Ching Chiaw Choo, Issam Harik Published online on: 26 Sep 2013

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1 Tis artile was downloaded y: On: 27 De 2018 Aess details: susription numer Puliser: CRC Press Informa Ltd Registered in England and Wales Registered Numer: Registered offie: 5 Howik Plae, London SW1P 1WG, UK Te International Handook of FRP Composites in Civil Engineering Manooer Zogi Conrete Columns Reinfored Internally wit Fier Reinfored Polymer Bars Puliation details ttps:// Cing Ciaw Coo, Issam Harik Pulised online on: 26 Sep 2013 How to ite :- Cing Ciaw Coo, Issam Harik. 26 Sep 2013, Conrete Columns Reinfored Internally wit Fier Reinfored Polymer Bars from: Te International Handook of FRP Composites in Civil Engineering CRC Press Aessed on: 27 De 2018 ttps:// PLEASE SCROLL DOWN FOR DOCUMENT Full terms and onditions of use: ttps:// Tis Doument PDF may e used for resear, teaing and private study purposes. Any sustantial or systemati reprodutions, re-distriution, re-selling, loan or su-liensing, systemati supply or distriution in any form to anyone is expressly foridden. Te puliser does not give any warranty express or implied or make any representation tat te ontents will e omplete or aurate or up to date. Te puliser sall not e liale for an loss, ations, laims, proeedings, demand or osts or damages watsoever or owsoever aused arising diretly or indiretly in onnetion wit or arising out of te use of tis material.

2 24 CONTENT INTRODUCTION Conrete Columns Reinfored Internally wit Fier Reinfored Polymer Bars Cing Ciaw Coo and Issam Harik Introdution Strengt interation of onrete olumns Strengt interation eavior of onrete olumns internally reinfored wit fier reinfored polymer ars Design of FRP Reinfored Conrete Columns Slenderness effet Notations Referenes Te strengt interation (P n M n ) eavior of onrete olumns reinfored internally wit fier reinfored polymer (FRP) ars is quantified erein ased on ultimate strengt approa y onsidering stress equilirium, strain ompatiility, and material onstitutive laws. Te study sows tat rupturing of FRP ars in onrete olumns is proale, prior to onrete reaing its ultimate strain and strengt, resulting in sudden and explosive-type failure. Design reommendations are provided erein in partiular to preventing tensile rupture of FRP ars in tese olumns. More stringent design requirements tan are disussed ere will ave to e onsidered su as wen long-term and sustained loadings are present. STRENGTH INTERACTION OF CONCRETE COLUMNS A strutural design of a onrete olumn is quite a ompliated proess. Te evaluation, owever, of a given olumn setion and reinforement is quite straigtforward. Beause olumns rarely arry axial load alone, su as wen loads applied off enter, moments indued at joints, et., olumn strengt evaluation must aount for diret axial load and flexure. As su te onrete olumn strengt diagram is typially a plot of interations etween axial load and flexural apaities, in wi te asissa (i.e., te x- or orizontal axis) often sows te ending moment or flexural strengt wereas te ordinate (i.e., te y- or vertial axis) represents te axial strengt of a onrete olumn. Te strengt evaluation proess is illustrated in Figure 24.1 for one partiular strain distriution, were a loation of te neutral axis is seleted, te orresponding strains, stresses, and fores of reinforements and onrete an e alulated appropriately to determine te axial load apaity, P n and ending moment apaity, M n = P n e, aout te plasti entroid of te ross setion [1]. 447

3 448 Te International Handook of FRP Composites in Civil Engineering Extreme ompression (varies) Tese values of P n and M n represent one point on te interation diagram. Additional points on te interation diagram an e generated y repeating aforementioned proedures wit additional loations of te neutral axis or oter depts, ea yielding a unique set of P n and M n. Te ultimate or maximum ompressive strain at te outermost extreme ompression fier of onrete is generally set to e per ACI Code [2]. Figure 24.2 sows a typial nominal strengt interation for a tied onrete olumn reinfored wit onventional steel reinforement. Several points in te interation urve for a steel reinfored onrete olumn are igly distinguisale: (1) Point A pure axial load. Tis represents te largest nominal axial load te olumn an arry, witout moment and (2) Point B alaned ondition. Tis point orresponds to a strain distriution wit onrete reaing maximum strain value of at te extreme ompression fier on one fae, and a tensile strain of steel reaing yield strain, ε y, at te extreme tensile layer fartest from te extreme ompression fier of onrete; and (3) Point C pure flexure. Tis point represents te nominal flexural apaity of te olumn, witout te presene of an axial load. P n N.A. e C s T s C Conrete stress (a) Setion () Strains () Internal and resultant fores FIGURE 24.1 Calulation of P n and M n : (a) olumn ross setion, () strain distriution, and () internal resultants. Axial load strengt A FIGURE 24.2 Strengt interation diagram B C Bending strengt (M n ) Uniform ompression ε y Balaned ondition

4 Conrete Columns Reinfored Internally wit Fier Reinfored Polymer Bars 449 STRENGTH INTERACTION BEHAVIOR OF CONCRETE COLUMNS INTERNALLY REINFORCED WITH FIBER REINFORCED POLYMER BARS Testing of full-sale reinfored onrete olumns internally reinfored wit glass reinfored polymer (GFRP) ars under axial load sowed tat tese olumns eaved very similar to tat of onventional steel reinfored onrete olumns and demonstrated tat te use of longitudinal GFRP ars did not ave an adverse effet on te performane of te olumns [3]. Under pure axial load ondition, te failure of te GFRP reinfored onrete olumns was initiated y te rusing of te onrete, wit te axial strains measured eing mu iger tan tose oserved in te steel RC olumns [3]. Strengt interation eavior of onrete olumns internally reinfored wit FRP ars was investigated using ultimate strengt approa y onsidering stress equilirium, strain ompatiility, and te material onstitutive laws of onrete and FRP ar, similar to te approa used for steel RC olumns, and te following oservations were made wit referene to Figure 24.3 [4]: Te moment resistane, in most ases, of a FRP reinfored onrete olumn, wi laks a alaned point, inreases as axial load inreases from pure axial load to pure flexure; unlike a steel reinfored onrete olumn tat exiits a simultaneous redution in moment and axial load resistanes from alaned ondition to pure flexure (see Figure 24.2). Tis is espeially apparent as te amount of FRP or reinforement ratio (ρ f ) in te olumn inreases. Depending upon te ultimate or maximum usale tensile strain, ε fut, of te FRP type onsidered, e.g., GFRP ars in Figure 24.3, tensile rupture of te FRP ar termed rittle tension failure ould our prior to onrete reaing its maximum usale strain, ε u, and ene not allowing onrete to attain its maximum usale strengt. Wile oneivaly FRP ars ould experiene ompression rupture, su failure is deemed less likely ompared to rittle tension failure as te ultimate or maximum γ Bending axis P * CFRP R/C n P * n ρ f 1% 3% 5% 8% P n E ft = E f = 21,300 ksi (147 GPa) ε fut = 1.6% and ε fu = 0.8% e f ć = 5000 psi ( MPa) P P n * = n 1 1 ; and M * M n = n fć 2 f ć GFRP R/C 0.4 ε 0.2 =ε u and ε f= ε fut Brittle-tension failure 0.0 M * n 0.0 M * n (a) () ρ f 1% 3% 5% 8% E ft = E f = 6500 ksi (45 GPa) ε fut =1.4% and ε fu =0.7% FIGURE 24.3 Strengt interation eavior of (a) CFRP RC olumns and () GFRP RC olumns. (From Coo, C.C. et al., ACI Strut. J., 103(3), 452, Wit permission.)

5 450 Te International Handook of FRP Composites in Civil Engineering P n * E f /E ft =1 E f /E ft = 0.5 ρ f =1% ρ f =1% E f /E ft =0 usale ompression strain, ε fu, of te FRP is often mu larger tan te ultimate or maximum usale ompression strain of onrete. Noneteless, tis failure mode sould e eked if FRP were to e used in onrete olumns. Due to te anisotropi and nonomogeneous nature of FRP, it is diffiult to otain aurately te ompression properties of te material. Reportedly, FRP reinforements generally ave lower ompressive strengt and stiffness ompared to teir tensile ounterparts. In Figure 24.3, te ε fu = 0.5ε fut (i.e., > ε u = 0.003) did not ause FRP rupture in ompression. On te one and, lower stiffness in ompression resulted in redution in strengt interation of FRP RC olumns to a ertain degree as illustrated in Figure Hene, it as een suggested tat negleting te ontriution of ompression reinforement, a pratie tat is ommon in onrete design, may e onservative wen defining te strengt interation of tese olumns [4]. DESIGN OF FRP REINFORCED CONCRETE COLUMNS E f /E ft =1 E f /E ft =1 E f /E ft = 0.5 ρ f =8% E f /E ft = 0.5 ρ f =8% E f /E ft =0 0.0 M* n 0.0 M* n (a) P * n ε =ε u and ε f =ε fut E f /E ft =0 Te primary onern of using FRP as te only longitudinal reinforement type in a onrete olumn may ave een te potentially explosive and atastropi nature of te olumn if its failure was initiated y rupturing of tese internal FRP ars. Su failure is pereived to e lak of warning, wi is igly undesirale. Rupturing of FRP ars is oneivaly proale as demonstrated in Figures 24.3 and 24.4, were it ours prior to onrete rusing or reaing its ultimate strain P n * P * n 1.2 Typ. : ε fut = 1.6% 0.0 M* n 0.0 M* n () Typ. : ε fut = 1.4% FIGURE 24.4 Effet of ompression moduli to tension moduli ratios (E f /E ft ) on ross-setional strengts of olumn reinfored wit CFRP and GFRP ars. (a) CFRP RC olumn ross setions, E ft = 21,300 ksi (147 GPa). () GFRP RC olumn ross setions, E ft = 6,500 ksi (45 GPa). (From Coo, C.C. et al., ACI Strut. J., 103(3), 452, Wit permission.)

6 Conrete Columns Reinfored Internally wit Fier Reinfored Polymer Bars 451 For FRP reinfored onrete olumns, failure initiated y onrete rusing may e te preferred mode, as it as een demonstrated in flexural memers reinfored wit FRP ars exiiting onrete rusing tat a more progressive and less atastropi failure was exiited [5,6]. To prevent rittle tension failure, a FRP reinfored onrete olumn will need to e provided wit a ertain amount of reinforement [7]; as sown in Figure 24.3, a GFRP reinfored onrete olumn wit a reinforement ratio, ρ f = A f /A g, of 1% experiened rittle tension failure at a ertain level of axial load on te strengt interation ut no su failure in te ones wit iger reinforement ratios, 3% or more. A FRP reinfored onrete olumn an e safeguarded from rittle tension failure if te amount of reinforement or reinforement ratio provided, ρ f, is greater tan a ertain minimum, ρ fmin, and tis minimum may e greater tan te 1% set in te ACI 318 Code for steel reinfored onrete olumns [7]. Te design aids of Figure 24.5a to f, omined appropriately wit te different adjustment fators to aount for type of onrete, FRP ars and teir layout, allow one to quikly estimate te (a) (),000,000 f ć = 5000 psi ( MPa) E f /E ft = 1.0 γ f ć = 5000 psi ( MPa) E f /E ft = 0.8 FIGURE 24.5 Tensile elasti modulus tensile strain (E ft ) interation diagrams for retangular onrete olumns reinfored wit FRP ars of different modular ratio, E f /E ft (a) 1.0, () 0.8, respetively. γ (ontinued)

7 452 Te International Handook of FRP Composites in Civil Engineering,000 f ć =5000 psi ( MPa) E f /E ft =0.6 () (d),000 ρ fmin of retangular-sape olumn ross setions reinfored wit FRP [7]. Te aids are generated y estimating te maximum tensile strain for a partiular type of FRP,, tat ould develop for a given onrete and layout, orresponding to te pure flexure ondition [7]. Te range of FRP onsidered in tese aide overs most availale ones ommerially [7]. If a onrete olumn is provided wit a ρ f greater tan or equal to te ertain minimum, ρ f min, determined from tese aids, ten rittle tension failure of te onrete olumn an e averted. Sould te omputed ρ f min e less tan 1%, it is reommended ten to take ρ f min equal to 1% similar to te one speified for steel reinfored onrete olumn per ACI 318 Code [7]. To estimate ρ f min, one would need te stiffness (i.e., tensile and ompressive Young s moduli, E ft and E f ) and maximum usale or ultimate tensile strain, ε fut, of te FRP ar used in te design of a reinfored onrete olumn. Sine te tensile strain,, of te FRP would e a funtion of te type of onrete and layout of tese ars, te maximum usale or ultimate tensile strain, ε fut, wi is f ć =5000 psi ( MPa) E f /E ft =0.4 FIGURE 24.5 (ontinued) Tensile elasti modulus tensile strain (E ft ) interation diagrams for retangular onrete olumns reinfored wit FRP ars of different modular ratio, E f /E ft () 0.6, (d) 0.4, respetively. γ γ

8 Conrete Columns Reinfored Internally wit Fier Reinfored Polymer Bars 453,000 f ć =5000 psi ( MPa) E f /E ft =0.2 (e) (f),000 typially provided y te manufaturer, must first e modified as follows, in order to estimate ρ f min using one of te aids in Figure 24.5 [7]: ε * = α α ε (24.1) fut γ fut α and α γ sown in Equation 24.1 are fators to aount for onrete strengt, f, oter tan MPa or 5000 psi and te layout of FRP ars, γ, as defined in Figure 24.5, respetively [7]. As pointed out earlier, te default value of α is 1.0 wen f is equal to MPa or 5000 psi, or oterwise omputed using te following expressions for oters [7]: γ f ć =5000 psi ( MPa) E f /E ft =0 FIGURE 24.5 (ontinued) Tensile elasti modulus tensile strain (E ft ) interation diagrams for retangular onrete olumns reinfored wit FRP ars of different modular ratio, E f /E ft (e) 0.2, and (f) 0.0, respetively. (From Coo, C.C. et al., ACI Strut. J., 103(3), 460, Wit permission.) γ

9 454 Te International Handook of FRP Composites in Civil Engineering Wen 21 MPa f < MPa or 3000 psi f < 5000 psi: α f f = α = or 1. 0 (24.2) Wen MPa f < 55 MPa or 5000 psi f < 8000 psi: α f f = α = or 1. 0 (24.3) Te default value of α γ is 1.0 wen γ is equal to 0.9, or oterwise omputed using te following expression for oters [7]: Wen 0.45 γ 0.9: αγ = γ 1.0 (24.4) It sould e noted tat Figure 24.5 is derived y assuming tat FRP reinforement is uniformly distriuted on all end faes of a retangular olumn ross setion. Terefore, te use of Figure 24.5 to estimate te minimum reinforement ratio, ρ f min, of a retangular olumn ross setion wit FRP reinforement only at te two end faes sould e onservative. Example A square, tie, sort onrete ( f = 5500 psi or 38.5 MPa) olumn of 24 in. 24 in. wit ars uniform distriuted on all faes is to te reinfored wit GFRP ars wit te following properties (provided y te manufaturer): Tensile modulus of elastiity, E ft = 6500 ksi (45.5 GPa) Maximum/ultimate tensile strain, ε fut = 1.4% (0.014) For design, onservatively assume E f = 0.6 E ft and ε fu = 0.5ε fut (unless tese properties are also provided). Te minimum reinfored ratio, ρ f min, required for tis olumn ross setion will e? Solution Sine ε fu = 0.5ε fut = = > ε u = (maximum usale onrete ompression strain per ACI 318 Code), terefore premature ompression rupture of GFRP ars is unlikely. Modifiation fator α for f = 5500 (38.5 MPa) > 5000 psi ( MPa): From Equation 24.3, α = ( 5500) = A reasonale 2.5 in. of onrete over measuring from te surfae to te entroid of reinforing ars is assumed per ACI 318 requirements gives γ= 24 ( ) = , say 0. 8 OK Terefore, modifiation fator α γ for γ = 0.8 < 0.9. From Equation 24.4, α γ = (0.8) = 1.06 Hene, te adjusted ultimate tensile strain of GFRP ars to e used in onrete olumn design is, from Equation 24.1, ε * α α ε = = fut γ fut

10 Conrete Columns Reinfored Internally wit Fier Reinfored Polymer Bars 455 Setting te adjusted value ε* fut = = and giving tat te moduli of elastiity ratio is E E f ft 3900 ksi = = 6500 ksi 06. Te estimated ρ fmin is approximately 1.4%, otained from Figure Terefore, if ρ f provided ρ fmin = 1.4% (or 0.014A g ), ten rittle tension failure sould e prevented. SLENDERNESS EFFECT A parametri study of slender FRP reinfored onrete olumns sowed tat tese types of olumns may e more suseptile to instaility failure tan steel reinfored onrete olumns [8]. Te design pratie of using moment magnifiation fators appears to appliale to FRP reinfored onrete olumns; owever, it is reommended tat te slenderness limit of 22 for steel reinfored onrete olumns ent in single urvature e redued to 17 for FRP reinfored onrete olumns [8]. NOTATIONS A f total area of FRP ars in a olumn ross setion A g olumn ross setion area = E f elasti modulus of FRP ars E f elasti modulus of FRP ars in ompression E ft elasti modulus of FRP ars in tension f onrete ompression strengt M n nominal moment P n nominal axial load α strain modifiation fator dependent on f α γ strain modifiation fator dependent on γ ε u ultimate or maximum usale onrete ompression strain (= per ACI ) ε f ompression strain developed in te FRP ars tensile strain developed in te FRP ars ε fu ultimate/maximum usale ompression strain of FRP ars ε fut ultimate/maximum usale tensile strain of FRP ars γ ratio of distane etween entroid of outer layers of rears to eigt of retangular olumn ross setion in te diretion of ending ρ reinforement ratio (= A s /) for steel reinfored onrete olumns ρ f reinforement ratio (= A f /) for FRP reinfored onrete olumns minimum reinforement ratio for FRP reinfored onrete olumns ρ f min REFERENCES 1. Wigt, J.K. and MaGregor, J.G. Reinfored Conrete: Meanis & Design, 5t edn., Pearson Prentie Hall: Upper Saddle River, NJ, ACI Committee 318, Building Code Requirements for Strutural Conrete (ACI ) and Commentary (ACI 318R-11), Amerian Conrete Institution, Farmington Hills, MI, De Lua, A., Matta, F., and Nanni, A. Beavior of full-sale glass fier-reinfored polymer reinfored onrete olumns under axial load, ACI Strutural Journal, 107(5), , Septemer Otoer Coo, C.C., Harik, I., and Gesund, H. Strengt of retangular onrete olumns reinfored wit fierreinfored polymer ars, ACI Strutural Journal, 103(3), , May June 2006.

11 456 Te International Handook of FRP Composites in Civil Engineering 5. Nanni, A. Flexural eavior and design of reinfored onrete using FRP rods, ASCE Journal of Strutural Engineering, 119(11), , GangaRao, H.V.S. and Vijay, P.V. Design of onrete memers reinfored wit GFRP ars, Proeedings of te Tird International Symposium on Non-Metalli (FRP) Reinforement for Conrete Strutures (FRPRCS-3), Japan Conrete Institute, Sapporo, Japan, Vol. 1, 1997, pp Coo, C.C., Harik, I., and Gesund, H. Minimum reinfored ratio for fier-reinfored polymer reinfored onrete retangular olumns, ACI Strutural Journal, 103(3), , May June Mirmiran, A., Yuan, W., and Cen, X. Design for slenderness in onrete olumns internally reinfored wit fier-reinfored polymer ars, ACI Strutural Journal, 98(1), , January Feruary 2001.

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