360R-06 Design of Slabs-on-Ground
R-34 ACI COMMITTEE REPORT
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| 360R-34 ACI COMMITTEE REPORT
slabs. Stiffer, more widely spaced reinforcement permits lower reinforcement percentages to be used satisfactorily. This is typically achieved with ASTM A 497 deformed wire reinforcement or ASTM A 615 deformed bars, widely spaced. Other deformed bar reinforcement is also acceptable, such as reinforcement defined in ASTM A 996 and A 706. 8.3.4 Maximum reinforcement—The objective of full shrinkage compensation is to attain restrained member expansive strains equal to or greater than the restrained shrinkage strains. Kesler et al. (1973) cautioned that the maximum level of reinforcement should be approximately 0.6% because, at that point, restrained expansion strains equaled restrained shrinkage strains. This maximum ratio does not depend on the yield strength of the reinforcement. To prevent concrete from shrinking more than the restrained expansion, lighter percentages of steel are recommended. Should high steel ratios be required for structural design conditions, higher expansion levels in the concrete, as measured by ASTM C 878 prisms, would be required. The required level of ASTM C 878 prism expansion strains can be determined by using Fig. 8.3. The figure shows the relationship between prism expansions, internal reinforce- ment percent, volume-surface relationship, and resulting concrete slab expansions. The figure enables one to estimate the anticipated member shrinkage strains using the volume- surface ratio for different slabs and different reinforcement percentages. If the resulting slab expansions are greater than the resulting shrinkage strains for a given volume-surface relationship, then full shrinkage compensation is obtained. This prism value is the minimum value that should be specified or verified in the lab with trial mixtures; the minimum recommended amount of concrete expansion for slabs-on-ground measured in accordance with ASTM C 878 is 0.03% (Russell 1973). 8.3.5 Alternative minimum restraint levels—Russell concluded that restrained expansion should be equal to or greater than restrained shrinkage (Keeton 1979). The concrete shrinkage depends on aggregate type and gradation, unit water content, volume-surface ratios, * and environ- mental and other conditions. The expansion strain depends largely on the expansion capability of the concrete mixture, which in turn depends on cement factor, curing, admixture, and the level of internal and external restraint. Therefore, the minimum reinforcement required to properly control expansion for shrinkage compensation depends on the potential shrinkage of the slab and the restrained prism expansion of the concrete mixture measured according to ASTM C 878. For a given volume-surface ratio and a minimum standard prism expansion level (verified with trial batch data), internal restraint levels provided by less than 0.15% steel in a typical 6 in. (150 mm) slab can be used (ACI 1980). If the slab expansion is greater than the shrinkage strain for a surface-volume ratio of 6:1, using Russell’s data (American Concrete Institute 1980), full compensation can be achieved. Circumferential curves depicting shrinkage strains for volume-surface ratios for other slab thicknesses are also shown in Fig. 8.4 . Care should be exercised when using low reinforcement ratios. If light reinforcement is used, it may accidentally be depressed into the bottom third of the slab, which can lead to subsequent warping and cracking. Light, but stiff, reinforce- ment can be obtained by using larger bars or wire at a wider spacing. The maximum spacing of reinforcing bars should not exceed three times the slab thickness. For smooth wire reinforcement, the spacing should not be more than 14 in. (360 mm), even though a wider spacing is easier for workers to step through. Deformed welded wire reinforcement can be spaced in the same manner as reinforcing bars. If tests and design calculations are not used, the minimum 0.15% reinforcement is often specified. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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