Durability, Deterioration, and Service Life
This domain tests deterioration mechanisms and whether you can convert an exposure condition into concrete requirements and a defensible service-life estimate rather than simply specifying higher strength.
Mechanisms
- Chloride-induced corrosion: chlorides reach the steel, depassivate it and the steel corrodes with volume expansion that spalls cover. Ingress is often modeled with Fick's second law and a chloride diffusion coefficient.
- Carbonation: carbon dioxide lowers pore-water pH until the steel depassivates; depth follows x = k times the square root of time.
- Freeze-thaw: damage requires critical saturation plus cycles; a proper air-void system with a spacing factor of about 0.20 mm or less protects the paste.
- Alkali-silica reaction: reactive silica plus alkalis plus moisture; control with low-alkali cement, SCMs and moisture exclusion.
- Sulfate attack and delayed ettringite formation follow sulfate exposure and high early-age temperature.
Translating exposure into concrete
ACI 318 exposure classes set maximum w/c, minimum f'c and minimum cover: typically 0.40 and 35 MPa for chlorides, and greater cover for cast-against-earth or marine zones. Test methods include ASTM C1202 rapid chloride permeability, surface resistivity and chloride ponding.
Worked example
With a carbonation coefficient of 3 mm per square root year, depth at 50 years is 3 times 7.07, about 21 mm. Cover of 40 mm delays depassivation; cover of 20 mm is already carbonated, so the reinforcement is corroding.
Traps
Equating high strength with durability; ignoring cover while chasing w/c; specifying air content but not the air-void spacing factor; overlooking that SCMs improve chloride resistance but need longer curing.