Spalling Concrete: Causes, Diagnosis, and Treatment

Spalling — concrete breaking away to expose corroded reinforcement — is the slow failure mode of reinforced concrete buildings. Spotting it early turns a R4,000 repair into a R400,000 one.

The mechanism

Steel reinforcement embedded in concrete corrodes when moisture and oxygen reach it. The corrosion expands the steel, splitting the concrete cover. Once started, the process accelerates: more exposed steel, more corrosion, more spalling.

The two main causes

Carbonation

Atmospheric CO₂ slowly reacts with cement, reducing alkalinity. When the carbonation front reaches the reinforcement, the protective passive layer breaks down. Mostly affects older buildings (40+ years) and structures with thin concrete cover.

Chloride attack

Salt from de-icing, marine environments, or contaminated aggregate breaks down the passive layer regardless of carbonation. Highly aggressive and faster than carbonation.

Other causes

• Freeze-thaw damage in saturated concrete
• Sulfate attack
• Alkali-silica reaction (ASR) — uncommon in UK

Identification

• Rust staining on concrete surface
• Cracking along reinforcement lines
• Loose concrete that can be tapped off
• Exposed reinforcement

Diagnostic testing

• Cover survey (electromagnetic): maps reinforcement depth
• Carbonation depth (phenolphthalein test): shows extent of carbonation
• Chloride sampling: quantifies salt content
• Half-cell potential survey: indicates active corrosion zones

Repair methods

Patch repair

Loose concrete removed, reinforcement cleaned and treated, repair mortar applied. Cost-effective for small areas.

Cathodic protection

Sacrificial anodes or impressed current systems stop corrosion electrochemically. Long-life solution for major structures.

Inhibitor treatment

Surface-applied migrating corrosion inhibitor reaches embedded steel. Suits early-stage carbonation.

Concrete encasement

For severely degraded sections, encasement in new reinforced concrete jacket.

Cost

• Diagnostic survey: R30,000–R100,000
• Patch repair: R7,000–R14,000 per m²
• Cathodic protection: R3,000–R8,000 per m²
• Major structural repair: R9,000–R24,000 per m²

MCFAR assesses and specifies concrete repair for residential and commercial buildings.

Frequently Asked Questions