360R-06 Design of Slabs-on-Ground
R-40 ACI COMMITTEE REPORT
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| 360R-40 ACI COMMITTEE REPORT
where P e = effective prestress force per tendon, lb (N); f p = minimum average residual prestress (required compressive stress), psi (MPa); W = slab unit strip width, 12 in./ft (1000 mm/m); H = slab thickness, in. (mm); and P r = from Eq. (9-1) , lb/ft (N/m). 9.5.3 Floating slab—A slab-on-ground that is isolated from all restraining elements that would resist contractions and expansions due to drying shrinkage, elastic shortening due to prestress, or temperature effects. The crack-control reinforcement can only be effective if the slab is allowed to shorten due to drying shrinkage and the elastic shortening due to the prestressing force. Any slab restraints such as columns, walls, footings, and loading docks should be isolated so as not to prevent the slab movement. Also, the concrete placement layout should be designed in such a way that the adjacent placements do not restrain the relative slab movements. Any dowels or other reinforcement going through a joint should have a compressible material on the side of relative movement. 9.5.4 Tendon stressing—The stressing sequence should be adjusted to the project requirements. While one-time stressing may be used for most residential slabs after the concrete has reached sufficient strength to transfer the force from the anchorages, a more gradual force introduction may be needed for industrial floors. Two stressing stages may be necessary to prevent early shrinkage cracks from appearing. Typically, the initial (partial) stressing should be completed within 24 hours after concrete placement. 9.5.5 Tendon layout—Depending on the slab usage (flexible rack layout, random traffic, heavy racks with fixed layout) and the placement layout and sequence (long narrow strips, rectangular sections), post-tensioning can be provided in one or both directions. One-way post-tensioning is common for narrow strip placements. Two-way post-tensioning is used for random traffic areas and for rectangular placement areas. Typically, post-tensioning tendons are placed in the direc- tion of vehicle traffic. Sometimes the post-tensioning tendons cross the traffic joint to keep it tied. This enhances the durability of the traffic joint and eliminates a need for more severe measures such as dowels or armored joint. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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