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Materials
Concrete; reinforced, unreinforced
Steel; mild, stainless, cast, wrought
Timber; hard, soft, treated, untreated
Plastics; fibre reinforcement, coatings
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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
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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
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Life of structure determined by:
Design; concept & parameters/assumptions
Original construction quality (design, materials, QA)
Environment; general, local
In-service usage
Maintenance
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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
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Aggressive Environments
- Chloride Exposure
- Pollution
- High Temperatures
- Aggressive Soils/Sewerage
- Freeze/Thaw
- Electrochemical
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Chloride Attack
Causes pitting corrosion
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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
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Steel corrosion - formula for anodic & cathodic
- Fe2+ ----->Fe2+ + 2e-2H2O + O2 + 4e- -----> 4OH-

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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
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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)
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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)
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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
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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)
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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
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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
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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
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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
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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
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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
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Timber Repair
- Aesthetics
- Relevant standards
- Protected species
- Lower life expectancy than original material
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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
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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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