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Mapping the Critical Path to a Successful Polishing Project

Feature Article: Designers Corner

By Dan Farmer,

Achieving a Quality Polished concrete project does not require magic but it does require a systematic approach and careful attention to details. Since the concrete itself provides the finished surface, there is no opportunity to "cover past sins" with a finish product manufactured to high quality standards in a controlled environment. Therefore polished concrete presents little leeway for shortcuts or careless preparation. Preparation for polished concrete begins with the careful and systematic selection of materials and activities that go into producing a uniform, stable and crack-free product to which the polisher can apply his craft and then protecting this finished surface for the balance of the construction duration to deliver a clean undamaged product on opening day.


Sub-grade preparation is critical because it is the raft which everything rides on. It must be uniform, stable and predictable. This is best performed under the guidance of geotechnical engineers specializing in predicting the behavior of soils under different conditions. This in turn normally requires the completion of a subsurface investigation to determine soil types and depths. The site should be excavated to sub-grade and then proof-rolled in the presence of the geotechnical engineer to determine stability and compaction. If necessary, unsuitable soil should be removed and replaced with earth suitable for engineering fill. Any abandoned existing structures which have the potential to introduce water under the completed slab should be removed and their trenches filled and compacted. The sub-grade should be uniformly compacted to the level recommended by the geotechnical engineer (usually to at least 98% of maximum dry density within 3% of optimum moisture content in accordance with ASTM Specification D 698 (standard proctor) and one hundred percent (100%) for the upper 24 inches beneath slabs). Utility piping below slabs should be bedded in drainage stone or sand. Trenches for utilities and over excavated holes for foundations and other below-slab structures must be re-compacted with the use of vibratory plate compactors to eliminate the opportunity for differential settlement. Sub-grade should be protected from degradation by water until slab is poured.


Base stone should be placed on top of the sub-grade and compacted to a uniform thickness of not less than 4 inches. Since the base stone’s primary function is to provide a channel for moisture to escape and to cushion the slab against the hydraulic forces of subsurface moisture and soil expansion, it should be open graded stone such as ASTM #57 or 67 stone or washed, evenly graded mixture of crushed stone or crushed or uncrushed gravel, with 100% passing a 1-1/2" sieve and not more than 5% passing a No. 4 sieve.


A vapor retarder is not typically required since there is no secondary finish applied to the top of the slab which requires protection from subgrade moisture. Likewise, elimination of the vapor retarder reduces the tendency for shrinkage cracks and curling since water can now dissipate from both top and bottom surfaces of the slab at a comparable rate.


If required by code, the vapor retarder should have a Class C rating in accordance with ASTM E1745 Standard for Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs and be no less than 10 mil thick in accordance with ACI 302.1R.


Reinforcement and jointing should be designed in accordance with the stresses that the slab will be subject to once it is cured and the facility occupied. Factors such as exposure to direct sunlight and interior temperature variations as well as live and dead loading should be considered. Additionally, the floor slab should be disconnected from the other structural components of the building to prevent stress-induced cracks. At a basic level, reinforcing should be 6x6 1.0/1.0 conforming to the requirements of ASTM A185 and supported on chairs conforming to the recommendations of CRSI. Reinforcing should never be laid directly on subgrade and pulled up into concrete as slab is poured. This results in irregular placement and uneven stress. It also potentially raises the reinforcement where it may be exposed by the grinding process.


Prepare design mixes for each type and strength of concrete in accordance with ACI 318, "Building Code Requirements for Structural Concrete," Section 5.3 and with applicable provisions of ASTM C94. Design mixes for polished concrete should have a low water to cement ratio around 0.45 and a low slump of 3 to 4 inches. While this may result in the necessity to use vibratory screeds to aid placement, it will result in a much more durable slab with more uniform distribution of aggregate throughout the cement matrix. Size color and quantity of aggregate should be chosen with both mix design and final aesthetic effect in mind.


The pre-concrete conference should take place at least 35 days prior to the start of the concrete construction schedule. The Concrete Contractor and the Polishing Contractor along with the General Contractor, the Architect and others whose scope of work impacts the placement and finishing of the concrete, shall conduct a meeting to review the proposed mix designs and to discuss the required methods and procedures necessary to achieve the required concrete quality. Specified flatness and how it is to be achieved, as well as chemical hardeners and or sealers to be applied should be discussed. The contractor shall send a pre-concrete conference agenda to all attendees 20 days prior to the scheduled date of the conference.


Use mechanical vibrating equipment, supplemented by hand spading, rodding, or tamping to consolidate placed concrete. Avoid dropping concrete more than approximately 3 feet to prevent segregation of aggregate. The equipment and procedures used to consolidate the concrete shall comply with the recommended practices of ACI 309 and suit both the type of concrete and project conditions. Until the placing of a panel or section is completed, deposit and consolidate concrete slabs in a continuous operation within construction joints. Consolidate concrete during placing operations so that it is thoroughly worked around reinforcement and other embedded items and into corners. Maintain reinforcement in the proper position during placement operations.

Bring slab surfaces to the correct level with a straightedge and strike off. Use highway bull floats or straightedges to smooth the surface, leaving it free from humps and hollows. Do not sprinkle water on the plastic surface. Do not disturb the slab surfaces before starting finishing operations.

Apply a trowel finish to all interior slab surfaces unless otherwise noted. The concrete shall be placed, consolidated, struck-off and leveled to the proper elevation using a laser screed, or vibratory screed. Begin floating when surface water has disappeared or when concrete has stiffened sufficiently to permit operation of power driven floats, or both. Consolidate surface with power driven floats or by hand floating if area is small or inaccessible to power units. Cut down high spots and fill low spots with highway straightedge. Uniformly slope surfaces to drains. Immediately after leveling, refloat surface to a uniform, smooth, granular texture.

Construction joints, in slabs shall have smooth round, square or diamond dowels with one end greased to permit movement but prevent differential settlement. Place construction joints perpendicular to the main reinforcement.

Construct isolation joints in slabs on grade wherever there is contact between slabs on grade and vertical surfaces such as walls, columns or stairs.

Contraction (control) joints in slabs on grade should be laid out to a maximum spacing of 36 times slab thickness each way. Contraction joints should be either premoulded or cut within

12 hours of finishing in order to be effective.


After placing and finishing the concrete, start initial curing as soon as free water has disappeared from concrete surface. Keep continuously moist for not less than 7 days. Maintain slab temperature above 50°F. Use moisture retaining cover and water misting to maintain moisture rather than curing compounds as it leaves behind no residue that the polisher must deal with.


Begin final curing immediately after initial curing and before the concrete has dried. Continue final curing in accordance with ACI 301. Avoid rapid drying at the end of the final curing period.


Now that a good base slab has been provided, the concrete polishing contractor can perform his craft without having to overcome the inherent defects in the slab and can provide the finished product that the Owner is searching for.