Cathodic protection of reinforced concrete

How bridges get built · Finishing, articulation and maintenance · Cathodic protection of reinforced concrete

Also called: ICCP, impressed current cathodic protection, galvanic anodes, sacrificial anodes, cathodic prevention

How it is done

Survey the structure: half-cell potential mapping, chloride profiles, cover, delamination and electrical continuity of the reinforcement; divide it into zones by exposure and steel density.
Design the system to EN ISO 12696 or AMPP (NACE) SP0290: anode type and layout, current density per zone, power units, reference electrodes and monitoring, cable routes and anode life.
Repair delaminated and spalled concrete with low-resistivity cementitious mortar (not insulating epoxy or polymer-rich mortars), and bond the reinforcement wherever tests show breaks in continuity.
Install the anodes: titanium mesh or ribbon in an overlay or in slots, discrete anodes in drilled holes, or thermally sprayed zinc or conductive coatings on the surface; embed reference electrodes and route cables to junction boxes.
Test for short circuits between anode and steel before the overlay or encapsulation is placed, then complete it.
Energise zone by zone, let the steel polarise, and adjust the current until the protection criteria are met; record potentials and currents.
Monitor remotely or by visits (frequently in the first year, then at least annually), adjust the current, and replace power units and galvanic anodes as they reach the end of their lives.

Key numbers

Current densityabout 2–20 mA/m² of steel surface for protection of chloride-contaminated concrete; about 0.2–2 mA/m² for cathodic prevention
Protection criteria (EN ISO 12696)instant-off potential more negative than -720 mV (Ag/AgCl), or depolarisation of at least 100 mV within 24 h, or 150 mV over a longer period
Driving voltageusually below about 24 V DC
Anode lifetitanium mesh or ribbon 50 years or more; conductive coatings and thermally sprayed zinc about 10–25 years; embedded zinc galvanic anodes about 10–20 years

Where it fits

Chloride-contaminated decks, crossheads, columns and piles in de-icing or marine exposure.
Structures where patch repairs keep failing at their edges.
Cathodic prevention of new marine structures with long design lives.

Where it does not

Prestressed concrete without strict potential limits (hydrogen embrittlement of high-strength steel).
Structures with discontinuous or epoxy-coated reinforcement unless continuity can be made.
Owners who cannot keep power and monitoring going for decades.

Choosing it

Choose impressed current CP over patch repair alone when chlorides are high over large areas: patching only moves the anodes to the edges of the repairs (incipient anode effect) and the damage returns within years.
Choose galvanic anodes over impressed current for local repairs, small elements, or where no power supply and monitoring can be maintained; accept a shorter life and less control.
Choose impressed current CP over replacement when the structure is otherwise sound and its replacement would be costly or disruptive, and the owner will maintain a powered system.
Prefer electrochemical chloride extraction or realkalisation where a one-off treatment is acceptable and permanent monitoring is not wanted.

Plant, pace and money

PlantConcrete repair equipment, continuity testing, anode installation (mesh, ribbon, drilled anodes, arc-sprayed zinc), transformer-rectifiers, remote monitoring units.

ProductivityIndicative: zones installed over weeks; a large viaduct over one or more seasons.

CostIndicative: impressed current systems about USD 150–500 per m² of protected concrete surface including repairs and monitoring; galvanic patch anodes about USD 30–100 each installed.

Risks and controls

What goes wrong

Short circuits between anode and steel.
Hydrogen embrittlement of prestressing steel by overprotection.
Acidification at the anode-concrete interface at high current density.
Systems left switched off for lack of maintenance.
Stray current corrosion of unbonded steel.

Quality assurance

Continuity tests, short-circuit checks before overlay, commissioning potentials and depolarisation tests, periodic monitoring reports.

Origins

Humphry Davy protected the copper sheathing of Royal Navy ships cathodically in 1824. Richard Stratfull of Caltrans applied impressed current to a salt-damaged bridge deck at Sly Park Road, California, in 1973, and titanium mesh anodes and thermally sprayed zinc made the method practical for decks and substructures from the 1980s and 1990s.

Examples

Bridge decks in North American salt states since the 1970s, chloride-damaged motorway substructures in the UK, and coastal and Gulf marine substructures.

Case studies

Sly Park Road bridge deck, near Placerville, CaliforniaUSA · 1973Richard Stratfull (Caltrans) applied impressed current through a conductive asphalt overlay to stop corrosion of a salt-damaged deck, the first cathodic protection of a bridge deck.

Yaquina Bay Bridge, Newport, OregonUSA · 1990sOregon DOT protected the 1936 coastal arch bridge against marine chloride with thermally sprayed zinc anodes operated as impressed current, a system then used on other Oregon coast bridges.