corrosion in reinforced concrete · 2017-06-22 · •bs en iso 12696 provides 4 criteria: •no...
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CED Working Day
Chris Atkins
Corrosion in Reinforced
Concrete
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1. Cement
Clay and limestone heated in a furnace to drive off
water, some can be replaced with slag / pfa
2. Crushed Rock
Filler, cheaper than cement, doesn’t generate heat
3. Water
Chemically reacts, cement goes hard
What is concrete?
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1. Good compressive strength
2. Poor tensile strength
3. Add steel to cope
Reinforced
Prestressed
Post Tensioned
Why’s it reinforced?
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1. When casting concrete heats up then cools
Early Thermal Shrinkage
2. Over time moisture levels equilibriate
Long Term Drying Shrinkage
3. Structural Requirements
Bending
Shear
Where does the tension come from?
L
Temp
L
TimeApplied Load
Deflected Shape
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Shrinkage
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Settlement
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Chemical Attack
• Sulphates
• Typically in the ground or sewage
• Acids
• Concrete is Alkaline (pH 13.5)
• Soft Water, Aggressive Carbon Dioxide
• Concrete has lots of calcium in it
• Alkali Aggregate Reaction
• Alkali in concrete reacts with the aggregate
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Salt Recrystalisation / Frost
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Fire Damage
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Abrasion
• Concrete is considered hard wearing
• Everything can be worn away given enough time
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Mechanical Damage
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Mostly
• Steel Rusts
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Corrosion – Eg Steel
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How Do Metals Corrode?
• Different voltages form on the metal
• Anode= lower voltage
• Cathode = higher voltage
• Electrons flow from anode to cathode in the metal
• Electrons released by turning into ions in solution
at the anode
• Electrons consumed at cathode by generating
alkalinity
Salt Solution
ZIN
C
CO
PP
ER
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Potentials
Zinc as
+ve
Stainless
as –ve
Col 2 +
756 +
Col 1
Zinc
Anode
Iron
Stainless
Voltmeters subtract the value in the negative
terminal from the value in the positive terminal
For European corrosion measurements, the
reading is termed as positive vs negative, or
positive against negative
121-843
-619
-756
-756
-93
34
44
0
0
0
0
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Potential Measurements
• Use a reference electrode
• Provides a stable value to measure against
• Steel connected to positive
• Reference connected to negative (com)
-347
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-347
IR Error
DC Supply
Current flows from anode to
rebar
Anode
-750
-1000
-1250
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Corrosion
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Corrosion of steel in concrete
• Concrete is a high pH – Steel passive.
• Chlorides cause local depassivation
• Punch holes in the passive film
• Internal or External
• Carbonation reduces pH
• Rust is bigger than steel
• Concrete spalls off
• Coatings can prevent carbonation and chloride ingress
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Corrosion of steel in concrete
Cathodes
Iron dissolves into
solution, electrons release
into the metal
Electrons from the anode react
with water and oxygen and
generate alkalinity
Anodes
Fe Fe2+ + 2e- 4e- + O2 + 2H2O 4 OH-
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CATHODES
ANODES
Repaired
CATHODE
SALTY and ANODIC
Repairing Chloride Induced Corrosion
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Cathodic Protection, Impressed vs Galvanic
22
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Decision To Repair
• BS EN 1504 covers process, cross referenced to BS EN ISO 12696 for
additional investigations to confirm suitability for CP
• Review available records
• Visual / Delam survey
• Chlorides / Carbonation can be undertaken if required
• Cover survey mandated
• Continuity testing mandated
• Half-cell survey mandated
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Cathodic Protection
½O2 + H2O + 2e- (metal) 2OH- (aq.)
Fe (metal) Fe2+ + 2e-
e-e-e-
+ve
Cl- Cl- Cl-
WATER WATER
OH-
OH-
OH-
OH-
WATER
SO42- SO4
2- SO42-
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Criteria
• BS EN ISO 12696 provides 4 criteria:
• No value more negative than -1100mV (-900mV for prestressed
concrete) due to hydrogen embrittlement
• Instant off more negative than -720mV OR
• 100mV decay over 24 hours OR
• 150mV over longer periods
• All measured against a silver/silver chloride 0.5M reference electrode
• Decay probes can also be used
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Competence of People
• BS EN 15257 (soon to be ISO 15257)
• Level 1 – follows method statements, takes readings (ISO level 2)
• Level 2 produces method statements, interprets readings (ISO level 3)
• Level 3 does design, commissioning and fault finding (ISO level 4)
• Supported by contractors, suppliers and designers
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Summary
• Steel Rusts
• Chlorides and carbonation
• CP fixes it
• Saves breakouts, propping, access and materials
• Codes available
• For design, materials, installation and competence of people
• Training available
• From lunch time seminars to full week training