Plain or slightly reinforced concrete can be used in the form of precast shell roofing. A suitable frame is made to cast the shell unit. A hessian cloth is spread over it. The cloth is given a sort of sag to maintain the shape of the shell. To regulate the thickness a thin frame of 2.5 cm height is placed. Chicken mesh cut to fit this mould is placed on it. Cement concrete is poured and manually compacted. The frame is made to rest to permit the hessian to sag with wet concrete. Edge beams are also casted. The final positions of the edge beam and the steel are shown in Fig. 9.4. To obtain a level surface the depression is filled with concrete or earth. The shell units may be 1 m × 1 m to about 3 m × 3 m in size.
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