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Transition phase and its effect

Short answer

The transition zone (ITZ) is a 10–50 µm porous layer around aggregates, high in oriented CH and microcracks from wall effect and bleeding. It weakens concrete: reduces strength, modulus, durability; causes nonlinear behavior; increases permeability. Improve with low w/c, silica fume, and proper curing.

What do you understand by the transition phase of concrete? Explain the effect of the transition phase on the properties of concrete.

The transition phase (transition zone, ITZ) is the thin shell of cement paste, about 10-50 µm thick, immediately surrounding the aggregate particles. Its structure differs from that of the bulk paste.

How it forms

  1. Because of the "wall effect", cement grains cannot pack closely against the large aggregate surface, so the zone is richer in water and has a higher w/c.
  2. A film of bleed water accumulates around the aggregate (especially under flat, large particles) and gets trapped.
  3. In this water-rich space, larger crystals of calcium hydroxide (oriented with the c-axis perpendicular to the aggregate) and ettringite grow, and less C-S-H forms. The result is a zone with high porosity, a low density and microcracks (from shrinkage and thermal movement).
 aggregate | ITZ (porous,  | bulk paste
           | CH crystals)  | (dense C-S-H)

Effect on concrete properties

  • Strength: concrete is weaker than either aggregate or paste; microcracks start in the ITZ at about 30-40% of ultimate load. Tensile and flexural strengths are very sensitive to it.
  • Stress-strain relationship: non-linear, due to progressive bond cracking.
  • Elastic modulus: lower than predicted from the aggregate and paste.
  • Durability and permeability: the porous, interconnected zone allows ingress of water and aggressive ions; affects frost, chloride and sulphate resistance.
  • Fracture: governs crack paths and brittleness.

It can be improved by a low w/c ratio, silica fume and fly ash, a superplasticiser, rough clean aggregate, and good compaction and curing.

✓ Answer checked by Claude Opus 5.5 (high effort)

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