Introduction
Not every crack in concrete is an emergency — and not every hairline can be ignored. The repair method depends on crack type, width, pattern, activity, and whether reinforcement is already corroding behind the face.
This article is a field guide for site engineers and supervisors: how to classify what you see, when to expose steel, and how to choose between sealing, injection, and section restoration before you open a bag of mortar.
Key takeaways
- Map crack type and activity before specifying a product — width alone is not enough.
- Spalled cover often hides corroding reinforcement; sound and expose before packing mortar.
- Non-structural cracks may need flexible sealing; section loss needs bonded polymer-modified repair.
- Active structural cracks need engineering input before cosmetic filling.
Crack types that matter
Plastic shrinkage cracks appear early, often parallel on slabs, and are usually shallow. Settlement and drying shrinkage produce more irregular patterns as the element loses moisture or support.
Structural cracks — from overload, inadequate reinforcement, or foundation movement — tend to be wider, may step through masonry, and often align with stress paths. Mark whether a crack is active: tell-tales, dated photos, or simple paper gauges over a few weeks tell you more than a single site visit.
- Record width, length, orientation, and dampness at each mapped crack.
- Note if the crack closes overnight or after rain — activity changes the repair.
- Flag any crack wider than ~0.3 mm on structural members for engineering review.
Check for exposed steel
Spalled cover, rust staining, and hollow-sounding patches often mean the bar is already losing section. Sound the surface with a light hammer; delaminated concrete returns a dull tone.
Expose steel to bright metal where corrosion is confirmed, clean to the specified standard, and treat with a compatible primer or corrosion inhibitor before placing repair mortar. Packing mortar over rusty steel traps the problem and shortens service life.
Choose a repair path
Non-structural, dormant cracks may be sealed with a flexible sealant or low-viscosity resin depending on width and exposure. Section loss and honeycombing need polymer-modified repair mortar bonded with a primer, built in layers to the original profile.
Injection is appropriate when you need to restore continuity in a dormant crack without cutting out large volumes — but it is the wrong tool for active structural movement. When in doubt, repair the cause (drainage, overload, cover) before you chase cosmetics.
- Seal dormant non-structural cracks; do not rigid-fill active joints.
- Use bonded polymer-modified mortar for cover replacement and section loss.
- Stop and escalate if cracks indicate frame or foundation distress.
Document before you repair
A short inspection record — photos, crack map, and proposed method — protects the owner and the applicator. Include product names, batch references, and curing notes when work starts.
If the member is part of a bridge, industrial frame, or water-retaining structure, treat undocumented crack filling as a liability, not a shortcut.
What to do next
Start with classification, not with the nearest repair bag. Expose and treat steel when corrosion is present, match the repair chemistry to the crack behaviour, and bring a structural engineer in whenever cracks suggest the frame — not just the cover — is moving.
Primadonn repair mortars and primers are specified for bonded section restoration once the diagnosis is clear. Diagnosis first; product second.
