360R-06 Design of Slabs-on-Ground
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| 5.1.3 Sawcut contraction joints—Sawcut contraction
joints are used to limit random, out of joint, floor slab cracking. Joints are usually located on column lines, with intermediate joints located at equal spaces between column lines, as shown in Fig. 5.1 . The following factors are normally considered when selecting spacing of sawcut contraction joints: • Method of slab design; • Thickness of slab; • Type, amount, and location of reinforcement; • Shrinkage potential of the concrete (cement type and quantity; aggregate size and gradation, quantity, and quality; w/cm; type of admixtures; and concrete temperature); • Base friction; • Floor slab restraints; • Layout of foundations, racks, pits, equipment pads, trenches, and similar floor discontinuities; • Environmental factors such as temperature, wind, and humidity; and • Methods and quality of concrete curing. As previously indicated, establishing slab joint spacing, thickness, and reinforcement requirements is the responsibility of the designer. The specified joint spacing will be a principal factor dictating both the amount and the character of random cracking to be experienced, so joint spacing should always be carefully selected. For unreinforced slabs-on-ground and for slabs reinforced only for limiting crack widths, other than continuously reinforced, Fig. 5.6 provides recommendations for joint spacing based on shrinkage values as determined by ACI 209R. Sawcut contraction joints should be continuous across intersecting joints, not staggered or offset. The exception to this rule would be the column isolation joint, shown in Fig. 5.3 . The aspect ratio of slab panels that are unreinforced, reinforced only for crack width control, or made with shrinkage- compensating concrete should be a maximum of 1.5 to 1; however, a ratio of 1 to 1 is preferred. L- and T- shaped panels should be avoided. Floors around loading docks have a tendency to crack due to their configuration and restraints. Figure 5.7 shows one method that can be used to minimize slab cracking at reentrant corners of loading docks. Plastic or metal inserts are not recommended for creating a sawcut contraction joint in any exposed floor surface that will be subjected to wheeled traffic. Sawcut contraction joints in industrial and commercial floors are usually formed by sawing a continuous slot in the slab to form a weakened plane below which a crack will form ( Fig. 5.8 ). Further details on sawcutting of joints are given in Section 5.3 . tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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