Repairing damaged concrete and exposed reinforcement
Spalled concrete showing brown steel is not a cosmetic problem: rust expands to as much as six times the steel's original volume, and that expansion is what blew the cover off in the first place — and it will keep blowing it off if you patch the shape and leave the cause. A correct repair treats the steel before it fills the hole.
A hard boundary before starting: this guide covers cover repair, spalling, honeycombing and rebar protection. Structural cracks — diagonal in beams, horizontal in columns, or any crack that widens over time — are the structural engineer's call before any trowel: repair mortar restores section, it does not restore load capacity that was designed by calculation.
Diagnosis: sound before you break
Hammer-sound the area and well around it: a hollow ring means detached cover even where the face looks intact — the repair's true boundary is the hollow-sound boundary, not the visible spall, and it usually runs much wider. Chalk it before breaking.
Breaking out: square edges, no feather edges
Break back to sound, solid concrete, at least 10 mm deep everywhere. Cut the repair's perimeter square-edged (a ~10 mm saw cut, then chisel) — a feather edge tapering to nothing always breaks off, because the mortar there is too thin to hold itself. The finished opening is square-shouldered, not a shallow dish.
The steel: expose it behind, clean it to bright metal
If rust wraps the bar, cleaning its face is not enough: expose the bar 15–20 mm clear behind so the new mortar surrounds it completely — half a bar buried in its old rust keeps expanding behind your repair. Clean with a wire brush or mechanical abrasion to bright grey metal and dust off. A bar whose section is visibly eaten is the engineer's call — supplementary reinforcement may be needed.
An EN 1504-3 class R3 mortar is formulated alkaline to rebuild the passivating environment the steel lost — so full encapsulation in the mortar is the protection, with dedicated rebar primers added where the project specification calls for them.
Pre-wetting: saturated, surface-dry
Wet the opening hours before repairing, and again after — dry old concrete sucks the mortar's water and starves the cure exactly at the bond line. At application the surface must be saturated and dark with no free water sheen: standing water at the bottom raises the water ratio at the interface and weakens it just as surely.
Filling: compacted layers, not one pour
Mix to the bag's water exactly and fill by compaction: press the mortar in with the trowel — behind the bar first, then in front — the aim is driving air out of every corner, not dressing the face. Deep repairs go in layers at the thickness the datasheet sets, each combed before it stiffens to key the next, the next placed once the last has set but not dried. Rule off the final face and close it with a sponge float ready for render or paint.
Curing: three damp days
Repair mortar is a relatively thin layer with an exposed face — the fastest thing on the whole site to dry out early. Keep it damp for three days: periodic misting, wet hessian, or taped-down polythene, with direct sun and wind screened in summer. A repair that dried on its first day hairline-cracks and lets go at its edges — and you are back at zero in two months.
The repair mistakes that bring the problem back
- Patching the visible spall and leaving the hollow ring around it.
- Feather edges tapering to nothing — they always break away.
- Cleaning the bar's face and leaving the rust behind it to keep expanding.
- Mortar onto a dry surface that drinks its water at the bond line.
- Filling a deep opening in one uncompacted pass.
- No moist curing — the repair dries out and delaminates within weeks.
- Repair mortar over a structural crack no engineer has seen.
Frequently asked
Where does a bonding bridge fit into repairs?
R3 mortar bonds to a saturated, roughened surface unaided in most cases. A bonding bridge or slurry is added on very smooth substrates or where the project spec requires one — and then the mortar goes on while the bridge is still wet, "wet on wet".
How do I repair honeycombing after striking formwork?
Same principle: break the friable zone back to dense concrete — do not just rub the face — then square edges, pre-wetting and compacted layers. Deep honeycombing that exposes bar bundles or passes through the section is an engineer's decision, not a trowel's.
Can I paint or render straight over the repair?
After the moist cure plus adequate subsequent drying (a week typically, by thickness and weather). Before rendering, treat the repair as a new substrate: absorption test and primer as needed — exactly as in the preparation guide.
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Last updated: 2026-08-09