Tips for Ensuring a Flat and Level Concrete Slab-on-Grade

A persistent challenge of slab-on-ground and elevated slab construction is achieving floor flatness (Ff) and floor levelness (Fl) to ensure safety, effective 

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functionality, and aesthetics.

Examples of effective functionality are

  Can loaded forklifts move smoothly over joints?

  Are the two-story racks level?

  Will a robot’s movement be stymied by differential deflection at a joint?

• Can the 2’x2’ thinset tile be leveled?

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. In other words, it takes teamwork.

Here are some tips for effective teamwork for your next slab-on-ground or elevated slab project

• Design for the floor’s intended use. There is guidance about specifying realistic Ff and Fvalues in Section 4.8 of ACI 117-10 Specification for Tolerances for Concrete Construction and Materials.

• Fully cover the reinforcing steel. If the rebar is too close to the surface (i.e., ¾” or less), the floor will undulate over and between bars. It is easier to achieve a flat floor with a 7 in. floor vs. a 4 in. floor 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. The concrete takes longer to discharge so has less uniform properties, which affect the ability of the contractor to achieve a uniform finish.

• Turn on the lights! Concrete contractors pouring floors are often left in the dark. Literally. Lighting impacts how the concrete contractor can see what he/she is doing when finishing a floor. A general contractor’s attention to lighting may help flatten and level a slab.

• Make room for bigger finishing equipment. On a similar note, the larger the concrete finishing equipment, the easier it is to achieve Ff and  Fnumbers. However, larger machines require larger openings to get the equipment to the slab– even if the equipment is disassembled and reassembled once inside the building. Permanent windows, garage doors, and pouring the floor later in the schedule are reasons why the concrete contractor is relegated to using smaller finishing equipment, which makes achieving high Ff and Fl numbers difficult.

• Be conscientious about curing. A vapor barrier placed directly beneath the slab will contribute to a moisture gradient through the slab, which causes differential curing and that further causes curling/warping and a greater potential for cracking. A vapor barrier placed 18 in. or more beneath the slab can still protect users from radon but may not prevent moisture transfer, which will delay application of planned floor finishes. A short-term time penalty is the minor cost of better long-term performance.

• Pay attention to the slump. Concrete uniformity is important to achieving Ff and Fl numbers. When the first truck of concrete is delivered at a 4 in. slump and the next at a 10 in. slump, the concrete contractor will have a challenging time achieving a uniform finish. If all the concrete is delivered at a 4-5 in. slump, the concrete contractor has good and uniform concrete to work with, and it will assist them in achieving a flatter and more level floor. While not ideal, if all the concrete is delivered at a 10 in. slump, at least the contractor has uniform concrete to work with.

If you have questions about an upcoming slab-on-grade project, don’t hesitate to contact me at mvancura@betonconsultingeng.com. We want your project to be successful.

Chris Sjolander of Northland Concrete and Masonry contributed to this blog post.