STRENGTHENING OF REINFORCED CONCRETE BEAMS UNDER TORSION USING CFRP SHEETS

Transcription

1 STRENGTHENING OF REINFORCED CONCRETE BEAMS UNDER TORSION USING CFRP SHEETS El Mstafa Higazy*, Ain Shams University, Egypt Mahmud El-Kateb, Ain Shams University, Egypt 36th Cnference n OUR WORLD IN CONCRETE & STRUCTURES: August 2011, Singapre Article Online Id: The nline versin f this article can be fund at: This article is brught t yu with the supprt f Singapre Cncrete Institute All Rights reserved fr CI Premier PTE LTD Yu are nt Allwed t re distribute r re sale the article in any frmat withut written apprval f CI Premier PTE LTD Visit Our Website fr mre infrmatin

2 36 th Cnference n Our Wrld in Cncrete & Structures Singapre, August 14-16, 2011 STRENGTHENING OF REINFORCED CONCRETE BEAMS UNDER TORSION USING CFRP SHEETS El Mstafa Higazy * and Mahmud El-Kateb * Department f Structural Engineeering Ain Shams University Cair, Egypt melkateb@gmail.cm Keywrds: Strengthening, Beams, Trsin, FRP Abstract. Repair and strengthening f bridges, buildings, and ther civil engineering structures have becme necessary due t aging, envirnmentally induced degradatin, increases in service lads, changes in use f the structure, design and/r cnstructin errrs, changes in design cde regulatins, and seismic retrfits. Many buildings and bridge elements are subjected t significant trsinal mments that affect the design, and may require strengthening. Reinfrced cncrete (RC) beams may be deficient in trsinal shear capacity and in need f strengthening. These deficiencies may ccur fr several reasns, such as insufficient stirrups resulting frm cnstructin errrs r inadequate design, reductin in the effective steel area due t crrsin, r increased demand due t a change in ccupancy. FRP cmpsites have shwn great prmise as a state-f-the-art material in flexural and shear strengthening as external reinfrcement. Hwever, little attentin has been paid t its applicability in trsinal strengthening f RC beams in terms f bth experimental and numerical research. The main bjectives f the current study are t investigate the trsinal behavir f RC beams strengthened with externally bnded carbn fiber reinfrced plymer (CFRP) sheets, and t identify the influence f the investigated parameters affecting the trsinal behavir n the effectiveness and feasibility f strengthening. The experimental prgram recunts an verall investigatin f trsinal strengthening f five rectangular reinfrced cncrete beams with externally bnded carbn fiber-reinfrced plymer (CFRP) sheets under the effect f cmbined flexure, shear and trsin. One specimen is kept withut strengthening as cntrl specimen, while fur specimens are strengthened with CFRP cmpsite wraps. The main parameters investigated are the CFRP warps spacing and number f plies. The cncrete grade and steel reinfrcement rati are kept cnstant fr all specimens. In the experimental findings, the increase in the ultimate trsinal mment fr different strengthening cnfiguratins, perfrmance imprvement and crack patterns are presented. Ain Shams University

3 El Mstafa Higazy and Mahmud El-Kateb 1. INTRODUCTION In recent decades, due t aging f mst structures and fr sustainability f satisfactry perfrmance f the existing structural system, repair and strengthening f bridges, buildings, and ther civil engineering structures have becme necessary. Generally, mst structures require repair and strengthening fr several reasns such as; envirnmentally induced degradatin, increases in service lads, changes in use f the structure, design and/r cnstructin errrs, changes in design cde regulatins, and seismic retrfits. Particularly, sme reinfrced cncrete (RC) elements such as beams may be deficient in trsinal shear capacity and in need f strengthening. These deficiencies may ccur fr several reasns, such as insufficient stirrups resulting frm cnstructin errrs r inadequate design, reductin in the effective steel area due t crrsin, r increased demand due t a change in ccupancy. In current practice, trsinal strengthening f cncrete members is achieved by ne f the fllwing methds; (1) increasing the member crss-sectinal area cmbined with adding f transverse reinfrcement, (2) using externally bnded steel plates and pressure gruting the gap between plate and cncrete element, and (3) applying an axial lad t the member by pst-tensining. Althugh these methds will cntinue t be used in many mre instances, fiber reinfrced plymer (FRP) cmpsites prvide anther ptin fr strengthening. FRP cmpsites have shwn great prmise as a state-f-the-art material in flexural and shear strengthening as external reinfrcement. Studies f flexural and shear strengthening f RC beams using FRP have been frmed since the early nineties f the last century 1. Hwever, little attentin has been paid t its applicability in trsinal strengthening f RC beams in terms f bth experimental and numerical research due t the specialized nature f the prblem and the difficulties in cnducting realistic tests and representative analyses. This paper attempts t fill that gap in the experimental area by investigating the trsinal behavir f RC beams strengthened with externally bnded CFRP sheets (wraps) under the effect f cmbined flexure, shear and trsin. The main bjective f the current study is t identify the influence f the investigated parameters affecting the trsinal behavir n the effectiveness and feasibility f strengthening. 2. EXPERIMENTAL PROGRAM 2.1 Specimen s details The experimental prgram recunts an verall investigatin f five rectangular reinfrced cncrete beams f 120x300 mm crss sectin and mm lng. The dimensins f the test specimens and the reinfrcement details are shwn in figure 1. 5#6/m 2#10 1#16 3#16 5#6/m 1#16 2#10 5#6/m 300 3#16 Figure 1: Dimensins and reinfrcement details f test specimens 120

4 El Mstafa Higazy and Mahmud El-Kateb The main parameters investigated are the CFRP cmpsite wrap spacing and the number f plies. One specimen was kept withut strengthening as cntrl specimen, while fur specimens were strengthened with 100 mm width CFRP cmpsite all arund wraps which were placed at spacing f 100 mm and 150 mm. Tw beams were wrapped using ne layer and the ther tw beams were wrapped using tw layers f the CFRP cmpsites. The cmpressive strength f cncrete (f c`) was 20 MPa and was kept cnstant fr all specimens. Test specimen matrix is listed in table 1. Specime n Wrap spacing N. f layers BR1 Cntrl specimen BR2 1 ply BR3 2 plies BR4 1 ply BR5 2 plies Table 1: Test specimen matrix In this research, Sikawrap Hex 230C CFRP sheets and Sikadur 330 impregnatin resin were used. All materials are supplied by Sika. The mechanical prperties f the CFRP sheets are given in table 2. In preparatin f the reinfrced cncrete beam fr fiber wrapping, the edges were runded s that there were n sharp crners that wuld cntribute t the reductin f the fiber strength. Thickness Tensile Strength Tensile Mdulus Ultimate Strain (mm) MPa MPa % , , Table 2: Mechanical prperties f CFRP sheets 2.2 Test Setup Specimens were eccentrically laded with tw cncentrated lads at bth thirds f the beam span. Eccentric lads were transferred t the beam using tw steel jackets specially designed t accmmdate and transfer the applied lad withut being failed r distrted. Beams were fixed frm bth ends t the labratry steel frame against vertical and transverse displacements but were free t have ther displacements. A lad cell is placed in the middle f the tw steel jackets t divide the applied lad n bth lading pints equally using a rigid steel beam. A diagrammatic sketch fr the test setup is shwn in figure 2.

5 El Mstafa Higazy and Mahmud El-Kateb 1 2 Rigid beam Lad cell Lab. steel frame Fixatin blts Steel jacket Steel jacket 2 1 Elevatin Lad cell Lab. steel frame Rigid beam Fixatin blts Steel jacket Specimen Sec. (1-1) Sec. (2-2) Figure 2: Diagrammatic sketch fr the test setup 2.3 Instrumentatin Displacement transducers were the main devices used fr measuring the impsed and resulting displacements in the test. Linear variable displacement transducers (LVDTs) f maximum gauge length f 100mm and precisin f 1/100 f mm were used t measure the varius types f displacement that make up the ttal displacements f the specimens. The cmbinatin f LVDTs was installed at the lading lcatin, and at supprt t measure the cmpnents f displacement in tw directins, in rder t evaluate the angle f rtatin f the beam. Electrical strain gauges were the main devices used fr measuring the strain in the test. Three electrical resistance strain gauges with 10mm gauge length were bunded t the lngitudinal and transverse reinfrcement bars in apprpriate lcatins. In additin, tw strain gauges were bunded t each CFRP wrap n bth its sides. Als, tw strain rsettes were set n the specimen side in the laded part t evaluate the principal strains in cncrete. All the LVDTs and strain gauges were cnnected t a channel bx. Then, all the data were recrded by a cmputer cntrlled data acquisitin system n lad intervals f 5.0 kn.

6 El Mstafa Higazy and Mahmud El-Kateb 3. EXPERIMENTAL RESULTS 3.1 Defrmatinal Behavir The defrmatinal behavir studied in this sectin emphasis relatinship between the trsinal mment carried by the test specimen versus the maximum twist angle f the specimen crss sectin which has fund t be at lading lcatin. The ultimate twist angle, the crrespnding trsinal mment and the maximum strain in FRP wraps fr all specimens are listed in table 3. Als, the trsinal mment twist angle relatinships fr all specimens are pltted in figure 3. Specimen Ultimate twist angle (Degree) Ultimate trsinal mment (kn.m) Max. strain in FRP wraps (µε) BR NA BR BR BR BR Table 3: Summary f results Trsinal mment (kn.m) BR1 BR2 BR3 BR4 BR Twist angle (Deg.) Figure 3: Trsinal mment versus twist angle relatinships The defrmatinal behavir f all specimens fllwed the same trend and it was nticeable that the FRP wraps had n effect n beam stiffness in early lading stages. Specimen BR1, which is the cntrl specimen in this research, fllwed almst a linear behavir fr the first fur lading steps up t a twist angle f (abut 7.0% f the ultimate twist angle) then fllwed a parablic trend up t failure at ultimate twist angle f and a crrespnding trsinal mment f 10.8 kn.m.

7 El Mstafa Higazy and Mahmud El-Kateb Specimen BR2, which had the lwest amunt f FRP wraps, fllwed almst a linear behavir up t a twist angle f (abut 6.9% f the ultimate twist angle) then fllwed a parablic trend up t failure at ultimate twist angle f and a crrespnding trsinal mment f kn.m. The trsinal capacity f BR2 was imprved t 104.2% f that f the cntrl specimen while the twist angle was reduced t 78.5% f the ultimate twist angle in cntrl specimen at same lading level. Specimens BR3 and BR4 almst shwed an identical behavir and defrmatin values. Specimen BR3 failed at ultimate twist angle f and a crrespnding trsinal mment f 11.7 kn.m. The trsinal capacity f BR3 was imprved t 108.3% f that f the cntrl specimen while the twist angle was reduced t 72.5% f the ultimate twist angle in cntrl specimen at same lading level. BR4 failed at ultimate twist angle f and a crrespnding trsinal mment f 11.7 kn.m. The trsinal capacity f BR4 was imprved t 108.3% f that f the cntrl specimen while the twist angle was reduced t 73.2% f the ultimate twist angle in cntrl specimen at same lading level. Specimen BR5, which had the largest amunt f FRP wraps, shwed as expected the best behavir. It was defrmed linearly up t a twist angle f (abut 7.2% f the ultimate twist angle) then fllwed a parablic trend up t failure at ultimate twist angle f and a crrespnding trsinal mment f 12.6 kn.m. The trsinal capacity f BR5 was imprved t 116.7% f that f the cntrl specimen while the twist angle was reduced t 66.4% f the ultimate twist angle in cntrl specimen at same lading level. Figure 4: Specimen BR2 prir t failure 3.2 Crack Patterns The cntrl specimen BR1 shwed a wider range f crack prpagatin and a faster rate f crack prgressin than ther specimens due t the absence f any FRP wrapping alng the beam. All the strengthened fur specimens shwed almst the same trend f crack prgressin, this culd be attributed t the relatively similar wrapping scheme and the identical type f reinfrcement details. Althugh, final stages f lading prir t failure shwed slightly different cracking behavir. Diagnal cracks initiated and prpagated thrugh the laded third f the beam frm the lading pint twards the supprt. At twist f nearly 75% f the ultimate twist, cracks were cmpletely develped. In subsequent lading steps, there was a gradual built-up f cracks preceded by widening f existing cracks and lss f stiffness. Als, delaminatin f FRP wraps was nticed at failure.

8 El Mstafa Higazy and Mahmud El-Kateb Figure 5: Crack patterns and mdes f failure 4. ANALYSIS 2 fib Bulletin 14 states that an externally bnded FRP wraps will prvide cntributin t the trsinal capacity nly if full wrapping arund the element s crss sectin is applied, s that the tensile frces carried by the FRP n each side f the crss sectin may frm a cntinuus lp. Ultimate trsinal mment calculatins are based n the fiber rientatin and the mde f failure. In case were the FRP wraps are in vertical (transverse) directin f the beam, and diagnal cracks are assumed t be inclined 45 t the lngitudinal beam axis, the cntributin f FRP wraps t ultimate trsinal mment is determined by using the effective strain in the fibers and can be predicted by the fllwing expressin: Mt frp = 2 εfrp Afrp Efrp Ac / S (1) Where εfrp is the strain in fiber; AFRP is the area f FRP wrap; EFRP is the mdulus f elasticity f the FRP material; AC is the grss area f the cncrete crss sectin; S is the center t center spacing f FRP wraps. Als, the ultimate trsinal mment f the reinfrced cncrete beam crss sectin (Mt cnc) can be 3 determined by the ACI , using equatin (11-18) as fllws: ( Vu 2 T p V ) + ( u h2 ) 2 φ ( c f c` ) bw d bw d 1.7 Ah (2) Then the ttal ultimate trsinal mment capacity f a strengthened beam crss sectin can be determined using equatins (1) and (2) as fllws: Mt calc = Mt frp + Mt cnc (3) Table 4 lists a cmparisn between the calculated value f the ultimate trsinal mment and the experimental value, all in kn.m. Calculated values shwed relatively a gd agreement with the measured values. Specimen Mt frp Mt cnc Mt Calc Mt Experimental Variatin BR % BR % BR % BR % Table 4: Cmparisn between calculated and test results

9 El Mstafa Higazy and Mahmud El-Kateb 5. CONCLUSIONS An experimental prgram cmprised testing five rectangular crss sectin reinfrced cncrete beams was cnducted t evaluate the efficiency and feasibility f strengthening beams subjected t trsin using CFRP wraps. Based n the btained results, the fllwing cnclusins can be furnished: Strengthening reinfrced cncrete rectangular beams subjected t trinal lads using CFRP wraps helped in imprving the verall perfrmance f strengthened beams. Strengthening beams with CFRP wraps helped in imprving the trsinal capacity f beams up t 116.7% f its nn-strengthened value. Strengthening beams with CFRP wraps helped in increasing the trsinal stiffness by decreasing the twist angle up t 66.4% f its nn-strengthened value. There is n effect f CFRP wraps n beam trsinal stiffness in lading stages befre cracking and in early lading stages after cracking. Even with lwer amunt f FRP, using single ply f wraps with small wrap spacing helped t achieve the same imprvement in perfrmance f that when using tw plies f wraps with larger wrap spacing. Hence, it is mre ecnmic and feasible. REFERENCES [1] A. Ghbarah, M. N. Ghrbel, and S. E. Chidiac, "Upgrading Trsinal Resistance f Reinfrced Cncrete Beams Using Fiber-Reinfrced Plymer", Jurnal f Cmpsites fr Cnstructin ASCE, Vl. 6, N. 4, Nvember 1, pp [2] fib Bulletin 14. Externally bnded FRP reinfrcement fr RC structures, fib - Internatinal Federatin fr Structural Cncrete, Lausanne, Switzerland [3] American Cncrete Institute (ACI) Cmmittee 318. (2008). Building,cde requirements fr structural cncrete. ACI , American Cncrete Institute, Detrit. [4] Adrian K. Y. Hii and Riadh Al-Mahaidi, "Trsinal Capacity f CFRP Strengthened Reinfrced Cncrete Beams", Jurnal f Cmpsites fr Cnstructin ASCE, Vl. 11, N. 1, February 1, pp [5] Saravanan Panchacharam and Abdeldjelil Belarbi, "Trsinal Behavir f Reinfrced Cncrete Beams Strengthened With FRP Cmpsites", First FIB Cngress, Osaka, Japan, Octber 13-19, 2002 [6] Mehran Ameli, Hamid R. Rnagh, and Peter F. Dux, "Behavir f FRP Strengthened Reinfrced Cncrete Beams Under Trsin", Jurnal f Cmpsites fr Cnstructin ASCE, Vl. 11, N. 2, April 1, pp