Determining Concrete Hardness

The hardness of the concrete surface has a direct impact on which metal bond diamond tool you will start grinding with when polishing or preparing for a coating. Determining the surface hardness should be performed on every project when the first cut is with a metal bond tool.

Mohs Hardness

The Mohs hardness scale is a widely used method for measuring the relative hardness of minerals. Developed by German mineralogist Friedrich Mohs in 1812, the scale assigns a numerical value to the hardness of a mineral - in our case concrete - based on its ability to be scratched by another mineral. The scale ranges from 1 to 10, with 1 being the softest and 10 being the hardest.

Mohs Hardness Test

The Mohs hardness test is a way of measuring the scratch resistance of concrete by comparing it with other minerals or tools with known hardness. To perform the test on concrete, drag a reference pick across the concrete surface, and then observe the scratch left behind. The last pick that will scratch the surface will give you the concrete surface hardness.

A Mohs hardness test kit is a set of 8 picks with different hardness values (2-9) that are used to determine the surface hardness. The test is simple but not exact. When testing the hardness of the concrete surface, it will get you in the hardness ballpark to select the metal bond tooling to start with.

Before testing the hardness, make sure there are no sealers or curing compounds on the surface. Film-forming materials will impact the results and must be removed to obtain accurate hardness values.

Metal Bond Tooling

When using a diamond tool to grind, drill, or cut concrete, the industrial diamond does all the cutting work. During the cutting, drilling, or grinding process, fresh diamonds need to be continuously exposed to perform their job. In all diamond tooling the material that bonds or holds the diamond, whether it be a grinding tool, saw blade, or drill bit, varies in composition. The hardness of the concrete has an impact on one of the bonding materials, the metal in the metal-bonded tool.

When the metal bond tool is used, the concrete needs to wear away the metal bonding material holding the diamonds, exposing fresh-cutting diamonds. If you do not expose fresh diamonds regularly when grinding, the tool becomes inadequate for cutting. If the diamonds are exposed before they are worn, they will cut extremely well but the tooling will have a very short service life.

Matching the metal bond material to the hardness of the concrete is when the Mohs Hardness Test comes into play. The softer the concrete, the harder the metal bond material needs to be. Soft concrete is very aggressive and if it is cut with a soft bond tool, the bond material will wear away extremely fast before the diamonds have been worn and need to be replenished. Changing tooling often will cost time and buying additional unnecessary tooling will cost money.

Manufacturers can label their metal bond tooling in two different ways: hardness of the concrete or hardness of the metal bonding material. These designations are completely opposite to each other so make sure when ordering metal bond tooling you understand how they are labeling them.

Mohs Hardness Concrete Hardness Metal Bond Tooling

Conclusion

Knowing the hardness of the concrete surface will help you select the bond of your initial metal cuts before putting a grinder on the slab. Is it a foolproof method? No, but is an educated start. If possible, determine the concrete hardness during the initial site visit. This will help in deciding which bond should be brought to the job to minimize wasting valuable time. We have been on projects when the incorrect bond was brought, and the contractor needed to drive back to the shop or have them shipped next-day air from the supplier to the site which costs time and money.

Niagara Machine carries all the necessary supplies to ensure you correctly measure concrete hardness and choose the right tooling for your grinder. From the Hardness test kit to the metal-bond diamonds, we have it all and can help you decide the best tools for preparation.