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Causes of deterioration; effects of steel corrosion

Short answer

Physical causes include freeze-thaw, thermal effects, shrinkage, abrasion, erosion and salt crystallization. Chemical causes are sulphate and acid attack, alkali-aggregate reaction, chloride ingress, carbonation and leaching. Corrosion leads to expansive rust, cracking, spalling, loss of steel section and bond.

Explain the physical and chemical causes of concrete deterioration. What are the effects of corrosion of steel in concrete?

Physical causes

Physical causes act by mechanical, thermal or moisture processes without changing the chemical composition of cement paste.

CauseMechanismEffect and control
Freezing and thawingWater in pores freezes, expanding by 9%, causing hydraulic pressure; repeated cyclesScaling, cracking, D-cracking. Use air entrainment (4 to 7%), low w/c, sound aggregate
Thermal effects (temperature cycle, fire)Differential expansion/contraction; above 300 °C paste loses water, above 500 °C Ca(OH)2 decomposesCracking, spalling, loss of strength. Joints, cover, insulation
Shrinkage and moisture movementDrying shrinkage restrained by steel/supportsCracks, which open the way for aggressive agents. Low w/c, curing, joints
AbrasionWear by traffic, sliding or rollingSurface loss in pavements, floors. Hard aggregates, high strength, surface treatments
Erosion and cavitationHigh velocity water with silt, or vapour bubbles collapsingPitting of spillways. Smooth surface, high-strength concrete
Salt crystallisationSalt solution crystallises in pores on dryingPressure causing scaling and efflorescence. Dense concrete, membranes
Wetting and dryingMoisture gradients cause cyclic stressCracking and surface deterioration

All physical processes increase permeability and speed up chemical attack, so low w/c, good curing and compaction help in all of them.

Chemical causes

Chemical causes are the reaction of cement paste or aggregate with substances from outside or inside the concrete.

CauseReactionEffect and control
Sulphate attackSulphates (soil, groundwater, sea) react with Ca(OH)2\text{Ca(OH)}_2 and C3A to form gypsum and ettringiteExpansion, cracking, softening. Use sulphate-resistant cement (IS 12330), PPC, low w/c, dense concrete (IS 456 Table 4)
Alkali-aggregate (silica) reactionAlkalis of cement react with reactive silica of aggregate to form an expansive gel which swells with moistureMap cracking, pop-outs. Use non-reactive aggregates, low-alkali cement, fly ash
Acid attackAcids dissolve Ca(OH)2\text{Ca(OH)}_2 and C-S-H into soluble saltsLoss of material, softening. Dense concrete, protective coatings
Chloride attackChlorides destroy the passive film of steelCorrosion of reinforcement. Limit chloride (IS 456 Table 7, 0.6 kg/m3^3 for RCC)
CarbonationCO2+Ca(OH)2→CaCO3+H2O\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}Lowers pH, then corrosion. Low w/c, cover
Leaching and efflorescenceSoft water dissolves Ca(OH)2\text{Ca(OH)}_2 and carries it outLoss of strength, white deposits
Sea waterCombined action of sulphate, chloride, crystallisation, abrasionUse PPC/slag, w/c 0.45 or less, cover
BiologicalBacteria produce sulphuric acid in sewersSurface erosion

The rate is controlled by permeability: the denser the concrete, the slower the attack.

Effects of corrosion of steel in concrete

  1. Expansive rust creates tensile stress; cracks run along the bars, followed by spalling of the cover and delamination.
  2. Loss of bar cross-section (pitting) reduces the tensile capacity, ductility and fatigue life of the member.
  3. Loss of bond between steel and concrete, so anchorage fails.
  4. Rust stains, exposure of bars, more ingress of water and chlorides (accelerating the process).
  5. Loss of serviceability (deflection, cracks) and finally structural collapse in severe cases.

✓ Answer checked by Claude Sonnet 5.5

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