126 The Masterbuilder - December 2012 • www.masterbuilder.co.in

FRP Strengthening Technique for Retrofitting Concrete Structures

The population of the modern developed world depends on a complex and extensive system of infrastructure to maintain economic prosperity and quality of life. The

existing public infrastructure of developing countries as well as developed countries has suffered from decades of neglect and overuse, leading to the accelerated deterioration of bridges, buildings, municipal and transpor-tation systems, and resulting in a situation that is approaching a global infrastructure crisis. Much of our infrastructure is unsatisfactory in some respect, and public funds are not generally available for the required replacement of existing structures or construction of new ones. One of the primary factors which has led to the current unsatisfactory state of our infrastructure is corrosion of reinforcing steel inside concrete (Refer Figure 1), which causes the delamination or spalling of concrete, loss of steel reinforcement, and in some cases failure. Since infrastructure owners can no

longer afford to upgrade and replace existing structures using the same materials and methodologies as have been used in the past, they are now looking to newer technologies and rehabilitation schemes, such as non-corrosive externally-bonded FRP reinforcement, that will prolong the useful service lives of concrete structures and reduce maintenance costs.

In the last ten to fifteen years, FRP materials have emerged as promising alternative repair materials for reinforced concrete structures, and they are rapidly becoming materials of choice for strengthening and rehabilitation of concrete infrastructure. FRP plates or sheets can be bonded to the exterior of concrete structures using high-strength adhesives to provide tensile or confining reinforcement which supp lements that provided by internal reinforcing steel. In addition, FRP strips, rods, and tendons can be inserted

Special Correspondent

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with an adhesive into grooves cut in structural members in an application called near-surface mounting (NSM). FRP materials are non-corrosive and non-magnetic, and can thus be used to eliminate the corrosion problems invariably encountered with conventional repair materials such as externally-bonded steel plates. In addition, FRPs are extremely light, strong, highly versatile, and comparatively easy to install, making them ideal materials for the repair and

supplements that provided by the internal steel stirrups ( Refer Figure 3). The fibres are oriented perpendicular to the longitudinal axis of the beam.

3. Confining Reinforcement: In this application, columns are wrapped in the circumferential direction with FRP sheets ( Refer Figure 4). Under compressive axial load, the column expands (dilates) laterally and the FRP sheets develop a tensile “confining” stress that places the concrete in a state

Figure 1: Damage due to rebar corrosion of concrete structure

strengthening of concrete structures.

Common FRP-Strengthening Applications

There are currently three main applications for the use of FRPs as external reinforcement of reinforced concrete structures:

1. Flexural Strengthening: In this application, FRP materials are bonded to the tension and/or side faces of a concrete beam to provide additional tensile reinforcement and to increase the strength of the member in bending (Refer

Figure 2. Typical flexural strengthening of a reinforced concrete T-beam using externally bonded FRP reinforcement.

Figure 2). The fibers are oriented along the longitudinal axis of the beam.

2. Shear Strengthening: In this application, FRP materials are bonded to the side faces of a concrete beam (often in the form of U-wraps) to provide shear reinforcement which

Figure 3. Typical shear strengthening of a reinforced concrete T-beam using externally bonded FRP reinforcement.

Figure 4. Typical axial strengthening of a circular reinforced concrete column using an externally bonded FRP wrap.

of triaxial stress. This significantly increases the strength and ductility of the concrete and the column. The fibres are most commonly oriented perpendicular to the longitudinal axis of the member.

A Brief of FRP Material

General: FRP materials, originally developed for use in the automotive and aerospace sectors, have been considered for use as external reinforcement for concrete structures since the 1970’s. However, it is really over the last 10 years or so that FRPs have begun to see widespread use in civil engineering projects. This is due to drastic reductions in FRP material and manufacturing costs and to numerous research projects that have demonstrated their many advantages over conventional materials. Many types and shapes of FRP materials are now available in the construction industry. For the purposes of external reinforcement of concrete, there are essentially two classes of FRP materials currently available: plates and sheets. Plates are rigid FRP strips that are manufactured using a process called pultrusion. Sheet FRPs are supplied as flexible fabrics of

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raw (or pre-impregnated) fibres. The sheet FRP materials are applied by saturating the fibres with an epoxy resin and

Installation Techniques: Although a variety of techniques can be used to apply external FRP reinforcement to reinforced concrete structures, two similar techniques are most widely used. The first of these is referred to as wet lay-up. In this technique, flexible sheets or fabrics of raw or preimpregnated fibres are saturated with an epoxy adhesive resin and placed on the surface of the concrete. As a result, the resin acts both as the adhesive and as the FRP matrix. The second technique involves the adhesion of pre-cured rigid FRP strips or plates to the surface of the concrete

Figure 5: Various available shapes of FRP

laying-up the sheets onto the concrete surface. In both of the above applications, the FRP materials used are usually unidirectional (with all fibres oriented along the length of the sheet). Refer Figure 5 shows various types and shapes of currently available FRP materials.

Constituents: FRP materials are composed of high-strength fibres embedded in a polymeric matrix. The fibres, which have very small diameters and are considered continuous in practice, provide the strength and stiffness of the composite, while the matrix, which has comparatively poor mechanical properties, separates and disperses the fibres. The primary function of the matrix is to transfer loads to the fibres through shear stresses that develop at the fibre-matrix interface, although the matrix is also important

Figure 6. Stress-strain relationships for fibres, matrix, and FRP.

for environmental protection of the fibres. In concrete strengthening applications, the fibres are typically carbon (graphite), glass, or aramid (KevlarTM), and the matrices are typically epoxies. Figure 6 shows typical stress-strain curves for fibres, matrices, and the FRP materials that result from the combination of fibres and matrix.

Figure 7. Carbon FRP sheets applied as external shear U-wrap reinforcement for a concrete bridge girder using the wet lay-up procedure.

using an epoxy adhesive. In this technique the adhesive does not become the matrix for the FRP, so a well-defined bond-line is created. The second technique is more akin to conventional rehabilitation techniques using steel plates, but does not offer the flexibility enjoyed by the wet lay-up procedure. Both procedures have been used with success in field applications (Refer Figure 7 and 8).

Properties: Unidirectional FRP materials used in external

Figure 8. Carbon FRP strips applied as external flexural reinforcement for a concrete bridge girder. Light bands are glass FRP U-wraps applied as external shear reinforcement over the flexural reinforcement.

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strengthening applications are typically linear elastic up to failure, and do not exhibit the yielding behaviour that is displayed by conventional reinforcing steel. This is shown in Figure. 2-5, which demonstrates the significant differences in the tensile behaviour of FRPs as compared with steel. FRP materials generally have much higher strengths than the yield strength of steel, although they do not yield, and have strains at failure that are often considerably less than steel. The specific properties of different FRP materials vary a great deal from one manufacturer to another and depend on the fibre and matrix type, the fibre volume content, and

the orientation of the fibres within the matrix, among other factors. It is beyond the scope of this module to discuss different FRP reinforcing materials in detail. However, Table 1 and Figure 9 give material properties for a number of typical currently available FRP strengthening systems.

Advantages: FRP materials for use in concrete strengthening applications have a number of key advantages over conventional repair materials such as steel. In many cases, these advantages make FRPs the only possible solution to a strengthening problem. Some of the most important advantages include:

1. FRP materials do not corrode electrochemically, and have demonstrated excellent durability in a number of harsh environmental conditions;

2. FRP materials have extremely high strength-to weight ratios. FRP materials typically weigh less than one fifth the weight of steel, with tensile strengths that can be as much as 8 to 10 times as high;

3. FRP materials are extremely versatile and are quickly and easily installed. This reduces the downtime required for repair and offsets indirect repair costs; and

4. FRP materials are electromagnetically inert. This means that they can be used in specialized structures such as buildings to house magnetic resonance imaging (MRI) or sensitive communications equipment, etc.

FRP System Fiber Type Weight [g/cm2] Thickness [mm]Tensile Strength

[Mpa]Tensile Elastic Modulus [GPa]

Strain at Failure [%]

Fyfe Co. LLC [www.fyfeco.com]

Tyfo SHE-51 Glass 930 1.3 575 26.1 2.2

Tyfo SCH-35 Carbon - 0.89 991 78.6 1.3

Mitsubishi[www.mitsubishichemical.com]

Replak 20 Carbon 200 0.11 3400 230 1.5

Replak 30 Carbon 300 0.17 3400 230 1.5

Replak MM Carbon - 0.17 2900 390 0.7

Replak HM Carbon 200 0.14 1900 640 0.3

Sika [www.sikacanada.com]

Hex 100G Glass 913 1.0 600 26.1 2.2

Hex 103C Carbon 618 1.0 960 73.1 1.3

CarboDur S Carbon 2240 1.2-1.4 2800 165 1.7

CarboDur M Carbon 2240 1.2 2400 210 1.2

CarboDur H Carbon 2240 1.2 1300 300 0.5

Watson Bowman Acme [www.wabocorp.com]

MBrace EG 900 Glass 900 0.35 1517 72.4 2.1

MBrace CF 530 Carbon 300 0.17 3500 373 0.94

MBrace AK 60 Aramid 600 0.28 2000 120 1.6

*Additional information can be obtained from the specific FRP manufactures Table 1. Selected Properties of Typical Currently Available FRP Strengthening Systems*

Figure 9. Stress-strain plots for various FRP strengthening systems

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Evaluation of Existing Structures

Causes of Deficiencies: Deficiencies in existing concrete structures can be due to a variety of factors. Some of the more common factors that have contributed to the deterioration of concrete infrastructure:

1. Environmental effects on structural behaviour, which include chloride-induced corrosion of conventional reinforcing steel in concrete, freeze-thaw cycling, and wet-dry cycling (all of which can contribute to cracking and deterioration);

2. Evolution of design loads, such that structures cannot safely carry loads required by updated versions of design codes;

3. Evolution of design guidelines from working stress (older design methodology) to limit states (current design methodology), such that structures designed using older procedures are inadequate when evaluated under current guidelines; and

4. Increase of traffic and loads due to more cars and heavier trucks on roads and highways.

Evaluation: Obviously, a major component of any structural rehabilitation, upgrade, or strengthening project is concerned with evaluation of the existing structure. To develop an appropriate FRP strengthening strategy, an assessment of the existing structure should first be conducted to determine the condition of the concrete, to identify the causes of deficiencies, to establish the existing load carrying capacity of the structure, and to evaluate the feasibility of using externally-bonded FRP systems for repair. The evaluation of an existing structure should be carried out with extreme care, and should be concerned with the following information:

1. The as-built drawings including all past modifications; 2. The actual size of the concrete elements; 3. The actual properties of the existing materials including 4. The surface tensile strength of the concrete; 5. The location, size, and cause of cracks and spalls; 6. The location and extent of any corrosion of the reinforcing

steel; 7. The quantity and location of the existing reinforcing

steel; and 8. An appropriate evaluation of the applied loads.

Failure Modes: There are four potential flexural failure modes for externally-strengthened reinforced concrete flexural members:

1. Concrete crushing before yielding of the reinforcing steel; 2. Steel yielding followed by concrete crushing; 3. Steel yielding followed by FRP rupture; and 4. Debonding of the FRP reinforcement at the FRP/concrete

interface.

It is not always clear at the outset of a design or analysis which of the above failure modes will govern. Thus, an assumption must be made and the failure mode checked. If the assumption is incorrect, a different failure mode is assumed and the analysis is repeated. In the current discussion it is assumed that the fourth failure mode, FRP debonding, will not occur and can be ignored (in practice this assumption is assured through the use of specialized anchorage techniques).

An example of Column Strengthening Using FRP

Existing concrete columns under pure compressive loads can be strengthened by externally-bonded FRP wraps by wrapping the columns in the circumferential direction (refer to Figure 10). When the column is subjected to axial load, it shortens longitudinally but dilates (expands) laterally. This dilation causes tensile stress to develop in the FRP wrap, and this tensile stress confines the concrete and places it in a state of triaxial (3-dimensional) stress. The result of this stress condition is that both the load capacity and deformation capability of the concrete in the column are significantly improved, leading to stronger and more ductile structural members (Refer Figure 11). The design of concrete columns strengthened with externally-bonded FRP wraps is performed using empirical equations derived primarily from test data. The applicability of the procedures presented herein is currently limited to the following types of members and load conditions:

Figure 10. Schematic showing confinement mechanism for axial strengthening of circular reinforced concrete columns using externally-bonded FRP wraps.

Figure 11. Comparison of stress-strain behaviour of circular and rectangular FRP-wrapped and unwrapped concrete columns.

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1. Strengthening of relatively undamaged concrete columns;2. Strengthening of short columns subjected to concentric

axial loading; and 3. Fibres oriented perpendicular to the column axis

(circumferentially).

Specifications and Quality Control

Although the strengthening of existing structures using externally-bonded FRP materials is a relatively simple technique, the proper installation of the FRPs is essential to ensure the adequate performance of the repaired structure. Since the installation procedures differ from one system to another, appropriate specifications for the external reinforcement of a specific structure should be clearly defined. The project specifications should include requirements to provide a quality assurance plan for the preparation of the construction site, as well as for the installation and curing of all FRP material systems. The following items all require detailed and specific consideration in any FRP strengthening project:

Specifications

- Approval of FRP materials;- Handling and storage of FRP materials;- Staff and contractor qualifications;- Concrete surface preparation;- Installation of FRP systems;- Adequate conditions for FRP cure; and- Protection and finishing for the FRP system.

Quality Control and Quality Assurance

- Material qualification and acceptance;- Qualification of contractor personnel;- Inspection of concrete substrate;- FRP material inspection; and- Testing to ensure as-built condition.

A Few Case Studies of Field Applications

Maryland Bridge

The City of Winnipeg implemented a trial application of carbon fibre reinforced polymer sheets as a first step in upgrading the shear capacity of the Maryland Street Bridge in Winnipeg, Manitoba. The twin five-span continuous precast prestressed concrete structures were designed and constructed in 1969. However, analysis using current codes indicated that the shear strength of the I-shaped girders was not sufficient to withstand today’s increased truck loads. Four girders were strengthened using FRP sheets which were placed vertically (with a horizontal layer placed across the top and bottom of the web for anchorage). Horizontal and vertical strain gauges were applied so that the structure could be monitored on an ongoing basis. (Refer Figure 12)

John Hart Bridge

In one of the largest strengthening projects of its kind, carbon fibre reinforced polymer (CFRP) sheets have been used to upgrade the shear capacity of the John Hart Bridge in Prince George, British Columbia. The bridge required shear strengthening in order to support heavier truck loads. It consists of seven simply supported spans with six prestressed concrete girders per span. The 42 girders are each 1500 mm deep with a typical I-shaped cross-section.

Figure 12. One of the Maryland Bridge’s girders after being strengthened for shear using carbon FRP sheets. The FRP was subsequently painted to give it the appearance of concrete.

Figure 13. The John Hart bridge after strengthening for shear using externally-bonded carbon FRP sheets. Light gray sections are locations where FRP reinforcement was installed.

The girders were strengthened with FRP sheets covering a four metre length at each end. By strengthening 64 girder ends, the shear capacity or the bridge was increased by between 15 and 20 percent. The project was completed in six weeks, during which time the bridge remained completely accessible to traffic. (Refer Figure 13).

Country Hill Boulevard Bridge

In 1996, a material testing program at the University of Calgary began to review the strengthening effects of carbon fibre reinforced polymer (CFRP) strips on existing bridge beams. In 1997, the City of Calgary decided to strengthen the Country Hills Boulevard Bridge over the Deerfoot Trail

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in north-east Calgary. One of the main problems with the bridge was that its thin deck would be over-stressed in lateral bending under truck loading due to the skew angles at the abutments and pier. Conventional strengthening with the addition of steel reinforcement would have required breaking the deck into strips, adding reinforcement, and re-concreting each strip. It was important to have one lane open at all times during construction and it was feared that that failure could occur. To avoid this problem and to strengthen the deck in a nondestructive way, strengthening with CFRP strips was chosen. Carbon FRP strips were installed in eight areas of the slab found to be in need of strengthening. Strips were bonded to the top surface of the bridge deck to provide additional strength in negative bending. Carbon FRP strips were particularly useful for this application, since the bridge’s asphalt wearing surface could be reinstalled over the strengthening materials. (Refer Figure 8.3)

strength, lightweight and corrosion resistance characteristics of FRP make it ideal for retrofitting applications. Retrofitting of reinforced concrete (RC) structures by bonding external steel and FRP plates or sheets is an effective method for improving structural performance under both service and ultimate load conditions. It is both environmentally and economically preferable to repair or strengthen structures rather than to replace them totally. With the development of structurally effective adhesives, there have been marked increases in strengthening using steel plates and FRP laminates. FRP has become increasingly attractive compared to steel plates due to its advantageous low weight, high stiffness and strength to weight ratio, corrosion resistance, lower maintenance costs and faster installation time. The research in usage of FRP is in advanced stage in all the countires and codes will incorporate the results too. Nevertheless FRP is a magical innovation in restoring all the important structures to use by the way of retrofitting and renovation and every nation is making use of this.

Reference

- Jackson DR, Islam M, Hurley FJ, Alvarez FJ. Feasibility of evaluating fiber reinforced plastic (FRP) wrapped reinforced concrete columns using ground penetrating radar (GPR) and infrared (IR) thermography. Demonstration Project No. 84-2. Washington, DC: US Department of Transportation, Federal Highway Administration; 1999.

- Kaiser H, Karbhari VM. Non-destructive testing techniques for FRP rehabilitated concrete. I. A critical review. Int J Mater Product Technol 2004;21(5):349–84.

- Kaiser H, Karbhari VM, Sikorsky C. Non-destructive testing techniques for FRP rehabilitated concrete. II. Assessment. Int J Mater Product Technol 2004;21(5):385–401.

- Karbhari VM, Sikorsky C, Lee LS. Field monitoring and degradation assessment of FRP bridge rehabilitation using level IV NDE techniques. In: Proceedings of the structural health monitoring workshop, SHM/ISIS, Winnipeg, Canada, September, 2002.

- Lee LS, Sikorsky C, Karbhari VM. Remaining service life of FRP rehabilitated structures. In: Proceedings of the 60th SAMPE technical conference, Anaheim, CA, May 2004. 13pp.

- AASHTO LRFD Bridge Design Specifications. American Association of State Highway and Transportation Officials, 3rd ed., 2004.

- ACI Committee 318. Building code requirements for reinforced concrete (ACI 318-99) and commentary (ACI 318 R-99). Farmington Hills, Michigan: American Concrete Institute; 1999. p. 391.

- www.isiscanada.com- ACI Committee 440. Guide for the design and construction

of externally bonded FRP systems for strengthening concrete structures. ACI 440.2R-02. Farmington Hills, Michigan: American Concrete Institute; 2002. p. 44.

- California Bridge Design Specification. California Department of Transportation, 2004.

- Toutanji H, Zhao L, Zhang Y. Flexural behaviour of reinforced concrete beams externally strengthened with CFRP sheets bonded with an inorganic matrix. Eng Struct 2006;28(March):557–66.

- Kachlakev D, McCurry DD. Behavior of full-scale reinforced concrete beams retrofitted for shear and flexural with FRP laminates. Composites 2000;31:445–52.

Figure 14. Strengthening the Country Hills Boulevard Bridge with externally-bonded carbon FRP strips for flexure in negative bending.

Refer Figure 15 for retrofitting of a bridge deck using FRP.

Conclusion

Rapid deterioration of infrastructure became a principal challenge facing all nations worldwide. Both serviceability and ultimate load carrying capacity of most of the existing concrete structures and bridges became inadequate to meet the users demand. An attractive solution is strengthening of the existing structures using FRP materials. High tensile

Figure 15. Rehabilitation of bridge deck using externally bonded FRP. (a) Adhesively bonded prefabricated strips and (b) unidirectional fabric impregnated and bonded using the wet layup process.

Retrofitting FRP