Quick Answer

Use a Mohs scratch test with common objects (fingernail, coin, knife, file) or a rebound hammer to estimate concrete hardness, then translate it to a bond: soft concrete → hard (soft-cutting) bond; hard concrete → soft bond. Testing one small area in each pour saves money on the wrong tooling.

Key Features of Hardness Testing

• Mohs scale for concrete: typically 3–7. Fingernail (2.5) scratches soft slabs; a steel file (6.5) only scratches very hard slabs.
• Rebound hammer (Schmidt hammer): measures surface hardness and correlates with compressive strength (PSI/MPa).
• Consistency check: test several points — a slab can be hard in one bay and soft in another.
• Surface condition: curing compounds, coatings or dust affect readings; test on clean, bare concrete.

How to Choose the Bond

Concrete Hardness	Typical PSI	Recommended Bond
Soft	under 3,500 psi	Hard / soft-cutting bond
Medium	3,500–5,000 psi	Medium bond
Hard	5,000+ psi	Soft / aggressive bond

How to Test Step by Step

1. Clean a 1 m² area of dust, laitance and curing compound.
2. Scratch with a coin, then a knife, then a file; note which one cuts.
3. Take 5 rebound hammer readings and average them.
4. Match the result to a bond and order a small test set before bulk buying.

Frequently Asked Questions

Q: Is a Mohs test accurate enough for buying tooling?

A: It is a reliable field indicator. Combine it with the rebound hammer for a confident bond decision.

Q: Can I test hardened concrete with a nail?

A: A nail should not scratch quality concrete — if it does, the surface is soft or has laitance.

Q: Does hardness change with depth?

A: Yes. Surface hardness can differ from deeper concrete; test where you will actually grind.

Conclusion

A 10-minute hardness test prevents expensive mistakes. Pair the right bond with quality tooling such as the WerkMaster Single Bar Segment (JOY-WMS-B1) or the Diamond Trapezoid Grinding Disc (JOY-TP-R3) for consistent, predictable grinding.