Notes

Why Little Craters Appear in Concrete

Small cone-shaped holes, often with a broken stone sitting in the bottom. What causes popouts, which ones are only cosmetic, and which ones are worth a closer look.

Updated October 2026 · Oak Creek Concrete

They usually appear after the first real winter. A new drive or walk that looked perfect in the autumn comes through to spring with a scattering of small, neat holes in it, each one shaped like a shallow cone, and in the bottom of many of them a piece of stone that looks as if it has been snapped in half.

That is exactly what has happened. These are popouts, and they are one of the few concrete defects where the cause is sitting in plain sight at the bottom of the hole. Understanding what went on is the quickest way to decide whether you have a cosmetic nuisance or something worth raising with whoever poured your concrete driveway.

What a popout is

The National Ready Mixed Concrete Association defines a popout as a small, generally cone-shaped cavity in a horizontal surface, left behind after a piece of aggregate close to the top has fractured under pressure from inside. Part of the broken particle is frequently still attached at the base of the cone. The cavities it describes range from about a quarter of an inch to a few inches in diameter.

Concrete is mostly stone by volume, held together by hardened paste. A particle that happens to sit a fraction of an inch below the finished surface has only a thin skin of mortar over it. If that particle swells or splits, the skin goes with it.

The two ways it happens

The industry sheet separates popouts into two families, and they look slightly different once you know what to look for.

Physical popouts are the common kind. A few particles in some natural gravel deposits are porous and light for their size, and they soak up water like a sponge. When the slab is wet and the temperature drops, the water inside those particles freezes and expands, the particle fractures, and it lifts the mortar above it clean off. The sheet names low-density chert as the most frequent culprit, and it notes that this kind is typically seen on outdoor flatwork in freeze-thaw climates, usually within the first year of placement.

Chemical popouts come from a reaction between alkalis in the concrete and certain silica-bearing stones, known as alkali-silica reaction. It produces a gel that absorbs water and swells. These cavities tend to be small and are often marked by a spot that looks discoloured or permanently damp. The stone at the bottom is less likely to be neatly split. Some of them can appear within days of the pour rather than waiting for winter.

A close cousin: the popoff

There is a third thing that gets mistaken for a popout. Sometimes the mortar over a stone flakes away and leaves the stone itself intact and exposed. The same NRMCA sheet calls that a popoff and treats it differently, because it points at the installation rather than the gravel: poor finishing, a lack of curing, water trapped above the particles, or a weak bond between mortar and stone. It also notes that deicing salts can make this kind of flaking worse.

So when you look into the hole, the stone tells you something. Broken stone at the bottom suggests the particle burst. A whole, undamaged stone with the mortar gone from around it suggests the surface was never as strong as it should have been, which is a fair conversation to have with the installer.

Why good concrete still gets them

This is the part that frustrates people. Specifications for concrete aggregate set maximum limits on troublesome particles, but they do not demand zero, and the sheet is candid that it is generally neither easy nor cheap to remove every one of them from a natural deposit.

Its list of preventive steps is sensible: a lower water to cement ratio so the concrete is less likely to become saturated, no water added to the surface during finishing, no hard steel trowel on outdoor slabs, proper curing, sealing the surface, and smaller maximum stone size. Then comes the honest line. Those measures can reduce how often popouts happen. They will not necessarily eliminate them.

Whether the gravel in any particular local mix carries these particles is a question for the ready-mix supplier, and we are not going to guess at the geology of southeast Wisconsin quarries without a source. What we can say is that where problem particles are in the only affordable aggregate, the sheet notes that some level of popouts on walks and paving is generally accepted.

When to worry and when not to

What you see	What it usually means
A handful of clean cones, split stone at the base	Physical popouts; cosmetic
Small pits with a damp or stained ring	Possibly chemical; worth an opinion
Stones left whole, mortar gone around them	Popoffs; a surface quality question
Popouts across much of the slab	Water can now get in more readily

The industry view is that most popouts are aesthetic and do not affect the structural performance or service life of the slab. A few scattered cones in a driveway are, in practical terms, a blemish.

Two situations deserve more attention. A surface peppered with them has lost a good deal of its protective skin, and the sheet warns that this makes it easier for water and harmful chemicals to get in, which can lead on to other deterioration. And chemical popouts may be an early sign of a durability problem that shows up later in the life of the slab.

What repair looks like

Before anybody fills holes, the guidance suggests working out the cause, and on a serious case that means taking core samples and having a petrographer examine them. That is a laboratory step, not a driveway-side opinion, and it is only warranted where a lot is riding on the answer.

For ordinary cases the method is straightforward. Chip out what is left of the particle, clean the cavity, and fill it with a proprietary repair mortar such as a dry-pack or epoxy mortar, following the maker's own directions. The catch is appearance: the sheet notes it will be difficult to match the colour of the existing concrete, so patched popouts tend to stay visible as small spots.

Where there are simply too many to patch one at a time, a thin bonded overlay can restore a uniform surface. That is a resurfacing job with its own conditions, set out in what resurfacing costs.

If you are about to pour

You cannot choose the stone in a mix yourself, but you can ask. Ask the contractor whether popouts have been a problem with the supplier they use, whether the outdoor surface will be broomed rather than hard troweled, and how the slab will be cured. Those answers tell you whether somebody has thought about it.

And if you already have a slab with a few new craters in it after its first winter, it is almost certainly still a sound slab. Anyone recommending a full tear-out on the strength of scattered popouts alone should be asked to explain why. For a second look at your own drive, start with the free driveway estimate and mention what you found in the holes.