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Concrete · From the field
Why concrete cracks and spalls in Maryland — and which cracks matter
All concrete cracks. The useful question is whether yours cracked where it was supposed to, and whether the surface coming away is cosmetic or structural. Here is how to read a slab before anybody quotes you to replace it.
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Quick answer
Why is my concrete driveway cracking?
Concrete shrinks as it cures and keeps moving with temperature for its whole life, so every slab develops cracks — the question is where. Control joints are tooled or cut in to decide that in advance, and a slab that cracks along its joints is behaving correctly. Cracks that wander across panels, step vertically, or open wide enough to put a coin in are telling you something else: usually that the subgrade under the slab has moved, or that the joints were too far apart or never cut at all.
Concrete cracks. Control joints decide where
Concrete loses volume as it cures and continues to expand and contract with temperature for as long as it exists. Because it is strong in compression and weak in tension, that movement has to be released somewhere, and if nobody decides where, the slab decides for itself — randomly, diagonally, and across the middle of the panel where you will be looking at it. Control joints are the decision being made deliberately: a deliberate line of weakness, tooled in while the concrete is plastic or saw-cut shortly after, where the inevitable crack is invited to happen out of sight at the bottom of a groove.
Joints have to be deep enough and close enough together to work. Too shallow and the slab cracks somewhere else anyway; too far apart and a panel large enough to need two cracks will produce one of its own between them. Reading a driveway therefore starts with the joints: a slab with clean joints at sensible spacing and no other cracking is a slab doing exactly what it was designed to do, however many lines are in it. People are often relieved to hear that the grid of cracks they have been worrying about is the grid somebody put there.
The flip side is the slab with no joints at all, or joints at ten or twelve feet on a surface that needed them closer. Those produce the wandering map cracks that homeowners find alarming, and they are not a material defect — the concrete did what concrete does. They are also usually not a structural emergency. A slab that has cracked into pieces but whose pieces are still at the same level is intact in the way that matters; it is carrying load, it is just not pretty.
Spalling is a surface problem with a chemical cause
Spalling is different from cracking and gets confused with it constantly. It is the surface of the concrete flaking, pitting or sheeting away in thin layers, exposing the aggregate underneath, and it typically shows up in the areas where cars park and snow gets shovelled. The cause in this part of Maryland is overwhelmingly de-icing salt. Chloride-laden meltwater soaks into the top few millimetres of the slab, then freezes, and the expansion happens inside the surface paste rather than on top of it. Repeat that through a winter and the paste gives up.
Resistance to this is designed in at the point of placement, not afterwards. Air-entrained concrete contains deliberate microscopic voids that give freezing water somewhere to expand into, which is why exterior flatwork in a freeze-thaw climate should be specified with it and interior slabs usually are not. Finishing matters as much: overworking the surface or working water into it brings fines and water to the top and leaves a weak skin that will spall whatever the mix was. A good exterior slab is a correct mix, placed without extra water, finished without being polished to death, cured rather than just left, and sealed before its first winter.
Where a slab is already spalling, the honest answer depends on depth. A shallow surface loss on a sound slab can be ground and resurfaced with an overlay that bonds to what is left, and that is a genuine repair rather than a cover-up. Where the spalling has gone deeper, where it has reached reinforcement, or where it is widespread rather than in the salt zones, an overlay goes over a surface that is still coming apart underneath it and will lift. That is the point where we say replace, and we would rather say it than take money for an overlay with a season in it.
Settlement cracks are the ones to take seriously
The crack that actually indicates a problem is the one with a vertical step in it. Run your hand across a crack: if both sides are at the same height, the slab has cracked but not moved, which is a shrinkage or thermal crack and generally a cosmetic matter. If one side sits lower than the other, the ground under that side has gone somewhere, and the slab is now a hinge rather than a plane. That is a settlement crack, and unlike the others it gets worse on a schedule.
Where the ground went is usually findable. The most common culprit on newer houses is a utility trench — water, sewer, gas or electric — backfilled quickly without being compacted in lifts, which then consolidates for years afterwards under the slab that was poured over it. Next most common is a downspout or a sump discharge putting roof or basement water into the subgrade beside the slab and washing fines out from under it. Then there is organic material left in the fill: a buried stump, roots, topsoil that was never stripped, all of which lose volume as they break down.
This matters because the repair follows the cause and not the appearance. Filling a stepped crack with sealant addresses nothing; the ground will keep moving and the filler will tear. Where the void is confined and the slab is otherwise sound, lifting and supporting it from underneath can be the right answer. Where the subgrade is broadly wrong, or the slab has broken into several pieces at different heights, the slab comes out, the subgrade gets corrected and compacted, and new concrete goes down. The concrete page carries the ranges we price removal and replacement to.
What the Maryland winter actually does to flatwork
The climate here is harder on concrete than a colder, drier one would be, for the same reason it is hard on pavers: it is the number of freeze-thaw crossings, not the depth of the cold. A slab in this county passes through freezing repeatedly over a winter, and each crossing is a cycle of the water inside the surface expanding and relaxing. Add the chloride from de-icing and the water stays liquid to a lower temperature, penetrates further and freezes deeper into the paste, which is precisely why the apron where the snow shovel ends up fails first on an otherwise sound driveway.
Drainage decides how much of that water is available to do damage. A slab pitched to shed water dries between events. A slab that holds a puddle, or that sits lower than the ground beside it, keeps the surface wet going into every freeze, and the places where puddles sit are reliably the places where spalling appears first. The same applies underneath — a subgrade that stays saturated has less bearing capacity and heaves more, so poor drainage drives both failure modes at once.
The practical consequences are small and worth doing. Sealing exterior flatwork before its first winter and periodically afterwards slows chloride ingress considerably. Using sand or a calcium-magnesium acetate product rather than rock salt on a new slab, especially in its first winter, avoids the worst of the chemistry while the concrete is still gaining strength. Keeping gutters and downspouts discharging past the slab rather than onto it removes most of the water that was going to be the problem. None of it is glamorous and all of it buys years.
Repair, resurface or replace
Repair is right where the slab is structurally fine and the damage is local. Hairline and moderate cracks with no vertical displacement can be cleaned out and filled, which keeps water and salt out of the crack and stops it ratcheting wider through freeze-thaw. A joint that has lost its sealant gets resealed. A corner that has broken off a sound slab can be rebuilt. These are maintenance jobs, they are cheap, and they are worth doing early rather than at the point where the crack has become a step.
Resurfacing is right where the surface has failed and the slab has not. A bonded overlay or a polymer-modified resurfacer takes a spalled but structurally intact driveway back to a continuous surface, and it can be broomed, textured or stamped. The conditions are real, though: the existing slab has to be sound, the surface has to be properly prepared rather than just washed, the overlay has to be thick enough for what will drive on it, and the existing joints have to be carried through the new surface or they will telegraph a crack up through it within a year.
Replacement is right when the subgrade is the problem, when pieces of slab sit at different heights, when spalling is deep or widespread, or when the slab is simply at the end of a long life. We try to be straight about this in both directions. There is no sense replacing a driveway that has a decade left in it because it has cracks along its joints, and there is equally no sense overlaying a slab that is in several pieces so that it looks new for one winter. The site visit is mostly about working out which of the three you are actually in.
If you only skim
The short version
- Every slab cracks. Control joints decide where, and cracking along the joints is correct behaviour.
- A crack with both sides at the same height is cosmetic. A crack with a vertical step means the ground moved.
- Spalling is salt plus freeze-thaw attacking the surface paste — air entrainment, careful finishing, curing and sealing are the defences.
- Settlement usually traces to an uncompacted utility trench, water being discharged beside the slab, or organic material in the fill.
- Seal exterior flatwork before its first winter, and keep rock salt off new concrete.
- Repair local damage, resurface a failed surface on a sound slab, replace when the subgrade is the problem.
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Straight answers
What homeowners ask next
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Run your hand across it. If both sides are level with each other, it is a shrinkage or thermal crack — worth cleaning out and sealing so water and salt stay out of it, but not a structural matter however long it is. If one side sits lower than the other, the subgrade under that side has moved and the crack will keep opening. That one is worth having looked at, because the repair depends on finding where the ground went rather than on the crack itself.
If the slab underneath is sound, yes, and a bonded overlay is a real repair rather than a cosmetic one. The conditions matter: the existing concrete has to be structurally intact, the surface has to be mechanically prepared rather than pressure-washed, and the existing control joints have to be carried up through the new surface. Where spalling is deep, has reached reinforcement, or covers the whole slab rather than the salt zones, an overlay will lift and we will tell you so instead of selling it.
Most often the mix, the finishing or the first winter. Exterior flatwork in a freeze-thaw climate should be air-entrained, and a slab that was not will struggle. Overworking the surface, or adding water at the truck to make it easier to place, leaves a weak skin that spalls whatever the mix said. And rock salt on concrete in its first winter, before it has gained full strength, does a disproportionate amount of damage. Two driveways poured the same week can age very differently on those three variables alone.
Only in specific circumstances, and a thin topping over old concrete is not one of them. A bonded overlay on a prepared, structurally sound slab is legitimate. Adding a new slab on top of a cracked or settled one is not, because the new concrete inherits every joint and movement in the old one and cracks over them, usually in the first year. If the slab underneath has failed, it comes out. We would rather quote the removal than warranty a pour that is going to crack.
Across Frederick County and Maryland within about 90 minutes of Frederick — eight counties from Hagerstown across to Annapolis. Maryland only; we do not work in Washington, DC. Call (301) 524-1729 or send photos of the slab through the quote form and we will tell you which of repair, resurfacing or replacement we think you are looking at.
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