Specifying new concrete slabs for polishing

The Concrete Polishing Council (CPC) guide specification requires a preinstallation conference before placing concrete in areas scheduled for polishing. ACI SPEC-310.1-20 requires a preconstruction conference when specified.

Agree on the intended finish, acceptance criteria and responsibilities at this stage. Our preinstallation coordination guide covers the meeting and approval requirements.

For slabs on ground, the Concrete Polishing Association of America (CPAA) recommends fine grading the subgrade uniformly flat with a laser device. Rises and depressions can vary slab thickness and increase cracking potential. Depressions can also allow bleed water to pool over the vapor barrier, causing differential curing and drying that leaves surface blemishes.

Manufacturer mix guidance and project specifications can impose different requirements. RetroPlate’s mix recommendations, for example, apply to its polishing system; the Ashford Formula’s pozzolan threshold limits warranty coverage. They are not interchangeable limits for all polished concrete.

The cement, supplementary cementitious materials and admixtures guide compares the product-specific provisions. Where a project requires performance-based concrete specifications, resolve conflicts with prescriptive manufacturer guidance before placement; our concrete mix specification guide addresses that coordination.

Below-slab moisture control and the finish’s moisture sensitivity need separate attention. On projects governed by GSA P100, May 2024, the moisture-control provisions require a vapor barrier directly beneath slab-on-grade construction and where concrete structures separate conditioned and unconditioned spaces.

Moisture can affect topical color, densifier penetration, shine, cure marks and efflorescence, as Jennifer Faller describes in her Concrete Decor article on moisture and polished finishes.

Our vapor retarders and moisture-related finish guide explains the source-specific requirements and finish risks.

Flatness at placement does not necessarily describe the slab when grinding starts. CPC guidance identifies flatness at the time of polishing as important to selecting aggregate exposure and calls for early coordination of changes over time, including curling in slabs on ground.

See placement flatness versus flatness at polishing before selecting the final exposure class.

CPC Position Statement #5 identifies differences between machine-troweled and manually troweled surfaces: machine troweling produces a denser, harder and darker surface. Different densities also absorb added color differently.

The National Ready Mixed Concrete Association (NRMCA), in CIP 14, warns that finishing while concrete is bleeding or has water on the surface causes defects such as dusting, scaling, crazing, delamination and blisters.

Deeper grinding may reduce appearance differences caused by surface density, but it can change the intended finish and add cost.

Curing affects appearance as well as concrete development. NRMCA CIP 11 identifies wrinkles in plastic-sheet curing covers as a cause of dark streaks or mottling.

Do not assume that a densifier also serves as a curing compound.

Before polishing under ACI SPEC-310.1-20, confirm that curing followed the Contract Documents and complete the specified surface preparation. The curing compounds and densifier compatibility guide covers the product restrictions and preparation requirements.

Slab age alone does not establish readiness. ACI SPEC-310.1-20 requires confirmation that concrete compressive strength meets the Contract Documents before polishing. Its minimum 28-day cure provision applies to the mockup area.

CPC also warns that portland-limestone cement mixtures may need additional time to develop acceptable surface hardness. The strength, age and polishing-readiness guide separates these checks from a fixed calendar date.

Suspended slabs can deflect and develop high and low areas that produce different grinding depths and aggregate exposure. A ground-level mockup does not reproduce those conditions.

American Society of Concrete Contractors authors recommend locating the mockup on the suspended slab itself. Our suspended-slab polishing guide covers the appearance and coordination issues.

CPC Position Statement #1 calls for protection after slab placement and again after polishing. It places that responsibility with an organization present through construction that can control other trades’ activities and schedules.

Identify that responsibility before work begins. The construction protection guide covers both protection periods.