This type of roof is widely used in the modern construction. Here steel bars and concrete are used to form the roof. This consists of an RCC roof slab resting on walls or beams. For short spans (length to width ratio greater than 1.5) the slab is treated as one-way slab otherwise it is treated as a two-way slab and accordingly steel reinforcements are provided. The thickness and steel reinforcement required are based on the span and loading conditions. In case the slab is required to be supported by RCC beams, the size and the steel reinforcement of the beams are determined and provided accordingly (Fig. 23.8).
At the top level of the wall and beam, centring sheets are placed horizontally by means of grid over the centring sheets. A clear cover of 15 mm at the bottom and above the centring sheet is provided. Cement concrete mix (usually a mix of 1 : 2 : 4 is used) is placed on the centring sheets after adequate mixing with water. The concrete is rammed well by tampers or a vibrator is used to attain the maximum density and the required thickness. After about 12 hours of laying, the exposed surface is cured with water for 21 days.
In case of large spans, they are divided into bays, and ribs are provided to stiffen the slabs. These ribs are monolithically casted with the slab and function as beams in the shape of T, usually referred to as T-beams. The slab portion is termed the flange of T-beam. The width of the rib should be adequate to house the tensile reinforcement of beam. All modern constructions employ T-beam slabs for public buildings (Fig. 23.9).
Leave a Reply