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Water and Wastewater Protective Coating Systems | Article 13 of 24

Concrete Moisture, Outgassing, Dew Point, and Environmental Control

Moisture and changing environmental conditions can produce pinholes, bubbles, loss of adhesion, condensation, incomplete cure, and premature lining failure even when the substrate appears properly prepared.

Concrete Is Not a Sealed, Inert Surface

Hardened concrete contains pores, capillaries, cracks, and voids. These spaces can hold water, water vapor, and air. Moisture may enter from the original concrete mixture, rainfall, cleaning, groundwater, leaking process equipment, immersion service, or the soil surrounding a buried structure.

When an impermeable or low-permeability lining is applied, moisture and air within the concrete do not simply disappear. Their movement must be understood and controlled before the lining is installed.

Sources of Concrete Moisture

• Water remaining from concrete placement and curing
• Water used for cleaning, waterjetting, or surface preparation
• Rain, flooding, leakage, or condensation
• Groundwater or moisture moving through below-grade walls and floors
• Process water entering through cracks, joints, penetrations, or failed waterstops
• Moisture trapped behind an existing coating or repair
• Water introduced by damp repair mortar or cementitious resurfacing
• High-humidity air absorbed by a porous prepared surface

Moisture Content, Moisture Vapor, and Hydrostatic Pressure Are Not the Same

Moisture content describes water present within the concrete at the time and location of measurement.

Moisture-vapor movement is the movement of water in vapor form through the concrete in response to differences in vapor pressure.

Hydrostatic pressure results from liquid water exerting pressure against or through the structure.

A product described as moisture tolerant may accept a damp surface during application. That description does not automatically mean it can resist active leakage, hydrostatic pressure, or continuing negative-side moisture.

Understand Positive-Side and Negative-Side Moisture

The positive side is the side from which the coating is intended to resist the service liquid. The negative side is the opposite side of the concrete.

A lining inside a tank may successfully resist the stored water while moisture entering through an exterior wall moves toward the back of the lining. This negative-side moisture can carry dissolved salts and create pressure at the coating-to-concrete interface.

Investigate groundwater, exterior drainage, wall penetrations, cracks, construction joints, and adjacent process areas. A coating should not be expected to correct an uncontrolled water source that lies behind it.

Moisture Testing Must Match the Structure

No single moisture test answers every question. Some methods indicate surface or near-surface moisture. Others evaluate conditions deeper within a floor slab. Results apply to the locations and times tested and should be interpreted in relation to the structure, exposure, coating system, and manufacturer’s limits.

Before testing, identify the required method, instrument, calibration, test locations, quantity, conditioning time, environmental conditions, acceptance limit, and reporting procedure.

Plastic-Sheet Moisture Indication

ASTM D4263 uses a sealed plastic sheet to indicate capillary moisture in concrete. Darkening of the concrete or moisture beneath the sheet indicates that moisture is present under the test conditions.

This method is an indication, not a quantitative measurement of moisture content, internal relative humidity, vapor-emission rate, or hydrostatic pressure. A surface that does not show visible moisture beneath the sheet is not automatically suitable for every coating system.

Use the method only as permitted by the specification and coating manufacturer, and report the environmental conditions, locations, duration, and observed results.

Relative-Humidity Testing in Concrete Slabs

ASTM F2170 provides a quantitative method for measuring relative humidity within concrete floor slabs using in-situ probes. This test is commonly associated with flooring work and can provide useful information about moisture within a slab.

The measured relative humidity should be compared with the written limit established for the complete coating or flooring system. The contractor should not substitute a generic percentage obtained from another product or project.

In-situ slab testing is not automatically applicable to vertical tank walls, ceilings, curved structures, or every water and wastewater application. Confirm that the selected method is appropriate for the structure being evaluated.

Electronic Moisture Meters

Non-destructive electronic meters can help compare moisture conditions across a surface and identify areas that differ from surrounding concrete. Readings can be influenced by aggregate, salts, reinforcement, surface profile, temperature, density, thickness, and instrument design.

Unless the applicable procedure and manufacturer establish otherwise, use these instruments primarily for comparative surveying rather than treating the displayed value as an absolute concrete-moisture percentage.

Moisture Testing Is a Snapshot

Moisture conditions can change after testing. Rain, groundwater, leaking valves, cleaning operations, temperature changes, loss of ventilation, and return of nearby processes can alter the substrate.

Reinspect and retest when conditions change, when the surface has been exposed to new moisture, or when the time permitted between testing and coating has been exceeded.

What Is Concrete Outgassing?

Outgassing occurs when air or vapor within concrete pores moves outward while a primer, resurfacer, or lining is still wet. The escaping gas can form bubbles, craters, pinholes, or channels through the applied material.

Porous concrete, bugholes, recently repaired areas, rapid temperature increases, direct sunlight, heated enclosures, and changes in atmospheric pressure can increase the risk.

Rising Temperature Increases Outgassing Risk

When the concrete warms, air within its pores expands and tends to move outward. If coating is applied while the substrate temperature is rising, the escaping air may pass through the wet film.

Applying during stable or falling substrate temperatures can reduce outgassing risk when the coating manufacturer permits that practice. However, falling temperature can also bring the surface closer to the dew point and create condensation.

Temperature direction is therefore only one part of the decision. The contractor must also monitor dew point, relative humidity, surface temperature, product limitations, and expected conditions throughout application and cure.

Controlling Outgassing

• Monitor the substrate-temperature trend before and during application.
• Avoid rapid heating of the concrete after coating begins.
• Shade surfaces exposed to direct sunlight when appropriate.
• Fill bugholes, voids, and open pores with the specified resurfacer or filler.
• Use the specified primer and apply it at the required coverage.
• Work primer into porous surfaces when the manufacturer requires it.
• Observe induction time, recoat interval, film thickness, and application technique.
• Inspect the primer before applying subsequent coats.
• Repair pinholes using the coating manufacturer’s approved procedure.

A Thicker Coat Does Not Automatically Solve Outgassing

Applying additional material over active outgassing may produce larger bubbles, entrapped air, sagging, excessive exotherm, solvent entrapment, or incomplete cure. The cause must be brought under control.

Follow the approved repair procedure for pinholes and bubbles. This may require opening defects, preparing the affected area, applying filler or primer, recoating, and repeating holiday inspection.

Dew Point and Condensation

Dew point is the temperature at which air becomes saturated and moisture begins to condense. Condensation can form when the substrate temperature reaches or falls below the dew point.

A surface does not need to look visibly wet for condensation to interfere with a coating. A microscopic moisture film can reduce adhesion, affect cure, promote flash rust on steel, or create amine blush and surface defects in certain resin systems.

Maintain the substrate-temperature separation above dew point required by the project specification and coating manufacturer. A commonly encountered requirement is at least 5°F, or approximately 3°C, but the governing documents must control.

Measure the Surface, Not Only the Air

Air temperature alone does not establish whether coating conditions are acceptable. Concrete and steel surfaces can be colder or warmer than the surrounding air because of stored water, soil contact, sunlight, nighttime cooling, ventilation, or process conditions.

Record at minimum:

• Air temperature
• Substrate temperature
• Relative humidity
• Calculated or instrument-displayed dew point
• Substrate temperature minus dew-point temperature
• Weather, ventilation, heating, and dehumidification conditions
• Time, location, instrument identification, and person taking the reading

Instrument Use and Verification

Environmental instruments must be suitable for the required measurement range and maintained according to the manufacturer’s instructions. Sensors need time to stabilize after moving between environments.

• Confirm calibration or verification status before use.
• Protect sensors from coating overspray, dust, and direct heat.
• Take readings near the work surface, not only at the entrance to the structure.
• Measure representative high, low, shaded, exposed, and damp locations.
• Allow contact probes sufficient time to stabilize.
• Investigate unusual readings instead of discarding them without explanation.

Conditions Must Remain Acceptable During Cure

Environmental control does not end when spraying or rolling stops. Many coatings remain vulnerable to condensation, low temperature, high humidity, water exposure, and contamination while they cure.

Maintain required conditions through the manufacturer’s stated cure period or until the coating can safely tolerate the expected environment. Account for overnight temperature drops, weather changes, loss of temporary heat, shutdown of dehumidification equipment, and changes in ventilation.

Temporary Heating Can Introduce New Hazards

Unvented combustion heaters can add moisture and combustion products to the work area. Direct heat can warm one portion of a structure while leaving other areas below the permitted temperature. Heating equipment can also introduce ignition and confined-space hazards.

Temporary environmental-control equipment must be selected, located, ventilated, monitored, and operated as part of the project’s safety and quality-control plan.

When Coating Should Not Proceed

• The concrete exceeds the coating system’s documented moisture limit.
• Active leakage or hydrostatic pressure has not been controlled.
• Condensation is present or likely to form during application or cure.
• The substrate temperature is outside the permitted range.
• Relative humidity exceeds the coating manufacturer’s limit.
• Rapid warming creates unacceptable outgassing.
• Weather or ventilation conditions cannot be maintained.
• Testing is incomplete, inconsistent, or no longer representative.
• The parties have not resolved conflicting requirements in writing.

Contractor’s Field Checklist

• Have all known and possible moisture sources been investigated?
• Is active leakage or hydrostatic pressure controlled?
• Is the specified moisture-test method appropriate for the structure?
• Are test locations and frequencies representative?
• Are results within the coating manufacturer’s written limits?
• Is the substrate warming, cooling, or stable?
• Are air temperature, surface temperature, humidity, and dew point acceptable?
• Can acceptable conditions be maintained throughout application and cure?
• Have outgassing risks and porous areas been addressed?
• Are all readings, locations, times, instruments, and corrective actions documented?

Knowledge Check

1. Does a moisture-tolerant primer automatically resist hydrostatic pressure?

No. Moisture tolerance during application and resistance to active water or hydrostatic pressure are different performance requirements.

2. Why can applying a coating while concrete is warming cause pinholes?

Air and vapor within the concrete expand as the substrate warms and may escape through the wet coating, producing bubbles, craters, or pinholes.

3. Why must environmental monitoring continue after application?

The coating can remain vulnerable to condensation, temperature changes, excessive humidity, water exposure, and contamination until it has cured sufficiently.

Technical References and Further Study

• ASTM D4263-24, Standard Practice for Indicating Moisture in Concrete by the Plastic Sheet Method. This practice indicates the presence of capillary moisture before coating application but does not provide a quantitative moisture value.
• ASTM F2170-19a, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In-Situ Probes. This method quantitatively measures relative humidity at selected locations within concrete slabs.
• ASTM F1869-23, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride. Confirm that this method is appropriate for the application and accepted by the coating manufacturer.
• AMPP SSPC-Guide 23-2025, Field Methods for the Determination of Moisture in Concrete and Masonry Walls and Ceilings, EIFS, and Stucco.
• ASTM D3276-21, Standard Guide for Painting Inspectors—Metal Substrates. This guide includes environmental and inspection considerations relevant to coating work.
• The coating manufacturer’s current technical data sheets, safety data sheets, moisture limits, environmental requirements, primer instructions, recoat intervals, and cure requirements.

Standards and product requirements may be revised. Confirm the current edition, the appropriate test method, and the coating manufacturer’s project-specific limits before application.

Professional responsibility: This article provides foundational education and does not replace a project-specific moisture investigation, environmental-control plan, coating specification, manufacturer instruction, or professional evaluation of water intrusion and structural conditions. When moisture sources or test results are uncertain, stop and obtain written direction before coating.

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