AirSprayTech Academy Certificate Program
Commercial and Industrial Floor Coatings for Professional Contractors
Concrete Moisture and Floor-Coating Failure
Article 04 of 24
Concrete can look dry while still containing enough moisture to damage
a flooring system. Professional contractors evaluate moisture before
installation, document the results, and compare them with the flooring
manufacturer's written acceptance requirements.
Dry-Looking Concrete Is Not Proof of a Dry Slab
Concrete is porous. Its internal pore structure can hold moisture and
permit moisture vapor to move through the slab. That movement may
continue even when the floor surface looks and feels dry.
Once a low-permeability flooring system is installed, the drying
conditions at the top of the slab change. Moisture and soluble
compounds can accumulate beneath the new system. If the selected
flooring cannot tolerate those conditions, blistering, loss of
adhesion, staining, efflorescence, or other failures may follow.
Moisture testing does not guarantee performance, but installing without
appropriate investigation leaves the contractor and owner without
critical information.
Where Concrete Moisture Comes From
Concrete moisture is not limited to newly placed slabs. Both new and
existing concrete may be affected by multiple sources.
Moisture Within the Concrete
- Water originally used in the concrete mixture
- Water introduced during curing
- Cleaning or process water absorbed by the slab
- Leaks, flooding, or exterior weather exposure
- Moisture trapped beneath an existing flooring system
Moisture From Below
- Soil moisture beneath an on-ground slab
- A missing, damaged, or ineffective vapor retarder
- Groundwater or poor site drainage
- Plumbing or process leaks below the slab
- Changes in building drainage or surrounding grade
Moisture conditions can change with seasons, HVAC operation, rainfall,
groundwater, building use, and cleaning practices. One result describes
conditions at a particular location and time; it does not describe
every possible future condition.
Moisture Vapor Is Not the Same as Hydrostatic Pressure
These terms are frequently used as if they mean the same thing, but
they describe different conditions.
| Condition |
General Description |
| Moisture vapor movement |
Water vapor moves through the concrete pore structure in
response to moisture and vapor-pressure differences.
|
| Capillary moisture |
Liquid water moves through connected pores because of
capillary action.
|
| Hydrostatic pressure |
Liquid water applies pressure against the slab or structure,
commonly because of groundwater, water accumulation, or
inadequate drainage.
|
| Surface condensation |
Water forms on the concrete when its surface temperature is
at or below the dew point of the surrounding air.
|
A topical moisture-mitigation product intended for vapor control should
not automatically be assumed capable of resisting active water
intrusion or hydrostatic pressure. Active water must be identified and
corrected through appropriate drainage, waterproofing, repair, or
engineering measures.
How Moisture Contributes to Flooring Failure
Moisture-related failure does not always produce the same appearance.
Several mechanisms may operate alone or together.
Loss of Adhesion
Moisture, alkalinity, contamination, weak concrete, or pressure beneath
the flooring system may reduce adhesion at the concrete-to-coating
interface. The system may peel away cleanly or remove a weak layer of
concrete with it.
Osmotic Blistering
Soluble materials beneath a relatively impermeable coating can attract
moisture and contribute to pressure within blisters. Contamination,
salts, cleaning residue, and other soluble compounds may be involved.
Efflorescence and Mineral Deposits
Moisture moving through concrete may transport dissolved salts. When
the water evaporates, white or light-colored mineral deposits can
remain at cracks, joints, coating defects, or exposed concrete.
Alkaline Conditions
Moisture can transport alkaline compounds toward the surface.
Prolonged exposure to elevated alkalinity may attack materials that
are not designed for those conditions.
Incomplete Cure or Property Development
Some flooring materials are sensitive to moisture, condensation, or
environmental conditions during application and early cure. The result
may be poor adhesion, discoloration, surface defects, or incomplete
development of the intended properties.
Recognize Common Warning Signs
These conditions do not prove that moisture is the only cause, but they
justify further investigation:
- Blisters or bubbles beneath the flooring
- Peeling or loss of adhesion
- Darkened concrete beneath removed coating
- White crystalline deposits or efflorescence
- Recurring dampness at cracks or joints
- Discoloration that returns after cleaning
- Corrosion or staining around embedded metal
- Failures concentrated along cracks, joints, drains, or slab edges
- Successful flooring in one area and failure in another
- Failures that worsen seasonally or after HVAC changes
Moisture-related symptoms should be evaluated together with coating
adhesion, substrate condition, contamination, preparation, system
compatibility, film thickness, and installation records.
Common Concrete-Moisture Evaluation Methods
No single test answers every moisture question. The project
specification and flooring manufacturer should establish the required
test method, number of tests, test locations, conditioning, and
acceptance criteria.
| Method |
What It Indicates |
Important Limitation |
| In-situ relative-humidity testing |
Measures relative humidity within the concrete slab at a
prescribed depth under standardized conditions.
|
Requires correct hole depth, preparation, equilibration,
location, calibration, and reporting.
|
| Anhydrous calcium-chloride testing |
Estimates the moisture-vapor emission rate from a defined
concrete surface area during the test period.
|
Measures conditions near the slab surface and must be used
only within the method's applicable conditions.
|
| Plastic-sheet indication |
May reveal visible darkening or condensation beneath a
sealed plastic sheet.
|
It is qualitative and should not be treated as a numerical
moisture-acceptance test.
|
| Electrical moisture meter |
Can assist with comparative surveying and locating areas
that deserve further investigation.
|
Readings can be affected by concrete composition, depth,
salts, reinforcement, and instrument design.
|
| Surface temperature and dew point |
Helps determine whether condensation may form during
preparation, application, or cure.
|
Does not measure internal slab moisture or moisture vapor
movement through the concrete.
|
In-Situ Relative-Humidity Testing
ASTM F2170 testing uses probes placed in drilled holes to evaluate
relative humidity within a concrete floor slab. The prescribed test
depth depends on the slab's drying conditions.
Reliable testing requires:
- The required number of tests for the project area
- Locations that represent the slab and known problem areas
- Correct hole depth and diameter
- Removal of drilling dust from the hole
- Proper sleeves, probes, and sealing
- Required equilibration before readings
- Calibrated equipment with current records
- Documented slab and ambient conditions
The measured value must be compared with the flooring manufacturer's
written limit for the complete system being considered. A limit for
one product should not automatically be applied to a different product
or manufacturer.
Anhydrous Calcium-Chloride Testing
ASTM F1869 testing estimates the amount of moisture emitted from a
defined area of concrete during a specified test period. Results are
commonly reported as pounds of moisture per 1,000 square feet over
24 hours.
Test accuracy depends on following the standard, including:
- Proper slab and building conditioning
- Mechanical preparation of the test area as required
- Accurate timing and weighing
- A properly sealed test enclosure
- Suitable spacing and number of tests
- Documentation of ambient conditions and test locations
This method evaluates conditions near the concrete surface during the
test period. It does not directly describe moisture conditions
throughout the slab.
The Plastic-Sheet Method
ASTM D4263 uses a plastic sheet sealed to the concrete for a specified
period. Visible condensation or darkening beneath the sheet indicates
moisture is present.
The absence of visible moisture does not prove that the slab meets a
flooring manufacturer's numerical moisture requirements. The method is
best understood as an indication test, not a substitute for required
quantitative testing.
Testing Must Represent the Floor
One test near the entrance cannot describe a large building. Moisture
conditions may vary because of:
- Different concrete placements or slab thicknesses
- Changes in the vapor retarder
- Exterior walls, doors, and loading docks
- Drains, trenches, plumbing, or process equipment
- Previous flooring or adhesive
- Leaks, flooding, or cleaning practices
- Different HVAC conditions
- Sunlight, shade, or seasonal exposure
- Local repairs and replaced concrete
The testing plan should include representative areas and known problem
locations. Every test must be identified on a floor plan so the result
can be connected to a specific location.
Building Conditions Affect Test Results
Moisture tests should be performed under the environmental conditions
required by the applicable test method and project specification.
Testing an unconditioned building and then installing flooring after
the HVAC system changes the environment may produce misleading
expectations.
Record at least:
- Air temperature
- Relative humidity
- Concrete surface temperature
- HVAC operating condition
- Exterior doors and windows
- Recent cleaning, leaks, or weather exposure
- Date, time, test location, and instrument identification
Do Not Invent an Acceptance Limit
A moisture number has meaning only when it is compared with a written
acceptance requirement for the proposed flooring system. The
contractor should not create a limit, rely on an unrelated product's
limit, or assume that a primer automatically makes every moisture
condition acceptable.
Obtain the flooring manufacturer's current written requirements. When
results exceed those requirements, stop and obtain written direction
before proceeding.
Moisture-Mitigation Systems
A moisture-mitigation system is generally installed beneath the
flooring system to reduce the effect of moisture vapor and associated
conditions. It is not simply an extra coat of ordinary primer.
A specified mitigation system may require:
- Mechanical preparation to a defined concrete surface profile
- Removal of all coatings, adhesives, curing compounds, and contamination
- Sound concrete with properly treated cracks and penetrations
- Specified temperature and environmental conditions
- Accurate mixing and application at the required coverage
- Pinholes, holidays, or porosity to be corrected
- Compatible primers, underlayments, and flooring materials
- Inspection and documentation before subsequent layers
Confirm whether the system is intended for the measured moisture
condition, concrete alkalinity, slab type, service environment, and
proposed flooring. Obtain written manufacturer approval when project
conditions fall outside published requirements.
When the Floor Should Not Be Coated
Installation should be delayed when:
- Required moisture testing has not been completed
- Test results exceed the system's written limits
- Active water or hydrostatic pressure is present
- The source of recurring moisture is unknown
- Condensation is present or likely during application
- The concrete or environment is outside application limits
- Moisture-mitigation requirements have not been resolved
- The owner has not provided written direction for a known risk
A schedule deadline does not change the condition of the concrete.
Document the results and obtain a technically supportable resolution
before installation.
Moisture Documentation Checklist
- Identify the proposed flooring and mitigation systems.
- Obtain the manufacturers' current moisture-acceptance limits.
- Identify the required test methods and responsible party.
- Confirm required building conditioning before testing.
- Prepare a test-location plan for representative and suspect areas.
- Use calibrated instruments and retain calibration information.
- Record test depth, location, date, time, and environmental conditions.
- Photograph and map each test location.
- Compare results with the written limits for the complete system.
- Document leaks, standing water, efflorescence, and other warning signs.
- Obtain written recommendations when results exceed published limits.
- Retain reports with the project installation and quality records.
Key Takeaway
Concrete can appear dry while still presenting a serious moisture risk.
Use the required test methods, test representative locations, document
the building conditions, and compare results with the flooring
manufacturer's written requirements.
Moisture testing does not remove risk. It reveals information
needed to manage that risk.
Knowledge Check
1. Why is a dry-looking concrete surface not proof that the slab is
ready for coating?
Show answer
Concrete can contain and transmit moisture through its internal
pore structure even when no moisture is visible at the surface.
2. What is the difference between moisture vapor movement and
hydrostatic pressure?
Show answer
Moisture vapor movement involves water vapor moving through the
concrete. Hydrostatic pressure involves liquid water exerting
pressure against the slab or structure.
3. What does ASTM F2170 testing evaluate?
Show answer
It evaluates relative humidity within the concrete slab using
in-situ probes installed at the prescribed depth.
4. Why is the plastic-sheet method not a substitute for required
quantitative moisture testing?
Show answer
It provides a qualitative indication of visible moisture or
darkening but does not provide the numerical result required by
many flooring manufacturers.
5. What should happen when test results exceed the flooring system's
written moisture limit?
Show answer
Installation should stop until the condition is resolved through
drying, an approved mitigation system, system redesign, or other
written direction from the appropriate parties.
Technical References
Consult current editions, the project specification, and the flooring
manufacturer's written requirements. Relevant references may include:
-
ASTM F2170: Standard test method for determining
relative humidity in concrete floor slabs using in-situ probes.
-
ASTM F1869: Standard test method for measuring
moisture vapor emission rate of concrete subfloor using anhydrous
calcium chloride.
-
ASTM D4263: Standard test method for indicating
moisture in concrete by the plastic-sheet method.
-
ASTM F710: Standard practice for preparing
concrete floors to receive resilient flooring.
-
ASTM D4258: Standard practice for surface
cleaning concrete for coating.
-
ICRI Technical Guideline No. 310.2R: Selecting
and specifying concrete surface preparation for sealers, coatings,
polymer overlays, and concrete repair.
-
The flooring and moisture-mitigation manufacturers' current
technical data sheets, safety data sheets, testing requirements,
limitations, and written system recommendations.
Standards and manufacturer instructions may be revised. Verify the
required edition and project requirements before using any reference.
