1. Introduction
Surface Applied Corrosion Inhibitors (SACI) are specialist materials used to reduce or control corrosion of steel reinforcement embedded within reinforced concrete. They are generally applied directly to the concrete surface and are designed to penetrate through the concrete cover and reach the reinforcing steel.
SACI systems are particularly useful when reinforced concrete structures are showing early or established signs of reinforcement corrosion, especially where chloride contamination or carbonation has reduced the natural protection provided by the concrete.
Unlike conventional concrete repair, which generally involves removing damaged concrete and treating exposed reinforcement, SACI provides a relatively non-invasive method of improving corrosion resistance over larger areas. It can therefore form an important part of a wider concrete repair and corrosion-management strategy.
Typical applications include multi-storey car parks, bridges, balconies, façades, marine structures, industrial buildings, retaining structures, tunnels and other reinforced concrete assets exposed to aggressive environments.
2. What Are Surface Applied Corrosion Inhibitors?
Surface Applied Corrosion Inhibitors are chemical compounds formulated to penetrate the concrete and provide protection to embedded reinforcing steel.
They are normally supplied as liquids, creams, gels or other surface-applied formulations. The material is applied to the prepared concrete surface by methods such as:
- Roller application.
- Brush application.
- Spray application.
- Low-pressure spray.
- Other manufacturer-approved application methods.
After application, the inhibitor migrates through the concrete by mechanisms such as diffusion, capillary absorption or other chemical transport processes. The objective is for the active components to reach the reinforcement and modify the electrochemical conditions at the steel/concrete interface.
Depending on the product chemistry, inhibitors may either form a protective film on the reinforcement or modify the electrochemical reactions associated with corrosion.
SACI products should be selected based on the concrete condition, chloride exposure, reinforcement depth, permeability and environmental conditions. Manufacturer-specific application requirements should always be followed.
3. Why Are SACI Systems Necessary?
Reinforced concrete normally provides reinforcing steel with a highly alkaline environment. Under these conditions, a passive oxide layer forms on the steel and helps prevent significant corrosion.
However, this protection can be compromised by environmental exposure.
Two of the principal mechanisms are carbonation and chloride ingress.
Carbonation occurs when carbon dioxide from the atmosphere penetrates the concrete and reacts with alkaline components. As carbonation progresses towards the reinforcement, the alkalinity around the steel can fall sufficiently for the passive condition to be lost.
Chlorides can penetrate concrete from sources such as de-icing salts, marine environments, contaminated materials and industrial exposure. When chloride concentrations at reinforcement level become sufficiently high, localised corrosion can initiate.
Once corrosion starts, the steel expands as corrosion products form. This expansion can generate internal pressure and eventually cause:
- Concrete cracking.
- Delamination.
- Spalling.
- Exposed reinforcement.
- Loss of reinforcement cross-section.
- Loss of bond between concrete and reinforcement.
- Further moisture and chloride ingress.
Conventional patch repairs can remove visibly damaged concrete but may not eliminate corrosion risk in surrounding concrete that remains contaminated.
SACI can help address this residual corrosion risk by introducing corrosion-inhibiting chemistry into otherwise retained concrete.
4. How SACI Helps Reinforced Concrete Structures
The principal purpose of SACI is to reduce the rate of reinforcement corrosion.
After penetrating the concrete, the inhibitor is intended to reach the embedded steel and interfere with the electrochemical corrosion process. Depending on the product, this may involve strengthening or maintaining the passive condition of the steel, reducing anodic or cathodic reactions, or creating a protective environment around the reinforcement.
The benefits can include:
Reduction in corrosion activity
SACI can reduce the corrosion rate of reinforcement where suitable environmental and concrete conditions exist.
Extension of service life
By slowing corrosion, SACI can delay further deterioration and potentially extend the useful service life of the concrete structure.
Reduced cracking and spalling
Controlling reinforcement corrosion reduces the formation of expansive corrosion products and therefore helps reduce the risk of future corrosion-related cracking and spalling.
Protection of retained concrete
SACI can be applied to areas where concrete is being retained, helping to manage corrosion risk beyond localised repair locations.
Reduced disruption
Surface application generally requires considerably less physical intervention than extensive concrete breakout, making SACI attractive for occupied buildings, car parks and other structures where disruption needs to be minimised.
5. Typical Application Process
The effectiveness of a SACI treatment depends heavily on correct preparation and application.
A typical process includes:
Step 1 – Condition survey: The structure is assessed to identify cracking, delamination, corrosion, chloride contamination, carbonation and other deterioration.
Step 2 – Concrete preparation: Surfaces are cleaned to remove dirt, coatings, grease, laitance, biological growth and other contaminants that could prevent penetration.
Step 3 – Repair of defective concrete: Loose, delaminated or structurally unsound concrete is generally removed and repaired before or in conjunction with inhibitor treatment.
Step 4 – SACI application: The inhibitor is applied at the manufacturer's specified coverage rate and in the required number of coats.
Step 5 – Penetration and curing: The material is allowed to penetrate and react in accordance with the manufacturer's requirements.
Step 6 – Subsequent protection: Where specified, the treated concrete may receive a compatible protective coating, waterproofing system or other surface treatment.
6. Factors Affecting Performance
The performance of SACI depends on several factors, including:
- Concrete permeability.
- Concrete moisture content.
- Reinforcement depth.
- Crack width and distribution.
- Chloride concentration.
- Carbonation depth.
- Concrete quality.
- Environmental exposure.
- Application rate.
- Surface preparation.
- Product chemistry.
- Compatibility with existing coatings and repairs.
Highly impermeable concrete or deeply embedded reinforcement may make penetration more difficult. Consequently, SACI should not be specified simply as a generic treatment without first establishing that the selected product is appropriate for the particular structure.
7. SACI as Part of Concrete Repair
SACI is normally most effective when used as part of a comprehensive corrosion-management system rather than as a standalone solution for severely damaged concrete.
For example, a refurbishment scheme for a reinforced concrete car park may incorporate:
- Concrete condition survey.
- Removal of loose and delaminated concrete.
- Reinforcement cleaning and treatment in exposed repair areas.
- Localised structural concrete repairs.
- SACI application to retained concrete.
- Crack repair.
- Protective concrete coating.
- Waterproofing improvements.
- Drainage and outlet repairs.
- Regular inspection and maintenance.
This integrated approach addresses both existing physical damage and the conditions contributing to future corrosion.
8. Advantages and Limitations
The main advantages of SACI include relatively straightforward surface application, reduced concrete breakout, limited disruption and the ability to treat larger areas of retained concrete.
It can be particularly useful where the structure is generally sound but has an increased risk of reinforcement corrosion.
However, SACI has limitations. It does not replace the need to remove loose or structurally unsound concrete. It cannot restore reinforcement that has already suffered significant section loss, and it should not be relied upon to correct structural deficiencies.
Penetration can also be affected by concrete density, moisture and reinforcement depth. The long-term effectiveness of a treatment depends on the product, application quality and continuing exposure conditions.
9. Inspection and Quality Control
Quality control should be implemented throughout the works.
The contractor should record:
- Product manufacturer and identification.
- Batch numbers.
- Concrete surface preparation.
- Weather and substrate conditions.
- Surface moisture condition where relevant.
- Application rate.
- Number of coats.
- Coverage achieved.
- Areas treated.
- Cure/protection requirements.
- Any deviations from the approved method statement.
Where appropriate, testing or specialist assessment may be undertaken to demonstrate penetration or treatment effectiveness.
The treated surface should also be inspected for compatibility with any subsequent coatings or waterproofing systems.
10. Overall Benefits
When appropriately selected and correctly applied, SACI can provide an effective method of managing reinforcement corrosion in reinforced concrete.
Its greatest advantage is that it allows corrosion protection to be extended beyond individual repair patches. This can help reduce the risk of corrosion continuing in adjacent retained concrete and can complement conventional concrete repair techniques.
SACI can therefore contribute to:
- Improved durability.
- Reduced corrosion rates.
- Reduced future concrete deterioration.
- Extended service life.
- Lower long-term maintenance requirements.
- Reduced extent of concrete breakout.
- Reduced disruption during refurbishment.
- Improved protection of existing reinforcement.
11. Conclusion
Surface Applied Corrosion Inhibitors are an important technology for the repair and protection of reinforced concrete structures affected by corrosion risk.
By penetrating the concrete and reaching embedded reinforcement, SACI is intended to modify the electrochemical conditions responsible for corrosion and reduce the rate of deterioration. This makes the technology particularly valuable for structures where chloride contamination, carbonation or environmental exposure presents an ongoing corrosion risk.
SACI should, however, be considered as one component of an overall corrosion-management strategy. Successful refurbishment should combine appropriate investigation, concrete repairs, reinforcement treatment, corrosion inhibition, protective coatings, waterproofing and effective maintenance.
When properly specified, applied and monitored, SACI can help preserve existing reinforced concrete, extend service life and reduce the frequency and extent of future corrosion-related repairs.