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Once pH testing is done and you have high readings there seems to be a lot of different opinions on what needs to be done.   A concrete slab that is carbonated and ready to receive a flooring adhesive should have a pH of about 8.0.  This means the surface of the concrete has had minimal moisture vapor movement.  Excessive moisture vapor movement will bring additional hydroxides (alkalis) to the surface and will cause the pH level to increase.  The surface of the concrete needs to be exposed to carbon dioxide, which neutralizes the existing pH on the concrete’s surface and will penetrate down about one-millimeter.  If you were to dig down beneath the carbonation layer you will find the internal pH of the slab to have a pH of about 12.0.  Remember the pH scale is a logarithmic scale with each number being 10-time greater than the previous one.  Example:  a ph of 7.0 is neutral and a pH of 10.0 is 1,000 times greater than a pH of 7.0.  Most adhesives on the market have a pH tolerance of 9.0 with some going as high as a pH of 10.0.

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pH testing

pH testing is a relatively simple procedure.  Photo 1 shows pH testing equipment.

1. Clean the concrete to bare concrete.  This means to clean the concrete to a residual free, dust free, porous surface. Photo 2 shows a porous concrete surface.

2. Do not dig down into the concrete below the carbonation layer.

3. Place a 1 – 1-1/2-inch puddle of distilled or de-ionized water on the surface of the cleaned concrete. Photo 3 shows a pH test.

4. Allow the water to stand for one minute.

5. Place the pH strip into the water and remove immediately.

6. Compare the color of the pH strip with the color chart to determine the pH

So what do you do if your pH readings are high? First and most important, make sure your moisture testing was done correctly.  High pH and high moisture vapor emissions are very common and travel together.  If you’re using a calcium chloride testing procedure it is extremely easy to end up with false positives (low readings) unless you follow the ASTM F-1869 protocol to the letter.  It is much the same with the in-situ RH probe; if your equipment is out of calibration or you fail to follow the ASTM F-2170 protocol, you can get incorrect readings.  Remember the reading for the in-situ probe is supposed to be at the 40 percent level of the thickness of the slab, not the gradient of the hole. 

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Rinsing the slab

Once the moisture testing values are confirmed, and the pH is high, you can try to neutralize the slab by rinsing with clean neutral water, using the following procedure.

1. Start with a clean, porous concrete

2. Spray a small area with the clean neutral water. Photo 4 shows rinsing the slab.  If in doubt about the water take a pH paper and test the water.

3. Immediately after the application of the water, thoroughly wet vacuum the area rinsed to remove any excess water. Photo 5 demonstrates wet vacuuming.

4. Allow it to dry for 24 hours and retest to be sure the slab is neutralized.

In most cases the rinsing with clean neutral water will eliminate the high pH.  If the pH is still high after the rinse, then it is necessary to neutralize the pH.  This is done by using an acid rinse.  When choosing an acid, be careful of what type of acid you use.  Here are some reasons why not to use the following:

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Some manufacturers recommend the use of vinegar (acetic acid) and water.  I do not recommend the use vinegar, because if there is any vinegar residue left in the slab it will continue to break down the Portland cement, which is the binder use to hold the concrete together.  Long term exposure to acetic acid will break the Portland cement down.

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Muriatic acid

Muriatic acid is a diluted version of hydrochloric acid and is very dangerous to use especially indoors.  The recommended mix ratio for muriatic acid is 10:1 ten parts water to one part acid.  The problem is the fumes that come off the acid wash are corrosive to metals.  So every thing in the area made of metal needs to be removed or protected.  I was involved in a court case where muriatic acid was used to rinse a concrete slab and the fumes destroyed some high tech machinery due to the corrosive effect to the metal and cost several million to replace.  Like acetic acid, if any residue is left in the concrete it will destroy the Portland cement.  

Acid washing

My recommended choice for acid washing is to use a common product found in almost every grocery store.  The product is club soda (carbonic acid).  It is mild, safe to use and effective in neutralizing pH. Photo 6 shows spraying of the Club Soda.

1. Spray the Club Soda on the surface of the concrete.

2. Wet Vacuum the excess with a wet vacuum.

3. Immediately rinse the Club Soda (do not allow it to dry on the concrete) with clean neutral water.

4. Wet vacuum the excess water and allow it to dry 24 hours.

5. Test the surface to be sure the pH is neutralized.