Tuesday, November 24th, 2015

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Recommendations to Achieve High-Quality Exposed Concrete Floors

by Jason M. Thompson

With concerns over the life-cycle costs of flooring and adhesion to concrete substrates, as well as the ever-increasing interest in sustainable design, exposing concrete floors as a finished condition is almost a default position for many of the projects we’re working on these days.  Since we specify concrete for structural purposes, we’re often asked to weigh in on, and sometimes even lead, the efforts to ensure the finished quality of exposed concrete floors is the best it can be.

When exposing concrete as an aesthetic finished product, there are four areas that must be addressed – detailing, specifications, execution, and expectations.  Here are a few best practices that we’ve come to employ in recent years:

Detailing:

For slabs on grade, go with 5 inches thick minimum; thicker if the slab encases radiant heating tubing.  Provide joints per ACI 360R, with proper timing to mitigate cracking but prevent raveling.

For slabs on metal deck, don’t bother jointing – there’s too much shrinkage restraint from the flutes.  Use a minimum 4-inch-thick topping thickness.  Reinforce with #3 or #4 bar and 0.5% minimum by topping volume, with max spacing of 12 inches on center.  Control flexural cracking with a heavier deck gage and closer beam spacing.

Specifications:

Use a shrinkage reducing admixture, like Eclipse by Grace.  Use a reasonable W/C ratio – 0.45 is a good target.  Target slump = 4 inches +/- 1 inch.  Consider microfibers for polished slabs if the specified depth of grind will expose large aggregate.  Avoid cement replacement (fly ash, silica fume, or slag).  For polished floors, make sure to comply with CPAA recommendations and any applicable basis-of-design specs (e.g. RetroPlate, Husqvarna, etc.).  No air entrainment.

For polished floors, specify Exposure Level (1 thru 4) and Surface Texture (A-1 thru C-3) per CSDA Standard 115.

For slabs on grade, eliminate vapor retarder if allowed by ACI.  For slabs on metal deck, specify vented deck.  Both measures help equalize bleed water migration and curing between top and bottom surfaces.

Execution:

Mandate a pre-construction meeting with the GC, trade-assist subcontractors, and owner.  Mandate mock-ups of all conditions (including metal deck) and make sure all specs are followed.

For polished slabs on grade, comply with FF50 and FL30 (minimum values not less than FF35 and FL20) per CPAA.  For polished elevated slabs, comply with FF30 and FL20 (minimum values not less than FF15 and FL10).  Expect pushback on the latter – it may require that the contractor laser screed from columns.

Use an evaporation retarder immediately after finishing and begin curing immediately after that.  Always wet cure with fog or soaked burlap.  If possible, don’t pour in extreme temps or windy conditions.

Keep other trades off the slab if possible.  Cover the slab to protect it during subsequent construction.

Expectations:

Determine early in design whether there will be exposed slabs and where.  Engage the owner and contractor to make them a part of the process during design.  Concrete cracks despite all valiant efforts to ensure otherwise, so make sure the owner has reasonable expectations.

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