Screening and Acceptance Testing Are Different

A non-destructive electronic moisture meter can be moved across a floor and used to collect many readings in a relatively short time. This makes it useful for identifying comparative moisture patterns and selecting locations for additional testing.

However, most electronic meter readings do not provide a direct measurement of internal slab relative humidity or moisture-vapor-emission rate. Unless the project documents and product manufacturer expressly permit the meter as an acceptance method, its readings should be treated as comparative screening information.

ASTM F2659

ASTM F2659 provides guidance for the preliminary evaluation of the comparative moisture condition of concrete, gypsum cement, and other floor slabs and screeds using a non-destructive electronic moisture meter.

The words preliminary and comparative are important. The meter can help compare one area with another, but the reading should not automatically be reported as a concrete moisture-content percentage, relative-humidity percentage, or moisture-vapor-emission rate.

How Electronic Moisture Meters Work

Different meters use different electrical principles. Depending on the design, the instrument may respond to electrical impedance, capacitance, resistance, radio-frequency behavior, or another property affected by the material beneath the sensor.

Water changes the electrical behavior of concrete. However, water is not the only factor that can influence the meter. Concrete density, aggregate, salts, reinforcing steel, surface materials, depth of response, and instrument design can also affect readings.

Two General Meter Configurations

Always confirm that the instrument is intended for concrete or the substrate being evaluated. A meter scale designed for lumber should not be interpreted as a concrete moisture percentage.

What Electronic Meters Do Well

• Survey large floor areas quickly
• Identify relative wet and dry patterns
• Locate possible leaks or water pathways
• Compare perimeter, interior, drain, and joint areas
• Help select locations for quantitative testing
• Track changes in selected areas over time
• Support failure investigations
• Provide immediate field information without drilling numerous holes

A dense grid of comparative readings can reveal patterns that three or four isolated quantitative tests might miss. The contractor can then position required tests in both typical and higher-reading areas.

What Electronic Meters Cannot Establish by Themselves

• The internal relative humidity of the slab under ASTM F2170
• The moisture-vapor-emission rate under ASTM F1869
• The source of the moisture
• The presence or condition of an underslab vapor retarder
• Whether hydrostatic pressure exists
• The future moisture condition after the slab is covered
• Whether every product in the proposed system will perform
• A universal pass-or-fail decision for coating installation

Factors That Can Affect Meter Readings

Preparing for a Comparative Survey

Before collecting readings, review the floor plan and identify areas with different construction or exposure histories. Separate slabs, additions, repairs, equipment pads, below-grade areas, and previous flooring installations may need to be evaluated as distinct sections.

The surface should be accessible, reasonably clean, and compatible with the instrument's instructions. Dirt, liquid water, metal debris, adhesive residue, coatings, and uneven surfaces can interfere with consistent readings.

Information to Gather Before Testing

• Instrument manufacturer, model, and serial number
• Current verification or calibration information
• Meter operating instructions and substrate limitations
• Floor plan and total survey area
• Known construction joints and slab placements
• Previous flooring, coatings, patches, or repairs
• Locations of drains, plumbing, exterior walls, and equipment
• History of leaks, rain exposure, flooding, or washdown
• Ambient temperature and relative humidity
• Required quantitative test methods and acceptance limits

Developing a Moisture Map

A moisture map converts individual readings into a visible pattern. Mark the floor into a consistent grid and assign every reading a location number.

The grid spacing should be close enough to reveal meaningful changes. Large open areas may permit wider spacing, while areas around drains, walls, joints, wet spots, and previous failures may require closer readings.

1. Divide the floor into sections. Separate areas with different concrete placements, elevations, exposures, or floor histories.
2. Establish a reading grid. Use columns and rows, measured coordinates, or another repeatable location system.
3. Check the instrument. Perform any reference check or verification required by the manufacturer.
4. Acclimate the equipment. Allow the meter to adjust to the test environment as required.
5. Use consistent technique. Maintain the same orientation, contact, pressure, and reading time.
6. Record every value. Do not record only the highest or lowest readings.
7. Mark unusual conditions. Note cracks, joints, stains, metal, coatings, patches, drains, or standing water.
8. Identify comparative patterns. Highlight areas that read higher or lower than the surrounding floor.
9. Select quantitative test locations. Include typical areas and representative higher-reading areas.

Establishing a Comparative Baseline

Electronic meter readings are most useful when compared with other readings collected on the same project using the same instrument and procedure.

A lower-reading area can provide a comparative baseline, but it should not automatically be labeled dry or acceptable. The baseline itself may still contain more moisture than the proposed system permits.

Likewise, a higher-reading area should not automatically be declared wet or failed. Reinforcement, salts, aggregate, patches, or other variables may contribute to the difference. Use the pattern to direct further investigation.

Surface Moisture and Condensation

An electronic meter may respond strongly when liquid water or condensation is present on the surface. Before attributing the condition to moisture rising through the slab, compare the surface temperature with the dew-point temperature.

Condensation may form when warm humid air contacts cool concrete. This can occur near loading doors, refrigeration equipment, cold-storage areas, uninsulated pipes, and spaces affected by HVAC startup or shutdown.

Record ambient temperature, relative humidity, surface temperature, and dew point. Follow the coating manufacturer's minimum required separation between the substrate temperature and dew point.

The Plastic Sheet Method

ASTM D4263 describes a practice for indicating moisture in concrete using a plastic sheet sealed to the surface. After the prescribed exposure, the area is examined for visible moisture or darkening.

The plastic sheet method can indicate that moisture is present under the conditions of the test. It does not measure internal relative humidity or calculate a moisture-vapor-emission rate.

Electronic Meters Versus Quantitative Tests

Common Electronic-Meter Mistakes

• Calling a comparative reading an actual moisture percentage
• Calling the meter value an RH result
• Using a wood scale to evaluate concrete
• Surveying only the visibly dry areas
• Failing to map reading locations
• Comparing readings collected with different instruments or settings
• Ignoring reinforcement, metal deck, salts, patches, or coatings
• Using inconsistent contact pressure or meter orientation
• Testing over surface water without noting the condition
• Using screening results as the sole installation-acceptance decision
• Failing to perform the quantitative test required by the specification

Reporting a Comparative Survey

A survey report should make clear that the readings are comparative unless the instrument and approved procedure establish otherwise.

Include the Following Information

• Project name, address, date, and surveyed area
• Name and qualifications of the operator
• Instrument manufacturer, model, serial number, and setting
• Verification or calibration information
• Applicable ASTM guide or project procedure
• Surface materials and preparation condition
• Ambient temperature and relative humidity
• Concrete surface temperature and dew point when applicable
• A floor plan showing every reading location
• All recorded values—not merely selected readings
• Cracks, joints, drains, repairs, coatings, and unusual conditions
• Locations recommended for quantitative testing
• A statement explaining the limitations of the survey

Contractor Field Checklist

• Confirm that the meter is intended for the substrate.
• Read the current manufacturer instructions.
• Verify the instrument before beginning the survey.
• Identify the required quantitative acceptance test.
• Inspect and document the surface condition.
• Establish a consistent survey grid.
• Use the same meter, setting, orientation, and technique.
• Record every reading and location.
• Note metal, salts, patches, coatings, and surface water.
• Compare higher readings with visible site conditions.
• Use the survey to select representative quantitative test locations.
• Do not approve installation solely from an unapproved screening method.

Knowledge Check

Technical References

Use the edition required by the project specification and follow the current written instructions issued by the instrument and coating-system manufacturers.

• ASTM F2659-23 - Standard Guide for Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement, and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter.
• ASTM D4263-24 - Standard Practice for Indicating Moisture in Concrete by the Plastic Sheet Method.
• 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 Concrete Slab Moisture Testing Program - Education and certification covering standardized concrete slab moisture-test procedures.
• Current operating instructions issued by the electronic meter manufacturer and current technical data for the specified coating, flooring, adhesive, and moisture-mitigation systems.

These references provide technical guidance but do not replace the project specification, governing regulations, manufacturer requirements, or evaluation by a qualified professional. Final testing, system-selection, and application decisions must be based on current documents and documented site conditions.