Concrete slabs come in a variety of forms depending on their locations. Ground slabs take the load directly to the ground. Plain slabs (similar in shape to ground slabs) take the load directly from the floor and transmit it to the beams. In other cases, joists, poured as part of plain slabs, carry the loads to the beams. Joists are used to strengthen the middle portion of the slab.

Figure 7-6.—Devices used to support horizontal reinforcing bars.

Concrete slab reinforcements (fig. 7-5) are supported by reinforcing steel in configurations called slab bolster and high chair. Concrete blocks made of sand-cement mortar can be used in place of the slab bolster. The height of the slab bolster is determined by the concrete protective cover required. If the concrete surface is to be in contact with the ground or exposed to the weather after removal of the forms, the protective covering of concrete over the steel should be 2 in. Other devices used to support horizontal reinforcing bars are shown in figures 7-6, 7-7, and 7-8. Wood blocks should be

Figure 7-7.—Precast concrete block used for reinforcing steel support.

Figure 7-9.-Steel in place in a wall.

substituted for the metal supports only if there is no possibility of the concrete becoming wet or if the construction is known to be temporary. WALL REINFORCEMENT.— Placement of steel reinforcement in load-bearing walls is the same as for columns except that the steel is erected in place and not preassembled. Horizontal steel is tied to vertical steel at least three times in any bar length. The wood block is removed when the form has been filled up to the level of the block, as shown in figure 7-9.

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