been conducted on mem- The test parameters considered in these studies are listed in bers with spans comparable to those used for bridges, and Table 2.5. The major test parameters are (a) the geometry of fewer tests have investigated the influence of scale (i.e., model- the beam used in the experiments, (b) beam type, (c) proper- scale versus large-scale) on the shear behavior of members ties of concrete and steel reinforcement, (d) types of FRP, and strengthened with FRPs. Furthermore, because FRP is gener- (e) strengthening schemes. As seen from Table 2.5, most of ally used to strengthen damaged structures, attention needs these studies have focused on rectangular beams, although to be given to the effects of existing cracks on the behavior of most RC bridge girders have a T-section with integrated deck the strengthened member. slabs. The shape of the cross section is related also to the The previously developed analytical models were based strengthening scheme. For example, rectangular beams are on the studies listed in Table 2.5, the majority of which con- commonly strengthened by fully wrapping the member, an sidered only small-scale testing. Therefore, this research aimed impractical solution for T-beams due to the presence of the at expanding the experimental database with results from flange. Therefore, more focus should be placed on T-beams tests on full-scale T-beams, which are more representative

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12 Table 2.3. Models that account for non-uniform strain distribution in FRP. Reference Equations Author (Year) f frp ,ed h frp , e ( sin + cos ) V frp = 2 t frp w frp frp s frp f frp , ed = D frp frp ,max Debonding model: 2 1 - cos zb 2 if 1 frp , z dz sin D frp = zt = h frp ,e frp ,max d 2 -2 1- if > 1 E frp frp ,max, d = 0.315 w L f c' f frp t frp Chen and Teng (2003a and 2003b) Rupture model: zb z dz 1+ D frp = = zt h frp ,e z ,max 2 f frp 0.8 f frp if max E frp frp ,max = f frp 0.8 max E frp if > max E frp Strip spacing limitation: h frp ,e (1 + cot ) w frp s frp - min 2 sin 300 mm for complete wrap: cos V f = cr E f t f z sin for composite strips: bf cos V f = cr E f t f z s f sin sin h/2 Carolin and Taljsten (2005b) f (y)dy = -h/2 max h fu cr = min bond cos 2 c max cos 2

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13 Table 2.3. (Continued). h frp V frp = 2 D f t frp w frp E frp f ,max s frp 1 for 1.4 -2 1 Cao et al. Df = 1- for 1.4< < 3 frp 1 - 0.2 ( - 1.4 ) 2 (2005) 2.05 for 3 0.427 w 4 f c' f ,max = E frp t frp * Terms are defined in notations section. Table 2.4. Models derived from mechanics-based approaches. Reference Equations Author (Year) V f = ht p Q13 2 + Q231 + (Q 12 2 + Q221 ) tan (c ) Malek and hv Fy Saadatmanesh Vs = Es y Av for y < (1998) tan (c ) s Es hv Fy Vs = Fy Av for y tan (c ) s Es Discrete formulation: Vr = 0.25k 2 f c' ( Acf tan f + Acw tan w ) + Tv ns + TFRP Continuous formulation: ds Vr = k f c' Ac (Tv + TFRP ) - Tv s k = 2.1( f c' ) -0.4 Tv = Av f vy 2 w frp sin Deniaud and Cheng TFRP = d FRPtE frp max RL + ( ns + 1) cos sin (2001, 2004) s frp tan w 2 w frp s TFRP = d FRPtE frp max RL sin + cos sin s frp ds Maximum allowable strain in FRP: 3 fc' d FRP 0.16 max = (%) ultFRP ( tE ) 0.67 frp (ka sin )0.1 Remaining bonded width over initial width ratio: 0.4 d FRP RL = 1 - 1.2 exp - k e Leff sin (continued on next page)

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14 Table 2.4. (Continued). Side bonding: 1 sin w f VRd , f = min {0.9 d , hw } f fed 2t f Rd sin p f design effective stress for side bonding: 2 z rid ,eq leq f fed = f fdd 1 - 0.6 min{0.9d , hw } z rid ,eq sf zrid ,eq = min {0.9d , hw } - le - sin f fdd / E f U-wrap or complete wrapping: 1 wf VRd , f = 0.9d f fed 2t f ( cot + cot ) Rd pf design effective stress for U-wrapping: 1 le sin f fed = f fdd 1 - 3 min{0.9d , hw } Monti and Liotta design effective stress for complete wrapping: (2005) 1 l e sin 1 l e sin f fed = f fdd 1 - + ( R f fd - f fdd ) 1 - 6 min {0.9 d , hw } 2 min {0.9 d , hw } Ef tf where: f ctm = 0.27 Rck 2 / 3 le = 2 fctm 0.80 2E f Fk f fdd = f ,d tf 2 - wf p f where: Fk = 0.03kb f ck f ctm kb = 1 1 + w f 400 w f min ( 0.9d , hw ) sin ( + ) sin lb l f fdd ( lb ) = f fdd 2- b (for lb < le) le le r r R = 0.2 + 1.6 c where 0 c 0.5 bw bw a 2 + d 2 - (1 - 2 ) a (a + d ) - a 2 2 = if < cu 2d 2 ( a2 + d 2 ) (a 2 + d2 ) -a 1 = (1 - ) if Sim et al. (2005) cu 2 (a 2 +d 2 ) 2 1 1 = if > cu 2 2 V and Af Ap f py = = v sy + ( sin + cos ) bh b e cu b t cu * Terms are defined in notations section.

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15 Table 2.5. Summary of experimental studies. Properties and Parameters Concrete Type of Number of Tests Geometry Type of Beam and Steel FRP Strengthening Scheme Rectangular Section Two-Side Bonding Concrete Strength Beam Spanning Beam Spanning Beam Spanning Complete Wrap Regular Beams Angle to Long. Angle to Long. Reinforcement Reinforcement 7 ft 2.5) U-Wrap Aramid L>13 ft Carbon L< 7 ft Strips Glass Author Year Berset 1992 2 Uji 1992 4 Al-Sulaimani 1994 4 et al. Ohuchi et al. 1994 13 Chajes et al. 1995 5 Sato et al. 1996 3 Araki et al. 1997 8 Funakawa et 1997 3 al. Kamiharako 1997 1 et al. Miyauchi et 1997 4 al. Sato et al. 1997 2 Taerwe et al. 1997 3 Taljsten 1997 3 Umezu et al. 1997 15 Chaallal et al. 1998 2 Mitsui et al. 1998 6 Triantafillou 1998 9 Khalifa et al. 1999 6 Kachlakev 1999 3 and Barnes Khalifa et al. 2000 4 Deniaud and 2001 5 Cheng Li et al. 2001a 5 Li et al. 2001b 9 Park et al. 2001 2 Chaallal et al. 2002 10 Khalifa and 2002 4 Nanni Li et al. 2002 9 Micelli et al. 2002 10 Pellegrino 2002 9 and Modena Beber 2003 28 Diagana et al. 2003 8 Hsu et al. 2003 3 (continued on next page)

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16 Table 2.5. (Continued). Properties and Parameters Concrete Type of Number of Tests Geometry Type of Beam and Steel FRP Strengthening Scheme Rectangular Section Two-Side Bonding Concrete Strength Beam Spanning Beam Spanning Beam Spanning Complete Wrap Regular Beams Angle to Long. Angle to Long. Reinforcement Reinforcement 7 ft 2.5) U-Wrap L>13 ft Aramid Carbon L< 7 ft Strips Glass Author Year Taljsten 2003 6 Adhikary et 2004 8 al. Xue Song et 2004 12 al. Cao et al. 2005 10 Carolin and 2005a 18 Taljsten Miyajima et 2005 4 al. Monti and 2005 16 Liotta Sim et al. 2005 9 Zhang and 2005 10 Hsu Barros and 2006 5 Dias Bousselham 2006a 20 and Chaallal Pellegrino 2006 8 and Modena Lees and 2007 8 Kesse Leung et al. 2007 12 Alrousan et 2009 4 al. Arteaga et al. 2009 15 Gamino et al. 2009 7 Rizzo and De 2009 1 Lorenzis Note: (1) Shaded cells denote considered parameters. (2) Control specimens without FRP strengthening are not included in the table. of bridge girders and consider the effects of other param- sider the bond characteristics at the interface between FRP eters such as pre-cracking and the amount of transverse and concrete substrate to predict the shear contribution of reinforcement. FRP when the expected failure is caused by debonding of FRP. Consideration of the bond mechanism, the intermedi- ate crack (IC) debonding, the bond stress-slip relationship, 2.4.2 Bond Behavior of the effective bond length, and the bond strength is required FRP-Concrete Interface for the development of improved shear design equations. The performance of shear strengthening of concrete gird- The most important role of the interface bond between the ers by using externally bonded FRP sheets depends on the FRP sheets and concrete is to transfer shear stresses from exist- interface bond behavior between the FRP sheets and the con- ing concrete structures to externally bonded FRP sheets for crete substrates. Many analytical models attempted to con- both shear and flexural strengthening. The bond properties

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17 of FRP sheet-concrete interfaces have been widely studied. and the local bond stress-slip relationship. Bond behavior is Various test methods have been developed to evaluate the influenced by the mechanical and physical properties of the average interfacial bond strength. These methods include concrete, FRP composite, and adhesive; the influencing fac- single-lap-type, double-lap-type, bending-type, and inserted- tors are listed in Table 2.6. type tests, as shown in Figure 2.1. Among the interface param- In evaluating FRP-concrete interface bond behavior, the eters evaluated are average shear bond strength, effective bond bond stress-slip (-s) relationship is the most important length, maximum shear bond stress, interfacial fracture energy, factor. For FRP sheets bonded to concrete, this relationship Load Load Load Concrete Rebar FRP Notch FRP Concrete Bond Length (a) Single-lap shear bond test (b) Double-lap shear bond test Load Steel Plate Load Load FRP Concrete Notch FRP Bond Length (c) Bending-type shear bond test (d) Inserted-type shear bond test Figure 2.1. Test methods to evaluate the bond strength.

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18 Table 2.6. Factors influencing the bond behavior at FRP-concrete interface. Elements Influencing Factor Modulus of elasticity, thickness, surface Concrete condition, strength, drying shrinkage, water content Modulus of elasticity, strength, thickness, Continuous Fiber Sheet stiffness, length/width of sheet, weave FRP Bonding Resin application Modulus of elasticity, strength, glass Primer transition temperature, spread Putty Loading condition Bending, shearing, punching, cyclic Ambient temperature, moisture, sun light Environmental actions radiation, etc. is determined by the strain distributions in the FRP, and exceeds the effective bond length, no further increase in fail- the local bond stresses measured in the FRP sheets. Several ure load can be achieved. However, a longer bond length may empirical -s relationships have been proposed including a delay complete debonding and thus improve the ductility. elasto-plastic model (Sato et al., 1997 and De Lorenzis et al., Several studies have been performed to determine effective 2001); a bilinear model based on interfacial fracture energy bond length. Figure 2.4 shows the effective bond lengths cal- (Yoshizawa et al., 2000); a model based on the Popovic's culated by analytical models and equations stipulated in expression (Nakaba et al., 2001); and a shear softening model many current code and design guidelines versus the rigidity (Sato et al., 2000). of FRP reinforcement (Ef f). As shown in the figure, most The experimental studies have shown that the bond shear studies have reported that effective bond length increases as stress at the FRP-to-concrete interface increases rapidly with the stiffness of FRP sheets increases. However, two studies increases in the interfacial slip until it reaches the peak stress (Maeda et al., 1997 and ACI 440.2R-08) show a different (bond strength) as illustrated in Figure 2.2. After this point, trend, probably because these models were derived using a interfacial softening (or micro-cracking) starts, together with very limited experimental database. a decrease in the interfacial shear stress and an increase in the The analytical models for effective bond length and bond interfacial slip. strength were derived based on small-scale tests; the bond There is no agreement among researchers on the shape behavior of full-size beams may be different than that pre- of the model, however, use of fracture mechanics implicitly scribed by these models. Thus, full-scale tests would provide leads to a very simple generic expression that considers only data to calibrate/improve these models. the FRP stiffness and interfacial fracture energy (defined as the area beneath the bond stress-slip curve) for the determi- 2.4.3 Anchorage Systems to Enhance nation of bond capacity. Debonding occurs first within the the Effectiveness of FRP effective bond length (defined as a length over which the Strengthening Systems majority of the bond stress is maintained, see Figure 2.3) as a result of debonding of a very thin layer of concrete rather When a proper anchorage system is not provided, failure than debonding at the FRP/concrete interfaces. When the of FRP-strengthened reinforced concrete members is com- bonded length of FRP along the FRP-concrete interface monly manifested by debonding of the FRP. Therefore, vari- Bond Stress Bond Stress Bond Stress Slip Slip Slip (a) Cutoff type (b) Bilinear type (c) Tensile softening type Figure 2.2. Various bond stress-slip models.

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19 Le technique requires no surface preparation work but con- siderable labor for cutting the grooves. 99.99 % of Max Load Another system used to prevent debonding of FRP is anchor 99.9 % of Max Load spikes (Eshwar et al., 2003; Eshwar et al., 2008; Orton, 2007; Local bond stresses 96.0 % of Max Load Niemitz, 2008). Each anchor spike consists of a precured fiber 80 % of Max Load portion and a dry fiber portion (see Figure 2.6). The anchor 50 % of Max Load spikes may be constructed in situ. First, fibers are bundled 20 % of Max Load together, and half of the fiber length is covered with plastic or duct tape. The uncovered bundled fibers are then impreg- nated and thoroughly saturated with resin. Finally, the satu- rated fibers are passed through a circular hole in a steel plate, or die, to obtain the desired diameter of the anchor spikes. The dry fibers are used for bonding purposes and trimmed to Free Loaded end the appropriate length according to specific requirements. Figure 2.3. Concept of effective bond length based Following surface preparation of the concrete, holes of the on stress distribution (Ueda and Dai, 2005). desired diameter and depth are drilled and partially filled with saturant. The laminate is then applied, and while it is wet, the precured portion of each spike anchor is inserted into ous types of anchorage systems, including the near surface the holes. The dry fibers are spread around the layer in a mounted system (NSM), fiber reinforced polymer anchor circular fashion, and a layer of saturant is then applied (see spikes, additional horizontal strips, and various mechanical Figure 2.6). anchorage systems have been studied to evaluate their effect An additional horizontal FRP strip applied on top of the ver- on FRP failure by debonding. tical FRP strips has also been used as an anchorage system Many experimental studies have demonstrated the effec- (Hutchinson and Rizkalla, 1999; Schnerch, 2001). This tech- tiveness of the NSM system (Khalifa and Nanni, 2000; De nique is very easy to install and requires no more labor than Lorenzis, 2002; Micelli et al., 2002). In this system, a bent other anchorage systems. However, different levels of effective- portion of the end (or a region near the end) of the FRP ness have been reported. Schnerch (2001) reported that the hor- reinforcement is embedded into the concrete, as shown in izontal strip neither delays nor prevents debonding, and it does Figure 2.5. For fiber sheets, the bend is created during wet not increase the contribution of the FRP to the shear strength lay-up, and in the case of laminates, it is pre-formed. This of the beam at failure. Test results reported by Hutchinson 12 Maeda et al. (1997) Neubauer and Rostasy (1997) Niedermeier (1996) 10 JCI (2003) - Sato Model JCI (2003) - Iso Model Chen and Teng (2001) Effective Bond Length, Le (in.) 8 Niu and Wu (2000) Kanakubo et al (2003) -1 Kanakubo et al (2003) -2 Ueda and Dai (2004) 6 Ueda and Dai (2005) Foster and Khomwan (2005) Miller and Nanni (1999) 4 Ben Ouezdou et al. (2008) ACI 440 (2008) CSA S806 (2002) 2 fib-TG9.3 (2001) Appendix 1 fib-TG9.3 (2001) Appendix 2 Eurocode 8-3 (2004) Concrete Society (2004) 0 100 150 200 250 300 350 400 450 Stiffness of FRP reinforcement, Ef tf (kip/in) Figure 2.4. Effective bond length versus FRP rigidity.

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20 The four methods used to anchor the FRP sheets as shown in Figure 2.7 are (a) nail type, (b) semi-closed type, (c) sub- variation of semi-closed type, and (d) closed type. Sato et al. (1997) concluded that the shear strength of beams can be improved by transverse wrapping of FRP sheets if adequate anchoring is provided by steel plates and bolts and recommended the use of long anchor bolts that penetrate the full web. This anchorage system, however, creates stress con- centrations where the anchors are placed, and the bolts lead to discontinuity of the FRP system. Matthys (2000) conducted a project to strengthen four continuous reinforced concrete beams in shear and flexure, by supporting them with masonry columns. Bond/anchorage tests indicated a 44% increase in anchorage capacity with the use of steel bolted connections. Mechanical anchorage resulted in a less brittle failure mode due to the transition to an external tensioning system after debonding and to increased displacements resulting from CFRP slip. Schuman (2004) conducted a comprehensive study on Figure 2.5. Construction of a NSM Anchorage System. anchorage systems for shear strengthening of reinforced concrete (RC) beams. A mechanical anchorage system was and Rizkalla (1999) indicated that using the horizontal strip applied to increase the shear contribution of CFRP systems increased the shear contribution of FRP by 16 percent. by embedding anchor rods into the cross section with various Mechanical anchorage systems have been used widely to bearing plates, (e.g., GFRP plate). The anchorage systems are prevent premature FRP debonding. Steel angles, steel or FRP possible, the four methods described by Schuman (2004) can composite plates, and anchor bolts are examples of most be summarized as (1) complete wrapping through the flange commonly used mechanical anchorage systems. (called complete wrap), (2) FRP laminate extended into the Sato et al. (1997) conducted a series of tests using various flange, (3) bonded steel anchors with bearing plates (called anchoring methods to develop a shear strengthening tech- two-side bonding), and (4) GFRP plate anchors. nique for beams and a method of estimating their effective- The two particularly important conclusions from Schuman's ness. The test results showed that sufficient strength can be (2004) research are (1) the FRP composite alone provides the achieved only if the CFRP sheets are mechanically anchored. T-beam with little additional ultimate load and displacement capacity and creates a more brittle failure mode and (2) the use of properly embedded and sized anchors allows the vertical ties to remain intact during failure. These anchors then force a more ductile compression zone and ensure a shear/flexural failure mode. Schuman (2004) also concluded that short anchors (Fig- ure 2.8) lead to an increase in load carrying and displacement capacity and cause the CFRP reinforcement to be activated before the steel reinforcement yields but deeper anchors (Fig- ure 2.8) allow the CFRP reinforcement to be activated earlier, thus delaying yielding in the steel stirrups. For all mechanical anchorage systems configurations, the embedment length, diameter of the anchor, and the bearing strength of the plate are primary considerations. Regarding embedment length, longer anchors are more effective, but they increase the amount of labor required and present the risk of damaging the concrete and steel reinforcement. The diameter of the anchor must be chosen based on the failure modes of the connection (e.g., bearing of the plate, spalling of Figure 2.6. Anchor spikes. concrete, and yielding or rupture of the anchor).