CIVE1173 Deterioration Mechanisms

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higgsboson
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224439
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CIVE1173 Deterioration Mechanisms
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2013-06-23 10:34:21
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Deterioration Mechanisms
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Week 2 lecture notes summary. Exam
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  1. Materials
    Concrete; reinforced, unreinforced

    Steel; mild, stainless, cast, wrought

    Timber; hard, soft, treated, untreated

    Plastics; fibre reinforcement, coatings
  2. Aggressive Environments
    • Types of attack
    • - chloride exposure; marine environment,
    • de-icing salts, admixtures
    • - pollution; CO2, NOx, SOx
    • - high temperatures; UV exposure
    • - aggressive soils/sewerage; sulphates
    • - freeze/thaw
    • - electrochemical; DC rail power, cathodic protection systems

    Deterioration mechanisms
  3. Structure longevity
    5 years; temporary, short-term repairs

    10 years; medium-term repairs

    20 years; roads

    50 years; most infrastructure

    100 years; major or critical infrastructure

    “Forever”; heritage
  4. Life of structure determined by:
    Design; concept & parameters/assumptions

    Original construction quality (design, materials, QA)

    Environment; general, local

    In-service usage

    Maintenance
  5. Durability
    Environmental exposure;  acid rain, water table changes, new factories, increased traffic, pollution

    Costs; 15% of all (343,000) reinforced concrete bridges structurally deficient

    Estimated repair costs $4.0billion for the next 10 years

    • Estimated repair and maintenance budget for developed countries is 3-5% GDP (includes Australia)
    • 2 tonnes of concrete for every person in the world
  6. Aggressive Environments
    • Chloride Exposure
    • Pollution
    • High Temperatures
    • Aggressive Soils/Sewerage
    • Freeze/Thaw
    • Electrochemical
  7. Chloride Attack
    Causes pitting corrosion
  8. Steel corrosion
    caused by chlorides, acids, bacteria

    precautions; stainless, coatings (galv’, plastic, paint), allowances (additional thickness)

    in concrete steel is passive

    oxide film formed on steel due to high pH

    pH in range 12-14

    pore water in concrete acts as electrolyte (Na+, K+, Ca2+, H+, OH-)

    passive film around reo is broken down – area starts to corrode (anode) – area of steel with intact film feeds reaction (cathode)

    • corrosion initiation; Cl concentration & Cl/OH ratio, cement composition, steel type (mild corrodes more easily), defects (breaks down passive layer), moisture content (current
    • will flow), temperature (every +10c doubles rate)

    Water & Oxygen needed; saturated or dry structures won’t corrode

    p25 Week 2 notes for typical corrosion cell
  9. Steel corrosion - formula for anodic & cathodic
    • Fe2+ ----->Fe2+ + 2e-
    • 2H2O + O2 + 4e- -----> 4OH-
  10. Carbonation attack
    CO2 dissolves in pore water to form weak carbonic acid which lowers pH of concrete.  Result is depassivation of reo. Corrosion MAY occur
  11. Corrosion
    • General; very small anodes/cathodes separated by only few mm; red/orange expansive oxides; corrosion rates less than 10 microns/year; usually caused by carbonation (chlorides unlikely)
    • Pitting; small anodic areas & large remote cathodes requires low resistivity in concrete
    • up to 8 times faster than general corrosion
    • may lead to serious section loss usually black or dark brown – less expansive
    • Electrochemical; stray current from long sections of cable near adjoining reo
    • Cathodic protection systems (unconnected steel corroded by CPS)
  12. ASR (AAR - Alkali Aggregate Reaction) Attack
    • Aggregates can react with OH- in the pore water to produce Alkali Silica Reaction (ASR)
    • Requires; high alkali content (Na2O,
    • K2O), moisture, reactive aggregate
    • Test susceptibility; EN 206 – 1, ASTM
    • 295 (petrographic), Rilem AAR 3, BS 812 Part 123 (expansive prism)
    • Mitigation; non reactive aggregate, limit alkali content, blended cement (PFA, silica fume), lithium compunds (LiNO3)
  13. Sulphate Attack
    • Reacts with cement components
    • Sea water
    • Sulphate in ground water (diffuses
    • into concrete in the pore water)
    • Acid sulphate soils
    • Sewerage
    • Products; ettringite, gypsum, thaumasite
    • - Gypsum
    • - Ettringite
    • - Thaumasite
  14. Gypsum Attack
    • Calcium hydroxide and sulphate react to form gypsum; CaSO4. 2H2O
    • Gypsum will subsequently react with calcium aluminate hydrate (C-A-H) to form ettringite (expansive reaction)
    • And with calcium carbonate and C-S-H gel to form thaumasite (Weakens strength and makes concrete friable)
  15. Sulphate Attack – Anion Effect
    • Most common anion form is Na+
    • Anaerobic bacteria reduces the sulphate to sulphide (e.g. sewer)
    • Sulphide is oxidised to form acid
    • Acid then dissolve the cement matrix
  16. Leaching
    • Water flows through cracks in concrete
    • Minerals in cement paste leach out (mainly Ca2+, also Na+ & K+)
    • White precipitate on surface is efflourescence
    • Eventually leads to loss of strength
  17. Salt scaling
    • Dissolved salts; sea water, ground water
    • Water sucked into concrete and move via capillary action
    • Water evaporate from the surface; high temps & winds, coastal structures particularly at risk
    • Salts precipitate in pores in concrete causing expansion
  18. Freeze/Thaw
    Freezing water expands leading to “pop-outs”

    • Increases ease of water entry leading to
    • increased rate of deterioration

    Use air entrainment to allow expansion of water into voids
  19. Thermal exposure
    UV degrades plastic & coatings; ASTM D1005 & D104, site exposure & test cabinets

    Corrosion rate x2 for +10c

    Microbiological species feed & breed at higher temps
  20. Timber Deterioration
    Deterioration; moisture (dry out = shrinkage, water exposure = rot), insects (termites, beetles), fungi (dry & wet rot)

    Defect location will determine effect on timber

    Timber species are susceptible to different deterioration mechanisms

    Defect location affects result (mid-span significant in bending, end significant in shear)

    Deterioration specific to species

    Treatment will reduce decay

    Lab testing not in situ
  21. Timber Repair
    • Aesthetics
    • Relevant standards
    • Protected species
    • Lower life expectancy than original material
  22. Stone and Masonry - weathering
    • Moisture; freeze/thaw, leaching
    • Acidic gases/acid rain
    • Increased loading; exceed structural capacity, frequent loading leading to fatigue
    • Skills / trades no longer available
    • Match aesthetic
    • Current standards / hidden strengthing
  23. Plastics and coatings
    • Some bacteria can digest plastics
    • UV exposure deterioration
    • Moisture affecting bond & performance
    • Damage to coating means increased corrosion rate
    • Application
    • Poor QA affects performance
    • Environmental conditions will affect adhesion (moisture is death)
    • Performance of coatings is specific to environment (go robust to be sure)

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