360R-06 Design of Slabs-on-Ground
DESIGN OF SLABS-ON-GROUND 360R-53
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- 13.8—Base and vapor retarders/barriers
| DESIGN OF SLABS-ON-GROUND 360R-53
Therefore, it may be better to anchor walls to a separate slab under the finished floor slab with at least 6 in. (150 mm) of base material between the two slabs to minimize joints in the finished floor slab. This is not often done, but is recommended where reduction of cracks and joints is important. Besides isolating the slab-on-ground from walls, columns, and column footings, the slab should be isolated from guard posts (bollards) that penetrate the floor and are anchored into the ground below. The slab should be isolated from any other slab shrinkage restraints, such as drains. A compressible material should be specified full slab depth around all restraints to allow the slab to shrink and move relative to the fixed items. Electrical conduit and storm drain lines should be buried in the subgrade so that they do not either reduce the slab thickness or restrain drying shrinkage. Restraint parallel to joints due to conventional round dowels can be eliminated by the use of square, diamond- plate, or rectangular-plate dowel systems with formed voids or compressible isolation material on the bar/plate sides to allow transverse and longitudinal movement while transferring vertical load. Refer to Chapter 5 for more information. 13.8—Base and vapor retarders/barriers A permeable base, with a smooth, low-friction surface helps reduce shrinkage cracking because it allows the slab to shrink with minimal restraint. A relatively dry base also allows some of the water from the bottom of the slab to leave by acting as a blotter before the concrete sets. A vapor retarder/barrier should be used where required to control moisture transmission through the floor system. If used, a vapor retarder/barrier in direct contact with the slab may increase slab curling. A vapor retarder/barrier aggregate, blotter system design, or both, should be evaluated as set forth in Chapter 3 . One option more fully discussed in Chapter 3 is for the retarder/barrier to be covered with at least 4 in. (100 mm) of reasonably dry, trimmable compactible granular material to provide a permeable, absorptive base directly under the slab. Using 4 in. (150 mm) or more of this material over the retarder/barrier, however, will improve constructibility and minimize damage. Nicholson (1981) showed that serious shrinkage cracking and curling can occur when concrete slabs are cast on an impervious base. If the base is kept moist by groundwater or if the slab is placed on a wet base, then this will increase the moisture gradient in the slab and will increase curl. If the aggregate material over the vapor retarder/barrier is not dry enough at concrete placement, however, it will not act as a blotter and can aggravate curling and moisture problems. Thus, in spite of the inherent problems with placing the concrete directly on the vapor retarder/ barrier, it is better to do so if there is a chance that the aggregate blotter will not be relatively dry; refer to the discussion in Chapter 3 . If crushed stone is used as a base material, the upper surface of the crushed stone should be choked off with fine aggregate material to provide a smooth surface that will allow the slab-on-ground to shrink with minimum restraint. If polyethylene is required only to serve as a slip sheet to reduce friction between slab and base, and the base is to remain dry, then the polyethylene can be installed without a stone cover. Holes should be drilled in the sheet (while the sheet is still folded or on a roll) at approximately 12 in. (300 mm) centers to allow water to leave the bottom of the slab before the concrete sets. Figure 13.3 shows the variation in values for base friction. In post-tensioned slabs over two sheets of polyethylene, the friction factor may be taken as 0.3. For long (over 100 ft [30 m]) post-tensioned slabs, 0.5 might be used to account for variations in base elevation over longer distances. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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