cathodic protection (cp) of steel in concrete -...
TRANSCRIPT
Why CP of concrete
• Traditional patching and repair will re-
introduce cracks, spalling and damage,
• Risc and safety regarding the integrity of
the construction, and such as falling
pieces of concrete,
• Costs.
Steel in concrete• Steelpassivation in an alkaline environment with pH ≥ 9,5
• Portland cement hydration > CSH + Ca(OH)2 with pH = 12 - 13
1 2 3 4 5 6 7 8 9 10 11 12 13
acid alkaline
passivation
Main causes of concrete
cracks and spalling
• Carbonation Acidification of the concrete (lowering pH) due
to CO2 with Ca(OH)2 into CaCO3
(calciumcarbonate pH=8-9)
• Chlorides (salts) additive (CaCl2) accelerator
Sand contaminated with salt,
Salt penetration, seawater, salt-spray, etc
Carbonationprocess
CO2
Carbon Dioxyde in
the air penetrates
the concrete.
Lowering the
alkalinity
CO2
CO2
The
carbonationprocess
continues slowly
Until reaches
the steel
reinforcement CO2
CO2
Reinforcement
starts to corrode
CO2
CO2
Cracking
and spalling
continues.
Pieces of
concrete
comes off
Chlorides
• Corrosion is possible in an alkaline
environment,
• Chlorides forms acids during pitting proces
• Chlorides are hygroscopic en pulls moist
into the concrete,
• Anode/cathode surface ratio of pitting is
very small = increased corrosion velocity
Potentialmapping
• If no visual damage can be seen, but reinforcement corrosion might be present.
• Selective repair method for economic reasons.
Repair methods in case of
chloride initiated corrosion
• Reinforcement has to be completely
recovered with fresh (chloride-free)
concrete or mortar.
• Chloride-extraction,
• Cathodic Protection (CP).
Basics of cathodic protection
• By making the potential (in Voltage) of the
reinforcement more negative then the
most negative corrosion potential :
cathodic reaction.
• NACE and EN standard : < -850 mV CSE.
• Cathodic electrochemical reaction :
O2 + 2H2O + 4e => 4OH- (high pH).
Types of CP of concrete
• Galvanic systems.
• Impressed current systems with discrete
anodes, titaniumwire and conductive
coatings.
Galvanic system
• Reinforcement directly connected to the zinc
anode,
• Principle based on sacrificial anode (zinc)
system.
electrons
anode cathode
- +
electrolyte
ions
galvanic cell
galvanic system
• No wiring installation or external powersource needed.
• Simple and easy installation.
• Cost-effective
• Offers sufficient current in dry environment (acc. European standard EN 12696).
• Repair mortars : mineral based sand–cement mixtures
• Clean coating free concrete surface.
• No use of polymere based repair-mortars.
• Zinc can be overcoated with acrylic based topcoat.
• Use sealant or avoid situations when running water is present.
• Longevity appr. 21 years.
Engineering
• Determine amount of steel surface within
concrete constructiondesign & field-inspection,
• Minimal currentdensity of 20 mA/m2 steelsurface
in concrete (min 100 mV depolarization by help
of reference cells -EN12696),
• Reinforcement must be electric continuous,
• Determine amount and location of Reference
cells within the concrete.