Repair mortar: fixing concrete and masonry

Repairing damaged concrete or masonry does not start with the mortar — it starts with the diagnosis. The short answer: a repair mortar based on CPJ 45 is dosed at no less than 350 kg/m³ of 0/4 sand, onto a sound, clean, dampened substrate — and it is useless if the cause of the damage (leak, corrosion, movement) keeps acting. A repair that ignores the cause is a plaster cast on a broken leg.

Diagnose before repairing

Damage has signatures. Fine shrinkage cracks in a facade mostly need surface sealing; spalling with exposed, rusty rebar signals corrosion spreading behind — the serious case, requiring the steel to be cleared all around and often a specialised contractor; honeycombing (porous zones from poor placing) is straightforward to re-profile; moving, through-going cracks belong to the structural engineer, not to a bag of cement. Assess your situation honestly before opening the bag.

Preparing the substrate: the step that decides everything

Preparation is what separates a ten-year repair from a shell that debonds in six months. The method: chip back to sound concrete (never fill over friable material), feather the edges to avoid right-angle joints, clean by blowpipe or washing to strip dust and loose particles, then dampen to saturation the day before and again just before filling — or apply a bonding agent per its data sheet. A clean, rough, damp substrate is half the job done.

Batching the mortar

For everyday re-profiling: CPJ 45 + 0/4 sand, mixed stiff, dosed at 350 kg/m³ of sand minimum — a rich mortar compensates for thin sections. For fixings and small non-structural touch-ups, CPJ 35 also fits (fixing and repair mortars: 200-300 kg/m³ per its data sheet) when available — production varies by campaign, check stock before planning. Thin or heavily loaded repairs use ready-mixed repair mortars, factory-batched with shrinkage-controlling additives: formulated products DAM does not manufacture — we produce the cement inside them.

Protecting the repair

A repair mortar is cured like concrete: shielded from sun and wind, kept damp in the early days, it develops strength without cracking. And it is not loaded right away: let it set before stressing, formworking or folding the repaired zone into a wider cycle. Rushing the last step wastes every hour of preparation upstream — curing and patience are the final two ingredients.

For repair mortars and site volumes: CPJ 45, CPJ 35 on request subject to availability — free personalized quote.