Polished Concrete Seed-and-Cream Method

Why this matters

Architectural polished concrete sells on aggregate exposure: pebbles, granite chips, recycled glass, or specified decorative aggregates set in the surface and revealed by grinding. The seed-and-cream method is the way to control exactly what aggregate the polisher exposes. Place a standard concrete slab and the surface aggregate is whatever the producer's coarse aggregate happens to be; broadcast a curated decorative aggregate into the still-plastic surface and then trowel a cream coat over it, and the polisher reveals exactly the look the architect drew. The method requires precise timing on a placement-day window, and a finisher who understands that the polishing crew is depending on what the slab crew does in the next 90 minutes.

What the polishing crew needs from the placement

A polishable seed-and-cream slab needs three properties: the seeded aggregate sits at a controlled depth in the surface (not floating on top, not driven deep into the matrix); the cream coat above the aggregate is hard and dense, not a wear coat applied later; the floor is flat enough that the polisher does not have to grind through the cream coat in low spots to reach the aggregate elsewhere. ACI 117 specifies floor flatness (FF) and floor levelness (FL) numbers; a polished floor target is FF 50 / FL 30 minimum, FF 60 / FL 40 for premium installations, measured by floor profilograph per ASTM E1155.

Step 1: Mix design and pour

Specify a low water-cement ratio mix (0.45 maximum), 4,000 psi minimum 28-day strength, with a water reducer for workability and a Type B retarder if the pour will exceed 2 hours of placement. Coarse aggregate at 3/8 to 3/4 inch maximum; pea-gravel grade is acceptable when the seeded aggregate is the decorative show. Avoid mixes with high fines from manufactured sand because the cream coat must rise cleanly during finishing. Reinforcement is welded wire fabric or fiber per the structural engineer; no surface reinforcement near the finished face.

Place by chute or pump, screed flat using a laser screed or vibratory truss screed. Bull float once to consolidate. Wait for the bleed water to break.

Step 2: Seed the decorative aggregate

When the surface has firmed to the point that bleed water has fully evaporated but the surface still accepts a print (typically 30 to 90 minutes after screeding depending on weather), broadcast the seeded aggregate evenly across the surface. Standard residential broadcast rate: 1 to 3 pounds per square foot depending on aggregate size and desired coverage. Use a hopper spreader or a hand-broadcast pattern; visual coverage targets 60 to 80 percent of the surface at the broadcast stage.

Seeded aggregates: tumbled marble or granite chips 1/4 to 1/2 inch (architectural standard), recycled glass cullet (premium decorative, sourced from companies such as Trinic and EcoGranite), beach pebbles 3/8 to 1 inch for heavy-look exposures, recycled aluminum or copper chips (specialty). Match the aggregate hardness to the desired polish; soft aggregates (limestone, recycled glass) polish faster but show wear; hard aggregates (granite, marble) take longer to expose but hold the polish for decades.

Step 3: Embed the aggregate

Walk in or hand-tamp the aggregate to drive each piece flush with the surface, leaving the top 1/8 to 1/4 inch of the aggregate just below the cream coat. Use a tamping float or a magnesium darby; do not use a steel trowel at this stage because steel pushes the aggregate too deep. The target depth is determined by how aggressively the polisher will grind: 1/8 inch embedment for a salt-and-pepper exposure, 1/4 inch for a heavy-aggregate exposure.

A common error is broadcasting too late; if the surface is past the soft-print stage, the aggregate will not seat and will pop loose during polishing. Verify by walking the slab in flat-soled boots; a properly timed surface accepts the print of each step at about 1/16 inch.

Step 4: Apply the cream coat

Once the aggregate is seated, apply a cement-rich cream coat that brings the surface up to about 1/16 inch above the highest aggregate. The cream coat is finished with a power float and power trowel pass. The polisher will grind through the cream coat to expose the aggregate; controlling cream-coat thickness and density is what controls the consistency of the exposure across the slab.

Cream-coat options: power-trowel the existing surface cream that comes from working the slab (zero added material, cheapest, lowest control); broadcast a dry-shake mineral hardener (Mapei Mapecrete, Master Builders MasterTop, BASF Emer-Tip) at 1 to 2 pounds per square foot and float it in for higher density and color uniformity (this is the architectural standard); broadcast a colored dry-shake (charcoal, black, red, custom) for a tinted cream layer that contrasts the aggregate.

Step 5: Cure for polishing

Cure the slab in a way that does not contaminate the surface. Spray-applied curing compounds (ASTM C309) leave a film that has to be ground off; the polishing process removes it as part of the early grind. Water curing (wet burlap, plastic sheeting) is cleaner but requires monitored moisture for 7 days. Polished slabs targeted for early polishing can cure under plastic sheeting for 14 days before any grinding starts; for tight schedules, 7 days minimum.

Step 6: Polish and densify

The polishing crew starts with metal-bond diamonds at 30, 60, then 120 grit to expose the aggregate. Densifier (lithium silicate per ASTM C1262 / typical product such as Prosoco Consolideck LS) is applied at the 200 grit stage to lock the cream and aggregate into the cementitious matrix. Polish progresses through resin-bond diamonds 100, 200, 400, 800, and 1500 grit for a typical Class 3 architectural polish; 3000 grit for Class 4 mirror finish per the Concrete Polishing Council classification.

Acceptance and protection

Verify the finished surface to the project specification: aggregate exposure depth, gloss reading (taken by 60-degree gloss meter, ASTM D523, typical target 60 to 80 for Class 3 polish), distinctness of image (DOI) per ASTM E430 for premium projects. Protect the finished floor with Ramboard or equivalent until project handover; polished concrete is a finished floor, and any trade walking on it before handover risks scratches that require re-polishing to remove.

References

1. ACI 117, "Specifications for Tolerances for Concrete Construction and Materials," floor flatness and levelness.
2. ACI 302.1R, "Guide to Concrete Floor and Slab Construction," and ACI 302.2R for slabs that will receive polished or moisture-sensitive finishes.
3. ACI 310, "Guide to Decorative Concrete," seeded aggregate methods.
4. ASTM E1155 (FF/FL measurement), ASTM C309 (curing compounds), ASTM D523 (gloss).
5. Concrete Polishing Council (CPC) of ASCC, "Polished Concrete Glossary and Specifications" including Class 1 through 4 polish classifications.