Code of practice for construction of floor and roof with joists and filler blocks, Part 2: With hollow clay filler blocks
1981 Edition

According to IS 6061 Part 2, precast concrete joists should maintain a consistent width, with length and depth tailored to the span and depth of the filler blocks. Typical widths range from 100 mm to 150 mm, depths depend on span (commonly 150 mm to 300 mm), and spacing between joists is generally 450 mm to 600 mm center-to-center. The maximum usual span is about 4.5 meters. Joists are to be placed at specified intervals spanning supports such as beams or walls, ensuring proper load distribution. Detailed dimensions and spacing can be found in Appendix A of the standard.

The quality standards for hollow clay filler blocks are primarily established in IS 3951 (Part 1) - 1975, which specifies the requirements for structural hollow clay tiles used as filler blocks. IS 6061 (Part 2) references IS 3951 for filler block quality and further details the construction practices for floors and roofs using these blocks. Together, these standards ensure the blocks meet strength, durability, and performance criteria necessary for structural applications.

To guarantee durability of precast joists under IS 6061 Part 2, casting should occur on a level, shaded surface protected from environmental factors. Inner moulds and base surfaces need treatment to facilitate removal and smooth finishes, with reinforcement placed accurately for cover. After casting, mould sides must not be removed before 24 hours, and joists should be kept moist by spraying water for the first 24 hours onsite. Subsequently, joists are transferred to curing yards for continuous moist curing for at least 7 days. After curing, joists are stored in shaded areas and allowed to dry for a minimum of three weeks before installation. Handling must avoid shocks, ensure uniform support, and use spacers during stacking to prevent deformation.

For the topping concrete slab placed over hollow clay filler blocks, IS 6061 Part 2 specifies minimum reinforcement percentages of 0.15% steel area along the joists and 0.20% across them, with bar spacing limited to 300 mm maximum. Reinforcement bars should be tied to stirrups projecting from the joists to ensure composite action between the topping slab and the precast joists. The topping concrete thickness is typically at least 50 mm, and blocks should be wetted prior to placing concrete. Reinforcement can be mild steel bars or welded wire mesh, designed to mitigate cracking and promote structural integrity.

IS 6061 Part 2 recommends that the maximum span for precast reinforced or prestressed concrete joists generally should not exceed 6 meters. Joists are to be spaced and supported appropriately by walls or beams, with load design considering self-weight of joists and filler blocks, topping concrete, finishes, and imposed loads in accordance with relevant IS codes such as IS 875 for live loads. Proper handling and storage practices help maintain structural performance. Adhering to these limitations ensures safe and durable floor and roof assemblies.