In contractual aspect, based on the requirement of General Specification of Civil Engineering Works (1992 Edition), the tolerance of concrete cover is between +5mm and –5mm and engineers should not accept sub-standard work because they do not possess the authority to change the acceptance criteria. In case engineers consider contractor’s proposal acceptable in technical point of view, consent has to be sought from the employer regarding the changes in acceptance criteria.
From technical point of view, the effect on cracking due to an increase in concrete cover should be considered. In general, there are three main parameters which govern crack width, namely tensile strain at the point considered, the distance of longitudinal bar to the concerned point and the depth of tension zone.
For the second factor, i.e. proximity of longitudinal bars to point of section, the closer a bar is to this point, the smaller is the crack width. Therefore, closely spaced bars with smaller cover will give narrower cracks than widely spaced bars with larger cover. Therefore, with an increase of concrete cover, the crack width will increase which is undesirable.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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Is there any guidelines for designing the chairs in terms of its minimum diameter and minimum numbers required per sq. mt for the raft foundations ? Also what should be the minimum spacing between the chairs.