Code of practice for construction of floors and roofs using precast concrete waffle units
1983 Edition

Scope and Sampling according to IS 10505

• Applicability: Defines dimensional criteria for slab units.
• Sampling (Clause 10.1 & Appendix A): Units are randomly selected from production lots as outlined in Table 1.

Table 1: Sample Sizes and Rejection Limits

Sampling Procedure (Clause A-2.1):

• Randomly select units and inspect dimensional compliance.
• If defective units ≥ first rejection number → reject the lot.
• If defective units < first rejection number → proceed with second sampling equal in size.
• Combine defective counts from both samples:
  • If combined defective units < second rejection number → accept the lot.
  • Otherwise → reject the lot.

Dimensional Tolerances (Clause 9.1):

• Length and breadth: ±5 mm or ±0.1% of dimension, whichever is greater.
• Thickness: ±2 mm.

This systematic approach ensures dimensional conformity and quality assurance for precast waffle slab units.

Detailed Scheme under IS 10505 for Precast Waffle Units

1. Sampling & Quality Assurance (Clause 1.2, Table 1)

• Number of test units depends on lot size.
• Rejection criteria based on defect counts.

2. Deflection Criteria (Clause 3.2)

• Must adhere to IS 456:1978 Clauses 22.1, 22.2, and 23 to ensure serviceability.

3. Reinforcement Detailing (Clause B-4)

• Follow IS 10505 and IS 456 for reinforcement placement and anchorage.

4. Design Procedures (Appendix B)

• Load considerations as per Appendix B of IS 10505.
• Structural analysis under Clause B-2.2.

Use this framework for robust quality management and compliant design of precast waffle floor and roof assemblies.

Material Requirements as per IS 10505

• Materials must comply with IS 456-1978 standards.

Dimensional Tolerances (Clause 9.1):

• Length/Breadth: ±5 mm or ±0.1% dimension, whichever exceeds.
• Thickness: ±2 mm.

Sampling and Testing (Clauses 1.2, 2.1; Tables A-1 & A-2):

• Sampling sizes and rejection limits vary with lot size as per Table 1.
• Units failing dimensional criteria are considered defective.
• Acceptance based on defect counts in initial and secondary samples.

Maintain these standards to assure material quality and dimensional accuracy in precast waffle units.

Key Aspects of Structural Design under IS 10505

• Materials: Use concrete and steel conforming to IS 456-1978.

• Load Factor for Limit State (Clause 4.4): Design load at demoulding and handling stages is 1.5 times the collapse load.

• Design of Waffle Floors/Roofs (Clause 2.2): For spans greater than 150 mm and rib spacing exceeding 750 mm, design as slab and grid beam systems. Shear forces at the interface of precast and in-situ concrete must be calculated; provide shear keys or reinforcement to prevent separation.

• Deflection Control (Clause 4.1): Shear stresses in ribs must conform to IS 456-1978 provisions.

This ensures both safety during handling and acceptable service performance in waffle slab structures.

Manufacturing Guidelines for Precast Units (IS 10505)

• Mould Construction (Clause 5.1): Bottom and side moulds may be constructed from timber, masonry, concrete, steel, FRP, or approved plastics. Surfaces must be smooth and dimensionally accurate. Reinforcement such as chicken mesh or fibers in masonry moulds enhances durability. Admixtures may be added to improve concrete properties.

• Strength and Stability (Clause 4.2): Precast units must resist stresses during handling, stacking, transport, and placement, including the weight of in-situ concrete joints. Account for delayed strength development when using Portland Pozzolana Cement.

• Reinforcement (Clause 6 & Appendix A): Follow IS 456-1978 for detailing; apply sampling and quality control as per Appendix A.

These practices contribute to producing durable and dimensionally precise precast waffle elements.

Reinforcement Guidelines for Waffle Units (IS 10505)

• Detailing should comply with Clause 25 of IS 456:1978.

• Unit Geometry: Precast waffle units typically have inverted trough shapes (square, rectangular, or triangular). Ribs may be tapered outward to facilitate demoulding and ensure monolithic action with cast-in-place joints.

• Reinforcement Placement: Main bars are embedded within ribs of the precast unit. Additional reinforcement is installed in cast-in-situ joints to maintain structural continuity.

• Typical Cross-Section: Thickness and rib dimensions depend on span and loading. Minimum screed thickness on top is maintained for functional and structural integrity.

• Mould Materials: Timber, steel, FRP, plastic, masonry, or concrete moulds with smooth, precise finish. Use of fibre mesh in masonry moulds enhances durability.

Follow these reinforcement practices to achieve durability and structural monolithicity.

Concrete Mix Recommendations (IS 10505)

• Minimum concrete grade: M20 as per IS 456-1978.
• Materials must meet IS 456-1978 specifications.
• Use of admixtures is permitted to enhance strength.
• Moulds should be fabricated from approved materials with smooth top surfaces.

Typical Mix Proportions (Reference IS 456:2000):

Mix design steps per IS 10262 include selecting target strength, water-cement ratio, cement and aggregate quantities, and adjustments for workability and durability.

Casting and Curing Protocols (IS 10505)

• Concrete Mix: Minimum grade M20 as per IS 456:1978.

• Mould Types: Bottom moulds: timber, masonry, concrete, steel, FRP, plastic. Side moulds: timber, steel, FRP, plastic. Ensure smooth, accurate top surfaces. Reinforce masonry moulds with chicken mesh or fibres.

• Curing: Follow IS 456:1978 guidelines. Minimum wet curing period of 7 days. Low-pressure steam curing permitted for accelerated strength gain.

Proper curing ensures durability and strength of precast waffle slabs.

Dimensional Tolerances Summary (IS 10505)

• Length and breadth tolerance: ±5 mm or ±0.1% of dimension, whichever is greater.
• Thickness tolerance: ±2 mm.

Sampling and Testing:

• Sampling sizes and rejection criteria depend on lot size as per Table 1.
• Defective units are those failing dimensional requirements.
• Acceptance based on defect counts in one or two samples.

Size Specifications:

• Modular lateral dimensions as per IS 6820-1972.
• Minimum flange and web thickness: 25 mm.
• Depth varies with structural design considerations.

These criteria ensure precise and consistent unit dimensions.

Sampling and Testing of Precast Units (IS 10505)

• Sampling is conducted based on lot size with random unit selection.
• Sample sizes and rejection thresholds are defined in Table 1.
• If defects exceed limits in the first sample, a second sample is tested.
• Lot acceptance depends on combined defect counts.

Dimensional tolerances are ±5 mm or ±0.1% for length/breadth and ±2 mm for thickness.

This procedure guarantees statistical verification of dimensional compliance prior to acceptance.

Transportation and Erection Guidelines (IS 10505)

• Lifting Sling Placement: Rope slings should be positioned at one-fifth the length from each end of the precast unit.

• Trolley Overhang Limit: Maximum overhang permitted is one-fifth of the unit length.

• Lifting Methods: Manual, chain pulley blocks, hoists, or cranes may be used.

• Strength and Stability: Units must sustain stresses during handling, stacking, transport, and placement, including design and dead loads. Account for delayed strength when using Portland Pozzolana Cement.

• Load Testing: Conduct floor and roof load tests as per IS 456:1978.

These measures ensure safe handling and structural integrity throughout transportation and erection.

Curing of In-Situ Concrete Joints (IS 10505 & IS 456)

• Minimum 7 days of wet curing followed by at least 7 days drying prior to loading or finishing.
• Wet curing methods include water ponding, wet hessian, or continuous sprinkling as outlined in IS 456.
• Low-pressure steam curing is permissible for early strength development.
• After curing, apply cement slurry coating to joints to seal hairline cracks.
• Use minimum M20 grade concrete for joint casting.

These steps ensure the durability and strength of joints between precast units.

Fixture and Opening Requirements in Precast Units (IS 10505)

• Accurate installation of holes, openings, and embedded fixtures (fan hooks, inserts, conduits) must occur during precasting with sufficient embedment.
• Post-casting drilling or cutting is prohibited without Engineer-in-Charge approval.
• Designers must specify all fixture and opening requirements within precast units or in-situ joints.
• Unit dimensions should be modular as per IS 6820-1972, with minimum flange and web thickness of 25 mm.

These provisions ensure proper integration of fixtures without compromising structural integrity.

Floor Finish Specifications (IS 10505)

• Minimum floor finish thickness is 75 mm to provide adequate impact resistance and acoustic performance.
• Floor finishes should comply with relevant IS standards such as IS 456:2000, IS 2571 (terrazzo), and IS 1237 (cement concrete flooring).
• Fix fixtures and openings during precasting; avoid post-casting modifications without approval.

Proper floor finishing enhances durability, aesthetics, and functionality of precast slab floors.

Roof Treatment Guidelines (IS 10505)

• Minimum floor thickness for roof slabs is 75 mm for impact and acoustic resistance.

• Waffle slab roofs with spans up to 6 m and rib widths ≥ 100 mm, rib spacing ≤ 750 mm may be analyzed as solid or flat slabs per IS 456.

• For spans exceeding 6 m or rib spacing over 750 mm, the roof must be designed as a slab and grid beam system with shear reinforcement at precast-in-situ interfaces.

• Waterproofing and thermal insulation must be appropriate to local climate, following relevant IS codes (e.g., IS 3370 for waterproofing, IS 3792 for insulation).

These treatments ensure long-term durability and comfort in waffle slab roof systems.