How to Fix a Cracked Concrete Driveway Before the First Freeze

There is a crack in your driveway. You have walked over it since about March, you have registered it roughly four hundred times, and every single time you have thought: I should do something about that. Then you have carried the shopping inside and forgotten about it until the next time.

Here is the thing nobody tells you. That crack has a deadline, and the deadline is the first hard freeze. Not because the crack is dangerous, but because of what happens to it the first night the temperature drops below freezing with water sitting inside it.

Why the first freeze is the deadline

Water expands by roughly nine percent when it turns to ice. That is not a rounding error. It is a wedge.

So the sequence goes like this. It rains. Water runs along the surface of the slab and finds the crack, because the crack is the lowest path available. The temperature drops overnight and the water inside freezes, expanding, pushing the two faces of the crack apart by a fraction of a millimetre. In the morning it thaws, the gap is now very slightly wider than it was, and so it holds a little more water next time.

Repeat that forty or fifty times over a winter and you do not have a hairline any more. You have a gap you can get a finger into, with crumbled edges, and in a few years, a section of slab that has broken free and moved. This is called freeze-thaw cycling, and it is the single biggest reason concrete driveways in cold climates fail.

The repair, done now, is twenty minutes and a few dollars. The repair, done after three winters of that, is a resurfacing quote.

First: is this a crack you should be fixing yourself?

This is the part most guides skip, and it matters more than any technique. Some cracks are a sealing job. Some are a symptom, and sealing those is like painting over damp.

Seal it yourself if: it is a hairline or narrow crack, under about a quarter of an inch wide. Both sides sit level with each other. It has not obviously changed in the time you have been looking at it. It runs across a slab rather than tracing the whole height of a wall. These are shrinkage and settlement cracks, and they are normal in concrete — concrete cracks as it cures, and that is not a defect.

Get somebody to look at it if: it is wider than a quarter of an inch. One side sits higher than the other, so you can feel a lip with your foot. It runs the full height of a foundation or basement wall. It is wider at one end than the other in a way that suggests the ground beneath has moved. Or it has visibly grown since last year.

What you actually need

Less than you think. There is a whole aisle of equipment for this job and most of it is for contractors doing a hundred metres of commercial floor.

• A stiff brush, or a wire brush for anything stubborn
• A vacuum, or a leaf blower, or someone willing to blow hard
• A screwdriver or an old chisel to pick out loose material
• A bottle of self-levelling crack filler
• A dry day, above freezing, with no rain forecast for the rest of it

You do not need a mixer, a trowel, a kneeling board, or a second person. If a product requires all that, it is a different product for a different job.

The job, step by step

1. Pick your day. Dry, above freezing, and nothing coming in the next few hours. This is genuinely the step people get wrong, and it is the one that decides whether the repair holds. Filler needs a clean dry surface to bond to. A crack that is damp at the bottom will take the filler and let go of it by spring.

2. Get the loose material out. Run a screwdriver or an old chisel along the crack to dislodge the crumbled edges and whatever has worked its way in — grit, moss, old sealant, leaf matter. You want the sides of the crack to be sound concrete, not dust. This feels like it is making the crack worse. It is not. It is making the repair stick.

3. Clean it properly. Brush it out, then blow or vacuum the dust away. Dust is the enemy here: filler poured onto dust bonds to the dust, and the dust is not attached to anything. If you have a leaf blower, this takes about four seconds and is the most satisfying part of the job.

4. Check it is dry. If you cleaned it with water, or it rained in the last day or two, wait. Deep cracks hold water at the bottom long after the surface looks dry. A warm dry day does the work for you.

5. Pour. Run the filler along the crack steadily. A self-levelling product does the spreading for you — it finds its own level in the gap, which is the entire point of it. Fill to just below the surface on the first pass if the crack is deep; it is easier to add a second pass than to clean up an overfill.

6. Leave it alone. Resist smoothing it over repeatedly. Self-levelling filler is still moving for the first few minutes, and poking at it leaves marks that set permanently. Pour, check it has levelled, walk away.

7. Give it longer than the label says before driving on it. Surface dry and fully cured are different things. In cold weather everything takes longer than the packet assumes, because the packet assumes a warm workshop.

The five mistakes that make a repair fail

Filling a wet crack. The most common one by a distance. The filler cannot bond to a damp surface, so it sits in the gap like a plug rather than bonding to the sides, and the first freeze pops it out.

Skipping the clean-out. Pouring straight into a dirty crack is the second most common, and it fails for the same reason: the filler bonds to loose material, and loose material moves.

Doing it too cold. Below freezing, filler does not cure properly. It may look fine for a week. It will not last the winter.

Overfilling. A proud bead of filler standing above the slab gets caught by a snow shovel, a plough blade or a car tyre, and gets torn out, taking the repair with it. Flush or very slightly below is what you want.

Waiting for spring. Spring is when you discover how much damage the winter did. The whole value of this job is that it happens before, not after.

While you have the bottle out

The driveway is the obvious one, but it is rarely the only crack on a property, and you will not want to do this twice. Walk round with the bottle and look at:

The garage floor. Takes road salt off the car all winter, which is harder on concrete than the weather is.

The steps and the threshold. Small cracks here are the ones that turn into chipped edges, because water gets in and then something stands on it.

Where a path meets the house. That join opens up every year and funnels water straight down beside the foundation, which is the last place you want it.

The join where a flat roof meets a wall. Shed roofs, extension roofs, porch roofs. This is where the first drip of the winter usually comes from.

Basement and foundation walls. A damp patch that returns every autumn normally starts at a crack you can only find when it is actively wet. Which, inconveniently, is exactly when you cannot seal it — so mark it now and come back on a dry spell.

What the result will actually look like

Grey. Close to ordinary concrete, which is what most driveways and garage floors are, so on those surfaces the repair stops announcing itself within a few weeks of weathering.

It will not be invisible, and anyone who tells you otherwise is selling. On a coloured, stained, brushed or power-floated finish you will see the line. That is a fair trade for keeping water out of a slab, but you should know it before you pour rather than after.

What we sell, and what we would tell a neighbour

We stock Waterproof Concrete Crack Filler in two sizes. The small one is for a few cracks around a step, a sill or a patch of garage floor. The larger one is the one to take if you are walking the length of a driveway. It pours, it self-levels, it goes off at the surface in about half an hour and cures in roughly two.

And to be straight with you: this is a fifteen dollar product. We are not going to pretend it is the most exciting thing in the shop. It is simply the job on the autumn list with the hardest deadline and the worst consequences for missing it, which is why it is worth a Saturday morning rather than another year of walking past it.

If the crack is in that second list further up — wide, lipped, moving, running the height of a wall — please do not buy this. Get it looked at. We would rather lose the sale than have you seal over something that needed a builder.

The rest of what gets a house through winter is in One Less Chore, including the cordless snow shovel for when the driveway you just sealed disappears under six inches of snow.