Last reviewed 30 Sept 2026 · 8 min read
Principles of repair
A repair is a treatment of a damaged structure so that it recovers its function and durability. Four principles govern good repair:
- Find and remove the cause — a leaking joint, chlorides, poor drainage, scour or overload. If the cause remains, the damage returns.
- Remove all unsound concrete — every delaminated and contaminated area, until sound concrete is reached.
- Treat the reinforcement — clean the rust, replace or supplement damaged bars, and protect the steel.
- Restore the section with compatible materials — of similar stiffness, thermal movement and permeability to the parent concrete, and with good bond.
The design of the repair includes a repair strategy — patch, protect, strengthen, or replace — chosen after the assessment; the service life expected from the repair and its life-cycle cost are considered.
Steps of a patch repair
FormulaTypical sequence
- Mark out the area of delamination (hammer sounding) and extend the limit into sound concrete; saw-cut the edges to about 10–20 mm depth (avoiding feather edges).
- Break out the damaged concrete (pneumatic chipping, or hydro-demolition for large areas — no micro-cracking of the remaining concrete), down to at least behind the bars (a clearance of about 20 mm around the corroded bars) so that the full circumference of the bar can be cleaned and the mortar can bond all around.
- Clean the reinforcement to a bright metal finish (wire brush, needle gun, abrasive blasting); replace or supplement bars with a significant section loss (commonly more than about 10–20 %) by welding or lapping new bars.
- Coat the steel with a zinc-rich or epoxy anti-corrosive primer (two coats); avoid coating the concrete area that is to bond to the mortar.
- Prepare the substrate — clean, remove dust and oil, roughen, and make it saturated surface-dry; apply a bonding agent (polymer-modified slurry or epoxy) as specified.
- Place the repair material in layers, compact well, finish, and cure (wet curing or curing compound) — curing is essential.
- Protect the repaired area with a protective coating (anti-carbonation or anti-chloride) and treat the surrounding concrete if it is contaminated.
Repair materials
| Material | Description and use |
|---|---|
| Cementitious repair mortar (polymer-modified) | Cement + fine aggregates + polymers (SBR latex, acrylic) or silica fume; low shrinkage, good bond, for patches up to about 50 mm thick, applied by hand |
| Micro-concrete / flowable repair concrete | Free-flowing, non-shrink, fine aggregate; poured into formwork for deeper repairs, column and beam jackets |
| Shotcrete / guniting | Concrete or mortar sprayed at high velocity; good for large areas, thin layers and overhead work; wet-mix and dry-mix processes |
| Non-shrink (expansive) grout | For filling voids, pedestals and under bearings; cementitious or epoxy grouts |
| Epoxy mortar and epoxy resin | High strength and bond; for structural repairs, anchors and bonding new to old concrete; sensitive to moisture and temperature during application |
| Epoxy injection resins (low viscosity) | Inject into dormant structural cracks to restore the monolithic behaviour |
| Polyurethane and acrylic injection | Flexible or foaming; for leaking, moving or wet cracks and joints; seal water paths |
| Polymer coatings and sealers | Anti-carbonation coatings, hydrophobic impregnation (silane/siloxane), elastomeric crack-bridging coatings, and waterproofing membranes |
| Corrosion inhibitors | Migrating inhibitors applied on the surface which penetrate to the steel; or admixed in repair mortar |
| Cathodic protection systems | Anodes and cables described below |
| Fibre reinforcement | Steel or polypropylene fibres in shotcrete and mortar to control cracking |
Compatibility. A repair mortar should have an elastic modulus, thermal-expansion coefficient and shrinkage similar to the concrete: a much stiffer or more shrinking repair cracks or debonds. A repair material with low permeability placed in a patch can drive corrosion to the surrounding concrete (incipient anode or ring anode effect) — the surrounding concrete may require treatment or galvanic anodes may be placed around the patch.