Crack control methods as per IS 456

Cracking is a serviceability limit state: cracks spoil the appearance and let moisture reach the steel, causing corrosion. IS 456-2000 does not require a crack width calculation for every member; instead it controls cracking by detailing rules and, where needed, by a check on crack width.

Methods of crack control

1. Limit the bar spacing (cl. 26.3.3(b), cl. 43 and Table 15).
   • In slabs, the horizontal distance between parallel main bars should not exceed $3d$ or 300 mm, whichever is less; for distribution bars $5d$ or 450 mm.
   • In beams, the clear distance between tension bars is limited by Table 15 (it depends on the service stress, i.e. on $f_y$ and the redistribution of moments), e.g. a maximum clear spacing of about 180 to 300 mm.
2. Use small-diameter bars well distributed around the tension zone instead of a few large bars.
3. Side-face reinforcement: in beams with overall depth above 750 mm, provide 0.1% of the web area on each face, spaced not more than 300 mm or the web thickness, to control cracks on the sides (cl. 26.5.1.3).
4. Minimum steel: provide at least the minimum tension steel (e.g. $0.85bd/f_y$ for beams, 0.12-0.15% for slabs) so that cracking does not occur suddenly.
5. Limit the cover to the tension reinforcement to 50 mm; larger cover needs extra mesh to control cracks. Adequate cover is also needed for durability.
6. Limit the crack width: the surface width of a crack should not exceed 0.3 mm in members where cracking is not harmful (0.2 mm in aggressive environment, 0.1 mm in severe) - cl. 35.3.2. It is calculated by the formula of Annex F when required.
7. Control shrinkage and temperature cracks: proper curing, low water-cement ratio, contraction/expansion joints and distribution steel.