CONTRIBUTION OF EXTERNALLY APPLIED FRP COMPOSITES TO CONCRETE SHEAR TRANSFER

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1 CONTRIBUTION OF EXTERNALLY APPLIED FRP COMPOSITES TO CONCRETE SHEAR TRANSFER Saenz, N., Pantelides, C.P., and Reaveley, L.D. Department o Civil and Environmental Engineering, University o Utah, Salt Lake City, U.S.A. ABSTRACT This investigation is onerned with the determination o the ontribution o iber-reinored polymer (FRP) omposites to onrete shear transer. The FRP omposite was applied externally to unraked plain onrete push-o test units designed to ail along a known shear plane. The FRP omposite used was arbon iber abri with epoxy resin and was applied perpendiular to the shear plane. Test units with three shear-to-transverse stress ratios were onstruted to enompass various shear transer appliations. The experiments inluded several FRP omposite wrapping shemes and our FRP reinorement ratios. The shear strength ontributed by the onrete-frp shear rition interation is ound to be a untion o the onrete-to-onrete shear rition oeiient and the eetive FRP omposite tensile strain. The shear transer units reinored with arbon FRP omposites inreased shear transer apaity by a ator rom 1.32 to 3.25 times that o the as-is onrete units. 1. INTRODUCTION Shear transer onrete test units internally reinored with steel stirrups were used to determine the horizontal design o preast shear onnetions; this researh, in addition to other indings, was used to develop the shear rition hypothesis [1]. Initially raked and unraked steel reinored onrete onnetions have been studied [2-4]; modiied shear rition equations and boundary onditions or its appliability were proposed; ohesion and rition eets or unraked onnetions and limits on ultimate shear strength were also introdued. The shear rition onept has been studied or high-strength reinored onrete [5, 6]. Shear onnetions or wall panels have been studied using shear transer units, where evaluation o the shear rition oeiient ator was o interest [7]. In Fiber Reinored Conrete (FRC), steel or polypropylene ibers are mixed with onrete. The shear strength and dutility o high-strength FRC onrete using shear transer units has been studied [8]; speimens with polypropylene ibers had a lower inrease in the ultimate load but greater dutility ompared to speimens with steel ibers. In another study, steel iber FRC onrete inreased the shear transer apaity up to 60% o the onrete ompressive strength [9]. The Iosipesu test, adopted rom ASTM D5379 [10], has been used to determine the shear transer strength o RC members with CFRP omposites [11]. Shear transer strength or onrete internally reinored with Glass FRP (GFRP) omposite stirrups has also been studied, where some plastiity was observed beause o gradual delamination o the GFRP omposite [12].

2 In the present researh, CFRP omposite strips are used to strengthen the onrete externally at a known ailure plane to resist stresses in shear transer. Thirty-six shear transer units were built and tested with these objetives: (1) investigate the eiieny o CFRP wrapping sheme oniguration; (2) investigate the inluene o CFRP reinorement ratio; (3) determine the inluene o shear-to-transverse stress ratio on shear transer, and (4) understand the undamental behavior o CFRP omposite external onnetions in shear transer. 2. DESCRIPTION OF TEST UNITS The test units were designed to ail in shear at a known plane, as shown in Fig. 1; the transverse stress, σ y, and shear stress, τ xy, are: P σ y = Cσ ; bl P τ xy = Cτ (1) bh where C σ = C τ = 1 i uniorm stress distribution is assumed or both ases. From Eq. (1), the shear-to-transverse stress ratio is: k = σ τ x y = y L h (2) Three dierent shear-to-transverse stress ratios were onsidered. The onrete units had onstant L = 305 mm, and b = 127 mm, with k = 1.26, 1.50, and 1.85, orresponding to h = 165 mm (Type I units), 203 mm (Type II units) and 241 mm (Type III units), respetively. To ensure ailure o the units in the shear plane, reinoring steel was plaed away rom the shear plane so no other mode o ailure suh as lexural, ompression or bearing apaity might be exeeded. Figure 1 shows typial steel reinorement or units with a k-ratio o Material Properties The test units were all ast in one bath along with onrete ylinders, whih were tested at the same time as the test units so that hardening due to onrete aging would not be a variable. The onrete ompressive strength,, ranged rom 34 to 37 MPa. Mild steel reinorement was used with an y = 410 MPa whih was not plaed at the shear ailure plane as shown in Fig. 1, so it would not inluene the results. A CFRP omposite with an epoxy-resin matrix was used; the arbon iber was a kg/m 2 high-strength, unidiretional abri with these properties: ilaments per tow = 12,000; tensile strength = 4.76 GPa; tensile modulus = 234 GPa; density = 1800 kg/m 3 ; and elongation = 1.5%. A high-modulus, high-strength epoxy-resin was used with these properties ater 72 hours o uring at 60 C: tensile strength = 72.4 MPa; tensile modulus = 3.16 GPa; density = 1157 kg/m 3 ; and elongation at break = 4.8%. The tensile material properties o the CFRP omposite laminate were determined ollowing ASTM D3039 [13]. The average mehanial properties o the CFRP omposite laminate were: tensile strength u = 903 MPa; tensile modulus, E = 68 GPa; tensile strain u = 1.33%; and ply thikness t i = 1.00 mm. A high-strength, high-modulus strutural adhesive was used to enhane

3 Figure 1. Typial shear transer test unit setup and steel reinorement or Type II units bond between the onrete surae and CRFP omposite laminate, with these properties: tensile strength = 37 MPa; tensile modulus = 2.83 GPa; elongation at break = 1.3%; modulus o rupture = 46.2 MPa; and shear strength = 25.5 MPa. A 1 mm thik layer was spread with a plasti spatula on the onrete suraes where the CFRP omposite would be applied. Surae Preparation and Layup From previous experiments, it is known that the most eetive surae preparation tehnique or this type o appliation is high-pressure water jet with a high-strength adhesive/epoxy primer [14]. The water stream is delivered at a onstant rate at a high-pressure (280 MPa) to the onrete surae using a rotating maniold to expose the aggregate. A wet-layup proedure was used to apply the CFRP omposite. To saturate the arbon iber, a ustom saturating mahine was used to maintain a onstant iber volume ratio. Test Setup The experiments were onduted as monotoni tests under a ompressive load, P, as shown in Fig. 1, using displaement ontrol at mm/s. To prevent loal ompressive ailure, a 6 mm thik high-density polyethylene plate was inserted inside the steel aps. Eletroni data was reorded every 0.02 s; the ollowing instruments were used in the data aquisition proess: (a) Linear Variable Displaement Transduers (LVDT), and (b) strain gauges. 3. EXPERIMENTAL RESULTS Four dierent wrapping shemes were tested with the same CFRP reinorement ratio and unit type. Units o Type II (k = 1.50) and a CFRP reinorement ratio o 0.6% were used or omparing various wrapping shemes. The CFRP reinorement ratio is deined as:

4 ti w ρ = (3) b h where t i = ply thikness; w = ply width; b = width o retangular ross setion; and h = shear plane height, as shown in Fig. 2(d). For eah o the our wrapping shemes, two units were tested to investigate the inluene o the wrapping sheme on shear strengthening eiieny: (1) Type IIA: Four-sided wrapped with three evenly distributed single layer 25 mm-wide strips, as shown in Fig. 2(a); (2) Type IIB: Four-sided wrapped with a single layer 75 mm-wide strip, as shown in Fig. 2(b); (3) Type IIC: Four-sided wrapped with a double layer 38 mm-wide strip, as shown in Fig. 2(); and (4) Type IID: Two-sided wrapped with a single layer 75 mm-wide strip, as shown in Fig. 2(d). Type IIA: Four-sided wrapped with three evenly distributed single layer strips The maximum shear load, P max, was and kn and the maximum horizontal slip at the shear plane was 4.04 and 2.46 mm or the two units. Figure 3(a) shows a typial unit at ailure, where diagonal tension raks developed. Debonding o the CFRP laminate at the shear plane was observed, whih extended 76 mm on eah side o the shear ailure plane deined in Fig. 1. Type IIB: Four-sided wrapped with single layer strip The maximum shear load, P max, was and kn and the maximum horizontal slip at the shear plane was 0.38 and 1.91 mm or the two units. Figure 3(b) shows the typial unit at ailure. Both units developed diagonal tension raks; debonding o the CFRP laminate at the shear plane was observed, whih extended 102 mm on eah side o the shear ailure plane. Type IIC: Four-sided wrapped with double layer strip The maximum shear load, P max, was and kn and the maximum horizontal slip at the shear plane was 1.52 and 1.75 mm or the two units. Figure 3() shows the typial unit at ailure. Debonding o the CFRP laminate at the shear plane was observed, whih extended 89 mm on eah side o the shear ailure plane. Type IID: Two-sided wrapped with single layer strip The maximum shear load, P max, was and kn and the maximum horizontal slip at the shear plane was 1.19 and 2.13 mm or the two units. Figure 3(d) shows the typial unit at ailure, aused by bond ailure o the CFRP laminate. This type o ailure was observed on both sides o eah unit. The bond ailure mehanism was brittle and hene, diagonal tension raks were not learly observed. Figure 4 summarizes the experimental results in terms o normalized shear stress and CFRP tensile strain normalized by the CFRP ultimate tensile strain. For units with the our-sided wrapped sheme, the tensile strain in the CFRP omposite loated at the shear plane reahed 6985 µs at ailure; the highest strain reahed at the side o the unit (b = 127 mm) was 1100 µs. There is no evidene that additional shear strength was developed due to the our-sided wrapped onrete with CFRP omposite, as shown in Fig. 4.

5 (a) (b) () (d) Figure 2. Test unit detail: (a) Type IIA; (b) Type IIB; () Type IIC; (d) Type IID (a) (b) ( (d) Figure 3. Typial unit at ailure: (a) Type IIA; (b) Type IIB; () Type IIC; (d) Type IID v u Type IIA Type IIB Type IIC Type IID e u Figure 4. Normalized ultimate shear strength-cfrp eiieny or varying wrapping shemes

6 Aording to ACI 440 [15], the our-sided wrapping sheme is the most eiient FRP omposite wrapping sheme and the two-sided is the least eiient. Based on the experimental results, two parameters are aeted by the wrapping sheme: (1) Shear strength, and (2) CFRP tensile strain. ACI 440 [15] makes no distintion between shear strength and CFRP tensile strain eiieny as a untion o the wrapping sheme. From Fig. 4, shear strength shows no signiiant dependene on the wrapping sheme used. Tensile strain levels in the CFRP reinorement show some dependene on the wrapping sheme, whih an be up to 19% higher or our-sided ompared to two-sided wrapped units. Tests with Varying FRP Composite Ratio For this series o tests, the two-sided wrapping sheme Type IID o Fig. 2(d), with a single CFRP layer on eah ae, was used. Four CFRP ratios deined in Eq. (3) were used: 0.3%, 0.6%, 0.9% and 1.2%. Two units or eah CFRP ratio and eah o the three shear-to-transverse stress ratios were tested; in addition, two units were tested or eah o the three shear-to-transverse stress ratios without CFRP omposite shear reinorement. The two-sided wrapping sheme was hosen to investigate the inluene o CFRP ratio on shear strengthening beause it is simpler and provides essentially the same shear strengthening as the our-sided wrapping sheme. 4. CFRP COMPOSITE EFFECTIVE TENSILE STRAIN The CFRP omposite is stressed ater the onrete shear rition or horizontal slip apaity o the as-is units is exeeded. The unit ails in two suessive stages: irst, the imposed shear stress is ontrolled by the onrete alone until the onrete shear rition apaity is reahed; seond, the additional imposed shear stress is ontrolled by the CFRP omposite ating as a lamping ore, whih indues additional aggregate interlok/shear rition until the bond between laminate and onrete ails. In all tests, the highest strain in the CFRP omposite ourred at the shear ailure plane, and varied approximately linearly rom the shear ailure plane (highest strain) to the edge (least strain) as shown in Fig. 5. The CFRP omposite had suiient length exeeding the required bond length. The tensile strain along the midheight o the CFRP omposite in the horizontal diretion was symmetri about the shear ailure plane. Figure 6 shows the eetive CFRP omposite tensile strain, deined as the eetive strain at ailure, e, over the ultimate 4000 Pu = kn C S3 Strain (µs) 2000 L2 267 R3 R R5 L1 127 (As-Built Fail. Load) R Distane rom Center (mm) Figure 5. Typial CFRP omposite tensile strain distribution in the horizontal diretion

7 e u Type I Type II Type III ρ E Figure 6. CFRP tensile strain eiieny CFRP tensile strain, u, versus the normalized stiness o the CFRP laminate; the latter is deined as the CFRP reinorement ratio times the modulus o elastiity, E, divided by the ultimate tensile stress,. The CFRP eiieny ranges rom 0.18 to 0.48, and eah o the our u dierent CFRP reinorement ratios is distinguishable. The normalized stiness value o 0.20 orresponds to ρ = 0.3%, the smallest CFRP reinorement ratio used in these tests. Equation (4) establishes an average CFRP strain-eiieny, whih varies with CFRP normalized stiness. Using the least squares method, the CFRP strain-eiieny is: u 0.27 e ρ E ξ = = 0.26 ; 0.90 ρ E u u u (4) 5. SHEAR TRANSFER CAPACITY From the experimental results, the test unit was observed to ail in two suessive stages: (a) the shear stress was ontrolled by the onrete alone until the onrete shear rition apaity was reahed; and (b) the additional imposed shear stress was resisted by the CFRP omposite ating as a lamping ore, whih indued additional aggregate interlok/shear rition until the bond between laminate and onrete ailed. To determine the onrete shear rition strength and the additional shear rition strength provided by the onrete-cfrp interation, the relation between ultimate shear to onrete ompressive strength, normalized by the onrete ompressive strength, 7. From Fig. 7, using the method o least squares: u u ρ u v u, versus CFRP stiness, is alulated and presented in Fig. v = ρ + (5) The irst term in Eq. (5) is the shear rition strength ontributed by onrete-cfrp shear rition interation, where is the shear rition interation oeiient, and ρ is the eetive u

8 0.30 v u R 2 = 0.91 Type I Type II Type III ρ u Figure 7. Normalized ultimate shear stress versus normalized CFRP stiness CFRP omposite tensile stress, whih is the lamping stress provided by CFRP reinorement. The seond term in Eq. (5) is the onrete shear rition strength, where is the omponent or bond and asperity shear. Type I units with a shear-to-transverse stress ratio k = 1.85, or a CFRP reinorement ratio ranging rom 0.3% to 1.2%, inreased the shear transer apaity by a ator rom 2.12 to 3.25; or Type II units with k = 1.50, the inrease was 1.32 to 2.08 times; and or Type III units with k = 1.26, the inrease was 1.50 to 2.41 times. From Fig. 7, the upper bound shear stress or the shear transer units tested o all three types was 0.28, regardless o the shear-to-transverse stress ratio. 6. CONCLUSIONS Shear transer tests were arried out or initially unraked onrete that was strengthened with varying CFRP omposite wrapping shemes. There was no evidene that additional shear strengthening was developed due to a our-sided wrapped sheme ompared to a two-sided sheme. However, up to 19% higher CFRP tensile strain eiieny was observed or our-sided wrapped ompared to two-sided wrapped units. The maximum CFRP tensile strain at ailure or the our-sided wrapped units varied rom 18% to 53% o the ultimate tensile strain and was a untion o its stiness. The ailure mode was debonding o the CFRP laminate; the wrapping sheme type did not aet the shear-to-horizontal slip relationship. Units with a two-sided wrapped sheme were tested or varying CFRP reinorement ratios. The imposed shear stress was ontrolled by onrete alone until the onrete shear apaity was reahed; the additional imposed shear stress was resisted by the CFRP omposite ating as a lamping ore, induing additional aggregate interlok/shear rition until the bond between CFRP laminate and onrete ailed. The CFRP omposite was not signiiantly stressed or loads below the onrete shear apaity, so the priniple o superposition between onrete shear rition apaity and shear rition apaity due to onrete-cfrp interation an be applied.

9 The CFRP omposite tensile strain distribution along the CFRP strip or the two-sided wrapped units was symmetri with respet to the shear plane, and was approximately triangular; in addition, the length o the CFRP strip was suiient to develop up to 48% o its tensile apaity. The shear transer units reinored with CFRP omposites having a reinorement ratio o 0.3% to 1.2% inreased shear transer apaity by a ator rom 1.32 to 3.25 times. The inrease in shear transer apaity was a untion o the shear-to-transverse stress ratio. An upper bound or the shear apaity o shear transer units was established as 28% o the onrete ompressive strength, regardless o the shear-to-transverse stress ratio. The ailure mehanism in all the tests, regardless o the wrapping sheme, was bond ailure o the CFRP omposite laminate. ACKNOWLEDGMENTS The writers aknowledge the inanial support o the National Siene Foundation under Grant No. CMS The writers aknowledge in-kind support rom Sika Corporation and Eagle Preast In.; they also aknowledge the assistane o graduate students at the University o Utah. Reerenes 1. Birkeland, P. W. and Birkeland, H. W., Connetions in preast onrete onstrution, ACI J. Pro., 63/3 (1966), Mattok, A. H., Hobek, J. A. and Ibrahim, I. O., Shear transer in reinored onrete, ACI J. Pro., 66/2 (1969), Mattok, A.H. and Hawkins, N. M., Shear transer in reinored onrete-reent researh, PCI J., 17/2 (1972), Mattok, A. H., Li, W. K. and Wang, T. C., Shear transer in lightweight reinored onrete, PCI J., 21/1 (1976), Mattok, A. H., Shear rition and high-strength onrete, ACI Strut. J., 98/1 (2001), Kahn, F. L. and Mithell, A. D., Shear rition tests with high-strength onrete, ACI Strut. J., 99/1 (2002), Foerster, H. R., Rizkalla, S. H. and Heuvel, J. S., Behavior and design o shear onnetions or load bearing wall panels, PCI J., 34/1 (1989), Valle, M. and Büyüköztürk, O., Behavior o iber reinored high-strength onrete under diret shear, ACI Mater. J., 90/2 (1993), Allos, A. E., Shear transer in ibre reinored onrete, Int. Con. on Reent Developments in Fibre Reinored Cements and Conretes, Eds. R. N. Swamy and B. Barr, Elsevier Appl. Si., New York, (1989), ASTM, Standard test method or shear properties o omposite materials by the V-Nothed beam method, D5379/D5379M-98, (1998), West Conshohoken, Pennsylvania. 11. Dolan, B. E., Hamilton III, H. R. and Dolan, W., Strengthening with bonded FRP laminate, ACI Con. Int., 20/6 (1998), Burgoyne, C. and Ibell, T., Use o iber-reinored plastis versus steel or shear reinorement o onrete, ACI Strut. J., 96/6 (1999), ASTM, Standard test method or tensile properties o polymer matrix omposite materials, D3039/D3039M-00e1, (2000), West Conshohoken, Pennsylvania. 14. Pantelides, C. P., Volnyy, V. A., Gergely, J. and Reaveley, L. D., Seismi retroit o preast onrete panel onnetions with arbon iber reinored polymer omposites, PCI J., 48/1 (2003), Amerian Conrete Institute, Guide or the design and onstrution o externally bonded FRP systems or strengthening onrete strutures, ACI Committee Report 440.2R-02, (2002), Farmington Hills, Mihigan.