Experimental study on the capacity of RC beams strengthened in shear by CFRP-sheets

Transcription

1 Fourth International Conerence on FRP Composites in Civil Engineering (CICE2008) 22-24July 2008, Zurich, Switzerland Experimental study on the capacity o RC beams strengthened in shear by CFRP-sheets E. Grande, M. Imbimbo & A. Rasulo Department o Mechanics, Structures and Environment, University o Cassino, Italy ABSTRACT: The present paper shows and discusses some o the results obtained within an experimental campaign carried out on RC beams strengthened in shear by externally bonded FRP sheets. In particular, the objective o the paper is to analyze the inluence o the shear spanto eective depth ratio on the behaviour o the strengthened beams. ith this aim, the obtained results in terms o strength, cracking distribution and modes o ailure, have been compared on the basis o the dierent slenderness values. Moreover, in order to study the eectiveness o the theoretical ormulations proposed by codes, the obtained results have been compared with the theoretical ones deduced by the ACI and the CNR code ormat recommendations. 1 INTRODUCTION The recent publication o code-ormat recommendations on the use o composite materials as reinorcement o structures certainly testiies the grade o awareness the research community has reached in the development o analysis and design tools required or the use o this strengthening technique. Nevertheless, many aspects remain to be investigated. Among these, the inluence o the slenderness ratio on the resistance mechanism o RC beams strengthened in shear by FRP represents an important issue. In act, the theoretical ormulations proposed by codes or evaluating the contribution o FRP reinorcement do not take into account the eect o the slenderness ratio o beams. In order to investigate the eect o the slenderness ratio on the FRP contribution, the present paper shows and discusses some o the results obtained within an experimental investigation carried out on RC beams strengthened in shear by externally bonded FRP sheets and characterized by dierent shear span-to eective depth values (denoted in the paper as slenderness λ) and strengthening conigurations (Table 1). In particular, the objectives o the paper is to analyze the inluence o the slenderness ratio on the behaviour o the strengthened and not strengthened beams. The results o the tests in terms o shear capacity are compared to the design ormulations provided by the ACI (2002) and the CNR-DT200 (2004) code-ormat recommendations. 2 EXPERIMENTAL CAMPAIGN The examined beams are part o an experimental campaign conducted on twenty-ive specimens devoted to investigate the eect o dierent aspects on the contribution o FRP-strengthening in terms o shear capacity o RC beams (Rasulo and Imbimbo, 2006; Grande et al., 2007). In particular, in this paper ten ull-scale beams characterized by a rectangular cross-section o 250 mm wide and 450 mm deep have been examined. The longitudinal steel reinorcement o the beams consisted o our 22 mm diameter plus two 20 mm diameter steel bars at the bottom, and two 20 mm diameter steel bars at the top o the beam s section. The reinorcement was designed in order to assure a shear dominate ailure mode in all specimens. The web reinorcement consisted o 8 mm diameter closed steel stirrups placed with 400 mm spacing. Three series o beams are examined. The irst, corresponding to a slenderness equal to 4, is constituted by our beams 3600 mm long and tested as simple supported beams subjected to a - 1 -

2 three-point loading (Figure 1). The second series, corresponding to a slenderness equal to 3, is constituted by our beams 2800 mm long and tested as simple supported beams subjected to a three-point loading (Figure 1). The third series, corresponding to a slenderness equal to 2.5, is constituted by two beams 2800 mm long and subjected to a our-point loading (Figure 1). Seven beams (see Table 1) were strengthened in shear with CFRP sheets placed perpendicularly to the beam longitudinal axis. The sheets were applied over the external surace o the specimens by adopting three dierent strengthening conigurations: : complete wrapping with composite sheets surrounding completely the cross section and an adequate overlapping at the junction to prevent opening; U: improved U jacketing with main composite sheets surrounding only three sides o the section and an additional strip placed along the upper part o beam to improve the anchorage o FRP sheets; S: side bonding with composite sheets placed only at the beam lateral surace. The remaining three specimens were the not strengthened control beams. Each specimen is labelled by a our character designation: RSn, RSnU, RSnS and RSnNR, where R denotes that the beam has a rectangular section; Sn indicates the stirrup spacing, assuming, in our case, the value S4, standing or 400 mm;, U, S and NR indicate the strengthening coniguration, previously named as complete wrapping (), improved U jacketing (U), side bonding (S) or not strengthened (NR). Details o specimens and strengthening coniguration are illustrated in Table 1. Table 1: Tested specimens Label Slenderness Strengthening coniguration RS4NR RS4 RS4NR RS4 RS4U RS4S RS4NR RS4 RS4U RS4S 2.5 (L=2.8 m) 3 (L=2.8 m) 4 (L=3.6 m) NR U+ S A concrete mix consisting o Portland cement was used. The average cylindrical compression strength, c, was equal to 21 MPa at the time o beam tests. A steel with an eective yield strength, sy, equal to 500 MPa (corresponding to the Italian standardized FeB44 grade), was adopted. Commercial carbon iber reinorced plastic sheets (Sikarap- Med Mod 400C N) with a Young s modulus o 392 GPa and an ultimate tensile strain about 0.6%, as certiied by the producer, were used or strengthening the beams. The composite application was one ply with a design thickness (consistent with carbon iber amount) o mm and a two components (epoxy resin and hardener) epoxy resin. In all specimens a 100 ton hydraulic jack was used to apply the load and during testing one or two initial unloading cycles were perormed in order to measure the stiness degradation due to concrete cracking. The midpoint delections were monitored by a vertical displacement - 2 -

3 transducer and strain gauges were mounted both on the longitudinal steel bar surace (at dierent positions) and on two dierent stirrups (Figure 1). Reaction rame Load cell Hydraulic jack spherical hinge Reaction rame S2b S1b S2b S1b P3b P2b P1b Displacement transducer P3b P2b P1b D support stirrup spacing 400mm: a=1470mm; b=1510mm stirrup spacing 300mm: a=1460mm; b=1490mm stirrup spacing 200mm: a=950 mm; b=1150mm a b Figure 1. Test set-up and specimen instrumentations 470 a b 3 TEST RESULTS AND DISCUSSION 3.1 FRP-Strength contribution The ultimate load o the specimens are summarized in Table 2, where is also reported the relative strength increase, calculated as dierence between the ultimate load o the strengthened beam and the ultimate load o the corresponding control beam, divided by the latter. The ratio between the ultimate load F tot o the beams divided by the ultimate load F NR o the corresponding unstrengthened ones is plotted in Figure 2 in unction o the slenderness ratio o beams. From this igure is clear the dependency o the strength increase on the on the slenderness ratio. In particular, comparing the strength values o the examined beams, the ollowing observations can be carried out: The increase o the strength due to the FRP reinorcement is greater or the beams with slenderness ratio λ=3 in the case o rapping and U-jacketing conigurations. Similar values characterize the beams with side bonding conigurations. The beam with λ=2.5 presents the minimum value o the strength increase due to the FRP strengthening. Similar considerations can be made also in terms o the FRP shear contribution (deduced by the dierence between the strength o the strengthened beams and the strength o the corresponding not strengthened ones). In particular, the beams with λ=3 and λ=4 present FRP shear contributions range between 24% (λ=4 -side bonding coniguration) and 52% (λ=3- wrapping coniguration); the beam with λ=2.5 shows a FRP shear contribution equal to 18%. Table 2: Test results slenderness label Strength (kn) Strength increase (%) 2.5 NR NR U S NR U S

4 2.5 2 F tot /F NR U+ S stirrup spacing: 400 mm slenderness 3 4 Figure 2. FRP strength contribution The experimental values have been compared with the design ormulas (neglecting any saety partial actor) contained in the major code-ormat recommendations (CNR-DT , ACI44. IR ), even i them do not consider the inluence o slenderness. The (CNR-DT 200, 2006) proposes two dierent design equations respectively or the case o side bonding (1) and U-jacketing or wrapping coniguration (2): 1 sinβ w V = min FRP { 0.9d;h w} 2 t (1) ed γ sin θ p Rd 1 w V = min FRP { 0.9d; h w} 2 t ed ( cot β+ cot θ) (2) γ p Rd where: d: beam eective depth; ed : design eective strength o the FRP shear strengthening; t : thickness o FRP strip/sheet (on single side); β: angle o inclination o strip/sheet; θ: crack angle; p : strip spacing; w : strip width. γ Rd : a partial actor that depends on the resistance model (in the case o shear/torsion it is assumed equal to 1.20). These equations are based on the work o (Monti et al. 2004) according to which the Moersh resisting mechanism is considered only in the case o reinorcement type U or, while or side-bonding is assumed that the role o FRP is that o bridging the shear crack. The (ACI 440.2R-02, 2002) proposes the ollowing design equation or the cases o side bonding, U-jacketing and wrapping conigurations: V ( ) 2t w d sinα+ cosα e = (3) FRP p where: d : depth o FRP shear reinorcement; α: angle o inclination o strip/sheet; e : eective stress in the FRP; E : tensile modulus o elasticity o FRP; ε e : eective strain level in FRP reinorcement. and assumes dierent equations or evaluating the eective strain level in FRP: ε e = ε u (completely wrapped members) (4.a) ε e =κ ν ε u (bonded U-wraps or bonded ace plies) (4.b) where ε u is the design rupture strain o FRP reinorcement and κ v is the bond-dependent coeicient or shear. These equations are based on the work o (Khalia et al., 2000) where the contribution o the FRP system to the shear strength o the reinorced member is based on the ibre orientation and an assumed crack pattern

5 In Figure 3 the values o the experimental FRP shear resisting action (V FRP ), computed as dierence between the ultimate shear orce o the strengthened beam and the ultimate shear orce o the corresponding control beam, are shown. In the same igure, the theoretical values o the FRP shear resisting action evaluated according to the ormulations contained in (CNR- DT200, 2006) and in (ACI 440.2R-02, 2002) are also reported. As previously observed, the experimental values show a variability with slenderness, not captured by the code-ormat predictive equations, represented by the horizontal lines. From Figure 3 it is also interesting to notice that the code values are greater than the experimental ones or all specimens V FRP (kn) U+ S stirrup spacing: 400 mm theoretical values: CNR-DT V FRP (kn) () (U+) (S) U+ S stirrup spacing: 400 mm theoretical values: ACI440 slenderness Figure 3. FRP shear contribution: comparison between theoretical and experimental values 3.2 Cracking pattern In Figure 4 is reported the cracking pattern at collapse o the specimens considering the dierent slenderness and strengthening coniguration. In particular, the igure reers only to the hal span o each beam where the shear rupture is occurred. The global cracking layout seems to be more inluenced by FRP application than by the beam slenderness, apart or the case o λ=2.5 which presents its own peculiarities. This observation conirms the act that the ailure mechanism (Table 3) does not vary substantially between beams with λ=3 and λ=4. λ NR U+ - S - Figure 4. Crack patterns at the collapse (in approximate scale) Obviously crack angles tend generally to be steeper with λ=3. This observation could suggest that in the beams with λ=3 a minor number o steel stirrups are involved in the resistant mechanism and, consequently, a greater contribution o the FRP is expected. In the case o the - 5 -

6 beam with λ=2.5 it is also important to underline that a dierent load scheme is used during the test. In particular, in this case the central part o the beam is characterized by a negligible value o the shear. This is probably the main responsible o the dierent trend o the FRP contribution in comparison to the beams with greater slenderness values. The pattern also conirms that non reinorced beams (NR) have several cracks distributed all along the shear span, whilst reinorced beams (, U, S) tend to concentrate the damage along one crack. Table 3: Failure modes slenderness label Failure modes 2.5 FRP in tension FRP in tension 3 U+ FRP in tension with debonding S FRP debonding FRP in tension 4 U+ FRP in tension with debonding S concrete crushing; FRP debonding 4 CONCLUSIONS Shear strengthening o RC beams through FRP involves several resisting mechanisms, since concrete, transversal steel reinorcement and laterally bonded FRP sheets cooperate together to resist to external action. Previous studies have explored the interaction between steel stirrup spacing and FRP, showing a clear trend: the FRP contributes the most when there is less amount o transversal steel. hen considering the eect o slenderness (with ixed amount o steel working in shear) the interaction studies is, instead, between the contribution provided by the concrete (through the classic arch and beam actions) and the contribution provided by FRP. In this case the trend is less clear. The slenderness seems to have no inluence over ailure mode and cracking pattern, two aspects that surely govern the strength response. But when considering the FRP contribution our experimental campaign shows a peak strength at intermediate slenderness, even i this result may be aected by the not homogenous testing conditions (our vs. three-point loading). 5 REFERENCES Rasulo, A. and Imbimbo, M., 2006 The inluence o transverse steel and FRP conigurations on the shear behaviour o RC beams, Conerence Fib06, Naples, Italy. Grande, E., Imbimbo, M. and Rasulo, A., 2007 Experimental behaviour o RC beams strengthened in shear by FRP sheets, Conerence FRPRCS - 8, July 2007, Patrasso, Greece. Monti, G., Santinelli, F. and Liotta, M.A., 2004 Shear Strengthening o Beams with Composite Materials, Proc. 2nd International Conerence on FRP Composites in Civil Engineering - CICE Adelaide, Australia. Khalia, A. and Nanni, A., 2000 Improving shear capacity o existing RC T-section beams using CFRP composites, Cement and Concrete Composites, vol.22(3), CNR-DT200, 2006 Guide or the design and construction o externally bonded FRP systems or strengthening existing structures. Materials, RC and PC structures, Masonry structures, National Research Council o Italy, Rome, Italy. ACI R-02, 2002 Guide or the Design and Construction o Externally Bonded FRP Systems or Strengthening Concrete Structures ACI - American Concrete Institute, Farmington Hills, MI., USA