Why Measure pH and RH Levels in Concrete Floors?
Flooring contractors who work over concrete slabs need to consider both the pH (power of hydrogen) and the relative humidity (RH) of the concrete before installing any flooring, or they risk costly flooring failures.
- What Is the pH Level of Concrete?
- The Basics of pH in Concrete
- RH: The Reliable Indicator of Moisture Inside the Concrete
- Rapid RH Concrete Moisture Testing System
- pH Testing
What Is the pH Level of Concrete?
Freshly poured concrete has a pH of 12 to 13. Over time, calcium hydroxide in the concrete reacts with carbon dioxide in the air to form calcium carbonate, which lowers the pH at the surface. Many flooring adhesive manufacturers require a surface pH of 10 or below, though requirements vary by product.
The Basics of pH in Concrete
pH measures how acidic or alkaline (basic) a water-based solution is.
To make concrete, producers mix aggregate and cement with water, and the chemistry of that water changes as the concrete cures. When installers use adhesives to bond a floor covering to concrete, they must follow the manufacturer’s pH guidelines.
If they don’t, the adhesives, finishes, and flooring materials can debond, blister, stain, or develop other serious performance problems.
Concrete additives, along with the concrete’s natural hydration process, also affect how acidic or alkaline the moisture in a slab is. Acids release hydrogen ions (positively charged) in water, and bases release hydroxide ions (negatively charged).
The pH scale runs from 0 to 14. Battery acid, for example, has a pH of about 0, while liquid drain cleaner has a pH of about 14.
A fresh slab at pH 12 to 13 is therefore well above what most adhesives tolerate. That high pH comes from the calcium hydroxide formed during hydration, and it drops only as the calcium hydroxide reacts with carbon dioxide at the surface.
RH: The Reliable Indicator of Moisture Inside the Concrete
A concrete slab dries from the top down, so its surface can seem much drier than the concrete below it. That’s why you need to test inside the concrete before laying the floor covering, so the adhesives you choose will bond as they should.
This “inside” test, known as the in-situ RH test, shows the true moisture condition of the slab. The method grew out of research at Lund University in Sweden in the 1990s, which established the depth at which a reading represents the slab as a whole. That work is the basis for ASTM F2170, the standard that sets out how the test is performed.
Moving ahead with a flooring installation without reliable information about the slab’s moisture condition is risky. If the slab doesn’t have enough time to dry before the flooring goes in, repair and litigation costs from a moisture-related flooring failure can spiral out of control.
Keep in mind that flooring and adhesive manufacturers publish guidelines that installers must follow for each product. Changes in local weather, humidity, and seasons can also affect the outcome.
Fortunately, simple tools exist for testing both RH and pH.
Free Download: 4 Reasons Why Your Concrete Is Taking Forever to Dry
Rapid RH Concrete Moisture Testing System
Wagner Meters developed the Rapid RH® to make in-situ RH testing fast, accurate, and cost-effective. Contractors install its calibrated RH sensors at 40% of the slab’s depth (for a slab drying from one side, or 20% for a slab drying from two sides) and read the data with a digital, wireless reader and the DataMaster™ L6 app, which works with most mobile devices.
The Rapid RH is designed to make it easy to take multiple readings at each test location and determine when a slab has reached an acceptable RH for the flooring to be installed. The Rapid RH Starter Kit is a good way to get started.
pH Testing
Wagner Meters recommends using its pH test kit as part of your Rapid RH moisture assessment.
Slab moisture and pH are related but measured separately. A wetter slab carries more alkali to the surface as moisture moves up through it, which is why the two are often discussed together. But neither reading predicts the other, which is exactly why both are worth taking. As we’ve seen, high pH, just like high moisture, can make a slab unsuitable for flooring installation.
That’s why flooring installers and contractors should get a reliable assessment of both before giving the go-ahead on a job.
The in-situ RH test measures conditions within the slab rather than just at its surface, which is why it’s widely considered the most reliable indicator of a slab’s moisture condition. Pair it with a pH test, and you’ll have the information you need for a safe, long-lasting flooring installation.
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 October 19th, 2026
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Hello Jason,
I am a gardener. I am putting together a plan of new flowers in the openings of Old concrete blocks, descending a staircase, outside from the front yard to the back yard. It looks as though these cement blocks are well over 10 years old. I am trying to work with what is there. Most New England plants are acid loving. From one of the articles above, it sounds like older, dried out cement blocks would be more in the neutral pH of 7/8? Do I need to consider only alkaline plants in this part of the design? This is the “statement” piece of the design. Do I have to line the various openings of the cement blocks? I’m concerned…
In case I can’t find my way back to this site, can you also send your reply to: gardens02138@gmail.com
THANK YOU!
I found this by accident but so glad I did!
I have been thinking of purchasing a portable pH meter!
Nancy
Nancy:
Thanks for the question. I can say, I’m not a gardener, but I commend those that are capable. I have never tested old concrete blocks, but I would have to believe they would be close to neutral. Beyond that, I’m not sure I’m have much more to add. Good luck.
my slab was poured in 1979 does it still need testing before installing carpet
Sonia:
Thank you for the question. Any concrete SHOULD be tested prior to installation, but there are many that would argue if there is one type of product that has the least possibility of having moisture issues it would be a breathable carpet product.
Hello sir
Thanks for all those information, but I have one question which is:
PH value in fresh concrete is between 11 to 13 I want to know after the reaction between calcium hydroxide and carbon dioxide in the air reduce the PH on surface of concrete. So what is the value of PH will be.
will be less than 10 or what?
Abdurahim:
Thanks for the question. In my experience, yes, after the reaction and carbonation layer develop you typically will see numbers closer to 7-8.
What is the difference between Seal and prep concrete slab floors to mitigate high moisture content in concrete and mitigate unacceptable ph levels in the concrete?
Mark:
Thanks for the question. I’m not sure I fully understand your question, but I will answer it as I understand it. The difference between Seal and prep…..with most moisture mitigation products that are topically applied to a high RH/pH concrete slab, the concrete needs to be prepped to help with the bonding of the slab and the product. Prep can be shot blasting, scarifying, sanding, or anything recommended by the manufacturer. Thanks.
Well written and to the point. I appreciate the detail in this article!
I recently received information from a GC that the flooring would not be covered under warranty due to high levels of RH. I noticed your website mentions “Changes in local weather, humidity, and seasons can also influence the outcome.” The measurements were taken in the spring during after significant moisture. Can you speak to how much the local weather may influence the readings? Is there a recommended way of adjusting measurements to account for these varriables?
Thanks,
Toby:
A relative humidity sensor placed in concrete is measuring humidity, relative to temperature. There isn’t a cut and dry, “up 2 degree’s in temperature results in a 4% drop in RH%”. The measurement in concrete doesn’t work off of a normal type of psychrometric chart. This is why the standard for utilizing sensors in concrete to measure RH% for sufficient dryness requires ambient air to be at “service conditions” or the conditions that the space will experience when occupied. The other thing to keep in mind is that concrete is like a sponge and will absorb and expel moisture as conditions in the air change, prior to flooring being installed.
Thank you Jason for taking time to share your thoughts. Much appreciated.
Hi thank you for sharing the invaluable insights. I have a number of issue which could be related to the concrete slab foundation.
(1) White deposits appearing on surface of brickwork on the lower course of bricks on one side if the house. The white deposits could be scratched off quite easily. Also, the mortar is coming loose leaving gaps in between.
(2) Also the soil next to the course of brickwork is constantly damp even up close to the surface, though further out from the brickwork it’s already dried out.
It is unusual as we have extremely dry weather since the occasional downpour of about a month ago where a fair amount if water may have got around the perimeters of the foundation slab. Internally, the porcelain tiles that were laid about a year ago appears to be intact. (There’s no water leaks from pipes, neither any broken storm water. Not sure how to check the internal grey water pipes for leaks).
(3) In one of the rooms, there is a recurring smell resembling damp plaster however the paintwork did not appear to have been compromised. I had placed a 300gm Sunfresh moisture absorber to extract moisture simultaneously at two spots. There is around 15mls collected in each container the last month which I am not sure if it is considered excessive or abnormal.
Grateful to hear your suggestions and insights. Kind regards.
Esther:
Thanks for the comments. It sounds like much of this may be just normal moisture migration, whether due to condensation or latent moisture in the materials. Of the issues you mentioned, the loose mortar would be one I would definitely investigate. I would look to find a reputable building inspector to help evaluate this situation. Good luck.
This is a helpful article. I had a company put vinyl flooring over my concrete and it is coming up in less than a month. They came and said I must have got it wet. They re-glued the places that were coming up but it is doing the same thing. I do not believe they did this testing before applying the flooring and pressure sensitive glue. Now that it is still coming up after re gluing they are assuming I have a leak under my floor. Of course I don’t know for sure, but I think it is happening from an error on their part based on your article and some others I have read.
Any thoughts you have would be appreciated.
Kay:
Thanks for the question. It’s hard to say what the “issue” is without having some diagnostic information. In this situation, I can’t even begin to evaluate whether moisture is the culprit without testing results and with this, any flooring/adhesive product I am aware of requires some type of moisture testing prior to installation. So talking to the adhesive and flooring manufacturer about their requirements should give you the information you need to then ask the installer for your specific information. Once you have this information, making a determination of cause may be easier.
I bought some kahrs engineered wood flooring. Arrived January 2018. Grande collection manor Oak. 3/4 inch T & G.
when the shipment arrived, I noticed that manufacturing date may be Nov. 2010. Is that acceptable? do these boxes (even though packed) have a shelf life? How would I make sure that boards are safe to use. Thank You.
Alison:
Thanks for the comment. Depending on the storage conditions and/or the amount of time the product is allowed to acclimate to the environment it is being installed in, things are probably ok. That being said, I would recommend contacting the manufacturer to get their input.
Thanks,
Jason