Clause 26.4 specifies the minimum concrete cover (clear distance from the nearest surface of concrete to the surface of the nearest reinforcement bar) required for protection against corrosion and fire. Cover depends on the exposure condition (Table 16) and fire resistance period (Table 16A). The more stringent value governs.
Key Requirements
•Nominal cover shall not be less than the diameter of the bar
•Cover for durability per Table 16 — varies by exposure condition
•Cover for fire resistance per Table 16A — varies by fire rating and member type
•The governing cover is the maximum of: durability requirement, fire requirement, and bar diameter
•Tolerance on cover: +10 mm, -0 mm (cover shall not be less than specified)
For a main bar of diameter φ, cover ≥ φ in all cases. For precast members manufactured under controlled conditions, cover may be reduced by 5 mm.
Table 16A — Nominal Cover for Fire Resistance (Clause 26.4.3)
Fire Rating
Beams (mm)
Slabs (mm)
Columns (mm)
0.5 hour
20
20
40
1 hour
20
20
40
1.5 hour
20
25
40
2 hour
40
35
40
3 hour
60
45
40
4 hour
70
55
40
Practical Notes
✓For most residential/commercial buildings (moderate exposure, 2-hour fire): slabs 35 mm, beams 40 mm, columns 40 mm. Many engineers round up to 25 mm (slab), 25 mm (beam), 40 mm (column) for mild exposure — check your exposure classification carefully.
✓Cover is measured to the nearest bar surface — including stirrups. If stirrup is 8 mm, beam cover to main bar = nominal cover + 8 mm.
✓Excessive cover (> 75 mm) requires skin reinforcement or face bars to prevent surface cracking — see IS 456 Clause 26.5.1.3.
Common Mistakes
⚠Specifying 15 mm cover for slabs — Table 16 requires 20 mm minimum even for mild exposure. 15 mm is the old IS 456:1978 value.
⚠Not accounting for stirrup diameter — if you specify 25 mm clear cover, the actual cover to the main bar is 25 + 8 (stirrup) = 33 mm. Ensure the drawing shows cover to the outer bar.
⚠Ignoring fire resistance requirements — for 2-hour fire rating, beam cover increases to 40 mm (Table 16A), overriding the 30 mm durability cover for moderate exposure.