Durability of FRP Composites for Construction

1
ISIS Educational Module 8
Durability of FRP Composites for Construction
Produced by ISIS Canada
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Module Objectives

• To provide students with a general awareness of
  important durability consideration for FRPs
• To facilitate and encourage the use of durable
  FRPs and systems in the construction industry
• To provide guidance for students seeking
  additional information on the durability of FRP
  materials

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Outline
ISIS EC Module 8
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Introduction Overview
Section
1

• The problem
• In recent years, our infrastructure systems
  have been deteriorating at an increasing and
  alarming rate

New materials that can be used to prolong and
extend the service lives of existing structures ??
Fibre Reinforced Polymers (FRPs)
ISIS EC Module 8
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Introduction Overview
Section
1

• Key uses of FRPs in construction

1. Internal reinforcement of concrete

Corrosion of steel reinforcement in concrete
structures contributes to infrastructure
deterioration
Use non-corrosive FRP reinforcement

1. External strengthening of concrete

Provide external tension or confining
reinforcement
(FRP plates, sheets, bars, etc.)
ISIS EC Module 8
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Introduction Overview
Section
1

• What is FRP?

• FRP is a composite

Composite combination of two or more materials
to form a new and useful material with enhanced
properties in comparison to the individual
constituents (concrete, wood, etc.)

• FRPs consist of
• Fibres
• Matrix

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Introduction Overview
Section
1
Polymer matrix

• Polymer matrix

As the binder for the FRP, the matrix roles
include

• Binding the fibres together
• Protecting the fibres from environmental
  degradation
• Transferring force between the individual fibres
• Providing shape to the FRP component

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Introduction Overview
Section
1
Polymer matrix

• Commonly used matrices

Internal reinforcing applications

• Vinylester fabrication for FRP reinforcing bars

(superior durability characteristics when
embedded in concrete)
External strengthening applications

• Epoxy strengthening using FRP sheets/plates

(superior adhesion characteristics)
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Introduction Overview
Section
1
Fibres

• Fibres

Provide strength and stiffness of FRP

• Protected against environmental degradation by
  the polymer matrix
• Oriented in specified directions to provide
  strength along specific axes (FRP is weaker in
  the directions perpendicular to the fiber)
• Selected to have

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Introduction Overview
Section
1
Fibres

• Three most common fibres in Civil Engineering
  applications
• Glass
• Carbon
• Aramid (not common in North America)

• Required strength and stiffness
• Durability considerations
• Cost constraints
• Availability of materials

• Selected based on

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Introduction Overview
Section
1
Fibres

• Glass fibres

• Inexpensive
• Most commonly used in structural applications
• Several grades are available
• E-Glass
• AR-Glass (alkali resistant)
• High strength, moderate modulus, medium density
• Used in non weight/modulus critical applications

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Introduction Overview
Section
1
Fibres

• Carbon fibres

• Significantly higher cost than glass
• High strength, high modulus, low density
• E 250-300 GPa standard
• E 300-350 GPa intermediate
• E 350-550 GPa high
• E 550-1000 GPa ultra-high
• Superior durability and fatigue characteristics
• Used in weight/modulus critical applications

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Introduction Overview
Section
1
Fibres

• Aramid fibres

• Moderate to high cost
• Two grades available 60 GPa and 120 GPa elastic
  moduli
• High tensile strength, moderate modulus, low
  density
• Low compressive and shear strength
• Some durability concerns
• Potential UV degradation
• Potential moisture absorption and swelling

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Mechanical Properties
Section
1
Type of fibre and matrix
FRP mechanical properties are a function of
Fibre volume content
Orientation of fibres
Here we are concerned mainly with unidirectional
FRPs!
ISIS EC Module 8
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FRP vs. Steel
Section
1
Mechanical Properties

• FRP properties
• (in general versus steel)
• Linear elastic behaviour to failure
• No yielding
• Higher ultimate strength
• Lower strain at failure
• Comparable modulus (carbon FRP)

CFRP
GFRP
Steel
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Quantitative Comparison
Section
1
Typical Mechanical Properties
Based on 2001 data for specific FRP rebar
products
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Introduction Overview
Section
1
FRP

• Physical, mechanical, durability properties of
  FRPs

• Overall properties and durability depend on

• The properties of the specific polymer matrix
• The fibre volume fraction
• (i.e., volume of fibres per unit volume of
  matrix)
• The fibre cross-sectional area
• The orientation of the fibres within the matrix
• The method of manufacturing
• Curing and environmental exposure

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Introduction Overview
Section
1
Examples of FRP
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Introduction Overview
Section
1
FRPs

• In the design and use of FRP materials
• The orientation of the fibres within the matrix
  is a key consideration
• Most important parameters for infrastructure
  FRPs
• ? Uniaxial tensile properties
• ? strength and elastic modulus
• ? FRP-concrete bond characteristics
• ? transfer and carry the tensile loads
• ? Durability

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Introduction Overview
Section
1

• What is durability?

• The ability of an FRP material to
• resist cracking, oxidation, chemical
  degradation, delamination, wear, and/or the
  effects of foreign object damage for a specified
  period of time, under the appropriate load
  conditions, under specified environmental
  conditions

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CAUTION!
Section
1

• Data on the durability of FRP materials is
  limited
• Appears contradictory in some cases
• Due to many different forms of FRPs and
  fabrication processes
• FRPs used in civil engineering applications are
  substantially different from those used in the
  aerospace industry
• Their durability cannot be assumed to be the same
• Anecdotal evidence suggests that FRP materials
  can achieve outstanding longevity in
  infrastructure applications

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Introduction Overview
Section
1
Durability

• Environments

• All engineering materials are subject to
  mechanical and physical deterioration with time,
  load, and exposure to various harmful
  environments
• FRP materials are very durable, and are less
  susceptible to degradation than many conventional
  construction materials

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Introduction Overview
Section
1
Durability

• Factors affecting FRPs durability performance

• The matrix and fibre types
• The relative portions of the constituents
• The manufacturing processes
• The installation procedures
• The short- and long-term loading and exposure
  condition (physical and chemical)

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Introduction Overview
Section
1
Durability

• Potentially harmful effects for FRP

Environmental Effects
Physical Effects
Moisture Marine Environments
Alkalinity Corrosion
Sustained Load Creep
DURABILITY OF FRPs
Heat Fire
Cyclic loading Fatigue
Cold Freeze-Thaw Cycling
Ultraviolet Radiation
POTENTIAL SYNERGIES
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Moisture Marine Exposures
Section
2

• FRPs are particularly attractive for concrete
  structures in moist or marine environments
• FRPs are not susceptible to electrochemical
  corrosion
• Corrosion of steel in conventional structures
  results in severe degradation

HOWEVER

• FRPs are not immune to the potentially harmful
  effects of moist or marine environments

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Moisture Marine Exposures
Section
2
Moisture

• Some FRP materials have been observed to
  deteriorate under prolonged exposure to moist
  environments
• Evidence linking the rate of degradation to the
  rate of sorption of fluid into the polymer matrix
• All polymers will absorb moisture
• Depending on the chemistry of the specific
  polymer involved, can cause reversible or
  irreversible physical, thermal, mechanical and/or
  chemical changes
• It is important to recognize that
• Results from laboratory testing are not
  necessarily indicative of performance in the field

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Moisture Marine Exposures
Section
2
Moisture

• Selected factors affecting moisture absorption in
  FRPs

• Type and concentration of liquid
• Type of polymer and fibre
• Fibre-resin interface characteristics
• Manufacturing / application method
• Ambient temperature
• Applied stress level
• Extent of pre-existing damage
• Presence of protective coatings

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Moisture Marine Exposures
Section
2
Moisture

• Overall effects of moisture absorption

Moisture absorption
Plasticization of the matrix caused by
interruption of Van der Walls bonding between
polymer chains

• Reduced matrix strength, modulus, strain at
  failure toughness
• Subsequently reduced matrix-dominated properties
  Bond, shear, flexural strength stiffness
• May also affect longitudinal tensile strength
  stiffness
• Swelling of the matrix causes irreversible damage
  through matrix cracking fibre-matrix debonding

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Moisture Marine Exposures
Section
2
Moisture

• Typical moisture absorption trend for a matrix
  polymer

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Moisture Marine Exposures
Section
2
Moisture

• Strength loss trend of typical FRPs due to
  moisture absorption

Note no strength reductions in some
lab studies
Further research needed
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Moisture Marine Exposures
Section
2

• Potentially Important degradation synergies

• Moisture absorption
• Sustained stress
• Elevated temperatures

Stress-induced micro-cracking of the polymer
matrix
Moisture-induced micro-cracking of polymer matrix
in a GFRP
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Moisture Marine Exposures
Section
2
Fibres

• The effect of moisture on fibres performance

• Glass fibres

Moisture penetration to the fibres may extract
ions from the fibre and result in etching and
pitting. can cause deterioration of tensile
strength and elastic modulus

• Aramid fibres

Can result in fibrillation, swelling of the
fibres, and reductions in compressive, shear, and
bond properties. Certain chemicals such as sodium
hydroxide and hydrochloric acid can cause severe
hydrolysis

• Carbon fibres

Do not appear to be affected by exposure to moist
environments
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Moisture Marine Exposures
Section
2
Resins

• FRPs can be protected against moisture absorption
  by appropriate selection of matrix materials and
  protective coatings

• Vinylester

currently considered the best for use in
preventing moisture effects in infrastructure
composites

• Epoxy

also considered adequate

• Polyester

Available research also suggests poor performance
and should typically not be used
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Alkalinity Corrosion
Section
3
Alkalinity

• Effects of alkalinity on FRPs performance

• The pH level inside concrete is gt 11 (i.e.,
  highly alkaline)
• Becomes important for internal FRP reinforcement
  applications within concrete (particularly for
  GFRP)

• Protection by matrix
• Level of applied stress
• Temperature

Damage to glass fibres depends on
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Alkalinity Corrosion
Section
3
Alkalinity

• Degradation mechanisms for GFRP reinforcement

• Reduction in tensile properties
• Damage at the fibre-resin interface

Alkaline solutions cause embrittlement of the
fibres
Alkaline solutions
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Alkalinity Corrosion
Section
3
Alkalinity

• The effect of alkaline environments on fibres

• E-glass fibres

• Strength reduction of 0 75 of initial values

• AR-glass fibres

• Significant improvement in alkaline
  environments, but

• Aramid fibres

• Strength reduction of 10 50 of initial values

Need further research

• Carbon fibres

• Strength reduction of 0 20 of initial values

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Alkalinity Corrosion
Section
3
Corrosion

• Galvanic Corrosion

• FRPs are not susceptible to electrochemical
  corrosion
• Certain FRPs (e.g., CFRPs) can contribute to
  increased corrosion of metal components through
  galvanic corrosion

Galvanic corrosion accelerated corrosion of a
metal due to electrical contact with a
nonmetallic conductor in a corrosive environment
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Alkalinity Corrosion
Section
3
Corrosion

• Guarding against galvanic corrosion

• CFRPs should not be permitted to come in to
  direct contact with steel or aluminum in
  structures

• Internal reinforcement
• place plastic spacers
• between steel and CFRP bars

• External strengthening
• apply a thin layer of epoxy or GFRP sheet
  between CFRP and steel

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High Temperatures Fire
Section
4

• FRP materials are now widely used for
  reinforcement and rehabilitation of bridges and
  other outdoor structures
• FRPs have seen comparatively little use in
  building applications
• FRP materials are susceptible to elevated
  temperatures
• Several concerns associated with their behaviour
  during fire or in high temperature service
  environments
• Extremely difficult to make generalizations
  regarding high temperature behaviour
• Large number of possible fibre-matrix
  combinations, manufacturing methods, and
  applications

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High Temperatures Fire
Section
4

• FRPs used in infrastructure applications suffer
  degradation of mechanical and/or bond properties
  at temperatures exceeding their glass transition
  temperature

• Glass transition temperature, Tg
• the midpoint of the temperature range over which
  an amorphous material (such as glass or a high
  polymer) changes from (or to) brittle, vitreous
  state to (or from) a rubbery state (ACI 440 2006)

• All organic polymer materials combust at high
  temperatures
• Most matrix polymers release large quantities of
  dense, black, toxic smoke

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High Temperatures Fire
Section
4

• Potential problems of FRPs under fire

External FRP strengthening
Internal FRP reinforcement
Too thin for self-insulating layer, loss of bond
at T gt Tg
Sudden and severe loss of bond at T gt Tg
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High Temperatures Fire
Section
4

• Mechanical properties of FRPs deteriorate with
  increasing temperature
• Critical temperature commonly taken to be Tg
  for the polymer matrix
• Typically in the range of 65-120ºC
• Exceeding Tg results in severe degradation of
  matrix dominated properties such as transverse
  and shear strength and stiffness
• Longitudinal properties also affected above Tg
• Tensile strength reductions as high as 80 can be
  expected in the fibre direction at temperatures
  of only 300ºC
• Important that an FRP component not be exposed
  to temperatures close to or above Tg during the
  normal range of operating temperatures

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High Temperatures Fire
Section
4

• Degradation of mechanical properties is mainly
  governed by the properties of the matrix

• Carbon fibres

No degradation in strength and stiffness up to
1000 ºC

• Glass fibres

20-60 reduction in strength at 600 ºC

• Aramid fibres

20-60 reduction in strength at 300 ºC
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High Temperatures Fire
Section
4

• Deterioration of mechanical and bond properties
  for GFRP bars

Critical temperature (T gt Tg)
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High Temperatures Fire
Section
4

• The use of FRP internal reinforcement is
  currently not recommended for structures in which
  fire resistance is essential to maintain
  structural integrity
• Exposure to elevated temperatures for a prolonged
  period of time may be a concern with respect to
  exacerbation of moisture absorption and
  alkalinity effects

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Cold Temperatures
Section
5

• Potential for damage due to low temperatures and
  thermal cycling must be considered in outdoor
  applications
• Freezing and freeze-thaw cycling may affect the
  durability performance of FRP components through
• Changes that occur in the behaviour of the
  component materials at low temperatures
• Differential thermal expansion
• between the polymer matrix and fibre components
• between concrete and FRP materials
• Could result in damage to the FRP or to the
  interface between FRP components other materials

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Cold Temperatures
Section
5

• Exposure to subzero temperature may result in
  residual stresses in FRPs due to matrix
  stiffening and different CTEs between fibres and
  matrix

• Stiffness
• Strength
• Dimensional stability
• Fatigue resistance
• Moisture absorption
• Resistance to alkalinity

Matrix micro-cracking and fibre-matrix bond
degradation
May affect FRPs
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Cold Temperatures
Section
5

• Increased severity of matrix cracks
• Increased matrix brittleness
• Decreased tensile strength

• Increasing of freeze/thaw cycles

HOWEVER
The effects on FRP properties appear to be minor
in most infrastructure applications
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Ultraviolet Radiation
Section
6

• Ultraviolet (UV) radiation damages most polymer
  matrices

• Aramid fibres significant
• Glass fibres insignificant
• Carbon fibres insignificant

• The effects of UV on

• Thus, potential for UV degradation is important
  when FRPs are exposed to direct sunlight

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Ultraviolet Radiation
Section
6

• Photodegradation UV radiation within a certain
  range of specific wavelengths breaks chemical
  bonds between polymer chains and resulting in

• Discoloration
• Surface oxidation
• Embrittlement
• Microcracking of the matrix

• UV-induced surface flaws can cause

• Stress concentrations ? may lead to premature
  failure
• Increased susceptibility to damage from
  alkalinity moisture

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Ultraviolet Radiation
Section
6

• Combined effects of UV and moisture on FRP bars

• CFRP tensile strength reduction of 0-20
• GFRP tensile strength reduction of 0-40
• AFRP tensile strength reduction of 0-30

• Protection of FRPs from UV radiation

• UV resistant paints
• Coatings
• Sacrificial surfaces
• UV resistant polymer resins

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Creep Creep Rupture
Section
7

• Creep A behaviour of materials wherein an
  increase in strain is observed with time under a
  constant level of stress (L final length)

L1
L1
P P L gt L1
P P L L1
with creep
ideal
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Creep Creep Rupture
Section
7

• Relaxation a reduction in stress in a material
  with time at a constant level of strain (P
  final load)

L1
L1
1
1
P gt P1 L L1
P P L L1
ideal
with relaxation
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Creep Creep Rupture
Section
7
Creep

• Effects of creep on the performance of FRPs

• Fibres ? relatively insensitive to creep in
  absence of other harmful durability factors
• Matrices ? highly sensitive to creep

Thus, creep is potentially important for FRP
(Because loads must be transferred through the
matrix)
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Creep Creep Rupture
Section
7
Creep

• For good performance under sustained loads
• Use an appropriate matrix material
• Take care during the fabrication and curing
  processes
• Creep behaviour of different FRP materials is
  complex and depends on
• Specific constituents and fabrication
• Type, direction, and level of loading applied
• Exposure to other durability factors such as
  alkalinity, moisture, thermal exposures
• Few standard test methods for creep testing FRP
  materials
• Difficult to make generalizations about FRPs
  creep performance

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Creep Creep Rupture
Section
7
Creep Rupture

• Under certain conditions creep can result in
  rupture of FRPs at sustained load levels that are
  significantly less than ultimate

Called Stress Rupture, Creep Rupture, or Stress
Corrosion

• Creep rupture is influenced largely by the types
  of fibres and susceptibility to alkaline
  environments (glass FRPs in particular)

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Creep Creep Rupture
Section
7

• Endurance time the time to creep rupture of FRPs
  under a given level of sustained load

Endurance time

• Other factors influencing endurance time include
• Elevated temperature
• Alkalinity
• Moisture
• Freeze-thaw cycling
• UV exposure

Endurance time
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Creep Creep Rupture
Section
7

• Creep rupture stress limits for FRP reinforcing
  bars (50 years creep rupture strength)

• GFRP 29-55 of initial tensile strength
• AFRP 47-66 of initial tensile strength
• CFRP 79-93 of initial tensile strength

Note Laboratory testing is not necessarily
representative of field performance
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Fatigue
Section
8

• Fatigue all structures are subjected to repeated
  cycles of loading and unloading due to

• Traffic and other moving loads
• Thermal effects (differential thermal expansion)
• Wind-induced or mechanical vibrations

• Fatigue performance of most FRPs is as good as or
  better than steel

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Fatigue
Section
8

• Good fatigue performance of FRPs depends on

• Toughness of the matrix
• Ability to resist cracking

• Performance of FRPs under fatigue load

• CFRP best
• GFRP good
• AFRP excellent

• NOTE Fatigue performance of FRP reinforced
  concrete appears to be best when GFRP
  reinforcement is used

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Reduction Factors
Section
9

• Numerous factors exist that can potentially
  affect the long term durability of FRP materials
  in civil engineering and construction
  applications
• Durability factors remain incompletely understood
• Reduction factors in existing design codes and
  recommendations
• Applied to the nominal stress and strain
  capacities of FRPs
• limit the useable ranges of stress and strain in
  engineering design

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Reduction Factors (FRP bars)
Section
9

• For non-prestressed FRPs

Reduction Factor
Exposure Condition
Material
Document
0.60
All
AFRP
CHBDC, 2006
0.75
All
CFRP
0.50
All
GFRP
0.75
All
All
CSA S806-02
0.90
Not exposed to earth and weather
AFRP
ACI 440.1R-06
0.80
Exposed to earth and weather
1.00
Not exposed to earth and weather
CFRP
0.90
Exposed to earth and weather
0.80
Not exposed to earth and weather
GFRP
0.70
Exposed to earth and weather
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Reduction Factors
Section
9

• Sustained (service) stress levels are limited to
  avoid creep rupture and other forms of distress

Stress limit ( of ultimate)
FRP Bars
Document
35
AFRP
CHBDC, 2006
65
CFRP
25
GFRP
30
GFRP
CSA S806-02
30
AFRP
ACI 440.1R-06
55
CFRP
20
GFRP
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Specifications Durability of FRP Bars
Section 10

• ISIS Canada has recently published a product
  certification document
• Specifications for Product Certification of Fibre
  Reinforced Polymers (2006)
• Test methods are given for quantitatively
  defining the durability of FRP reinforcing bars
  for concrete
• Classifies FRP bars into different durability
  categories (e.g. D1, D2, etc.)

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Specifications Durability Criteria
Section 10
Property Specified limits
Void content 1
Water absorption 1 for D2 FRP bars and grids 0.75 for D1 bars and grids
Cure ratio 95 for D2 bars and grids 98 for D1 bars and grids
Glass transition temperature DMA 90C, DSC 80C for D2 bars and grids DMA 110C, DSC 100C for D1 bars and grids
Alkali resistance in high pH solution (no load) Tensile capacity retention 70 for D2 bars and grids tensile capacity retention 80 for D1 bars and grids
Alkali resistance in high pH solution (with load) Tensile capacity retention 60 for D2 bars and grids tensile capacity retention 70 for D1 bars and grids
Creep rupture strength Creep rupture strength 35 UTS (Glass) 75 UTS (Carbon) 45 UTS (Aramid)
Creep Report creep strain values at 1000 hr, 3000 hr and 10000 hr
Fatigue strength Fatigue strength at 2 million cycles 35 UTS (Glass) 75 UTS (Carbon) 45 UTS (Aramid)
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Case Study Field Evaluation of GFRP
Section 11

• Laboratory experiments have suggested that FRPs
  may be susceptible to deterioration under many
  environmental conditions
• Field data are scant for FRPs used in
  infrastructure applications
• Available field data indicate that in-service
  performance can be much better than assumed on
  the basis of laboratory testing

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Case Study Field Evaluation of GFRP
Section 11

• ISIS Canada Research project to study in-service
  performance of glass FRP reinforcing bars in
  concrete structures in Canada
• Joffre Bridge (Sherbrooke, Quebec)
• Crowchild Bridge (Calgary, Alberta)
• Halls Harbour Wharf (Halls Harbour, Nova
  Scotia)
• Waterloo Creek Bridge (British Columbia)
• Chatham Bridge (Ontario)
• Samples studied for evidence of deterioration
  using various optical and chemical techniques

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Case Study Field Evaluation of GFRP
Section 11

• There are many methods to investigate durability
  performance of GFRP reinforcing bars

• Optical Microscopy (OM)
• Scanning Electron Microscopy (SEM)
• Energy Dispersive X-ray Analysis (EDX)
• Infrared Spectroscopy (IS)
• Differential Scanning Calorimetry (DSC)

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Field Evaluation of GFRP
Section 11
Case study

• Optical Microscopy (OM)

• To visually examine the interface between the
  GFRP reinforcing bars and the concrete

After 8 years of exposure to alkalinity,
freeze-thaw, wet-dry, and chlorides
Crowchild Trail Bridge
Chatham Bridge
No evidence of damage or deterioration
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Field Evaluation of GFRP
Section 11
Case study

• Scanning Electron Microscopy (SEM)

• To conduct highly detailed visual examination of
  GFRP

After 8 years of exposure to alkalinity,
freeze-thaw, wet-dry, and chlorides
Crowchild Trail Bridge
Chatham Bridge
No evidence of damage or deterioration
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Field Evaluation of GFRP
Section 11
Case study

• Energy Dispersive X-ray Analysis (EDX)

• To determine if any chemical changes had occurred
  in glass fibres or in polymer matrix

After 8 years of exposure to alkalinity,
freeze-thaw, wet-dry, and chlorides
No Sodium or Potassium are present
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Field Evaluation of GFRP
Section 11
Case study

• Other techniques

• Infrared Spectroscopy (IS)
• to determine the extent of alkali-induced
  hydrolysis of the matrix
• No evidence of damage or deterioration
• Differential Scanning Calorimetry (DSC)
• to determine the glass transition temperature of
  a polymer material
• No evidence of damage or deterioration

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Durability Research Needs

• The durability performance of FRP materials is
  generally very good in comparison with other,
  more conventional, construction materials
• However, it should be equally clear that the
  long-term durability of FRPs remains incompletely
  understood
• A large research effort is thus required to fill
  all of the gaps in knowledge

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Durability Research Needs

• Moisture
• Effects of under-cure and/or incomplete cure of
  the polymer matrix
• Effects of continuous versus intermittent
  exposure to moisture when bonded to concrete
• Alkalinity
• Determination of rational and defensible standard
  alkaline solutions and alkalinity testing
  protocols and database of durability information
• Development of an understanding of alkali-induced
  deterioration mechanisms
• The potential synergistic effects of combined
  alkalinity, stress, moisture, and temperature are
  not well understood, particularly as they relate
  to creep-rupture of FRP components.

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Durability Research Needs

• Fire
• Non-destructive evaluation methods for
  fire-exposed composites
• Fire repair strategies
• Development of relationships between tests on
  small scale material samples at high temperature
  and full-scale structural performance during fire
• Fatigue
• More fatigue data on a variety of FRP materials
• Mechanistic understanding of fatigue in
  composites in conjunction with various
  environmental factors
• Development of a rational and defensible short
  term representative exposure to evaluate
  long-term fatigue performance

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Durability Research Needs

• Synergies
• Potentially important synergies between most of
  the durability factors considered in this module
  remain incompletely understood
• Research needed to elucidate the
  interrelationships between moisture, alkalinity,
  temperature, stress, and chemical exposures

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Additional Information
Additional information on all of the topics
discussed in this module is available
from www.isiscanada.com
ISIS EC Module 8