Do Nothing Else Future Prevention • Impermeable repair material • Sealants/waterproofing • Improved Improved drainage • Penetrating Penetrating inhibitors
Surface Repairs
Durable repair material
Spall
Crack Heavily corroded rebar
Contaminated concrete with chlorides or carbonation Delamination
Bonding new to old Reinforcing steel protection Reinforcing steel cleaning Concrete surface conditioning Removal of contaminated concrete & undercutting of exposed steel Edge conditioning
Damage due to Corrosion Corrosion
2
Removal Concrete Removal of Damaged Concrete
Patch Patch with New Concrete
Edited from ICRI Movie on Surface Repair
3
Simple Patch Configuration Wrong
Right
4
Patch Repairs – Check List 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)
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6
7
8
Repair/Substrate - Incompatibility
No chlorides High pH
Chloride & Lower pH
Creation Creation of of a New New Galvanic Galvanic Cell Cell Formation of ‘incipient’ anode adjacent to the new patch
Cathode: O 2 + 2H 2O + 4e
4OH -
Anode: 2Fe 2Fe 2+ + 4e-
9
Spall in Slab
Patch Repair
“Anodic Ring” Ring” Effect
10
Improving the Performance of Patch Repairs Treating the Surface H2O
Cl
Membranes
No chlorides High pH
Chloride & Lower pH
H2O vapour
Reduces moisture content in the slab & prevents further chloride ingress.
But does it reduce corrosion?
11
Improving the Performance of Patch Repairs Treating the Surface H2O
Cl
Dense overlay No chlorides High pH
Chloride & Lower pH
H2O vapour
Reduces moisture content in the slab & prevents further chloride ingress.
But does it reduce corrosion?
Improving the Performance of Patch Repairs Treating the Surface
Penetrating Sealer
H2O No chlorides High pH
Cl Chloride & Lower pH
H2O vapour
Reduces moisture content in the slab & prevents further chloride ingress.
But does it reduce corrosion?
12
Improving the Performance of Patch Repairs Treating the Steel Rebar
EpoxyEpoxy-coating the exposed steel
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?
Improving the Performance of Patch Repairs Treating the Steel Rebar Coating the exposed steel with zinc paint
Zn → Zn 2
+
+
2e
−
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?
Improving the Performance of Patch Repairs Installing a Discrete Sacrificial Anode
Norcure System Courtesy of http://www.norcure.com
13
Norcure Embedded Galvanic Anode
Sacrificial metal (zinc) core
Cementitious matrix Tie wires
Improving the Performance of Patch Repairs 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 the time to the next repair?
Courtesy of http://www.norcure.com
Courtesy of http://www.norcure.com
14
Courtesy of http://www.norcure.com
Courtesy of http://www.norcure.com
Courtesy of http://www.norcure.com
15
Courtesy of http://www.norcure.com
Courtesy of http://www.norcure.com
Courtesy of http://www.norcure.com
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Courtesy of http://www.norcure.com
Courtesy of http://www.norcure.com
Removing all the Chloride Contaminated Concrete
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18
Hydromilling
19
Complete Slab Replacement
Cathodic Protection
Sacrificial Anode 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 -
H O 4
→
-
e 4 + O 2 H +
Zn → Zn 2
+
+
2e
−
2
O
20
Impressed Current
2e-
+
2 H 2O → 2 H
+
O2 + 2e
−
“Inert Anode”
2 NaCl → 2 Na
+
+
Cl2
+
2e
−
2e-
1 O + H O + 2e 2 2 2
−
→
−
2OH
H 2O + e
−
−
H + OH
→
21
22
Requirements for Impressed Current CP
23
24
Summary of CP Requirements
• Direct current supply • Impressed current anode • Electrolyte (concrete) • Cathode (rebar) • Electrical and electrolytic conductivity
Control & Monitoring Current 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 Fe → Fe 2
+
+
2e
−
2e-
2e-
I I
Apply Cathodic Protection Protection
Cl -rich patch Fe → Fe 2
+
+
2e
−
2e-
2e-
I I
Apply Cathodic Protection Protection
e-
e-
I I
26
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.
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 the probe (electron flow in reverse direction)
Cl -rich patch Fe → Fe 2
+
+
2e
−
2e-
2e-
I
27
The CP system is then connected.
Fe → Fe 2
+
+
2e
−
2e-
2e-
I
As the system current is increased the current flow to the probe will be reduced as the system becomes more negative.
e-
1 O + H O + 2e 2 2 2
−
→
−
2OH
Fe → Fe 2
+
+
2e
−
e-
e-
I
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.
2e-
1 O + H O + 2e 2 2 2
−
→
−
2OH
1 O + H O + 2e 2 2 2
−
→
−
2OH
e-
e-
I
28
Measuring potential versus an embedded reference half-cell (e.g. Ag/AgCl, Mn/MnO2)
