Using FRP External Strengthening of Concrete

3/22/2021

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External Strengthening of Concrete

Using FRP

Diego Romero, PhD, PE, SE

Walker Consultants

Eri Vokshi, PE

Sika

LEARNING OBJECTIVES

• Identify applications suitable for FRP

strengthening

• Understand basics of the ACI 440.2R code

and limitation of FRP strengthening

• Understand industry best practices for

concrete surface preparation and application

of FRP materials.

AGENDA Introduction to FRP

Strengthening with FRP

Design Considerations

Surface Preparation

Installation

Coatings and Fire Protection

Q&A

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THE BEGINNING

• Initially developed in Japan

and Europe in the 1980s

• FRP use picked up after the

1995 Hyogoken-Nambu

Earthquake in Japan

• Used as an alternative to

external reinforcing

techniques such as steel

plate bonding or column

jacketing

WHAT IT FRP?

• Fiber Reinforced Polymer (FRP) = Fiber Mesh

+ Resin

• Not to be confused with FRCM (Fabric

Reinforced Cementitious Matrix)

Glass

• Cost Effective

• Reasonable Strength

• Used in passive applications (seismic)

• Great in Dry or Acidic Environments

• Electrical insulator

Carbon

• Expensive

• High Strength

• Used in actively loaded applications

• Great in wet or high alkalinity environments

• Electrical Conductor

Aramid

• Due to its high fracture energy, used for impact resistance and blast mitigation

• Electrical insulator

FABRIC TYPES

• Uni-directional fabrics

– Typically used in shear or flexural strengthening applications

• Bi-directional fabrics

– Beam-column joint strengthening

– Combined shear and flexural strengthening

– Two-way slabs

– Saves labor, higher cost

UNI-DIRECTIONAL

BI-DIRECTIONAL

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BENEFITS

• Corrosion resistance

• Strength

• Quick turnaround

• Ease of application– Lightweight

– Non-destructive

– Limited access

– Concealed




Surface Preparation

Installation


Q&A

WHY

STRENGTHEN?

• Design deficiencies

• Deterioration/Damage

• Increased Demand

– Slab openings

– Change of use

• Seismic upgrade

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FRP VERSUS CONVENTIONAL

STRENGTHENING

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Surface Preparation

Installation


Q&A

DESIGN

• ACI 440.2R-17: Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures– Provides design guidelines

– Installation requirements

– Detailing requirements

– QA/QC

– Design Examples

• ACI 562-19: Code Requirements for Evaluation, Repair, and Rehabilitation of Concrete Buildings

– Allows use of FRP for repair/strengthening

• Typically, FRP is called out as performance design

– Contractor engages specialty engineer to produce calculations and shop drawings.

DESIGN PHILOSOPHY PER ACI 440.2R

Limit-states design principles

Acceptable levels for ultimate

state

Acceptable levels for service state

Analysis of different

failure modes

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STRENGTHENING GENERAL

PRINCIPLES

• FRP materials work only in tension.

• The FRP strengthening system shall be located in areas where tensile stresses occur.

• FRP systems do not yield, they rupture. Behavior is linear elastic until rupture.

• FRP material properties are impacted by environmental factors.

• The choice and the design of the system are made by an experienced engineer.

FRP LIMITATIONS

“Supplemental Reinforcement”

Existing Capacity Load demand:

1.1DL + .75LL≥

FRP LIMITATIONS

“Limits during a fire event”

Nominal resistance

of structure at

elevated temp

Load demand:

1.0DL + 1.0LL≥

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Simply supported

Fixed support Intermediate support Cantilevers

MAIN TYPES OF STRENGTHENING

FLEXURAL

1 TON

C

T

Application of FRP

Strengthening system

1 TON 1 TON 1 TON

C

C

C

C

C

T

T

T

T

T

Application of FRP

Strengthening system

FLEXURAL STRENGTHENING

Steel

Contribution

Steel

Contribution

FRP

Contribution

FRP

Contribution

FAILURE MODE CONSIDERATIONS FOR


Crushing of the concrete in compression before

yielding of the reinforcing steel

Crushing of the concrete in compression before

yielding of the reinforcing steel

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Yielding of the steel in tension followed by

rupture of the FRP laminate

Yielding of the steel in tension followed by

rupture of the FRP laminate

Yielding of the steel in tension

followed by concrete crushing

Yielding of the steel in tension

followed by concrete crushing



Debonding of the FRP from the concrete substrate

(FRP debonding)

Debonding of the FRP from the concrete substrate

(FRP debonding)

Shear/tension delamination of the concrete cover

(cover delamination)

Shear/tension delamination of the concrete cover

(cover delamination)



▪ Step 1: Calculate the FRP system strain design limits

▪ Step 2: Determine the existing state of strain on the soffit

▪ Step 3: Determine the debondment strain in FRP

▪ The design strain of FRP accounting for debonding failure mode must be less than the

rupture strain of the FRP system

FLEXURAL DESIGN STEPS PER

ACI 440.2R

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▪ Step 4: Determine the effective strain in FRP reinforcement

▪ Step 5: Determine effective stress of FRP

▪ Step 6: Calculate Strain in the existing reinforcing steel to determine flexural reduction factor


ACI 440.2R

▪ Step 7: Check service stresses in the reinforcing steel


ACI 440.2R


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FLEXURAL STRENGTHENING IN MRI

ROOM

1-Vertical stirrups are the most

common solution.

1-Vertical stirrups are the most

common solution.

2-Inclined bars are quite

efficient, but their installation is

usually more complex.

2-Inclined bars are quite

efficient, but their installation is

usually more complex.21

SHEAR STRENGTHENING

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SHEAR STRENGTHENING

Steel and

concrete

Contribution

Steel and

concrete

Contribution

FRP

Contribution

FRP

Contribution

▪ Can be discrete strips or a continuous strips

SHEAR STRENGTHENING

▪ Step 1: Calculate bond reduction coefficient

▪ Step 2: Determine effective strain on FRP

SHEAR STRENGTHENING DESIGN

PER ACI 440

Predicts

debonding

Theoretical

failure strain of

aggregate

interlock loss

Predicts FRP

rupture due to

stress

concentrations

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▪ Step 3: Determine shear contribution due to FRP

▪ Step 4: Check that contribution due to FRP and steel is less than ACI 318 criteria

SHEAR STRENGTHENING DESIGN

PER ACI 440

wf

sf

wf

sfα

SHEAR STRENGTHENING

Pier Caps

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CONFINEMENT STRENGTHENING

ε’c

f ’cc

εccu

f ’c

Unconfined

ε’t

CFRP confinement

• Increase in both axial and

flexural strength

• Commonly used for seismic

retrofitting

• Most efficient

reinforcement in circular

columns

Increased confinement

pressure = Higher apparent

compressive strength

AXIAL STRENGTHENING AND

REPAIR CONFINEMENT

Source: https://abc-utc.fiu.edu/mc-events/michigan-dots-use-of-externally-bonded-frp-systems-for-bridge-element-strengthening/




Surface Preparation

Installation


Q&A

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FRP DISCUSSED TODAY

Epoxy Wet Lay-up Systems

Pre-saturated Wet Lay-up Systems

Dry Lay-up Systems

Pultruded Plates

FRP INSTALLATION REQUIREMENTS

Removal of existing contaminated concrete

Selection of proper repair materials

Surface preparation requirements for FRP

Proper saturation and application of FRP

Post application testing

SURFACE PREPARATION

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Repair

Spalls

Cracks

Voids

Mechanical Preparation

Abrasive/Hydro Blasting

Needle Scabbling

Grinding

Cleaning

Compressed Air

Brush

Vaccuum

SURFACE PREPARATION STEPS

Industry Focus & Aid

Extremely Sensitive Topic

ICRI => 30+ years Servicing Industry

www.icri.org

Standard concrete repair must come first

Repair any spalls and delaminations

Choose repair materials – ICRI 320.2

Low shrinkage

Compatible modulus

Adequate strength

Fast Curing (added bonus)

GUIDELINES

Must repair structural cracks >10 mils

Epoxy Injection/Gravity Feed

Sikadur 35 Hi Mod/Sikadur 31 Hi Mod

Root cause should be determined prior to

repair

Cracks subject to active hydrostatic or osmotic

pressure (i.e., running water) should not be

treated

with epoxy products.

Cores should be taken to ensure proper

penetration and adhesion.

REPAIR - CRACKS

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Must ensure 100% contact of FRP to

substrate

Fill with fast setting epoxy gel

Bug Holes

Rock Pockets

Honeycombs

Sikadur 30

1" thickness max

REPAIR - VOIDS

Useful Guides:

Surface Prep for Deteriorated Concrete = #310.1R

Surface Prep Methods = #310.2R

MECHANICAL PREPARATION


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"Methods are not critical, results are!“

Methods

Abrasive blasting (best)

Typically avoid silica

Grinder

Needle Scabbler

Open pores – allows excellent adhesion

Remove laitance – eliminate bond

breakers


Outside corners rounded to ½" min.

Inside corners – epoxy filet – ½" radius

Smooth (but open) & Level


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Prior to application, all substrates must

be clean and free of dust

Substrate shall be:

Brushed – stiff bristles to get into the

pores

Air blasted – oil free

Vacuumed – to achieve dust free

surfaces

CLEANING

40 F minimum, and rising!

Warm material

95 F maximum

Need hot weather protocol

Avoid direct sunlight

Cool material

Substrate moisture - <4% via Tramex

Concrete must be 21-28 days old

SURFACE PREP – SITE CONDITIONS

Minimum tensile strength =

200 psi

Substrate failure

TESTING SUBSTRATE

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Surface Preparation

Installation


Q&A

EPOXY RESIN BASICS

HOW EPOXIES WORK

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Produce Heat during mixing and cure

Mixing time is critical – 3 minutes

In warmer temperature conditions, cure rate is faster.

Can cool to slow them down

Mass dependent

Remove from mixing bucket ASAP

Once cured, thermosetting resins cannot be re-worked by heating.

[To avoid confusion: Thermoplastics

can be re-worked by heating.

Epoxies are not thermoplastics.]

EPOXIES ARE THERMOSETTING RESINS

1/2” drive, low speed rotary drill (400 –

600 rpm typical)

Appropriate epoxy mixing paddle

e.g. “Jiffy” or “Exo-mixer”

Spatula - To scrape side walls of

containers

MIXING TOOLS

MIXING – LIKE THIS

Full units only!

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MIXING - NOT LIKE THIS!

OR THIS...

OR THIS...

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AND DEFINITELY NOT LIKE THIS!

FRP INSTALLATION TOOLSHTTPS://US-43.WISTIA.COM/MEDIAS/RY4MAKSBP5

FIELD SATURATED FRP SYSTEMS

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FIELD SATURATED FRP SYSTEM

The ORIGINAL FRP system

Longest in the market and most trusted

Most common resin used is epoxy

Saturation is done in the field

Durable in various environments

Repair Imperfections


Prep Work


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Mix & Apply Epoxy Primer


Cut fabric to size


Saturate Fabric with Resin – Table or Saturator


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Remove air bubbles


APPLICATION VIDEO – WET LAY-UP SYSTEM

HTTPS://US-43.WISTIA.COM/MEDIAS/GHZBX2U7I8

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PRESATURATED SYSTEMS

What is a pre-saturated system?

Pre-saturated systems have been used in aerospace industry

Consistent resin to fabric ratio

Reduction in Labor

Application efficiency

Non hazardous

Saturating resin is an aliphatic PU, which are inherently durable and UV resistant

PRE-SATURATED FRP SYSTEM

PRESATURATED SYSTEM

Prep Concrete

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PRE-SATURATED SYSTEM

Mix epoxy primer and apply on the substrate

Prime Concrete

PRESATURATED SYSTEM

Open foil pouches when ready to apply

PRESATURATED SYSTEM

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Cut “wet” fabric if necessary

PRESATURATED SYSTEM

Lay Wrap

PRESATURATED SYSTEM

Quality in the field

Known Resin to Fabric Ratio

Within 5%

Material Certs for engineers

Strengths

Modulus

R:F Ratio

ISO 9001 Plant

Full Traceability

Drop in replacement for current

products

Reduction in Labor

Reduce 5-6 man crew

by 2-3 men


Increase work rate by 20-30%

4 day project down to 3

No need to move saturated fabric

around

Ease of delivery

Single source

Non hazardous

Can be easily transported or air freighted

ADVANTAGES

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Order & Ship Resin

Order & Ship Fabric

Prepare concrete

Bring Saturator on site

Mix Epoxy Primer

Prime Concrete

Fabric is cut on site (if necessary)

Set up saturator

Fabric is then saturated (saturator or table/rollers)

Piece by piece, saturated fabric transported and given to installers

Applied to primed surface

Left to cure

Clean up saturator and site

Dispose of Resin pails

FIELD SATURATED SYSTEM –

INSTALLATION STEPS

Order & Ship Resin

Order & Ship Fabric

Prepare concrete

Bring Saturator on site

Mix Epoxy Primer

Prime Concrete

Fabric is cut on site (if necessary)

Set up saturator

Fabric is then saturated (saturator or table/rollers)

Piece by piece, saturated fabric transported and given to installers

Applied to primed surface

Left to cure

Clean up saturator and site

Dispose of Resin pails

PRE-SATURATED SYSTEM –

INSTALLATION STEPS

APPLICATION VIDEO – PRESATURATED SYSTEM

HTTPS://US-43.WISTIA.COM/MEDIAS/YFIMPJOB32

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DRY LAY-UP

A simplified wet lay-up application method

Applicable for very thin carbon or glass fabric systems

Reduction in labor


DRY LAY-UP INSTALLATION METHOD

Prep Concrete

DRY LAY-UP INSTALLATION

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Apply epoxy primer


Apply dry fabric on the primer and roll it to to ensure epoxy comes through the

fabric.


Apply epoxy on top of the installed system to ensure full saturation


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Sand broadcasting (if required)


APPLICATION VIDEO – DRY LAY-UP SYSTEM

HTTPS://US-43.WISTIA.COM/MEDIAS/3O9DGYTV6N

FRP PLATES

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Sika® CarboDur® Plates are pultruded CFRP

Designed for strengthening concrete, timber and masonry structures

Higher stiffness than the wet lay-up systems

Lightweight

Non-corrosive

FRP PLATES

CLEANING CFRP STRIPS

CUTTING STRIPS ON-SITE

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Pre-mix components

Low speed drill

Uniformly blended

MIXING EPOXY RESIN

APPLYING EPOXY TO SUBSTRATE

Applying Epoxy onto CFRP

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Set strip by hand

Work from one end to the other

Moderate pressure

STRIP INSTALLATION

ROLLING CFRP ONTO CONCRETE

Moderate pressure

Ensures intimate contact

APPLICATION VIDEO – FRP PLATES

HTTPS://US-43.WISTIA.COM/MEDIAS/XPAZ061WDY

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Surface Preparation

Installation


Q&A

PROJECT COMPLETION

PROTECTIVE COATINGS

Before After

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Delamination

Limits of delaminated FRP area to

ensure adequate performance

Material Testing

Tensile tests of laminate from

field

Bond

Pull off tests to determine bond

strength to concrete

Minimum 200 psi (1.4 MPa)

QC ACCEPTANCE CRITERIA

Pull testPull test

FIRE PROTECTION OF FRP

Quantifies flame spread rate

Quantifies amount of smoke generated

ASTM E84STANDARD TEST METHOD FOR SURFACE BURNING

CHARACTERISTICS OF BUILDING MATERIALS

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Intumescent coating is sacrificial element in test

Class A rating achievable for interior or exterior use

System tested up to 18 layers of FRP/epoxy system – Class A rating

ASTM E-84 TEST

ASTM E119 Standard Test Methods for Fire Tests of Building Construction

and Materials

E119 Curve

0

500

1000

1500

2000

2500

0 60 120 180 240 300 360 420 480

Time (min)

Tem

p (F)

Service temp. epoxy < 140F

NRC Canada – Test facility, Ottawa, CA

Testing administered by Queens Univ.

UL witnessed test

T beams

Columns

Systems tested

CFRP fabrics

GFRP fabrics

CFRP plates

Sikacrete-213F

Fire-resistive mortar

STRUCTURAL FIRE ENDURANCE TEST

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Structural Fire Endurance test

UL witnessed test

Systems tested with CFRP systems

Fireproof Shotcrete

Hydraulic bound, vermiculite-based, fire-

resistive mortar

Contains aggregates and admixtures which

are highly effective in resisting heat from

fires

Applied by wet spray process

FIREPROOFING - SHOTCRETE

COLUMN TEST

Square columns

Columns provided with fire insulation to

protect FRP wrap

Columns tested under sustained

concentric axial service load in the column

furnace at NRC

Tested to 2,000F

IRC/NRC Column Furnace and

Column after Fire Testing

Purpose: Obtain ASTM E119 fire

endurance ratings / verify

models and UL Rated System

UL LISTINGS – 4 HR. RATING

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As restoration evolves, emerging technologies such as FRP’s are cost

and time effective solutions for reinforcement of infrastructure

Other advantages include: ease of application, non-corrosive, and

conformability

Proper repair and application is critical to ensure successful and

long-lasting reinforcement

Proper planning, setting-up, tools, conditioning the products are key

to a successful install

CONCLUSIONS

Thank you for participating!

Any Questions?

Diego Romero, PhD, PE, SE

[email protected]

Phone: (312) 633-4260

Eri Vokshi, PE

[email protected]

Mobile: (561) 254-8472

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Documents

Using FRP External Strengthening of Concrete