Reinforced concrete fails in two principal ways. Carbonation progressively lowers the alkalinity of the concrete pore solution until the passive oxide layer protecting steel reinforcement breaks down. Chloride ions, whether from seawater spray, tidal splash or airborne salt, penetrate the cover zone and initiate corrosion directly once their concentration at the bar surface exceeds a threshold. Both mechanisms are electrochemical at their core, and both are governed by the quality and depth of the concrete cover. Anti-carbonation coatings address neither mechanism by stopping corrosion once it starts; they work by reducing the rate at which the aggressive species reach the reinforcement in the first place.
Why Carbonation Matters
Concrete pore water is highly alkaline, typically sitting at a pH above 12.5 due to calcium hydroxide and other hydration products. At that pH, steel reinforcement sits within a stable passive film. Atmospheric carbon dioxide dissolves into moisture within the concrete pore network and reacts with calcium hydroxide to form calcium carbonate, a process called carbonation. The reaction is not structurally damaging in itself; carbonated concrete is often denser than uncarbonated concrete. The problem is that the reaction consumes alkalinity. Once the carbonation front reaches the reinforcement and the pH drops below roughly 9, the passive film becomes thermodynamically unstable and corrosion can initiate wherever oxygen and moisture are present.
The depth of carbonation follows an approximate square-root-of-time relationship, which means early protection has a disproportionate effect on service life. Concrete with low water-to-cement ratio and adequate cover carbonates slowly; older structures built with higher w/c ratios and minimal cover are often already carbonated to or beyond the bar depth. In Australian conditions, facades facing prevailing weather tend to wet and dry cyclically, which accelerates carbonation compared with permanently wet or permanently dry surfaces.
AS 3600 classifies exposure conditions and prescribes minimum cover and concrete strength accordingly. Exposure class B1 covers near-coastal environments within one kilometre of the coast; B2 covers coastal frontage and areas subject to salt spray. Inland urban environments generally fall into A1 or A2. The cover and concrete grade requirements in AS 3600 Table 4.10.3 are designed to provide adequate durability over a nominal design life, but they assume the concrete is placed and cured correctly. Poorly cured cover concrete, honeycombing, or inadequate compaction can reduce the effective barrier substantially below what the specification intended.
Chloride Ingress: A Separate but Related Problem
Chloride attack differs from carbonation in mechanism but shares the same vulnerability: thin or permeable cover concrete. Chloride ions do not lower pore pH; they destabilise the passive film directly by locally breaking it down at the steel surface, producing pitting corrosion that can be severe even when the surrounding concrete remains alkaline. This is why chloride-induced corrosion can appear in concrete that shows no visible carbonation and no general pH drop.
Ingress occurs by diffusion through the pore solution, by absorption into partially dry concrete during wetting cycles, and by capillary suction at the surface. Tidal and splash zones are the most aggressive because the concrete alternately wets with chloride-laden water and dries, concentrating chlorides near the surface. Structures within a few hundred metres of the coast accumulate airborne chloride deposition even without direct water contact.
AS 3600 Exposure Class C applies to concrete in contact with seawater or subject to severe coastal exposure. The standard's durability provisions for Class C require higher concrete grades and greater cover than inland exposures, reflecting the severity of the environment. For existing structures that were not designed or built to these standards, or that have reached the end of their original design life, surface-applied protection becomes the practical remediation route.
How Anti-Carbonation Coatings Work
An anti-carbonation coating is a film-forming system applied to the concrete surface that presents a high resistance to CO2 diffusion. The key performance metric is the equivalent air layer thickness for CO2, expressed as the SD value (diffusion-equivalent air layer thickness in metres). A coating with a high SD value for CO2 effectively extends the concrete cover by the equivalent of many metres of open air, dramatically slowing the advance of the carbonation front.
The coating must simultaneously allow water vapour to pass outward from the substrate. Concrete structures in service contain moisture from original construction water, from rain ingress, and from condensation. If a coating presents a high resistance to water vapour as well as to CO2, moisture trapped beneath the film builds vapour pressure during temperature cycling and eventually causes blistering, delamination, or both. A coating that blisters has failed as a protective barrier regardless of its theoretical CO2 resistance, because the broken film allows direct ingress.
The vapour permeability requirement is expressed as the SD value for water vapour, and a well-specified anti-carbonation coating should have a high SD for CO2 paired with a low SD for water vapour. Manufacturer datasheets express this as a ratio or list both values separately. Specifiers should check both figures, not just the CO2 resistance claim.
Fosroc's range includes products formulated specifically for this duty. As with any product in this category, the datasheet values for CO2 equivalent air layer thickness and water vapour permeability should be reviewed against the project exposure classification and the substrate condition before specifying. CSA supplies the Fosroc construction chemicals range and can provide technical data sheets on request.
Surface Preparation and Application
No coating performs better than the preparation beneath it. Carbonated or contaminated concrete must be assessed before coating; if active corrosion is already present, the coating alone will not arrest it. Corroding reinforcement generates expansive corrosion products that will fracture the cover and the coating above it regardless of the film quality. Active corrosion requires repair, typically by breaking out delaminated cover, treating or replacing corroded steel, and reinstating the cover with a compatible repair mortar before any protective coating is applied. The post on concrete repair mortars in this series covers substrate preparation in more detail.
For structures where carbonation has not yet reached the reinforcement, or where the carbonation depth is still within an acceptable margin, coating the sound concrete surface is a rational preventive measure. Surface preparation typically involves pressure washing to remove laitance, biological growth, and surface contamination, followed by any localised crack repair or filling. Cracks wider than the manufacturer's stated limit should be sealed before coating, as a film-forming coating will not bridge active cracks reliably.
Application is generally by brush, roller, or airless spray in two or more coats to achieve the specified dry film thickness. Dry film thickness governs the SD value; an under-applied coating will not achieve the rated CO2 resistance. Independent inspection of wet film thickness during application is worth specifying on larger projects.
Where Silane and Siloxane Impregnations Suit Instead
Film-forming anti-carbonation coatings are not always the right answer. In some situations, particularly where the concrete surface is subject to abrasion, UV exposure that degrades organic films, or where aesthetics require a natural concrete appearance, a silane or siloxane impregnation is more appropriate.
Silane and siloxane penetrants do not form a surface film. They penetrate the concrete pore structure and react with the pore walls to render them hydrophobic. Water is repelled at the surface, reducing capillary absorption and limiting chloride ingress by the absorption mechanism. They do not provide significant CO2 resistance because they do not block gas-phase diffusion; CO2 is not repelled by a hydrophobic surface in the way liquid water is. Their primary value is in reducing chloride uptake in wetting-and-drying environments, and they are particularly suited to splash zones, bridge soffits, and coastal facades where chloride absorption is the dominant threat.
Silane penetrants require a dry or near-dry substrate for effective penetration. Applied to a wet substrate, they sit at the surface and cure without penetrating, providing little benefit. The choice between a film-forming anti-carbonation coating and a penetrating silane or siloxane impregnation depends on the dominant degradation mechanism, the substrate condition, the aesthetic requirements, and the maintenance regime available over the asset's life.
Service Life and Maintenance
Anti-carbonation coatings extend service life by slowing the rate of carbonation front advance, not by stopping it permanently. The coating itself has a finite service life governed by UV exposure, thermal cycling, surface abrasion, and the quality of the original application. Most manufacturers specify a maintenance recoat interval; this should be built into the asset management plan from the outset rather than treated as an unexpected cost.
AS 3600 does not prescribe coating systems directly; it sets the durability requirements that the overall concrete system, including any surface treatment, must satisfy. Where a coating is being used to compensate for inadequate cover or a higher-than-anticipated exposure class, an engineer should confirm that the combined system meets the intended design life. Remedial specifiers working on existing structures should engage a structural or materials engineer to assess the current carbonation depth, chloride profile, and residual design life before selecting a protection strategy.
Realistic service-life extension depends on the substrate condition at the time of coating, the quality of preparation, the product selected, and the maintenance regime. Manufacturer data and independent testing provide the basis for these projections; no credible specifier should accept a service-life claim without understanding the test conditions behind it.
Specifying for Australian Conditions
Australia's climate range from tropical north to temperate south means that a single specification does not suit all projects. High UV intensity in Queensland and Western Australia degrades organic coatings faster than in southern states. Coastal exposure in Sydney or Melbourne involves different salt deposition rates than tropical marine environments in Darwin or Cairns. The AS 3600 exposure classification is a starting point, but local knowledge of the specific site conditions should inform the final product selection.
For asset owners and facade engineers working through the specification process, the practical steps are: establish the exposure classification under AS 3600, assess the current carbonation depth and chloride profile if the structure is existing, determine whether carbonation or chloride ingress is the dominant mechanism, select a product with verified CO2 SD value and vapour permeability data that suits the mechanism, confirm surface preparation requirements, and programme maintenance recoat intervals into the asset management plan.
CSA supplies the Fosroc range of protective coatings and concrete repair products. Technical data sheets, safety data sheets, and application guides are available through the trade desk at constructionsupplies.group/au, where you can also enquire about product availability and project quantities.