Crack injection or route and fill — which does your concrete crack need?

Crack width decides the repair method. A concrete crack under about a millimetre is injected with a penetrating adhesive that travels into the crack on its own. A crack over about five millimetres is routed open with a grinder and filled with polymer cement over adhesive, the same way a patch is built. Between one and five millimetres you’re in the small-crack range, and the choice depends on movement and on whether water is running through it.

The mistake is treating all cracks as one job. A hairline crack and a five-millimetre crack behave completely differently, and they fail differently when they’re repaired the same way.

Why does crack width change the method?

Crack width changes the method because a narrow crack cannot be filled and a wide crack cannot be injected. Below a millimetre there is no room to place a stiff material — nothing with body to it will travel into the crack — so the repair has to be a liquid that penetrates by itself. Above five millimetres there is enough volume that a liquid alone won’t hold; the crack needs to be opened, cleaned and rebuilt with material that has strength.

The three bands we work to:

Crack width	Name	Method
Under 1 mm	Hairline	Injected with a permeable, penetrating adhesive
1–5 mm	Small crack	Injected or routed, depending on movement and water
Over 5 mm	Wide crack	Routed open with a router and grinder, then filled

One thing that catches people out: routing a crack changes which band it’s in. Open a four-millimetre crack to clean it properly and you may finish with a seven-millimetre channel. At that point it’s a wide crack and gets treated as one, whatever it measured before the grinder went near it.

Why do cracks move so much more than the slab around them?

A crack moves proportionally far more than the concrete around it, because the crack is thin and the movement is not. That’s the reason crack repairs fail more often than patch repairs.

Work the arithmetic. If the concrete on either side pulls apart by two millimetres as it contracts, and the crack itself is five millimetres wide, the crack has just opened by forty percent of its own width. The slab moved a fraction of a percent. The crack moved half again.

So a crack filler is being asked to survive strain that nothing else on the deck experiences. Anything rigid in there will break, which is why the material and the bond matter far more in a crack than they do in a flat patch — the same bond-versus-anchors problem that makes ordinary patches fail, only exaggerated.

How is a wide crack repaired?

A wide concrete crack is routed open, coated with adhesive, and filled with polymer cement — the same materials and the same sequence as a bonded patch, in a narrower channel. A router and grinder open the crack into a clean, sound-sided channel; loose material and debris come out; adhesive goes onto the exposed faces; polymer cement fills the channel and seals it.

The reason it mirrors concrete patching is that the requirement is the same. The fill has to bond chemically to both faces so that it moves with the concrete rather than sitting between two walls that are moving without it. A rigid filler wedged into a moving crack is a repair with a defined lifespan.

Cracks can run a long way. On the airside concrete at Hamilton International Airport we handled cracks as long as thirty feet, using Polyforce Plus Adhesive with Polyforce Polymer Cement.

How is a hairline crack repaired?

A hairline crack is repaired by injecting a low-viscosity adhesive that penetrates into the crack and seals it from the inside, with nothing routed out. We often use Polyforce Plus together with a material called Joint Seal.

Worth saying plainly, because it confuses people: Joint Seal is a product made by someone else that happens to share a name with our company. There’s no relationship. It’s just called Joint Seal.

On a mid-rise garage on Fairview Mall Drive in North York we ran crack repair alongside vertical expansion joint work on the garage walls and ceiling, using cold-applied polyurea.

What happens if water is running out of the crack?

A crack with water actively coming through it cannot be surface-patched, because the water pushes the repair off the concrete before it bonds. The water has to be dealt with first, either by injecting a material that cures in wet conditions or by stopping the flow, and only then can the crack be closed.

This is one of the practical arguments for adhesives that cure in wet and moist environments. In a parking garage the cracks worth repairing are usually the ones with water in them, and a material that needs a dry substrate is not much use on a leaking wall.

What does this mean for a garage repair quote?

Nearly every parking garage job we do involves patching or crack repair, and usually both — they run together because they come from the same cause. Water and road salt get through the deck, the steel corrodes, the concrete cracks and spalls.

So when a garage quote separates crack repair into linear feet and patching into square feet, that’s normal and correct. What’s worth asking is which crack method is priced, and at what width the contractor switches between them. A quote that prices everything as injection, or everything as routing, hasn’t measured the cracks.

What to do next

On your next walk, take a crack gauge or even a ruler and note the widths, not just the locations. Under a millimetre, one to five, over five — three columns, and the counts in each. That single measurement changes the scope more than almost anything else on the sheet.

Also note which cracks are wet. Those move to the front of the queue regardless of width. We scope parking garage concrete repair that way across Toronto and the GTA, and the crack survey is usually the part that surprises the board.