Do I need to fix cracks before coating?

Almost every slab we look at has cracks in it. Most of them are nothing. A few of them change the whole conversation.

The distinction is not size. It is whether the crack is still moving.

The three kinds

Shrinkage cracks

Concrete shrinks as it cures and again as it dries over years. Where that shrinkage is restrained, it cracks.

These are typically thin, meandering, and flat: the two sides sit level with each other. They are stable, meaning they finished moving long ago.

These repair well and stay repaired. Chased out into a V, filled with a rigid polyurea or epoxy filler harder than the surrounding concrete, ground flush. Under a full flake broadcast they disappear.

Control joints

Not cracks at all. Deliberate saw cuts or tooled lines that tell the slab where to crack, so it does so in a straight line rather than randomly.

These have to keep doing that job after the coating goes on. Filling a live joint rigidly means the slab cracks somewhere else, usually in a worse place.

So joints get treated with a semi-rigid filler that supports the edges against traffic while still allowing movement, and they remain visible as lines. That is the correct outcome, not a compromise. Anyone promising a seamless floor over live joints is storing up a crack.

Structural and settlement cracks

The ones that matter.

Wider, often running across the slab in a fairly straight line, and crucially with displacement: one side sits higher than the other, or the gap opens and closes with the seasons.

That means the ground under the slab has moved, or is still moving. A tree root, settlement of fill, water washing out material underneath, or foundation movement.

These come back. Filling and coating over one hides it for a season or two and then it telegraphs through, because the cause has not stopped.

How to tell which you have

Run a finger across it. If you can feel a step between the two sides, that is displacement and it is a settlement crack.

Look at where it runs. A crack that continues up a wall, or lines up with a crack outside, is structural.

Look at the width along its length. A shrinkage crack is fairly consistent and thin. A settlement crack is often wider at one end.

Check whether it has moved. If you have photographs from a few years ago, compare. Or mark the ends with a pencil and look again in six months, though most people do not want to wait that long.

What repair actually involves

Chasing. The crack is cut open into a V or a U with a crack chaser, which sounds counterintuitive but gives the filler something to grip and removes weak material at the edges.

Cleaning. Vacuumed out completely. Filler over dust bonds to dust.

Filling. A rigid polyurea or epoxy filler, chosen to be harder than the surrounding concrete so the repair supports load rather than compressing under it.

Grinding flush. Once cured, ground level with the slab as part of the general preparation.

Spalled and pitted areas get the same treatment at larger scale: loose material out, rebuilt with a patching compound, ground flat.

What happens if you skip it

A coating is a few dozen mils thick. It has no structural strength and it will not bridge anything.

Skip the repair and the crack shows through as a visible line almost immediately, because the coating follows the profile of what is underneath. Any edge that is loose will take the coating with it when it breaks away.

An unfilled crack also lets water reach the underside of the coating around it, which spreads the failure outward.

The honest conversation

On a slab with settlement cracking, there are three options and only one of them is good.

Address the movement first. Slab jacking to lift a settled section, or dealing with the drainage or the root that caused it. Then coat.

Coat anyway, knowing it will return. Legitimate if you understand what you are buying. Sometimes that is a reasonable trade for five good years.

Accept the crack as a feature. On a polished floor or an industrial slab, cracks filled with a contrasting semi-rigid polyurea can be made to read as deliberate lines. Some clients like it.

What is not an option is being told the coating will fix it.

What we do

Every crack on your slab gets looked at and classified before quoting, and you get told which ones will disappear and which ones will come back.

That conversation costs us jobs occasionally, because "this crack will show again in two years" is not what anybody wants to hear before spending thousands. It is considerably better than the same conversation two years later.

Spalling, which is a different problem

Cracks get all the attention but spalling is more common on older Birmingham slabs and it needs a different fix.

Spalling is where the surface layer has broken down and flaked away, leaving shallow craters. It is caused by freeze-thaw cycling, by de-icing salt at some point in the slab's life, or by a poor original finish where too much water was worked into the surface.

The important characteristic is that the material around a spalled area is often weak too, well beyond the visible damage. Coating over it means bonding to concrete that is already failing.

The repair is to take out all the loose and friable material, which usually means the patch grows once you start, then rebuild with a patching compound and grind flush. On a badly spalled slab this can be a substantial part of the job.

It is also why we would rather see a floor than quote from a photograph. Spalling looks like staining in a picture.

More at garage floor coating and, if your existing floor has failed, repair and recoating.