Fiber Reinforced Polymer (FRP) for the Repair & Retrofit of Existing Structures & for New Construction

List of Tables Table 1: Material Properties of Glass Fibers Federal Highway Administration 12 Table 2: Material Properties of Amarid Fibers Federal Highway Administration 12 Table 3: Mat

ALBERT NERKEN SCHOOL OF ENGINEERING

Fiber Reinforced Polymer (FRP) for the Repair & Retrofit of Existing Structures

& for New Construction

A thesis submitted in partial fulfillment

of the requirements for the degree of

Master of Engineering

Jessica Galbo, E.I.T

May 8,2012

Professor Jameel Ahmad, Ph.D

Chairman of Civil Engineering at Cooper Union

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THE COOPER UNION FOR THE ADVANCEMENT OF SCIENCE AND ART

ALBERT NERKEN SCHOOL OF ENGINEERING

This thesis was prepared under the direction of the Candidate's Thesis Advisor and has

received approval It was submitted to the Dean of the School of Engineering and the

full Faculty, and was approved as partial fulfillment of the requirements for the degree

of Master of Engineering

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Chairman of Civil Engineering at Cooper Union

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Abstract

Fiber reinforced plastic and fiber reinforced polymer (FRP) materials are becoming more widely used and accepted in the repair and retrofit of existing structures, and have limited applications for new construction This thesis identifies FRP's characteristics, including its properties and behavior, its current applications, how to design with FRP, and research being done for FRP's further development

FRP itself is a composite material made of fibers and resins which was first developed in the 1930s The fibers provide structural strength, and the resins help to distribute forces within the FRP and protect the system from moisture and corrosion

FRP can be used for retrofit, rehabilitation, and repair of existing structures, or in new construction Structures are most often retrofit with FRP using externally bonded FRP laminates to provide flexural strength, shear strength, or confinement for service loading and seismic loading FRP can also be applied externally to prevent areas prone to corrosion or environmental damage because it is air tight, waterproof, and corrosion-resistant FRP can be used as a composite with concrete internally in the form of FRP reinforcing bars for new construction These sections will not corrode or spall due to the noncorrosive properties of FRP compared to conventional reinforcing steel FRP can also be molded or pultruded into sections used as composites with concrete, or sections made exclusively of FRP These sections have never been used for large scale projects, and typically have been constructed for research purposes by various state departments of transportation

Case studies of several types of FRP applications are presented and investigated in this thesis to determine its anticipated benefits and limitations, and conclusions and recommendations are presented, regarding the use of FRP materials

3.0 FRP for Retrofit, Rehabilitation, & Repair of Existing Structures 16

List of Tables

Table 1: Material Properties of Glass Fibers (Federal Highway Administration) 12 Table 2: Material Properties of Amarid Fibers (Federal Highway Administration) 12 Table 3: Material Properties of Carbon Fibers (Federal Highway Administration) 13

List of Figures

Figure 1: Stress-Strain Diagrams for Fibers (Federal Highway Administration) 13 Figure 2: Industrial Plant with FRP Strips for Flexural Strength 21 Figure 3: Photos of Peaks and Valleys of Roofing with FRP Strips 21 Figure 4: FRP Plate Strengthening of Church Street Bridge Pier Cap 22 Figure 5: Photos of Typical Rebar Arrangements in Columns (Left) and Beams (Right) 23 Figure 6: KY3297 Bridge - FRP Strips for Shear Strengthening 28 Figure 7: Challenger Middle School Footbridge: FRP U Wrap for Shear Strengthening 29 Figure 8: Mander et al - Relationship Between Confining Stresses & Axial Strength 35 Figure 9: McKinley Tower: Conventional Repair vs Equivalent FRP Wrapping Repair 38 Figure 10: Parking Garage Column FRP Wrapping for Axial Strength Increase 38 Figure 11:1-40 Bridge FRP Wrap for Corrosion Protection 43 Figure 12: FRP Rebar Arrangement - Longitudinal and Shear Rebar 44 Figure 13: Moment Capacity of Beam considering Rebar Development Lengths (23) 46 Figure 14: 53rd Avenue Bridge FRP Deck Reinforcing Bars 49

Figure 18: Construction of Concrete Filled FRP Tubes and FRP decking Neil Bridge 54

Figure 21: Mile High Road Bridge constructed with Hybrid Composite Beam Superstructure 56 Figure 22: U Shape FRP and Concrete Beams Installed in Refugio TX byTxDOT 57

Figure 24: Load Testing of FRP Bridge at Fort Bragg with Ml AbramsTank 58 Figure 25: FRP Bridge Substructure Installed at Fort Eustis 59

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1.0 Introduction

1.1 Statement of Problem

The average age of bridges in the United States is 43 years, many of which were built anticipating a design life of 50 years whom are approaching the end of their design lives When the 1-35 Bridge in Minneapolis collapsed in 2007, it caused alarm for all aging infrastructure and created

an awareness of the deteriorated condition of infrastructure in the US Regular inspections must be made to assess the conditions of bridges, and maintenance must be performed when structural integrity may become compromised The conditions of bridges in the United States, as of 2009, were given a Report Card rating of "C" by the American Society of Civil Engineers (ASCE); 26% of bridges are considered structurally deficient or functionally obsolete The ASCE has called for a balance between immediate repairs, preventative measures, repair/retrofit of deficient bridges, and replacement when necessary to keep bridges in a good state, and to maximize their lifespans When proper measures are taken to construct infrastructure with long lifespans, and proper maintenance occurs over the bridge's lifespan the overall cost, called a "lifecycle cost" will be minimized This means that sometimes the upfront investment cost is higher during construction, but over the structure's life it will perform better and have a significantly extended life, requiring fewer replacements and large retrofit measures, and have lower total costs (5)* Repair, rehabilitation, and construction must be done in cost effective ways, which will become even more demanding as much of the country's infrastructure reaches the end of their intended design lives This calls for the full utilization of advances in technology, and new materials, such as Fiber Reinforced Polymers (FRP)

One strategy to optimize the use of available funding to maintain infrastructure is to only spend money when it's absolutely necessary Find and use the lowest cost designs and materials to

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minimize upfront investments, and to only perform repairs when absolutely necessary But this strategy is not proactive or preventative; it is retroactive, and typically results in high lifecycle costs

Another strategy to construct and maintain infrastructure is to make consistent investments, whenever necessary This means choosing a high upfront cost option for construction when it is anticipated the benefits will make the structure more durable, and the invested money will be regained due to lower maintenance costs, and a longer lifespan Consistent maintenance is to

be performed to find and eliminate problems before they advance to a state that is not easily repaired which can compromise the structure and significantly decrease its intended lifespan

Measures should be taken to increase the lifespan of infrastructure in order to:

• maintain the safety and functionality of our current infrastructure,

• reduce life-cycle construction costs,

• protect the environment from harmful construction byproducts including the release of carbon dioxide into the atmosphere from cement mixing, and to preserve raw materials

One method by which infrastructure's lifespans can be increased is through the utilization of FRP materials FRP can increase the lifespans of existing concrete and steel infrastructure by providing structural upgrade where necessary and providing protection from sources of deterioration

FRP materials are versatile, as they can be used to repair existing infrastructure experiencing problems with deterioration, rehabilitate overstressed structures, and for new construction FRP is extremely durable due to its material properties, not susceptible to corrosion, and longer lasting with little required maintenance if installed correctly FRP is lightweight, making it ideal for rehabilitation projects on existing structures, for which any additional loads can cause overstresses; therefore FRP can contribute significant strength without increasing structural loads and demands

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on the structure Because of FRP's durability, it is ideal for use in new construction, whose lifespan will require fewer large maintenance and repair projects due to reduced susceptibility to severe deterioration

Despite knowledge amongst engineers of FRP's existence and potential benefits, it is not commonly used, and has not earned acceptance by many agencies or contractors for regular use in construction This thesis is intended to provide insight to the engineer on not just the benefits of FRP, but how it works, how to design with it, and examples of its use in earlier projects With this knowledge, engineers can be more informed of the appropriateness of FRP for use in projects and can make recommendations to clients and contractors in situations where FRP would be beneficial

to a project

1.2 Purpose of Thesis

Fiber reinforced plastic and fiber reinforced polymer (FRP) materials are becoming more widely used and accepted in the repair and retrofit of existing structures, and have limited applications for new construction This thesis identifies FRP's characteristics, including its properties and behavior, its current applications, how to design with FRP, and research being done for FRP's further development Case studies of its uses will be presented and investigated to determine its anticipated benefits and limitations, from which conclusions will be drawn and recommendations presented

2.0 FRP Applications, Composition, and Properties

The growing popularity of FRP products led to better product development, including new vinyl and epoxy resins, and new fibers made of materials such as amarid and carbon with different strength properties Improvements were made in glass refinement techniques, and strand quality increased Fibers were also being produced in continuous mats with both uni-directional and multi­directional fiber weaves Epoxy resins offer superior quality, and better corrosion and weathering resistance Epoxy resins were expensive, so they were not first used for the boating industry, but rather in the aerospace industry Carbon fibers were first developed in the UK Royal Aircraft Establishment, in 1963 Eventually FRP materials were used by the boating, aircraft, and automobile industries, because it allowed for lightweight, corrosion resistant products Early products constructed of FRP materials were considerably over designed, due to uncertainties with the material's strength Now FRP composites are used to make all types of boats, aircrafts, automobiles, and consumer products such as tennis rackets, fishing rods, ladders, and bathtubs

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FRP was first used in infrastructure in the early 1980s in Europe and Asia, and was later used in the United States in the 1990s FRP was considered for use due to observed problems with conventional reinforcing steel and concrete systems, where corroded steel could compromise the structural system FRP was considered as an alternative that could eliminate the corrosion problems which deteriorate infrastructure Some of the early projects using FRP included FRP bridge decks, and bridges stressed with FRP tendons Since then FRP has been used on existing structures to provide seismic upgrade to bridges and buildings, increase strength, and provide resistance to corrosion and chemicals when applied in the form of sheets (16)

2.2 Composition and Properties

The two major components of FRP are fibers and resins The fibers provide strength to the FRP material, and the resins transfer stresses between the individual fibers, and provide chemical resistance and anti-corrosive properties of FRP

2.2.1 Fibers

Fibers provide structural strength to the FRP material The fibers are individual strands which have anisotropic material properties, in contrast to concrete and steel which are isotropic Isotropic materials have the same material properties independent of their orientation, and anisotropic materials have material properties which vary across its axes Fibers' strength properties exist in the direction along its strands These fibers are woven into different fabric patterns, which can provide strength in a direction parallel to the length of the fabric, perpendicular to the length of the fabric, or components in both directions by changing the direction of the fibers within the fabric The most commonly used fibers are glass, carbon, or amarid

Glass fibers are the least expensive of the three materials, and carbon is the most expensive Glass fibers come in three different classes: E-glass, S-glass, and C-glass E-glass is most commonly used in infrastructure projects Glass fibers have been tested and shown to creep under sustained

loads, but it can be designed in a way to prevent this from occurring to a level which may become problematic Glass fibers have the lowest strength and lowest elongation capacities, and for those reasons it is typically used for corrosion prevention rather than strength enhancements of existing structures It is made from raw materials that are plentiful, most notably sand, and therefore is a sustainable product

Table 1: Material Properties of Glass Fibers (Federal Highway Administration)

Amarid fibers have high tensile strength and elastic modulus, excellent creep and shrinkage resistance, and a lower density than glass or carbon fibers Because of their elongation capabilities they are good for impact They are made synthetically, and extremely resistant to heat, fire, chemicals, and corrosion

Table 2: Material Properties of Amarid Fibers (Federal Highway Administration)

Carbon fibers have very high fatigue and creep resistance, and lower thermal expansion coefficient than glass or amarid fibers It is more brittle than glass or amarid Usually high strength carbon fibers are used for structural strengthening for flexure and shear (11)

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Table 3: Material Properties of Carbon Fibers (Federal Highway Administration)

Density (g/cm3)

Young's Modulus (GPa)

Tensile Strength (GPa)

Tensile Elongation (%)

1.8

230 2.48 1.1

1.9

370 1.79 0.5

2.0 - 2.1

520 - 620 1.03 - 1.31

0.2

All of these fibers exhibit linear elastic behavior until their ultimate strength is reached This means that the material will behave elastically and then experience sudden brittle failure FRP is not ductile like steel and does not behave plastically For this reason, most designs use allowable strengths that are considerably lower than the ultimate strength of the fibers, but considering their extremely high strengths, as high as 600 ksi, they are up to ten times stronger than a comparable steel section Therefore despite its brittle failure mechanism, FRP can still be a reliable design and construction material due to its high strength, and ultimate strength which is much higher than that

of steel's plastic strength When strength is desired more than deformation capacity, then a carbon fiber can be used due to its high strength and lower ultimate strain capacity When the deformation capacity is desired as much as or more than the strength, such can be the case with seismic retrofitting, an E-glass fiber can be used (11)

2.2.2 Resins

Fibers are saturated in resins Resins help transfer stresses between the fibers, and protect the fibers from damage such as UV rays, moisture, and chemical exposure Two main types of resins are thermosets and thermoplastics

Thermoplastic resins are flowable when heated, and harden upon cooling Thermoset resins cure from being heated, from being part of a two part system with a catalyst reaction, or both For those reasons thermoset resins are used for composite materials Different thermosets appropriate for use in FRP are polyesters, phenolics, and epoxies

provide excellent corrosion resistance, but are more expensive than polyester resins (27)

Epoxy Resins

These resins generally have larger tensile strengths, greater bond strengths, and better resistance to corrosion than Polyester Resins They cure by a two part system, a resin and a hardening catalyst Once these are mixed together the mixture creates heat which begins the curing

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process Epoxy resins generally take up to a full week to completely cure while Polyester resins will cue within 6-8 hours Epoxy resins will absorb less water than polyester resins For reasons discussed above, Epoxy resins have superior characteristics to Polyester Resins, but cost more (12)

3.0 FRP for Retrofit, Rehabilitation, & Repair of Existing Structures

FRP is a structural material which can be designed to provide reinforcement of sections for flexure, shear, and confinement Generally, concrete and steel can also be used to encase or reinforce underperforming elements, but sometimes the additional weight of these materials causes additional overstresses in the element being repaired or in the structure's foundation FRP is extremely lightweight compared to concrete and steel It can reinforce overstressed elements without adding additional considerable load to a structure, allowing for minimal impacts to the existing structure's stability Additionally, when an existing structure is prone to deterioration, an FRP wrap used in conjunction with conventional repair methods can stop and prevent deterioration from occurring within the section

3.1 FRP for Tensile Reinforcement

FRP strips can be attached to the horizontal faces of flexural beams in order to provide additional tensile capacity In order to understand the mechanics of an FRP system, first the flexural behavior of steel beams, and conventionally reinforced concrete beams must be understood

3.1.1 Concept

When a moment is induced in a beam, across that beam's cross section both compressive and tensile stresses are induced The total induced tension equals the total induced compression For beams of uniform material, the stress at any given depth of the beam's cross section can be calculated using the formula

the extreme fibers of the beam do not exceed their yield strength, therefore the beam still behaves elastically

With beam of a single material with a cross section that is symmetrical across its bending axis, this results in a neutral axis at the beam's centerline, and equivalent compressive and tensile stresses and strains in either side of the beam's cross section Ductile materials like steel behave well in tension and compression, as long as the possibility of buckling is prevented through the design of adequate stiffeners spaced along the beam's length A typical yield strength of steel is 60 kips per square inch, with some higher strength steels having yield strengths of 70 kips per square inch Brittle materials like concrete behave well in compression but have very limited tensile capacity, and do not yield and instead crack Concrete's maximum tensile stress is known as the force of rupture, and is calculated, per 2010 AASHTO LRFD code as

fr = 7.5 4Tc

Where fc = the compressive strength of the concrete, in pounds per square inch Concretes can have

variable strengths depending on mix proportions, but typical concrete strengths used in cast in place structural construction are 3000 pounds per square inch (psi) to 4000 psi For example, a 4000 psi concrete has a force at rupture of only 150 psi Considering the limited available tensile strength of the concrete as compared to its compressive strength (150 psi, vs 3000 psi) concrete beams designed for flexure contain flexural reinforcement, typically steel Important features of this steel are its high tensile capacity, and its ductile properties to prevent brittle failure of the beam An engineer designs this steel to provide the beam's entire tensile capacity, without relying on the concrete for tensile strength as it is a brittle material with very limited capacity in tension

The flexure in a reinforced concrete beam is allowed by the balance of forces across the cross section, with the total tension forces provided by the steel being completely counteracted by

the compression of concrete on the other side of the beam's neutral axis The moment capacity of the cross section is calculated by multiplying the total available tensile capacity of the steel in the beam (As*fy), by the distance between the centerline of the reinforcing steel, and the centerline of

the compression zone in the concrete (d - a/2) This number is usually factored, and as dictated by current AASHTO LRFD codes, is multiplied by a factor of 0.9 for the beam's nominal moment capacity

Illustrative Example: Flexural Capacity of Conventionally Reinforced Concrete Section

h = total section depth; d = section depth minus the concrete covering the steel; fy = yield strength

of steel; b = width of base; As= area of steel

h = 24" Tall beam, d = 21"; b = 18"; f Y = 60 ksi; f c = 4 ksi; As = 8 in 2

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the tension side of the beam The panels are procured with resins, and are attached to a surface with adhesives or mechanical anchors such as bolts The tensile capacities of the panels and strips are accounted for considering their strength per layer of fiber (2)

Illustrative Example of a Concrete Beam with Steel Reinforcing bars and FRP Strips

n = number of layers of FRP laminate; fL = strength of FRP laminate; tL = thickness of one FRP

laminate layer; wL = width of applied laminate

h = 24" Tall beam, d = 21"; b = 18"; f Y = 60 ksi; f c = 4 ksi; As = 8 in 2 ; n = 1; f L = 350 ksi; t L = 1/16"; w L = b

3.1.3 Construction and Application Methods

The existing surface must be cleaned and repaired before any FRP is applied For steel, the surface shall be blast clean For concrete, any contamination and corrosion must be removed and

repaired before application of FRP This will remove and replace concrete and steel components which can experience volumetric changes due to corrosion or alkali silica reactions By mitigating these volumetric changes, debonding of FRP from the beam's surface is prevented

Once the existing beams have been sufficiently repaired typically a layer of adhesive is applied to the surface, and then FRP can be installed With FRP strips, the strips are typically delivered to site impregnated with resins, or are impregnated with resins on site, also called "wet layup" and then applied to the surface Precast and cured panels are applied, and typically also bolted or anchored in place to ensure adequate bond to the substrate The exposed FRP strips and panels should then have a UV protective coating applied, as FRP can degrade slowly with UV exposure After that application a layer of paint can be applied to the surface of the FRP to help it blend in with the adjacent materials such as steel or concrete

3.1.4 FRP for Flexure Case Studies

In 2002 in Denver Colorado a fire damaged the roof of an industrial plant The fire damage resulted in the permanent loss of strength in the concrete roofs reinforcing steel In order to repair the roof, the damaged cover concrete was removed using hydro-demolition, and was replaced using shotcrete The roofs existing condition was assessed using SAP finite element modeling, and the flexural deficiencies in the roof were determined FRP sheets were applied where negative moment reinforcing was required on the peaks of the roofs, and where positive moment reinforcing was required on the underside of the valleys of the roofs.(18)

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Figure 2: Industrial Plant with FRP Strips for Flexural Strength

Figure 3: Photos of Peaks and Valleys of Roofing with FRP Strips

East Church Street Bridge NYSDOT Cap Beam Strengthening Study

East Church Street Bridge in Elmira, NY was built in 1954 It is a four span bridge which carries two lanes In 1997 during an inspection it was found that the cap beams were exhibiting cracking This cracking is due to additional dead loads the structure has been loaded with since construction The deck now has an overlay, and concrete barriers whose dead loads were not accounted for in the bridge's original design NYSDOT used FRP plates to strengthen the structurally deficient cap beam of its third pier for an evaluation of its performance and cost effectiveness On one side of the cap beam, plates were attached using epoxy layer and drilled anchor bolts into the concrete cap beam On the other side of the cap beam the panels were attached using the same epoxy layer, and plates were applied and clamped to the FRP and concrete to ensure contact during

curing After they were installed they got an application of tar coat to prevent water seepage between the plates and concrete, and a layer of paint to resemble concrete and protect the FRP from UV radiation

After construction was completed, the structure was load tested, and the capacities of the cap beams were found to have decreased the service load stresses in the negative moment reinforcing steel by 10% and the positive moment steel by 6% This study proved that FRP can be a cost effective solution ($18,000) compared to conventional methods ($150,000), which can also be easily installed, requiring limited lane closures (25)

est Shear pj

St|el girder (Typ.) Bridge deck East

Positive moment region plate (Typ.) region plate Negative moment

3.2 FRP Wrapping for Shear Reinforcement

Typical reinforced concrete column and beam design includes longitudinal steel reinforcing This longitudinal steel acts compositely with the concrete to provide flexural capacity to the section,

as previously discussed in section 3.1, FRP for Flexure

Reinforced concrete section detailing also includes transverse reinforcing This transverse reinforcing can be in the form of hoops or continuous spiral reinforcing in circular cross sections in columns In rectangular sections, such as beams, the confinement is typically stirrups For beams or columns which are subject to shear, this transverse reinforcing contributes to the section's shear capacity as described further below

Figure 5: Photos of Typical Rebar Arrangements in Columns (Left) and Beams (Right)

3.2.1 Concept

Typically shear in a section is provided by the combined strength of the concrete and the transverse steel When a beam is subject to flexure, according to ACI code guidelines the concrete's shear capacity is:

Vc = 2 * J f t * bw* d

The width of the base of the section is represented by bw When a section is subject to

combined axial compression and flexure, according to the ACI, the shear resistance of the concrete

is calculated as:

Nu is the total axial force in the beam in pounds, and Ag is the gross cross sectional area in

square inches When the applied shear in a section exceeds the concrete section's shear resistance, then additional shear capacity is required Conventionally this is achieved with transversely spaced steel reinforcing bars According to the ACI the shear capacity of transverse reinforcing is calculated as:

Where As = the cross sectional area of the shear reinforcing, Fy = the steel's yield stress, d = the depth of the cross section in the dimension of the applied shear, s = the spacing of the transverse reinforcing along the longitudinal axis of the beam, and a= the inclination angle of the reinforcing within the beam, where reinforcing completely parallel to the direction of applied shear has an a of 90° The total shear capacity of the section is the combined capacities of the steel and concrete shear

are typically used for retrofitting, where a flexural beam subject to shear concerns is not accessible from all sides because of its location directly beneath a structural floor or roofing system in a building, or directly below a bridge's deck The beam is only accessible from three sides, and in cases

as such FRP strips can be applied to the section in "u shapes" only wrapping the accessible sides When a column requires upgrade of shear capacity typically all sides of the column are accessible and the entire perimeter is wrapped

When it comes to the effectiveness of the wrapping system, "u shaped" wraps provide the same shear capacity in the direction parallel to the wrap's two vertical faces as a wrap applied around the entire perimeter provides in that direction The "u shaped" strips are thought to have higher potential for debonding from the concrete's surface than a wrap applied around the entire perimeter which is allowed to cure into one continuous piece and also achieve surface bonding to the concrete

The shear strength achieved through the application of FRP to a section is calculated similarly to that of a section reinforced with steel

The ultimate strength of the FRP is defined as below:

F FRP — &FRP * £F R P

The modulus of elasticity of the FRP is provided by the FRP manufacturer The ultimate strain the FRP can experience is also provided by the manufacturer, but the section may fail before this ultimate strain is reached The ultimate strain at failure depends upon the failure mode of the section

When a section is wrapped around its entire perimeter, it is believed that the loss of concrete's aggregate interlock will occur before the ultimate strain is attained in the FRP fiber Therefore the maximum strain used to calculate the ultimate strength of the FRP should be the lesser of 0.004, or 75% of the manufacturer's provided ultimate strain capacity, per ACI 440.2R-08 code when a section is wrapped around its entire perimeter

When a section is reinforced with "u shaped" strips, it is assumed that debonding of the FRP fiber will occur before aggregate interlock shear failure within the concrete For this reason, the effective strain used to calculate the strength of the FRP is to be the lesser of 0.004 and the FRP's ultimate strength multiplied by a bond reduction coefficient The bond reduction coefficient is a function of both FRP stiffness and concrete strength, and is calculated as presented below:

11900 * eFRPj

23300

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The bond reduction coefficient is calculated as kv Lc is the active bond length of the FRP strips, and

kx and k2 are modification factors based on concrete strength and FRP wrapping arrangement

(American Concrete Institute)

3.2.3 Construction

FRP can be applied to a beam in three different configurations to provide shear reinforcement:

• Perimeter wrapping

• "U Wrap" or strips attached in U shapes

• Strips or plates placed on both vertical faces

All three configurations can be designed to provide the same shear strength to the section The differences between the three are the likely failure mechanisms, which is accounted for in ultimate strain considerations as discussed in section 3.2.2

FRP cannot be applied to concrete that contains contamination Any concrete contaminated with chlorides, corrosion, or alkali silica gel must be removed and repaired with new materials This may require as much as full removal and replacement of the concrete cover Additionally FRP cannot

be applied to sharp corners FRP manufacturers can provide the required minimum corner radius on the section to which the FRP is being applied

3.2.4 FRP for Shear Case Studies

a layer of resin, the FRP material, and an additional layer of resin The FRP system brought the shear strength of the superstructure up to demands, and extended the remaining life of the bridge from 5 years to 20 years.(26)

Bottom Vfew

Figure 6: KY3297 Bridge - FRP Strips for Shear Strengthening

Challenger Middle School Foot Bridge Repair

Challenger Middle School in Tucson, Arizona has two buildings connected by a footbridge Near the bridge's supports the beams exhibited shear cracking It was determined that over their winter break the entire repair would have to be completed Due to the aggressive required

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schedule, FRP strips were chosen for retrofit The strips were applied in U shapes, and work was finished in 3 days (21)

Figure 7: Challenger Middle School Footbridge: FRP U Wrap for Shear Strengthening

3.3 FRP Wrapping for Confinement Reinforcing

In columns and other compression members, transverse reinforcing is typically provided This transverse reinforcing provides both shear capacity, and confinement to the axially loaded section When a concrete section is confined, the buckling of the section's longitudinal rebar is prevented, and experiments have confirmed that the confined concrete's ultimate strength and strain capacity increase This allows the section to have greater moment capacity and deformation capacity when the column is subjected to lateral loads, such as during a seismic event

The tops and bottoms of existing columns may not contain adequate confinement reinforcing, as the design codes in effect during their construction did not require an amount that would allow the section to be ductile enough to demonstrate elastic or plastic behavior during a seismic event These older columns may behave in a brittle fashion with their existing designs An applicable seismic retrofit will wrap the plastic hinge regions of a column for strengthening and so it can achieve ductile behavior during a seismic event

3.3.1 Concept

In 1988 Mander et al (15) provided stress strain relationships for concrete confined by transverse reinforcing These relationships showed that with transverse reinforcing and its passive lateral confining pressure, concrete's ultimate strength and ultimate strain increases This behavior

is desired when lateral forces are applied to a structural column, under constant compression When lateral forces are applied, the shear and moments in the column increase The increased strength and deformation capacity in a column with transverse reinforcing makes it able to take higher loads, and can be used to prevent brittle failure of the structural element When seismic forces are applied

to structures whose columns have inadequate confinement reinforcing, typical failure mechanisms include brittle shear failure and failure within the column's plastic hinge zone When confinement

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reinforcing is adequate, the shear capacity of the column is increased, and the column is able to form plastic hinges at its top and bottom, which is a ductile failure mechanism Ductile failure mechanisms are desirable because they allow forces to be redistributed through the structure without brittle failure associated with normal concrete behavior

Before the confined compressive strength of concrete can be determined, three things must be calculated:

1 The lateral pressure from transverse reinforcing

2 The confinement effectiveness coefficient

3 The effective lateral confining pressure

1) Lateral Pressure from Transverse Reinforcing

The internal forces imposed on a section can be solved for using a free body diagram When half of the section is assessed, the force through the transverse reinforcing equals the total internal force imposed on the section The total internal force can then be divided by the area to equal the internal pressure This equation is written for circular and rectangular sections as follows:

fyh*Asp = f1*S*ds

Where fyh is the yield strength of the transverse reinforcing steel, Asp is the total cross sectional area

of the transverse bar, f! is the lateral confining pressure within the hoop, s is the spacing between transverse reinforcing from center to center along the section's longitudinal axis, and ds is the

diameter of the circular section Solving for fjthis equation becomes:

„ _ fyh * ASp

h ~ S * ds

In the case of a circular section, two sides of the transverse reinforcing contribute towards confinement pressure, therefore the bar's cross sectional area times two = Asp When a section is

rectangular, and has rectangular ties confining the core, there can be multiple legs of confinement in each direction, not just perimeter transverse reinforcing This impacts the equation only in the input

of Asp where Asp equals the total cross sectional area of transverse rebar If there are three legs of

confinement ties, then 3 times the individual bar diameter contributes towards confinement This also means that rectangular sections can be unequal confining pressures along each horizontal direction In this case, the confining pressures should be calculated in both directions, depending upon the amount of transverse rebar present

2) Confinement Effectiveness Coefficient

The effectiveness of the confinement must then be calculated The theory about confining bars is they only effectively confine the entire core at the exact elevation of the bar For this reason

a confinement effectiveness coefficient is calculus as follows where Ae is the effectively confined

concrete core area, and Acc is the area of the concrete core:

In circular sections, the concrete core area is calculated as:

The effectively confined core area midway between confinement hoops can be calculated as follows:

Where s' is the distance between the face to face of adjacent transverse reinforcing The confined

core area inside the transverse reinforcement, where p cc is the volumetric ratio of longitudinal steel reinforcing to confined concrete core, is calculated as:

k e =