360R-06 Design of Slabs-on-Ground
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| 9.6.4.1 Strip placements—Normally, every other strip is
placed, and the adjacent slabs are placed in a second phase. This allows the initial slabs to shorten in their long (and short) direction before the adjacent slab is placed. Typically, no dowels or other reinforcement are necessary across this joint unless a load transfer is required, as in areas of joint- crossing traffic. To significantly improve the durability of such a joint, post-tensioning perpendicular to the joint can be provided in this area. The short direction joint on the end of a long strip may open more than is desirable. To offset this effect, a placement strip can be left open as long as practical to allow the majority of shortening to occur before the place- ment strip is closed. Also, stage stressing reduces this long- direction shortening as the young concrete is only partially loaded. The elastic shortening decreases with the age of concrete at load transfer. 9.6.4.2 Placement of rectangular sections—All of the criteria from the section for strip placements apply. Typically, post-tensioning is provided in both directions, and provisions should be made for the tendons crossing the joints so as not to restrain the shortening of the adjacent slabs. This can be achieved by using compressible sleeves around the tendons. 9.6.5 Special considerations—The prestressing tendons should be located within the upper middle 1/4 of slab thickness while maintaining a proper concrete cover. Occasionally, the engineer may determine that the tendons need to be in another location based on other considerations. A higher position of the tendons in the slab will reduce the risk of surface cracking, and a lower position will reduce the potential of cracking under concentrated loads. The tendons can be supported on support bars, especially when one-way post-tensioning is used. Support bars can also serve as crack-control reinforcement. Special slab-on- ground chairs of the required height are used to ensure that the location of the tendons will remain unchanged during concrete placement. |