Floor flatness and levelness matter in such applications as manufacturing facilities, big box stores, and warehouses. ACI 117.1R-14 defines floor flatness (Ff) as “deviation of a surface from a plane” and floor levelness (Fl) as “deviation of a line or surface from a horizontal line or surface.” Flat floors make it easier to maintain tolerances in manufactured products such as airplane wings. Robots and forklifts travel more smoothly—and safely—over flat floors. Levelness matters when stacking pallets of goods in a warehouse or big box store. That is, the higher the stack, the more it tilts if the floor isn’t level.
ACI requires measurement of flatness and levelness within 72 hours of placing the slab. In practice, contractors want to measure them before they cut the joints—usually within 24 hours of placement. Unfortunately, slabs do not stop shrinking, curling, and warping after that. But if you follow our advice, you can still get the floor you want.
ACI 117 lists five categories of flatness. A conventional floor might be used in a mechanical room, where neither functional nor aesthetic requirements mandate more than ordinary flatness. A moderately flat floor would be appropriate in a carpeted office. Where you’re going to install thin-set tile, you’ll want a flat floor. In a warehouse with high speed forklift traffic, you’ll want a very flat floor. And if you’re designing a warehouse floor for random traffic, you’ll want a super flat floor.
Designing for floor flatness and levelness
Achieving the desired slab-on-ground or elevated slab flatness and levelness requires a skilled and experienced contractor. But the concrete contractor cannot be 100% successful without knowledgeable contributions from the designer/specifier, general contractor, and concrete supplier. That is, it takes teamwork. Here are some things the designer can do to help.
• Design for the intended use. Section 4.8 of ACI 117-10, “Specification for Tolerances for Concrete Construction and Materials,” provides guidance for specifying realistic Ff and Fl values. You want a floor that’s flat and level enough, but not more—that just adds to the cost and may unnecessarily delay the project.
• Fully cover the reinforcing steel. If it’s too close to the surface (that is, ¾ in. or less), the floor will undulate over and between reinforcing bars.
• Make the slab thick enough. It is easier to achieve a flat floor with a 7 in. thickness than a 4 in. thickness because one concrete truck is needed to fill a bay of a 7 in. floor but must be moved several times for a 4 in. floor. When the concrete takes longer to discharge, it has less uniform properties. That makes it harder for the contractor to achieve a uniform finish.
Specifying the concrete
Specify a maximum shrinkage of 0.04%. The easiest way to achieve that is to use the aggregate suspension method, along with the largest aggregate size practical. And if you specify seven days’ moist curing or longer, you’ll minimize shrinkage, cracking, and curling. For contracting purposes, it’s best to make curing a separate pay item. Internal curing using saturated lightweight aggregate can also help.
Instead of water/cement ratio, specify a maximum water content of 250 lbs/yd3. Limit the total cementitious materials content by specifying maximum as well as minimum strength. And substitute fly ash or slag cement for some of the portland cement.
Consider using steel fibers instead of reinforcing bars. Fibers do a better job of controlling cracking; you may even be able to do away with joints altogether. Also, with fibers you don’t have to worry about supporting the reinforcing bars to keep them at the right elevation. All concrete cracks, but low-shrinkage concrete will crack less. Steel fibers will keep the cracks narrow and tight so they’re less problematic.
It’s a lot easier to make a flat slab on ground than a suspended slab. Due to creep, suspended slabs continue to deflect between the supports over time. A low-shrinkage concrete will creep less, but less is not none at all. If you must have a flat suspended slab, you’ll need to go back later to grind and/or top the slab to make it flat. You can either have it flat or of uniform thickness—not both.
Tips for the contractor
• Turn on the lights! Concrete contractors placing floors are often left in the dark—literally. A general contractor’s attention to lighting can really aid in achieving floor flatness and levelness.
• Make room for bigger finishing equipment. The larger the concrete finishing equipment, the easier it is to achieve Ff and Fl numbers. Naturally, larger machines require larger openings to get the equipment to the slab. Permanent windows, garage doors, and placing the floor later in the schedule can all force the concrete contractor to use smaller-than-ideal finishing equipment.
• Be conscientious about curing. A vapor barrier directly beneath the slab will contribute to a moisture gradient through the slab. Such a gradient will result in differential curing, which in turn causes curling, warping, and possibly cracking as well. Placing the vapor barrier 18 in. or more below the slab will still protect users from radon, but may not prevent moisture transfer. If plans call for flooring or coatings, you’ll have to wait longer for acceptable moisture conditions. However, that delay will give you better long-term performance.
• Pay attention to slump. Concrete uniformity is important to achieving Ff and Fl numbers. When the first load is delivered at a 4 in. slump and the next at a 10 in. slump, the concrete contractor will have a hard time achieving a uniform finish. If all the concrete is delivered at a 4- to 5 in. slump, the concrete contractor has uniform concrete to work with, making it easier to achieve a flat, level floor.
• Limit equipment loads until the concrete gains strength. Otherwise you’ll damage the slab.
Chris Sjolander of Northland Concrete and Masonry contributed to this blog post.