Three phase system and transition zone

Concrete is a heterogeneous composite at the microscopic level, treated as a three-phase system:

   +-----------------------------+
   |  hcp (binding medium)       |
   |      +---------+            |
   |      | AGGREG- |<- transition
   |      |   ATE   |   zone (ITZ)
   |      +---------+            |
   |  ( ) pores / voids          |
   +-----------------------------+

1. Aggregate phase

Occupies 70-75% of the volume. It governs the unit weight, modulus of elasticity and dimensional stability. In normal concrete it is stronger than the paste, so it does not govern strength. Its size, shape, grading, texture and porosity influence workability and bond.

2. Hydrated cement paste (hcp, binding medium)

Contains C-S-H gel (50-60%), calcium hydroxide (20-25%), calcium sulphoaluminates (15-20%), unhydrated cement grains and pores (gel pores, capillary pores, air voids). It binds the aggregates and gives strength and durability.

3. Transition zone (interfacial transition zone, ITZ)

A thin layer, 10-50 µm thick, around the aggregate particles. It is the weakest link in the system.

• Formation: next to the aggregate there is a "wall effect", so cement grains pack loosely, and bleed water collects under the aggregates. The zone therefore has a higher w/c, so large crystals of $Ca(OH)_2$ and ettringite grow in a preferred orientation, with more porosity and microcracks.

Effect of the transition zone on the properties of concrete

• Strength: concrete fails at a stress well below the strength of both the paste and the aggregate, because microcracks begin in the ITZ. Bond cracks form there at about 30-50% of the ultimate load and later join up with cracks in the mortar. The ITZ is also the main reason concrete is much weaker in tension than in compression.
• Stress-strain: concrete behaves nonlinearly even though the aggregate and the paste are each linear elastic, because of microcrack growth in the ITZ.
• Modulus of elasticity: lowered; the porous ITZ is a weak link in the load path, so the modulus of concrete is lower than predicted from the moduli of the aggregate and the paste.
• Permeability and durability: the porous ITZ forms a pathway for water and ions, so it reduces durability (chloride, sulphate and frost attack).
• Bond between aggregate and paste, and fracture toughness, are reduced.

Remedy: use a low w/c, silica fume or fly ash (which fill the ITZ and consume Ca(OH)$_2$), a superplasticiser, good compaction and curing, and clean, rough aggregate.