Size of Cracks

Cracks are judged by the crack width at the surface of the concrete which decreases towards deformed rebars. Long years of research and experience showed that crack with upto 0.4 mm do not significantly harm corrosion protection of rebars, if the concrete cover is sufficiently thick and dense. 

Polluted air especially CO2 causing carbonation and SO2 forming acids or chlorides from deicing salts damage the concrete independent of cracks.  Structures must be protected against such attacks having cracks or no cracks. 

Cracks are harmful for the image of the engineers if they are easily visible, because layman consider them as a damage. 

For the environmental criteria allowable crack width can be as follows: 

Low aggressive environment 0.5 mm easily visible.  Medium aggressive environment 0.4 mm scarcely visible.  High aggressive environment 0.3 mm for the unarmed eye. 

These values are valid for a normal concrete cover of 30 mm and bar diameters upto 25 mm. For a larger cover the allowable crack width can be increased suitably.