360R-06 Design of Slabs-on-Ground
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| 9.3—Introduction
Slabs-on-ground may be prestressed using bonded or unbonded tendons that are post-tensioned and anchored after the concrete has obtained sufficient strength to withstand the force at the anchorage. The primary advantages of a post- tensioned slab-on-ground are: • Increased joint spacing—only construction joints are necessary—no sawcut contraction joints; • Avoidance of shrinkage cracks and significant reduction of cracking through active prestress; • Lower life-cycle cost, fewer joints to maintain, and higher durability due to precompression; • Enhanced serviceability and no facility down time for repairs of joints; • Better preservation of floor flatness and levelness by minimizing the number of joints and joint curling; • Decreased slab thickness; • Increased load capacity; • Resilience and recovery capability from overloading; and • Reduction of superstructure cracking. Regarding the recovery capability of post-tensioned concrete slabs, it is not likely that a ground-supported slab can be deflected sufficiently to exceed the yield strength of the prestressing steel, which means that cracks due to overload are likely to close up after the load is removed. Competent construction supervision and coordination is required for post-tensioned slabs. The post-tensioning system (PTI 2006) used for industrial floor applications should meet PTI “Specifications for Unbonded Tendons” (PTI 2000). Tendons for all applications should be properly placed, stressed, and anchored. Also, the concrete properties should meet the design criteria to be able to receive the forces introduced through the post-tensioning anchorages. Slab penetrations made and drilled anchorage devices placed after construction should be coordinated with tendon locations to avoid severing tendons. This can be done using metal detectors or similar devices to locate the tendons in an existing slab. Post-tensioning of ground-supported slabs began in the early 1960s. In 1967, the first three ground-supported slabs using a system of post-tensioned reinforcement approved by the Federal Housing Administration were installed in Houston. In January 1968, tests on a 20 x 40 ft (6 x 12 m) prestressed residential ground-supported slab were reported (Thompson and Anderson 1968). These tests and previous experience with completed construction led to the first general approval for the use of prestressed post-tensioned ground-supported slabs throughout the United States in June 1968 by HUD. The only requirement placed on the use of this method of reinforcement was that a rational design be provided by a registered professional engineer. Since June 1968, millions of square feet of ground-supported concrete slabs for residential, commercial, and industrial applications have been constructed using post-tensioned prestressed concrete. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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