How to Determine Concrete Slab Thickness

Knowing the thickness of a concrete slab can be important for several reasons. For new construction, you might need to know for QA analysis. If you’re working with an existing slab, you might need to know so you can anchor new equipment.

One common reason is for proper moisture testing to ensure appropriate dryness for flooring finish installation.

In this article, we’ll cover some topics around concrete slab thickness including:

• Common Methods of Determining Slab Thickness
• Here’s how to determine concrete slab thickness
• Testing by a Consultant
• Slab Thickness and Concrete Moisture Testing
• Some Reasons for Needing to Know the Thickness of a Concrete Slab

Common Methods of Determining the Concrete Slab Thickness

If you’re looking at new construction, you should be able to check the construction documents to find the thickness of the slab. If the structure isn’t new, you might still be able to find the archived construction documents. Check with the owner, the general contractor or the architect.

If there’s some renovation or repair work being done, there might be some trenching in the slab for new plumbing or electrical. If there is, you can check the opening of the slab there.

If there’s no other way to determine the thickness, and drilling a small hole is acceptable, then you can measure the thickness with a piece of wire.

How to Determine Concrete Slab Thickness

1. Drill a small hole vertically through the slab.
2. From a piece of wire that you think is long enough to go through the slab, bend a hook into the end that’s narrow enough to go into the hole.
3. Insert the hooked end into the hole until you feel the bottom edge of the slab with the hook.
4. Mark the top edge of the slab on the piece of wire.
5. Pull out the wire and measure from the hook to the mark.

Testing by a Consultant

If it’s going to be difficult to determine the slab thickness yourself, you can hire a consultant to use non-destructive testing (NDT) technology. These technologies use sound or electromagnetic fields that do not damage the concrete. These technologies include:

1. Impact-echo
2. Ultrasonic pulse-echo
3. Ground-penetrating radar
4. Magnetic Imaging Tomography

Your consultant will determine which method is most suitable for your situation. The tests can be conducted quickly and easily with small, portable equipment.

Slab Thickness and Concrete Moisture Testing

One of the most common reasons you might need to know the thickness of a concrete slab is for moisture testing. Any time you’re going to install a flooring system on a concrete slab, you need to test the slab for moisture.

All concrete contains moisture, and if the moisture level is too high, it can cause serious damage to the floor.

For the most accurate moisture test, the slab thickness must be known. Concrete moisture testing has been studied since the 1960s, and researchers have developed a scientifically proven test for measuring moisture levels in a slab.

The test requires sensors for measuring relative humidity to be inserted into the slab at very specific depths. For slabs drying from one side, the depth is 40% of the thickness. For slabs drying from two sides, the depth is 20% of the thickness. For that reason, knowing the thickness of the concrete slab is critical.

This test is called “the relative humidity test using in situ probes,” and it’s the basis for the ASTM F2170 standard. Wagner Meters provides an in situ concrete testing system that conforms precisely to ASTM F2170.

The Wagner Meters Rapid RH® L6 system uses single-use sensors for speed, economy, and ease of use. The L6 sensors come calibrated and documented from the factory.

Once the sensors are installed in the slab and equilibrated for the required 24 hours, repeat readings can be taken without additional equilibration time. And unlike reusable probes, the L6 sensors never need calibration.

The Rapid RH Total Reader® reads, displays, and transmits temperature and relative humidity data via Bluetooth® to the DataMaster™ L6 app. The DataMaster L6 app stores, displays, and reports the data on your iOS or Android mobile device. From your mobile device, you can email PDF format reports to your client and all interested parties.

Backup copies of your readings are stored in the cloud and in the sensors that are permanently installed in the slab. This unbroken digital path from the sensor to the final report, plus automatic data backup ensures the highest data integrity, accuracy, and peace of mind.

Learn more about how to conduct moisture tests for concrete floors in our more in-depth article.

Some Reasons for Needing to Know the Thickness of Concrete Slabs Are

• QA analysis
• Anchoring new equipment
• Moisture testing

Several Methods to Determine the Thickness of a Concrete Slab Are

• Checking construction documents
• Measuring an exposed edge
• Drilling a hole for a wire probe
• Testing by a consultant

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Jason Spangler

Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.

Last updated on July 15th, 2024