The mechanics of installing finished flooring on concrete slabs may be straightforward, but what is not so straightforward is determining whether the condition of the concrete is suitable for the installation. The two factors that determine the concrete’s suitability for accepting the finished flooring product are relative humidity (RH) and pH. These factors are related. When the RH rises the pH tends to rise as well.

Many suppliers of both the flooring and the adhesives used to attach flooring to the concrete slab have RH and pH criteria that the installer must follow to ensure that the installation will be successful and have a long life.

We understand that there are experts in the industry that do not believe that measuring pH is necessary as long as the RH levels are low. However, we are supplying helpful information in this article for people that are required to do this test because it is required by suppliers or is specified as a test that needs to be completed.


What is pH?

The “p” stands for “potential” and the “H” stands for “hydrogen.” So, pH is used to measure the concentration of hydrogen ions in a substance or solution. Most people are familiar with the effects of substances with high hydrogen ion concentrations and those with low hydrogen ion concentrations. Lemon juice and your car battery fluid are acidic and have low hydrogen ion concentrations while lye and baking soda, alkaline (bases), have high hydrogen ion concentrations.

The pH scale runs from 0 to 14 with water being in the middle with a pH of 7. Acids have pH levels lower than 7 down to 0 while alkalines run from 7 to 14. Or to put it another way; the level of hydrogen ions in an acidic solution is higher than that in pure water while the level in alkaline (base) solutions is lower than that in pure water. The pH scale is logarithmic. This means that for every unit increase or decrease in pH there is a 10 times increase or decrease respectively, in hydrogen ions in the solution.

What can pH do to concrete and to hardwood flooring adhesive?


Flooring Adhesive

The surface pH of the concrete slab on which flooring products are to be installed can react adversely with the adhesive used to attach the flooring and cause damage to the flooring if the pH exceeds the adhesive vendor’s pH limits.

Since flooring installers are mostly concerned with the effect of pH on the adhesive used to attach the flooring to the concrete slab, they are only interested in measuring the pH at the concrete surface to ensure that it complies with the adhesive manufacturer’s requirements. Typically, the adhesive vendors specify a pH of 10 or less. Since freshly poured concrete has a pH between 11 and 13, the concrete must be given time to cure and for the pH to decrease due to interaction with the CO2 in the air.

Failure to comply with the flooring adhesive vendors pH requirements can lead to softening of the adhesive, adhesive dry-out, and bubbling, all of which result in failure of the adhesive to properly attach the flooring to the concrete slab. Any or all of these effects can result in undesirable effects on the finished flooring system.


pH Testing for Compliance with Adhesive Vendors pH Requirements

There are at least two ways to test for the pH of the concrete surface prior to installing hardwood flooring – the lo-tech method and the hi-tech method.


The Lo-Tech Method

One of the attractions of this method is that very little equipment is required. There are no batteries that can die, no delicate equipment that can get damaged or go out of calibration under field conditions, and it’s inexpensive.

The lo-tech equipment consists of a color-coded pH scale and a roll of pH test paper. There is one color on the scale for each unit change in pH from 0 to 14. 

To use the lo-tech method, proceed as follows, but also refer to ASTM F710 and to the resilient flooring manufacturer’s written instructions for guidelines on acceptable testing methods and acceptable pH levels.

  • To test for pH at the surface of a concrete slab, use wide range pH paper, its associated pH chart, and distilled or deionized water. 
  • Place several drops of water on a clean surface of concrete, forming a puddle approximately 1 in. (25 mm) in diameter.
  • Allow the puddle to set for 60 (+/- 5) seconds, then dip the pH paper into the water. 
  • Remove immediately, and compare to the chart to determine pH reading. 


Hi-Tech Method

The advantage of using the Hi-Tech method is that the results are obtained with a digital device that allows for numerical measurements. To use the hi-tech method proceed as follows, but refer to ASTM F710 and the resilient flooring manufacturer’s written instructions for guidelines on acceptable testing methods and acceptable pH levels.

  • Calibrate the sensor using 7.0 and 10.0 pH solutions. 
  • Place several drops of distilled or deionized water on a clean surface of concrete, forming a puddle approximately 1 in. (25 mm) in diameter.
  • Allow the puddle to set for 60 (+/-5) seconds. 
  • Put the tip of the sensor probe into the water and read the pH. 
  • Record the pH reading and the concrete temperature. 

Since moisture tests on concrete are taken on at least three surface locations for the first 1000 ft2 of the slab and an additional location for each additional 1000 ft2 or fraction thereof, pH tests would be best taken at the same locations for consistency sake.

Test results by a number of investigators have shown reasonable agreement between the pH values given by the lo-tech and the hi-tech methods.