Why
Why Concrete Driveways Crack
Four different things produce a line in a driveway, and only two of them are problems. Working out which one you are looking at is the entire diagnosis, and it decides whether the answer costs nothing or costs a driveway.
Almost every homeowner who calls us about a crack has already decided it is bad news. Often it is not news at all. Concrete is a material that cracks as a matter of course, and the trade spends most of its effort deciding where those cracks will go rather than preventing them, because preventing them is not on offer. What separates a driveway that is fine from one that is failing is not whether it has cracked. It is which of four mechanisms did it.
One: it shrank, which was always going to happen
Fresh concrete contains more water than the chemical reaction needs, and as that surplus leaves, the slab gets marginally smaller. A driveway is far too large and far too gripped by the ground beneath it to shrink evenly, so the tension has to be relieved somewhere.
That is what joints are. A control joint is a deliberate weakness, cut or tooled into the slab so the inevitable crack forms along a line somebody chose rather than one the slab picked. Cut to roughly one quarter of the slab thickness, the joint gives the crack a place to start and it obligingly goes there, below the surface where nobody sees it.
So a fine line running along a joint is not damage. It is the joint doing its job. The thing worth checking is spacing: as a working rule joints belong at 24 to 36 times the slab thickness and no more than 15 feet apart. On a four-inch residential slab that puts them comfortably inside that maximum. Where somebody has poured a wide expanse and cut too few joints, the slab relieves itself wherever it likes, and that is where the straight line across the middle of a bay comes from.
Two: the ground moved, and the slab went with it
This is the one that matters. A driveway is not a structure standing on footings; it is a slab sitting on whatever was compacted beneath it. When that material settles, washes out or swells, the concrete above has no choice but to follow.
The tell is height. Put a straight edge across the crack. If the two sides are still level with one another, the ground below is holding and you are almost certainly looking at mechanism one. If one side sits lower than the other, or the two edges rock when you stand on them, the ground moved, and no amount of attention to the concrete will fix a problem that is not in the concrete.
Around here the usual culprits are drainage and compaction. Water that runs under a slab rather than off it takes fine material with it and leaves a void, and a base that was placed in one thick lift rather than compacted in layers never reaches the density it needed. Both take several seasons to show, which is why this failure tends to surface after the warranty conversation rather than before it.
Three: something too heavy stood on it
Residential driveways are built for cars. A four-inch slab over a properly prepared base carries domestic traffic for decades without complaint, and the same slab can be broken by one afternoon of something it was never specified for. Skip lorries, concrete trucks delivering to a neighbour, a caravan left on one jack, a fully loaded builder's tipper turning on the apron.
The signature is a crack that radiates or maps from one point rather than running edge to edge, and it is usually accompanied by somebody remembering exactly when it appeared. There is no repair for this other than replacing what broke, but it is worth identifying because it says the rest of the driveway is fine.
Four: the surface came apart rather than the slab
Flaking, pitting, exposed stone underfoot, a texture that has gone rough. Strictly this is not cracking at all, but people describe it the same way on the phone, so it belongs here.
The slab underneath is usually sound. What has failed is the top few millimetres, and the common causes are a surface closed too early during finishing, water worked into the top during floating, or deicing salt applied while the concrete was still young. That last one is preventable and worth knowing about before the first winter rather than after it.
What to do with the diagnosis
Only two of the four actually need work. A shrinkage crack along a joint needs nothing, though sealing it keeps water and salt out of the opening and that is cheap insurance. Surface failure on a sound slab is a resurfacing conversation, provided there is enough good concrete beneath for a new layer to grip.
Movement is the one that decides the money. One bay that has dropped is a bay that comes out and goes back, with the base put right underneath it first. Several bays that have all done the same thing are telling you the base is the problem across the whole lot, and at that point replacing panels one at a time simply buys you the same conversation next year. That threshold, and what each route actually costs, is the subject of repair or replace a concrete driveway.
If the slab is sound and only sitting low, the answer is lifting rather than removing, which is a different trade and a fraction of the money. We do not do it and we still say so, because the alternative is selling somebody a driveway they did not need.
What actually reduces the odds
Nobody can promise a crack-free driveway and you should be wary of anyone who does. What a good build does is make the cracks land where they were meant to and keep the ground underneath still:
- A base compacted in layers, not tipped in and levelled, so it reaches density all the way down.
- Joints cut on time and to depth, at sensible spacing rather than wherever the day ran out.
- Water sent somewhere deliberate, because drainage is what protects the base you just paid for.
- A mix suited to the exposure, since a driveway in this climate takes freeze-thaw and salt that an interior slab never sees.
- No deicers at all through the first winter. Not calcium chloride, not magnesium chloride. Both appear on the list of products never to use on concrete, and the magnesium version tested worst for strength loss.
If your driveway is already past that point, concrete repair is where the work lives, and the first visit is spent establishing which of the four you have before anybody discusses a number.