Repairing concrete before epoxy starts with classification, not filler. A dormant shrinkage crack, a moving isolation joint, a shallow spall, and a slab with differential settlement may all look like “a gap in the floor,” but they require different decisions.

Direct answer: repair sound, nonmoving defects with a material and detail approved for the selected flooring system. Preserve or “honor” moving joints and active cracks when the system requires movement accommodation. Stop and obtain professional evaluation when cracking suggests continued movement, settlement, heaving, corrosion, or structural distress.

An epoxy floor coating is not, by itself, a structural repair. Filling a line so it disappears on installation day does not prove that the underlying cause has stopped.

First identify the defect

ICRI defines an active crack as one whose cracking mechanism is still at work or that is still moving. A dormant crack is not expected to become active, or its movement is small enough not to affect the repair. That distinction is more useful than width alone.

Condition What it means for a coating decision Typical next step
Dormant/static crack Movement is not expected to defeat the approved repair Prepare and fill using the flooring-system detail
Active/moving crack A rigid repair or continuous brittle coating may crack again Determine movement and use an engineered movement detail or professional design
Contraction/control joint An intentionally weakened plane that regulates shrinkage cracking Follow the system designer’s joint detail; do not assume “fill and forget”
Expansion/isolation joint Separation intended to permit relative movement Usually preserve movement through the flooring assembly with the specified sealant/detail
Spall or popout Concrete is missing or locally unsound Remove unsound material and rebuild with a compatible repair system
Settlement, heaving, or faulting The two sides have changed elevation or the support may be moving Stop cosmetic repair and investigate the slab/support condition

ACI and ICRI use precise joint terms. A contraction joint creates a weakened plane to control where shrinkage cracking occurs. Expansion and isolation joints are separations intended to accommodate movement. Treating all three as ordinary cracks erases their designed function.

Inspection before choosing a repair

Map the defect before opening it with a tool. Record:

  • location, length, width, and whether the crack continues through walls, columns, or adjacent slab panels;
  • vertical displacement, curling, heaving, or a change in level across the line;
  • staining, efflorescence, dampness, corrosion products, or active water;
  • whether the line is a sawcut or construction detail rather than random cracking;
  • previous filler, sealant, coating, or failed patch;
  • changes across seasons, loads, temperature cycles, or recent settlement.

A one-time photograph cannot prove dormancy. Crack gauges, dated measurements, and review of the building history can provide better evidence. Widespread map cracking, displacement, recurring opening, exposed reinforcement, or cracks associated with structural elements exceed routine coating preparation.

A practical repair sequence for nonmoving defects

The exact geometry and material come from the repair and flooring manufacturers, but the workflow is consistent.

  1. Define the objective. Is the repair intended to restore a coating-ready surface, seal a dormant crack, rebuild a spall, or transfer structural load? These are not interchangeable goals.
  2. Remove unsound and incompatible material. Old filler, weak edges, laitance, oil, and loosely bonded concrete cannot be left as the foundation of a new repair.
  3. Prepare the geometry only as specified. Some systems call for routing, sawcut shoulders, or an exposed-aggregate profile. Others do not. Opening every hairline crack can remove sound concrete without improving the repair.
  4. Control and remove dust. Routing and grinding concrete can generate respirable crystalline silica. Use the task-appropriate engineering controls and OSHA/NIOSH guidance described in the concrete grinding guide, not a generic “wear a mask” rule.
  5. Install a compatible material. Observe the exact mix ratio, substrate condition, depth/width limits, placement method, cure time, and temperature range in the current product documents.
  6. Reprofile the repair if required. A glossy patch or raised edge may not provide the surface required by the next primer or coat.
  7. Verify the transition. The repaired area and surrounding slab must be sound, clean, appropriately profiled, and within the flooring system’s recoat requirements.

This work belongs within the broader concrete preparation sequence. Repair does not replace contaminant removal, moisture evaluation, profiling, or final cleaning.

Repair-material categories are not interchangeable

Material category Possible role Important limitation
Low-viscosity resin Gravity filling or injection in a specifically qualified crack Does not accommodate movement merely because it penetrates deeply
Epoxy patching paste or mortar Rebuilding static cracks, spalls, edges, or voids within its documented geometry Cure, profile, moisture, and overcoat compatibility are product-specific
Polyurea or other rapid-setting filler Fast joint/crack treatment in approved flooring assemblies “Fast” does not establish movement capability or coating compatibility
Flexible sealant/joint system Accommodating movement at designed discontinuities Usually requires a joint geometry, backing, and coating termination detail
Cementitious repair mortar Rebuilding concrete or larger defects Moisture condition, cure, tensile bond, and time before resin coating must be verified

Do not select solely by the label “concrete crack filler.” A coating manufacturer may require one material for static cracks, another for moving joints, and a separate transition detail through the finish.

Cure and recoat compatibility matter

A patch that feels hard may still be outside its approved coating window. Verify:

  • minimum and maximum placement dimensions;
  • required substrate moisture condition and primer/bonding agent;
  • cure time at the actual slab and air temperature;
  • whether blush, residue, or surface gloss must be removed;
  • required profile before the flooring primer;
  • chemical compatibility with the primer and build coat;
  • whether a test patch or pull-off test is required.

Product-specific numbers stay product-specific. For example, a patching paste data sheet may list a short pot life and a particular cure schedule; those values do not apply to other epoxy, polyurea, or cementitious repairs.

When the crack should remain visible in the system

Movement joints are normally detailed through the resinous floor instead of buried under a continuous rigid film. The final detail may include a sawcut, backer rod, flexible sealant, reinforced transition, or another manufacturer-approved assembly. The correct solution depends on joint type, anticipated movement, traffic, chemicals, hygiene requirements, and system thickness.

Even a well-detailed static crack can reflect through a coating if it becomes active. Sherwin-Williams’ crack-treatment guidance explicitly separates static and dynamic conditions; Sika’s system guidance likewise calls for moving joints and cracks to be honored. These are system principles, not guarantees against future movement.

Red flags for professional evaluation

Pause ordinary floor preparation when you find:

  • differential settlement, heaving, faulting, or recurring displacement;
  • cracks through structural members or near columns and load-bearing walls;
  • rust staining, exposed reinforcement, or concrete delamination;
  • active leakage or hydrostatic pressure;
  • rapid change, wide openings, or a repeating structural pattern;
  • uncertainty about whether a joint was designed for movement;
  • an industrial floor where traffic, chemicals, sanitation, or shutdown risk makes failure consequential.

A licensed structural engineer or qualified concrete-repair professional can determine cause and repair objective. A flooring installer can then coordinate the coating transition with that design.

What to look for when selecting tools and materials

Future commercial recommendations on this page should follow the defect diagnosis. Useful selection criteria include:

  • published defect type and movement limits;
  • minimum/maximum repair geometry;
  • bond and substrate-preparation requirements;
  • cure and overcoat documentation for the intended flooring system;
  • routing or grinding tooling that matches the prescribed geometry;
  • dust collection compatible with the exact tool;
  • mixing and dispensing equipment appropriate to the material;
  • SDS-defined handling controls and PPE.

No product link can resolve an unidentified moving crack. Choose the detail first, then the compatible repair components.

Final decision checklist

  • Is this a crack, designed joint, spall, or evidence of slab movement?
  • Is it dormant, or is there evidence the mechanism is still active?
  • Has unsound or contaminated material been removed?
  • Does the repair product document this defect, geometry, substrate, and overcoat?
  • Will a movement joint be honored through the floor rather than rigidly bridged?
  • Have cure, profile, cleanliness, and primer compatibility been verified?
  • Is engineering or repair-specialist input warranted before coating?

Return to the epoxy flooring systems hub to place repair within moisture assessment, profiling, primer selection, and complete system design.

Sources and technical references

Accessed September 2026. Product documents can change; verify the current revision before specifying or installing a system.

  1. Concrete Repair Terminology — International Concrete Repair Institute
  2. RAP-1: Structural Crack Repair by Epoxy Injection — American Concrete Institute
  3. Crack Treatment Guidelines — Sherwin-Williams High Performance Flooring
  4. Sika MB flooring primer product information — Sika USA
  5. Sikafloor-150 Epoxy Patching Paste product data sheet — Sika USA