AirSprayTech Academy Certificate Program
Moisture in Concrete: When a Lining Should Not Be Applied
Protective Linings for Industrial Coating Contractors - Article 13 of 20
Concrete can look dry while containing enough moisture to interfere with primer penetration, lining cure, and long-term adhesion. Contractors must identify where the moisture comes from, use the specified test method, and know when the correct decision is to stop rather than coat.
Return to the Course OverviewDry-Looking Concrete Can Still Be Wet
Concrete contains a network of pores and capillaries capable of holding and transporting moisture. A surface can appear dry while moisture remains deeper in the slab, wall, tank, or containment structure.
Moisture can move toward the surface after a lining is applied. Because many linings have low permeability, vapor or liquid pressure can build at the concrete-lining interface.
The result may be pinholes, bubbles, blisters, soft primer, loss of adhesion, discoloration, or complete delamination.
Where Concrete Moisture Comes From
Before deciding how to handle moisture, determine its likely source.
- Water remaining from the original concrete mixture
- Wet curing or recent cleaning
- Groundwater beneath a slab or behind a wall
- A missing, damaged, or ineffective vapor retarder
- Rain, flooding, or weather exposure
- Leaking pipes, joints, roofs, tanks, or process equipment
- Water entering through cracks or penetrations
- Hydrostatic pressure from the opposite side of the structure
- Condensation caused by environmental conditions
- Normal process or washdown operations
Drying may solve moisture left from cleaning. Drying will not permanently correct groundwater, an active leak, or hydrostatic pressure.
Moisture Can Damage a Lining in Several Ways
Loss of adhesion
Moisture can prevent primer from properly wetting or penetrating the concrete. Some lining materials may react with water or fail to develop their intended bond.
Blistering
Moisture vapor or dissolved materials can create pressure beneath a relatively impermeable lining. Blisters may develop soon after installation or after the structure enters service.
Osmotic effects
Soluble materials in the concrete or at the interface can attract moisture through the lining. This movement can contribute to liquid-filled blisters and loss of adhesion.
Pinholes and bubbles
Warming concrete can release air and moisture vapor from its pores. When primer or lining is still wet, the escaping material can create pinholes, craters, or bubbles.
Incomplete or defective cure
Moisture can interfere with the cure of certain resin systems. The resulting material may remain soft, cloudy, tacky, foamed, or poorly bonded.
Surface Moisture Is Not the Whole Story
Different test methods evaluate different aspects of concrete moisture. A test that indicates no moisture at the surface does not necessarily prove that the concrete contains no internal moisture or future vapor drive.
| Test or Observation | What It Can Indicate | Important Limitation |
|---|---|---|
| Visual inspection | Visible dampness, darkening, deposits, or active leakage | Cannot establish acceptable internal moisture |
| Plastic-sheet indication | Moisture developing beneath a sealed sheet | Qualitative indication; not a universal acceptance test |
| Non-destructive electronic meter | Comparative readings and possible moisture variation | Readings may be influenced by concrete composition, depth, and embedded materials |
| In-situ relative humidity | Relative humidity at a specified depth inside a floor slab | Must follow the test procedure and may not apply to every wall, tank, or structure |
| Calcium-chloride test | Moisture-vapor emission from the tested floor area during the test period | Does not directly measure moisture deeper within the slab |
| Core, probe, or project-specific testing | Condition at selected depths or locations | Requires an approved procedure and qualified interpretation |
The Plastic-Sheet Method
A plastic sheet sealed to concrete can provide a qualitative indication of moisture developing at the surface. After the specified exposure period, the area is examined for visible moisture or darkening.
A passing plastic-sheet observation does not prove that the concrete meets every lining manufacturer's moisture requirement. Temperature, test duration, location, recent surface drying, and environmental conditions can influence what appears beneath the sheet.
Use this method only for the purpose allowed by the specification and the lining manufacturer.
In-Situ Relative-Humidity Testing
In-situ relative-humidity testing uses probes placed in holes at specified depths in a concrete floor slab. When performed correctly, the method provides information about the slab's internal moisture condition.
The technician must follow the current test standard for hole depth, quantity, location, cleaning, conditioning, calibration, equilibration, and reporting. Improperly placed or contaminated holes can produce misleading results.
This method was developed for concrete floor slabs. Applying its results to tank walls, vertical concrete, overhead surfaces, or unusual structures requires project-specific direction.
Moisture-Vapor Emission Testing
A calcium-chloride test measures the amount of moisture vapor emitted from a defined concrete floor area during the test period. The result is expressed as a moisture-vapor emission rate.
Surface preparation, building conditions, test placement, and test duration affect the validity of the result. The test does not directly measure all moisture stored deeper in the concrete.
The specified test method and manufacturer's published limit must be used. Do not convert an emission-rate result into an assumed relative-humidity value.
Electronic Moisture Meters
Non-destructive electronic meters can help locate areas that appear wetter or drier than surrounding concrete. They are useful for comparative surveys and identifying locations that may require additional testing.
Reinforcing steel, conductive materials, concrete density, aggregate, surface condition, and instrument design can affect readings. Unless specifically approved as the acceptance method, an electronic meter should not be used by itself to release the concrete for lining.
Hydrostatic Pressure Is a Different Problem
Hydrostatic pressure occurs when liquid water pushes through or against the concrete. It may be found in below-grade walls, pits, tunnels, basins, tanks, and slabs exposed to groundwater.
A coating applied to the side opposite the water source may be subjected to pressure trying to push it away from the concrete. Drying the visible surface does not remove that pressure.
Warning signs include:
- Active seepage or flowing water
- Recurring damp areas after drying
- Efflorescence or mineral deposits
- Water entering through cracks and joints
- Previous coatings blistered or detached with damp concrete beneath
- Seasonal changes related to rainfall or groundwater
Moisture Behind Walls and Containment Structures
Moisture testing is often discussed in connection with floor slabs, but tanks, walls, containment structures, and wastewater facilities present additional challenges.
Moisture can enter through soil, cracks, pipe penetrations, joints, leaking equipment, or the exterior face of the structure. Test methods developed for interior floor coverings may not fully describe these conditions.
The owner, designer, lining manufacturer, or qualified moisture professional should establish the evaluation method and acceptance criteria for unusual structures.
Recent Washing and Waterjet Preparation
Pressure washing, waterjetting, wet abrasive blasting, and wet cleaning add moisture to the concrete. Even when no standing water remains, pores and cracks may still contain water.
Drying time depends on concrete density, depth of wetting, temperature, humidity, airflow, structure thickness, and drainage. A fixed waiting period does not prove that the substrate meets the lining requirements.
Retest after wet preparation using the required method. Do not rely only on surface color.
Temperature and Outgassing
Concrete pores contain air and moisture vapor. As the substrate warms, this material expands and can escape into a wet primer or lining.
Applying during stable or falling substrate temperature may reduce outgassing, provided all manufacturer and specification limits are met. This practice does not correct excessive internal moisture, active leakage, or hydrostatic pressure.
Porous concrete may also require an approved pore-filling primer, grout coat, or resurfacing material. These materials must be selected as part of the complete lining system.
When the Contractor Should Stop
Do not apply the lining when:
- Required moisture testing has not been completed.
- Test results exceed the manufacturer's or specification's limits.
- Different test methods provide unexplained conflicting results.
- Active leakage, seepage, or standing water is present.
- Hydrostatic pressure is suspected but has not been evaluated.
- The concrete was recently washed and has not been retested.
- Efflorescence or dampness returns after cleaning.
- The moisture source has not been identified.
- Environmental conditions can produce condensation.
- The manufacturer has not approved the proposed moisture-mitigation system.
- The project team has not provided written direction for an unresolved condition.
Moisture-Mitigation Systems
Certain products are designed to reduce the effect of moisture vapor on flooring or coating systems. These may include moisture-tolerant primers, resin-based mitigation layers, cementitious systems, drainage systems, or other engineered assemblies.
A mitigation product is not permission to coat every wet surface. Its use depends on:
- The moisture source and severity
- The substrate and structure type
- The test results and approved limits
- The intended lining and chemical service
- Positive-side or negative-side water pressure
- Surface preparation and repair requirements
- Compatibility between every layer
- Written manufacturer approval and warranty conditions
Moisture-vapor management is a specialized subject. Contractors should not create a field-designed system by combining unrelated products.
Test Locations Matter
Moisture is rarely distributed evenly. Test locations should represent the complete work area and include locations likely to have higher moisture.
Consider testing near:
- Exterior walls and below-grade areas
- Drains, trenches, pits, and sumps
- Cracks and construction joints
- Plumbing and process penetrations
- Previously blistered or delaminated coatings
- Areas exposed to leaks or washdown
- Different concrete placements and repair areas
Record each test location so the result can be tied to a specific part of the structure.
Test Conditions Must Represent Service
Building temperature, humidity, ventilation, and equipment operation can affect moisture behavior and test results. Testing an open, unconditioned building may not represent conditions after the building is enclosed and operating.
Follow the conditioning requirements of the selected test method. Document air temperature, surface temperature, relative humidity, test duration, instrument identification, calibration status, location, and result.
Do Not Invent a Universal Moisture Limit
There is no single moisture number that approves every lining over every type of concrete. Different products tolerate different conditions, and different tests produce different measurements.
Acceptance must be based on:
- The exact test method specified
- The lining manufacturer's published limit
- The project specification
- The structure and service environment
- Written approval for any proposed mitigation system
Document the Decision
A moisture report should include:
- Project and structure identification
- Date, time, and technician
- Concrete age and known construction information
- Recent cleaning, weather, leaks, and process exposure
- Test method and current standard followed
- Instrument identification and calibration information
- Number and location of tests
- Temperature and humidity conditions
- Individual results, not only an average
- Manufacturer or specification acceptance limits
- Areas released, rejected, or requiring additional investigation
If the work proceeds under a written exception or approved mitigation plan, attach that authorization to the project record.
Contractor Field Checklist
- Has the likely source of moisture been identified?
- Is active leakage or hydrostatic pressure present?
- Is the required test method stated in the specification?
- Are the required test locations and quantities established?
- Are instruments calibrated and technicians qualified?
- Do test conditions comply with the method?
- Are individual results below the approved product limit?
- Has concrete been retested after washing or waterjetting?
- Have recurring dampness and efflorescence been investigated?
- Has the lining manufacturer approved any mitigation system?
- Are all results and decisions documented?
- Has the moisture hold point been released before priming?
Knowledge Check
1. Can concrete be approved because it looks and feels dry?
2. Are relative humidity and moisture-vapor emission the same measurement?
3. Will drying the surface correct hydrostatic pressure?
4. Can an electronic meter be useful even when it is not the acceptance test?
5. Who establishes the acceptable moisture limit?
Key Takeaway
When moisture conditions are unknown or unacceptable, the professional decision is to stop the lining application.
Contractors must identify the moisture source, use the specified test method, compare results with the exact lining requirements, document the findings, and obtain written approval before proceeding. A rushed coating schedule does not make wet concrete dry.
Technical References
- ASTM International, ASTM D4263 - Indicating Moisture in Concrete by the Plastic Sheet Method .
- ASTM International, ASTM F2170 - Determining Relative Humidity in Concrete Floor Slabs Using In-Situ Probes .
- International Concrete Repair Institute, Concrete Slab Moisture Testing Program .
- International Concrete Repair Institute, ICRI Guideline 710.3 - Guide for the Mitigation of Moisture in Concrete Floor Slabs .
- AMPP, Concrete Moisture Testing and Mitigation .
- AMPP, Avoiding Premature Coating Failures on Concrete .
Standards, product limits, and testing procedures can change. Consult the current editions and the current instructions for the exact lining system. Final lining selection, moisture acceptance, mitigation, and service suitability must be confirmed by the specification, manufacturer, owner, designer, or qualified moisture or corrosion professional.
Coming Next
Article 14 of 20 - Environmental Conditions, Dew Point, and Condensation Control
The next article explains how air temperature, surface temperature, relative humidity, dew point, ventilation, weather changes, and condensation affect lining application and cure.