PRAXIS XXI Project nº CEG 3/3.1/2572-95Retrofitting reinforced concrete columns with FRP materials is a recent but established technique based on their high strength/weight ratio and on other relative advantages, including the fact that FRP external shells prevent or mitigate environmental degradation of the concrete and consequent corrosion of the steel reinforcement. These techniques require, nevertheless, further studies and this paper presents and discusses experimental results obtained in tests of RC columns jacketed with GFRP under monotonic and cyclic axial compression, topic especially relevant when interventions on seismic zones are required. In order to evaluate the effect of the loading type on strength and ductility, 27 circular columns with 750mm height and 150 mm diameter, either of plain concrete or of RC, were tested. Among these circular columns, 11 were confined with two plies of CFRP whereas six were confined with three plies of GFRP. Comparison of results with proposed empirical models is shown.authorsversionpublishe

Rodrigues, C. C.

Silva, M. G. da

English

Repositório da Universidade Nova de Lisboa

1 INTRODUCTION  Rehabilitation and strengthening of existing columns of reinforced concrete with FRP wraps has been growing rapidly, requiring further studies on the re-liability of their use, both in terms of mechanical properties and in durability. Design expressions that fully explore the properties of the composites have to be based on experimental work and on modelling that respects failure modes, ductility gains, interfa-cial pressure and other important aspects. The pre-sent study adds some information on those topics and examines test data against recently proposed empirical formulations.  Concrete is a brittle material that deforms in-elastically under compression, due to progressive loss of stiffness caused by voids and micro cracking. Under axisymetric stresses  (σx=σy), micro cracking is related to confinement and the following phases can be identified: (i) Stiffening, when dσ/dε>0;  (ii) Peak stressing, when dσ/dε reaches zero and damage accumulation leads to σmax;  (iii) Softening associated with dσ/dε<0 and the formation of cracks and/or shear bands. Restraining lateral deformation, a higher strength capacity is ob-tained as well as higher ductility. Confinement may be provided by transversal reinforcement (stirrups) or external wraps of FRP or steel and the behaviour of the specimen differs for each case. In the latter so-lution, the elastic behaviour of FRP’s contrasts with yielding of steel and has to be reflected on the mod-elling and on the estimate of interfacial pressure.      Many available data have been obtained for low values of the aspect ratio, λ=height/diameter, typi-cally λ=2, raising some doubts on generalisation of those results. Besides well-known shortcomings of such scaling for compressive tests based on which failure modes are to be analysed, the relative stiff-ness of the outer composite shell vs. concrete ap-pears over estimated. The aspect ratio for the tests reported in this note is λ=5, avoiding the above ob-jection to frequently cited results (Harmon &       Slaterry 1992, Mirmiran & Shahawy 1997, Samaan et al. 1998).  Earlier results on lateral confinement and on re-sponse to cyclic loading are briefly outlined below to frame adequately the results obtained at UNL. Models for confinement of RC columns by FRP shells still need improvement. In fact, core confine-ment provided by transverse reinforcement in RC columns has been extensively studied and quantified in Codes, although some procedures are still de-bated. It has been referred, for instance, that the re-sponse of cylinders subjected to equivalent levels of pressure depends on how that lateral pressure is transmitted and not on its magnitude alone (Samaan The behaviour of GFRP reinforced concrete columns under monotonic and cyclic axial compression Carlos Chastre Rodrigues PhD Student, Universidade Nova de Lisboa, Portugal, c-chastre@mail.fct.unl.pt Manuel Gonçalves da Silva Full Professor, Universidade Nova de Lisboa, Portugal, mgs@mail.fct.unl.pt  ABSTRACT: Retrofitting reinforced concrete columns with FRP materials is a recent but established tech-nique based on their high strength/weight ratio and on other relative advantages, including the fact that FRP external shells prevent or mitigate environmental degradation of the concrete and consequent corrosion of the steel reinforcement. These techniques require, nevertheless, further studies and this paper presents and dis-cusses experimental results obtained in tests of RC columns jacketed with GFRP under monotonic and cyclic axial compression, topic especially relevant when interventions on seismic zones are required. In order to evaluate the effect of the loading type on strength and ductility, 27 circular columns with 750mm height and 150 mm diameter, either of plain concrete or of RC, were tested. Among these circular columns, 11 were con-fined with two plies of CFRP whereas six were confined with three plies of GFRP. Comparison of results with proposed empirical models is shown. et al. 1998). Consequently, the stiffness of the FRP jackets is of great importance and has to be ac-counted for in the modelling (Sarkani et al. 1999), contrary to common practice. It also raises questions on applicability of Mander et al. (1988) model ex-tension to FRP confinement. The balance of axial strain energy of confined concrete with hoop strain energy plus axial strain energy of the unconfined column, fails in the case of FRP wrapping, when lat-eral strain energy in concrete cannot be neglected.  This paper presents and discusses several experi-mental results of GFRP reinforced concrete columns under monotonic or cyclic axial compression in or-der to evaluate the effect of the monotonic and cy-clic loading on strength and ductility. 2 TESTING SET-UP Tests on 27 circular columns with 750mm height and 150 mm diameter either of plain concrete or of RC with 6φ6mm – S400 longitudinal steel, and φ3mm stirrups at spacing equal to 5, 10 or 15 cm were carried out. Among these circular columns, 11 were confined with 2 plies of CFRP (Rodrigues & Silva 2001) whereas 6 were confined with 3 plies of GFRP.  Concrete, tested in standard cubes, led to cylindri-cal strength f’c0=37.7MPa. Glass fibers were ac-quired from TYFO and, from the manufacturer, the mechanical characteristics were Ej=27.6 GPa, fj=552MPa, tply=1.27mm and ultimate strain of 2.0%.   Axial tests were conducted with a 5000kN press belonging to the National Laboratory of Civil Engi-neering, partner in the project. Three vertical dis-placement transducers were placed at mid-height and two pairs of two strain gauges were placed so as to measure vertical and circumferential deformation. In three of the specimen, additional twelve strain gauges were placed to evaluate eventual vertical variation of vertical strain. Monotonic and last cycle tests were displacement controlled at a velocity of 10 μm/s and continued beyond failure to record the stress-strain (fc-ε) curves until a force of solely 50kN was attained. Cyclic tests were run at a rate of 0.2N/mm2/s until beginning of last cycle when changed to displacement control. Figure 1 shows a column prior to test and after failure. 3 RESULTS Table 1 presents discrete values of strength (fc) from results of the experimental investigation of GFRP re-inforced concrete columns under uniaxial cyclic compression. The value of axial strain in rupture (εcc) is also indicated.    Figure 1 – Instrumented column of GFRP prior to test and after failure.   Table 1.  Results of Uniaxial Compression Tests Specimen Stirrups GFRP Load f’c0 f’cc εcc  number   Type (MPa) (MPa) (%) C1 - - Monotonic 23.1 - 0.20C2 - - Monotonic 24.4 - -C3 - - Monotonic 32.2 - 0.20C21 - 3 layers Monotonic - 77.6 1.28C22 - 3 layers Monotonic - 114.2 2.46C23 - 3 layers Cyclic - 115.7 3.13C27 - 3 layers Cyclic - 112.4 2.21C7 φ3 // 0.10 - Monotonic - 26.4 0.20C8 φ3 // 0.10 - Monotonic - 30.4 0.20C9 φ3 // 0.10 - Monotonic - 24.9 0.20C24 φ3 // 0.10 3 layers Monotonic - 116.3 2.23C25 φ3 // 0.10 3 layers Cyclic - 91.3 1.66  Figure 2 trough 5 display results of stress (fc), ax-ial strain (εc), lateral strain ( lε ), volumetric strain (εV) and dilation rate (μ) obtained on tests. Figure 2 shows ccf ε− , lε−cf  and Vcf ε− curves for con-fined concrete with GFRP (C21; C22). Column C24 has 6φ6 for longitudinal reinforcement and φ3//0,10 for stirrups.   As described for CFRP (Rodrigues & Silva 2001), following concrete failure, the GFRP jacket holds the column and provides stiffness that allows an in-crease of the strength and still higher increase of ul-timate strain. The monotonic curve can be approxi-mated by a bilinear curve, as seen. The volumetric strain, εv = 2 lε +εc, can be correlated to the failure and post-failure of the concrete, with volume de-creasing for fc near f’c0 in the present case.   C24GFRPφ3//0,100204060801001200,0 0,5 1,0Dilation RateAxial Stress (MPa)C21GFRP0204060801001200,0 0,5 1,0Dilation RateAxial Stress (MPa) C22GFRP020406080100120-2% -1% 0% 1% 2% 3%StrainAxial Stress (MPa)εcεvεlC24GFRPφ3//0,10020406080100120-2% -1% 0% 1% 2% 3%StrainAxial Stress (MPa)εcεvεlC21GFRP020406080100120-2% -1% 0% 1% 2% 3%StrainAxial Stress (MPa) εcεvεl(a) (b) (c) Figure 2 – Stress-Strain response (fc vs. εc, lε , εV) for plain (C21,C22) & RC (C24) jacketed columns. Monotonic loading.  Observed Behaviour of columns of plain concrete can be divided in phases. Firstly, increasing axial strain causes decrease of volume and lateral expan-sion at low rate. Then concrete fails, volume re-verses change and the jacket enters its main function of confinement, with dilation rate increasing almost linearly with stress. Finally, total load capacity is exhausted and the column fails through rupture of the jacket. Reinforced concrete shows a different behaviour near failure with εv decreasing continuously, whereas⏐εv⏐reaches a maximum at failure for plain concrete. (a) C22GFRP0204060801001200,0 0,5 1,0Dilation RateAxial Stress (MPa) (b) (c) Figure 3 – Dilation ( lε /εc) curves for plain (C21, C22) and RC (C24) jacketed columns. Monotonic loading.  Figure 4 shows curves ccf ε− , lε−cf  and Vcf ε−  for confined plain concrete columns with GFRP (C23; C27) or RC (Strφ3//0.10) under cyclic loads.  Specimen C25 experienced strain gauges failure and lε  not recorded in the upper range. Comparison of figures 2 and 4 indicates that the envelopes for cyclic loads follow trends approaching the mono-tonic curves.  Dilation curves shown in figures 3 and 5 require further study and more data for interpretation.  C23GFRP020406080100120-2% -1% 0% 1% 2% 3%StrainAxial Stress (MPa)εcεvεlC27GFRP020406080100120-2% -1% 0% 1% 2% 3%StrainAxial Stress (MPa)εcεvεlC25GFRPφ3//0,10020406080100120-2% -1% 0% 1% 2% 3%StrainAxial Stress (MPa)εcεvεlC25GFRPφ3//0,10020406080100120-1,0 -0,5 0,0 0,5 1,0 1,5Dilation RateAxial Stress (MPa)C23GFRP020406080100120-1,0 -0,5 0,0 0,5 1,0 1,5Dilation RateAxial Stress (MPa)C27GFRP020406080100120-1,0 -0,5 0,0 0,5 1,0 1,5Dilation RateAxial Stress (MPa)(a) (b) (c) Figure 4 – Stress-Strain response (fc vs. εc, lε , εV) for plain (C23, C27) and RC (C25) jacketed columns. Cyclic loading.  4 COMPARISON WITH EMPIRICAL MODELS Several procedures have been proposed to estimate the strength and the ultimate strain of concrete con-fined with FRP, under axial loading (Karbhari et al. 1997, Mirmiran & Shahawy 1997, Samaan et al. 1998, Spoelstra & Monti 1999).  Expressions submitted by Samaan et al. (1998) and Spoelstra & Monti (1999) provide also the stress-strain curves and are selected for application hereafter.   (a) (b)  (c) Figure 5 – Dilation ( lε /εc) curves for plain (C23, C27) and RC (C25) jacketed columns. Cyclic loading.    a) Samaan et al. (1998) The bilinear relationship advanced by Samaan et al. (1998) is displayed in figure 6 based on (1):  ( )( )cccc EfEEEEf εεε25.115.1021211+⎥⎥⎦⎤⎢⎢⎣⎡⎟⎟⎠⎞⎜⎜⎝⎛ −+−=  (1)  Initially, the jacket acts passively and the slope ( ∂ fc/ ∂ ε) corresponds solely to the vertical response of concrete and is given by:    3950 01 cfE = [MPa] (2) For fc near the maximum stress of unconfined concrete, f’c0, the effective modulus (Ej) and the thickness (tj) of the FRP also play a role, as indi-cated by: [ ]MPa    3456.161.245 2.002 DtEfE jjc×+=  (3) The strenght of the confined concrete relates to f’c0 through the equation: 7.00' 0.6 lccc fff +=  (4) that derives from s modification of the linear expres-sion proposed by Richart et al. (1929). Analogously, an estimate of the ultimate strain can also be found: 20Effcccc−′=ε  (5) where:  [ ]MPa    258.6371.0872.0 00 ++= lc fff  (6) b) Spoelstra & Monti (1999) The model suggested by Spoelstra & Monti (1999) is based on Popovics (1973) formula: xrrxff ccc +−=1' (7) where: cccxεε= ;  )( secEEErcc−= ; ccccfEε'sec =  (8-a,b,c) ⎜⎜⎝⎛⎟⎟⎠⎞⎟⎟⎠⎞⎜⎜⎝⎛−′′+= 15100cccccc ffεε  (9) Strain εc0, corresponding to the maximum com-pressive stress of unconfined concrete fc0, is taken as 2‰. The elasticity modulus at the origin is: '05000 cc fE = [MPa].  (10) Maximum stress f’cc for confined concrete, based on Mander et al. (1988) modified by Karbhari et al. (1997) is given by: ⎟⎟⎠⎞⎜⎜⎝⎛′−′++−′=′000 294.71254.2254.1clclccc ffffff (11) where lf  is the confinement pressure due to the FRP jacket and given by static equilibrium: Dtff FRPl⋅=2  (12)  Mander et al. (1988) model [4] is applicable to concrete confined by steel, except in the early stages when steel is still in the elastic range. Spoelstra & Monti (1999) adapted formula due to Pantazopoulou & Mills (1995) to take into consideration the elastic behaviour of FRP through the entire range of strain: ( ) ( )( )llll ffffEfccccccc ,2,,εβεεεε −=  (13) the relationship of fc and lε  with the current strain cε  and confining pressure lf  is given by the au-thors.  The constant β is a property of concrete: cococfEεβ 1−=  (14) Once lε  is computed from Equation (11), the strain jε  in the confining jacket can be found, as well as stress jjj Ef ε= . The updated value of lf  can be used for a new estimate of lε  through Equa-tion (13), the iterations continuous until lf  con-verges.  The procedure is repeated for each cε , until the  complete  stress-strain  curve  is  covered (Spoelstra & Monti 1999). The response of an FRP-wrapped RC specimen obtained with Spoelstra & Monti (1999) model can be seen in Figure 6, along with a comparison with Samaan et al. (1998)  model. Figure 7 shows curves ccf ε− , lε−cf  and Vcf ε−  for  Spoelstra & Monti (1999) model compared with experimental results (C24) and compared with ccf ε−  curve of  Samaan et al. (1998)  model. Figure 8 shows dilation curves of GFRP confined concrete obtained with Spoelstra & Monti (1999) model compared with experimental results (C24).  020406080100120-2% -1% 0% 1% 2% 3%StrainAxial Stress (MPa)εcεvε l Spoelstra & MontiSpoelstra & MontiSpoelstra & MontiSamaan et al.  Figure 6 – Empirical Stress-Strain responses of GFRP confined concrete (Spoelstra & Monti 1999, Samaan et al. 1998) models.  020406080100120-2% -1% 0% 1% 2% 3%StrainAxial Stress (MPa)εcεvε l C24C24C24Spoelstra & MontiSpoelstra & MontiSpoelstra & MontiSamaan et al. Figure 7 – Empirical Stress-Strain models compared with ex-perimental results (C24).  C240204060801001200,0 0,5 1,0Dilation RateAxial Stress (MPa)Spoelstra & Monti Figure 8 – Dilation curves of GFRP confined concrete obtained with Spoelstra & Monti (1999) model compared with experi-mental results (C24).  The Spoelstra & Monti (1999) curves show a fairly good agreement with experimental results. 5 FINAL REMARKS The study provides interesting results on GFRP rein-forced cylindrical columns, obtained for specimen with adequate aspect ratio (height/diameter). Both columns of plain concrete and of RC, wrapped with GFRP jackets, were tested under axial load until complete failure. Load was applied both monotoni-cally and in cycles and the changes of volumetric expansions, lateral and axial strain with axial stress were recorded. Results show: a) Relatively good agreement of results with empirical formulas proposed by Spoelstra & Monti (1999). Results are less accurate for model proposed by Samaan et al. (1998). b) Behaviour of columns of plain concrete can be divided in phases. Firstly, increasing axial strain causes decrease of volume and lateral expansion at low rate. Then concrete fails, volume reverses change and the jacket enters its main function of confinement, with dila-tion rate increasing almost linearly with stress. Finally, total load capacity is ex-hausted and the column fails through rupture of the jacket. c) Reinforced concrete shows a different behav-iour near failure with εv decreasing continu-ously, whereas⏐εv⏐reaches a maximum at failure for plain concrete.  Studies continue seeking the improvement of modelling and the generation of results applicable to design. 6 ACKNOWLEDGMENTS Support for this study was provided by the Portu-guese Foundation for Science and Technology under PRAXIS XXI Project nº CEG 3/3.1/2572-95. The writers are  grateful  to  Mr. Rui Neves  and  Mr. Bernardino from LNEC and Mr. Gaspar from UNL for their help with the experiments. REFERENCES Harmon, T.G. and Slaterry, K.T. (1992); “Advanced composite confinement of concrete”, Proc. Advanced Composite Mat. In Bridges & Struct., CSCE, pp292-302. Karbhari, Vistasp, M., Gao, Yanqiang (1997); “Composite Jacketed Concrete Under Uniaxial Compression – Verifica-tion of Simple Design Equations”. Journal of Materials in Civil Engineering, Vol. 9, nº 4, Nov., Paper nº 13873. Mander, J.B., Priestley, M.J.N. and Park R. (1988); “Theoreti-cal stress-strain model for confined concrete”, Journal of Structural Engineering, ASCE, 114 (8), pp.1804-1826. Mirmiran, A. and Shahawy, M.(1997); “Behaviour of Concrete Columns Confined by Fiber Composites”, Journal of Struc-tural Engineering, vol.123, No. 5, May, pp. 583-590. Pantazopoulou, S. J. and Mills, R. H. (1995), “Microstructural aspects of the mechanical response of plain concrete”, ACI Materials Journal, 92-M62, November-December, 605-616. Popovics, S. (1973), “Numerical approach to the complete stress-strain relation for concrete”, Cement and Concrete Research, 3(5), 583-599. Richart, F. E., Brandtzaeg, A. and Brown, R. L. (1929), “The failure of plain and spirally reinforced concrete in com-pression”, Engineering Experiment Station Bulletin, 190, University of Illinois, Urbana, USA, April. Rodrigues, C. C.; Silva, M. G. (2001); “Experimental investi-gation of CFRP reinforced concrete columns under uniaxial cyclic compression”; In Proceedings of the 5th Interna-tional Conference on FRP for Reinforced Concrete Struc-tures; Cambridge, UK. Samaan, M., Mirmiran, A. and Shahawy, M.(1998); “Model of Concrete Confined by Fiber Composites”, Journal of Struc-tural Engineering, vol.124, No. 9, September, pp. 1025-1031. Sarkani, S. et al., (1999); “Strengthening RC Structures Using FRP”, Proc. Structural Faults + Repair, 8th. Intl. Conf., London, 13 -15 July, UK. Spoelstra, M. R. and Monti, G., (1999), “FRP-confined con-crete model”.  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The behaviour of GFRP reinforced concrete columns under monotonic and cyclic axial compression

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The behaviour of GFRP reinforced concrete columns under monotonic and cyclic axial compression

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