Measuring Concrete Hardness

As we all know “concrete gets hard”, but how is that measured and what does that mean to you when installing polymer coatings or systems?

The two most common tests are:

1. Rebound Test Hammer Method

2. MoHS (Measure of Hardness Scale) Scratch Test

The rebound hammer method, also commonly referred to the Schmidt Hammer is a test to check the impact strength of a concrete substrate. This hammer has a piston that is engaged easily by the user, then when pushed on to the concrete “fires”. The scale on the hammer then produces a reading in PSI (pounds per square inch) of the tested area. Low readings could signify faulty strength concrete or hollow spots. These need to be addressed prior to doing any surface prep.

image credit: http://www.huatecgroup.com/china-2_207j_60mpa_concrete_test_hammer_schmidt_hammer-1174652.html

Once a contractor begins the surface preparation, it has been determined they accept the slab “as is” and are responsible for completing their proposed Scope of Work. This tools retails between $350-$600 and is a very useful tool.

The second test is more common today, especially with the large amount of concrete polishing that is being done.

A standard test kit contains nine (9) different tipped “pencil” looking picks. These are typically numbered #2 through #9. Diamonds are rated 10 on the MoH scale and are the hardest mineral known to man. Conversely talc being soft (baby powder) has a MoH rating of 1. See the below graph.

The test is performed by starting at a low pick #, such as 3 and making a two inch line coming toward you with downward pressure (try to think of breaking your lead in pencils). The higher numbered pick you use that leaves a noticeable “scratch” is determined to be that concretes MoH rating. I recommend testing multiple areas of your project.

This information is of vital importance as it will help you to select the correct diamonds for your prep, saving you time and money.

Lastly, it should be noted that all concrete in the United States DOES NOT have the same hardness! Factors such as available aggregates and climatic differences will produce different MoH ratings. In the Midwest ratings typically range from #5-#7, where in areas such as Florida or Arizona it is common to get #3 and #4 readings Know your enemy and you will be much more successful and profitable.

image credit: http://bedrocksupplies.com/products/mohs-hardness-test-kits.html

Written by Mike Lenk, VP of Sales

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