Durability of FRP Composites for Construction

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

2. Composites FRP For Construction 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 ISIS EC Module 8

3. Composites FRP For Construction Introduction & Overview Reduction Factors Case Study Specifications Moisture & Marine Exposures Alkalinity & Corrosion High Temperatures & Fire Creep Fatigue Cold Temperatures & Freeze-Thaw UV Radiation Outline ISIS EC Module 8

4. Composites FRP For Construction 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

5. Composites FRP For Construction Introduction & Overview Section: 1 Key uses of FRPs in construction: • Internal reinforcement of concrete Corrosion of steel reinforcement in concrete structures contributes to infrastructure deterioration Use non-corrosive FRP reinforcement • External strengthening of concrete Provide external tension or confining reinforcement (FRP plates, sheets, bars, etc.) ISIS EC Module 8

6. Composites FRP For Construction High-strength fibres Polymer matrix 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 ISIS EC Module 8

7. Composites FRP For Construction 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 ISIS EC Module 8

8. Composites FRP For Construction 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) ISIS EC Module 8

9. Composites FRP For Construction 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: ISIS EC Module 8

10. Composites FRP For Construction 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: ISIS EC Module 8

11. Composites FRP For Construction 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 ISIS EC Module 8

12. Composites FRP For Construction 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 ISIS EC Module 8

13. Composites FRP For Construction 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 ISIS EC Module 8

14. Composites FRP For Construction 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

15. Composites FRP For Construction 2500 2000 1500 Stress [MPa] 1000 500 1 2 3 Strain [%] 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 ISIS EC Module 8

16. Composites FRP For Construction Material 200 GPa Failure Strain 2-2.6 % 50-74 GPa 147-165 GPa 2-4.5 % 30-55 GPa >10 % Elastic Modulus 1-1.5 % Ultimate Strength Glass FRP 517-1207 MPa Carbon FRP 1200-2410 MPa Aramid FRP 1200-2068 MPa Steel 483-690 MPa Quantitative Comparison Section: 1 Typical Mechanical Properties* * Based on 2001 data for specific FRP rebar products ISIS EC Module 8

17. Composites FRP For Construction 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 ISIS EC Module 8

18. Composites FRP For Construction Unidirectional glass FRP bar Glass FRP grid Carbon FRP prestressing tendon Glass fibre roving Carbon fibre roving Introduction & Overview Section: 1 Examples of FRP ISIS EC Module 8

19. Composites FRP For Construction 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 ISIS EC Module 8

20. Composites FRP For Construction 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” ISIS EC Module 8

21. CAUTION! Composites FRP For Construction 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 ISIS EC Module 8

22. Composites FRP For Construction 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 ISIS EC Module 8

23. Composites FRP For Construction 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) ISIS EC Module 8

24. Composites FRP For Construction 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 ISIS EC Module 8

25. Composites FRP For Construction 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 ISIS EC Module 8

26. Composites FRP For Construction 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 ISIS EC Module 8

27. Composites FRP For Construction 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 ISIS EC Module 8

28. Composites FRP For Construction 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 ISIS EC Module 8

29. Composites FRP For Construction < 1% % Mass Gain 0 1 2 Time (years) Moisture & Marine Exposures Section: 2 Moisture Typical moisture absorption trend for a matrix polymer: ISIS EC Module 8

30. Composites FRP For Construction 100 % % Strength Retention 10 0 5 Time (years) 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 ISIS EC Module 8

31. Composites FRP For Construction 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 ISIS EC Module 8

32. Composites FRP For Construction 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 ISIS EC Module 8

33. Composites FRP For Construction 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 ISIS EC Module 8

34. Composites FRP For Construction pH > 11 GFRP bar Alkalinity & Corrosion Section: 3 Alkalinity Effects of alkalinity on FRPs’ performance: • The pH level inside concrete is > 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 ISIS EC Module 8

35. Composites FRP For Construction GFRP bar 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 ISIS EC Module 8

36. Composites FRP For Construction 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 ISIS EC Module 8

37. Composites FRP For Construction 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 ISIS EC Module 8

38. Composites FRP For Construction Steel bar CFRP bar Spacer Steel girder GFRP sheet CFRP sheet 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 ISIS EC Module 8

39. Composites FRP For Construction 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 ISIS EC Module 8

40. Composites FRP For Construction 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 ISIS EC Module 8

41. Composites FRP For Construction 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 > Tg Sudden and severe loss of bond at T > Tg ISIS EC Module 8

42. Composites FRP For Construction 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 Tgresults 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 ISIS EC Module 8

43. Composites FRP For Construction 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 ISIS EC Module 8

44. Composites FRP For Construction High Temperatures & Fire Section: 4 Deterioration of mechanical and bond properties for GFRP bars: Critical temperature (T > Tg) ISIS EC Module 8

45. Composites FRP For Construction 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 ISIS EC Module 8

46. Composites FRP For Construction 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 ISIS EC Module 8

47. Composites FRP For Construction 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’ ISIS EC Module 8

48. Composites FRP For Construction 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 ISIS EC Module 8

49. Composites FRP For Construction Ultraviolet Radiation Section: 6 Ultraviolet (UV) radiationdamages 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 ISIS EC Module 8

50. Composites FRP For Construction 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 ISIS EC Module 8