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Planning The New Concrete Mix For Polishing Concrete Floors

Posted on February 24, 2014 at 3:25 PM

We at Concrete Art-FX, have been asked numerous times by clients, GCs, and architects alike as to what is the optimal concrete mix and guidelines to a new concrete floor pour to get the best possible result for concrete polishing?.

 Since the answer is not a simple one liner, we decided to turn to Canadian Polished Concrete Association for their recommendation on this question.

Although it is not mandatory to follow the recommendations and guidelines below to have a beautiful polished concrete floor, but it is certainly excellent advice that will eliminate all the potential errors we sometimes encounter by concrete contractors that have no knowledge of polished concrete process and how it works.

CONCRETE MIX DESIGN

 

In addition to ACI standards for mixing concrete, CPAA recommends the following:

For the portion of the concrete that will be polished, each mix ingredient should be from the same source, from the same respective batch, and each delivered to the concrete producer in one delivery.

Type I portland cement according to ASTM C150 - 09 Standard Specification for Portland Cement.

A water-to-cement (W/C) ratio is .45.

One of the strengths of diamond polished concrete is the range of aggregates that can be exposed, therefore a uniformly graded mix of not less than 3 aggregate sizes - fine, intermediate, and large - will yield the best visual quality. It should be understood that since aggregates are heavy and there is a cost for transporting aggregate from its source to the batch plant is a factor in the cost of the concrete, locally available aggregates are used to produce concrete. This is an important design consideration because, depending on the project location, there may be few, if any, opportunities for other aggregates to be used in the concrete mix.

The mix should not be air entrained.

Inclusion of admixtures, plasticizers, slag, fly ash, or other products replacing portions of the portland cement in the concrete mix is not recommended. If any of these are used, the total volume should not exceed 20 percent of the portland cement volume.

Admixtures should not be calcium chloride based.

If an integral color is included in the concrete mix, the minimum batch size should be 3 cubic yards.

During batching, the incoming material consistency should be monitored and controlled.

The compressive strength should not be less than 3,500 psi.

PLACING CONCRETE

In addition to ACI standards for placing concrete, CPAA recommends the following:

Transit mixer drums should be properly washed out after each concrete mix discharge and before a new batch is loaded.

Water added by the transit mixer driver should be monitored and controlled.

During mixing, transporting, and placing the concrete mix, monitor and control the temperature to not more than 85 degrees F.

The slump at the point of discharge should be 5 inches, plus or minus 1/2 inch.

FINISHING CONCRETE

In addition to ACI standards for finishing concrete, CPAA recommends the following:

After placement of the concrete mix, strike off the surface using a laser screed, then bull float at 90 degrees to the screed pull direction, vib and consolidate, and level to specified elevation. A 10 foot check rod is recommended, however, if not available bull floats should be 6 foot long; smaller bull floats may be used on sloping surfaces.

When placing concrete mix at edges, use a 36 inch long metal or wood edged screed and run parallel with the formwork or edge after the initial screeding and before floating. Hand floating should be parallel to the edge and performed in 24 inch increments to avoid lifting or depressing the surface. Avoid pulling excessive amounts of the concrete mix to the edges by either not using hand tools more than 24 inches from the edge, or floating in a fan direction.

When little or no bleed water is present and concrete mix has sufficiently hardened to support finishing equipment without causing imperfections in the surface, begin machine floating using pans and make two passes.

To improve the possibility of achieving the specified flatness/levelness requirements, check and re-straighten if necessary using a 10 foot or longer highway straight edge or bump cutter.

When machine floating edges, use pans and overrun the formed edge by 5 inches. For both walk-behind and riding equipment, make the first pass along the edge with the left side, or cutting side, of the equipment to pull down high spots of the surface. Make a second pass along the edge with the right side, or filling side, of the equipment to fill low spots in the surface.

Steel trowel the surface in three passes without burning the surface or burning the aggregate (plastic trowel blades will prevent burning the aggregate).

Lightly hand or machine tool edges construction joints and exercise care that edges are not depressed or chattered along bulkheads, formed edges, columns, and pipe penetrations.

Do not dust the finished surface with dry portland cement or sand to accelerate curing and drying.

CONCRETE CURING AND DRYING

CPAA recommends evaporation control and wet curing concrete slabs according to ACI 308R-01: Guide to Curing Concrete (Reapproved 2008) without the use of topically applied curing compounds. While topically applied curing compounds may assist with curing during the first few days after concrete placement, they retard concrete drying in the weeks and months after curing and may cause the slab to be too wet when the time comes for diamond polishing or floor covering application.

Densifiers and hardeners should not be applied to concrete.

FLOOR FLATNESS/LEVELNESS CRITERIA

CPAA recommends specifying the following tolerances:

Specified Overall Value Minimum Local Value FF Floor Flatness 50 35 FL Floor Levelness 30 20 The floor flatness and levelness should be tested within 8 hours after completion of the final troweling operation according to ASTM E1155 - 96(2008) Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers by an independent testing agency experienced with the testing procedure and possessing the necessary equipment.

Additionally, a remedy for out-of-tolerance work should be specified.

CONTRACTION JOINTS

Based on independent studies, CPAA recommends saw-cutting slabs as soon as possible after finishing using a saw blade that has a triangular arbor configuration to reduce edge raveling or dislodging aggregates at the following spacing to minimize slab curling and cracking:

Slab thickness, inches Spacing, feet on centers each way 4 10 6 12 8 15 Since concrete shrinks during curing and drying in two directions, cracks are minimized when the area between contraction joints is as close to a square as possible.

 

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