Gymnasium Concrete Floor
Gyms may have wood floors or poured sports floors; either way the balls don’t bounce properly if the floors aren’t flat. So, how flat is flat? Flooring manufacturers and the subcontractors who install the flooring require the substrate to be of acceptable flatness (and dryness) before the flooring installation begins. The architect specifies in the design documents how flat the concrete slabs must be.
FF/FL is the construction shorthand for the flatness and levelness measurement of a flooring surface and is the American Concrete Institute (ACI) standard. FF, or Flatness F-Number, is a numeric value that defines the maximum floor bumpiness allowed over a 2 ft (0.6 m) distance; FL, or Levelness F-Number, defines the tilt or pitch of a floor over a 10 ft (3 m) distance. The higher the F-Number value, the more level or flat the slab. For example, an FF/FL of 40 30 may be acceptable flatness and levelness (the first number is always the flatness). For gym floors in recreation centers, a 50 FF is not uncommon.
If an F-Number is specified for flatness, it must be at least the equivalent of the Maple Flooring Manufacturers Association (MFMA) standard 1/8 in. (0.3 cm) in 10 ft (3 m) radius tolerance, which is roughly a 40 FF. Flatness measurements are taken within 72 h of the slab placement, because slab flatness can change over time as the concrete cures, or dries out. Preferably, measurements are taken as soon as each day’s placement is dry enough to bear foot traffic; the results can then be used to make corrections in any construction problems before placement is repeated the next day.
Flatness and levelness are more readily achievable for "slab-on-grade" concrete than for "elevated slabs," or concrete slabs poured on a prepared surface on the ground (as opposed to upper floors). Accordingly, elevated slabs typically are measured only for the single FF number.
Concrete floor slabs above grade are either (1) composite construction, consisting of light gauge corrugated metal deck, reinforcing steel mesh and concrete, or (2) poured-in-place concrete. The wet concrete is heavy, and when poured the weight causes the metal deck on which it rests to deflect, or sag slightly. When the elevated slab cures, or dries out enough to take weight, or load, other than itself, it is a little lower in the middle of the span.
To counteract some of the deflection and keep the slab as flat as possible, shoring can be placed under the midpoint of the span of the metal deck. These measures typically are supplemented by filling in low spots on top of the slab if necessary.
Prior to flooring installation, the installer measures the substrate flatness before accepting the conditions as satisfactory for the final surface. The FF readings can be taken in 1 ft (0.3 m) intervals in 11 ft (3.4 m) sections using equipment specialized for that purpose; however, many installers may rely on the basic straightedge method.
Concrete and Flooring: The Dryness Chase
The concrete under gym floors not only has to be really flat, it has to be really dry. If it’s not dry enough, the flooring that goes on top of it may have serious problems, such as debonding, blistering, buckling, or adhesive failure caused by water vapor.
On the construction site, the race is on to dry out the concrete slab in time for the flooring installation, whether the floors are wood, synthetic sports floor, or vinyl tile. Each flooring material has an industry standard for how dry the concrete substrate has to be before it is acceptable, and each product manufacturer may have its own requirement.
Although concrete looks monolithic and impenetrable, it is actually quite porous to water vapor. The water that is added to the concrete mix to make it fluid enough to easily pour into place eventually evaporates, leaving tiny voids throughout the slab. Water vapor migrates through these voids, moving from areas of higher relative humidity to areas of lower humidity, such as from the ground below the slab on grade to inside the building.
The properties of the concrete are affected by a wide range of factors, such as weather; amount of water in the mix of the cement, water, sand, lime, and aggregate (stone) that concrete is made of; the mix design; and the amount of time between mixing and placement. Some or all of these factors can conspire to cause a floor slab to be too moist to accept the flooring material.
So, what do you do about this problem if you are the CM? You dry the slab out until the numbers are right. The numbers are the measurement of the concrete dryness, and low numbers are needed for flooring material adhesion and performance. Typically, testing is conducted with either surface moisture meters or standard test kits that collect the moisture in a 1 square foot (SF) (0.09 m2) area over a 24 h period. The moisture emitted by the slab is absorbed by a measured amount of calcium chloride and weighed. The weight is expressed in pounds per 1,000 square feet (SF); the lower the number, the drier the slab. Sensors embedded in the body of the concrete are the most accurate moisture measurement method but are less commonly employed for this building type.
The drying-out process can take a long time, but the flooring installation has to wait for the moisture emission levels to be low enough, or else the flooring will fail in some way. For example, wood floors may require results to be 3.5 lb (1.6 kg) or less to be right for the flooring installer. For rubber and vinyl floors, 3.0 lb (1.4 kg) or less is generally the standard. Moisture testing is conducted by a trained and certified independent testing agent. The test results are reviewed by the architect or a knowledgeable consultant to determine whether or not the surface is ready for the flooring installation.
Drying out the concrete is mostly a matter of time, but the process can be accelerated by methods employed immediately after the slab is poured and other methods after the building is enclosed. Floor tests must be made after the air-conditioning system is operating and has been at service conditions for at least 48 h, allowing time for water vapor to migrate and humidity to stabilize inside the building.
A dry slab starts with a dry construction site and continues with a vapor barrier properly placed under the slab on grade. The vapor barrier is a sheet of plastic at least 6 mm thick that typically is placed between the stone or sand base and the concrete. The product is protected during construction because punctures in the barrier allow the water vapor through and may make the moisture content of the slab hard to overcome.
After the slab is poured, the concrete is kept moist to improve the hydration process, or curing. Moisture-retaining blankets facilitate the curing better than wet curing compounds. After some months of drying while other construction activities proceed, the slabs still may not test out. If so, isolation of the slab area-plus dehumidifiers and fans-usually does the job.