Bridge Product Gateway
External Reinforcement Systems – Concrete Repair, Strengthening, and Seismic Retrofit

Introduction

FRP products were first used to reinforce concrete structures in the 1950s. During the next two decades, the quality of the FRP materials improved considerably, manufacturing methods became more automated and material costs decreased. The use of these materials for external reinforcement of concrete bridge structure’s started in the 1980s, first as a substitute to steel plate bonding and then as a substitute for steel confinement shells for bridge columns.

The technology for external retrofitting was developed primarily in Japan (sheet wrapping) and Europe (laminate bonding). Today there are more than 1000 concrete slab/steel girder bridges in Japan that have been strengthened with sheet bonding to the slabs. Also, many thousands of bridge columns have been seismically upgraded with the same materials. Ongoing development of cost-effective production techniques for FRP composites has progressed to the level that they are ready for the construction industry. Reduced material cost, coupled with labor savings inherent with its low weight and comparably simpler installation, relatively unlimited material length availability, and immunity to corrosion, make FRP materials an attractive solution for post strengthening, repair, seismic retrofit, and infrastructure security.

The principles behind externally bonding FRP plates or wraps to concrete structures are very similar to the principles used in application of bonded steel plates. In general, the member’s flexural, shear, or axial strength is increased or better mobilized by the external application of high tensile strength material.

Reasons for applying FRP systems as an external reinforcement for bridge structures:

• Capacity upgrade due to a change in use of a structure

• Passive confinement to improve seismic resistance

• Crack control

• Strengthening around new openings in slabs

FRP composite systems have been applied to many structural elements including beams, columns, slabs and walls as well as many special applications such as chimneys, pipes and tanks. More recently this technology has been applied to infrastructure security applications relating to hardening and blast mitigation of structures.

FRP composite systems can be categorized based on how they are delivered to the site and installed. External FRP composites systems come in a variety of forms under the general categories of 1) wet lay-up systems, and 2) precured systems. The FRP composite system and its form should be selected based on the acceptable transfer of structural loads, load capacity, and ease and simplicity of installation.  

Features and Benefits

Repair
FRP composite systems can be used to repair damaged concrete structures. The FRP is used in combination with resin crack injection, cementitous repair mortars, epoxy grouts, etc., to repair the section and restore it to pre-damaged load ratings. Repair of concrete structures caused by corroding steel rebar can be accomplished provided the corroded elements are repaired or replaced and the source of corrosion is addressed. The repair of any element in a structure must be approached as project-specific. The type of composite, the number of layers, the orientation of fibers, and the preliminary work and surface preparation all depend on the design goals and type of structural element as determined by the project.

Strengthening
FRP composite systems can be used to strengthen undamaged concrete structures that require greater load capacity due to functional changes, additional loads, code changes or other reasons. The FRP is placed on tensile surfaces in a manner similar to steel plate bonding for strengthening or embedded into saw cut grooves near the concrete surface. FRP composite systems can add shear and flexural strength to beams and slabs for both positive and negative moment conditions. Strengthening of existing concrete structural members with FRP composites is accomplished by utilizing the tensile strength and stiffness of the composite and the strain compatibility of the composite to the existing member. The design must include proper selection of the adhesive used to bond the FRP reinforcement to the surface of the concrete to be strengthened. As in repair, the type of composite, the number of layers, the orientation of fibers, and the preliminary work and surface preparation all depend on the design goals and type of structural element as determined by the project.

Seismic Retrofit
FRP composite systems have been used extensively in seismic zones for confinement of concrete columns and walls. A number of FRP systems have been qualified for use by State DOT’s for wrapping circular and rectangular bridge columns. Improvements in ductility factors of up to 10 fold have been realized through the use of FRP column wrapping. Specific FRP systems, offered by some of the manufacturers referenced below, address seismic requirements according to the load capacities anticipated and geometric considerations of the building structure. In addition, FRP systems can be used for stabilizing hollow clay tile, brick and other unreinforced and lightly reinforced masonry walls in life-safety applications where vital egress and exit paths in buildings are required.

Codes and Specifications

The designer or end-user considering the use of FRP composite systems for the repair, strengthening or seismic upgrade of existing structures should reference:

USA

• ACI 440.2R-02, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures, American Concrete Institute, 2002.

Europe

• Europe fib Bulletin 14, Externally Bonded FRP Reinforcement for RC Structures, Federation Internationale du Beton, 2001, ISSN 1562-3610.

Canada

• CSA S806-02, Design and Construction of Building Components with Fiber-Reinforced Polymers, Canadian Standards Association, May 2002, ISBN 1-55324-853-8.

Design Guide Manuals

• Strengthening Reinforced Concrete Structures with Externally Bonded Fibre-Reinforced Polymers, ISIS Canada, www.isiscanada.com
   

FRP Systems

Overview of Wet Lay-up Systems

Wet lay-up FRP systems consist of dry unidirectional or multidirectional fiber sheets or fabrics that are impregnated on-site with a saturating resin. The saturating resin is used to provide a binding matrix for the fiber and bond the sheets to the concrete surface. Wet lay-up systems are saturated with resin and cured in place and in this sense are analogous to cast-in-place concrete. Three common types of wet lay-up systems are listed below:

Products

• Dry unidirectional fiber sheets with the fiber running predominantly in one planar (0 axis) direction

• Dry multidirectional fiber sheets or fabrics with fibers oriented in at least two planar directions

• Dry fiber tows that are wound or otherwise mechanically applied to the concrete surface. The dry fiber tows are impregnated with resin during the winding operation.

Installation/Application

1. Prepare Substrate – The concrete, masonry or steel surface must be properly prepared prior to bonding. There shall be no spalling or delamination in the case of concrete and the corners must be ground to a minimum radius of 10mm (3/8 in.). The bearing substrate surface is typical roughened by grinding or sandblasting. Sandblasting is normally used on steel. Any unevenness in the concrete or masonry is removed with a mineral-based re-profiling mortar.

2. Prime Concrete Surface – Some systems require that the dust-free surface be coated with a primer.  For steel a sandblasted surface yields the best results.

3. Apply Epoxy – To saturate the sheet and simplify installation, the adhesive/matrix resin is applied to the front and back of the material. Mix and apply epoxy onto surface using a roller or brush.  Once the material has been wet-out with the liquid resin, the material may be rolled for ease of transport and/or application to the place of application.

4. Place FRP Sheet on Structure – Unroll sheet rolls onto the structural element being strengthened. Tension is maintained to minimize intrusion of air entrainment behind sheet. Placing one layer at a time, apply pressure to wrap using a roller. A hard rubber roller with ridges (air relief roller) works well for this application.

5. Apply Epoxy to Sheet Surface – A topcoat of epoxy can now be applied to the sheet to fully saturate the material. If applicable, quartz sand can be applied to topcoat prior to curing to provide a textured surface for plaster or painting.

Overview of Precured Systems

Pre-cured FRP systems consist of a wide variety of composite shapes manufactured in the system supplier’s facility and shipped to the job site. Typically, an adhesive is used to bond the precured flat sheets, rods or shapes to the concrete surface or inserted into slots cut into the wall. The adhesive used to bond the precured system to the concrete surface must be specified by the system manufacturer.  Adhesive selection is critical in that the adhesive  provides for the proper transfer of load between the surface of the concrete and the cured reinforcement.  Common types of pre-cured systems are listed below:

Products

• Precured unidirectional laminate sheets in the form of large flat plate stock or as thin ribbon strips coiled on a roll.

• Precured multidirectional grids coiled on a roll or precut in sheet form.  Grids are able to be cut easily in the field using tin-snips.

• Precured shells in the form of shell segments cut so they can be opened and fitted around columns or other elements. Multiple shell layers are bonded to the concrete and to each other to provide seismic confinement or strengthening.

Precured Grids

Grids are generally produced with glass fiber or carbon fiber and are wet-out with a resin and fully cured in the manufacturer’s factory. Grids are produced in a variety of constructions from unidirectional to a balanced construction. Grid reinforcements can be supplied with a wide range of tensile strength properties. Tensile strengths in both the warp and weft direction can be varied.  Typically grid thickness ranges from 0.01”– 0.100” and grid-opening sizes from 0.125 to 2 in. The product is typically supplied in rolls although it may be cut to length and supplied in sheet form. The product can be cut to size with a heavy-duty scissors or tin snips. The grids are produced so the degree of cure and alignment is controlled. The equipment used to produce these structural grids insures that warp and weft continuous fibers are uniformly tensioned for consistent and uniform fiber loading.

Installation/Application

Typically an adhesive is used to bond the precured flat sheets, rods or shapes to the concrete surface or inserted into slots cut into the substrate. The system manufacturer must specify the adhesive used to bond the precured system to the concrete surface.  Adhesive selection is critical for the proper transfer of load between the surface of the concrete and the precured reinforcement.  If an adhesive is not used, the system manufacturer will provide details on the method for affixing their reinforcement system to the surface of the concrete. Applying precured systems is similar to wet lay-up system.  The procedures are as follow:

1. Prepare Substrate – The concrete, masonry or steel surface must be properly prepared prior to bonding. There shall be no spalling or delamination in the case of concrete and the corners must be ground to a minimum radius of 10mm (3/8 in.) or the recommendations of the precured system manufacturer. The substrate surface is typically roughened by grinding or sandblasting. Sandblasting is normally used on steel. Any unevenness in the concrete or masonry is removed with a mineral-based re-profiling mortar. Trowelable adhesives fill small holes or uneven surfaces before applying FRP systems.

2. Prime Concrete Surface – Some systems require the dust-free surface be coated with a primer.  Follow the system manufacturers recommended procedure.  For steel, a sandblasted surface yields the best results.

3. Apply Epoxy – The majority of systems require an adhesive be applied to the surface of the concrete, masonry or steel prior to the FRP precured reinforcement being pressed into the surface of the adhesive. If this is not the case, follow the system manufacturer’s recommended procedures.  The adhesive thickness is critical to the performance of the system.  Follow the system manufacturers recommendation regarding the thickness of the adhesive to be applied. The adhesive is applied to the surface using a steel or plastic trowel or squeegee. Each adhesive has a working time at a specific temperature. This information is available from the system manufacturer.

4. Place FRP Precured material on the Structure – After the adhesive has been applied to the surface of the concrete, masonry or steel, the precured FRP material should be placed in contact with the adhesive and pressed into the surface of the adhesive following recommendations from the system manufacturer.

5. Cosmetic Topcoat– A topcoat of epoxy can now be applied to the sheet to provide a cosmetic finish. If applicable, quartz sand can be applied to the topcoat to provide a textured surface for plaster or painting.

Note: Installation/Application of procured grids follow the method described above. The surface of the structure to be strengthened will have a trowelable epoxy adhesive applied to the surface after the surface has been prepared. The grids are then pressed into the surface of the epoxy adhesive with the excess adhesive coming through the holes in the grids as the grid is wrapped around the column or onto other surfaces to give the correct number of layers to meet the design requirements. Following the completion of the application of the grids, the excess epoxy adhesive on the surface is smoothed to encapsulate the exterior of the grid structure to create the finished structural grid system to create the strengthening system.

Precured Shapes for Near Surface Mounted (NSM) Application

Pre-cured NSM rod/shape systems can generally be used as an alternative for reinforcing concrete and masonry structures similar to surface laminates. NSM Rods/shapes provide a more discrete solution to strengthening structures in that they generally are inserted into the masonry or concrete structure via slot or saw cuts. Generally, shapes can vary in size depending on application but typically are provided in round and rectangular cross-sections. The shapes are manufactured in the system supplier’s facility and shipped to the job site. The shapes generally feature a surface treatment to facilitate bond between the FRP and adhesive or grout. An epoxy adhesive or cementitious grout is used to bond the precured rods in the groove cut into the surface Adhesive selection is critical in that the adhesive provides for the proper transfer of load between the wall and the cured reinforcement.  A cosmetic surface can then be added to completely hide the strengthening system. Since the products are embedded into the substrate and bonded on three sides of the FRP shape, development lengths for NSM strengthening may be shorter.  NSM rods/shapes may also be anchored into adjacent members and the opportunity of upgrading elements in their negative moment region is opened-up, as the FRP shape is not exposed to potential mechanical damage typical of floor or deck systems.  FRP rods/shapes using the NSM technique does not require extensive surface preparation and installation time may be less than other systems.  The American Concrete Institute Committee 440 is presently reviewing modifications to ACI440.2R-02 to document design methodologies for FRP/NSM strengthening.

Installation/Application

After assessment of the condition of the existing structure and design by a competent professional, installation of the NSM FRP strengthening is performed according to the following general guide:

1. Cut Groove – Using a diamond blade saw or grinder, a groove 1.5 times the bar diameter (in the case of a rectangular FRP shape, 1.5 times the depth and 3 times the thickness) is cut as prescribed.  The use of two diamond blades on the saw arbor may be necessary.

2. Prepare Groove – The groove is masked with masking tape or similar product to prevent excess adhesive from marring the surface. The groove is thoroughly cleaned using a vacuum and/or compressed air.

3. Apply Adhesive – Structural adhesive gel or grout is filled in the groove.  Care should be taken to avoid entrapped air voids.

4. Place FRP rod/shape Into Groove – After the adhesive has been applied into the groove, the rod is placed and pressed into the groove to insure proper location of the rod/shape.

5. Finish – After the FRP rod/shape is seated into the groove, the adhesive is smoothed and any additional adhesive is added. General clean up and removal of the masking.

Concrete Repair Suppliers

American Composites Manufacturers Association   1010 North Glebe Road, Arlington, VA  22201
P: 703-525-0511  F: 703-525-0743  E: info@acmanet.org
New York Office  600 Mamaroneck Avenue, Suite 429  Harrison, NY  10528 
P: 914-381-3572   F: 914-381-1253