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In simple R.C.C. flooring, the R.C.C. slab bends downwards, causing tension at the bottom fibers’ at the mid – span. Due to this mild steel bars reinforcement is placed at the bottom of the slab, keeping a minimum clear cover of 15 mm. Half these bars are bent up near ends to take up negative bending moment caused due to partial fixidity at the ends. This main reinforcement is placed in the direction of the span of the slab, which is equal to the width of the room, especially when the length of the room is more than 1.5 times the width of the room. Such a slab is known as one way reinforced slab. Nominal reinforcement (known as temperature / distribution reinforcement) is placed in the perpendicular direction. Hooks are placed at the end of each plain bar, though these are not required in ribbed bars.
The bearing of the slab in the wall should neither be less than its thickness, nor less than half the width of the wall. Figure 1. shows one way reinforced slab. Such slabs are quite suitable and economical for spans up to 5 m. The slab is cast on timber or steel shuttering. After erecting the centering, properly bent reinforcement is placed in position. Distance pieces of stone or concrete are placed between the reinforcement and the shuttering plate so that proper cover is maintained. Cement concrete of appropriate mix (usually M20 mix or 1:1 ½ :3 mix) is then poured and well compacted. The slab is then properly cured. Shuttering is removed only when the concrete has fully set.
When the length of the room is less than 1.5 times the width of the room, the slab spans/ bends in both the directions. It is essential to provide reinforcement in both the directions. Such a slab is known as a two – way reinforced slab, such as the one shown in fig below. At the corner, suitable mesh reinforcement is provided at the top and bottom, to prevent their lifting. The plan of the reinforcement of a two – way slab, at its top and bottom.
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