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“Moisture Testing Of Concrete Floor Slabs”

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Wagner Meters is the proud sponsor of the above named webinar.  We realize that there is so much important information jam packed into this 60 minute video that we decided to create a guide which pulls out all the key points, strategies and industry best practices presented within and display it in an easy to read format.

If you have any comments or questions regarding this webinar, please direct your inquiry to our Flooring Division Sales Manager, Jason Spangler

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Section 1:  Components In A Floor System

Levels of a floor system from the ground up are as follows:

-Subgrade soil

-Possible compacted sub base

-Vapor retarder

-Capillary break

-Concrete floor/Slab

-Patching and leveling compounds, adhesives and primers

-Finished flooring such as lenolium, vinyl, wood or carpet

Types of floors relative to the ground:

-Floors above grade, elevated floors

Structural floor slab-concrete floor that supports itself

Concrete in steel deck-a building framed in steel, corrugated or ribbed steel sheets support the concrete floor

-Slabs on ground or slabs on grade

-Slabs below grade


All levels are sensitive to moisture-when wetter they expand, when dryer will contract

Floor coverings, adhesives, primers and patching compounds-sensitive to moisture and high PH breakdown

Section 2: Sources of moisture in concrete slabs

-Above the slab

Ambient relative humidity in the room

Precipitation-rain, snow or fog that can get on the slab

Water used for curing the concrete

Construction water-washing spills during other construction tasks

-Below the slab

Moisture vapor from the soil rising into the bottom of the slab

Is the reason why subslab vapor retarder is applied

-Within the slab

curing and batch water-ratio of water to cement batched into the cement

Is the number one factor in determining length of time needed to dry out the concrete

Section 3: Industry Accepted Methods For Drying out Concrete Slabs

First, who should perform moisture testing on concrete floors?

An independent testing agency, to avoid bias and pressures resulting from a tight construction schedule.

A testing certification is available from the International Concrete Repair Institute-ICRI

Failure to adhere to standard testing procedures-data varies beyond acceptable tolerances and impacts ability to make good decisions re. installation of flooring

Test results should be reported to all contractors and engineers deciding whether to proceed with floor installation or not.

Manufacturers of flooring materials publish required moisture limits in concrete subfloors.

PH of concrete floors should not exceed 9 or 10-if higher will interfere with performance or breakdown of adhesives.


Remedies to control moisture

-Retard moisture below the slab, possibly install a capillary break layer and should also include a vapor retarder at best

-Top of the slab, install a topical moisture mitigation system to prevent flooring failures

Drying rate on the surface of the slab depends on:

-Relative humidity in the air Over the concrete


-Air flow across the surface

Assist acceleration of natural drying by artificially creating lowest relative humidity possible, warmer air flow and better air circulation by using fans and heaters.

Concrete goes through 3 drying stages:

-Initial placement of concrete is full of water, and has a PH of 12-13.

Water has to evaporate at the surface first.

-Liquid water from deeper down moves up to the surface.

-Moisture vapor begins to diffuse, move up through the concrete, and evaporates at the surface.

Section 4: Methods for testing moisture in concrete slabs

  • ASTMT4263-Plastic sheet test

Requires taping an 18 In. square plastic sheet on surface

Checking for moisture droplets underneath plastic

Results depend on:

-Temperature of the slab surface

-Dew point of concrete

If droplets occur:

Moisture level in slab is too high

If no droplets then:

Moisture level may not be acceptable-inconclusive results


  • Mat bond test

Uses a 3 Ft. square of flooring adhered to the concrete slab to evaluate the bond

If bonding fails or breaks down, moisture is too high.

If bond holds, test is still inconclusive as lower levels of the slab may be a lot wetter.

Reality-on construction sites, can’t be left long enough to get conclusive results

  • ASTMF2659-hand held electrical resistance and impedance meters

Evaluates moisture at the surface-top 1 inch of the concrete

Factors that can impact readings:

-Type of aggregate used in the concrete

-Degree of hydration of the cement paste

Test only is a potential indicator of moisture.

Provides a numeric  value but not a quantitative measurement of the moisture.

Challenge-Putting out an electromagnetic field which interacts with the steel in the concrete such as reinforcement or wire mesh can show an indication that the concrete is soaking wet.

That is why it can only measure the top inch to avoid interacting with the metal below.

No measurement of moisture is possible below that level.

Test is only good as a survey tool.


  • ASTMF1869-Calcium chloride test used for over 50 years

Not used in other countries

Uses salt crystals  to absorb moisture vapor from  the surface of the slab beneath a sealed dome

The resulting Moisture Vapor Emission Rate or MVER measures only the top ½ to ¾ inch of the slab and is highly sensitive to ambient conditions just before testing.

If the result is around 10lbs. then internal RH is probably high.

If the result is around 5lbs. or lower there is no correlation with internal RH.

Results obtained does not provide a reliable indication of the long-term moisture condition of the slab surface.

No scientific tests have concluded how much crystal is needed or how deep they must be inserted in the slab, nor the dimensions of the kits.

Final assessment, the test is too inaccurate.

  • ASTMF2420-the lid or the hood test is used in the United Kingdom and now is published as a standard test in the United States

Measures the relative humidity of the air space under an insulated hood sealed to the concrete floor for a minimum of 72 hours.

Measures only the moisture in the upper portion of the slab unless left in place for a long time-weeks

  • ASTMF2170-the relative humidity probe test

Measures moisture “within” a concrete slab

At 40% depth represents the amount of moisture that will exist throughout the slab  after the floor covering is installed and the moisture equalizes

Only test that provides an indication of the moisture that will exist in the concrete subsequent to installing the flooring

Section 5: What is Relative Humidity And Why Measure It?

Meaning of RH:

The ratio of how much moisture is actually in the atmosphere is compared to what the air could hold.

If the humidity is high you feel the dampness, if lower you feel comfortable, the same is true in concrete.

So RH is what we feel compared to what the air could hold if it were saturated.


20 years of scientific testing shows that RH is the most accurate assessment of moisture condition within a slab.

It is the most predictive tool relating to performance of floor coverings and adhesives.


Sensitivity of RH to ambient floor conditions

Several studies at the University Of Illinois and in Denmark and Sweden suggest that when you make a RH measurement, if the building temperature changes the RH in the slab will change very little.


Predicting when the slab will be ready for floor covering

It is suggested to place probes in the slab when it is new and check the probe readings every week, a drying curve will show how quickly the concrete is drying out. Doing this helps contractors better predict when the slab may reach the desired point of dryness in order to install the flooring. Decisions can be made by contractors/installers whether to take remedial action to step up the process of drying the slab.

Section 6: All About RH Probes

How many tests should be made and where should the tests be placed?

Perform 3 tests for the first 1000Sq. Ft. of floor area and 1 test for each additional 1000 Sq. Ft..

Example:  A floor with 10000SQ. Ft. needs 12 testing sites.

Where to place the probes?

Various areas across the floor especially areas of high moisture including near dividing lines of different construction pours.

For slabs on grade and below grade include a test location within 1 meter of each exterior wall.

Beware of where pipes and duct work are located.

Stay away from placing probes near doorways and open windows, results will be biased incorrectly.

Types of RH probes for testing concrete

Reusable and single use probes

  • Reusable probes are inserted in a sleeve and installed in a hole drilled to the appropriate depth in the slab.

Results will drift for several hours and overnight.

Leap frogging of probes from one location to another, if not left long enough will cause skewed results.

Need to leave probe in each location for several hours.

Recalibration standards

Probes must be checked 30 days before use.  If probe is out of accuracy by more than 3%, standard requires recalibration or discarding probe.

Difference between checking probes and recalibration

Checking a probe is mandatory before initial use.  Recalibration is sending the probe to the manufacturer, making adjustments and a certificate is then issued traceable to a national standard.

Recalibration costs more in some cases than the actual probe purchase.

Shelf life

Sealed probes from the manufacturer with initial calibration is good for 2 years.

When in use  variety of situations-very wet, dry or dirty environment may shorten the life of a probe. Check for accuracy and discard or send back for recalibration.

  • Single use probes

No recalibration is needed, they are fast and reliable.

Installing single use probes

Drill to a depth of 40%with a rotary hammer drill and ¾” masonry drill bit.

Attach vacuum attachment to your vacuum and clean out the hole drilled.

Use ¾” brush to clean out loose pulverized concrete.

Repeat cleaning steps twice more.

Insert sensor into hole using insertion tool.

Push down on the insertion tool to force down the sensor into the hole, do not tap or hammer.

Insert a protective cap into the top of the sensor base sleeve and push down to the concrete surface.

If extensions are needed for thicker slabs assemble extensions and insert cap onto the top extension and push total assembly down into the hole until fully seated on the sensors.

To take a reading

Remove protective cap and insert LCD reader into the smart sensor, it will power up on its own.

Wait until the humidity reading automatically appears on the screen, then remove immediately.

The LCD display will toggle between the RH percent and degree temperature reading and will shut off after five minutes.

Record readings on the plotting chart included in each package.

When ready to install floor covering…

Remove the cap and place a stainless steel metal disc over the smart sensor.

Skim coat the hole using a patching compound compatible with the flooring manufacturer’s instructions.

Manufacturer comparison of results from using single probes

After 10 minutes 95% of final reading

After 20 minutes 98%

All tested at least 95% after 4 to 6 hours.

By 24 hours all company probes were reading the same and there was no change over the next 3 days.

RH probes can give a final answer in a short period of time.

They are quicker and more accurate than other systems which will help in the floor covering installation planning process.