Quick Answer

Hard concrete takes a soft bond, and soft concrete takes a hard bond. The rule sounds backwards, but it is simple: the bond must wear at the right rate to keep exposing fresh diamond. Match it wrongly and the tool either glazes over or sheds its diamonds too fast.

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Why the Rule Is Reversed

On hard concrete (granite, quartz or flint aggregate), a soft matrix wears quickly, constantly exposing fresh sharp diamond to attack the hard surface. On soft concrete (limestone, sandstone, soft shell aggregate), the abrasive paste wears the bond fast; a hard matrix resists that wear so diamonds stay locked in and work longer.

Identifying Your Concrete

• Scratch test: a hardened steel nail barely marks hard concrete and gouges soft concrete
• Hardness tester: measures in MPa and gives a clear number to match against bond charts
• Grinding feel: if the tool bounces or slows, the concrete is likely hard

Common Mistakes

• Using a soft bond on soft concrete: diamonds pop out in minutes, tool looks barely used but stops cutting
• Using a hard bond on hard concrete: the surface polishes instead of cutting, and the tool glazes

How to Confirm Before You Buy

Order a test plate first, grind a small area, and check both the cut rate and the segment wear pattern. A properly matched bond shows a steady, even wear line and consistent cutting. Ask your supplier for a bond chart that maps concrete hardness to bond type.

Frequently Asked Questions

Q: What if my floor has both soft and hard patches?

A>Use the bond for the majority, or switch between bonds on different passes and adjust speed.

Q: Can I make one bond work everywhere?

A>Medium bond is the safest default, but you lose efficiency at both extremes.

Q: How quickly should a segment wear on the right bond?

A>A typical metal bond segment on matched concrete lasts hundreds of square metres; if it disappears in hours, the bond is too soft.

Q: Does grit change the bond rule?

A>No. Bond is matched to the concrete; grit is matched to the stage of the job.

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Conclusion

Identifying concrete hardness and matching the bond is the highest-impact decision in grinding. Do it before you order, confirm with a test plate, and your tooling cost per square metre drops dramatically.