Code of practice for design and construction of floors and roofs using precast reinforced/prestressed concrete ribbed or cored slab unit
1982 Edition

According to the code, for ribbed slabs, the minimum rib width is 50 mm for spans up to 5 meters and 70 mm for longer spans, with internal slopes between 1/15 and 1/8 for ease of demoulding. For cored slabs, the minimum slab thickness must be at least 20 mm, with the smallest cross-sectional width excluding voids sufficient to resist shear stresses. These specifications ensure structural adequacy and manufacturability of the precast elements.

The standard specifies a minimum concrete grade of M15 as per IS 456:1978, with M20 or higher preferred for reinforced units. Prestressed concrete units require higher grades conforming to IS 1343:1980. Maximum aggregate size is limited to 12 mm for ribbed and cored slabs where flange thickness is less than 50 mm. These measures ensure sufficient strength and workability for the precast components.

Lifting hooks must be placed at one-fifth the length from each end of the precast unit, constructed from mild steel reinforcing bars with sufficient capacity to bear the unit's self-weight during demoulding, handling, and erection. After installation, hooks can be cut or bent into the joint concrete. Supports for stacking should be positioned at approximately one-sixth of the span from the ends, avoiding central support, while the main reinforcement is always kept at the bottom during transportation and stacking to ensure stability and safety.

The code mandates dimensional tolerances where cross-sectional dimensions must be within +3 mm or ±0.1% of the dimension, whichever is larger. Mould tolerances include length up to -10 mm, width up to -3 mm, height up to +3 mm, and diagonal and warp/bow limits of ±5 mm and ±3 mm respectively. Thickness of cored slabs should have a tolerance of ±d with a minimum of 20 mm. Sampling and testing procedures ensure these tolerances are maintained for quality control.

Precast units should follow curing practices per IS 456:1978, with a minimum of 7 days for grades M15 to M20 and longer for higher grades; low-pressure steam curing may be used for early strength gain. In situ concrete joints must be cured for at least 7 days using water or moist curing methods, followed by a drying period of 7 days, and application of cement slurry to seal hairline cracks. These steps ensure proper strength development and bonding between precast units and onsite concrete.