Chapter 2. Strengthening of Reinforced Concrete Structures with FRPs
2.5 Existing Design Procedures for Delamination
Following the successes o f many projects involved strengthening with FRP m aterials, infrastructure owners, regulators, contractors, and professional organizations began to consider FRPs in more retrofitting projects. However, widespread use is restrained by the lack o f design codes. This section reviews the most widely available design procedures and identifies the merits and deficiencies o f each publication.
2.5.2 Professional Organizations 2.5.2.1 American Concrete Institute
The technical committee 440 of the American Concrete Institute is currently preparing a design guideline document “Guide for the Design and Construction o f Externally Bonded FRP Systems for Strengthening Concrete Structures” . Sections o f the guidelines cover properties o f the retrofit systems, evaluation o f existing structures, structural design, shipping, storage, handling, quality control and maintenance. The evaluation o f existing structures section covers important criteria for field inspection and assessment o f existing load capacity. The recommendations for design o f columns, flexural members and walls are based on ACI-318 requirements. Failure design recommendations state to avoid brittle failures such as anchorage, delamination, and shear failure. To avoid delam ination failures, the guidelines recommend that the ultimate design tensile strain o f the FRP at
2 .S tre n g th e n in g o f R einforced C o n crete S tru c tu re s w ith FRPs
failure should be less than Km S fu where K m is a modification factor less than 0.9 and £/u is the rupture strain o f the FRP. The term Km is expressed in Equation 2.21.
n E f t f Km =
1 fo r n E f t , <214,000
428,000 f 1
107 000 ( z . z i )
’ f o r n E f t f >214,000
n E / t f 1 f
where n is the number o f FRP layers, Ej (MPa) and tf (mm), are the modulus o f elasticity and thickness o f the FRP, respectively. Equation 2.21 term recognizes that laminates with greater stiffness are more prone to delamination. Consequently, as the stiffness o f the laminate increases, the strain limitation becomes more severe. The term Km is based on a general recognized trend and on the experience o f engineers practicing the design o f externally bonded FRP systems. Research work is currently in progress at the Centre for Infrastructure Engineering Studies (CIES) at the University o f M issouri-Rolla to develop construction specifications and a process control manual for FRP bonded to concrete structures.
2.5.2.2 The Canadian Network of Centres of Excellence
The Canadian Network o f Centres o f Excellence on Intelligent Sensing for Innovative Structures, ISIS Canada, published “ Strengthening Reinforced Concrete Structures with Externally-Bonded Fibre Reinforced Polymers” [ISIS Canada Design Manuals, 2001].
The document presents the design procedures that have been developed and validated for strengthening o f concrete structures with FRPs. The basic equations and methodology are presented and examples o f applications are given to illustrate the various design
2 .S tre n g th e n in g o f R einforced C o n crete S tru c tu re s w ith FRPs
procedures. Anchorage provisions are provided. The anchorage length is evaluated using a maximum allowable bonding stress.
l d = bf Ef £f utf . (2.22)
b '- Tmax
= H f c (2-23)
where Ld is the development length o f the FRP reinforcement; bf is the width o f the externally bonded FRP reinforcement; b is the w idth o f the concrete section; E/ is the modulus o f elasticity o f the FRP reinforcement; tf is the thickness o f the FRP sheets and/or strips; s/u is the rupture tensile strain o f the FRP reinforcement; Tmax is the bond strength o f the FRP reinforcement; f ' c is the compressive strength o f the concrete; k is a proportionality factor depends on the type o f the FRP reinforcement, modulus o f rupture o f concrete and the type o f the joint application (bending or shear). A typical value o f k o f 0.184 is recommended in the manual based on test results o f Bizindavyi and Neale, 1999.
FRP specific issues such as peeling are not addressed.
2.5.2.3 German Institute for Construction Technology
The German Institute for Construction Technology published a document in 1998, which acted as a certification for FRP products and their applications in Germany [Deutches Insitut fur Bautechnik, 1998], General provisions for flexure, shear, and anchorage were provided. Anchorage was computed through an allowable anchorage force along a critical development length. The code also recommended shear strengthening through steel stirrups in cases o f insufficient shear capacity. Delamination failures were not addressed.
___________________________________2 . S tre n g th e n in g o f R einforced C o n crete S tru c tu re s w ith FR Ps
2.5.2.4 Japan Concrete Institute and Japan Society of Civil Engineers
The Japanese Concrete Institute published “Technical Report on Fibre Reinforced Concrete, 1997”. The publication provided guidelines for evaluating the contribution o f external FRP reinforcement to the shear capacity o f retrofitted concrete systems. The Japanese Society o f Civil Engineers published JSCE 307 Subcommittee on retrofitting design. Unfortunately, the publication is available in Japanese only.
2.5.2.5 International Conference of Building Officials
The International Conference o f Building Officials published AC 125 “Acceptance Criteria for Reinforced Concrete and Unreinforced Masonry Strengthened using Fibre Reinforced Composite Systems” [ICBO ES, 1997]. In this publication, FRPs were treated as additional steel reinforcement; specific design equations were provided for flexural strength, shear strength and anchorage. Anchorage was evaluated through a maximum allowable bonding stress, which was used to evaluate a minimum laminate length.
However, FRP specific issues such as peeling were not addressed.
2.5.3 Retrofit System Manufacturers 2.5.3.1 Sika
Sika manufactures the Sika Carbodur systems and published the Sika Carbodur manual
“Sika Corporation, 1997”. The design guidelines provided detailed equations for flexural strengthening o f beams and slabs. A computational spreadsheet was provided to aid the designer. No provisions were provided for shear strengthening. However, the procedures considered both delamination and anchorage. Delamination was considered through a
___________________________________ 2 . S tre n g th e n in g of Reinforced C o n crete S tru c tu re s with FRPs
critical shear force that caused vertical slip at concrete crack tips. The manual assumed that the ratio o f vertical to horizontal displacements remained constant. The provided equation did not take into account local fracture mechanisms at the crack tip. Also the use o f laminate strains relative to a crack opening displacement for evaluation o f dowel action did not account for other processes where delamination could occur due to degradation or incompatible materials. Consequently, based on the empirical nature of this approach, its application in the field is limited.
2.5.3.2 MBrace Composite Strengthening Systems
MBrace manufactures high strength unidirectional CFRP and GFRP sheets and published
“MBrace Composite Strengthening System Design Guide, 1998”. The guide addressed strengthening o f concrete structures using externally bonded CFRP and GFRP reinforcement. The guide was made to cover various types o f strengthening that were developed and tested for use in constmction. This included flexural strengthening, shear strengthening and guidelines for improving the ductility o f compression members.
Design provisions for using the system to strengthen unreinforced, conventionally reinforced, and prestressed concrete structures were also provided. However, the guide did not provide any information regarding the delamination process.
2.5.3.3 S&P Composite Reinforcing Systems
S&P Composite Reinforcing Systems manufactures CFRP laminates and sheets as well as GFRP sheets. The design guidelines published by the company evaluated the contribution o f external FRP reinforcement to the flexural capacity o f retrofitted concrete
___________________________________ 2 . Strengthening of R einforced C o n c re te S tru c tu re s w ith FR Ps
systems. The maximum elongation limit for design o f CFRP laminates was restricted to 0.6-0.8 percent to avoid peeling o f the FRP reinforcement based on experimental results conducted by other researchers.
2.5.4 Independent Researchers
Many researchers proposed design guidelines in addition to the professional organizations and manufacturers reviewed above. The m ajority o f the guidelines followed certain procedures along the lines o f flexural and shear evaluation based on the ACI recommendations. Differences were often found in addressing anchorage and delamination issues [Malek et al., 1998]. Anchorage evaluation was addressed through a critical plate-epoxy-concrete interfacial stress value [Ziraba et al., 1994]. Furthermore, factors o f safety involved in the design procedures were proposed by Kelly et al. (2000).
However, a document outlining the use o f available design and evaluation techniques including quantitative provisions for both delamination and anchorage is yet not available.