Concrete Assessment: Age, Condition, Porosity, and Contamination Checks

Why Concrete Assessment Matters

Epoxy does not “fix” bad concrete; it bonds to what you leave behind after prep. If the slab is too green, too weak at the surface, contaminated, or hiding delamination, the coating can peel, blister, or wear through quickly. A good assessment tells you whether the slab is a good candidate, what prep intensity is required, and where repairs or special primers are needed.

Slab Age and Cure: When Is Concrete Ready?

Minimum age vs. actual readiness

Many slabs are physically hard enough to walk on within days, but still releasing moisture and alkalinity for weeks. As a beginner rule, avoid coating new concrete until it has had adequate cure time and is dry enough for the epoxy system you plan to use.

• New slabs: If the slab is recently placed, verify the project spec for minimum cure time. If you cannot verify, treat it as higher risk and plan on moisture testing and more aggressive surface prep.
• Recent patches/repairs: Patch materials can cure differently than the slab. Check each patch area separately for hardness, dusting, and moisture behavior.
• Signs it may be too green: persistent dark damp areas, strong “wet concrete” look, soft surface that powders easily, or high pH residue/efflorescence.

Practical guidance

If the slab is new or you suspect moisture issues, do not rely on “it looks dry.” At minimum, perform porosity checks and look for moisture-related symptoms (darkening, damp edges, efflorescence). If moisture is suspected, plan for a moisture-mitigating primer system or postpone coating.

Condition and Surface Integrity: What You’re Really Bonding To

Surface hardness and weak layers (laitance/dusting)

Epoxy bonds best to sound, dense concrete with a clean, open profile. Two common problems are:

• Laitance: a weak, cement-rich layer at the surface (often from overworking, excess water, or finishing practices). It can look smooth and “nice,” but it is fragile and can shear off under a coating.
• Dusting concrete: a surface that continually produces fine powder. This indicates weak surface paste, poor curing, or abrasion breakdown. Coating over dusting concrete often leads to bond failure because the epoxy is glued to dust, not to solid concrete.

Spalls, scaling, and cracks

• Spalls: localized breakouts/pits, often from impact, corrosion of embedded steel, or freeze/thaw. These must be repaired; coating will telegraph them and may fail at sharp edges.
• Scaling: widespread flaking/peeling of the top layer, commonly from freeze/thaw, deicers, or weak surface paste. Scaling indicates the top layer is unsound and typically requires removal to reach solid concrete.
• Cracks: note whether they are hairline, wider, or show vertical displacement. Movement cracks can reflect through coatings; displaced cracks can indicate slab movement and may require structural evaluation.

Porosity and Profile Readiness

Epoxy needs a surface that is both clean and appropriately porous (open) so it can wet out and mechanically lock in. Concrete that is too tight (burnished, sealed, or coated) prevents penetration and reduces bond strength. Concrete that is too weak (dusting/laitance) provides poor substrate strength even if it is porous.

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Water drop absorption test (field porosity check)

This quick test helps you identify tight, sealed, or contaminated areas that repel water.

1. Choose locations: test multiple spots: center, edges, near doors, parking/tire areas, and any visibly different zones.
2. Clean the spot lightly: wipe loose dust with a clean dry cloth (do not use cleaners for this quick check).
3. Apply water: place a few drops (or a teaspoon) of clean water on the surface.
4. Observe for 60 seconds: note whether the water darkens the concrete and absorbs, or beads up.

Contamination Checks: What Can Block Bonding

Sealers and curing compounds

Many slabs have invisible bond breakers: acrylic sealers, silane/siloxane water repellents, waxes, or curing compounds. These can make the surface look normal yet cause epoxy fisheyes, craters, or peeling.

• Clues: water beading, uneven sheen, “slick” feel, or areas that resist wetting.
• Where to look: near exterior doors, along edges, and on newer slabs where curing compounds are common.

Paint and previous coatings

Epoxy over unknown coatings is risky unless you can confirm compatibility and adhesion. Old paint may be poorly bonded or softened by solvents, and epoxy will only be as strong as the layer beneath.

• Clues: peeling edges, different color patches, roller marks, or a film that scratches off.
• Rule of thumb: if you cannot reliably identify and verify adhesion of the existing coating, plan to remove it mechanically.

Tire marks and plasticizer contamination

Hot tires can leave rubber residue and plasticizers that interfere with adhesion and can cause localized softening or staining. These areas often appear as dark arcs where vehicles park or turn.

• Clues: dark tire-shaped marks, glossy “burnished” tracks, or areas where water beads more than surrounding concrete.
• Implication: these zones typically require aggressive degreasing and mechanical removal of the affected surface layer.

Oil and grease

Oil can soak into concrete pores and later migrate upward, causing fisheyes, craters, or delamination. Even if the surface looks clean, subsurface oil can remain.

• Clues: darkened spots, persistent stains, slippery feel, or recurring “wet-looking” areas after cleaning.
• Implication: plan for repeated degreasing plus mechanical profiling; severe saturation may require removal and patching.

Step-by-Step Inspection Routine (Field Walkthrough)

Step 1: Visual survey (map the slab)

Walk the entire floor and create a simple sketch. Mark each issue so you can plan prep and repairs.

• Look for: cracks, spalls, scaling, pop-outs, patched areas, joints, stains, tire zones, glossy/tight areas, and any damp-looking regions.
• Note transitions: where the finish changes (smooth to rough, light to dark) often indicates different porosity or contamination.
• Check edges and corners: these often have paint overspray, sealers, or moisture entry.

Step 2: Straightedge checks (high/low spots and flatness)

Use a long straightedge (or a level/straight board) to find humps and dips that will telegraph through epoxy and cause puddling or thin spots.

1. Place the straightedge in multiple directions across the slab.
2. Mark high spots (rocking straightedge) and low spots (gaps under the straightedge).
3. Circle areas where water would pond (common near drains, garage door lines, and repaired zones).

Practical note: epoxy is self-leveling only within limits; significant dips need patching/leveling, and humps often need grinding.

Step 3: Sounding for delamination (hollow spots)

Delaminated or debonded concrete layers can exist under a seemingly solid surface. Sounding helps you find hollow areas that must be removed.

1. Use a hammer, mallet, or a heavy metal tool to tap the surface in a grid pattern, especially around cracks, spalls, and patched areas.
2. Listen for changes: sharp “ring” usually indicates solid concrete; dull/hollow “thud” suggests delamination or debonded topping.
3. Outline hollow zones with chalk/marker for removal and repair.

Step 4: Identify spalls and scaling (define repair boundaries)

Not all damaged concrete is equal. Define whether you have isolated defects or widespread unsound surface.

• Spalls: mark each spall and probe edges; if edges crumble easily, the repair area must be enlarged to reach sound concrete.
• Scaling: if large areas flake under light scraping, plan on removing the entire weak layer rather than spot repairs.

Practical On-the-Spot Tests

Tape pull check (weak surface layers and poor adhesion of existing coatings)

This quick check helps identify laitance, dusting, or poorly bonded paint/old coatings.

1. Select the tape: use a strong tape (e.g., high-tack duct tape or packaging tape).
2. Prepare the area: lightly brush off loose dust (do not “clean” aggressively; you want to test what’s there).
3. Apply tape firmly: press hard to ensure full contact; rub it down.
4. Pull sharply: remove at a consistent angle with a quick pull.
5. Inspect tape and surface: look for concrete powder, sandy grains, or coating chips on the tape.

Scratch/probe check (quick hardness feel)

Use a screwdriver or similar tool to scratch the surface in questionable areas (dusting zones, near spalls, and smooth/tight finishes).

• If it gouges easily or produces sand-like grains: surface is weak and needs removal to sound concrete.
• If it only produces fine powder with resistance: likely sound, but still verify porosity and contamination.

Spot wetting check for contamination patterns

Combine the water drop test with observation: if water absorbs in most areas but beads in parking lanes or near a workbench, treat those as contamination hotspots requiring targeted degreasing and extra profiling.

Decision Rules: Proceed, Remediate, or Avoid

Proceed (good candidate) when most of the following are true

• Concrete is sound: no widespread scaling, no hollow/delaminated zones found by sounding, and spalls are isolated and repairable.
• Surface is not dusting: tape pull does not lift significant powder/sand; scratch test shows reasonable hardness.
• Surface is wettable/porous after planned profiling: water drop test shows absorption (or you have a clear plan to mechanically open tight areas).
• Contamination is limited and identifiable: oil/grease and tire areas are localized and can be treated with targeted cleaning and removal.
• No unknown, failing coatings remain (or you will remove them fully).

Remediate more aggressively when you see any of the following

• Water beading across large areas: likely sealer/curing compound or burnished finish; plan on more aggressive mechanical profiling until water absorbs uniformly.
• Dusting/laitance indicators: tape pull lifts significant powder or the surface scratches/gouges easily; remove the weak layer and reassess.
• Localized oil saturation: repeated staining, persistent dark spots, or beading in specific zones; plan repeated degreasing plus deeper mechanical removal, and consider replacing severely saturated concrete.
• Hollow-sounding areas: remove delaminated concrete to sound substrate and patch before coating.
• Flatness issues: significant low spots that will pond coating or high spots that will create thin film; grind/fill as needed.

Avoid coating (or pause the project) when these conditions exist

• Active moisture problems: persistent dampness, recurring efflorescence, or moisture entry you cannot control; coating may blister or debond without a verified moisture mitigation plan.
• Widespread unsound surface: extensive scaling, severe dusting across the slab, or large delaminated areas indicating the top layer is failing broadly.
• Unremovable/unknown bond breakers: you cannot remove a sealer/curing compound/old coating reliably, or the slab remains non-wettable even after attempted profiling in test areas.
• Structural movement indicators: cracks with vertical displacement, ongoing movement, or settlement signs that will likely continue and damage the coating system.

Use these rules to decide your next step: proceed with standard prep, increase prep intensity and repairs, or stop and address the underlying slab issues before investing in epoxy.