Floor flatness and levelness after 72 hours

Floor Flatness (Ff) and floor levelness (Fl) are required by ACI specifications to be measured within 72 hours of placing the slab. Concrete contractors prefer to grab Ff and Fl measurements before joints are cut (typically less than 24 hours). So, Ff and Fl are a measure of the contractor’s workmanship, the consistency of the concrete supplied, and the appropriateness of design elements such as rebar cover, slab thickness, and location of vapor barrier.

Unfortunately, slabs do not stop shrinking, curling, and warping after Ff and Fl have been measured.

Photo credit: Enduracon and John Cross, photographer

Beton’s recommendations for achieving long-term floor flatness

This list is by no means exhaustive and these recommendations need not be applied all at one time. The appropriateness of these measures depends on individual slab placement conditions, requirements for flatness, and topping/finishing design.

  • Consider internal curing with lightweight aggregate
  • Specify maximum shrinkage of 0.04%
  • Consider using macro fibers to keep cracks tight
  • Consider using macro steel fibers and eliminate reinforcing steel to ensure proper position of flexural reinforcement and hold cracks together
  • Limit equipment loads until concrete has gained full strength
  • Wet curing for minimum of seven days (longer is better)
  • Aggregate gradation to limit shrinkage and cement content
  • Uniform gradation (0.45 Power Curve)
  • Use maximum top aggregate size possible
  • Use a higher percentage of intermediate aggregate (3/8 in) than sand
  • Limit total cementitious content by specified strength requirements
  • Substitute a portion of Portland cement with fly ash or slag
  • Specify maximum water content of 250 lbs/cy (not w/c)