If a contractor proposes to increase concrete cover beyond contractual specification (i.e. 40mm to 70mm), shall engineers accept the proposal?

Normally40 mm cover being given to RCC columns,Though  Increase  the cover has the advantage of extra protection from the corrosion it may increase the dead load of the structure which is not acceptable as far as the design parameters are concerned

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Yes- because it has no special implication rather add to the strength, but became gold plating to the contractor