Why are crack width limits specified in reinforced concrete design?

Crack width limits (typically 0.1-0.3mm depending on exposure) are specified for: (1) Durability - wider cracks allow ingress of moisture, chlorides, and CO2 causing corrosion, (2) Aesthetics - visible cracks may be unacceptable, (3) Water-tightness - liquid-retaining structures require tight limits. Crack width depends on reinforcement stress, bar spacing, cover, and bond. It's controlled by providing adequate reinforcement area, limiting bar spacing, and proper detailing.