MAXIMUM ALLOWED CRACK WIDTH AS PER BS CODAL PROVISION IS 0.3MM FOR THE STRUCTURES IN CONTACT WITH SOIL AND NOT SUCH PROVISION S IS FOR INTERNAL MEMBERS

ARIF ALI SIDDIQUI
STRUCTURAL DESIGN ENGINEER
IMCC 8 JANTAR MANTAR MARG
NEW DELHI INDIA

Regarding this question and others of similar nature, I dont know what is recommended by the ACE, PCA etc but in the UK crack widths are controlled by the size, spacing and cover of the rebar. In general, having calculated the required area of reinforcement for flexural purposes, the anticipated crack widths can be predicted by a selection of the above factors.This principle has been used for many years now and has produced results matching predictions.
See the relevant chapters in BS8110 (concrete) and BS8007 (water retaining structures).
Also you can download Excel spreadsheets which contain crack calculation software amongst many others (www.structural-engineering.fs.net.co.uk and www.rcc-info.org.uk)
For general R.C structures, we adopt 0.3mm maximum width while for water retaining structures 0.1mm and 0.2mm may be used as the limiting value, depending on the importance of the structure/finishes etc.

Are you asking for slab on grade or pile supported slab at ground level?