Three Symptoms—Not One Diagnosis

A blister is a raised area in a coating film. Delamination is a separation between layers or within a material. Efflorescence is a deposit of salts left after moisture reaches a surface and evaporates.

These conditions can appear together, but they are not interchangeable. Efflorescence can indicate moisture and salt movement without coating failure. A delaminated coating may have no blisters. A blister can form for reasons unrelated to osmosis.

What Is Osmotic Blistering?

Osmosis is the movement of water through a semi-permeable material toward a solution containing a greater concentration of dissolved material. In a coating system, the coating film can act as the semi-permeable layer.

If water-soluble salts, contaminants, uncured components, or other dissolved materials are present beneath the coating, water can move toward that concentrated solution. Liquid accumulates at the interface, pressure develops, and the coating lifts into a blister.

Moisture may originate within the slab, beneath the slab, from cleaning operations, from exterior water intrusion, or from another source. The dissolved material may come from the concrete, surface contamination, preparation residue, coating components, or previous chemical exposure.

Conditions Needed for Osmotic Blistering

• A source of water or continuing moisture
• A coating film through which water can move
• Water-soluble material beneath or within the coating system
• A difference in dissolved-material concentration
• Sufficient coating adhesion and flexibility for pressure to create a blister

Removing only the blister does not remove these conditions. If the source of moisture and soluble material remains, additional blisters may form.

Not Every Blister Is Osmotic

What the Blister Can Tell You

The blister's size, frequency, distribution, contents, and failure plane can provide valuable clues. ASTM D714 provides a standardized method for evaluating the degree of blistering by size and frequency.

Document the Following

• Blister size and frequency
• Whether blisters are isolated or widespread
• Location relative to cracks, joints, walls, drains, and equipment
• Whether the blister is dry, gas-filled, or fluid-filled
• Color, clarity, odor, and apparent viscosity of any fluid
• pH and conductivity when qualified testing is performed
• Material attached to the underside of the removed coating
• Condition of the exposed concrete
• Whether the pattern changes with temperature, rainfall, or operations

Preserving Blister Evidence

Do not open every blister before the condition is documented. Photograph the area from a distance, at medium range, and close up. Include a ruler or other scale where useful.

Mark representative blister locations on a floor plan. When a formal investigation may follow, preserve unopened blisters and arrange for controlled sampling by the appropriate qualified professional.

If a blister is opened, use clean tools and containers when collecting fluid. Record who collected the sample, the exact location, date, time, and how it was stored.

Understanding Delamination

Delamination means that a bond has failed or a material has separated within itself. The most important initial question is: Where did the separation occur?

This location is called the plane of failure. It can reveal whether the immediate weakness was at the concrete-coating interface, between coating layers, within the concrete, or within a patching or leveling material.

Moisture and Delamination

Moisture can contribute to delamination by weakening susceptible adhesives, carrying alkaline compounds to the bond line, supporting osmotic pressure, or preventing proper adhesion during application.

Moisture can also damage the concrete surface itself. If near-surface concrete becomes weak or deteriorated, a strongly bonded coating may detach with a layer of concrete attached.

However, moisture should not be used as a convenient explanation for every bond failure. Poor profile, dust, curing compounds, sealers, oil, mixing error, missed recoat windows, and incompatible materials must also be evaluated.

Pull-Off Adhesion Testing

ASTM D7234 describes pull-off testing of coatings on concrete using portable adhesion testers. A loading fixture is bonded to the coating, the surrounding area may be cut according to the applicable procedure, and tensile force is applied perpendicular to the surface.

The numerical strength is useful, but the failure location is equally important. Two tests can produce similar strength values while failing in entirely different layers.

Pull-Off Test Records Should Include

• Test location and identification number
• Coating system and approximate thickness
• Dolly size and adhesive used
• Surface preparation for bonding the dolly
• Tester manufacturer, model, and calibration information
• Loading rate and test procedure
• Maximum recorded stress
• Percentage of each failure type across the test surface
• Photographs of the dolly, coating, and substrate after testing
• Environmental and substrate conditions

What Is Efflorescence?

Efflorescence is a generally white or light-colored crystalline deposit formed when moisture dissolves salts, transports them toward an exposed surface, and then evaporates. The salts remain after the water leaves.

Efflorescence demonstrates that moisture and dissolved material have moved through the concrete or adjoining construction. It does not, by itself, identify the water source.

Possible Moisture Sources

• Ground moisture beneath a slab
• Missing or damaged underslab vapor protection
• Rain entering through walls, roofs, or openings
• Plumbing or process-water leaks
• Washdown and sanitation operations
• Below-grade water intrusion
• Moisture remaining in new concrete
• Condensation or repeated surface wetting

Efflorescence Versus Other Deposits

Not every white deposit is efflorescence. The material may be coating degradation, cleaner residue, process contamination, mineral scale, chemical reaction products, or dust.

Location, solubility, appearance, moisture pattern, facility operations, and laboratory analysis may be needed to identify the deposit. Do not select a cleaning chemical until the material and substrate compatibility have been evaluated.

Why Cleaning Alone Often Fails

Removing efflorescence improves appearance but does not stop the moisture transporting the salts. If the source remains, the deposit may return.

Cleaning with water can dissolve the visible salts and carry them back into the concrete. Acid cleaning can introduce additional water, alter the concrete surface, create reaction products, and present worker and disposal hazards.

The repair plan should address the moisture source, remove deposits using an approved method, verify surface condition, and select a compatible system.

Mapping the Failure Pattern

Failure-Investigation Process

1. Make the area safe. Control loose flooring, trip hazards, sharp edges, chemicals, and wet surfaces.
2. Document undisturbed conditions. Take overview and close-up photographs before opening blisters or removing materials.
3. Map the distress. Relate it to walls, cracks, joints, drains, equipment, doorways, and previous repairs.
4. Classify the visible symptoms. Record blister size and frequency, delamination, deposits, softening, discoloration, and dampness.
5. Determine the failure plane. Identify which material or interface separated.
6. Preserve samples. Retain coating, adhesive, concrete, deposits, and blister fluid when appropriate.
7. Review project records. Examine product data, preparation records, moisture results, environmental logs, batch numbers, film thickness, and cure times.
8. Investigate moisture sources. Evaluate the slab, ground, leaks, exterior drainage, cleaning, process operations, and condensation.
9. Perform appropriate tests. Consider moisture, pH, soluble material, adhesion, surface strength, contamination, and laboratory analysis.
10. Develop a written repair plan. Correct the underlying cause before replacing the failed system.

What Not to Do

• Do not call every blister osmotic without supporting evidence.
• Do not blame moisture without considering preparation and application.
• Do not destroy all evidence before sampling.
• Do not assume efflorescence identifies the exact water source.
• Do not grind and spot-patch while moisture continues to enter the slab.
• Do not wash salts into the concrete without an approved procedure.
• Do not apply acid automatically to an alkaline or salt-contaminated surface.
• Do not report pull-off strength without reporting the failure plane.
• Do not guarantee that a topical membrane will stop hydrostatic pressure.
• Do not begin repairs until responsibilities and the repair scope are documented.

Developing the Repair Plan

A lasting repair must address the moisture source, soluble material, damaged concrete, failed coating, surface preparation, crack and joint details, and compatibility of the replacement system.

Depending on the findings, corrective work may include drainage repair, leak correction, removal of contaminated concrete, mechanical preparation, crack treatment, moisture mitigation, waterproofing, new patching, and installation of a complete compatible coating or flooring system.

A small mock-up or test area may be appropriate, but a successful short-term test patch does not automatically demonstrate long-term performance under changing moisture and service conditions.

Contractor Investigation Checklist

• Photograph the undisturbed failure.
• Map blistering, delamination, and deposits.
• Rate blister size and frequency when required.
• Record whether blisters contain liquid.
• Identify and photograph the plane of failure.
• Preserve representative samples.
• Review moisture, pH, and environmental records.
• Investigate leaks, drainage, washdown, and groundwater.
• Review surface preparation and coating application records.
• Confirm film thickness, mix ratio, cure, and recoat intervals.
• Obtain qualified laboratory or consulting support when needed.
• Correct the cause before installing the replacement system.

Knowledge Check

Technical References

Use the editions required by the project specification and follow current manufacturer instructions for testing, evaluation, and repair.

• ASTM D714-25 - Standard Test Method for Evaluating Degree of Blistering of Paints.
• ASTM D7234-22 - Standard Test Method for Pull-Off Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers.
• 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 F710 - Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring.
• ICRI Guideline No. 310.2R - Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair.
• ICRI Guideline No. 710.3 - Guide for the Mitigation of Moisture in Concrete Floor Slabs.
• Current technical data sheets, installation instructions, safety data sheets, and failure-analysis guidance issued by the specified coating, flooring, adhesive, repair, and moisture-mitigation manufacturers.

These references provide technical guidance but do not replace the project specification, governing regulations, laboratory analysis, manufacturer requirements, or evaluation by a qualified professional. Final failure conclusions and repair selections must be based on the complete body of available evidence.