A cracked or uneven concrete floor can indicate anything from harmless shrinkage to active subsoil movement. Diagnosis comes first — repair is only useful once the cause is fixed.
Common defects and causes
Hairline cracks (under 1mm)
Drying shrinkage, plastic settlement. Cosmetic, can be sealed with epoxy injection if water ingress.
Settlement cracks (1–5mm)
Differential subsoil settlement or undersized hardcore. Usually static after initial settlement period.
Active cracking (growing over time)
Indicates ongoing movement — clay shrinkage, drain leak, tree roots, or heave. Address cause before repair.
Slab heave (floor lifting in centre)
Most common cause: pyrite-bearing hardcore expanding ("mundic" or pyritic heave). Less commonly clay heave below slab.
Edge curling
Slab edges lift as moisture differentials cause differential drying. Aesthetic; rarely structural.
Repair methods
Crack injection
Epoxy or polyurethane resin injected under pressure into cracks. Seals against water and re-bonds concrete.
Self-levelling screed
Cementitious overlay used to reinstate level surface over minor unevenness.
Floor lifting (resin injection)
Expanding structural resin injected below slab re-levels heaved or settled slabs without demolition. Specialist contractors required.
Slab replacement
For severely compromised slabs, full removal and reinstatement with proper hardcore and DPM.
Pyrite-related heave
Common in parts of South Africa, parts of the South African with iron pyrite in hardcore. Identification: laboratory testing of hardcore sample (R6,000–R12,000). Remedy: usually full slab replacement and removal of affected fill.
Cost
- Crack injection: R800–R2,400 per metre
- Self-levelling screed: R500–R1,100 per m²
- Resin slab lifting: R1,600–R4,000 per m²
- Slab replacement (residential): R2,000–R4,000 per m²
- Slab replacement with hardcore removal: R3,600–R7,000 per m²
MCFAR diagnoses concrete floor defects and specifies repair.
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Request a QuoteFrequently Asked Questions
Should I lay new floor over a cracked slab?
Only if the cracks are static and the slab is sound. Active cracking will telegraph through new finishes.
How do I tell if my hardcore is pyritic?
Visual signs include slab heave and dark staining. Definitive answer requires laboratory testing.