The Effect of Curling on Floor Flatness by ProQuest


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									Concrete Technology                            Ward R. Malisch, P.E. and Bruce A. Suprenant, P.E.

The Effect of Curling on
Floor Flatness
F-numbers should                                  “Floor tolerance measurements
                                                should be made the day after
                                                                                       random cracks. The curl can result in
                                                                                       a loss of contact between the slab
be measured within                              a concrete floor is finished and         and subbase. Generally, the length
                                                before shoring is removed, in          of lost subbase contact is about 10
72 hours of floor                               order to eliminate any effects of      percent of the slab length (measured
finishing to best                               shrinkage, curling, and deflection.”    between joints) at each joint that
                                               However, when ACI 117-90 was            has load transfer (doweled or sawcut
reflect the concrete                         published, Section 4.5 included dif-      joints), and about 20 percent at each
contractor’s efforts.                        ferent provisions for the timing of the   joint with no load transfer. These
                                             measurements:                             percentages, however, are also a

            hen floor flatness and level-      • FL levelness tolerances shall be        function of joint spacing, concrete
            ness are measured using             measured within 72 hours after slab    properties, slab thickness and sub-
            F-number technology, timing         concrete placement.                    base stiffness. Upward curl at slab
of measurements is critical because          • Floor finish tolerances as measured      corners can be as high as 1 inch but
curling of slabs on ground increase             with a 10-foot straightedge must       is typically about 1⁄4 inch.
with time. The result is decreased              be done within 72 hours after slab       For the most part, contractors
F-numbers that may not meet speci-              concrete placement.                    understood that curling had an
fied values. Changes in F-numbers may         • There was no time requirement on        effect on F-number measurements.
present problems for the floor covering          measuring FF floor flatness, and the     F-number measurements made 6 to
contractor who has an expectation of            commentary gave a reason:              9 months after the concrete slab had
how flat and level the floor should be.          “Since neither deflection nor curling    been placed often indicated that the
The higher the initial FF number, the        will significantly change a floor’s FF      floor had indeed changed, because
greater the negative effect of curling.      value, there is no time limit on the      these F-numbers were lower than
Because of this, Section of          measurement of this characteristic.  ”    F-numbers measured within 72 hours
ACI 117-06, “Standard Tolerances for           The statement in the commentary         of placement.
Concrete Construction and Materials,   ”     indicating that “…neither deflection         Reports on two sets of F-number
requires that:                               or curling will significantly change       measurements — one for a University
    “Floor test surfaces shall be            a floor’s FF value” has since been         of Maryland gymnasium floor and
   measured and reported within              shown to be incorrect. This is why        another set for an industrial warehouse
   72 hours after completion of slab         ACI 117-06 required that all measure-     slab in Pennsylvania — illustrate the
   concrete finishing operations and                        ,
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