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Unformatted text preview: Part 2 of a 3-part series Control of shrinkage and curling in slabs on grade By Robert F. Ytterberg How to deal with warping and curling F i g u re 4. R e l a t- ing slab curling to c o n c rete shrink- age shows that the greater the shrinkage, the g reater the curl- ing (from Ref. 7). F i g u re 5. O n e year of shrink- age data for a 15- inch-thick slab as a function of rela- tive humidity with only the top sur- face exposed to drying (from Kee- ton as cited in Ref. 6). Slab curling is a function of the moisture gradient and temperature diff e re n t i a l across the slab thickness. Moist subgrades increase slab curling, but even a so- called dry subgrade keeps the bottom of the slab damper than the slab sur- face. This article continues a series started in the No- vember issue (page 825), and figures and tables are numbered consecutively with that beginning article. S lab edges that curl upward and cen- ters of slabs slightly depressed from their original elevation are common in slabs on grade. Curling makes a mockery of the commonly specified 1 / 8-inch flatness tolerance because slab edges can curl up- ward 1 / 8 inch or more above a spot only 4 to 5 feet from the edge. At slab corners the uplift can be greater 3 / 1 6 to 1 / 4 inch above a location 4 to 5 feet from the slab corner. Heavy loads on the slab can cause the un- supported, curled-edge cantilever to crack. Curling is caused by drying shrinkage and by moisture or temperature diff e r- ences across the thickness of the slab. Negative moisture gradients are usual in slabs on grade and they cause upward edge curling. Such moisture gradients and curling increase when: The slab is made from high-shrinkage c o n c r e t e The slab surface is exposed to low- humidity air The subgrade or subbase under the hardened slab has a high moisture c o n t e n t The concrete has a high modulus of e l a s t i c i t y The most common positive tempera- ture gradient with its downward edge curling is that caused by heat from the sun on the upper slab surface. Upward edge curling already caused by negative mois- ture gradients can be increased by cold slab surface temperatures or by hotter slab bottom temperatures. Vertical deflection is greater at slab corners than at slab edges simply because that is what hap- pens when two adjacent, perpendicular edges in a flat plane both roll upward. Definition of terms The terms curling and warping a r e used interchangeably in this article, in conformance with ACI terminology (Ref. 14), which defines them as follows: Curling The distortion of an original- ly essentially linear or planar member into a curved shape such as the warping of a slab due to creep or to differences in tem- perature or moisture content in the zones adjacent to its opposite faces....
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