Cracks in concrete are inevitable — but how you address them before coating determines whether they'll remain hidden or telegraph through your new floor. Understanding crack types and selecting the appropriate repair method is essential for successful flooring projects.
Types of Concrete Cracks
1. Plastic Shrinkage Cracks
Occur during initial curing when surface dries faster than the interior:
- Typically shallow (surface only)
- Random, map-like pattern
- Generally dormant (not moving)
- Usually cosmetic concern only
2. Drying Shrinkage Cracks
Develop as concrete cures and loses moisture over time:
- Often follow regular patterns
- May be deeper than plastic shrinkage cracks
- Typically dormant once fully cured
- Common at reentrant corners and changes in section
3. Settlement Cracks
Caused by differential movement in the substrate:
- Often wider and deeper
- May show vertical displacement
- Could be dormant or active
- Require structural assessment
4. Structural Cracks
Result from overloading, impact, or design deficiencies:
- May continue through full slab depth
- Often active (still moving)
- Require engineering evaluation
- May indicate larger structural issues
5. Control Joint/Expansion Joint Issues
When joints fail to function as intended:
- Cracking adjacent to joints
- Joint spalling or deterioration
- Joints filled with debris preventing movement
Crack Assessment
Key Questions
- Width: Hairline (<0.5mm), medium (0.5-2mm), or wide (>2mm)?
- Depth: Surface only or full depth?
- Movement: Dormant or active?
- Cause: Known or needs investigation?
- Pattern: Random, linear, or structural?
Movement Testing
Determine if crack is active before selecting repair method:
- Install crack monitors across representative cracks
- Monitor for 2-4 weeks minimum
- Movement >0.1mm indicates active crack
- Active cracks require flexible repair systems
Repair Methods
1. Routing and Sealing
Best for: Surface cracks, dormant cracks, aesthetic repair
Procedure:
- Widen crack with diamond blade or router (V-groove or square cut)
- Clean thoroughly with vacuum and compressed air
- Apply appropriate sealant (epoxy, polyurethane, or polyurea)
- Strike off flush with surface
- Allow to cure before coating
Materials:
- Rigid epoxy for dormant cracks under rigid coatings
- Flexible polyurethane for active cracks or flexible coatings
- Polyurea for rapid cure requirements
2. Epoxy Injection
Best for: Structural repair, full-depth cracks, restoring monolithic strength
Procedure:
- Install injection ports at regular intervals along crack
- Seal crack surface between ports
- Inject low-viscosity epoxy under pressure
- Work from one end to ensure complete filling
- Remove ports and finish surface after cure
Advantages:
- Restores structural integrity
- Penetrates full crack depth
- Creates monolithic bond
Limitations:
- Only for dormant cracks
- Requires skilled applicators
- Time-consuming for extensive cracking
3. Gravity-Fed Epoxy
Best for: Hairline cracks, large areas of fine cracking
Procedure:
- Clean surface thoroughly
- Flood low-viscosity epoxy over surface
- Work into cracks with squeegee or broom
- Remove excess before cure
- May require multiple applications
Best used when: Cracks are too numerous or fine for individual treatment.
4. Stitching
Best for: Structural cracks, preventing further propagation
Procedure:
- Drill holes on both sides of crack at angle
- Install steel or carbon fiber staples
- Fill holes and crack with epoxy grout
- Restores load transfer across crack
When required: Structural engineer has determined crack repair must restore load-bearing capacity.
5. Flexible Joint Treatment
Best for: Active cracks that will continue moving
Procedure:
- Route crack to appropriate width/depth ratio
- Install backer rod if deep
- Apply flexible sealant (polyurethane or silicone)
- Honor as movement joint in coating system
Important: Rigid coatings over flexible repairs will crack. The coating system must accommodate the joint.
Material Selection Guide
| Crack Type | Recommended Repair | Material |
|---|---|---|
| Hairline, dormant | Gravity-fed or ignore | Low-viscosity epoxy |
| Medium, dormant | Route and seal | Epoxy paste |
| Wide, dormant | Route and seal or inject | Epoxy or mortar |
| Any width, active | Flexible joint treatment | Polyurethane sealant |
| Structural | Injection + stitching | Structural epoxy |
Integration with Coating Systems
Timing
- Complete crack repair before any coating application
- Allow full cure of repair materials (typically 24-72 hours)
- Grind or sand repairs flush with surface
- Prime repaired areas before coating if required
Visibility
Even properly repaired cracks may remain visible through thin coatings:
- Different thermal movement than surrounding concrete
- Slight color variation in repair material
- Surface texture differences
Solutions: Thicker coating systems, broadcast aggregate finishes, or accepting crack visibility as inherent to the material.
Re-Cracking Risk
Dormant cracks properly repaired rarely re-crack. However:
- Ensure cause is addressed, not just symptom
- Active cracks will re-crack if filled with rigid material
- Consider crack-bridging primer systems for extensive cracking
Need Crack Assessment?
Our team can evaluate your concrete conditions and recommend the optimal repair approach.
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