Code of practice for construction of reinforced concrete shell roof
1962 Edition

IS 2204 Clause 4.4.1 specifies reinforcement bar diameters based on shell thickness as follows:

Additional points include a minimum bar diameter of 5 mm for all shells, with maximum diameters applicable only to unthickened shell sections. Total reinforcement depth should not exceed three times the maximum allowed diameter, and bar spacing must be no greater than five times the shell thickness. Unreinforced panel areas should also be limited to 15 times the square of shell thickness. These requirements ensure adequate reinforcement without compromising shell stability.

Per IS 2204 Clause 6.1, formwork for reinforced concrete shell roofs must be carefully designed and constructed to maintain the delicate shape of the thin shell surfaces. Differential settlement of supports and centering should be minimized to avoid distortion. For repetitive construction, panelized or mobile formwork units are recommended and should be designed by the contractor. Formwork surfaces need to be firm, smooth, and durable to allow reuse and possibly eliminate the need for plastering. If no decorative finish is planned, the surface pattern of the formwork can be intentionally left on the shell's underside to meet aesthetic or economic considerations.

According to IS 2204 Clause 5.2, the construction sequence for single shell roofs involves the following steps:

1. Erect formwork for edge members and traverses; place reinforcement and perform concreting.
2. Remove formwork while leaving necessary supports intact.
3. Erect full shell formwork; place steel reinforcement; concreting and curing.
4. Remove full formwork after curing.
5. Apply waterproofing and thermal insulation.

For multiple shells, the stages are similar but supports for edge beams, traverses, and shells remain until all shells in the series are complete, typically four to five shells per series. This sequence must be approved by the designer and strictly followed to ensure structural integrity and proper stress distribution.

IS 2204 specifies that concrete mixes conform to IS 456-1957 standards with the designer's specific requirements. When hand tamping is used, the concrete slump should be a minimum of 5 cm. The use of air-entraining agents is recommended to enhance workability without increasing water content. For compaction, hand tamping is suitable for mixes with adequate slump, while screed vibrators are generally used for mechanical compaction. Needle vibrators are reserved for thicker or edge sections where deeper compaction is needed. Panel layouts differ for singly and doubly curved shells, with the designer deciding on arrangements to control shrinkage and ensure proper compaction.

IS 2204 advises that construction joints in shell roofs should be positioned along curved shell lines, preferably at locations experiencing zero shear stress, such as the directrix at mid-span. Before placing new concrete at the joint, the surface must be thoroughly cleaned, typically by wire brushing, and treated with a cement slurry to promote bonding. While ideally the entire shell thickness is cast in a single operation, when joints are necessary, these precautions ensure structural continuity. Expansion joints must have a minimum clear gap of 2 cm filled with elastic filler, and waterproofing must continue uninterrupted across these joints to maintain durability.