Pop-Outs vs Spalling Decision Tree

Why this matters

A homeowner sees a chip on their driveway and calls. The contractor has 10 minutes to determine whether they're looking at a 25-cent fix, a 25-dollar fix, or a 25,000-dollar fix. Pop-outs (small conical craters with an aggregate fragment in the bottom) and spalling (sheet-like delamination of the surface) look superficially similar but have completely different causes, completely different repair paths, and completely different liability implications. ACI 201.2R and ACI 224.1R frame the technical distinctions; ICRI 310.2R covers the prep for whichever repair the diagnosis calls for. This article is the field decision tree.

The four defect categories that get confused

1. Pop-outs. Conical depressions typically 1/4 to 2 inches in diameter with a fractured aggregate particle visible at the base. Discrete, not progressive across a surface. Caused by an internal aggregate failure (reactive aggregate, freeze-thaw of a porous particle, or contaminant inclusion).

2. Surface scaling. Sheet-like loss of the top 1/16 to 1/4 inch of paste across broad areas. Often follows a finishing-trowel pattern or wind-exposure pattern. Caused by freeze-thaw of weak surface paste, typically because of low air entrainment, premature finishing trapping bleed water, or de-icer-driven osmotic stress.

3. Spalling (delamination). Sheet-like loss of 1/4 inch to 2 inches of concrete from the surface, often exposing aggregate or reinforcing steel. Caused by reinforcement corrosion expansion, deeper freeze-thaw than scaling, or impact damage.

4. Crazing. Network of fine surface cracks without material loss. Cosmetic only, addressed elsewhere in the crack-diagnosis decision tree.

The same homeowner might describe any of the first three as "spalling" on the phone. The visit determines the actual category.

Decision 1: Depth of material loss

Run a fingernail across the defect and a ruler vertically.

• Loss under 1/16 inch: surface scaling. Paste-only defect.
• Loss 1/16 to 1/4 inch: scaling progressing or shallow spalling.
• Loss 1/4 to 2 inches: spalling.
• Discrete conical craters 1/2 to 2 inches across with aggregate fragment at base: pop-outs.
• No material loss, just visible network of fine lines: crazing (see separate guide).

Decision 2: Pattern of distribution

Pop-outs are discrete. Scaling and spalling are distributed.

• Isolated craters with intact concrete between them, often correlated with visible aggregate inclusions: pop-outs. Count them per square yard - under 5 per sq yd is typically cosmetic only; above 20 per sq yd suggests systemic aggregate problem (alkali-silica reactivity or freeze-thaw of unsound aggregate).
• Continuous sheet of loss across a finished area: scaling or spalling. Note the boundary - sharp edges suggest mechanical/load damage; gradual edges suggest environmental.
• Loss correlated with reinforcing steel locations (visible from a rebar scanner sweep): corrosion-driven spalling. The expansion of corroded steel has fractured the cover.

Decision 3: Aggregate examination

For pop-outs, pull a few of the loose aggregate fragments and examine.

• Chert (hard, glassy, often white or tan, porous): freeze-thaw failure of unsound aggregate. Common in midwest aggregate sources without proper screening.
• Soft sedimentary fragments (shale, soft limestone, clay-bound siltstone): freeze-thaw or moisture-expansion failure.
• Crystalline silica-rich aggregate (granite, quartzite) with white reaction-product rim: alkali-silica reactivity (ASR). The reaction product is the diagnostic marker.
• Iron-stained fragments (rust ring around the aggregate): pyrite or iron-sulfide inclusion oxidizing.

ASTM C295 petrographic examination is the definitive test for aggregate cause but is rarely cost-justified on residential work. Visual identification of the fragment type usually answers the question for crew purposes.

Decision 4: Corrosion involvement

For surface losses with steel proximity:

• Use a cover meter or rebar scanner to locate reinforcing steel.
• If the spalled area is directly over a bar at 3/4 to 2 inch cover, corrosion expansion is the likely driver.
• Confirm with chloride testing (ASTM C1152) if available; chloride contents above 0.20 percent by weight of cement are the corrosion-initiation threshold for embedded steel.
• Visible rust staining at the spall boundary is presumptive evidence of corrosion-driven spall.

Corrosion-driven spalling is the highest-liability category. The repair has to address the corroded steel (passivation, sacrificial anode, or replacement) before patching the concrete, or the patch spalls off again within 1-3 years.

Decision 5: Freeze-thaw history

For scaling and non-corrosion spalling, the freeze-thaw exposure determines the diagnosis path.

• Exterior slab in Climate Zone 4-7 (cold) with visible scaling: freeze-thaw of under-air-entrained concrete is the working hypothesis. Confirm by checking the air content of a core sample if cost-justified (ASTM C457 hardened-air analysis).
• Exterior slab in Climate Zone 1-3 (warm) with scaling: less likely freeze-thaw; more likely premature finishing or weak surface from rapid drying.
• Interior slab with scaling: not freeze-thaw. Look at chemical exposure (acid washdown, de-icers tracked in), abrasion, or finishing defect.

ACI 201.2R Section 5 covers freeze-thaw mechanisms in detail; the diagnostic shortcut is exposure + climate + air content.

Decision 6: Repair path

Once classified, the repair branches:

For pop-outs:

• Under 5 per sq yd, cosmetic concern only: fill with polymer-modified patch mortar or leave uncorrected. Not progressive.
• 5-20 per sq yd: fill each with patch mortar (Mapei Planitop XS, Sika MonoTop, or equivalent), prep substrate per ICRI 310.2R.
• Above 20 per sq yd: systemic aggregate problem. Single-pop-out repair is futile; new ones will appear. Recommend full topping (1/2 to 1 inch overlay with proper aggregate) or replacement.
• ASR-confirmed pop-outs: ASR continues to react. Topping or replacement; surface repair only is not a permanent fix.

For surface scaling:

• Light scaling under 1/16 inch, cosmetic: penetrating sealer (silane/siloxane per ASTM C1583 testing) slows further damage. Not a structural repair.
• Moderate scaling 1/16 to 1/4 inch: bonded overlay (Sika Armatec 110 EpoCem bonding agent under a topping mix). Prep substrate to CSP 5-7 per ICRI 310.2R.
• Severe scaling above 1/4 inch: full surface removal and re-topping, or replacement.

References

• ACI 201.2R-16, Guide to Durable Concrete.
• ACI 224.1R-07, Causes, Evaluation, and Repair of Cracks in Concrete Structures.
• ACI 562-21, Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures.
• ICRI Guideline 310.2R, Selecting and Specifying Concrete Surface Preparation.
• ASTM C295/C295M, Standard Guide for Petrographic Examination of Aggregates for Concrete.
• ASTM C457/C457M, Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete.
• ASTM C1152/C1152M, Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete.