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| Photo 2 |
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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.
pH testing
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| Photo 3 |
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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.
Rinsing the slab
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| Photo 4 |
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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:
Vinegar
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| Photo 5 |
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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.
Muriatic acid
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| Photo 6 |
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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.
