AirSprayTech Academy Certificate Program
Commercial and Industrial Floor Coatings for Professional Contractors
Repairing Cracks, Joints, Spalls, and Damaged Concrete
Article 08 of 24
Cracks, joints, spalls, voids, and deteriorated concrete must be
understood before they are filled or covered. A repair succeeds when
it addresses the condition, remains compatible with the flooring
system, and supports the intended service—not merely when it hides the
defect.
First Identify What You Are Repairing
Cracks and joints are not interchangeable. A crack is generally an
unplanned separation in the concrete. A joint is intentionally formed,
tooled, or cut to accommodate construction sequencing, shrinkage,
isolation, or movement.
Filling every opening with the same rigid material can transfer movement
into the new flooring system. Before selecting a repair, determine:
- Whether the opening is a crack or a joint
- The joint's intended function
- Whether movement is expected
- Whether vertical displacement is present
- Whether moisture or contamination is entering the opening
- The condition of the surrounding concrete
- The traffic and service expected after installation
- How the flooring manufacturer requires the condition to be treated
Recognize Conditions Outside the Flooring Contractor's Scope
Flooring contractors should not make unsupported structural
determinations. Stop and request evaluation by the appropriate design
professional when conditions include:
- Significant vertical displacement
- Cracks that continue through walls, beams, or foundations
- Recurring or rapidly widening cracks
- Settlement, heaving, or slab movement
- Widespread delamination or severe concrete deterioration
- Corroding reinforcing steel
- Active water intrusion
- Damage that may affect structural capacity
A resinous flooring system is not a structural repair unless it has
been specifically designed and approved for that purpose.
Common Crack and Joint Types
| Condition |
General Purpose or Cause |
Repair Consideration |
| Shrinkage crack |
Concrete volume change during drying or early curing. |
Determine whether movement has stabilized and whether the
flooring system can tolerate future movement.
|
| Control or contraction joint |
Intended to encourage shrinkage cracking at a planned
location.
|
Follow the project and manufacturer's requirements for
filling, honoring, or treating the joint.
|
| Construction joint |
Separates concrete placements made at different times.
|
Evaluate load transfer, movement, edge condition, and
contamination before treatment.
|
| Expansion or isolation joint |
Allows movement between structural elements or around
columns, walls, and equipment bases.
|
Normally requires a detail that preserves intended
movement.
|
| Structural or active crack |
May result from settlement, loading, thermal movement,
restraint, or structural behavior.
|
Requires appropriate evaluation before a flooring repair
is selected.
|
| Spall |
Concrete loss caused by impact, joint failure, corrosion,
freeze-thaw exposure, or deterioration.
|
Remove unsound material and rebuild with a compatible
repair designed for the depth and service.
|
Document the Condition Before Repair
Preparation may expose cracks and damage that were not visible during
the initial survey. Mark each condition on the floor and record:
- Location and identification number
- Length, width, depth, and general pattern
- Horizontal or vertical displacement
- Moisture, staining, oil, or chemical residue
- Previous repair material and its condition
- Broken, rounded, or spalled edges
- Relationship to columns, walls, drains, machinery, and traffic
- Evidence of continuing movement
- Photographs before and after repair
This record helps distinguish included repairs from newly discovered or
concealed conditions.
Preparing Cracks for Repair
A repair material cannot bond reliably to dust, contamination, weak
concrete, or loose edges. Crack preparation may include:
- Confirming the approved repair detail.
- Removing existing filler, coating, contamination, and loose material.
- Opening or routing the crack only as required by the repair system.
- Removing weak edges until sound concrete is reached.
- Vacuuming dust and debris from the prepared opening.
- Confirming that moisture conditions are acceptable.
- Priming when required by the repair-material manufacturer.
- Installing the material at the required depth and geometry.
Uncontrolled widening can damage sound concrete or turn a narrow crack
into an unnecessarily large repair. Follow the specified repair detail
and equipment requirements.
Rigid and Flexible Crack Treatments
Rigid Repair
A rigid repair may be used for a stable condition when load
transfer, edge support, or continuity is required. Rigid
materials generally cannot accommodate substantial movement.
If the crack moves after installation, the crack may reflect
through the repair and flooring system.
Flexible Treatment
A flexible or elastomeric detail may be selected where limited
movement is anticipated. It may require a reinforced strip,
flexible membrane, termination detail, or visible joint.
Flexibility does not guarantee that unlimited or structural
movement can be concealed.
The flooring manufacturer should provide the approved detail for the
complete system. The repair material, primer, membrane, flooring, and
topcoat must remain compatible.
Joint Treatment Requires a Decision
Every joint should be deliberately assigned one of the following
treatments:
- Filled and coated when the approved system permits it
- Filled and later recut through the flooring system
- Honored through the completed flooring system
- Detailed with a flexible sealant or joint system
- Rebuilt because the edges or filler have failed
The decision depends on joint type, expected movement, traffic, cleaning,
chemical exposure, sanitation, appearance, and the flooring
manufacturer's requirements.
Expansion and isolation joints generally must retain their intended
movement. Coating rigidly across them can transfer movement into the
surrounding flooring.
Semi-Rigid Joint Fillers
Semi-rigid fillers are commonly used in appropriate interior concrete
floor joints to support joint edges under hard-wheel traffic. They are
different from soft elastomeric sealants used where greater movement
capability is required.
Installation considerations include:
- Joint age and expected slab shrinkage
- Joint cleanliness and dryness
- Removal of old filler and contamination
- Required depth and use of backer material
- Correct mixing and dispensing
- Overfilling and shaving requirements
- Temperature during installation and service
- Compatibility with the flooring system
- Timing relative to coating application
Filling a joint too early in the slab's life may result in separation
as the concrete continues to shrink. Follow the project documents and
filler manufacturer's written requirements.
Repairing Spalls and Broken Joint Edges
Spalled areas must be cut or removed back to sound concrete. Feathered
edges are often weak and may break under traffic, so many repair
materials require a defined edge depth or geometry.
A repair sequence may include:
- Marking the repair boundary.
- Locating embedded utilities and reinforcement.
- Saw-cutting the perimeter when required.
- Removing loose and deteriorated concrete.
- Cleaning exposed concrete and steel as specified.
- Controlling active moisture and contamination.
- Applying primer or bonding material when required.
- Placing and consolidating the repair material.
- Finishing to the required elevation and texture.
- Allowing sufficient cure before preparation or coating.
The repair should restore the intended floor geometry without creating
an abrupt ridge, depression, or weak featheredge.
Select Repair Materials for the Actual Condition
Repair material selection should consider:
- Repair depth and volume
- Concrete temperature and environmental conditions
- Moisture condition of the substrate
- Required cure and return-to-service time
- Traffic, impact, vibration, and point loading
- Chemical and thermal exposure
- Movement and flexibility requirements
- Compatibility with the primer and flooring system
- Coefficient of thermal expansion
- Required surface profile after cure
A fast-curing repair is not automatically a suitable repair. It must
bond to the prepared concrete, remain dimensionally stable, tolerate
service, and accept the subsequent flooring system.
Cementitious and Resinous Repairs
Cementitious Repairs
Cement-based repair materials may be appropriate for deeper
repairs, resurfacing, or areas where their physical properties
match the concrete and service requirements.
Moisture content, cure time, surface profile, and compatibility
must be acceptable before resinous flooring is installed.
Resinous Repairs
Epoxy, polyurethane, polyurea, or other resinous materials may
offer rapid cure, strong adhesion, or specialized performance.
Their stiffness, thermal behavior, depth limitations, moisture
tolerance, and compatibility must match the condition and
flooring system.
Repairing Deep Voids and Large Areas
Large or deep repairs should not be filled indiscriminately with a
material intended for thin patches. Excessive placement thickness may
cause heat buildup, shrinkage, cracking, incomplete cure, or material
waste.
Confirm:
- Minimum and maximum placement thickness
- Whether aggregate extension is permitted
- Whether the repair must be placed in lifts
- Required primer or bonding agent
- Working time and heat generation
- Cure between lifts or subsequent flooring layers
- Required edge geometry and reinforcement treatment
Follow the repair-material manufacturer's written installation
instructions and the approved project detail.
Moisture and Contamination Can Defeat the Repair
Cracks, joints, and spalls often collect oil, chemicals, cleaning
solution, and water. A clean-looking opening may still contain
contamination below the surface.
Do not install repair material into active water, oily concrete, loose
residue, or conditions outside the manufacturer's limits. Control the
source, remove contaminated material, and obtain an approved repair
recommendation.
Repair Elevation Matters
A repair that is too high may remain visible and interfere with traffic.
A repair that is too low may create a depression, collect liquids, and
require additional flooring material.
Repairs should be finished to the elevation and slope required by the
completed flooring system. Consider:
- Specified flooring thickness
- Required drainage slope
- Transition to surrounding concrete
- Aggregate broadcast or mortar thickness
- Final texture and cleanability
- Door, drain, curb, and equipment clearances
Prepare Repairs Before Flooring Installation
Many cured repair materials require mechanical preparation before
priming or coating. Smooth resin, curing film, laitance, or irregular
edges may interfere with adhesion.
Before flooring installation:
- Confirm the repair has cured sufficiently.
- Remove surface film, contamination, and irregular high spots.
- Create the required profile without damaging the repair.
- Vacuum dust and loose debris.
- Check the perimeter for feathered or unbonded edges.
- Verify elevation and drainage.
- Inspect for shrinkage cracks, voids, or pinholes.
- Confirm compatibility with the flooring primer.
Repair Acceptance and Documentation
Repairs should be inspected before they are hidden by the flooring
system. Record:
- Repair identification and floor-plan location
- Condition and dimensions
- Preparation method
- Repair product and batch information
- Mixing and installation time
- Concrete and ambient conditions
- Primer or bonding agent used
- Placement depth and number of lifts
- Cure time before preparation and coating
- Photographs before, during, and after repair
- Final inspection and acceptance
Documentation is particularly important when cracks may reappear or
movement-related conditions remain.
Concrete-Repair Checklist
- Determine whether the condition is a crack, joint, spall, void, or structural concern.
- Identify expected movement and the intended function of each joint.
- Mark, photograph, measure, and map every repair.
- Obtain professional evaluation for settlement, displacement, or structural damage.
- Remove loose material, contamination, and failed previous repairs.
- Prepare sound edges and the required repair geometry.
- Control moisture, active water, oil, and chemical contamination.
- Select repair material compatible with depth, movement, service, and flooring.
- Follow required mixing, placement, cure, and thickness instructions.
- Finish repairs to the proper elevation and slope.
- Mechanically prepare cured repairs when required.
- Inspect for voids, cracking, unbonded edges, and elevation problems.
- Document materials, conditions, installation, and acceptance.
- Protect accepted repairs until flooring installation.
Key Takeaway
Successful repair begins with correct identification. Determine whether
the condition is a stable crack, moving crack, functioning joint, spall,
void, contamination problem, or structural concern. Then select a
compatible repair detail that supports the flooring system and expected
service.
Do not simply fill the opening. Repair the condition you actually have.
Knowledge Check
1. Why should cracks and joints not automatically receive the same
repair?
Show answer
Cracks are unplanned separations, while joints may be intentionally
designed to accommodate construction, shrinkage, isolation, or
movement. Their function and expected movement determine the repair.
2. Why should expansion and isolation joints generally remain functional?
Show answer
They are designed to permit movement. Rigidly coating across them
may transfer that movement into the surrounding flooring system and
cause cracking or separation.
3. Why are weak featheredges undesirable in a concrete repair?
Show answer
Thin, unsupported repair edges can break under traffic and create
weak transitions. Many repair materials require a defined minimum
edge depth.
4. Why might a cured repair need mechanical preparation?
Show answer
Smooth resin, laitance, curing film, contamination, or irregular
high spots may prevent proper adhesion of the primer and flooring.
5. When should a flooring contractor request professional structural
evaluation?
Show answer
When there is significant displacement, settlement, heaving,
recurring movement, severe deterioration, corroding reinforcement,
or damage that may affect structural capacity.
Technical References
Consult current editions, the project specification, and the repair and
flooring manufacturers' written requirements. Relevant references may
include:
- ACI 546R: Concrete Repair Guide.
-
ICRI Technical Guideline No. 310.2R: Selecting and
specifying concrete surface preparation for sealers, coatings,
polymer overlays, and concrete repair.
-
ASTM C881/C881M: Standard specification for
epoxy-resin-base bonding systems for concrete.
-
ASTM D4258: Standard practice for surface cleaning
concrete for coating.
-
ASTM D4259: Standard practice for preparation of
concrete by abrasion before coating application.
-
ASTM D7234: Standard test method for pull-off
adhesion strength of coatings on concrete using portable pull-off
adhesion testers.
-
Applicable project drawings, joint details, structural repair
instructions, and design-professional recommendations.
-
The repair-material, joint-filler, and flooring manufacturers'
current technical data sheets, safety data sheets, installation
instructions, and compatibility recommendations.
Standards and manufacturer instructions may be revised. Verify the
required edition and project requirements before using any reference.
