Spalling is the breaking away of a material’s surface in flakes, chips, or larger fragments. It most commonly affects concrete, brick, and natural stone, but it also occurs on metal components like bearings and gears. The underlying cause is always the same: internal pressure or stress builds up until it exceeds the material’s strength, and pieces of the surface break off.
If you’ve noticed chunks missing from a sidewalk, a pitted concrete wall, or a flaking brick facade, you’re likely looking at spalling. Understanding what drives it helps you figure out whether it’s cosmetic or a sign of deeper structural trouble.
How Spalling Happens
Spalling starts below the surface. When forces inside a material push outward with enough pressure, the outer layer cracks and separates. In concrete and masonry, the most common trigger is water that has soaked into the material’s tiny pores. When that water freezes, it expands by about 9% in volume. If the pressure from that expansion exceeds the material’s tensile strength, cracks form. With repeated freeze-thaw cycles, those cracks widen, and surface fragments break loose.
The depth of water penetration determines how deep the damage goes. During freezing, the outer layer fractures to whatever depth the water reached and falls away. Surfaces that stay wet for long periods, whether from pooled water, snow accumulation, or poor drainage, are especially vulnerable because the moisture has time to soak in deeply before freezing.
Fire and extreme heat cause a different but related process. When concrete is exposed to high temperatures, moisture trapped in its pores turns to steam. That steam builds pressure inside the material’s tiny internal channels. At the same time, the heated surface expands while the cooler interior does not, creating thermal stress. When either the pore pressure or the thermal stress exceeds the concrete’s strength, pieces blow off the surface, sometimes explosively. This is why concrete structures exposed to fire often show dramatic surface loss.
Spalling vs. Scaling
People often confuse spalling with scaling because both involve surface damage on concrete. The difference is depth. Scaling happens right at the surface: thin flakes of the cement paste peel away, exposing the small stones (aggregate) embedded in the concrete. It looks like the top skin has been scraped off.
Spalling comes from deeper within the material. Instead of thin flakes, you see larger chunks or fragments breaking loose, often revealing the steel reinforcement bars underneath. If you can see rebar or the damaged area is more than an inch or two deep, you’re dealing with spalling rather than scaling.
What Causes Spalling in Buildings
Freeze-thaw cycles are the most visible culprit, but several other forces cause spalling in concrete structures.
- Corroding reinforcement. Steel rebar inside concrete rusts when moisture and oxygen reach it. Rust takes up more space than the original steel, so it pushes outward against the surrounding concrete. Over time, this internal expansion cracks the concrete cover and forces it off in chunks. This is the most common cause of spalling on bridges, parking garages, and older buildings.
- Insufficient concrete cover. The layer of concrete between the rebar and the outside surface acts as a protective barrier. U.S. building codes require at least 3 inches of cover for concrete poured directly against ground. When that cover is too thin, whether from a construction error or wear, moisture reaches the steel faster and corrosion-driven spalling begins sooner.
- Poor-quality concrete. Concrete mixed with too much water, not enough air-entraining additive, or improperly cured is weaker and more porous. That extra porosity lets water penetrate more easily, accelerating every type of spalling.
- Chemical exposure. Deicing salts, acidic runoff, and industrial chemicals dissolve the cement paste that holds concrete together and speed up rebar corrosion. Salt-heavy environments like coastal areas and northern roadways see some of the worst spalling damage.
Spalling in Metal and Mechanical Parts
Spalling isn’t limited to concrete. In mechanical engineering, it refers to small pits or craters that form on the surfaces of bearings, gears, and other metal components subjected to repeated rolling or sliding contact. Every time a bearing rolls under load, the surface absorbs stress. Over thousands or millions of cycles, tiny cracks develop just below the contact surface. Eventually, small pieces of metal flake away, leaving rough, pitted areas.
Several conditions accelerate this process. Poor lubrication allows metal-on-metal contact that concentrates stress. Abrasive particles or contamination in the lubricant grind into surfaces and create weak points. Corrosion, uneven loading, and vibration all contribute. Once spalling begins on a bearing raceway, it tends to spread quickly because the rough, pitted surface creates even higher stress concentrations on the remaining material. A spalled bearing typically produces increased noise, vibration, and heat, all signs that the component needs replacement.
When Spalling Becomes a Structural Risk
Minor surface spalling on a driveway or patio is a cosmetic issue. Deeper or more widespread spalling on a load-bearing structure is a different matter entirely. The Association of State Dam Safety Officials notes that large areas of crumbling concrete, deterioration deeper than 3 to 4 inches (depending on wall or slab thickness), and exposed rebar all indicate serious concrete degradation. Left unrepaired, this level of damage can compromise a structure’s stability.
The critical concern with advanced spalling is the loss of protective cover over reinforcement steel. Once rebar is exposed to air and moisture, corrosion accelerates dramatically. The rebar weakens, loses cross-sectional area, and can no longer carry its designed load. In a beam or column, this creates the potential for sudden failure. That’s why engineers tracking spalling on bridges, dams, and buildings keep written records of the size, depth, and spread of damage over time. Growth in any of those dimensions signals that the deterioration is active and progressing.
How to Prevent Spalling
Prevention starts with keeping water out. Applying a penetrating sealer to concrete surfaces fills the pores near the surface and dramatically reduces how much moisture can soak in. Silane-based sealers are a common choice because they penetrate into the concrete rather than forming a film on top, so they hold up well under foot and vehicle traffic. For structures already showing early signs of corrosion, surface-applied corrosion inhibitors can soak into hardened concrete and slow the rusting of embedded steel.
Good drainage matters just as much as sealants. Water that pools on a concrete slab has hours to soak in before it freezes. Sloping surfaces to shed water, maintaining functioning gutters and downspouts, and fixing low spots where puddles form are all straightforward ways to reduce freeze-thaw exposure.
For new construction, the most effective prevention is built in from the start. Using air-entrained concrete creates millions of tiny air bubbles inside the mix that give expanding ice room to push into, relieving pressure before it cracks the surrounding material. Ensuring adequate cover over rebar, following code requirements for the specific exposure environment, and limiting the water-to-cement ratio during mixing all produce concrete that resists spalling for decades. In harsh environments, epoxy-coated or stainless steel rebar adds another layer of corrosion resistance.
Repairing Spalled Surfaces
Small, shallow spalls on flatwork like driveways and patios can be patched with a polymer-modified repair mortar. The damaged area needs to be chipped back to sound concrete first, since bonding new material over crumbling old material won’t last. For deeper repairs where rebar is exposed, the corroded steel needs to be cleaned or replaced, coated with a corrosion-inhibiting primer, and then encased in fresh repair concrete.
On larger structures, repair gets more involved. Engineers assess whether the spalling is isolated or part of a pattern that suggests widespread internal corrosion. If the rebar throughout a structure is actively corroding, spot repairs will only buy time before new spalls appear nearby. In those cases, broader interventions like cathodic protection systems (which use a small electrical current to slow corrosion across an entire structure) or full-depth concrete replacement may be necessary. The earlier spalling is caught and addressed, the simpler and less costly the repair.