One of the questions we hear most often before a commercial floor coating project is: what happens to the cracks in my slab? It's the right question to ask. Concrete cracks are a reality in virtually every commercial and industrial slab, and how they're handled before the coating goes down directly determines whether the finished floor looks clean and holds up over time — or shows ghost lines and fails early.
Not All Cracks Are the Same
Before deciding how to treat a crack, you have to understand what type of crack it is. The treatment is different depending on whether the crack is structural or non-structural, active or dormant, and what the coating system going on top will be.
Hairline cracks are very fine surface cracks — typically less than 0.3mm wide — that occur in the top layer of the slab from shrinkage during curing, temperature cycling, or minor surface stress. They don't compromise the structural integrity of the slab and don't move significantly. Most hairline cracks in a commercial slab can be addressed with a flexible epoxy or polyurea crack filler that bridges the crack and prevents it from reflecting through the coating above.
Dormant cracks are wider cracks — typically 0.3mm to 3mm or more — that have stabilized and are no longer actively moving. These require more substantial treatment: routing the crack to create a uniform channel, cleaning thoroughly, and filling with a rigid or semi-flexible repair material depending on the coating system specified above. The routing step is important — it creates a defined, clean-edged repair profile that bonds better than filling a rough, tapered crack.
Active cracks are cracks that are still moving — expanding and contracting with temperature changes, load cycling, or ongoing slab movement. Active cracks require flexible repair systems and, depending on severity, may require engineering assessment. A rigid repair over an active crack will re-crack as the slab continues to move, usually within months of installation.
The Materials We Use
Polyurea crack filler is the standard material for most commercial floor crack repair. It's a two-component system that cures very rapidly — typically within minutes — allowing the project to continue with minimal downtime. Polyurea is flexible enough to accommodate minor slab movement without re-cracking, bonds well to concrete, and can be ground flush with the slab surface after cure. It's the right choice for most hairline and dormant non-structural cracks.
Epoxy injection is used for structural cracks where the goal is to restore the tensile strength of the concrete, not just fill the surface crack. Low-viscosity epoxy is injected under pressure into the crack, penetrating the full depth of the slab and curing to a rigid bond. This is a more involved process and is specified when a structural engineer has identified the crack as requiring strength restoration rather than surface treatment.
Cementitious repair mortars and hybrid concrete repair materials are used for spalled areas, damaged edges, and surface defects rather than cracks per se. They're applied to rebuild the slab surface where material has been lost, and they need to cure fully before the coating system is applied over them.
The Process on a Commercial Project
On a typical commercial floor coating project, crack and defect repair happens after mechanical surface preparation (diamond grinding or shot blasting) and before primer application. The sequence matters: grinding first exposes the full extent of cracks and defects that may not be visible on an unground slab surface. You can't accurately assess what needs repair until the surface has been prepared.
After grinding, we walk the slab and mark all cracks, spalls, and defects requiring repair. Cracks are either filled directly with polyurea or routed first depending on width and condition. Spalled areas are ground back to sound concrete and patched with the appropriate repair mortar. Everything is allowed to cure fully before the coating system begins.
Managing Expectations
Crack repair before coating minimizes crack reflection — but it doesn't guarantee zero visibility in every case. For very wide or heavily damaged slabs, or slabs with ongoing active movement, there are limits to what surface repair can achieve. In those situations, we give clients an honest assessment of what the finished floor will look like and what additional measures — if any — are practical. The goal is always a finished result you're satisfied with, which starts with accurate expectations before work begins.
If you have a commercial slab with significant cracking and you're considering a coating system, contact us. We'll assess the slab and give you a clear picture of what the repair and preparation scope involves.