05 corr repair

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Repair & Maintenance ofRepair & Maintenance ofConcrete StructuresConcrete Structures

5. Repair of Corrosion5. Repair of Corrosion--Damaged StructuresDamaged Structures

Emmons, 1994

Innovative Repair?Innovative Repair?

Cause Effect

• Chlorides at steel

• Carbonation

• Insufficient cover

• Poor quality concrete

• Cracking

• Rust staining

• Spalling

• Delamination

Corrosion

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Repair the Symptom• Replace delaminated concrete• Fix cracks• Overlay

Address the Cause• Remove chloride-contaminated

concrete• Remove chlorides (ECE)

Alter the Process• Cathodic protection

Future Prevention• Impermeable repair material• Sealants/waterproofing• Improved drainage• Penetrating inhibitors

Do Nothing Else

Surface RepairsSurface Repairs

Delamination

Contaminated concretewith chlorides orcarbonation

Spall

Heavilycorrodedrebar

CrackDurable repair material

Bonding new to oldReinforcing steel protection

Concrete surface conditioning

Removal of contaminated concrete &undercutting of exposed steel

Edge conditioning

Reinforcing steel cleaning

Damage due to CorrosionDamage due to Corrosion

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Removal of Damaged ConcreteRemoval of Damaged Concrete

Patch with New ConcretePatch with New Concrete

Edited from ICRI Movie on Surface RepairEdited from ICRI Movie on Surface Repair

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Simple Patch Configuration

RightRight

WrongWrong

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Locate embedded electrical conduit or post-tensionning tendons using pachometer or other techniques (e.g. radiography).

Conduct structural review before removal of significant amounts of concrete to determine if support (e.g. shoring) is required.

Exposed corroded rebar should be undercut to ensure adequate coverage and bond with new concrete (minimum clearance of ¾ inch or ¼ inch more than max aggregate size in patching concrete)

The full circumference of the exposed bar should be cleaned

Care should be taken to avoid damaging the bond around any exposed uncorroded bar

Loose rebar should be tied to other secure bars

If more than 25% of the cross section of a bar has been lost (or more than 20% of the cross section of two bars in close proximity) a structural review should be conducted to determine if repair or replacement of the bars is necessary.

Unless shotcrete repairs are anticipated, the edges of the patch should be cut straight and square with the surface to ensure maximum integrity of the patch (i.e. avoid feathered edges)

Patch Repairs – Check List

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Repair/Substrate Repair/Substrate -- IncompatibilityIncompatibility

No chloridesHigh pH

Chloride & Lower pH

Creation of a New Galvanic CellCreation of a New Galvanic Cell

Anode:2Fe → 2Fe2+ + 4e-

Cathode:O2 + 2H2O + 4e- → 4OH-

Formation of ‘incipient’ anodeadjacent to the new patch

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SpallSpall in Slabin Slab

Patch RepairPatch Repair

““Anodic RingAnodic Ring”” EffectEffect

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Improving the Performance of Patch RepairsImproving the Performance of Patch Repairs

No chloridesHigh pH

Chloride & Lower pH

Treating the Surface

MembranesH2O

H2O vapour

Reduces moisture content in the slab & prevents further chloride ingress.

But does it reduce corrosion? But does it reduce corrosion?

Cl

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No chloridesHigh pH

Chloride & Lower pH


H2O vapour



H2O Cl

Denseoverlay


No chloridesHigh pH

Chloride & Lower pH


H2O vapour



H2O ClPenetratingSealer

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Treating the Steel Rebar

Electrically isolates the steel in the patch and prevents it from becoming a cathode and reduces the risk of incipient anode formation.

But does it extend the time to the next repair? But does it extend the time to the next repair?

EpoxyEpoxy--coating the exposed steelcoating the exposed steel


Treating the Steel Rebar

The zinc coating will become a “sacrificial” anode and may prevent the formation of an incipient anode formation adjacent to the patch.

But does it extend the time to the next repair? But does it extend the time to the next repair?

Coating the exposed steel with zinc paintCoating the exposed steel with zinc paint

−+ +→ eZnZn 22


Installing a Discrete Sacrificial Anode

Courtesy of http://www.norcure.com

NorcureNorcure SystemSystem

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Sacrificial metal(zinc) core

Cementitiousmatrix

Tie wires

Norcure Embedded Galvanic Anode


Installed Galvanic Anode


The zinc becomes a “sacrificial” anode and provides cathodic protection to the steel in the vicinity of the patch.

Recent testing has demonstrated that this system does extend theRecent testing has demonstrated that this system does extend the time to the time to the next repair? next repair?


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Removing all the Chloride Contaminated ConcreteRemoving all the Chloride Contaminated Concrete

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Hydromilling

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Complete Slab ReplacementComplete Slab Replacement

Cathodic ProtectionCathodic Protection

−+ +→ eZnZn 22

O2

+ H

2O +

4e-→

4OH

-

Less noble metal (e.g. Zinc) connected to steel reinforcement and confers protection to the steel through “sacrificial” corrosion.

Note: the anode is consumed by the process.

Only suitable when the resistivity of the system is low and good electrolytic contact is maintained – e.g. submerged or buried structures, particularly marine structures

Sacrificial AnodeSacrificial Anode

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“Inert Anode”

2e-

−− →++ OHeOHO 2221

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2e-

−+ ++→ eOHOH 222 22−+ ++→ eClNaNaCl 222 2

Impressed CurrentImpressed Current

−− +→+ OHHeOH 2

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Requirements for Impressed Current CPRequirements for Impressed Current CP

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Summary of CP RequirementsSummary of CP Requirements

• Direct current supply• Impressed current anode• Electrolyte (concrete)• Cathode (rebar)• Electrical and electrolytic conductivity

Control & MonitoringControl & MonitoringCurrent Density

Typically current densities are in the range of 10 to 20 mA per m2 steel although higher values (up to 50 mA/m2) may be required in the most severe environments (e.g. poor quality concrete, low cover, high chlorides, high O2 environment, fluctuating moisture, high temperatures, extensive active corrosion on the steel)

Power Supply

It is usual to provide a number of small power supply units to protect individual (isolated) zones - typically 50 to 100 m2 area.

This usually translates to total current requirements of < 10 A and relatively low voltages of < 10 V (DC) to drive an individual zone (circuit). The actual voltage needed to deliver the required current will depend on the electrical resistivity of the concrete.

Supplies are usually transformer rectifier units that run on standard AC mains voltage and provide full control of the DC output currents and voltage.

Chess, 1998

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Cl-rich patch

I

−+ +→ eFeFe 22

2e-2e-

I

Cl-rich patch

I

−+ +→ eFeFe 22

2e-2e-

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Apply Cathodic Apply Cathodic ProtectionProtection

Ie-e-

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Apply Cathodic Apply Cathodic ProtectionProtection

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Macrocell Probes – based on creating a local aggressive condition around an isolated piece of steel. An area of concrete is cut out and the steel bar isolated.

Cl-rich patch

The isolated bar – “probe” – is then connected to the main reinforcement via a “zero-resistance” current meter.

The hole is filled with chloride-rich concrete (Cl in patch > Cl in parent concrete)

Cl-rich patch

The current flow between the probe and main reinforcement is measured. Current should flow from the main reinforcement to theprobe (electron flow in reverse direction)

−+ +→ eFeFe 22

2e-2e-

I

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The CP system is then connected.

−+ +→ eFeFe 22

2e-2e-

I

As the system current is increased the current flow to the probe will be reduced as the system becomes more negative.

−+ +→ eFeFe 22

e-e-

I

e-−− →++ OHeOHO 222

122

As the system current is increased the current flow to the probe will be reduced as the system becomes more negative. When the current flow between the probe and main reinforcement passes through zero, this is considered to be a satisfactory level of protection.

I

2e-−− →++ OHeOHO 222

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e-e-

−− →++ OHeOHO 2221

22

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Measuring potential versus an embedded reference half-cell (e.g. Ag/AgCl, Mn/MnO2)

Criteria:• Absolute potential• “Instant-off” potential• Polarization decay

CP RepairCP RepairStrategyStrategy

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Electrochemical Chloride Extraction

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Cellulose fibres soaked in ‘anolyte’ (e.g. sodium borate) sprayed on concrete surface

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CP RepairCP RepairStrategyStrategy

ApplyECE

Protect from further Cl

ingress

Yes

ECE RepairECE RepairStrategyStrategy

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Repair of Carbonation-Induced Corrosion

Reinstate damaged concrete

Apply Coating(e.g.CO2 Barrier)

ElectrochemicalRealkalization

NaNa++

KK++

CO2

NaNa++

KK++

CO2

CO2-resistantCoating

Realkalization

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BeforeBefore AfterAfter

Protecting the Repaired Structure

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REPAIR?REPAIR?Yes EXECUTEEXECUTE

REPAIRREPAIRDESIGNREPAIR

No

Adapted from Emmons, 1994

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We will look at the issues that affect these steps in the process later on in the course!

Broomfield, 1997

Broomfield, 1997

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Broomfield, 1997

Broomfield, 1997

REPAIR?REPAIR?Yes EXECUTEEXECUTE

REPAIRREPAIR

DESIGN/SELECTREPAIR

Adapted from Emmons, 1994

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05 corr repair

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