CONFINEMENT EFFICACY OF PARTIALLY AND FULLY WRAPPED CFRP SYSTEMS IN RC COLUMN PROTOTYPES

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1 CONFINEMENT EFFICACY OF PARTIALLY AND FULLY WRAPPED CFRP SYSTEMS IN RC COLUMN PROTOTYPES Ferreira, D.R.S.M. P.D. Candidate, Dep. o Civil Engeneering, University o Minho, Campus de Azurém 48-8 Guimarães, Portugal Barros, J.A.O. Associate Pro., Dep. o Civil Engeneering, University o Minho, Campus de Azurém 48-8 Guimarães, Portugal INTRODUCTION Since the beginning o the nineties, ibre reinorced polymers (FRP) have been used to increase the load carrying capacity and the energy absorption capacity o reinorced concrete columns [-]. The high speciic strength and stiness, the low thickness and weight, and the high resistance to corrosion o FRP materials are avourable properties justiying the increase use o these composites in the structural upgrading []. The ull wrapping FRP-technique is the most used to increase the load carrying capacity and the energy absorption capacity o reinorced concrete (RC) columns. However, preliminary tests with concrete elements submitted to direct compressive loading revealed that partial wrapping (strips o CFRP sheets) is a promising coninement technique [7]. Since in concrete columns deserving strengthening intervention there are always a certain percentage o steel hoops, the application o CFRP strips in-between the existent steel hoops can be an adjusted coninement technique with technical and economical advantages, when ull wrapping is taken or basis o comparison. To assess the eicacy o the partial wrapping technique, 8 prototypes o RC columns were conined by distinct CFRP arrangements and tested under direct compression. The experimental program was designed to evaluate the inluence o the ollowing variables on the compression behavior: stiness o the wet lay-up CFRP sheet; distance between CFRP strips; width o the CFRP strip; number o CFRP layers per each strip; concrete strength; percentage o the steel longitudinal and tranversal reinorcement ratios. The present work describes the experimental program and presents and analyzes the obtained results. Using the obtained experimental results, the applicability o a coninement model [8] was appraised or the RC columns partially wrapped. Keywords: concrete, CFRP, coninement CONFINEMENT ARRANGEMENTS AND EXPERIMENTAL PROGRAM The experimental program deals with direct compression tests with RC column elements o mm length and mm diameter. This program is composed by several groups o tests in order to evaluate the inluence o the ollowing parameters on the compressive strength and deormation capacity o RC elements submitted, predominantly, to compressive loading: concrete strength class (two average compressive strengths, MPa and 3 MPa); stiness o the coninement CFRP system (two CFRP sheets, one o 3 g/m o ibers and the other o g/m o ibers); width (W) and spacing (s ) o the CFRP strips; number o CFRP layers per strip (L); percentage o the longitudinal and transversal steel reinorcement ( ρ sl, ρ st ), see Tab.. Due to lack o space, only the groups o tests CSφ, CS3φ, C3Sφ8 and C3S3φ8, indicated in Tab., are analyzed in the present paper. In this designation, Cxx means specimens o a concrete o average compressive strength o xx MPa, while S and S3 indicate the type

2 o CFRP sheet, g/m and 3 g/m, respectively. Finally, φj indicates the diameter, in mm, o the steel longitudinal bars. Two groups were conined with a CFRP sheet o 3 g/m o ibers (CS3φ, C3S3φ8), while the specimens o the other two groups (CSφ, C3Sφ8) were conined with a CFRP sheet o g/m o ibers. The average concrete compressive strength ( cm ) o the groups CSφ and CS3φ was MPa, while the cm o groups C3Sφ8 and C3S3φ8 was 3 MPa. The two ormer groups were reinorced longitudinally with our steel bars o mm diameter, whereas our steel bars o 8 mm diameter were applied in the last two groups. Following the recommendations o the Portuguese Code or RC structures, the spacing o the steel hoops was considered times the diameter o the longitudinal bars. In the discrete coninement arrangements the strips were placed at mid distance between two consecutive steel hoops. Each group o tests is constituted by three series that are distinguished by the width o the CFRP strip: 4 mm (W4), mm (W) and mm (W ully-wrapped). As Fig. shows, the partially-wrapped specimens o CSφ and CS3φ groups are conined by ive strips (W4S and WS), while the partially-wrapped specimens o C3Sφ8 and C3S3φ8 groups are conined by six strips (W4S and WS). Each one o these test series is composed by two sub-series, one o three layers per strip (L3) and the other with ive layers per strip (L). Previous research revealed that, above ive layers per strip, the beneits in terms o specimen load carrying capacity and energy absorption capacity are marginal [9]. Tab.. Experimental program. s'/ mm mm mm mm mm s' SG w Ø// CFRP SG mm mm s' SG w SG Ø//.9 CFRP SG mm 4Ø s'/ 4Ø8 W [mm] 4 Designation s [mm] W4SL3 W4SL 7 WSL3 WSL W [mm] Designation WSL3 WSL Concrete average compressive strength: MPa Longitudinal bars: φ W [mm] Designation s [mm] 4 W4SL3 W4SL WSL3 WSL 4 W [mm] Concrete average compressive strength: 3 MPa Longitudinal bars: φ8 Designation WSL3 WSL Type o CFRP sheet CF S&P 4 ( gm/m ) CF3 S&P 4 (3 gm/m ) Group o test series CSφ CS3φ Type o CFRP sheet CF S&P 4 ( gm/m ) CF3 S&P 4 (3 gm/m ) Group o test series C3Sφ8 C3S3φ8

3 W4S WS WS W4S a) WS WS b) Fig.. Coninement systems: a) CSφ and CS3φ groups; b) C3Sφ8 and C3S3φ8. MATERIALS From direct compression tests carried out at 8 days with three concrete cylinder specimens o mm diameter and 3 mm height, average compressive strength o MPa and 3 MPa was obtained or the concrete o the groups CSφ, CS3φ, and C3Sφ, C3S3φ, respectively. The CFRP sheets used have the trade name o CF S&P 4 ( g/m o ibers) and CF 3 S&P 4 (3 g/m o ibers). According to the supplier, CF and CF 3 sheets have a thickness o.7 mm and.7 mm, respectively, and have a tensile strength higher than 37 MPa, and an elasticity modulus and an ultimate strain in the ibre direction o about 4 GPa and, respectively. To check the values o these properties, samples o CFRP were tested according to ISO recommendations []. The tensile specimen coniguration is represented in Fig.. To avoid localized racture at the specimen s extremities ixed to the machine grips, layers o the same CFRP sheet were epoxy-glued to these extremities, as represented in Fig.. The strains were measured rom a clip gauge o mm o measuring length and.% accuracy, see Fig. 3. The tests were carried out under displacement control at a rate o mm/minute.the obtained results are presented in Tab.. The values determined experimentally or the thickness, included in Tab., were used in the evaluation o the elasticity modulus and tensile strength o the CFRP sheets. 9 mm mm mm CFRP specimen Clip Gauge mm mm 7 mm Fig.. CFRP tensile test specimen. Fig. 3. CFRP specimen being tested in direct tension. 3

4 Tab.. CFRP properties (average o ive tests). CFRP Sheets Thickness (mm) Tensile strength (MPa) Ultimate strain (%) Elasticity modulus (GPa) CF CF TEST SETUP Three displacement transducers were positioned at degrees around the specimen and registered the displacements between the steel load plates o the equipment (see Fig. 4). This test setup avoids that the deormation o the test equipment is added to the values recorded by the LVDTs. Taking the values registered in these displacement transducers, the displacement at the specimen axis was determined or each scan reading [9], and the corresponding strain was obtained dividing this displacement by the measured specimen s initial height. To decrease the restriction imposed by the machine load plates to the radial expansion o the specimen s extremities, a system o two sheets o Telon with oil between them was applied in-between the bottom plate o the machine and the bottom specimen s extremity. The Telon system was not applied in-between the top plate and the top specimen s extremity, since this plate was connected to a spherical steel hinge. Strains in the CFRP iber direction were measured by strain gauges (SG and SG) ixed on the specimen according to the arrangements indicated in the sketches into Tab.. A detailed description o the test equipment and test procedures can be ound elsewhere [9]. LVDT Ø4 Ø LVDT Steel Load plate LVDT LVDT LVDT Ø Concrete specimen LVDT Specimen Steel Load plate Telón Fig. 4. Position o the LVDTs. EXPERIMENTAL RESULTS Groups CSφ and CS3φ Figs. and show the relationships between concrete stress and both the concrete axial strain and the CFRP strain in the iber direction or the series o the groups o tests CSφ and CS3φ. Each curve represents the average response registered in the two specimens that compose each series. The concrete stress is the ratio between the applied load and the specimen cross section. In these igures, represents the unconined plain concrete specimens, URC,φ the unconined reinorced (longitudinally and transversally) concrete specimens. In each graph, the CFRP coninement ratio, ρ = A /A c,t, is also included, where A = S W L t mm is the cross sectional area o the coninement system (t is the thickness o the CFRP sheet), and A c,t is the area o specimen longitudinal cross section (A c,t = mm ). In general, the stress-strain relationship o the conined specimens is composed by two quasi-linear branches, connected by a nonlinear transition branch. 4

5 3 W4SL3_SG W4SL3 3 3 W4SL3_SG W4SL3 URC,φ ρ =.3% URC,φ ρ =.% CFRP tensile strain(mm/mm) Specimen axial compressive strain (mm/mm) 3 W4SL_SG W4SL 3 ρ =.% URC,φ CFRP tensile strain (mm/mm) Specimen axial compressive strain(mm/mm) 3 WSL3 WSL3_SG W4SL_SG W4SL 3 3 URC,φ WSL3_SG ρ =.33% WSL3 3 URC,φ ρ =.7% URC,φ ρ =.% WSL 4 WSL_SG 3 3 URC,φ WSL3_SG WSL3 4 3 URC,φ WSL_SG URC,φ Fig.. Test group CSφ. ρ =.8% ρ =.34% WSL ρ =.7% WSL WSL_SG URC,φ WSL3_SG WSL3 4 3 URC,φ WSL_SG WSL URC,φ Fig.. Test group CS3φ. ρ =.44% ρ =.3% ρ =.88%

6 Tabs. 3 and 4 include de main indicators o the eicacy provided by the applied coninement systems. In these tables, is the compressive strength o unconined plain concrete specimens (), φ is the compressive strength o unconined reinorced concrete specimens (URC,φ), is the specimen axial strain corresponding to, φ is the specimen axial strain corresponding to φ, cc is the compressive strength (corresponding to the specimen s ailure) o conined specimens, cc is the specimen axial strain corresponding to cc, max is the maximum tensile strain in the CFRP iber s direction and u is the CFRP ultimate strain indicated in Tab.. Each value o Tabs. 3 and 4 is the average o the values obtained in the two specimens o each series. Tab. 3. Main indicators o the eicacy o the coninement systems in the CSφ test group. Specimen designation S L ρ [%] Uncon. Plain Conc. () Uncon. φ Rein. Conc. cc (MPa) 3.87 ( ). ( ) φ cc (µm/m) cc / φ cc / max φ (µm/m) max / u.7 ( ) ( ) φ W4SL W4SL WSL WSL WSL WSL Tab. 4. Main indicators o the eicacy o the coninement systems in the CS3φ test group. Specimen designation Uncon. Plain Conc. () Uncon. φ Rein. Conc. S L ρ [%] cc (MPa) 3.87 ( ). ( ) φ cc (µm/m) cc / φ cc / max φ (µm/m) max / u.7 ( ) ( ) φ W4SL W4SL WSL WSL WSL WSL

7 From the analysis o Figs. 7 and 8 and the results included in Tabs. 3 and 4 it can be concluded that cc / φ increased with ρ. Fig. 7 shows that this increase is almost linear in the considered ρ range. cc / φ has varied rom.7 in series conined with strips CF o 4 mm width and three layers per width (W4SL3), ρ =.3%, up to 4.8 in series ully-wrapped with ive layers o CF-3, ρ =.88%. The specimen ultimate axial strain, cc, has also increased signiicantly with ρ. The limits o cc / φ varied rom.4 to.. In Figure 8 the cc / φ o the ully-wrapped series were not included since their consideration will lose the linear increase trend between cc / φ and ρ. The increase o cc / φ with ρ was more pronounced in specimens o discrete coninement arrangements than in ully-wrapped specimens. The plastic deormation o the concrete in-between the CFRP strips can justiy this occurrence. cc/φ 4 3 W4S_CSi WS_CSi WS_CSi W4S_CS3i WS_CS3i WS_CS3i cc = 4.8ρ +.33 φ R = ρ[%] cc/φ W4S_CSi WS_CSi W4S_CS3 WS_CS3i cc φ R =.93 = 33.38ρ ρ[%] Fig. 7. cc / φ vs. ρ or CSφ and CS3φ groups. Fig. 8. cc / φ vs. ρ or CSφ and CS3φ groups. Groups C3Sφ8 and C3S3φ8 Figs. 9 and show the relationships between concrete stress and both the concrete axial strain and the CFRP strain in the iber direction or the groups o tests C3Sφ8 and C3S3φ8. Like in the CSφ and CS3φ groups o tests, in general, the concrete stress-strain relationship o the conined specimens o groups C3Sφ8 and C3S3φ8 is composed by two quasi-linear branches, connected by a nonlinear transition branch. Tabs. and include de main indicators o the eicacy provided by the applied coninement arrangements. The load carrying capacity o the test equipment was attained in the WSL series o C3Sφ8 group and in the WSL, WSL3 and WSL series o C3S3φ8 group, without the occurrence o the rupture o the specimens. Since the load carrying capacity o the equipment can be doubled i the tests are carried out in a non-closed loop control, the specimens o these series were again tested, up to its ailure, and the attained cc values are indicated in Tabs. and into square brackets. The relationship between cc / φ8 and ρ, represented in Fig., shows a linear increasing trend between these two parameters. In this igure, the cc corresponds to the compressive strength at the ailure o the specimens. Fig. shows that cc / φ8 has also a linear increase trend with the increase o ρ. Since in the manually controlled tests the strains were not measured, this igure only includes the results obtained in the specimens that ailed when the tests were carried out under closed loop control. 7

8 W4SL3_SG W4SL3_SG WSL3_SG WSL3 4 3 WSL3_SG W4SL3 URC_ WSL3_SG WSL3 7 ρ =.% ρ =.% W4SL3 W4SL3_SG W4SL3_SG 4 3 ρ =.4% CFRP tensile strain(mm/mm) Specimen axial compressive strain (mm/mm) 7 WSL3_SG WSL3 WSL3_SG 4 3 ρ =.3% WSL3_SG WSL ρ =.34% 4 3 ρ =.3% W4SL SG W4SL 4 3 W4SL_SG WSL_SG WSL_SG WSL WSL_SG WSL ρ =.% ρ =.34% W4SL W4SL_SG W4SL_SG WSL_SG WSL 7 WSL_SG WSL_SG WSL 7 ρ =.4% ρ =.3% 4 3 URC φ ρ =.4% 4 ρ =.88% Fig. 9. Test group C3Sφ8. Fig.. Test group C3S3φ8. 8

9 Tab.. Main indicators o the eicacy o the coninement systems in the C3Sφ8 test group. Specimen designation S L ρ [%] Uncon. Plain Concrete () Uncon. φ8 Rein. Concrete cc (MPa) 3.3 ( ) cc (µm/m) cc / φ8 cc / max co (µm/m) max / u.3 ( ) ( ) ( φ ) φ8 W4SL W4SL WSL WSL WSL WSL.7 7. [98.3].4.8 [3.] Tab.. Main indicators o the eicacy o the coninement systems in the C3S3φ8 test group. Specimen designation S L ρ [%] Uncon. Plain Concrete () Uncon. φ8 Rein. Concrete cc (MPa) 3.3 ( ) cc (µm/m) cc / φ8 cc / max co (µm/m) max / u.3 ( ) ( ) ( φ ) φ8 W4SL W4SL WSL WSL.3 WSL3 3.3 WSL [77.98] 7. [93.9] 7.88 [.]..8.8 [.38].8 [.8]..9 [3.] (SG3).7 (SG4)

10 cc/co W4S_C3Si8 WS_C3Si8 WS_C3Si8 W4S_C3S3i8 WS_C3S3i8 WS_C3S3i ρ[%] cc = 3.9ρ +.9 φ8 R =.9 Fig.. cc / φ8 versus ρ or the specimens o groups C3Sφ8 and C3S3φ8. cc/co W4S_C3Si8 WS_C3Si8 WS_C3Si8 W4S_C3S3i8 WS_C3S3i8 cc =.8ρ +. φ8 R = ρ[%] Fig.. cc / φ8 versus ρ or the specimens o groups C3Sφ8 and C3S3φ8. In series o equal ρ, such is the case o series WSL and WSL3, the coninement was a little bit more eective in the specimens ully wrapped, but the time consumed to ully wrapped the specimens was higher and the ailure modes o these specimens were more brittle. The last column o Tabs. and shows that, at the ailure o the specimens, which always occurred by the CFRP tensile rupture, the maximum tensile strain in the direction o the ibers, max, varied rom 7% up to 8% o the CFRP ultimate tensile strain, u. These values are just or specimens that ailed when the equipment was working in closed-loop control. As Lam and Teng have already reported [8], the variation o the strain ield in CFRP depends considerably on the distribution o the damage in the concrete specimen. Taking this into account and considering that only one or two strain gauges were applied, per specimen, or recording the CFRP strain variation, it is not surprising that a tendency was not determined or the max / u ratio. A high scatter was registered on the maximum strain values in the CFRP, since the recorded values only represent the areas where the strain gauges are placed, and are too dependent on the specimen ailure mode coniguration. These observations are also applicable to series CSφ and CS3φ, where max, varied rom 44% up to 84% o the u. NUMERICAL SIMULATION To simulate the behavior o concrete specimens ully wrapped with CFRP sheets, submitted to direct compressive loading, several analytical models have been proposed [8,, ]. To simulate the behavior o the partially conined concrete specimens tested in the present work, the model developed by Lam and Teng [8] was adopted. According to this model, the stress in the conined concrete (σ c ) is determined by the ollowing expressions (see Figure 3): σ ( E E ) c c = Ec c or c c t () 4 σ = c + E or c t c cc () where o was assumed equal to co, t is the strain at the transition between the domain o these two equations, with E being the slope o the equation (): t = (3) ( Ec E ) E cc = (4) cc

11 σc Axial stress, o= cc co Ec FRP conined concrete Unconined concrete E co t Axial strain, c Fig. 3. Stress-strain diagram o Lam and Teng model or FRP conined concrete. cc Due to lack o space, only the simulation o one series o the C3Sφ8 and C3S3φ8 groups are represented. Taking the cc / co, φ8 -ρ and cc / φ8 -ρ relationships included into Figs. and and adopting or E c the value (4 MPa) obtained rom the stress-strain curves o the specimens, the analytical and experimental stress-strain axial relationships (σ c - c ) are compared in Figure 4. For the remaining series the degree o accuracy o the simulation was similar to the one obtained or the simulated cases. W4SL3_C3Sφ8 WSL3_C3S3φ8 Experimental Model 7 Experimental Model Axial strain (mm/mm) a) b) Axial strain (mm/mm) Fig. 4. Comparison between analytical model and experimental results or the: a) W4SL3 o C3Sφ8 and b) WSL3 o C3S3φ8. CONCLUSIONS The present work dealt with an experimental research involving the use o carbon iber reinorced polymer (CFRP) wet lay-up sheets to increase the load carrying capacity and the deormation ability o reinorced concrete (RC) elements submitted to direct compressive loading. The experimental program was conceived to evidence the inluence o ully and partially wrapped coninement arrangements in the compression behavior o this type o elements. Series o tests o two concrete strength classes ( cm o and 3 MPa), two longitudinal steel reinorcement ratios (ρ sl o.4% and %), two transversal steel reinorcement ratios (ρ st o.4% and.9%), and two thicknesses or the CFRP sheets (t o.3 mm and.7 mm) were carried out to assess the inluence o these parameters on the coninement perormance provided by the coninement arrangements analized. In the partial wrapping systems, the distance and width o the CFRP strips were also parameters considered in the experimental program. From the results obtained in the CSφ and CS3φ group o tests (average compressive strength o MPa and conined by CFRP sheets o.3 mm and.7 mm thickness, respectively) the ollowing main observations can be pointed out: The compressive strength ratio, cc / φ increased rom.7 or ρ =.3% up to 4.8 or ρ =.88%; The cc / φ varied rom.4 or ρ =.3% up to. or ρ =.44%; The maximum strains in the CFRP iber direction varied rom 44% to 84% o the CFRP ultimate strain.

12 From the results obtained in the C3Sφ8 and C3S3φ8 (average compressive strength o 3 MPa and conined by CFRP sheets o.3 mm and.7 mm thickness, respectively) the ollowing main observations can be pointed out: The compressive strength ratio, cc / φ8 increased rom.37 or ρ =.% up to 3. or ρ =.88%; The cc / co ranged rom 3. or ρ =.% up to 7. or ρ =.3%; For the specimens that, when ailed, the strains in the iber direction o the CFRP were registered, the maximum strain varied rom 7% up to 8% o the CFRP ultimate tensile strain; For all test groups it was veriied that: A linear increasing trend was observed between cc / co and ρ ; For the partial-wrapping arrangements, the cc / co ratio increased almost linearly with ρ ; The increase o cc / co with ρ was not so pronounced in ully-wrapped specimens than in partially conined specimens, since in these last ones a high concentration o concrete plastic strain occurred in the concrete between CFRP strips. In general, or the specimens o equal ρ, the load carrying capacity o partially conined specimens was a little bit lower than the one o the ully conined specimens. Partial coninement arrangements were, however, easier and aster to apply than ull coninement arrangements. The Lam and Teng model predicted, with enough accuracy, the specimen compressive stress-strain relationship registered in the experiments. ACKNOWLEDGMENTS The authors wish to acknowledge the support provide by degussa Portugal and S&P Clever Reinorcement. The irst author would like to thank the inancial support provided by PRODEP action.3/n/99.4/. REFERENCES. Seible F, Priestley MJN, Hegemier GA, Innamorato D. Seismic retroit o RC columns with continuous carbon iber jackets. Journal o Composites or Construction, 997;(): -.. Xiao Y, M GR. Seismic retroit o RC circular columns using preabricated composites jacketing. Journal o Structural Engineering, 997; 3(): Xiao Y, Wu H, Martin, GR. Preabricated composite jacketing o RC columns or enhanced shear strength. Journal o structural Engineering, 999;4(3): Saai M., Toutanji HÁ, Li Z. Behavior o concrete columns conined with iber reinorced polymer tubes. ACI Material Journal, 999;9(4):-9.. Cole C, Belardi A. FRP jacketed Reinorced Concrete Columns. Technical report, Center or inrastrusture engineering studies, Univ. o Missouri, Dep. o Civil Engineering, Rolla, May ; USA.. ACI Committee 44. Guide or the design and construction o externally bonded FRP systems or strengthening concrete structures. American Concrete Institute,. 7. Barros JAO, Ferreira DRSM. Partial versus ull wrapping coninement systems or concrete columns. To be published in the International Conerence on Concrete Repair, Rehabilitation and Retroitting, South Arica, November. 8. Lam L, Teng JG. Design-oriented stress strain model or FRP-conined concrete. Construction and building materials Journal, 3;7: Ferreira DRSM, Barros JAO. Partial and ull coninement o concrete elements using CFRP sheets. Technical report 4-DEC/E-9, Dep. o Civil Eng., School o Eng., University o Minho, 4. (in Portuguese). ISO TC 7/SC N. Non-conventional reinorcement o concrete-test methods-part : Fiber reinorced polymer (FRP) sheets. International standard, 3.. Samaan M, Mirmiran A, Shahawy M. Model o concrete conined by iber composites. Journal o Structural Engineering, ASCE, 998;4(9): -3.. Toutanji HA. Stress-strain characteristics o concrete columns externally conined with advanced iber composites sheets. ACI Material Journal, 999;9(3):