Carbonation or Chloride Attack in Reinforced Concrete Structures: How to Tell the Difference

1. Introduction

Corrosion of reinforcement is one of the principal deterioration mechanisms affecting reinforced concrete structures. Two of the most common causes of reinforcement corrosion are carbonation-induced corrosion and chloride-induced corrosion.

Both mechanisms can produce similar visible symptoms; rust staining, cracking, delamination and spalling and therefore cannot always be distinguished from a visual inspection alone. However, the underlying mechanisms, spatial distribution of damage, chemical indicators and appropriate remedial measures are different.

Correctly identifying the cause of deterioration is essential because an inappropriate diagnosis can result in ineffective repairs and recurrence of corrosion.

This document provides a practical technical approach for distinguishing between carbonation and chloride attack in reinforced concrete structures.

2. Carbonation-Induced Corrosion

2.1 Mechanism

Concrete is normally highly alkaline, typically with a pore solution pH above approximately 12.5. This high alkalinity creates a passive oxide layer on embedded reinforcing steel, which protects the reinforcement from corrosion.

Carbon dioxide (CO₂) from the atmosphere penetrates the concrete and reacts with alkaline constituents, particularly calcium hydroxide:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

This process is known as carbonation.

As carbonation progresses inward from the concrete surface, the alkalinity of the concrete is reduced. When the carbonation front reaches the reinforcement, the passive condition of the steel can be lost. In the presence of moisture and oxygen, corrosion may then initiate.

2.2 Typical characteristics

Carbonation-induced corrosion is generally associated with:

Structures exposed to atmospheric CO₂.
Relatively dry or cyclically wet concrete.
Insufficient concrete cover.
Concrete with high permeability or poor curing.
Older concrete structures.
Reinforcement located within or close to the carbonation depth.
Corrosion that may occur relatively uniformly along reinforcement where carbonation has reached the steel.

Carbonation can occur in both internal and external concrete, although the rate is strongly affected by exposure conditions.

3. Chloride-Induced Corrosion

3.1 Mechanism

Chloride ions can penetrate concrete from external sources such as:

Marine environments.
De-icing salts.
Contaminated groundwater.
Chloride-bearing industrial environments.
Certain contaminated construction materials.

Chlorides can migrate through concrete by diffusion, capillary absorption and other transport mechanisms.

When a sufficient concentration of chloride reaches the reinforcement, the passive film on the steel can be locally disrupted. Corrosion can then initiate, often producing localised pitting corrosion.

Unlike carbonation, the concrete may remain highly alkaline even when chloride-induced corrosion occurs.

3.2 Typical characteristics

Chloride-induced corrosion is commonly associated with:

Marine structures.
Car parks and decks exposed to de-icing salts.
Bridge decks.
Splash and tidal zones.
Structures exposed to chloride-contaminated water.
Cracks that provide preferential pathways for chloride ingress.
Reinforcement located below concrete containing measurable chloride concentrations.

Chloride-induced corrosion may be highly localised, particularly during the initiation and early propagation stages.

4. Why Visual Inspection Alone Is Insufficient

The visible consequences of reinforcement corrosion are not unique to either mechanism.

Both carbonation and chloride attack can result in:

Rust staining.
Longitudinal cracking over reinforcement.
Delamination.
Concrete spalling.
Loss of reinforcement cross-section.
Reduced bond between reinforcement and concrete.

Consequently, observations such as "rust staining" or "spalling over reinforcement" demonstrate that corrosion is occurring, but do not by themselves establish whether carbonation or chlorides initiated the corrosion.

A diagnosis should therefore combine:

Environmental exposure assessment.
Concrete cover measurements.
Carbonation-depth measurements.
Chloride-content testing.
Concrete condition surveys.
Reinforcement condition assessment.
Where necessary, electrochemical testing.

5. Key Diagnostic Differences

Feature	Carbonation	Chloride Attack
Primary agent	Carbon dioxide	Chloride ions
Main effect	Reduction in concrete alkalinity	Local breakdown of steel passivity
Concrete pH	Reduced in carbonated zone	May remain highly alkaline
Typical source	Atmospheric CO₂	Marine exposure, de-icing salts, contaminated water
Progression	Carbonation front advances inward	Chloride concentration increases with depth/time
Corrosion pattern	Often relatively widespread along affected reinforcement	Frequently localised/pitting
Main field test	Phenolphthalein carbonation test	Chloride profile testing
Key measurement	Carbonation depth vs reinforcement cover	Chloride concentration at reinforcement depth
Visual appearance	Cracking, rust staining and spalling	Similar; potentially more localised and severe
Critical comparison	Has carbonation reached reinforcement?	Are chlorides present at sufficient concentration at reinforcement?

6. Carbonation Testing

6.1 Phenolphthalein test

The most common field method for determining carbonation depth is the phenolphthalein indicator test.

A freshly exposed concrete surface is sprayed with a suitable phenolphthalein solution. The test provides an indication of the boundary between relatively alkaline and carbonated concrete.

Typically:

Purple/pink: concrete remains sufficiently alkaline to react with the indicator.
Little or no colour change: concrete is carbonated or has insufficient alkalinity for the indicator response.

The distance from the exposed surface to the carbonation boundary is measured and recorded.

Important limitation

The phenolphthalein test does not directly measure pH or prove that reinforcement is corroding.

The indicator transition is also not identical to the exact pH at which steel depassivation occurs. It should therefore be regarded as a screening/diagnostic test, rather than a complete assessment of corrosion risk.

6.2 Comparing carbonation depth with concrete cover

One of the most useful diagnostic comparisons is:

Carbonation depth vs. reinforcement depth

For example:

Carbonation depth = 15 mm
Reinforcement depth = 35 mm

In this case, the carbonation front has not reached the reinforcement at the location tested. Carbonation is therefore unlikely to be the direct cause of corrosion at that particular location.

Conversely:

Carbonation depth = 40 mm
Reinforcement depth = 30 mm

Here, carbonation has reached and passed the reinforcement. Carbonation-induced loss of passivity is therefore a credible corrosion mechanism.

Because both carbonation depth and reinforcement cover vary spatially, multiple measurements should be taken rather than relying on a single location.

7. Chloride Testing

7.1 Chloride concentration profile

Where chloride attack is suspected, concrete samples should be taken at different depths and analysed for chloride content.

A typical investigation may involve samples corresponding approximately to:

Surface zone.
Near-cover zone.
Reinforcement depth.
Concrete beyond the reinforcement.

The resulting chloride profile can demonstrate whether chloride concentration increases toward the reinforcement.

A typical interpretation would be:

Concrete surface → increasing chloride concentration → reinforcement depth

This provides evidence of chloride ingress from the exposed surface.

7.2 Chloride threshold

The presence of chloride in concrete does not automatically mean that reinforcement corrosion has occurred.

The significance of the measured chloride concentration depends on factors including:

Cementitious system.
Chloride binding.
Concrete composition.
Exposure conditions.
Moisture availability.
Reinforcement condition.
Potential at the steel surface.
Test method and reporting basis.

Therefore, measured chloride concentrations should be interpreted against the applicable standards, specifications and established assessment criteria rather than relying on a single universal threshold.

8. A Practical Diagnostic Procedure

A structured investigation can be carried out as follows.

Step 1 – Establish the exposure history

Determine whether the structure has been exposed to significant sources of chlorides.

Examples include:

Seawater.
Sea spray.
De-icing salts.
Salt-contaminated groundwater.
Chloride-bearing industrial processes.

If no credible chloride source exists, carbonation becomes a more likely mechanism, although chloride contamination should not be ruled out without testing.

Step 2 – Map the visible deterioration

Record:

Cracks.
Rust staining.
Spalled areas.
Delaminated concrete.
Previous repairs.
Areas of exposed reinforcement.
Damp or persistently wet areas.

The location and distribution of deterioration can provide important clues.

Step 3 – Determine reinforcement cover

Use a suitable reinforcement detection/cover measurement technique to establish:

Reinforcement location.
Concrete cover.
Reinforcement arrangement.

Cover measurements should be correlated with carbonation measurements.

Step 4 – Measure carbonation depth

Expose fresh concrete at representative locations and determine the carbonation depth using a suitable indicator test.

Calculate or record:

Carbonation depth / reinforcement cover

This provides an initial indication of whether the carbonation front has reached the reinforcement.

Step 5 – Test for chlorides

Where chloride exposure is possible, obtain concrete samples for chloride analysis.

Ideally, determine a chloride concentration profile with depth, rather than relying solely on a single bulk sample.

Step 6 – Assess reinforcement corrosion

Where corrosion is suspected, inspect exposed reinforcement for:

Rusting.
Pitting.
Section loss.
Corrosion morphology.
Loss of bond.
Cracking associated with the reinforcement.

Electrochemical techniques may also be used to assess the likelihood and distribution of active corrosion.

Step 7 – Correlate the results

The diagnosis should be based on the combined evidence.

For example:

Case A – Carbonation

Carbonation depth exceeds reinforcement cover, no significant chloride source is identified, and chloride concentrations at reinforcement depth are low.

This strongly supports carbonation-induced corrosion.

Case B – Chloride

Carbonation has not reached reinforcement, but chloride concentrations at reinforcement depth are significant and the structure has a history of exposure to de-icing salts.

This supports chloride-induced corrosion.

Case C – Both mechanisms

Carbonation has reached reinforcement and significant chloride contamination is also present.

Both mechanisms may be contributing. This situation is particularly important because carbonation can alter chloride binding and transport behaviour, and the deterioration mechanism may not be attributable to a single cause.

9. Typical Investigation Findings

9.1 Example: Carbonation-dominated deterioration

A reinforced concrete façade has widespread longitudinal cracking and rust staining.

Investigation identifies:

Concrete cover: 20–25 mm.
Carbonation depth: 30–40 mm.
No significant chloride exposure.
Low chloride concentrations at reinforcement depth.
Corrosion occurring where carbonation has reached reinforcement.

Diagnosis: Carbonation-induced reinforcement corrosion is the most probable primary mechanism.

9.2 Example: Chloride-dominated deterioration

A multi-storey car park exhibits cracking and localised spalling around reinforcement.

Investigation identifies:

Concrete cover: approximately 35 mm.
Carbonation depth: approximately 10 mm.
Significant exposure to de-icing salts.
Chloride concentration increases with depth.
Chloride concentration at reinforcement depth is significant.
Localised corrosion and pitting are present.

Diagnosis: Chloride-induced reinforcement corrosion is the most probable primary mechanism.

9.3 Example: Combined deterioration

A coastal reinforced concrete structure exhibits extensive cracking and spalling.

Investigation identifies:

Carbonation reaching reinforcement in some areas.
Significant chloride contamination at reinforcement depth.
Localised pitting corrosion.
Areas with both carbonation and chloride ingress.

Diagnosis: The structure is experiencing combined carbonation and chloride-related deterioration. A single-mechanism diagnosis would be inappropriate.

10. Other Tests That May Be Required

For significant or complex structures, carbonation and chloride testing should form part of a broader condition assessment.

Potential supplementary investigations include:

Half-cell potential mapping

Provides an indication of the likelihood of reinforcement corrosion activity and can help identify areas requiring further investigation.

Concrete resistivity

Can provide information about the electrical resistivity of the concrete and the likelihood of corrosion activity under suitable conditions.

Corrosion rate measurements

Techniques such as linear polarisation resistance can provide an indication of the rate of reinforcement corrosion under appropriate conditions.

Concrete petrography

Microscopic examination can help identify:

Cracking mechanisms.
Cement paste characteristics.
Secondary deposits.
Aggregate-related deterioration.
Evidence of other deterioration mechanisms.

Compressive strength testing

Useful where structural capacity or general concrete quality is also under investigation.

Concrete permeability/transport testing

May be useful for understanding the susceptibility of the concrete to carbonation or chloride ingress.

11. Common Diagnostic Mistakes

Mistake 1 – Assuming rust means chloride attack

Rust staining only confirms or suggests reinforcement corrosion. It does not identify the initiating mechanism.

Mistake 2 – Assuming carbonation means corrosion

Carbonation may reach reinforcement without significant corrosion occurring if the moisture and oxygen conditions are unsuitable.

Mistake 3 – Using carbonation depth alone

Carbonation depth must be compared with reinforcement cover and the actual condition of the reinforcement.

Mistake 4 – Treating any chloride as evidence of chloride-induced corrosion

Chlorides can be present without having initiated corrosion. Their concentration, location, source and relationship to reinforcement must be considered.

Mistake 5 – Taking too few samples

Concrete deterioration is often highly variable. A single carbonation or chloride measurement may not represent the structure as a whole.

Mistake 6 – Ignoring cracks

Cracks can provide preferential pathways for CO₂, water and chlorides and can result in local deterioration that does not follow the expected penetration profile.

Mistake 7 – Assuming there is only one mechanism

Real structures can experience multiple deterioration mechanisms simultaneously.

12. Decision Framework

A simplified diagnostic decision process is:

Visible reinforcement corrosion

↓

Is there a credible chloride exposure?

No → Investigate carbonation and other possible mechanisms.
Yes → Carry out chloride testing.

↓

Has carbonation reached the reinforcement?

Yes → Carbonation may be contributing to loss of passivity.
No → Carbonation is less likely to be the direct cause at that location.

↓

Are significant chlorides present at reinforcement depth?

Yes → Chloride-induced corrosion is a credible mechanism.
No → Chlorides are less likely to be the primary cause.

↓

Compare with reinforcement condition and electrochemical evidence

↓

Determine whether the deterioration is:

Predominantly carbonation-induced.
Predominantly chloride-induced.
A combination of carbonation and chlorides.
Due to another mechanism.

13. Key Principle

The most useful distinction can be summarised as follows:

Carbonation reduces the alkalinity of concrete until the reinforcement loses its passive protection, whereas chloride attack can depassivate reinforcement while the surrounding concrete remains highly alkaline.

Therefore:

Carbonation diagnosis = measure carbonation depth + measure reinforcement cover + assess exposure and corrosion.

Chloride diagnosis = measure chloride concentration with depth + assess chloride exposure + assess reinforcement corrosion.

Neither mechanism should normally be diagnosed from visual evidence alone.

14. Conclusion

Distinguishing between carbonation-induced and chloride-induced reinforcement corrosion requires a combination of field observations, concrete testing, reinforcement assessment and knowledge of the structure's exposure history.

Carbonation is primarily identified by determining whether the carbonation front has reached the reinforcement. Chloride-induced corrosion is investigated by demonstrating the presence and distribution of chloride at reinforcement depth, together with evidence that the reinforcement has depassivated and begun to corrode.

In practice, the most robust assessment is not simply to ask whether carbonation or chlorides are present, but to establish:

What deterioration is occurring?
Where is it occurring?
What is the concrete cover?
How far has carbonation progressed?
What chloride concentration exists at reinforcement depth?
What is the condition of the reinforcement?
What environmental exposure has the structure experienced?
Are multiple deterioration mechanisms operating simultaneously?

This evidence-based approach provides a much stronger basis for selecting an appropriate repair strategy and predicting the likelihood of future corrosion.

Summary Table for Site Investigations

Investigation	Carbonation	Chloride attack
Visual condition survey	✓	✓
Exposure history	✓	✓
Reinforcement cover survey	Essential	Important
Carbonation depth	Essential	Useful
Chloride profile	Useful	Essential
Reinforcement inspection	✓	✓
Half-cell potential	Useful	Useful
Concrete resistivity	Useful	Useful
Corrosion-rate measurement	Where required	Where required
Petrography	Where required	Where required

The key diagnostic comparison is therefore:

Carbonation depth < reinforcement cover → carbonation has not reached the steel at that location.

Carbonation depth ≥ reinforcement cover → carbonation is capable of having removed the passive protection.

Significant chloride concentration at reinforcement depth + suitable exposure/corrosion evidence → chloride-induced corrosion is credible.

These comparisons should be made at multiple representative locations and interpreted in accordance with the relevant structural assessment standards and project requirements.