Reinforcement corrosion protection in concrete repair: coatings and inhibitors

Why reinforcement corrodes in concrete

In sound concrete, steel is protected: the high alkalinity of the pore solution forms a passive film on the surface of the reinforcement, and corrosion practically does not occur. Passivity is lost for two main reasons. Carbonation — carbon dioxide in the air neutralises the alkalinity, and when the carbonation front reaches the reinforcement, the steel starts to rust over its whole surface. Chlorides — from de-icing salts, saline soils and water, and salt-laden dust — break down the passive film locally even in alkaline concrete, and corrosion takes the form of pitting.

Rust occupies a larger volume than steel, so the concrete cover cracks and spalls off. But by the time spalling appears, corrosion usually affects a much larger zone than can be seen. The aim of a repair is not only to replace the damaged concrete but also to stop corrosion in the repaired and adjacent areas.

Protection principles of EN 1504-9

Of the eleven principles of EN 1504-9, five relate to reinforcement corrosion. Which of them are chosen depends on the cause and extent of the damage.

  • Principle 7 — preserving or restoring passivity. Carbonated or chloride-contaminated concrete is replaced with an alkaline repair mortar, and the cover is increased. This is the basis of ordinary spall repair.
  • Principle 8 — increasing resistivity. The concrete is made drier by hydrophobic impregnation or coatings, and the corrosion process slows down.
  • Principle 9 — cathodic control and principle 11 — control of anodic areas. These include reinforcement coatings and corrosion inhibitors that slow down the reactions on the steel surface.
  • Principle 10 — cathodic protection. Galvanic anodes or impressed current systems. This is an engineering solution that is designed separately; manufacturers approve some repair mortars for use together with such systems.

Materials for coating reinforcement are covered by EN 1504-7, which deals with active and barrier coatings for protecting existing reinforcement during repair.

What reinforcement is protected with

Cementitious coatings with inhibitors

One-component polymer-modified cementitious coatings with silica fume and corrosion inhibitors. They protect through their alkalinity, density and bond to the steel; many also serve as a bonding coat for the repair mortar. They are applied by brush or by spraying, as a rule in two coats. Examples: Sika MonoTop-910 N, Sika MonoTop-610, Mapefer 1K.

Epoxy-cement coatings

Three-component coatings based on cement modified with epoxy resin, with a corrosion inhibitor: higher resistance to the penetration of water and chlorides, and a long pot life. Example: SikaTop Armatec-110 EpoCem.

Zinc-rich primers

Epoxy primers with zinc dust that form part of certain repair systems. Example: Fosroc Nitoprime Zincrich Plus in the Renderoc system.

Penetrating corrosion inhibitors

Liquid products applied to the surface of undamaged concrete: they penetrate into the concrete and form a protective layer on the steel without the structure being broken out. They are used where the reinforcement is already at risk from carbonation or chlorides but the concrete is still intact — for example, around completed repairs to prevent new corrosion sites at the edge of a patch, and on architectural concrete that cannot be broken out. Such products have limits on chloride content and the condition of the concrete, and the penetration depth is checked on site. Example: Sika FerroGard-903+.

How to choose

  • Cause of corrosion. With carbonation, a correct repair that restores an alkaline cover, followed by surface protection, is usually sufficient. With chlorides, the scope of work is wider: contaminated concrete is removed beyond the visible spalls, the reinforcement is washed with pressurised water after cleaning, and inhibitors and cathodic protection are considered.
  • Repair system. The reinforcement coating is chosen within the same system as the repair mortar: this determines the intervals — onto a fresh or a dried coat — and the bond. Some coatings are not suitable as a bonding coat under rapid-hardening mortars.
  • Bond between reinforcement and concrete. The coating must not impair the composite action of steel and concrete. Manufacturers confirm this by testing to EN 15184: the bond of coated bars must be no less than 80 % of the bond of uncoated bars.
  • Working conditions. Temperature, humidity, access to the bars, the time until the mortar is placed.

Procedure

  1. Breaking out. The concrete around corroded reinforcement is removed so that the bar is exposed around its full circumference and can be cleaned and coated on the back; breaking out continues along the bar until undamaged steel is reached. For example, the Sika MonoTop-910 N data sheet specifies exposing a further 2 cm or so into the undamaged zone and, if corrosion has affected more than half of the circumference, freeing the bar at the back by at least 1 cm.
  2. Assessment of the section. The structural engineer decides whether bars with significant section loss are to be replaced or supplemented; new bars are anchored with products to EN 1504-6, which are covered in the “Anchoring and anchor grouting” section.
  3. Cleaning. Rust, mill scale, mortar residues and dust are removed by abrasive blasting or high-pressure water — usually to Sa 2 to ISO 8501-1, with higher requirements for some systems. The protective coating is applied immediately, without leaving the cleaned steel exposed for long.
  4. Application. Usually two coats by brush, fully covering the bar; the total thickness and the interval between coats are set by the manufacturer’s data sheet. With some systems it is acceptable for the coating to get onto the surrounding concrete, with others the sequence of operations must be observed — this is also stated in the data sheet.
  5. Repair mortar — onto the dried or the fresh coating, depending on the system and the role of the coat.

Common mistakes

  • Only the visible side of the bar has been cleaned and painted, and the rust behind it remains.
  • One thin coat instead of two, misses on the ribs and where bars cross.
  • Repair mortar placed before or after the permitted interval.
  • Only the visibly damaged concrete has been replaced, while the surrounding chloride-contaminated concrete has been left — a few years later, new spalls appear around the edges of the patch.
  • Inhibitor applied to damp concrete or concrete covered with a film, which it cannot penetrate.
  • No surface protection after the repair — carbonation and chloride ingress continue.

What to check with the supplier and the engineer

  • compliance with EN 1504-7 and test results to EN 15183 and EN 15184;
  • the number of coats, the thickness, the intervals and the sequence for placing the repair mortar;
  • for inhibitors — limits on chlorides, requirements for concrete moisture, the method of checking penetration depth;
  • compatibility with the repair mortars and protective coatings of the chosen system.

We carry out surveys, assess the causes of corrosion and select the system as part of our engineering services; protection of steel structures is described in the “Corrosion protection” section.

Materials in the catalogue

  • Sika MonoTop-610 — Sika — polymer-modified cementitious rebar protection and bonding primer with corrosion inhibitor.
  • Sika MonoTop-910 N — Sika — cementitious rebar corrosion protection and bonding primer for Sika MonoTop repairs.
  • SikaTop Armatec-110 EpoCem — Sika — epoxy-cement rebar protection with inhibitor and bonding primer for repair mortars.
  • Sika FerroGard-903+ — Sika — penetrating corrosion inhibitor impregnation for reinforced concrete without breaking out.
  • Fosroc Nitoprime Zincrich Plus — Fosroc — zinc-rich primer for exposed reinforcement in the Renderoc repair system.
  • Mapefer 1K — Mapei — one-component cementitious anti-corrosion coating for reinforcing steel to EN 1504-7.

This list is not exhaustive. The final choice is made on the basis of the manufacturer’s documents, taking into account the survey results.

Frequently asked questions

Can reinforcement be protected with an ordinary metal primer?

No. Reinforcement is protected with products tested to EN 1504-7 and for bond between bars and concrete to EN 15184, within the same system as the repair mortar. Primers for steel structures do not undergo such tests and may impair the composite action of the steel and the repair mortar.

Is a reinforcement coating needed if the bars have been cleaned well?

The data sheets of a number of repair mortars list coating the reinforcement as a measure to be taken “where necessary”. The decision depends on the cause of corrosion, the cover thickness and the aggressiveness of the environment; with chlorides, shallow cover and in an aggressive environment, a reinforcement coating is normally specified.

Does an inhibitor replace a repair?

No. Spalls, cracks and delaminations are repaired by conventional methods, and the inhibitor is used in addition — on undamaged concrete and around repairs, to slow corrosion where breaking out is not justified.

We will select a reinforcement protection system for your project — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.