Cathodic Protection of Steel in Concrete

Steel in concrete Steel passivation in an alkaline environment with pH ≥ 9,5 Portland cement hydration > CSH + Ca(OH)2 with pH = 12 – 13 Similar coefficient of linear expansion (13 vs. 14.5 um/m K ) Reasonable good adhesion of cement on steel surface 1 2 3 4 5 6 7 8 9 10 11 12 13 acid alkaline passivation

Pourbaix diagram for Iron at 25 degr. C CORROSION PASSIVATION

Concrete contains pores. Question : If steel and concrete is a good combination then how come steel corrodes in many occasions ? Concrete contains pores. too much water added will enhance pore forming. keep w/c factor strictly conform manufactures spec. cement type has an effect on pore forming during curing : Blast furnace slag and Fly Ashes are better then Portland cements for marine environment (denser concrete + and lower temperatures during curing ) Lower temperature during curing will produce less cracks due to internal and external temperature differences Never dismantle the concrete forms too early water needed for curing will evaporize in c. cover use temporary covers like plastic foil.

Main causes of concrete cracks and spalling Carbonation Acidification of the concrete (lowering pH) due to CO2 with Ca(OH)2 into CaCO3 (calcium carbonate pH=8-9) Ca(OH)2 + CO2 CaCO3 + H2O Chlorides (salts) additive (CaCl2) accelerator Sand contaminated with salt, Salt penetration, seawater, salt-spray, etc

Effect of anion on the inhibition of pitting pH1 carbonation pH2 Chloride content in concrete

Examples of damage

Old patchwork with damage

Traditional Patching Within a short period of time new cracks will form just beside the patched area. Not known how far the steel within the concrete is corroded. Symptoms control Non-durable rehabilitation technique.

Chlorides Corrosion is possible in an alkaline environment, Criteria for pitting: Active/Passive behavior, Chlorides forms acids within the pit during pitting proces Chlorides are hygroscopic en pulls moist into the concrete, Anode/cathode surface ratio of pitting is very small = increased corrosion velocity

Examples of chloride pitting

Why CP of concrete? Traditional patching and repair will re-introduce cracks, spalling and damage Due to pitting : Risc and safety regarding the integrity of the construction Costs : Properly performed traditional patching needs to replace all with chloride infested concrete by fresh mortar. Proved and effective system

Why can CP applied on concrete be so effective ? Due to the concrete’s inhomogenity chloride and oxygen concentrations differ strongly from location to location These concentration cells cause corrosion due to potential differences CP levelize the potential differences.

Concrete repair requirement for CP polymer or epoxy modified mortars should be avoided as much as possible. use mineral based mortars for repair. much less repair work is required by removing loose concrete only for surface applied anodes clean and coating free concrete surface (sand-blasting or high Temp steam cleaning).

Types of CP of concrete Impressed current systems with discrete anodes, titanium wire or conductive coatings. Galvanic systems with surface applied or descrete applied anodes.

Design and system criteria Determine amount of steel surface within concrete construction design & field-inspection, Minimal design current density of 20 mA/m2 steel surface in concrete (min 100 mV depolarization by help of reference cells -EN12696), Reinforcement must be electric continuous, If required for monitoring : determine amount and location of Reference cells within the concrete.

Steel to concrete surface area ratio [m2/m2] Generally : building, apartments : 0.6 – 0.8 civil engineering : bridge decks concrete jetties, parking garages 0.8 – 1 columns, pillars, beams 1.2 – 1.5

CP praxis – discrete anodes

CP praxis – conductive coating 1 2 3 4 5

Wire installation Kathodecontact

Rectifiers (powersource)

Galvanic system Advantages : 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 Points of attention : Clean coating free concrete surface. Avoid use of polymer based repair-mortars. Zinc can be overcoated with acrylic based topcoat. Use sealant or avoid situations when running water is present. Sacrificial system, limited durability but can be replaced.

Galvanic system Reinforcement directly connected to the zinc anode, Principle based on sacrificial anode (zinc) system.

ZLA - ZincLayerAnode This product is specifically developed for cathodic protection of steel reinforced concrete. ZLA is applied up on the concrete surface As soon as the electrical connection is set, the current will flow and the Zinc-Layer-Anode will sacrifice itself. The electrical connection can be opened and used for monitoring purposes. Additional data loggers can be installed to store data like current-consumption and reference cell potentials for depolarization measurements if required. A cathodic protection system which utilizes the ZLA is a galvanic system. Zinc has a lower (more negative) potential than steel, so the ZLA provides the current to inhibit the corrosion of steel in concrete. .

Applications in civil-works

Applications in civil-works

Apartments

NEW GALVANIC ANODES FOR PROTECTION OF STEEL IN CONCRETE Galvanic protection systems utilize sacrificial anodes that naturally generate an electrical current to mitigate corrosion of the reinforcing steel. In concrete structures, zinc anodes are typically used. We manufacture a wide range of zinc anodes specifically developed for concrete applications.

Our Engineering Activities Inspections of structures Corrosion Survey and Monitoring Corrosion Consultancy Product Development

Chloride analysis and a chloride penetration

Corrosion Survey Potential mapping Color cards

Pre-inspection activities Corrosion detection of pre-stressed or post-tensioned cables Magnetic Interference Fractographic Analysis Metallurgic studies Research of hystorical case studies