CONCRETE

Waffle Slab (Ribbed Slab)

Two-way ribbed slab with a grid of voids — long spans, low self-weight

Also calledwaffle slabribbed slabgrid slabcoffered slab
Related on InfraLens
CODES
IS 456
Definition

A waffle slab is a two-way ribbed slab cast over removable or permanent void formers (waffle pods), producing a regular grid of ribs with a thin topping. Removing concrete from the tension zone between ribs cuts self-weight by 30-40% versus a solid slab of equal stiffness, enabling long column-free spans (8-16 m) economically — common in showrooms, atria, auditoria and large floor plates.

The ribs act as a two-way beam grid; a solid drop panel (filled-in zone) is provided around columns to resist punching shear and the high negative moments there. Design follows IS 456 two-way principles with the ribs proportioned per Cl. 30 (ribbed/hollow-block slabs): rib width ≥ 65 mm, depth ≤ 4× width, clear rib spacing ≤ 1.5 m, topping ≥ 50 mm. Architecturally the coffered soffit is often left exposed.

Where used
  • Long-span column-free floors (showrooms, halls, atria)
  • Heavy-loaded industrial + commercial floors
  • Architecturally exposed coffered ceilings
  • Reducing foundation load via lighter floors
  • Transfer floors over large spans
Acceptance / threshold
Rib + topping proportions per IS 456 Cl. 30; solid head/drop panel at columns sized for punching shear (Cl. 31.6); deflection within Cl. 23.2 using effective stiffness of the ribbed section.
Frequently asked
Why use a waffle slab instead of a solid slab?
It removes concrete from the low-stress tension zone between ribs, cutting self-weight 30-40% while keeping two-way stiffness — allowing much longer column-free spans economically and lighter foundations.
Where are solid portions kept in a waffle slab?
Around columns a solid 'drop' / filled head is retained to carry punching shear and the large hogging moments; the voided waffle grid is used only in the mid-span low-shear region.
Related terms
One-Way Slab
Slab supported on two opposite edges, bending in one direction
Two-Way Slab
Slab supported on all four edges, bending in both directions
Self-Compacting Concrete (SCC)
Highly fluid concrete that flows under its own weight, fills formwork, and consolidates without vibration.
Segregation of Concrete
Separation of coarse aggregate from cement paste due to over-vibration, drop height >1.5 m, or low cohesion.
High Strength Concrete (HSC)
Concrete with characteristic strength above M60. Requires silica fume, low w/c (<0.35), and quality aggregate.
Honeycomb in Concrete
Voids in concrete from poor compaction or formwork leakage. Repair with grouting; severe cases require demolition.
Slab Design (RCC)
One-way (Lx/Ly>2) or two-way slab design per IS 456
Creep & Shrinkage (Concrete)
Time-dependent deformation of concrete under sustained load
Creep Coefficient of Concrete (θ)
Ratio of creep strain to elastic strain under sustained load — per IS 456 Cl. 6.2.5.1 — depends on age at loading and humidity.
Concrete Curing
Min 7 days for OPC, 10 days for PPC, 14 days for sulphate-resistant