The Effect of Curling on Floor Flatness

When floor flatness and levelness are measured using F-number technology, timing of measurements is critical because curling of slabs on ground increase with time. The result is decreased F-numbers that may not meet specified values. Changes in F-numbers may present problems for the floor covering contractor who has an expectation of how flat and level the floor should be. The higher the initial FF number, the greater the negative effect of curling. Because of this, Section 4.8.4.1 of ACI 117-06, "Standard Tolerances for Concrete Construction and Materials," requires that:

"Floor test surfaces shall be measured and reported within 72 hours after completion of slab concrete finishing operations and before removal of any supporting shores."

The required time limit applies to both F-numbers (FF and FL) and gap under-straightedge measurements made in accordance with ACI 117-06, but it wasn't always that way. Contractors should check to make sure what ACI document the project specs require. Even though ACI 117-06 is published, 80 percent of the projects are still using ACI 117-90 and a few still use ACI 117-81.

Requirements for timing of measurements

The first ACI tolerance document, ACI 117-81, stated that:

"Floor tolerance measurements should be made the day after a concrete floor is finished and before shoring is removed, in order to eliminate any effects of shrinkage, curling, and deflection."

However, when ACI 117-90 was published, Section 4.5 included different provisions for the timing of the measurements:FL levelness tolerances shall be measured within 72 hours after slab concrete placement.Floor finish tolerances as measured with a 10-foot straightedge must be done within 72 hours after slab concrete placement.There was no time requirement on measuring FF floor flatness, and the commentary gave a reason:

"Since neither deflection nor curling will significantly change a floor's FF value, there is no time limit on the measurement of this characteristic."

The statement in the commentary indicating that "?neither deflection or curling will significantly change a floor's FF value" has since been shown to be incorrect. This is why ACI 117-06 required that all measurements - FF, FL and the gap under a straightedge - be made within 72 hours. However, not all specifiers and contractors are aware of this change. The next section explores the effects of curling on F-numbers and why the change was made.

Curling effects on F-numbers

Slab curling is caused primarily by differences in moisture content or temperature between the top and bottom of the slab. The slab edges curl upward when the top surface is drier and shrinks more - or is cooler and contracts more - than the bottom. Curling is most noticeable at construction joints, but it can also occur at sawcut joints or random cracks. The curl can result in a loss of contact between the slab and subbase. Generally, the length of lost subbase contact is about 10 percent of the slab length (measured between joints) at each joint that has load transfer (doweled or sawcut joints), and about 20 percent at each joint with no load transfer. These percentages, however, are also a function of joint spacing, concrete properties, slab thickness and subbase stiffness. Upward curl at slab corners can be as high as 1 inch but is typically about 1/4 inch.

For the most part, contractors understood that curling had an effect on F-number measurements. F-number measurements made 6 to 9 months after the concrete slab had been placed often indicated that the floor had indeed changed, because these F-numbers were lower than F-numbers measured within 72 hours of placement.

Reports on two sets of F-number measurements - one for a University of Maryland gymnasium floor and another set for an industrial warehouse slab in Pennsylvania - illustrate the magnitude of changes that occurred over time. The F-number measurements were taken within 72 hours of placement, then 7 months later for the University of Maryland floor and 12 months later for the industrial warehouse slab.