Concrete Fundamentals and Deterioration Mechanisms
Domain focus
This domain tests how concrete behaves as a composite and why repairs fail when deterioration mechanisms are misdiagnosed. Expect questions on transport, corrosion, freeze-thaw, alkali-silica reaction (ASR), sulfate attack, and physical damage.
Core rules
- Concrete is a three-phase system: aggregate, hydrated cement paste, and the interfacial transition zone (ITZ). The ITZ is the weakest link and a main transport path.
- Reinforcing steel is protected by the high pH of pore solution, about 12.5-13.5. Carbonation and chlorides destroy passivity.
- Carbonation converts Ca(OH)2 to CaCO3; pH drops below about 9-10 and steel depassivates. Measure depth on freshly broken concrete with phenolphthalein.
- Chloride-induced corrosion is electrochemical. There is no single universal threshold; ACI 222R treats it as binder- and exposure-dependent.
- Freeze-thaw damage requires critical saturation, about 91.7 percent of pores, and repeated cycles; air entrainment provides relief.
- ASR needs reactive silica, available alkalis, and moisture. Products include gel, map cracking, and expansion.
- Sulfate attack forms ettringite or thaumasite; DEF is internal sulfate attack.
Worked example
A parking garage slab shows rust staining and delamination. Half-cell potentials are -420 mV CSE over the delaminated area and -150 mV elsewhere. Carbonation depth is 10 mm on 40 mm cover. Reasoning: the -420 mV zone has a high probability of active corrosion, but half-cell is probability, not rate. The 10 mm carbonation front has not reached steel, so chloride ingress or another mechanism is more likely; test chloride content and petrography. Repair must remove unsound concrete beyond the anode zone.
Traps
- Assuming carbonation is the cause whenever steel corrodes.
- Testing carbonation on a weathered surface.
- Treating a half-cell reading as a corrosion rate.
- Missing ASR because gel is not visible.