Spalling, Popouts and Pitting in a Concrete Floor: Causes, and Whether to Coat

Why it matters before a coating

A coating follows the slab. Over a damaged floor it traces every hole, which is why a worn, pitted or spalled floor is the case for a troweled epoxy mortar rather than a film. But the bigger risk is underneath the holes you can see. A coating bonds to the top of the concrete, so if that top layer is loose, or something below it is still pushing concrete off, the coating fails with it and the failure looks like the coating’s fault.

So the question for each kind of damage is not only how to fill it. It is whether the cause has stopped.

Start by sounding the floor

Drag a heavy chain across the slab or tap it with a hammer and listen. PCA puts it simply: a hollow sound indicates delaminated areas, and a ringing sound indicates intact concrete. NRMCA and PCA both point to ASTM D4580 for the practice. Mark every hollow patch before anyone grinds, because preparing the slab starts with cutting out what is unsound, and a hollow area that has not broken yet is invisible from above.

Popouts: a cone with a broken stone at the bottom

NRMCA describes a popout as a small, generally cone-shaped cavity left after an aggregate particle near the surface fractures from pressure inside it. Part of the broken particle is frequently still at the bottom. PCA gives the usual size as roughly 1/4 inch to 2 inches across. The cause is the stone itself: a porous, lightweight particle soaks up water and swells, or freezes, and breaks the surface over it. PCA names pyrite, coal, shale, soft limestone and chert among the usual culprits, and NRMCA says low density chert is the most common.

A second kind comes from chemistry. In alkali silica reaction (ASR), a gel forms around a reactive aggregate, absorbs water and expands. NRMCA says these popouts are typically small, often come with a small discolored or damp-looking spot, and the stone often does not split. Timing helps tell them apart: NRMCA says freeze driven popouts typically appear in the first year after placement, while some ASR popouts appear within a few days.

One caution is specific to coatings. In its list of ways to avoid popouts in new slabs, PCA advises against impervious floor coverings or membranes because they can aggravate popout development. That is written for young concrete with popout-prone aggregate, but a coating is exactly that kind of covering, so a slab still throwing popouts is not ready for one.

Popoffs and scaling: the surface mortar, not the stone

Mortar flaking, which PCA says is sometimes called popoffs, looks like popouts from a distance. The difference is at the bottom of the hole: PCA says flaking usually leaves no freshly fractured aggregate and few if any conical voids. The thin mortar over the top of a stone has come away and the stone is intact. NRMCA puts popoffs down to improper finishing or a lack of curing.

Scaling is the same loss spread over an area. PCA defines it as the general loss of surface mortar exposed to freezing and thawing, and NRMCA says it starts as small patches that later merge. NRMCA adds that concrete also peels without any freezing, often from troweling early, overworking the surface or finishing while bleed water is on it.

Scaling raises a question about the coating, not just the repair. PCA advises that impermeable materials, such as most epoxies, should not be used on slabs on ground or other concrete where moisture can freeze under the coating, because the freezing water can delaminate it, and recommends a breathable surface treatment instead. A scaled slab is evidence of those conditions. Where the damage is indoors and the cause was the finish, PCA says grinding or a thin bonded overlay can usually remedy a scaled surface if sound concrete is below it.

Delamination: a thin layer that has let go

NRMCA describes delaminations as separations of thin layers of the slab surface from the concrete below. PCA puts the layer at about 1/8 to 1/4 inch and gives the main cause as finishing the surface before bleeding has finished, which seals air and bleed water under a dense skin. NRMCA notes that a vapor retarder or a nonabsorbent base under the slab increases bleed water, and with it the risk.

This is the defect most likely to be blamed on a coating later. PCA says delaminations become apparent once the surface has dried and the area is crushed under traffic, and a broken out delamination leaves a hole that resembles a spall. Until then it looks like the rest of the floor, and only sounding finds it. A coating bonded to that layer is held on by nothing better than the layer. NRMCA says delaminated areas are repaired by removing the surface layer and patching, or, where it is extensive, by grinding and overlaying a new surface.

Spalls: something is pushing the concrete off

PCA calls spalling a deeper defect than scaling, often round or oval depressions or long cavities along joints, caused by pressure or expansion within the concrete, impact, fire, weathering or a poor bond between two layers. It names improperly constructed joints and corroded reinforcing steel as the two common causes. Which one you have decides the job.

• Along a joint. PCA says a spall there is usually the result of the joint functioning improperly, and that repairing it means restoring the joint to proper working condition, not filling the hole flush across it. Under forklifts and pallet jacks, an unsupported joint edge breaks down wheel by wheel, which is what a semi-rigid joint filler is there to prevent; the warehouse flooring page covers that traffic.
• Over steel. Rust takes up two to four times the volume of the steel it came from, NRMCA says, and that pressure pushes the concrete cover off. Rust staining in the hole, or reinforcing steel showing, means the cause is active and will keep going under a patch. PCA treats uncovered steel as past a surface repair.
• A broken out delamination. PCA notes that one leaves a hole resembling a spall. Sound the concrete around the edge: if it rings hollow, the layer is still letting go, and it is a delamination.

And pitting?

The word gets used for any small holes, but it is a description, not a diagnosis. PCA’s guide to slab surface defects lists twelve, and pitting is not one of them. Small pits in a floor are usually one of the defects above, and the bottom of the hole says which: broken stone means a popout, intact stone with the mortar gone means a popoff, and a thin layer gone with hollow-sounding concrete around it means a delamination breaking out.

The decision: patch, resurface, or find the cause first

1. Patch and coat when the damage is local, the cause has stopped and the concrete around it sounds solid: a handful of popouts, a few popoffs, a small delamination cut back to sound concrete. The patch goes in during prep and gets profiled with the rest of the floor.
2. Resurface when the damage is everywhere: popouts too many to patch, widespread scaling or delamination. A film would trace it all, so the floor needs a new wearing course first, from a bonded overlay or self-leveling underlayment to an epoxy mortar.
3. Find the cause before anything when it is still active: rust staining or exposed steel, damp or discolored popouts, new popouts or spalls appearing, or damage that is spreading. NRMCA’s advice for popouts is to assess the cause before planning repairs, with core samples evaluated by a petrographer. Steel corrosion and a slab that keeps deteriorating are beyond a coating crew’s call.
4. Do not use an impermeable coating where moisture can freeze under it, PCA’s warning for slabs on ground and other concrete exposed to freezing. That is the one case where the answer to “can I coat this” is no, whatever the repair.

Cracks follow the same logic with different evidence, and what caused that crack reads them the same way. If you are deciding what to put down over a damaged slab, the epoxy system builder takes a worn, pitted or spalled slab as one of its answers.