Q. Minimum and maximum distance between bars?

Minimum distance between bars and maximum distance between bars

In some codes, a minimum distance amid bars is specified to allow for sufficient space to provide accommodation for internal vibrators at the time of compaction. 

Inversely the restriction of maximum bar spacing is typically for controlling crack width. For a provided area of tension steel areas, distribution of steel reinforcement affects the pattern of crack formation. It is predominant to have smaller bars at closer spacing rather than larger bars at larger spacing to be effective in controlling cracks.

Henceforth the limitation of bar spacing beyond a definite value (means maximum distance between bars) aims at better control of crack widths.