Rule No. 8a asserts the futility of using the compressive strength and slump tests to control the quality of a slab on grade. Because these two concrete characteristics are irrelevant to a slab's performance, their management can provide little if any benefit.

Rule No. 9a: Curling is the defining behavior of every new concrete slab.

At the preslab meeting, my favorite discomforter goes to the supplier: “Oh, by the way, how much does your concrete curl?” The fact this eminently reasonable question still cannot be answered speaks volumes about the sorry technical state of our industry. Although various standard tests for measuring floor and pavement profiles exist, none directly address the quantification of slab curl. The following Curl Number Test fills this void.

Test panel. Any unloaded, unbroken, permanently exposed, rectangular area bounded by contraction joints.

Baseline test time limit. Within one hour after the test panel's bounding contraction joints have been chalk-lined.

Timing of retests. Any time after the baseline test has been completed.

Run diagonals. The two lines connecting the test panel's opposite corners.

Run directions. From south to north, or east to west, in that order of preference.

Layout. Strike chalk lines on both run diagonals. Permanently mark the centroid and the four points where the chalk lines intersect contraction joints. On each run diagonal, locate and permanently mark the start point and end point by measuring in both directions from the centroid to the whole foot nearest (but not beyond) the edge of the test panel. Make a sketch (see Test Panel Example) of each test panel showing its size, orientation, location, and run directions.

Measurement points. On each run diagonal, between the start point and the end point, locate measurement points at each whole foot. Let L equal the distance in feet from the start point to the end point. Number the start point -L/2, the centroid 0, the end point +L/2, and all intermediate points in whole number sequence accordingly.

Raw elevation measurements. On each run diagonal, from the start point to the end point, using an instrument accurate to ±0.005 inch, measure the raw elevation ri at each point i in inches.

Baseline elevation calculation. If the raw elevations ri be from an initial baseline test, compute each run diagonal's baseline elevations bi= ri– r0, where r0 is the raw elevation at the centroid. Record each run diagonal's bi values for future reference and terminate the analysis.

Retest elevation calculation. If the raw elevations ri be from a subsequent retest, compute each run diagonal's retest elevations yi= ri– r0- bi relative to its original profile, where r0 is the raw elevation at the centroid and bi is the baseline elevation originally recorded at point i.

Curl Number calculation. For each run diagonal, at each point i, let Mi = yi x i2, and S be the sum of all (L + 1) of these Mi values. The dimensionless Curl Number C for that run diagonal then equals:

Rule No. 9b: The higher the average Curl Number, the more the slab has curled.

Allen Face is the inventor of the F-number system, F-min system, Dipstick, F-Meter, D-Meter, and Screed Rail. He is also an ACI Fellow and a long-time member of ACI Committees 302, 360, and 117.

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