Concrete Moisture Meters vs. Relative Humidity Testing
When it comes to flooring installation on a concrete slab, a critical aspect often overlooked is moisture management. Knowing the precise moisture levels within your concrete is more than just good practice—it’s key to preventing future flooring failures and ensuring the longevity of your installation.
However, the question remains: How should you measure this moisture?
This comprehensive guide will dissect the functionality of two popular moisture-measuring methods: Concrete Moisture Meters and Relative Humidity Testing. We’ll also address some burning questions, such as:
- Do Moisture Meters Work on Concrete
- Using a Moisture Meter on Concrete
- Measuring Relative Humidity in Concrete
- Relative Humidity and Concrete Moisture Content
- The Best Way to Test Concrete Moisture
- Concrete Moisture Meter Verdict
This article aims to debunk myths, provide insights, and guide building professionals and homeowners to make informed decisions about moisture control in concrete. After all, making the right choice could mean the difference between a durable floor that lasts for years and a disaster waiting to happen.
Concrete moisture meters should never be used to make the final determination as to whether or not a concrete slab is dry enough for a flooring installation.
Whether you’re a building professional or a homeowner, using the right tools and methods to measure and control moisture in concrete is essential for ensuring the longevity and durability of your floors.
Do Moisture Meters Work on Concrete
Yes, concrete moisture meters work. But no concrete moisture meter of any type can give consistently accurate concrete moisture meter readings across the different mixes and densities of concrete. Other components (metal reinforcing bar, aggregate size, amount, etc.) can also cause false indications of moisture.
Using a Moisture Meter on Concrete
The C555 pinless moisture meter effectively measures moisture in concrete on the top ½ inch of smooth slabs.
Pin-type meters are not practical for measuring moisture in concrete because variable chemical and physical characteristics can cause false moisture readings. This is due to changes in electrical resistance that have nothing to do with moisture.
However, remember that surface moisture meters provide readings only from the uppermost layer of the concrete, which may not be sufficient to assess moisture conditions throughout the slab. For a more accurate understanding of moisture conditions within a concrete slab, it’s best to use RH testing methods.
Measuring Relative Humidity in Concrete
Professionals typically use in-situ probes or sensors to measure the relative humidity (RH) within a concrete slab. The process involves drilling a hole into the concrete slab, often about 40% of the slab’s thickness for an on-grade or above-grade slab.
Once drilled, an in-situ probe is inserted into the hole. This probe measures the humidity deep within the slab and provides a percentage-based RH reading. The percentage indicates how much moisture is in the air inside the concrete compared to the maximum amount of moisture the air could hold at the same temperature.
Relative Humidity and Concrete Moisture Content
While “relative humidity” and “moisture content” may sound similar, it’s important to distinguish between the two. RH refers to the moisture level in the air, specifically inside the concrete slab, and is expressed as a percentage.
On the other hand, moisture content generally refers to the total amount of water in the concrete itself, often calculated by weight.
The Best Way to Test Concrete Moisture
The in-situ relative humidity (RH) testing method is the most reliable way to test concrete moisture. It provides an accurate measurement of the final moisture condition of the slab if it were sealed with a flooring product at that time.
The ASTM F2170 standard, which specifies the RH testing method, is recognized by flooring and adhesive manufacturers as the most accurate way to test concrete moisture. This method inserts RH sensors into the concrete slab, providing a precise understanding of how much moisture is present.
Additionally, concrete moisture meters were sufficiently accurate (again, they are not). They only measure ‘moisture’ (not relative humidity) in a small area near the concrete slab’s surface. This type of surface ‘moisture’ testing tells us nothing about the conditions down in the slab.
Once a floor covering is placed on a concrete slab, the relative humidity in the concrete slab will equilibrate throughout the thickness of the slab.
Meaning a slab that may have been “dry” (low RH) at the surface (without a floor covering) will see a higher RH at the surface when the floor covering has been installed. This is where the problems occur. The calcium chloride test method (moisture vapor emission) and the UK-based ‘hood’ method (RH) have the same problem, as they are also surface tests.
The calcium chloride test method (moisture vapor emission) and the UK-based ‘hood’ method (RH) have the same problem, as they are also surface tests.
Remember that an uncovered concrete slab will have an RH gradient throughout its thickness until a floor covering is put on top. Under normal conditions, the RH at roughly 50% slab depth will be significantly higher than the surface. Unless the slab has been down for a long time and a vapor retarder is directly underneath the slab.
In reality, the surface of the concrete will more closely reflect the RH in the room or building. This does not indicate the potential for a concrete flooring failure due to the high RH deep in the slab. That RH will migrate to the surface when the concrete can no longer emit water vapor due to the non-permeable floor covering.
Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry
Again, most major floor covering and flooring adhesive manufacturers in North America and Europe already have concrete RH specifications in their installation guidelines.
Concrete Moisture Meter Verdict
At best, concrete moisture meters may have some practicality as relative (qualitative, not quantitative) measurement devices for possibly indicating the best placement for accurate, quantitative RH sensors within the concrete. Unlike other non-quantitative test methods, moisture meters for concrete are not final determination tools.
Learn more about RH probes.
So how can you ensure harmony between RH and MC?
Allow time for gradual acclimation when wood is moved to a new job site or location, particularly with exotic woods or woods that originated in markedly different climates. Or, if you have your own shop kiln for drying wood, set it for moderate changes over an extended schedule.
Monitor the changes with an accurate wood moisture meter.
Wood moisture meters from Wagner Meters provide easy, accurate, damage-free moisture measurement that lets you quickly determine MC of wood.
They also have the unique advantage of not being skewed by surface moisture. When RH is high and temperatures are cooler, a thin layer of condensation can form on the wood’s surface. Wagner’s IntelliSenseTM Technology reads past that moisture layer to provide a true picture of the wood’s internal MC.
Wood moisture meters can also be effective tools for determining unseen moisture-related problems in wood floors or other site materials where a change in MC could mean a water intrusion is in contact with the wood itself.
When RH and MC are in balance, your wood will perform at its best and retain its strength and beauty for years to come. Wood moisture meters from Wagner Meters provide a simple and effective way to determine when wood has reached a point of equilibrium with the RH of its environment.
As Sales Manager for Wagner Meters, Ron has more than 35 years of experience with instrumentation and measurement systems in different industries. In previous positions, he has served as Regional Sales Manager, Product and Projects Manager, and Sales Manager for manufacturers involved in measurement instrumentation.
Last updated on August 4th, 2023