Diagnosing and Fixing Common Problems: Cracks, Scaling, Dusting, and Unevenness

A Practical Troubleshooting Workflow

Concrete problems are easiest to fix when you approach them like a mechanic: (1) observe symptoms, (2) narrow likely causes, (3) choose prevention or repair that matches the cause. Avoid “one-size-fits-all” products—many repairs fail because the underlying issue (movement, moisture, freeze-thaw, or weak surface paste) wasn’t addressed.

Step 1: Observe and Document

• When did it appear? Same day, within a week, months later, or after winter?
• Where is it? Mid-panel, near re-entrant corners, along edges, over trenches/utility cuts, at joints, at downspouts.
• What does it look like? Hairline map pattern, long straight line, diagonal from a corner, random, parallel lines, flaking surface, powdery dust.
• Is it changing? Measure and mark crack width at a few points; re-check in 2–4 weeks and after rain/freeze.
• Is there displacement? Run a straightedge across cracks/edges to see if one side is higher (a trip hazard).

Step 2: Identify the Problem Category

Most common issues fall into two groups: cracks (movement or shrinkage) and surface defects (weak surface paste, freeze-thaw, finishing/curing problems). Some slabs have both.

Step 3: Choose Prevention or Repair

Pick the repair based on: (a) whether the concrete is still moving, (b) whether water is involved, and (c) whether the defect is cosmetic or structural/safety-related.

Crack Types: How to Tell Them Apart

Cracks are diagnosed by timing (same day vs. weeks/months), pattern (random, parallel, diagonal), and location (mid-panel, corners, over changes in support).

Plastic Shrinkage Cracks (Same Day)

When: Within a few hours of placing/finishing, often on warm, windy, or very dry days.

Continue in our app.

You can listen to the audiobook with the screen off, receive a free certificate for this course, and also have access to 5,000 other free online courses.

Or continue reading below...

Download the app

Pattern: Shallow, irregular, “spider” or fairly straight cracks that may be roughly parallel; typically not at joints; often across open areas.

Location clues: Broad exposed surfaces (slabs, pads) and areas that dried faster (sunny side, near edges).

Severity check: Usually hairline and shallow; rarely have displacement.

Repair (beginner-friendly):

• If hairline and cosmetic: leave it; keep water out if needed with a penetrating sealer once fully dry.
• If it collects dirt/water: use a low-viscosity concrete crack sealer (often a thin polyurethane or epoxy designed for hairline cracks). Clean, dry, and apply per label.

Prevention next time: control evaporation (windbreaks, shade), avoid finishing delays that leave the surface exposed, and protect the surface immediately after finishing.

Drying Shrinkage Cracks (Days to Months)

When: Often shows up in the first week to first few months as the slab dries and shrinks.

Pattern: Longer cracks that may run across panels; can start at re-entrant corners (inside corners of steps, notches, or around posts) and travel outward.

Location clues: Mid-panel cracks when joint spacing is too wide; cracks that “ignore” joints can indicate restraint or joint issues.

Severity check: Measure width. Hairline to ~1/16 in (1.5 mm) is common; wider cracks suggest movement or restraint.

Repair (beginner-friendly):

• Non-moving, narrow cracks: clean and seal with polyurethane crack sealant (good for outdoor slabs because it stays flexible).
• Wider cracks (but no displacement): use a backer rod (foam rope) to control sealant depth, then apply a self-leveling polyurethane sealant for horizontal surfaces.

Match to root cause: If water is getting into the crack and freezing, sealing matters more than cosmetics.

Settlement Cracks (Support Issues)

When: Weeks to months after placement, or after heavy rain, plumbing leaks, or soil consolidation.

Pattern: Often a single crack or a few cracks with one side lower; may follow a trench line or edge; can radiate from a corner if the corner dropped.

Location clues: Near downspouts, along fill areas, over utility trenches, at slab edges where soil washed out.

Severity check: Look for vertical displacement (lippage), rocking when you step near the crack, or widening over time.

Repair (beginner-friendly options):

• Stop the water: extend downspouts, regrade soil to shed water, add splash blocks, fix leaks.
• Trip hazard from offset: grind the high side (see “Unevenness” section) as a temporary safety fix.
• Crack sealing alone: only after drainage/support is addressed; otherwise the crack will reopen.

When to escalate: If the slab is actively settling, has significant voids, or the offset is large, professional slab lifting/leveling may be the right fix.

Structural/Overload Cracks (Load or Design Issues)

When: After heavy loads (vehicles on a thin slab, concentrated point loads), or gradually as loads repeat.

Pattern: Wider cracks, sometimes with branching; may align with wheel paths; may show displacement or crushing at edges.

Location clues: Driveways, areas with heavy equipment, near supports, or where thickness changes.

Severity check: Wide cracks, displacement, or cracks that continue to grow are red flags.

Repair note: Surface fillers won’t restore structural capacity. You may need partial replacement, thickened sections, or engineering guidance.

Surface Defects: Scaling, Dusting, Crazing, Spalling

Scaling and Flaking (Surface Peeling)

What you see: Thin flakes or chips coming off the top surface, often exposing sand/aggregate; commonly worse after winter.

Likely causes:

• Freeze-thaw damage when the surface stays wet and freezes repeatedly.
• Deicer salts that increase scaling risk, especially on young concrete.
• Weak surface paste from finishing with extra water, overworking the surface, or finishing while bleed water was present.

How to evaluate severity:

• Light scaling: small flakes, mostly cosmetic, no deep pitting.
• Moderate: widespread flaking with roughness that holds water.
• Severe: deep loss exposing coarse aggregate, recurring delamination, or edges breaking down.

Repair (beginner-friendly):

1. Remove loose material: stiff broom, scraper, or light chipping; do not patch over unsound concrete.
2. Clean thoroughly: pressure wash and let dry; remove salts/oils.
3. Patch/resurface: use a polymer-modified concrete patch or resurfacer rated for exterior freeze-thaw. Follow minimum thickness rules.
4. Cure/protect the repair: protect from rapid drying and from freezing per product instructions.
5. Reduce future wetting: improve drainage and avoid deicers when possible.

Common mistake: Applying a thin “paint-like” coating over scaling without removing loose paste—this usually peels.

Dusting (Powdery, Weak Surface)

What you see: Fine powder on the surface that rubs off on shoes; surface may look chalky and wear quickly.

Likely causes:

• Weak laitance (a fragile top layer) from overworking the surface or bringing too much paste to the top.
• Overwatering (adding water during finishing) that weakens the surface layer.
• Poor curing leading to a weak, porous surface.

How to evaluate severity:

• Light: minor dust, surface still hard.
• Moderate: noticeable wear paths, easy scratching.
• Severe: surface erodes, exposes sand, or continues producing dust after cleaning.

Repair (beginner-friendly):

1. Clean: sweep/vacuum; scrub with a degreaser if needed; rinse and dry.
2. Remove weak layer: if the top is very soft, lightly grind or mechanically abrade to reach sound concrete.
3. Apply a concrete densifier: lithium/sodium silicate densifiers can harden the surface by reacting with free lime. Apply evenly to a clean, dry surface as directed.
4. If severe: consider a thin bonded overlay/resurfacer after proper surface prep.

Match to root cause: If dusting is from moisture problems (water wicking up, constant wetting), densifier alone may not last—address drainage and moisture exposure first.

Crazing (Fine “Map” Cracks on the Surface)

What you see: A network of very fine hairline cracks, often visible when the surface is damp or at a low sun angle.

Likely causes: Surface drying faster than the underlying concrete, overly rich surface paste, or finishing practices that create a tight surface skin.

How to evaluate severity: Crazing is usually cosmetic and shallow; it typically does not have displacement and doesn’t go through the slab.

Repair (beginner-friendly):

• Cosmetic only: leave it, or use a penetrating sealer to reduce staining and water entry.
• If it traps dirt: clean and seal; avoid thick film coatings unless the surface is well-prepped and dry.

Spalling at Edges and Corners

What you see: Chunks breaking off at slab edges, corners, steps, or along joints; may start as small chips and grow.

Likely causes:

• Impact (shovels, snowplows, dropped loads).
• Freeze-thaw at saturated edges.
• Corrosion of embedded steel near edges (more common in older or improperly covered reinforcement).
• Joint/edge distress where water enters and freezes.

Repair (beginner-friendly):

1. Square up the repair area: chip back to sound concrete; undercut slightly if the patch product recommends it.
2. Clean: remove dust; rinse and let surface become “SSD” (saturated surface-dry) if required by the patch product.
3. Bond and patch: use a polymer-modified repair mortar suitable for edges; pack firmly and shape.
4. Protect: keep traffic off until cured; reduce water exposure at the edge (drainage, sealing).

Common mistake: Smearing patch over loose, cracked edges without removing unsound concrete—this leads to pop-off.

Unevenness: High Spots, Low Spots, and Trip Hazards

Diagnose the Type of Unevenness

• High ridge or hump: often finishing/screeding tolerance or localized heave.
• Low birdbaths (ponding): finishing tolerance, settlement, or poor drainage around the slab.
• Lippage at a crack or joint: settlement/heave or curling; can create a trip hazard.

Evaluate Severity (Safety First)

Use a straightedge and measure the vertical offset. If the offset creates a trip hazard (common threshold is around 1/4 in (6 mm) or more, depending on location and use), treat it as a safety issue even if the slab is otherwise sound.

Beginner Repairs

Grinding High Spots (Trip Hazard Reduction)

1. Mark the area: chalk the high ridge and the direction you want water to flow.
2. Choose tools: an angle grinder with a diamond cup wheel for small areas; a walk-behind grinder for larger areas.
3. Control dust: use a shroud and vacuum; wear proper respiratory protection.
4. Grind gradually: check often with a straightedge; avoid creating a dish that holds water.
5. Seal if needed: grinding opens the surface; a penetrating sealer can help reduce staining and water entry.

Filling Low Spots

For shallow depressions that hold water, use a patching/leveling compound rated for exterior use and thin applications. Surface prep is critical: clean, remove weak paste, and follow bonding instructions. If the low spot is due to ongoing settlement, filling may crack again unless drainage/support is corrected.

Improve Drainage (Often the Real Fix)

• Extend downspouts away from slabs.
• Regrade adjacent soil so water sheds away.
• Prevent water from flowing across the slab and pooling at edges.

Choosing the Right Repair Material (Quick Guide)

When to Call a Professional

Beginner repairs are appropriate for cosmetic issues and minor serviceability problems. Get professional help when any of the following are present:

• Trip hazards you can’t safely grind down without creating drainage problems.
• Wide cracks (commonly > 1/8 in / 3 mm), especially if they keep widening.
• Displaced cracks (one side higher/lower) indicating movement or loss of support.
• Major settlement (slab rocking, voids, rapid change after rain, sinkhole-like behavior).
• Structural concerns (cracks associated with heavy loads, broken corners that keep failing, or cracks that mirror underlying foundation movement).
• Rebar corrosion signs (rust staining, delamination, spalls exposing steel).

Quality Control Checklist for the Next Project (Prevent Repeat Problems)

Before the Pour

• Base and support: confirm uniform support; avoid soft spots and poorly compacted fill; plan for drainage so water doesn’t pond at edges.
• Water management: route downspouts and surface runoff away from the slab area.
• Joint plan: verify joint layout addresses re-entrant corners and panel sizes so shrinkage is “invited” to crack at planned locations.

During Placement and Finishing

• Watch the surface water: do not finish while bleed water is present; avoid adding water to “help” finishing.
• Timing discipline: start and stop finishing steps based on concrete readiness, not the clock.
• Flatness checks: use a straightedge early to catch humps/birdbaths while corrections are still possible.

After Finishing

• Curing adherence: start curing promptly and maintain it for the required duration; protect from rapid drying.
• Early protection: keep deicers off young concrete; prevent early freeze exposure; limit traffic until adequate strength.
• Post-cure inspection: after the first week and after the first major weather event, inspect for early signs of cracking, scaling, or drainage issues and correct water routing immediately.