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Separate age, compressive strength and surface hardness
Before polishing, check concrete strength and surface hardness against the project requirements. Slab age alone does not establish readiness. ACI SPEC-310.1-20 requires confirmation of the compressive strength specified in the Contract Documents. Its 28-day minimum cure applies to the mockup area, while Concrete Polishing Council (CPC) guidance addresses surface-hardness development in portland-limestone cement mixtures.
Our new concrete slab specification guide covers the mix, finishing and curing decisions that precede these checks.
| Source | Requirement or guidance | What it means for scheduling |
|---|---|---|
| ACI SPEC-310.1-20, §1.8.3.2 | Concrete in the mockup area must cure for at least 28 days. | This provision applies to the mockup, not every production slab. |
| ACI SPEC-310.1-20, §3.1.1.1(c) | Confirm that compressive strength meets the Contract Documents before polishing. | Check the project’s required strength; this clause supplies no universal value. |
| RetroPlate mix-design guide | Recommends concrete engineered to reach at least 4,000 psi (28 MPa) at 28 days. | This is a mix-design recommendation for the RetroPlate system, not a universal polishing start date. |
| CPC Position Statement #8 | For portland-limestone cement slabs that do not achieve Mohs hardness greater than 4.5, additional densifier treatments may be required to accelerate surface-hardness development. | This addresses surface hardness and possible treatment; it does not establish compressive strength or a universal acceptance threshold. |
The ACI requirements apply when incorporated into the Contract Documents; project-specific requirements govern a conflict. Our ACI 310.1-20 specification guide explains that relationship.
Plan and check surface hardness
CPC says portland-limestone cement mixtures may need additional time to develop acceptable slab hardness, without setting a mandatory wait, and that Type IL can be polished effectively with adjustments. The Mohs value in that guidance concerns the slab surface and does not replace the compressive-strength confirmation the project documents require.
Allow for slower hardness development with portland-limestone cement
CPC Position Statement #8 says mixtures containing portland-limestone cement (PLC) may need additional time to develop acceptable slab hardness—potentially 45 days or more. The time depends on:
- Concrete mixture design.
- Ambient temperatures.
- Placing and finishing techniques.
- Curing methods.
That guidance does not establish a mandatory 45-day wait or guarantee readiness on day 45.
CPC also states that Type IL cement can be placed, finished and polished effectively, with adjustments to mixture design, placing and finishing techniques, and polishing methods.
Check surface hardness separately from compressive strength
The Mohs value in CPC’s PLC guidance concerns the slab surface. It does not replace the compressive-strength confirmation required by the project documents.
For surface assessment and treatment considerations, see our surface hardness and soft concrete guide. The broader substrate acceptance guide covers the remaining checks before work starts.
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Frequently Asked Questions
Age alone does not establish readiness. ACI SPEC-310.1-20 requires production concrete to meet the compressive strength in the Contract Documents before polishing. Its 28-day minimum cure applies to the mockup area. RetroPlate separately recommends a mix designed to reach at least 4,000 psi (28 MPa) at 28 days.
No. CPC says PLC mixtures may need 45 days or more to reach acceptable hardness, depending on the mix, temperatures, placing, finishing and curing. It gives no fixed waiting period and does not rule out polishing Type IL concrete.
No. Surface hardness and compressive strength are different properties. CPC’s Mohs guidance for PLC slabs and possible additional densifier treatment does not replace confirmation of the compressive strength required by the Contract Documents.
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