CIVE1173 Deterioration Mechanisms

Concrete; reinforced, unreinforced

Steel; mild, stainless, cast, wrought

Timber; hard, soft, treated, untreated

Plastics; fibre reinforcement, coatings

• 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

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

Design; concept & parameters/assumptions

Original construction quality (design, materials, QA)

Environment; general, local

In-service usage

Maintenance

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

• Chloride Exposure
• Pollution
• High Temperatures
• Aggressive Soils/Sewerage
• Freeze/Thaw
• Electrochemical

Causes pitting 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

• Fe2+ ----->Fe²⁺ + 2e^-
• 2H₂O + O₂ + 4e^- -----> 4OH^-
• 

CO2 dissolves in pore water to form weak carbonic acid which lowers pH of concrete.  Result is depassivation of reo. Corrosion MAY occur

• 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)

• 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)

• 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

• 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)

• 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

• 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

• 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

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

UV degrades plastic & coatings; ASTM D1005 & D104, site exposure & test cabinets

Corrosion rate x2 for +10c

Microbiological species feed & breed at higher temps

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

• Aesthetics
• Relevant standards
• Protected species
• Lower life expectancy than original material

• 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

• 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)