Why Concrete Cracks, and Which Cracks Matter

Every concrete slab cracks. Yours, ours, the one at the bank, the one at the airport. Concrete shrinks as it dries, it is enormously strong in compression and weak in tension, and it gets poured in big flat pieces that are held at the edges by their own friction against the ground. Something has to give, and it gives by cracking.

So "is my concrete cracked" is never the useful question. The useful question is which kind of crack is that, and is it telling me something is wrong underneath. Most of the cracks people call us about are cosmetic. A few are not, and the difference is visible from standing height if you know what you are looking at.

The short version

Two things turn a crack from cosmetic to structural: width that changes over time, and height that differs across it. Everything else is mostly appearance.

Why concrete cracks at all

Concrete is not a solid that dries. It is a chemical reaction — cement and water forming crystals that lock the sand and rock together. That reaction consumes some of the water, and the rest leaves over the following weeks and months. As it leaves, the slab gets slightly smaller.

Slightly is enough. A driveway-length slab shrinking even a fraction of a percent has to shed that length somewhere, and the ground underneath will not let it slide freely. The bottom of the slab is gripped by the base; the top is free. The result is tension across the slab, and concrete pulls apart in tension at a fraction of the load it can carry in compression.

That is the whole mechanism. It is not a quality problem, it is not a mix problem in most cases, and no admixture or fiber makes it go away. What good work does is decide where the crack goes and keep it tight once it gets there.

Shrinkage cracks: the ones that do not matter

These are what most people are looking at.

A drying shrinkage crack is typically a hairline — you can see it, you often cannot feel it, and a business card will not go into it. It wanders rather than running straight. It shows up in the first weeks to months, not years. Crucially, both sides of it are at the same height. Run your palm across it with your eyes closed and you cannot tell where it is.

Those cracks are ugly and permanent and they are not going to do anything else. They do not grow into structural cracks. They do not mean the slab is thin or the steel was skipped. Left alone, a hairline shrinkage crack in a properly built driveway will still be a hairline shrinkage crack in twenty years.

There is a related version worth knowing about because it is common here. Plastic shrinkage cracking happens during the pour itself, on hot, dry, windy days — which describes a lot of Southern California afternoons and nearly every Santa Ana condition. The surface of the fresh slab loses water to the air faster than bleed water can rise to replace it, the top skins over and shrinks while the concrete beneath is still soft, and it tears. You get short, roughly parallel cracks in the surface, often an inch or two deep, sometimes visible before the crew has left.

They are cosmetic. They are also avoidable, which is why they annoy us: shading the pour, working in the cooler part of the day, fogging the surface, an evaporation retarder, and getting the curing started immediately all reduce them. If you see them on a fresh slab, the slab is fine and the pour day was fought rather than planned.

Crazing is the third cosmetic one — a fine, tight web of lines across the surface that looks like cracked glaze on a ceramic. It lives in the top layer of cement paste and goes nowhere. It shows up more on hard-troweled surfaces than broom finishes, and it becomes obvious when the slab is wet and nearly invisible when it dries.

Control joints, and why the crack goes where you cut it

Since the slab is going to crack, the job is to give it a line to crack along.

That is all a control joint is: a groove cut into the top of the slab, roughly a quarter of its depth, that makes a deliberate weak plane. The slab still cracks — it cracks down inside the cut, where the crack is hidden in a straight line that reads as part of the design instead of a scar across the middle of your driveway.

Three things decide whether joints work.

Depth. Too shallow and the slab is not weakened enough to prefer the cut. It relieves itself somewhere else and you get a random crack running past a perfectly good joint.

Spacing. Panels want to be close to square. A long, narrow panel will crack across its middle no matter how neatly the joints around it were cut, because the tension builds along the long dimension. The usual working rule is that panel sides run somewhere around two to three times the slab thickness measured in feet — a four-inch slab wants panels in the range of eight to twelve feet, closer to the low end if the mix is wet or the weather is hot. A drive strip five feet wide and thirty feet long needs joints across it every eight feet or so, not one down the middle.

Timing. Saw cutting is a window, not a task. Cut too early and the edges ravel and spall because the concrete has not gained enough strength. Cut too late and the slab has already relieved itself somewhere of its own choosing, and now you have both a crack and a joint. That window is hours, not days, and it moves with the weather — a hot day closes it early. Cutting joints on schedule sometimes means somebody coming back at an inconvenient hour, which is exactly why it gets missed.

If you have a random crack that runs parallel to a joint a foot or two away, you are looking at a timing miss or a spacing miss. It is a cosmetic outcome with a workmanship cause.

Re-entrant corners

An inside corner — where a driveway wraps around a garage return, where a patio notches around a post, where a walkway meets a wider pad — concentrates stress at the point of the corner. Shrinkage tension has nowhere to go but into that inside angle.

Cracks running diagonally out from inside corners are close to inevitable unless the layout anticipates them. A joint placed to run out of the corner gives the crack a line. Some crews add a short length of steel across the corner instead, or in addition, to keep whatever forms tight.

If you have a crack running out of an inside corner and it is tight and level, that is the geometry of the slab, not a failure. It is still worth noticing whether anyone tried to control it, because it tells you how much thought went into the joint layout.

The cracks that actually mean something

Here is where the tone changes. A crack matters when it is evidence that the ground under the slab is no longer supporting it evenly.

Vertical displacement. One side of the crack is higher than the other. You can feel it with a foot, and a straightedge laid across it rocks. This means the two pieces of slab are being held up by different amounts of support, which means the base under one of them has moved, washed out, or was never compacted. This is the single most important thing to look for and the easiest to check.

A dish or a low spot around the crack. Water stands there after a hose or a rain when it did not used to. The slab is settling into a void.

Width that grows. A crack that was a hairline last year and takes a fingernail this year is being driven by something ongoing. Mark the ends of it with a pencil and note the date — the cheapest diagnostic there is. A shrinkage crack has finished moving; a base problem has not.

Location that tells a story. Cracks that run along the edge of a slab, near a downspout, along the line of an old utility trench, at the low end of a drive that sheds water toward the house, or right where a delivery truck turns in are cracks with a cause you can point at. Water undermining a base almost always shows up at an edge first, because that is where the base is least confined.

Pattern. Several roughly parallel cracks across the direction of traffic, or a crack that outlines a rectangle in the middle of a panel, suggests the slab is bending over a soft area rather than shrinking.

What a crack repair actually is

This is the part people do not enjoy hearing. You cannot repair a crack in concrete. You can fill it, and filling it is worth doing for the right reasons, but nothing puts the slab back into one piece.

What filling accomplishes is keeping water out. Water in a crack gets into the base, and a base that gets wet under a crack becomes a base that settles under a crack, which is how a cosmetic problem becomes a structural one. On a driveway that sheds water across a crack, sealing it is preventive maintenance and it is cheap in effort.

The options, in ascending order of involvement:

• Leave it. A tight, level hairline in a well-drained slab is genuinely fine to ignore.
• Clean and seal it. A flexible crack filler keeps water out. It will still be visible. Anyone who tells you a filled crack disappears has not filled many.
• Rout and seal. Widen the crack into a clean channel with a saw or grinder, then fill it. It looks deliberate rather than accidental, holds the filler better, and is what we do on cracks wide enough to be obvious anyway.
• Grind a lip. Where one side has risen slightly and the edge is a trip hazard, the high side can be ground down. It solves the safety problem. It does not solve the base problem, and the lip will usually come back.
• Cut out and replace the panel. Where the crack has a structural cause confined to one area, that panel comes out, the base gets fixed, and a new panel goes in between existing joints. The color will not match. It never matches. New concrete against ten-year-old concrete is a visible repair and there is no honest way around it.
• Tear out and repour. Where the base failed across the slab, everything else is money spent on a surface that is going to do it again.

That color-match problem is worth sitting with, because it is the real argument for pavers on a yard where you expect ground movement. A settled paver field comes up, gets a corrected base, and goes back down with the same units and no visible repair. A settled slab gets a patch you will see forever.

What we will not promise

We will not promise you a crack-free driveway, and you should treat anyone who does with suspicion. They are either going to be unavailable when you call, or they do not understand the material well enough to be pouring it.

What good work buys you is a slab where the cracks are in the joints, the panels stay level with each other, and the surface is still one plane in fifteen years. That is achievable and it is what the base work, the chaired steel, the joint layout, and the drainage are all for. A crack-free slab is not on the menu.

What to ask about cracks

On a slab you already have:

1. Can you feel a step across it? If yes, that changes the conversation from cosmetic to structural before anything else is discussed.
2. Is it growing? Mark it and wait a season. This costs nothing and answers the most expensive question.
3. Where is the water going? Cracks near the low corner of a slab that drains the wrong way are not a mystery.

On a slab you are about to buy:

1. Where are the joints going, and when are they being cut? A contractor who has thought about your driveway can describe the panel layout before pouring it. One who has not will cut joints wherever looks reasonable when they get back to it.
2. What reinforcement, and how is it held up? Steel does nothing lying on the base. Chaired rebar is what keeps a crack tight and level once it forms.
3. How deep is the base and how is it compacted? In lifts, or it is not compacted. This is the answer that determines whether your cracks stay cosmetic.
4. What happens on a hot pour day? If the answer includes shading, timing, and getting the cure started immediately, someone has thought about plastic shrinkage. If the answer is a blank look, you may be getting surface cracks on day one.