3/22/2021 1 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 1 2 3
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3/22/2021
1
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
AGENDA Introduction to FRP
Strengthening with FRP
Design Considerations
Surface Preparation
Installation
Coatings and Fire Protection
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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AGENDA Introduction to FRP
Strengthening with FRP
Design Considerations
Surface Preparation
Installation
Coatings and Fire Protection
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
FLEXURAL STRENGTHENING
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
FAILURE MODE CONSIDERATIONS FOR
FLEXURAL STRENGTHENING
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)
FAILURE MODE CONSIDERATIONS FOR
FLEXURAL STRENGTHENING
▪ 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
FLEXURAL DESIGN STEPS PER
ACI 440.2R
▪ Step 7: Check service stresses in the reinforcing steel
FLEXURAL DESIGN STEPS PER
ACI 440.2R
FLEXURAL STRENGTHENING
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FLEXURAL STRENGTHENING
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