With the 2018 update to ASTM F2170 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes), now is the perfect time to revisit one requirement in the standard that hasn’t changed: Three concrete moisture sensors must be inserted for the first 1,000 square feet of flooring area, and an additional sensor for each subsequent 1,000 square feet of concrete area.

The relative humidity (RH) test method for determining the moisture condition of concrete slabs has been shown to be the most reliably accurate testing method available. That doesn’t mean every point of the F2170 standard makes obvious sense. Yet the 1,000-square-foot guideline wasn’t pulled from thin air. Before digging into the rationale behind the 1,000 square foot requirement, it’s helpful to understand how the ASTM F2170 standard came about.

Scientific Research Moves Industry Away from Surface Testing to in situ RH Testing

 Before the 1990s, most testing of the moisture condition of concrete slabs in the United States focused on surface moisture. The anhydrous calcium chloride test, for example, sat on top of the concrete slab and only detected the moisture gathering at the slab’s surface. Tests of this type tended to result in an unacceptable number of false readings for two main reasons: (1) surface readings are highly influenced by the ambient conditions around the slab; (2) moisture in a concrete slab typically occurs in a gradient instead of being the same throughout.

The 1990s were extremely important in the development of the F2170 standard because this was when researchers at the Technical University of Lund in Sweden ran scientifically-controlled testing on concrete and how it holds moisture. Their research verified that moisture exists in a gradient pattern throughout the slab. That’s because moisture tends to move from deep within toward the surface of the slab as evaporation occurs, and so the moisture condition at the surface is not necessarily indicative of the slab’s overall moisture.

The testing done at Lund also found that the most useful measurement, indicating the overall moisture condition once the concrete is sealed with a floor covering, is the RH level at 40% of the slab depth (or 20% if drying from two sides). Anyone familiar with the F2170 standard will recognize these numbers. These percentages indicate how deep into the slab an RH probe is to be inserted in order to comply with the F2170 standard.

Unearthing the 1000-square-foot Guideline’s Basis in Logic and Science

Just as the specific depths for placing RH probes were identified via rigorous scientific testing, the 1000-square-foot parameter is based on its own mixture of logic, math, statistics, and science.

The ASTM F2170 standard references “1000 square feet” two times in its instructions. First, it requires three in-situ RH probes be inserted in the first 1,000 square feet of flooring area being tested. Second, it requires one more sensor be inserted for every additional 1,000 square feet over the rest of the floor area.

So what’s so special about 1,000 square feet?

Here’s the math: The typical concrete slab is poured to a depth of four inches. For the typical truckload of wet concrete, this equates to an area of around 1,000 square feet. ASTM F2170 requires at least one RH sensor per 1,000 square feet because that maximizes the statistical likelihood that each truckload of concrete will get tested. (This is also the logic part.)

Now consider just a couple of the variables potentially affecting each truckload of concrete once it is delivered to the construction site. First, water may be added to the concrete to make it more workable when it is being poured. Afterward, while the concrete slab is drying, chances are it will be exposed to the weather, and this can significantly affect moisture levels too. Factors such as these can introduce variability, making it unwise to assume that moisture levels are consistent across the job site—even if all the concrete came from the same mix of cement, aggregate, admixture, and water. (This is the science part.)

The upshot: Anyone who wants to test the whole floor with accuracy and reliability needs to test all the concrete that was poured to create it.

All right, so why three sensors in the first 1,000 square feet?

More statistics: When drilling a hole for the RH sensor, there is a possibility of hitting a spot in the slab that’s mostly aggregate. If that happens, that sensor will likely give too low a reading. By putting three sensors in the first 1,000 square feet, this increases the likelihood that even if one sensor reads low, at least one of them will also hit a wet spot within the concrete. So with three sensors in that first 1,000 square feet, the odds of gaining an accurate assessment of the slab’s moisture condition greatly increase.

The ASTM F2170 recommendation to test moisture conditions within three feet from exterior walls rests on this same premise; that is, to cover all contingencies that may impact the moisture condition across the entire concrete floor.

Understanding Counsels Compliance

Separate from simply complying with the F2170 standard, inserting an RH sensor every 1,000 square feet also offers the general contractor and the flooring professional greater assurance of an accurate assessment of the slab’s true moisture condition. This information is vital in order to minimize the risk of a moisture-related flooring failure down the line.

If a flooring failure were to occur, having a map or diagram of the location of all RH sensors that were installed also provides supporting documentation that testing was done in compliance with the F2170 standard.

As should be clear, complying with the 1,000-square-foot requirement isn’t mere window-dressing. It’s at the center of conducting valid, accurate moisture testing of concrete. And that’s a critically important element for getting to the goal of beautiful, serviceable floors that will stand the test of time.