Proper planning and installation techniques combined to make a recent high tolerance, pile-supported structural slab a success. The slab was constructed in Tampa, Fla., for the new NexLube Blending Facility, an advanced oil re-refining operation that will process up to 28 million gallons of used oil annually. The facility’s main warehouse is a 56,000-square-foot building with both random-traffic areas and defined-path lift-truck traffic.


Every project has its challenges, and this one had more than its share. But a series of pre-slab construction conferences involving the owner, general contractor, concrete contractor, and floor consultant helped minimize potential problems. A few important agenda items discussed and resolved were:

  • The slab was designed to be 12 inches thick, supported by piles and pile caps, with #6 rebar at 10 inches on-center each way throughout the slab. The pile caps fell directly under the defined traffic aisles. If the piles and slab had been placed monolithically, it would likely have created differential shrinkage and cracking. To minimize this probability, the design was adjusted to allow the pile caps and slab to be placed independently.
  • We worked with the ready-mix supplier to design a mix optimized to eliminate aggregate gap grading which improved workability and finishability.
  • The team discussed the specified very narrow aisle (VNA) lift trucks and rack heights, which allowed a refinement in the floor tolerances as recommended in Table 1.1 in ACI 360R-10. Floor tolerances for the defined traffic portion of the project were specified to be F min 75 (see sidebar, Defining F min).
  • The construction schedule required that the slab be completed without a roof in the middle of the Florida summer—a challenge to say the least, due to sun, wind, and frequent afternoon thunderstorms. To minimize problems this could create, we used the hot weather concreting recommendations in ACI 305 and started placements in the late evening, allowing for finishing operations to be done overnight during the coolest part of the day. This is common practice in Florida, but not for a superflat floor.


The 16,500-square-foot defined-traffic portion of the building slab was divided into seven placement sections (aisles), 150 feet long (on average) and 10 to 15 feet wide. There were also two “turn-around” slabs for the VNA lift-trucks with similar dimensions and the same Fmin flatness specification.

Great care was taken in installing the side forms that created the construction joints between the defined traffic aisles and the intermediate slabs for the shelves to ensure they remained at the proper elevation during concrete placement operations. Stock steel L-metal was fastened to ¾-inch plywood and was adjusted within a thousandth of an inch of the proper grade using a high-quality optical level. Even tightening the screws changed the elevation requiring rechecking and adjusting. Because reinforcement ran continuously through the construction joints, additional supports for the reinforcement were added to minimize movement and vibration of the edge forms. To ensure accuracy before and during concrete placements the forms were checked for imperfections multiple times at 2-foot intervals using both an optical level and a Face Dipstick 2272.

Concrete for the first group of aisles was placed with a concrete boom pump. The screeding/strike-off method chosen was hand screed form to form. We might normally use a vibrating screed but with the high tolerances, we were concerned a vibrating screed would have moved the forms slightly, so the hand screeding was the better option.

Safety was a concern so plywood walk boards were placed alongside the edge forms to give crews a work platform above the continuous reinforcement. The intermediate slabs, where the racks will go, were also placed with a concrete boom pump, but screeding/strike-off was done using a Somero laser screed, the only wheeled traffic permitted on the adjacent, newly cast slabs.

Each placement was done in two lifts; staggering three loads of ready-mixed concrete (two on bottom lift, one on top). The initial lift was installed to half slab depth (6 inches) and vibrated to reduce the initial shrinkage from consolidation. Immediately after placement of the first lift, the second lift was placed half the distance of the first, vibrated, and then the strike-off began.

After initial strike-off came multiple transverse strike-offs by hand using magnesium straightedges both perpendicular and at 45 degrees. Numerous cuts and fills of the concrete surface followed, using 12-foot and 16-foot highway straightedges (bump cutters) to scrape, shave, or fill any imperfections that were visually out of tolerance.

Accompanying the re-straightening operations were machining operations using ride-on power trowels. Machines were operated in two directions using pan floats and combination blades. Attention was given to blade rpm and directional speed of machine operation. Controlling the rotational speed of the blades has a big impact on the flatness results. For this floor, proper timing of each step of the finishing operation imparted a nearly mirror-like finish.


Through proper planning and the specialized installation techniques, the VNA slabs for the new NexLube Blending Facility far exceeded the specified floor tolerances of Fmin 75. The profileograph revealed some aisles even exceeded Fmin 200 in both the transverse and longitudinal directions. These slabs were exceptionally flat—not one required any corrective grinding (which is common in construction of defined-traffic aisles). Interestingly, although traditional F-numbers are not truly relevant to defined-traffic floors, we checked the VNA aisles with the Face Dipstick and found that all lanes had an overall FF 229/FL 125, extremely high flatness numbers.

For more information on superflat floors, read these CC articles: “Understanding Specifications for Superflat Floors” by Terry J. Fricks, Concrete Construction, July 1995. Visit Also read “F is for Flatness (or Face)” by William D. Palmer Jr., February 2007. Find it at

Bryan M. Birdwell, is an owner of Birdwell & Associates LLC, Lakeland, Fla., which specializing in designing, consulting, and constructing jointless and reduced joint concrete floors and paving along with high tolerance floors. Their projects have been awarded 23 Golden Trowel Awards, four of which are world records.