360R-06 Design of Slabs-on-Ground
DESIGN OF SLABS-ON-GROUND 360R-29
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- CHAPTER 6—DESIGN OF UNREINFORCED CONCRETE SLABS 6.1—Introduction
| DESIGN OF SLABS-ON-GROUND 360R-29
interruption-free floor surface. Installing the joint filler flush with the top of the slab can best be achieved by overfilling the joint and shaving the top of the filler level with the slab surface after the material has hardened. CHAPTER 6—DESIGN OF UNREINFORCED CONCRETE SLABS 6.1—Introduction The thickness of unreinforced concrete slabs is determined as a plain concrete slab without reinforcement. Although the effects of any welded wire reinforcement, plain or deformed bars, post tensioning, steel fibers, or any other type of reinforce- ment are not considered, joints may be reinforced for load transfer across the joint. The slab is designed to remain uncracked due to loads applied on the slab surface. Normally, a safety factor of 1.4 to 2.0 is used relative to the modulus of rupture. It is important to note that, as set forth in ACI 318, slabs- on-ground are not considered structural members unless they are used to transmit vertical or horizontal loads from other elements of the building’s structure ( Chapter 11 ). Consequently, cracking, joint instability, and surface character problems are considered to be serviceability issues and are not relevant to the general integrity of the building structure. Concrete floor slabs employing portland cement, regardless of slump, will begin to experience a reduction in volume as soon as they are placed. This will continue as long as water, heat, or both, are being released to the surroundings. More- over, because the drying-and-cooling rates at the top and bottom of the slab are dissimilar, the shrinkage will vary with the depth, causing the as-cast shape to be distorted and reduced in volume. Resistance to formation of this distorted shape introduces internal stresses in the concrete that, if unrelieved, may cause the concrete to crack. Controlling the effects of drying shrinkage is critical to the performance of unreinforced concrete slabs. Two principal objectives of unreinforced slab-on-ground design are to avoid the formation of (out-of-joint) random cracks and to maintain adequate joint stability. The slab’s anticipated live loading will govern its thickness and cross-joint shear transfer requirements, while shrinkage considerations will dictate the maximum joint spacing. Application of present technology permits only a reduction in cracking and curling due to restrained shrinkage, not their elimination. ACI 302.1R suggests that cracking in up to 3% of the slab panels in a normally jointed floor is a realistic expectation. Refer to ACI 224R for further discussion of cracking in reinforced and unreinforced concrete slabs. A jointed, unreinforced slab-on-ground design seeks to optimize the finished floor’s serviceability by attempting to influence the shrinkage cracks to develop beneath the sawcut contraction joints. In industrial construction, this can result in a floor slab that will be susceptible to relative movement of the joint edges and joint maintenance problems when exposed to wheeled traffic. If the designer cannot be sure of positive long-term shear transfer at the joints through aggregate inter- lock, then positive load-transfer devices should be used at all joints subject to wheeled traffic. Refer to Section 5.2 for additional information. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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