It is important to start power floating early while the concrete is still plastic. However, starting too soon can create too much surface mortar reducing the floor flatness, especially when using a laser-guided screed. Laser-guided screeds can produce FF numbers in the high 40s and 50s, but power floating too soon can undo or reduce the surface flatness created by the laser-guided screed. Delay power floating until you leave no more than a 1/8-inch footprint in the concrete. Otherwise, you increase the risk of cutting troughs and creating surface waves that will reduce the floor flatness.
Timing can be judged by the height of the mortar ridge or “windrow” created by a ride-on trowel fitted with pan floats. Since the pans do not overlap, a mortar ridge is created between the pans. Starting too early creates excessive ridge heights, and the floor will require restraightening to restore the surface flatness. Ideally, ridge heights should be approximately 1/8 inch to ensure there is sufficient mortar to fill holes and low spots. When ridge heights start to exceed 1/8 inch, back off and wait; let the concrete become stiffer so you don’t generate and move too much mortar with the pans.
Always make the first float pass perpendicular to the direction of screeding and make subsequent passes perpendicular to the previous pass. Do not make float passes in only one direction; doing so creates waves and subsequent one directional passes makes the waves deeper. Alternating the direction of the float passes makes the floor flatter. Always overlap each pass by one-half the pan diameter to minimize troughing. If a mortar ridge exists from a previous pass, overlap the previous pass so the center of a pan is on the mortar ridge; removing mortar ridges makes the floor flatter.
To properly prepare a floor for troweling, multiple float passes are required. Make at least two float passes in two different directions (perpendicular to each other). Four or more passes in multiples of two makes the floor flatter, if done correctly. Some finishers claim making late float passes at 45-degree angles to earlier passes will make the floor flatter. Also, using large pans and heavy ride-on machines with multiple passes can make the floor even flatter.
Except for the first float pass, run the ride-on trowel at full speed so the machine is easier to control. Slow down on the first float pass to minimize the risk of troughing if the concrete is too soft or if you hit a wet spot. Avoid sharp, forward turns that cause the pans to dig into the soft surface creating low spots. Consider using a forward-backward pass technique to avoid forward turns on the first float pass. Remember, creating and moving too much surface mortar typically makes the floor less flat.
For edges, power float with a pan directly on or over the edge form. If the floor edges are low and need to be built up, make forward float passes along the edge. Otherwise, make backward passes with a pan on or over the form edge. Never let a machine run or sit in one spot too long and avoid going sideways with a ride-on trowel. Also, minimize activities that disrupt the floor surface such as walking in the fresh concrete after strikeoff or walking on the floor while still soft enough to leave footprints. Anything that disrupts the floor surface requires restraightening and typically makes the floor less flat.
Timing of FF/FL measurements
Due to concrete shrinkage and slab curling, FF/FL numbers diminish with time. Specifically, joints and cracks curl making the floor less flat with the passage of time. Therefore, flatness and levelness should be measured as soon as possible, preferably within 24-hours after concrete placement but no later than 72 hours. Otherwise, FF and FL numbers may not represent the true performance of your work.
Routinely placing very flat and super flat floors requires an understanding of floor flatness and how to properly use the equipment. More importantly, it requires practice and continuous self-evaluation. Always review your FF/FL numbers and critique your placing and finishing techniques. Then make adjustments and improve on your next placement.