Design and Construction of Floor and Roof with Precast Reinforced Concrete Planks and Joists - Code of Practice
1994 Edition

As per IS 13994 Clause 4.2.3:

• The minimum clear cover for reinforcement in precast planks is 15 mm.
• For precast joists, a minimum clear cover of 25 mm is specified. These covers protect the steel from corrosion and fire exposure. The clear cover denotes the distance from the concrete surface to the nearest reinforcement face, ensuring durability and structural integrity.

Partially precast joists are designed as either simply supported or continuous T-beams with a flange thickness of 60 mm, following IS 456:1978 design guidelines. During erection, joists must be supported on walls with a minimum bearing length of 100 mm. A central prop should be placed at mid-span immediately after installation, using a timber plank of at least 350 mm length, joist width, and 30 mm thickness at the bearing point. Joists should be aligned precisely on support blocks before propping. Transport and handling require lifting near the ends or at one-fifth span, maintaining the joist in a near-vertical orientation to prevent damage.

Design loads include:

1. Self-weight, accounting for the weight of the precast units plus any in-situ concrete toppings.
2. Imposed (live) loads, as specified in IS 875 (Part 2): 1987, based on the building's occupancy type.
3. Dead loads from finishes such as floor coverings or roof waterproofing, following IS 875 (Part 1): 1987.
4. Seismic load considerations require adherence to IS 4326:1993 for strengthening in earthquake-prone zones. The design employs the Limit State Method per IS 456:1978, ensuring safety and serviceability.

Continuity between precast planks and partially precast joists is attained by filling the haunches and gaps with M-15 grade in-situ concrete. Stirrups extending from the joists are tied with transverse reinforcement spanning across the haunch areas, creating a monolithic slab action. Negative reinforcement bars extend up to one-quarter of the span length on either side of supports to ensure moment transfer and continuity. Additionally, MS fan hooks securely anchor the precast elements to the cast-in-place concrete. Waterproofing layers are applied atop to prevent water ingress.

Recommended waterproofing methods include:

• Application of bitumen-based membranes such as bitumen felts, mastic, or glass fiber reinforced bitumen, conforming to IS 1346:1978, IS 4365:1967, IS 9918:1991, and IS 3036:1992.
• A protective lime concrete layer applied beneath finishes.
• Cement slurry wash applied at 4 kg cement per 10 m² over joists and haunches before pouring in-situ concrete.
• Overlaying the precast elements with a minimum 25 mm thickness of M-15 grade in-situ concrete to ensure watertight joints.
• Proper filling of haunches and reinforcement continuity to prevent leakage. These measures together provide durable waterproofing for precast floor and roof assemblies.