Concrete pH and Relative Humidity: Why Both Matter for Flooring
Understanding concrete pH and relative humidity (RH) in 4 steps:
pH measures how acidic or alkaline (basic) a water-based solution is. The pH scale commonly runs from 0 to 14, although readings can fall outside that range. A pH of 7 is neutral. Lower numbers are acidic, and higher numbers are alkaline. Strong acids like battery acid fall near the bottom of the scale, and strong alkaline products like liquid drain cleaner fall near the top.
Concrete is naturally alkaline, and that matters when you install flooring over it.
Interactive pH
Interaction is the point. Pure water is pH neutral, but when cement and water are mixed, the chemical reactions that harden concrete also make it highly alkaline. That alkalinity is normal. The challenge comes when you install a floor covering. Flooring and adhesive manufacturers specify the concrete surface pH their products can tolerate. If the surface pH is outside that range, the adhesive bond may break down, and the flooring project is jeopardized.
Moisture is a big part of the story. Moisture moving up through the slab can dissolve alkaline salts in the concrete, creating a high-pH solution at the surface, right where the adhesive is. If the moisture and pH are high enough, they can break down the adhesive. So a slab’s RH affects not only when it’s ready for flooring but also how well the adhesive holds up once the floor is installed.
Understanding RH
Managing moisture in the slab is essential for a successful installation. After concrete is placed, the cement hydrates, a chemical reaction that uses some of the mix water. The excess water has to leave the slab as it dries, usually through the top surface. The air around the slab affects how fast that happens: lower ambient RH speeds up evaporation, and higher RH slows it. This is why concrete needs adequate time to cure and dry before the floor covering goes down.
Testing the slab’s RH tells you when it has reached the level the flooring manufacturer specifies. A separate surface pH test tells you whether the pH is in range for the adhesive you’ve chosen. With both in range, you can lay down the floor covering with confidence that a properly selected adhesive will bond as it should.
Magnify
Now multiply that by a large project. A hospital, for example, can have many floors of concrete, and every area that gets a floor covering needs to be ready for it. When flooring fails on a project that size, the repairs and delays can be costly, and disputes over who is responsible can follow.
Concrete, adhesive, and flooring manufacturers publish guidelines that installers should follow before using a product. Jobsite conditions such as temperature and humidity also affect how fast a slab dries. Check that your chosen flooring product and adhesive are compatible with the slab’s measured RH and pH.
Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry
Measure
Simple assessment tools exist for both pH and RH. If you’ve ever maintained a fish tank, you already know that pH tests are simple. The Wagner pH Test Kit uses test strips and distilled water to assess concrete pH from 6.5 to 13.0. (ASTM F710 includes a surface pH test.)
Testing for RH takes more equipment. With the Rapid RH® system, you install calibrated Rapid RH L6 Smart Sensors in holes drilled to the depth ASTM F2170 specifies: 40% of the slab’s thickness for a slab drying from the top only, or 20% for a slab drying from both the top and bottom. Once the sensors are installed, they need to equilibrate for at least 24 hours (per ASTM F2170) before the final readings are taken. You read them with the Total Reader® and store the data in the DataMaster™ L6 app. Because the sensors stay in place, you can take readings as often as needed to see when the slab reaches the RH level the flooring manufacturer specifies.
Moisture and pH work together, so test for both. Knowing the slab’s RH and surface pH, and comparing them with the flooring manufacturer’s requirements, helps make sure the adhesive bonds as it should.
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 16th, 2026
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