Successfully reported this slideshow.

corrosion and protection of steel reinforced concrete

99

Share

Loading in …3
×
1 of 51
1 of 51

More Related Content

Related Books

Free with a 14 day trial from Scribd

See all

corrosion and protection of steel reinforced concrete

  1. 1. CORROSION AND PROTECTION OF STEEL REINFORCED CONCRETE PROVIDED BY: EMAD BEHDAD LECTURER: PROF.SHAMS
  2. 2. OUTLINE • INTRODUCTION • CORROSION PROCESS • TYPES OF CORROSION • CAUSES OF CORROSION • PROTECTION METHODS • CONCLUSION
  3. 3.  ASTM terminology (G 15) defines corrosion as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.” For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of the original steel and none of the good mechanical properties. Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.
  4. 4. Electrochemical process of steel corrosion in concrete
  5. 5. Volumetric change
  6. 6. • Ca, Na, K hydroxides in hydrated cement raise the pH to ~13.5  A dense protective ferric oxide (Fe2O3) passive film forms around the reinforcement Passive film develops on the bar surface • This passive film stops iron dissolution, and is stable at pH >10 pH >13
  7. 7. Yes if: (a) Concrete is always dry, then there is no H2O to form rust. Also aggressive agents cannot easily diffuse into dry concrete. (b) Concrete is always wet, then there is no oxygen to form rust. (c) Cathodic protection is used to convert all the reinforcement into a cathode using a battery. This is not easy to implement because anodic mesh is expensive, and this technology is not easy to install and maintain
  8. 8. (d) A polymeric coating is applied to the concrete member to keep out aggressive agents. These are expensive and not easy to apply and maintain. (e) A polymeric coating is applied to the reinforcing bars to protect them from moisture and aggressive agents. This is expensive and there is some debate as to its long- term effectiveness. (f) Stainless steel or cladded stainless steel is used in lieu of conventional black bars. This is much more expensive than black bars.
  9. 9. Can we avoid corrosion? No, not entirely: Concrete is not usually under water or continuously dry. Aggressive agents such as carbon dioxide, de-icing agents and/or sea water can diffuse into the best of moist concrete, and corrosion will eventually result.
  10. 10. COMMON CORROSION TYPES 1) Crevice Corrosion Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas). Oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal. Oxygen content of liquid at the mouth of the crevice which is exposed to the air is greater, so a local cell develops in which the anode, or area being attacked, is the surface in contact with the oxygen-depleted liquid.
  11. 11. Crevice Corrosion of Rebar Has Some Similarities with Filliform Corrosion The head of the advancing filament becomes anodic, with a low pH and a lack of oxygen, as compared with the cathodic area immediately behind the head where oxygen is available through the semipermeable film. Corrosion proceeds as the cathode follows behind the anodic head (from Corrosion Basics NACE).
  12. 12. 2) Pitting Theories of passivity fall into two general categories, one based on adsorption and the other on presence of a thin oxide film. Pitting in the former case arises as detrimental or activator species, such as Cl-, compete with O2 or OH- at specific surface sites. By the oxide film theory, detrimental species become incorporated into the passive film, leading to its local dissolution or to development of conductive paths. Once initiated, pits propagate auto-catalytically according to the generalized reaction, M+n + nH2O + nCl- → M(OH)n + nHCl, resulting in acidification of the active region and corrosion at an accelerated rate (M+n and M are the ionic and metallic forms of the corroding metal).
  13. 13. Chlorides Airborne, marine, industrial, groundwater, cast-in Cl– can penetrate through the passive film At Cl- > “threshold”, passive film breaks down, corrosion initiates Cl- “threshold” value is typically 0.05% by wt of concrete (0.02% prestressed concrete) Pitting corrosion Chlorides are main cause of reinforcement corrosion
  14. 14. Carbonation Ca(OH)2 + CO2 → CaCO3 + H2O
  15. 15. EFFECT OF CARBONATION  It can cause soft surface, dusting and color change  It reduces quality concrete  It reduces the concrete ability to protect reinforcement from corrosion (in an exposed environment)  It will result in additional shrinkage in carbonated region.
  16. 16. DETECTING CARBONATION  Depth of carbonation can be detected using an indicator.  A chemical such as Phenolphthalein sprayed on to freshly broken concrete.  Areas remaining alkaline will turn in a bright purply-pink color.  Carbonated areas of concrete will remain unchanged in color.
  17. 17. Cl- Cl- +ve Ions +ve Ions e- Fe Fe++ e- e- e- Rebar Rebar
  18. 18. Reinforcing steel corrosion Migration of chlorides, H20 Corrosion of the steel and O2 into the concrete, no reinforcement and corrosion and no damage to cracking and/or spalling concrete of concrete Degree of Corrosion Initiation Propagation (corrosion) Critical chloride threshold I Time
  19. 19. Cl- Cl- . Cl-. .- . Cl- .- Cl- . Cl- . Cl Cl . - . Cl- . Cl- Cl- . -. Cl Cl- Cl- . . . . pH >~10 . . . Cl . Cl- . . . . Cl- . - . - .- . .- . . . . . . . Cl - Cl Cl . Cl Cl- Cl e e - - Cathode . Cathode . . . Anode . . . . . . . . . ClElectrolyte - . Cl- . Cl- . . Cl . - Cl- . Cl- Cl- Cl. - . Cl- . Cl- Cl- Cl- Iron Oxygen Moisture Corrosion = Iron + Oxygen + Moisture Either  the pH falls due to carbonation or other chemicals  chlorides reach the steel above the threshold concentration  an electrical charge destroys the natural protection of the steel  Electrons flow and ions migrate  Rust expansion causes cracking  Rapid deterioration  Spalling
  20. 20. Rebar Spalling loss Cracks with Rust Delamination Staining
  21. 21. Abandoned Electric Pole
  22. 22. KISH ISLAND
  23. 23. BANDAR ABBAS
  24. 24.  chloride induced reinforcement corrosion in concrete exposed to seawater Corroded rebar from cracked concrete of a parking structure exposed to deicing salts
  25. 25. Reinforced steel in concrete cracking
  26. 26. CORROSION PREVENTION METHODS  REBAR COATING  SCARIFIED & PATCHED DECK AWAITS ANODE MESH  FLY ASH  HOT-DIP GALVANIZING  WIRELESS SENSOR FOR MONITORING CHLORIDE IN CONCRETE  INHIBITORS
  27. 27. REBAR COATING
  28. 28. EPOXY COATING PREVIEW MODEL
  29. 29. EPOXY‐COATED BARS Anode Cathode Reduces anode area Reduces cathodic area Increases threshold* REDUCED CORROSION Electrical Connection Ionic path • Electrical path between anode and cathode Makes ionic pathway longer
  30. 30. thermally sprayed coatings of Zn and Al, combat corrosion  For atmospheric, buried, and marine environment corrosion protection, Zn (TSZ), Al (TSA), and their alloys have proven that they provide long term corrosion protection and outperform most all other methods.  Anodic (TSZ/TSA) metal coatings applied to steel cathodes (more noble than Zn or Al), are referred to as cathodic or sacrificial protection coating systems.  These thermal spray coatings provide corrosion protection by excluding the environment (or electrolyte) and acting as a barrier coating (like paints, polymers, and epoxies), but unlike typical barrier coatings they also provide sacrificial anodic protection.
  31. 31. Zinc and zinc alloys are also sprayed directly onto concrete to protect the steel rebar within Arc spraying of zinc on a concrete bridge pier in the Florida Keys. In this case the zinc acts as sacrificial anode, although it is more frequently used in impressed-current systems. Three impressed-current zinc systems have already been installed by the Ministry of Transportation of Ontario in Toronto Sacrificial cathodic protection of steel in concrete by thermal zinc spraying
  32. 32. FLY ASH  using a Fly Ash concrete with very low permeability, which will delay the arrival of carbonation and chlorides at the level of the steel reinforcement.  Fly Ash is a finely divided silica rich powder that, in itself, gives no benefit when added to a concrete mixture, unless it can react with the calcium hydroxide formed in the first few days of hydration. Together they form a calcium silica hydrate (CSH) compound that over time effectively reduces concrete diffusivity to oxygen, carbon dioxide, water and chloride ions. By reducing ion diffusion, the electrical resistance of the concrete also increases
  33. 33. CATHODIC PROTECTION Impressed current (active) Sacrificial anode (passive)
  34. 34. TITANIUM ANODE MESH A. TYPICALLY ATTACHED TO THE CONCRETE SURFACE AND THEN ENCAPSULATED IN CEMENTITIOUS MATERIALS. B- EASILY CONFORMS TO THE STRUCTURE GEOMETRY. C- MOST USED IMPRESSED CURRENT ANODE FOR CONCRETE.
  35. 35.  Mixed Metal Oxide activated Titanium Anodes in the form of a ribbon mesh can be installed in close proximity and parallel to the reinforcement bars (rebar). MMO Ribbon Mesh
  36. 36. 1. Simple to Install. 2. No Power Supply Needed. 3. No Wiring or Conduit. 4. No Long-Term Monitoring or Maintenance
  37. 37. Conventional Patch Repair
  38. 38. Embedded Zinc Anode for Patch Repair
  39. 39. CATHODIC PROTECTION SACRIFICIAL ANODE
  40. 40. REFRENCES  Concrete Society Technical Reports TR 36 and 37
  41. 41.  www.corrocell.co.uk

×