What Is Concrete Spalling on a Stem Wall?

If the face of your Phoenix stem wall is flaking, chipping, or crumbling away in sheets, you’re looking at concrete spalling — and in Arizona it almost never means what the internet tells you. So what is concrete spalling, really? It’s the surface of the concrete breaking off, and here in the desert the cause underneath is usually rusting rebar, not the freeze-thaw damage that northern pages blame. Stem wall spalling repair is one of our core services, so this guide walks through what it is, why it happens locally, and what an honest fix looks like.

Spalling in plain English

Spalling is when hardened concrete loses pieces of its surface. It shows up as flaking concrete, thin chips popping off, a pockmarked or crumbling face, or larger chunks separating from the wall. On a home you’ll most often see it along the stem wall — the short concrete wall that rises from the footing and carries the base of your framing, the exposed band of concrete around the perimeter of the house at grade.

Early on, spalling looks cosmetic: a rough patch, sandy grit at the base of the wall, a shallow divot where a flake let go. That’s why so many homeowners ignore it. But spalling isn’t a paint problem — it’s the outer layer of structural concrete coming apart, and the reason is almost always hidden an inch or two below the surface. The visible flaking is a symptom, and treating it alone gets you nowhere.

What causes spalling in Arizona (it’s usually not freeze-thaw)

Search “concrete spalling causes” and most results lead with freeze-thaw: water seeps into the concrete, freezes, expands, and blows the surface apart over repeated winters. That’s a real mechanism — in Minnesota. In the Phoenix metro, where hard freezes are rare and shallow, freeze-thaw is almost never the driver, and blaming frost sends people toward the wrong repair.

In Arizona, the real story is corrosion. The steel reinforcing bar (rebar) cast inside the concrete rusts, and as it rusts it expands and pushes the surrounding concrete off from the inside — the mechanism behind the large majority of stem wall spalling across the Valley. For the deeper chemistry, see why rebar rusts and spalls in Arizona; the short version follows below.

Rusting rebar pushing the surface off

Concrete is naturally alkaline, and that high-pH environment normally forms a thin passive film on the embedded steel that keeps it from rusting. When that protection breaks down, the steel oxidizes. Here’s the part that does the damage: rust occupies several times more volume than the original steel. As the rebar corrodes it swells, generating enormous outward pressure on the concrete cover above it. The concrete can’t stretch, so it cracks, delaminates, and finally spalls off the face. The flaking you see outside is the steel underneath running out of room.

Spalling is a surface symptom of a steel problem underneath. The flake on the outside is the rebar telling you it’s rusting on the inside.

This is why a spalled stem wall so often reveals a rust-stained cavity, and in advanced cases the rebar itself — brown, scaled, and thinned — exposed to the air. Once you can see the steel, the corrosion has been working for a while.

Moisture, chlorides, and thin concrete cover

Corrosion needs moisture plus something to break down the steel’s passive protection. In Phoenix, both show up together:

• Moisture sources. Landscape irrigation spraying or pooling against the stem wall, monsoon downpours, roof runoff with no gutters, and poor drainage that lets water sit at the base. Desert soil doesn’t mean dry concrete — repeated wetting at grade is the single biggest accelerant.
• Chloride ions. Chlorides from soil, hard water, and softening salts migrate into the concrete and attack the steel’s passive film. Chloride-driven corrosion is the dominant spalling mechanism in our climate — not frost.
• Sulfates and carbonation. Sulfates in Arizona soils attack the cement paste, and carbonation — atmospheric CO₂ neutralizing the concrete’s alkalinity — lowers the pH until the steel loses its natural protection. Both make corrosion easier to start.
• Thin concrete cover. Concrete cover is the depth of concrete between the rebar and the outside surface — the steel’s first line of defense. When the original pour left the rebar too shallow, moisture and chlorides reach it faster and the wall spalls years sooner. Thin cover is one of the most common reasons two nearby homes spall on completely different timelines.

Put those together and you have the Arizona recipe: chlorides and moisture reach shallow-cover steel, the passive film fails, the rebar rusts and swells, and the face flakes off — no frost required.

Spalling vs. efflorescence vs. a crack — telling them apart

Three things get confused on stem walls, and knowing which one you have changes what you do next.

Spalling is a loss of material. You can feel the divot, sweep up the flakes, and often see rust-stained concrete where the surface used to be. It has depth.

Efflorescence is the opposite — a deposit, not a loss. It shows up as a white, chalky, powdery film. When water moves through concrete and evaporates at the face, it leaves dissolved mineral salts behind as that white bloom. The spalling vs efflorescence distinction matters: efflorescence doesn’t push the surface off, and you can usually brush it away. But it’s a real tell — it means moisture is regularly moving through that wall, exactly the condition that feeds corrosion and eventually spalling. It isn’t the damage; it’s the smoke alarm for the moisture that causes it.

A crack is a line, not a flaking area or a powder. Hairline cracks are common; wider or offset cracks can point to movement and are a different conversation. A crack can also become an entry point for the water that later drives spalling. See our overview of stem wall repair for how these problems are triaged together.

Why spalling gets worse if you ignore it

Spalling is progressive because the mechanism feeds itself. Once the surface flakes off, the remaining concrete cover is even thinner, so moisture and chlorides reach the rebar faster, the steel rusts faster, and it pushes off the next layer sooner. Each flake exposes more, and the pace picks up.

Left alone, a shallow spall becomes exposed rebar, the advanced stage. Now the steel corrodes in open air with no protection at all, losing the cross-section that carries load. A stem wall transfers the weight of the house to the footing and holds your anchor bolts and framing in place — enough section loss and that job is compromised. The same corrosive environment attacks embedded hardware too, which is why anchor bolts corrode in Arizona stem walls right alongside the rebar. None of this is cause for panic on day one — spalling is slow — but it’s a reason not to keep painting over it and hoping.

How spalling is repaired (remove, treat, rebuild, waterproof)

Here’s the honest part. Because spalling is driven by the steel underneath, patching the face alone just delays the next flake — the rust keeps swelling behind the patch and pushes it back off. A proper repair follows the sequence recognized concrete-repair standards are built around:

• Remove. Cut back all the spalled, cracked, and chloride-contaminated concrete until you reach sound material and fully expose the corroded steel. Half-measures here guarantee a repeat.
• Treat or replace the steel. Clean the rebar back to bright metal, or where it’s badly section-lost, replace it. This is the step face-patching skips — and it’s the step that actually stops the corrosion.
• Rebuild with non-corroding reinforcement. This is where GFRP composite rebar (glass-fiber-reinforced polymer) earns its place. GFRP doesn’t rust — it’s immune to the chloride-driven corrosion that started the problem — so rebuilding with it breaks the cycle instead of restarting the clock on ordinary steel. Repair mortar restores proper concrete cover.
• Waterproof. Seal the finished wall and, just as important, fix the moisture that fed the corrosion — redirect irrigation, add drainage, keep water off the wall. Repairing the concrete without fixing the water is a temporary win.

For a step-by-step look, see how to fix spalling concrete stem walls. Spalling repair is commonly priced around $28 to $48 per linear foot as a general range; what your wall needs depends on how far the corrosion has run, so a written estimate after someone looks at the wall is the only honest number.

If your stem wall is flaking, the sensible next step is to book an inspection before the spalling reaches the steel.

Frequently Asked Questions

What is concrete spalling?

Concrete spalling is when the surface of hardened concrete flakes, chips, or crumbles off, leaving a rough or pitted face and sometimes exposing the rebar underneath. On a home it usually appears on the stem wall around the base of the house — a structural surface breaking down, not a paint or finish issue.

What causes concrete to spall?

In Arizona, the main cause is rebar corrosion: the steel inside the concrete rusts, expands, and pushes the surface off from the inside. Moisture from irrigation, monsoon, and poor drainage, plus chloride ions, sulfates, carbonation, and thin concrete cover all speed it up. Freeze-thaw — the usual culprit in cold climates — is rarely the cause here.

Is spalling concrete serious?

It can become serious if ignored. Spalling is progressive: each flake thins the concrete cover, so the rebar rusts faster and loses load-carrying cross-section over time. It’s slow, not an emergency on day one, but it doesn’t heal on its own and gets more expensive the longer it runs.

Can spalling concrete be repaired?

Yes. A lasting repair removes the damaged and contaminated concrete, cleans or replaces the corroded steel, rebuilds with non-corroding GFRP composite rebar and repair mortar, then waterproofs the wall and fixes the moisture source. Patching only the surface delays the next flake because the rusting steel underneath keeps pushing.

What is the difference between spalling and efflorescence?

Spalling is a loss of material — concrete flaking or chipping off with real depth. Efflorescence is a deposit — a white, powdery mineral film left on the surface as water evaporates, which you can usually brush away. Efflorescence doesn’t damage the wall itself, but it’s a reliable sign of the moisture that drives corrosion and later spalling.

How much does it cost to repair spalling concrete?

Spalling repair is commonly priced around $28 to $48 per linear foot as a general range. The real figure depends on how deep the corrosion has gone and how much steel needs replacing. The accurate way to price it is a written estimate after we inspect the stem wall.