Recommended Guide Specification for the Design of Externally Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements (2010) / Chapter Skim
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A T T A C H M E N T A Recommended Guide Specification for the Design of Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements This Guide Specification is the recommendation of the research team for NCHRP Project 10-73 that was conducted at Georgia Institute of Technology. The Guide Specification has not been approved by NCHRP or any AASHTO committee; nor has it been formally accepted for the AASHTO specifications.
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as well as relevant research data dealing with externally bonded FRP reinforcement for reinforced and prestressed concrete structures are provided for those who wish to study the background material in depth. NCHRP Report 609 presents recommended construction specifications concerning the use of externally bonded FRP reinforcement for strengthening concrete structures.
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b = width of rectangular section (in) frpb = width of the FRP reinforcement (in.)
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' ccf = compressive strength of confined concrete (ksi) frpuf = characteristic value of the tensile strength of FRP reinforcement (ksi)
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cV = the nominal shear strength provided by the concrete (kips) frpV = the nominal shear strength provided by the externally bonded FRP reinforcement (kips)
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int = interface shear transfer strength (ksi) frp = resistance factor for FRP component of resistance 1.4 DESIGN BASIS C1.4 1.4.1 Bridge elements strengthened with externally bonded reinforcement shall be designed to achieve the objectives of constructability, safety, and serviceability, with regard to issues of inspectability, The resistance criteria in this Guide Specification were developed using modern principles of structural reliability analysis, which are consistent with those on which the AASHTO LRFD Bridge
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Structural reliability analysis takes the uncertainties in concrete, steel and FRP material strengths and stiffnesses into account using rational statistical models of these key engineering parameters. The criteria for checking safety and serviceability of structural members and components that have been strengthened with externally bonded FRP reinforcement are based on a target reliability index, , equal to 3.5 under inventory loading, which was the target value assumed in the development of the AASHTO LRFD Bridge Design Specifications.
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C1.5 This Guide Specification applies to strength limit states I and II, serviceability lim it states I, III and IV, Extrem e Event lim it states I and II, and the Fatigue lim it state, as defined in Article 3.4 of the LRFD Bridge Design Specifications . 1.5.1 Service Limit States C1.5.1 Structural me mbers shall satisfy LRFD Eq.
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Strength III - Load combination relating to the bridge exposed to wind velocity exceeding 55 mph. Strength IV - Load combination relating to very high C1.5.2 Design for strength limit states ensures that local and global strength and stability are provided to resist the specified load combinations that a bridge is expected to experience in its design life.
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The Fatigue II li mi t-state load combination is not applicable to concrete bridge structures reinforced with FRP system s as it is not generally applicable to concrete components and connections. 1.6 LOADS AND LOAD COMBINATIONS 1.6.1 Loads C1.6.1 The loads defined in LRFD Article 3.3.2 and characterized in LRFD Article 3.6 through 3.15 shall be applied for designing reinforced concrete and The loads required for the design and evaluation of concrete bridge structures reinforced with FRP syste ms are classified in the LRFD Specifications as prestressed highway bridge me mb ers strengthened with externally bonded FRP reinforcem ent perm anent and transient loads.
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Strength is the primary limit state for load rating; service and fatigue limit states are selectively applied in accordance with the provisions of the MBE. Live-load models for load rating include: Design Load: HL-93 Design Load per LRFD Specifications Legal Loads: AASHTO Legal loads, as specified in MBE Article 6A.4.4.2.1.1, and (2)
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ACI 440.2-R02 (2002) Guide for the design and construction for externally bonded FRP syste ms for strengthening concrete structures," Evaluation of Concrete Structures Prior to Rehabilitation," Am erican Concrete Institute, Farm ington Hills, Michigan.
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. "Live load model for highway bridges," Journal of Structural Safety, Volume 13, No.
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2.2.4 When cured under conditions identical to those of the intended use, the composite material system as well as the adhesive system, if used, shall conform to the following requirements:
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Figure C2.1 Effect of temperature on the properties of polymer composite materials (Zureick and Kahn, 2001) 2.2.4.2 The characteristic value of the tensile failure strain in the direction corresponding to the highest percentage of fibers shall not be less than 1%, when the tension test is conducted according to ASTM 3039.
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C2.2.4.4. The physical and mechanical properties of FRP materials and of the concrete structure reinforced with an externally bonded reinforced system are sensitive to various environmental conditions that can affect one or more of the followings: Chemical and/or physical changes in the polymeric matrix.
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on reinforced concrete beams strengthened with externally bonded FRP reinforcement A-16
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. "Rehabilitation of Reinforced Concrete Structures Using FiberReinforced Polymer Composites, ASM Handbook, Volume 21, Composites, pp.
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The strength in flexure of a reinforced concrete member that has been additionally reinforced by an externally bonded FRP plate is derived from the classic Bernoulli-Navier hypothesis that plane sections remain plane and perpendicular to the neutral axis during flexure. The stresses on the section can be determined from the constitutive relations for the concrete, reinforcing steel and FRP reinforcement, and the flexural strength at any section is determined from requirements for axial force and moment equilibrium at that section.
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c = strain in concrete cf = the 28 - day compression strength of the concrete (ksi) o = the concrete strain corresponding to the maximum stress of the concrete stress-strain curve cE = the modulus of elasticity of the concrete specified in Section 5.4.2.4 of the AASHTO LRFD Bridge Design Specifications (ksi)
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3.4 STRENGTH LIMIT STATES 3.4.1 Factored Flexural Resistance 3.4.1.1 Rectangular Sections The factored resistance, rM , of a steel-reinforced concrete rectangular section strengthened with FRP reinforcement externally bonded to the beam tension surface shall be taken as ckhT dckfAckdfAM frpfrp ssssssr 2 ' 2 '' 29.0 C3.4.1 The factored resistance is in line with the design strength determination in accordance with Article 5.7.3.2 of AASHTO LRFD Bridge Design Specifications, and is written so that the design strength for a reinforced concrete flexural member is simply augmented by the contribution of the externally bonded FRP reinforcement. This format ensures that when the FRP reinforcement is slight, the design strength approaches that of a flexural member without FRP reinforcement.
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h = depth of section (in) frpT = tension force in the FRP reinforcement (kips)
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C3.4.1.2 For most practical cases involving flanged sections strengthened externally with bonded FRP reinforcement to the tension surface, the depth of the neutral axis falls within the flange. When the neutral axis falls below the flange, the compression force exerted in the concrete is the sum of two components, one of which corresponds to the flange and one corresponds to the portion of the web under compression.
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debonding of the externally bonded FRP reinforcement, thereby enabling the development of a ductile mode of flexural failure. 3.4.3 Detailing requirements C3.4.3 Flexural members shall be detailed to permit the development of the factored resistance defined by Eq (3.4-1)
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This mode of failure has been demonstrated experimentally for beams with externally bonded steel plates (Jones et al., 1988; Oehlers and Moran, 1990) and FRP reinforcement (Malek et al., 1998; Lopez and Naaman, 2003; Yao and Teng 2007)
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. "Guide for the Design and Construction of Externally Bonded FRP System s for Strengthening Concrete Structures," Am erican Concrete Institute, Farm ington Hills, Michigan.
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. "An investigation of bond between concrete and externally bonded carbon fiber reinforced plastic plates," M.S.
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Reinforcement for Concrete Structures (FRPRCS-3)
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4.2 STRENGTHENING SCHEMES Reinforced concrete bridge elements shall be strengthened with externally bonded FRP reinforcement using one of the following methods: Side bonding U-jacketing U-jacketing combined with anchorage Complete wrapping Transverse reinforcement shall be provided symmetrically on both sides of the element with spacing not to exceed the smaller value of 0.4 dv or 12 inches, where dv is the effective shear depth defined in Article 5.8.2.9 of AASHTO LRFD Bridge Design Specifications C4.2 Typical FRP strengthening schemes for beams and columns are summarized as follows: Side bonding (Fig.
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C4.2-2) is the most common externally bonded shear strengthening method for reinforced concrete beams and girders.
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However, it is norm ally more practical to attach the external FRP reinforcem ent with the principal fiber direction perpendicular to the me mb er axis. Experim ental and analytical investigations of the behavior of reinforced concrete me mb ers strengthened in shear have revealed the following failure m odes: 1 - Steel yielding followed by FRP debonding 2 - Steel yielding followed by FRP fracture 3 - FRP debonding before steel yielding 4 - Diagonal concrete crushing Depending on the amount of usable steel shear reinforcement in the structural element, FRP debonding can occur either before or after steel yielding.
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sV = the nominal shear strength provided by the transverse steel reinforcement in accordance with Article 5.8.3.3 of the AASHTO LRFD Bridge Design Specifications; pV = component of the effective prestressing force in the direction of applied shear as specified in Article 5.8.3.3 of the AASHTO LRFD Bridge Design Specifications; frpV = the nominal shear strength provided by the externally bonded FRP system in accordance with Article 4.3; = 0.9 frp is a resistance factor, defined as follows: 0.40 for side bonding shear reinforcement; 0.55 for U-jacketing; 0.60 for U-jacketing combined with anchorages; 0.65 for complete wrapping. C4.3.1 The shear provisions in Article 4.3 draw upon the traditional ACI approach embodied by Chapter 11 of the ACI Standard 318-05, supplemented by the report of ACI Committee 440.2R-02 (ACI, 2002)
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4.3.2 Limitation on strength provided by concrete and steel The sum of Vc + Vs shall not exceed 0.25 c v vf b d , in which bv and dv are effective web width and shear depth, defined in Article 5.8.2.9, in which cf is expressed in ksi units. 4.3.3 Regions requiring externally bonded shear reinforcement C4.3.3 Except for slabs, footings and culverts, shear reinforcement shall be provided where the required strength exceeds 0.5 c pV V in which cV , pV , and are defined in Article 4.3.1, or where consideration of torsion is required by Eqs 5.8.2.1-3 or 5.8.6.3-1 of the AASHTO LRFD Bridge Design Specification.
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? s v s v w fr p t f R b w Figure C4.3.4.1 4.3.4 Strength provided by externally bonded FRP reinforcement 4.3.4.1 Nominal Strength C4.3.4.1
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utN = nominal tensile strength of the FRP reinforcement; sN = Strength of FRP reinforcement corresponding to a strain of 0.004 The term ka in eq 4.3.4.2.2-1 is a coefficient that defines the effectiveness of the specific anchorage system. In view of the limited available test data, on FRP reinforcement with mechanical anchorage systems, it is recommended that if the anchorage is engineered, the strength can be fully developed and ka = 1; otherwise, its strength contribution is unknown.
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The principles applied to strengthening in shear are also valid, for the most part, for the case of torsion. The user of these Guide Specifications is cautioned that, in contrast to the provisions in Articles 4.2 and 4.3, supporting experimental data on the enhancement of the capacity of a member to withstand torsion by externally bonded FRP reinforcement does not exist.
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For continuous FRP reinforcement 1 1 e frp frp frp tT N d x y (4.4.2-2) in which 1 10.66 0.33 1.5t y x 1x = lesser dimension of the member 1y = larger dimension of the member 4.4.1 Factored strength in torsion A-36
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In reinforced concrete construction, the crack faces are irregular and this slip is accompanied by separation of the crack faces. The slip movement and irregularities on the crack face introduce tension in the reinforcement that crosses the crack, and causes a clamping force to be developed normal to the crack.
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The contribution of the FRP reinforcement is included in the clamping force that appears in the expression for Vni, rather than additive to the factored shear strength. Similarly, the resistance factor for contribution of the FRP reinforcement is embedded in the clamping force.
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The clamping forces in Articles 4.5.3.2 and 4.5.3.3 have been modified to account for the presence of FRP reinforcement crossing the crack. To preserve the familiar format of the factored resistance, the resistance factor, frp , is included in the expression for the clamping force 4.5.3.2 – Where shear-friction reinforcement is inclined to the shear plane, such that the shear force produces tension in shear-friction reinforcement, Vni shall be computed by: sin cosniV C (4.5.3.2-1)
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4.5.3.3 – The clamping force, C , shall be determined as follows: vf yf frp frp frp frpC A f A E (4.5.3.3-1) In which vfA = area of steel reinforcement for shear-friction; yff = yield strength of steel reinforcement for shear-friction; frpA = effective area of FRP reinforcement for shear-friction; frpE = effective modulus of FRP reinforcement for shear-friction; frp = strain in FRP reinforcement for shearfriction, and frp = 0.65 The strain in the FRP reinforcement for shearfriction shall be taken as 0.004 unless test data are provided to support an alternative value.
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The value of fy used for design of shear-friction reinforcement shall not exceed 60 ksi. It is permitted to take permanent net compression across the shear plane as additive to the force in the shear-friction reinforcement, Avf fy, when calculating the required Avf
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(1999) , "Anchorage design for externally bonded carbon fiber reinforced polymer laminates", Proceedings of Fourth International Symposium on FRP Reinforcement for Concrete Structures, Baltimore, USA, 635-645.
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(1998) , "Shear strengthening of reinforced concrete beams using epoxy-bonded FRP composites", ACI Structural Journal, 95(2)
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. 5.2 METHODS FOR STRENGTHENING WITH FRP REINFORCEMENT 5.2.1 Columns shall be strengthened with FRP reinforcement using the complete wrapping method specified in Article 4.2.
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5.3.2 Short Columns in Compression Colum ns in compression shall be fully wrapped over the entire length. C5.3.2 The provisions in Article 5.3.2 apply to short columns in which second-order effects are negligible and the limit state of instability can be ignored.
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Relevant background related the maximum and minium values of confinement pressure in FRP reinforcement jackets in axially loaded columns is given by Thériault and Neale (2000)
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Externally bonded FRP reinforcem ent of colum ns strengthened to withstand end mo ments due to lateral load shall be reinforced over a distance from the colum n ends equal to the maxim um colu mn dim ension or the distance over which the mo me nt exceeds 75% of the maxim um required mo ment, whichever distance is larger.
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C5.4.3 Provisions in Article 5.4 are limited to members subjected to combined axial loading and bending where failures occur by concrete crushing in compression rather than reinforcement yielding in tension. If the eccentricity of axial force present in the member is greater than 0.10h for the spirally reinforced columns or 0.05h for tied columns, strengthening requires externally bonded FRP reinforcement to withstand force in the longitudinal direction of the columnin addition to its perimeter.
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0 004 . 0 where fe is the effective strain level in FRP reinforcement attained at failure fu is the design rupture strain of FRP reinforcement References ISIS (2001)
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