What In-Situ Relative-Humidity Testing Measures

In-situ relative-humidity testing measures the relative humidity of the air within a prepared hole in a concrete floor slab. The commonly referenced test method is ASTM F2170.

The test helps evaluate the slab's internal moisture condition. This is important because a concrete surface can appear dry while deeper portions of the slab contain substantial moisture.

When an impermeable or low-permeability flooring or coating system is installed, moisture can redistribute within the slab. An internal relative-humidity measurement can therefore provide information that a surface-only observation cannot.

Relative Humidity Is Not Moisture Content

Relative humidity is the amount of water vapor in air compared with the maximum amount that air can hold at the same temperature. It is expressed as a percentage.

An 85% relative-humidity reading does not mean that the concrete is 85% water. It means that the air within the test location has reached 85% of its water-vapor capacity at the measured temperature.

Temperature matters because warm and cool air have different water-vapor capacities. For this reason, the relative-humidity value and temperature should be recorded together.

Why Internal Testing Is Valuable

A slab drying primarily from the top develops a moisture gradient. The concrete near the exposed surface can become considerably drier than the concrete deeper within the slab.

Surface readings may change rapidly with weather, HVAC operation, sunlight, air movement, grinding, or recent cleaning. Internal relative-humidity testing is intended to evaluate conditions within the slab at a specified depth.

In-Situ Testing Can Help Identify

• Whether internal slab humidity exceeds the specified system limit
• Differences in moisture conditions across a large floor
• Areas requiring additional investigation
• Whether drying is progressing between separate testing periods
• Whether a moisture-mitigation system may need to be considered
• Conditions that should be documented before installation proceeds

Test Depth Matters

The probe hole must be prepared to the depth required by the current ASTM F2170 procedure. The required depth depends on whether the slab dries from one side or from two sides.

These percentages are measured from the surface through which the concrete is drying. The technician must determine the actual slab thickness and drying configuration before calculating the test depth.

How Many Tests Are Required?

Testing one convenient spot is not enough to characterize a large slab. Moisture conditions can vary because of slab thickness, placement sequence, exposure, previous flooring, exterior walls, sunlight, drainage, and building operation.

ASTM F2170 establishes a minimum number of test locations based on floor area. The commonly used minimum is three test locations for the first 1,000 square feet and at least one additional location for each additional 1,000 square feet.

Project specifications, manufacturers, owners, or observed site conditions may require more tests. Additional locations should be considered near exterior walls, wet areas, cracks, drains, plumbing, previous failures, and areas with different construction or exposure histories.

Building Service Conditions

The building environment influences the slab's moisture condition. Testing performed in an open or unconditioned building may not represent the conditions that will exist after occupancy.

The space should be maintained at the temperature and relative humidity expected during normal service for the period required by the test method and project documents. If normal service conditions cannot be achieved, the specification and current standard should be consulted before testing proceeds.

Record the Environmental Conditions

• Ambient air temperature
• Ambient relative humidity
• Concrete surface temperature
• Heating, ventilation, and air-conditioning status
• Whether doors and windows are open or closed
• Recent rain, flooding, cleaning, or water exposure
• Dates when normal service conditions were established

Selecting Test Locations

Test locations should represent the floor area—not merely the easiest places to drill. A written plan helps prevent convenient locations from replacing representative ones.

Avoid hidden electrical conduits, post-tensioning cables, embedded heating systems, plumbing, reinforcing steel, and other concealed components. Review drawings and use appropriate scanning or locating procedures before drilling.

Locations That May Require Special Attention

• Areas near exterior walls and loading doors
• Below-grade or perimeter areas
• Locations around drains and plumbing penetrations
• Previously failed flooring or coatings
• Areas exposed to rain or flooding
• Different concrete placements or slab thicknesses
• Sections previously covered by low-permeability materials
• Visible dampness, staining, efflorescence, cracks, or joints

Basic Testing Sequence

1. Review the governing requirements. Identify the required ASTM edition, product limits, test quantity, environmental conditions, and tester qualifications.
2. Prepare a test-location plan. Mark each location with a unique identification number and record it on a floor plan.
3. Determine slab thickness and drying condition. Calculate the required hole depth using the applicable procedure.
4. Check for embedded hazards. Review drawings and scan or locate utilities and reinforcing components as required.
5. Drill the hole. Use the specified diameter and depth while controlling concrete dust.
6. Clean the hole. Remove drilling dust according to the test-device and ASTM instructions.
7. Install and seal the sleeve. The sleeve must isolate the test depth from ambient room air.
8. Allow the required equilibration period. Do not record the official result before the minimum time required by the current standard.
9. Insert or read the sensor. Follow the sensor manufacturer's instructions without introducing outside air unnecessarily.
10. Record and report the result. Document relative humidity, temperature, location, time, equipment, calibration, and site conditions.

Equilibration Cannot Be Rushed

Drilling exposes concrete at the selected depth to the surrounding environment. The sealed test hole must be allowed to equilibrate before the official reading is taken.

ASTM F2170 currently requires a minimum equilibration period before recording test results. The technician must confirm the exact requirement in the edition governing the project. A reading taken too soon may not represent the required equilibrium condition.

Probe Calibration and Verification

Sensors must meet the accuracy and calibration requirements of the current test standard. Calibration documentation should be current, traceable to the probe or sensor, and retained with the project records.

Record the equipment manufacturer, model, probe identification, calibration or verification information, and the instrument used to read the sensor. Damaged, contaminated, expired, or questionable equipment should not be used.

Follow the probe manufacturer's storage, conditioning, reuse, and verification instructions. Do not assume that every sensor system has the same procedure.

Interpreting the Results

ASTM F2170 explains how to obtain an in-situ relative-humidity measurement. It does not establish one universal passing value for every coating, flooring material, adhesive, or membrane.

The measured result must be compared with the most restrictive applicable limit stated in the project specification and the current technical information for the complete proposed system.

A Passing RH Test Is Not a Complete Clearance

Relative-humidity testing addresses the internal moisture condition of the slab at the test locations and time. It does not prove that every other installation requirement has been met.

The contractor may still need to evaluate surface strength, profile, cleanliness, porosity, pH, soluble salts, oil, curing compounds, sealers, cracks, joints, temperature, dew point, and active water intrusion.

Common Testing Errors

• Testing before the building reaches the required service conditions
• Using too few test locations
• Selecting only convenient or apparently dry areas
• Failing to confirm the slab thickness
• Drilling to the wrong depth
• Leaving drilling dust in the hole
• Failing to seal the sleeve from room air
• Recording readings before the required equilibration period
• Using equipment without current calibration documentation
• Failing to record temperature with relative humidity
• Comparing results with the wrong product limit
• Reporting a screening measurement as an ASTM-compliant test

Reporting Requirements

A useful report should allow another qualified person to understand what was tested, where it was tested, how it was tested, and what conditions existed.

Include the Following Information

• Project name, address, and test date
• Name and qualifications of the testing technician
• ASTM standard and edition used
• Floor area and required number of tests
• Unique identification for every test location
• Floor plan showing the test locations
• Slab thickness and calculated test depth
• Whether the slab dries from one side or two sides
• Drilling and probe-installation date and time
• Reading date and time
• Relative humidity and temperature at each location
• Ambient temperature and relative humidity
• HVAC and building-service conditions
• Instrument model, sensor identification, and calibration information
• Visible moisture, damage, cracks, joints, or other unusual conditions
• Applicable acceptance criteria and the source of those criteria

Who Should Perform the Testing?

Some project specifications require testing by an independent agency or a qualified technician. The International Concrete Repair Institute offers a Concrete Slab Moisture Testing certification program covering ASTM moisture-test procedures.

Independence can be valuable when test results will determine whether a large installation proceeds or when warranty and liability issues are significant. Regardless of who performs the work, the required procedure must be followed and documented.

Safety During Probe Installation

Drilling concrete can expose workers to respirable crystalline silica. Follow applicable OSHA requirements, the project exposure-control plan, and the equipment manufacturer's instructions. Use appropriate dust collection, HEPA-filtered equipment, work practices, respiratory protection when required, eye protection, hearing protection, and other personal protective equipment.

Concrete slabs may contain electrical conduits, post-tensioning cables, radiant-heating components, plumbing, and reinforcing steel. Striking these components can cause serious injury and property damage. Review available drawings and use approved locating procedures before drilling.

Contractor Pre-Test Checklist

• Obtain the current governing ASTM test procedure.
• Confirm whether certified or independent testing is required.
• Verify that the building is at the required service conditions.
• Determine the total floor area and minimum number of tests.
• Identify the acceptance limits for every component in the system.
• Prepare a representative test-location plan.
• Confirm slab thickness and drying configuration.
• Locate embedded utilities and structural components.
• Verify equipment condition and calibration documentation.
• Prepare silica-dust controls and required PPE.
• Schedule the required equilibration period.
• Prepare a complete reporting form before testing begins.

Knowledge Check

Technical References

Use the edition required by the project specification and verify that testing personnel have access to the complete current procedure.

• ASTM F2170 - Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ Probes.
• ASTM F710 - Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring.
• ASTM F1869 - Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.
• ASTM F2659 - 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.
• ICRI Concrete Slab Moisture Testing Program - Education and certification for technicians performing standardized concrete slab moisture tests.
• OSHA 29 CFR 1926.1153 - Respirable Crystalline Silica standard for construction.
• Current technical data sheets and installation instructions issued by the specified coating, flooring, adhesive, primer, and moisture-mitigation system manufacturers.

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