Repairing Cracked Concrete, Properly

Cracked concrete is repairable — but only if the method matches the cause. Epoxy injection for structural cracks, routing and sealing for movement cracks, and full breakout when the reinforcement is compromised.

Repairing Cracked Concrete, Properly — practical guidance from the team running concrete repair and resurfacing on the Garden Route. Below is how to read a crack before you fill it, which product belongs in which crack, and why the strongest filler on the shelf is the wrong answer more often than it is the right one.

Read the crack before you buy anything

Every filler on the shelf will bond to clean concrete. Almost none of them will still be intact in two years if the crack is doing something the filler cannot accommodate. So the first job is not repair, it is diagnosis.

Four questions settle it. Is the crack still moving? Is it going all the way through? Is water coming through it? And is the reinforcement behind it corroding?

Movement is the one that governs product choice. A shrinkage crack that formed in the first months and has been stable ever since is a dormant crack, and a rigid repair will hold. A crack that opens in summer and closes in winter, or one that widens each year, is an active crack, and anything rigid you put in it will simply crack again — usually alongside the repair rather than through it.

Depth decides whether you are dealing with a surface defect or a structural one. A crazed, map-patterned surface is a curing problem in the top few millimetres. A single crack running through the full thickness and appearing on the underside of a slab is structural, and it is not a filler job.

Water and rust change the priority completely. A damp crack with brown staining is telling you the reinforcement behind it is corroding, and sealing the surface over corroding steel traps the problem and accelerates it.

For structural cracks the repair is injection rather than filling, and the resin systems in the South African market come mainly from Sika, a.b.e., Mapei and Fosroc. Low-viscosity epoxy is used where the crack is dormant and the object is to restore monolithic strength; a flexible polyurethane is used where the crack is live and will keep moving, because an epoxy in a moving crack simply cracks again alongside the repair. Establishing which of the two you have — by monitoring the crack over weeks rather than guessing on the day — is the part that decides whether the repair holds.

The five common crack patterns and what each one means

Cracks are legible if you know the vocabulary. Most of what we are called to look at falls into five patterns.

Fine random crazing across a slab surface, in a map or crocodile-skin pattern, is plastic shrinkage — the surface dried faster than the concrete below it cured. Cosmetic in itself, but it is a symptom of a weak, dusty top layer that will wear badly.

Straight cracks at regular spacing, often roughly where a joint should have been, are drying shrinkage finding its own relief. Usually stable once the concrete has finished shrinking.

A single crack running diagonally from a corner, or across a re-entrant corner at a doorway or column, is a stress concentration. Common, usually stable, and a candidate for routing and sealing.

Cracks with a step or offset, where one side sits higher than the other, mean the ground under the slab has moved or the slab is deflecting. That is a structural or geotechnical problem and filling it achieves nothing.

Cracks tracking the line of the reinforcement, often with rust staining and sometimes with the concrete face lifting or flaking, are corrosion. The steel is expanding as it rusts and pushing the cover off. Extremely common near the coast, and the only real repair is breaking out to the steel.

Matching method to crack

Once you know what the crack is, the method follows without much argument.

Epoxy injection restores structural continuity. It is used on dormant structural cracks where the point is to make the two sides act as one again. Injected under pressure through ports, with the surface sealed between them, it will fill a crack down to hairline width, and the bond is typically stronger than the surrounding concrete. It is also entirely rigid, which is why it is wrong for anything still moving.

Routing and sealing is the answer for active cracks. The crack is cut out to a clean square or V profile, cleaned, primed and filled with a flexible polyurethane or polysulphide sealant that can stretch as the crack works. You are not restoring structure here; you are creating a durable, waterproof joint where the concrete has decided to have one.

Polyurethane resin injection is for cracks that are actively leaking water. It reacts with water and foams to fill the void, stopping the leak. It is a water-stopping repair, not a structural one, and it is often followed by an epoxy or sealant repair once the concrete is dry.

Breakout and reinstatement is the honest answer for corrosion damage and for spalled or delaminated concrete. Cut back past the damage to sound material, expose the steel fully around its circumference, clean or replace it, apply a corrosion inhibitor and primer, and reinstate with a polymer-modified repair mortar. Slower, more expensive, and the only thing that actually works.

Crack condition	Method	What it achieves	Wrong if
Dormant, structural, dry	Epoxy injection	Restores structural continuity	The crack is still moving
Active / seasonal movement	Rout and seal, flexible sealant	Durable watertight flexible joint	You need structural continuity
Actively leaking water	Polyurethane resin injection	Stops the leak	Treated as a structural repair
Rust staining, spalling	Break out, treat steel, reinstate	Removes the cause	Surface-filled or coated over
Surface crazing, dusting	Grind and resurface / overlay	Sound, wearable surface	The substrate is delaminated

The strongest filler is usually the wrong question

“What is the strongest concrete crack filler?” is the most-asked question on this subject, and it contains the mistake. Strength is rarely the binding constraint. Compatibility with movement is.

Epoxy is the strongest of the common repair materials by a distance, and in a dormant structural crack it is the right answer for exactly that reason. Put the same epoxy in a crack that opens 0.5 mm every summer and it will not fail by being too weak. It will fail by being too stiff — the concrete will crack again a few millimetres to the side, and now you have two cracks and a rigid strip between them.

For a moving crack, the property that matters is elongation: how far the material can stretch before it tears. A good polyurethane sealant will accommodate substantial movement indefinitely. It is far weaker than epoxy and far more durable in that application.

The same logic applies to the cement-based fillers sold for big cracks. They are convenient and they bond adequately, but they shrink slightly as they cure and they have effectively no flexibility. In a wide, dormant, non-structural crack in a garden path, that is fine. In anything that moves or carries load, it is a repair with a short life.

Preparation, which is most of the job

Almost every failed concrete repair we are asked to look at failed at the preparation stage, not the product stage.

The crack has to be opened up. Filling a hairline crack from the surface puts a paper-thin film of material into the top millimetre, with no depth to grip and no cross-section to resist anything. Routing out to a proper profile — square-cut, with parallel sides and adequate depth-to-width — is what gives the repair material something to hold.

It has to be clean and dry. Dust, laitance, oil, old sealant and loose material all prevent bond. Compressed air or vacuum, not a brush that just redistributes the dust. Damp substrate kills the bond of most epoxies outright, and on the coast a slab can be damp long after it looks dry.

It has to be primed where the product calls for it, and the product has to be mixed properly. Two-part materials that are eyeballed rather than measured cure inconsistently, and the failure shows up months later as patches that are soft or brittle.

And on an external repair, the movement joints have to be re-established through the repair. Filling straight across an existing joint is how a neat repair develops a crack in a straight line the following season.

When repair stops being the right answer

There is a point where patching is throwing money at a slab that needs replacing, and it is worth naming it plainly.

If a large proportion of the surface is delaminated — a hollow, drummy sound when tapped, over a wide area — the top layer has separated from the concrete below and no overlay will stay attached to it. If the reinforcement has lost significant section to corrosion over a broad area, spot repairs will simply chase the corrosion front around the structure. If the slab is deflecting or has stepped, the problem is under it or in its design, and nothing applied to the surface will address that.

The other honest limit is economic. Breakout and reinstatement is labour-intensive. On a badly deteriorated domestic slab, the repair can approach the cost of a new one, and the new one comes with a fresh service life and no inherited defects.

We will say when that line has been crossed. It is a shorter conversation than a sequence of repairs that each last two years.

Why coastal concrete cracks and spalls sooner

The Garden Route is a harder environment for concrete than most of the country, and the mechanism is worth understanding because it changes what a good repair looks like here.

Airborne salt carries chloride. Chloride migrates into concrete through the pore structure and, once it reaches the reinforcement in sufficient concentration, breaks down the passive layer that normally protects the steel. From that point the steel corrodes, and corrosion products occupy several times the volume of the original steel. That expansion cracks the cover off from the inside.

Two things resist it: cover and density. Enough concrete over the steel, and concrete tight enough that chlorides move through it slowly. Both are decided at the pour, which is why a well-built slab in George lasts and a poorly cured one does not.

For repairs, it means chasing the visible damage is not enough. If chloride has penetrated the surrounding concrete, adjacent areas are already on the same path even though they look sound. On anything significant, testing the chloride profile before repairing tells you whether you are fixing a problem or the first instance of one.

Frequently asked questions

What is the strongest concrete crack filler?

Epoxy, by a clear margin — a correctly injected epoxy repair is typically stronger than the concrete around it. But strength is usually the wrong criterion. Epoxy is rigid, so in a crack that is still moving it will not fail by being weak, it will cause the concrete to crack again alongside the repair. For an active crack the property that matters is elongation, and a flexible polyurethane sealant is the correct choice even though it is far weaker.

How do you permanently fix cracks in concrete?

By fixing the cause, not the crack. A dormant structural crack in dry concrete can be permanently restored by epoxy injection. An active crack is permanently dealt with by routing it out and sealing it with a flexible sealant, so the movement has somewhere to go. Corrosion cracking is only permanently fixed by breaking out to the steel, treating or replacing it and reinstating with a repair mortar. Filling a crack whose cause is still active is a repair with an expiry date, whatever product goes in it.

Can hairline cracks in concrete be repaired?

Yes, though many do not need to be. Fine shrinkage cracks that are stable, dry and not showing rust staining are a characteristic of concrete rather than a defect. Where they matter — on a surface exposed to weather, or where chloride ingress is a concern — the options are low-viscosity epoxy or resin injection for structural cracks, or a penetrating sealer over a crazed surface. What does not work is smearing filler over the top; without routing there is no depth for the material to grip.

Can I pour concrete over old cracked concrete?

Only if the existing slab is sound and the cracks are stable. A bonded overlay needs a clean, sound, mechanically prepared substrate, and any crack in the base slab that is still moving will reflect straight up through the topping within a season. Where the substrate is delaminated or the reinforcement is corroding, an overlay just adds weight over a problem. An unbonded overlay on a separation layer, thick enough to work as a slab in its own right, is sometimes the answer — but it is a new slab, and it needs to be designed as one.

How big a crack can be repaired?

Width is rarely what disqualifies a repair. Wide dormant cracks are routine to rout and fill; very fine ones are actually harder because getting material into them requires injection. What disqualifies a repair is the cause: stepped or offset cracks indicating ground movement or deflection, widespread delamination, or extensive reinforcement corrosion. Those need the underlying problem addressed, and sometimes replacement.

What are the common mistakes when repairing concrete cracks?

Five account for nearly all of them. Not diagnosing whether the crack is active before choosing a product. Filling from the surface without routing out, so the repair has no depth. Working on a dusty or damp substrate so nothing bonds. Sealing over corroding reinforcement, which traps and accelerates the corrosion. And filling straight across an existing movement joint, which guarantees the repair cracks in a straight line the next season.

Will epoxy seal a crack in concrete?

It will, and it will do it very well in dry, dormant cracks — injected under pressure it fills down to hairline widths and restores structural continuity. It is the wrong material in two situations: cracks that are still moving, because epoxy has no flexibility, and cracks that are actively wet, because most epoxies will not bond to a damp substrate. Leaking cracks are stopped with polyurethane resin first, then repaired properly once dry.

Related reading

• Concrete spalling: identify, repair, retest
• Concrete curing in coastal climates
• Concrete MPa, plain
• Concrete slabs: thickness, MPa, reinforcement
• Concrete repair and resurfacing in George