Rehabilitation with NSM FRP Reinforcement

Transcription

1 This artile was downloaded by: On: 06 Nov 018 Aess details: subsription number Publisher: CRC Press Inorma Ltd Registered in England and Wales Registered Number: Registered oie: 5 Howik Plae, London SW1P 1WG, UK The International Handbook o FRP Composites in Civil Engineering Manoohehr Zoghi Rehabilitation with NSM FRP Reinorement Publiation details Khaled Soudki Published online on: 6 Sep 013 How to ite :- Khaled Soudki. 6 Sep 013, Rehabilitation with NSM FRP Reinorement rom: The International Handbook o FRP Composites in Civil Engineering CRC Press Aessed on: 06 Nov PLEASE SCROLL DOWN FOR DOCUMENT Full terms and onditions o use: This Doument PDF may be used or researh, teahing and private study purposes. Any substantial or systemati reprodutions, re-distribution, re-selling, loan or sub-liensing, systemati supply or distribution in any orm to anyone is expressly orbidden. The publisher does not give any warranty express or implied or make any representation that the ontents will be omplete or aurate or up to date. The publisher shall not be liable or an loss, ations, laims, proeedings, demand or osts or damages whatsoever or howsoever aused arising diretly or indiretly in onnetion with or arising out o the use o this material.

2 15 Rehabilitation with NSM FRP Reinorement Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b CONTENTS INTRODUCTION Khaled Soudki Introdution Materials...54 FRP Reinorement and Adhesive...54 Anhorage Systems...54 Installation Proedure or Non-Prestressed NSM Proedure or Prestressed NSM Bond Properties...56 Non-Prestressed NSM Strengthening...58 Flexural Strengthening...58 FRP Strain Limit...58 Analysis Assumptions...59 Flexural Analysis...59 Balaned Condition...60 Understrengthened Setion...60 Overstrengthened Member...60 Prestressed NSM Strengthening...63 General...63 Prestressing Stresses...63 Servie Stresses...63 Craking Moment...65 Flexural Strength...65 Servieability Requirements...68 Shear Design...69 Conluding Remarks...69 Reerenes...69 Rehabilitation and strengthening o onrete strutures using iber-reinored polymer (FRP) reinorement an be applied as an externally bonded (EB) system or a near-surae mounted (NSM) system. In the EB system, FRP laminates are bonded to the onrete surae using epoxy adhesives. To improve the bond strength between the onrete and the FRP, the onrete surae is usually treated by sandblasting. In the NSM system, FRP bars or plates are inserted into a groove that is made in the onrete surae with a onrete saw. The groove is then illed with epoxy adhesive to bond the FRP to the onrete. The use o NSM steel reinorement in the rehabilitation o onrete strutures dates bak to late 1940s (Asplund 1949). Advantages o using FRP reinorement in NSM strengthening appliations are numerous. NSM FRP does not require extensive surae preparation 53

3 54 The International Handbook o FRP Composites in Civil Engineering Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b work and is easy to handle due to their lightweights; better orrosion resistane and less onrete over is needed; NSM reinorement inside the epoxied groove is proteted rom physial damage, ire, or harsh environmental attak. The pratial appliations using the NSM system are more limited than the EB system beause o the lak o relevant provisions in international guidelines (ib 001, CNR 004, SIA , TR55 004, ISIS 008). To address this gap, the international engineering ommunity has arried aelerated researh on strutural aspets o NSM strengthening o onrete strutures: many researh work on the bond behavior and lexural strengthening o onrete members with non-prestressed NSM FRP reinorement has been reported, ewer studies have been arried on prestressed NSM strengthening has been reported in ewer studies, and very limited researh has been arried on the shear strengthening with NSM (Badawi and Soudki 006, El-Haha and Rizkalla 004, De Lorenzis and Teng 007). Reently, ACI 440.R guide (008) has inluded provisions to onsider the NSM system. This hapter will present a synthesis o the existing knowledge and design or the strengthening o onrete strutures with NSM reinorement. Design onsiderations related to bond and lexural strengthening with non-prestressed and prestressed FRP reinorement will be disussed. Design examples will be provided to illustrate the onepts presented. The reader should reognize that the proedures will be revised with new developments in NSM FRP systems. MATERIALS FRP Reinorement and Adhesive Two types o FRP reinorement are presently used or NSM systems: Carbon FRP (CFRP) and glass FRP (GFRP). CFRPs are haraterized by their high longitudinal tensile strength and their high modulus o elastiity (Table 15.1). FRP bars used in NSM appliations are ommerially available rom manuaturers like Sika, Switzerland, Hughes Brothers, the United States, and Pultral, Thetord Mines, Quebe, Canada. Typially, FRP round bars or strips are used depending on the manuaturer. The most ommon iller material used in the groove or NSM strengthening is the two-part epoxy. The role o the adhesive is to transer the shear stresses between the FRP reinorement and the onrete. Typial mehanial properties o the epoxy adhesives are listed in Table Anhorage Systems In prestressed NSM appliations, the FRP reinorement is gripped by means o lamp- or wedgetype anhors to maintain the prestressing ore. Desription o the anhors ollows. 1. Clamp anhor: onsists o grooved steel plates held together by bolts lamping the sleeveenased FRP and transerring the ore by shear-rition mehanism.. Split wedge anhor: onsists o a number o wedges itted into a barrel with a sot metal sleeve enasing the FRP to protet it rom nothing. TABLE 15.1 Material Properties o NSM FRP Systems Material Speii Gravity Tensile Strength (MPa) Tensile Modulus (GPa) Ultimate Elongation (mm/mm) Coeiient Therm. Exp (10 6 /C) GFRP CFRP Epoxy Steel >

4 Rehabilitation with NSM FRP Reinorement 55 Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b INSTALLATION FRP bars or strips are inserted into a groove that is made in the onrete surae with a speial onrete saw. Thereore, the soundness o the onrete inside the groove shall be assured beore installing FRP bars or plates. The groove is then illed with adhesive material to bond the FRP to the onrete. Sine the ore should be adequately transerred between the FRP and the onrete, the seletion and the appliation o adhesive materials must be areully onduted. As suh, the bond apaity during installation proedure is ritial. For more inormation about the NSM bond properties, reer to the Bond Properties setion. In this hapter, the general installation proedure and guidelines or non-prestressed and prestressed NSM strengthening systems are desribed. Proedure or Non-Prestressed NSM The typial proedure or NSM strengthening installation is illustrated in Figure The proedure may vary depending on the types o FRP and adhesive; the manuaturers speiiations should be onsulted. Installation o the NSM FRP reinoring bars begins by making the speiied grooves in the onrete over on the tension surae o the onrete member. A speial onrete saw with a diamond blade an be used to ut the grooves. Speial are is required so that the internal reinorement is not damaged during this proess. An appropriate groove size shall be seleted onsidering the size o the NSM bar or plate and the onrete over. Based on researh by Lorenzis and Nanni (00) and Hassan and Rizkalla (004), larger groove sizes produe higher bond strength than smaller groove sizes. On the other hand, the depth o the groove is limited by the bottom reinorement, and the width should not be exessive to limit onstrution osts. Groove size seletion is given in the Bond Properties setion. Ater making a groove, the grooves shall be leaned with ompressed air or by other means to remove debris and ine partiles to ensure proper bond between the epoxy adhesive and the onrete. Existing exessive raks or unsound onrete should be repaired. The adhesive material is then applied into the groove beore inserting the FRP bars or plates. The groove is halilled with the epoxy adhesive. The FRP reinoring bars or plates are inserted in the groove and lightly pressed to displae the epoxy adhesive. The groove is then illed with more paste, and the surae is leveled to prevent the stress onentration due to unevenness o the surae. The adhesive needs to be ully ured to beome hardened. The uring time varies depending on the type o adhesive. Proedure or Prestressed NSM Installation proedure or NSM prestressing is shown in Figure 15.. The groove is made in the soit o the onrete member with the onrete saw, as shown in Figure 15.a, and the adhesive is applied halway in the groove (see Figure 15.b). A prestressing assembly is attahed to the onrete member using an anhor system, and the FRP reinorement is prestressed, as shown T-beam Making a groove using onrete saw FRP inserted Epoxy (hal ill) Epoxy (omplete ill) FIGURE 15.1 Installation proedure o non-prestressed NSM-strengthened beams. Completed! Remove the exessive adhesive

5 56 The International Handbook o FRP Composites in Civil Engineering T-setion Groove (a) Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b Prestressing ore (b) Anhor (live end) () (d) (e) in Figure 15.. At this stage, the FRP bar is tensioned, and a ompressive ore is applied to the onrete at the same time. One the desired prestressing ore is reahed, the adhesive is ompletely illed in the groove and the exessive adhesive is leaned (see Figure 15.d). Ater uring o the adhesive, the prestressing assembly is removed and the transer o ore is ahieved by bond (Figure 15.e). Alternatively, the anhors are kept at the ends o the prestressed NSM reinorement. BOND PROPERTIES T-setion FRP Adhesive Adhesive Anhor (dead end) FIGURE 15. Prestressing proedure or prestressed NSM system. (a) Make NSM groove, (b) ill halway o the groove with adhesive, () set up prestressing assembly and apply the ore, (d) ill the groove ompletely with adhesive, and (e) remove prestressing assembly and the ut the bar. The bond between an NSM bar and the groove iller plays a key role in ensuring the eetiveness o NSM strengthening. The perormane o the bond depends on a number o parameters inluding the bar ross-setional shape and surae oniguration, the groove dimensions, the shear strength o the groove iller, the degree o roughness o the groove surae, prestress level in the bar, and whether the applied loading is stati or atigue (De Lorenzis and Teng 007). The groove iller is speiied by the manuaturer o the NSM system. Although researh was arried out on both epoxy and ement mortar, the most ommon type is epoxy. In this regard, ACI 440.R (008) reommends that the tensile adhesion strength o the iller should exeed 1.4 MPa and should exhibit ailure o the onrete substrate.

6 Rehabilitation with NSM FRP Reinorement 57 TABLE 15. Bond Stress or NSM Round Bars in Epoxy Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b The groove dimensions and the bar diameter play an important role in the mode o ailure. The ratio between groove size to atual bar diameter is deined as ator k. Table 15. lists typial values or k and bond strength or dierent FRP bars. When the ailure is at the iller onrete interae, the bond strength is alulated rom Equation 15.1a, but when ailure is by pullout o the bar rom the groove iller or by splitting o the over, the stress is alulated rom Equation 15.1b: where τ is the bond strength P is the ore in the bar d b is the bar diameter d g is the groove depth b g is the groove width l is the bonded length Type o FRP Surae Texture k P τ ( d + b ) l τ g P πdl b g (15.1a) (15.1b) Regarding the dimension o the groove, ACI 440.R (008) reommends that it should be at least 1.5 times the diameter o the FRP bar. For a retangular bar with an aspet ratio a b, a minimum groove size o 3.0a 1.5b is suggested, where a is the smallest bar dimension. For multiple FRP bars, the minimum lear groove spaing or NSM FRP bars should be greater than twie the depth o the NSM groove to avoid overlapping o the tensile stresses around the NSM bars. Also a lear edge distane o our times the depth o the NSM groove should be provided to minimize the edge eets that ould aelerate debonding ailure (Hassan and Rizkalla 003). The development length o an NSM FRP bar is alulated using an average bond strength o 6.9 MPa (ACI 440.R 008) as given in Equation 15.. P l ( a+ b) τ orretangular bars Bond Strength (MPa) CFRP Round Sandblasted Round Ribbed Round Spirally winded Round Sand oated GFRP Ribbed Ribbed (15.a)

7 58 The International Handbook o FRP Composites in Civil Engineering P l π τ d b orirularbars (15.b) Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b where τ is the bond strength P is the ore in the bar d b is the diameter o the irular bar a, b are the dimensions or the retangular bar l is the bonded length In prestressed NSM appliation, the release o the ore in the prestressing rod reates shear stresses in the surrounding epoxy. Weak bond between the FRP reinorement and the epoxy results in a longer transer length. I the bond is weak, the prestressing ore may be lost as the member is loaded due to slippage o the bar within the epoxy. The transer length o CFRP bars in prestressed NSM strengthening, with prestress level 40% 60% o their ultimate apaity, was ound to vary between 0 and 40 times bar diameter. The prestressing stress along the NSM CFRP bar in an epoxied groove an be estimated using Equation 15.3 (Badawi et al. 011): s Bx ( 1 exp ) (15.3) pre where s is the prestress stress in the rod or a given distane (x) rom the end o the bonded length pre is the maximum prestressing stress B is a ator obtained rom best it o experimental results. This ator aounts or dierent variables as epoxy type, epoxy thikness, and prestressing level. B 0.01 or CFRP in epoxy x is the distane rom the end o the bonded length NON-PRESTRESSED NSM STRENGTHENING Flexural Strengthening The apaity ondition in lexure that must be satisied or NSM FRP-strengthened members is that the resisting moment, M r (ϕ n M n ), shall be greater than the atored moment, ϕ M, as ollows: M φ M (15.4) r where M n is the nominal moment ϕ is the load ator ϕ n is the strength redution ator or material ator The ailure modes o NSM FRP-strengthened members inlude (1) onrete rushing, () FRP rupture, and (3) premature FRP debonding ailure. The lexural analysis o NSM-strengthened members shall be perormed onsidering these ailure modes. FRP Strain Limit In NSM-strengthened members, the ourrene o the debonding ailure is reported between approximately 60% and 90% o the ultimate strength ( rpu ) or strain o FRP material (ε rpu ),

8 Rehabilitation with NSM FRP Reinorement 59 respetively. Thereore, to aount or the debonding ailure mode, the ultimate FRP strain is redued by the FRP strain limit ator, ψ rp, to get the eetive FRP strain, ε e, as shown as ollows. εe ψrp εrpu (15.5) Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b The FRP strain limit ator, ψ rp, varies depending on many ators suh as the types o FRP material and adhesive, onrete soundness near the groove, the geometry o the groove, et. (see the Bond Properties setion). ACI 440.R (008) reommends using a strain limit ator o 0.7 or NSM systems. Analysis Assumptions The basi assumptions or the analysis are as ollows: 1. Peret bond exists between the onrete, adhesive, and the FRP with no slip.. Failure is by onrete rushing at ultimate ompression strain o or FRP ailure at an eetive FRP strain (set equal 0.7 times the FRP rupture strain) ater yielding o the internal steel reinorement. 3. Strain distribution over setion depth is linear (plane setion remains plane). 4. Conrete has a paraboli stress strain relationship in ompression. Tensile strength o onrete ater raking is ignored. 5. The stress strain urve or the steel reinorement is linear elasti-peretly plasti. 6. The FRP stress strain relationship is idealized as linear elasti to ailure. Flexural Analysis The lexural analysis o a setion with NSM FRP reinorement is based on ore equilibrium and strain ompatibility, as shown or a retangular setion in Figure The balaned amount o FRP reinorement, A b, is deined as the area o FRP at whih the onrete and FRP ail simultaneously. Depending on FRP reinorement, A, the lexural behavior an be lassiied as Balaned ailure: i A A,b, ailure will our by FRP ailure with the FRP strain at the eetive strain (ε e 0.7 * ε rpu ) and onrete ailure (ε u 0.003) simultaneously. Understrengthened setion: i A < A,b, ailure will our by FRP ailure (ε e 0.7 * ε rpu ) beore onrete ailure (ε u < 0.003). Overstrengthened setion: i A > A,b, ailure will our by onrete ailure (ε u 0.003) beore FRP ailure (ε < ε e ). To guard against brittle ailure, it is assumed that in all three ases, the internal steel reinorement will have reahed the yield strain beore FRP ailure or onrete rushing. This assumption shall be heked by the designer. d d s b A s A b ε ε s ε +ε bi N.A. β 1 C/ C α 1 ć b b T s A s y T A,b E ε FIGURE 15.3 Strain ompatibility and ore equilibrium in a non-prestressed setion.

9 60 The International Handbook o FRP Composites in Civil Engineering Balaned Condition At the balaned ondition, the ompression strain in the onrete ε is at the rushing strain ε u and the tensile strain in the FRP, ε, is at the eetive strain (ε e 0.7 * ε rpu ). Based on strain ompatibility and ore equilibrium equations or a retangular setion, the depth o the neutral axis, b, and the balaned FRP reinorement, A,b, an be alulated by Equations 15.6 and Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b b A εu ε + ε + ε b, e bi u d α b A E ε 1 b s y e (15.6) (15.7) Understrengthened Setion In an understrengthened setion (A < A,b ), the FRP strain ε reahes its ultimate design strain, ε e, with ompression strain in the onrete ε < ε u or The strain proile and ores in a retangular setion are shown in Figure For this ase, α 1 and β 1 are the mean stress ator and entroid ator o the paraboli onrete stress strain urve when the onrete strain at top iber does not reah its ultimate strain (Collins and Mithell 1987). ε ε ε ε (15.8a) (15.8b) 5500 ε αβ β ε ε ε 3ε ( ) ( ) 4 ε/ ε 6 ε / ε (15.9a) (15.9b) To determine the neutral axis depth,, an iterative approah is required by assuming the onrete strain at top iber, ε, and revising it until the ore equilibrium and strain ompatibility equations are satisied (Figure 15.3). The nominal moment or this ase is given in Equation Mn Asy ds AE e d + β1 β1 ε (15.10) Overstrengthened Member In an overstrengthened setion (A > A,b ), the ompression strain in the onrete, ε, reahes the ultimate strain, ε u (ε ε u 0.003) beore FRP strain ε reahes its ultimate design strain, ε e (ε < ε e ). The nominal moment or this ase is alulated using Equation 15.11: β1 Mn Asy ds AE d + β1 ε (15.11) The analysis presented in this setion is based on the retangular setion, and thereore i the rosssetional area o the members is dierent, appropriate derivations based on the ore equilibrium and strain ompatibility shall be arried.

10 Rehabilitation with NSM FRP Reinorement 61 Example 15.1: Moment in a Non-Prestressed Setion This setion shows the alulation proedure or the nominal moment o an RC slab strengthened with non-prestressed NSM FRP reinorement. Figure 15.4 is the ross setion o a slab pedestrian bridge, and the typial geometri and material properties are summarized as ollows: Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b Conrete: b 1000 mm, h 300 mm, 30 MPa Steel: A s 800 mm, E s 00,000 MPa, sy 400 MPa, d s 54 mm FRP: A 10.0 mm (4 6 mm diameter rods), E 140 GPa, ε rpu , d 95 mm Step 1. Determine nominal moment o the unstrengthened member Mn A s y ds α β A sy (800)(400) β1 13.5mm α b (0.805)(30)(1000) 1 β ( 800)( 400) kn m Step. Determine ailure mode o the setion Eetive FRP strain, ε e 0.7 * ε rpu 0.7 * (0.015) FRP strain, ε ε e Conrete strain, ε u Initial strain, ε bi 0 b εu ε + ε + ε A b, e bi u d α1b A E ε b sy Understrengthened setion (FRP ailure mode) d e ( 95) 65. 5mm ( )( 30)( 1, 000)( 65. 5) ( 800)( 400) 858mm ( 140, 000)( ) A 10mm < 858mm FIGURE 15.4 Typial ross setion o onrete slab. d s A b A s

11 6 The International Handbook o FRP Composites in Civil Engineering Step 3. Determine nominal moment o the strengthened member Use trial and error to ind while satisying ore equilibrium and strain ompatibility. Assume strain in onrete at top iber, ε ε Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b αβ β ε Fore equilibrium ondition C T s T + T Selet ε Selet ε ε ε ε 3ε d 4 ε/ ε 6 ε / ε e ε + ε ε ( ) ( ) ( ) d ε e 4 ( 0. 00/ 0. 00) (. 00/ 0. 00) ( 95) 47. mm α 1 bb 1 ( )( )( 47. )(, 1 000) kN A ( 800) ( 400) 30 kn s sy T AE ε ( 10)( 140, 000)( ) kN s e ε ε ε ε 075. αβ β mm C 63kN 065. αβ β mm C kN The nominal moment is alulated using Equation 15.10: kN T + T β1 Mn A s y ds AE e d β1 + ε s s C Revise T + T OK Revise M n kN m ( 60% inreaseinapaity)

12 Rehabilitation with NSM FRP Reinorement 63 PRESTRESSED NSM STRENGTHENING General Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b The undamental onept o a prestressed NSM FRP-strengthened member is to improve the servieability (raking, deletion) and moment by providing an ative prestressing ore to the onrete element (Figure 15.5). The tensile stress at the onrete bottom ae at mid-span under servie loads is redued or hanged into ompressive stress due to the eet o prestressing ore, as shown in Figure By using prestressed NSM strengthening, open raks shall lose, and the deletion o the member is redued. Prestressing Stresses The allowable stresses or prestressed NSM FRP reinorement shall be taken similar to those presribed by ACI 440.4R-04. For CFRP, the maximum allowable stress is 60% 65% o the ultimate tensile strength o the FRP reinorement. Regarding prestress losses, there are limited data or FRP reinorement in epoxy medium, and as suh, the designer should onsult the urrent literature or these values. Servie Stresses The servieability ondition or a prestressed NSM FRP-strengthened member requires the alulation o stresses under servie loads to ensure they are within permissible values in design guides suh as ACI The stresses at a setion are given as P Pe σ+ + + A S where σ is the stress in the onrete at distane y rom entroidal axis, MPa P is the prestress ore in the NSM FRP reinorement, N M is the moment due to servie loads, N mm e is the eentriity measured rom entroidal axis, mm A is the ross-setional area, mm S is the setion modulus, mm 3 FRP + Strain proile due to loading P P + M S Strain proile Final strain due to prestressing proile FIGURE 15.5 Stresses in a prestressed NSM FRP strengthening system. Prestressing ore (15.1)

13 64 The International Handbook o FRP Composites in Civil Engineering Example 15.: Flexural Stresses in an NSM Prestressed Slab Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b Compute the stresses in the prestressed NSM FRP-strengthened RC slab in Example The slab is arrying a servie moment o 60 kn m. The slab is prestressed with our 6 mm diameter CFRP bars eah stressed to 40% ultimate ater losses. The CFRP bars have ultimate strength rpu 100 MPa, modulus o elastiity, E rp 140 GPa, and rupture strain Conrete strength, 30MPa. Solution d d s A b A s P ( 04. ) A kN rpu e mm A bh mm bh S 6 6 Stresses in the onrete under servie loads e mm 3 P A MPa Pe S ( )( 145) MPa M S ( ) 40. MPa The stresses in the onrete at the top and bottom ibers under servie loading are thus: top: MPa σ T bottom: MPa σ B MPa OK MPa OK The onrete stresses in both extreme ibers are within permissible limits under servie loads. The allowable stresses are speiied in design guides, or example, ACI 440.4R (004). h

14 Rehabilitation with NSM FRP Reinorement 65 Craking Moment The raking moment, M CR, is alulated with the extreme tensile iber stress in the onrete set equal to the modulus o rupture, 06., as ollows: r Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b where S t is the setion modulus at tension ae, mm 3. M Example 15.3: Craking Moment in NSM Prestressed Slab Compute the raking moment or the slab in Example 15.. Flexural Strength CR P Pe + St 06. λ A S (15.13) M CR + 6 ( 033. ) ( ) ( ) 68kN m 68kN m > 60kN m(servie) The lexural strength o a prestressed NSM FRP-strengthened setion is determined by satisying the ore equilibrium and strain ompatibility while aounting or the initial ondition due to the prestressing eets. Figure 15.6 shows a prestressed retangular setion. The strain ompatibility is used to ompute the stress in the NSM FRP at ultimate ( u ). Strain in tendon at ultimate h d d s d C Initial strain proile due to prestressing b t ε ε + ( ε + ε ) < ε (15.14) u, pres,, load e N.A ε ε s ε rp e 07. * ε rpu Strain distribution Stress distribution Equivalent ores s ε CFRP(load) +ε CFRP(pre) +ε CFRP () FIGURE 15.6 Strain ompatibility and ore equilibrium in a prestressed setion. ε ε ś rp ś T s T rp C C s

15 66 The International Handbook o FRP Composites in Civil Engineering From strain diagram at ultimate d εload ε, u 1 (15.15) Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b From equilibrium A + A α 1 ab (15.16) u s y The solution proeeds by iteration to satisy Equations through The nominal moment o an NSM prestressed setion is alulated, ignoring the ompression steel, using Equation 15.17: Mn Asy( ds ) + ArpErp( ε, pres + ε, + ε, load )( d ) (15.17) where A s is the area o the tension steel reinorement A rp is the area o the FRP reinorement b is the beam width is the depth o neutral axis y is the stress in the tension steel reinorement M n is the nominal bending moment ε,pres is the strain in NSM FRP due to initial prestressing ore ε, is the strain in NSM FRP at deompression in onrete at level o bar ε,load is the strain in NSM FRP due to loading by ompatibility o strain Example 15.4: Flexural Strength o NSM Prestressed Slab Compute the ultimate moment o the NSM prestressed slab in Example 15.. The CFRP bars have ultimate strength rpu 100 MPa, modulus o elastiity, E rp 140 GPa, and rupture strain Conrete strength, 30 MPa. The prestress level ater losses is pe 40% rpu. Solution: 8 4 A 300, 000mm ; I mm ; e 145mm 8 4 A 300, 000mm I mm e 145mm P 0. 40, kN 5, 500 5, , 15MPa TotalFRPstrain ε ε + ε + ε load, a. ε,pres FRP strain due to prestressing E u, pres, pe E

16 Rehabilitation with NSM FRP Reinorement 67 b. ε, onrete strain at level o FRP tendon due to prestressing Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b P σ + A Pe I 3 ( ) ( )( 145) e 16. ε, E 3015 (+) 3 ( ) 145. ε,load onrete strain at level o tendon at ultimate Assume ε d ( ) ε (load) MPa e 16. MPa A + A sy mm αβ b ( 95 3) > ( εe) 3 For design, the maximum FRP strain 0.7 * ε rpu 0.7 * or 10,500 με ε ε + ε + ε u, pres,, load 10, ( ) ε load, 6 ε load,

17 68 The International Handbook o FRP Composites in Civil Engineering The two unknowns and ε are to be determined by trial and error approah as ollows: Assume ε 1000 µε αβ β mm ε Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b C T s T d ( ) αβ b 675, 613N 1 1 A 30, 000N s y A 176, 000N e T + T s 496, 000N C Revise Selet ε 800 µε α β β mm C αβ b 466, 793N T + T Revise s Selet ε 830 µε α β β mm β 3mm C αβ b 496, 76N T + T OK 1 1 The nominal moment is alulated as ollows: β1 β1 Mn A s y ds AE d + ε s kn m Comparing with the example in Example 15.1, the nominal moment o the prestressed setion is slightly smaller than the non-prestressed setion. However, the urvature (ε /) or the prestressed setion is , whih is hal that or the non-prestressed setion ( ). SERVICEABILITY REQUIREMENTS The servieability (raking and deletion) or an NSM-strengthened member shall satisy the appliable provisions in existing design guidelines. For example, ACI 440.R (008) limits the servie stress in onrete to 45% o the ompressive strength and the servie stress in the internal steel and 80% o the yield strength o the reinorement. The atigue stress and reep-rupture limits shall be based on ACI 440.R (008), whih speiies the stresses in a CFRP reinorement under servie loads to be below 55% o the ultimate strength o the CFRP.

18 Rehabilitation with NSM FRP Reinorement 69 Downloaded By: At: 03:35 06 Nov 018; For: , hapter15, /b SHEAR DESIGN There are very limited data on the shear behavior o NSM FRP-strengthened members (De Lorenzis and Teng 007). The eetive strain in the NSM FRP reinorement, mounted on the sides o the member, an be onservatively taken as an EB ae FRP plies ollowing the proedure desribed in ACI 440.R (008). The design engineer shall review the suitability o provisions in ACI 440.R (008) or EB FRP laminates and ACI 440.1R (006) or internal FRP stirrups. The designer shall onsult the published literature on shear strengthening as they beome available. CONCLUDING REMARKS NSM FRP reinorement is reently used or the rehabilitation and strengthening o onrete strutures. This hapter has oused primarily on the lexural behavior o onrete members strengthened with nonprestressed and prestressed NSM FRP reinorement. Many researh areas are still needed inluding servieability riteria, bond and development, shear strength, ire resistane and protetion, external prestressing and devies, et. For this emerging tehnology to beome widespread, speii provisions addressing NSM reinorement shall be inluded in international guidelines and speiiations. REFERENCES ACI (004). Prestressed onrete strutures with FRP tendons, ACI 440.4R-04, Amerian Conrete Institute, Farmington Hills, MI, 35pp. ACI (006). Guide or the design and onstrution o strutural onrete reinored with FRP bars, ACI 440.1R-06, Amerian Conrete Institute, Farmington Hills, MI, 44pp. ACI (008). Guide or the design and onstrution o externally bonded FRP systems or strengthening onrete strutures, ACI 440.R-08, Amerian Conrete Institute, Farmington Hills, MI, 80pp. Asplund, S.O. (1949). Strengthening bridge slabs with grouted reinorement, Amerian Conrete Institute (ACI), Strutural Journal, 0(4), Badawi, M. and Soudki, K.A. (006). Strengthening o RC beams with prestressed near surae mounted CFRP rods, Third International Conerene IIFC Composites in Civil Engineering, Deember 13 15, Miami, FL. Badawi, M., Wahab, N., and Soudki, K.A. (011). Evaluation o the transer length o prestressed near surae mounted CFRP rods in onrete, Journal o Building and Constrution Materials, 5(3), CNR (004). Guide or the Design and Constrution o Externally Bonded FRP Systems or Strengthening Existing Strutures, CNR-DT 00/004. CNR Advisory Committee on Tehnial Reommendations or Constrution, Rome, Italy, 144pp. Collins, M.P. and Mithell, D. (1987). Prestressed onrete basis, Canadian Prestressed Conrete Institute (CPCI), Ottawa, Ontario, Canada, 614pp. De Lorenzis, L. and Teng, J.G. (007). Near-surae mounted FRP reinorement: An emerging tehnique or strengthening strutures, Composites Part B, 38(), El-Haha, R. and Rizkalla, S.H. (004). Near-surae-mounted iber-reinored polymer reinorements or lexural strengthening o onrete strutures, ACI Strutural Journal, 101(5), ib (001). Externally bonded FRP reinorement or RC strutures Bulletin 14, International Federation or Strutural Conrete (ib), Lausanne, Switzerland, 130pp. Hassan, T. and Rizkalla, S. (003). Investigation o bond in onrete strutures strengthened with near surae mounted arbon iber reinored polymer strips, ASCE Journal o Composite or Constrution, 7(3), Hassan, T.K. and Rizkalla, S.H. (004). Bond mehanism o near-surae-mounted iber-reinored polymer bars or lexural strengthening o onrete strutures, ACI Strutural Journal, 101(6), ISIS (008). FRP rehabilitation o reinored onrete strutures, Design Manual No. 4, Version, ISIS Canada, Toronto, Ontario, Canada. Lorenzis, L. and Nanni, A. (00). Bond between near-surae mounted iber-reinored polymer rods and onrete in strutural strengthening, ACI Strutural Journal, 99(), SIA166 (004). Klebebewehrungen (Externally bonded reinorement), Shweizerisher Ingenieur- und Arhitektenverein SIA, Zurih, Switzerland, 44pp. TR55 (004). Design guidane or strengthening onrete strutures using ibre omposite materials, nd edition, Tehnial Report No. 55 o the Conrete Soiety, London, U.K., 10pp.

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