How is service life of concrete structures predicted for durability design?

Service life prediction uses deterioration models: (1) Chloride ingress - Fick's second law for diffusion, with threshold concentration and cover, predicts corrosion initiation time, (2) Carbonation - square root of time relationship with carbonation coefficient, (3) Propagation period - from initiation to unacceptable damage. Models require input parameters (diffusion coefficient, surface concentration) from testing or databases. Probabilistic approaches account for variability. Design adjusts cover, concrete quality, or provides additional protection (coatings, cathodic protection) to achieve target life (50-100 years).