360R-06 Design of Slabs-on-Ground
R-72 ACI COMMITTEE REPORT
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- Điều hướng trang này:
- APPENDIX 5—DESIGN EXAMPLES USING SHRINKAGE-COMPENSATING CONCRETE A5.1—Introduction
- A5.2—Example with amount of steel and slab joint spacing predetermined
| 360R-72 ACI COMMITTEE REPORT
M = f t S = f t Equate the moment capacity of the unreinforced slab to the post-tensioned slab 285 psi = 435 psi h = = 4.85 in. The 150 psi residual compressive stress could be increased to use a 4 in. thick slab or reduced so a 5 in. thick slab could be used. APPENDIX 5—DESIGN EXAMPLES USING SHRINKAGE-COMPENSATING CONCRETE A5.1—Introduction The material presented in this appendix is discussed in greater detail in ACI 223. Slab design using this material is divided into three parts. The first part is the selection of slab thickness, which can be done, for example, using Appendixes 1 , 2 , or 3 . This follows the assumption that the slab is being designed to remain essentially uncracked due to external loading. This is followed by design of the concrete mixture and the reinforcing steel to compensate for subsequent drying shrinkage. Because the net result of initial expansion and later shrinkage is to be essentially zero, no prestress is to be considered. The second part of the process—selection of the appropriate amount of reinforcement—is a critical part of the design. This reinforcement can be mild steel, as illustrated in this appendix, or post-tensioning tendons. ACI 223 recommends that the reinforcement be placed in the top 1/3 to 1/4 of the slab. The third part of the design is the determination of the required prism expansion to ensure shrinkage compensation, which leads to the design for properties of the concrete mixture. This is shown in Section A5.2. Expansion of the length of the slab is also determined. A5.2—Example with amount of steel and slab joint spacing predetermined The thickness of the slab, the joint spacing, and the amount of steel have been set as follows: Thickness = 6.0 in. Amount of mild steel = 0.36 in. 2 /ft, which is 0.5% Joint spacing = 80 ft The slab is assumed to dry on the top surface only; there- fore, the volume-surface ratio is 6.0 in. For complete shrinkage compensation, the amount of expansion will be equal to the anticipated amount of shrinkage, which must be determined first. For this example, the shrinkage is assumed to be equal to the prism expansion. Prism expansion = 0.046% = 0.00046 in./in. bh 2 6 -------- b 6 in. ( ) 2 6 --------------------- bh 2 6 -------- 285 psi(6 in.) 2 435 psi ---------------------------------- This expansion, as determined by ASTM C 878 from tests of the concrete mixture, is used with Fig. A5.1 to determine member expansion. Member expansion = 0.028% or 0.00028 in./in. This is to be combined with the joint spacing to determine the total slab motion and the required thickness of the joint filler. The design assumption is made that all motion occurs at one end of the slab. Motion = 0.00028 × 80 × 12 = 0.269 in. Use a joint filler that will compress at least twice this amount. Filler thickness = 2 × 0.27 = 0.54 in. ACI 223 discusses these features in greater detail. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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