Transition Zone in Concrete
- A small region next to the particles of coarse aggregate.
- Typically 10 to 50 μm thick around large aggregate
- Weaker than either of the two main components of concrete
- Exercises a far greater influence on the mechanical behavior of concrete than is reflected by its size
- Higher water-cement ratio closer to the larger aggregate than away from it (i.e., in the bulk mortar)
- Crystalline products of hydration such as ettringite and calcium hydroxide in the vicinity of the coarse aggregate consist of relatively larger crystals, and therefore form a more porous framework than in the bulk cement paste or mortar matrix.
- The plate like calcium hydroxide crystals tend to form in oriented layers
- With the progress of hydration, poorly crystalline C-S-H and a second generation of smaller crystals of ettringite and calcium hydroxide start filling the empty space that exists between the framework created by the large ettringite and calcium hydroxide crystals. This helps to improve the density and hence the strength of the interfacial transition zone.
Strength of transition zone
- At early ages the volume and size of voids in the interfacial transition zone will be larger than in bulk mortar; thus, weaker in strength.
- With increasing age the strength of the interfacial transition zone may become equal to or even greater than the strength of the bulk mortar. This may occur as a result of crystallization of new products in the voids of the interfacial transition zone by slow chemical reactions between the cement paste constituents and the aggregate, formation of calcium silicate hydrates in the case of siliceous aggregates, or formation of carboaluminate hydrates in the case of limestone. Such interactions are strength contributing because they also tend to reduce the concentration of the calcium hydroxide in the interfacial transition zone.
- Large calcium hydroxide crystals possess less adhesion capacity, not only because of the lower surface area and correspondingly weak van der Waals forces of attraction, but also because they serve as preferred cleavage sites owing to their tendency to form an oriented structure
- A major factor responsible for the poor strength of the interfacial transition zone in concrete is the presence of micro-cracks
Influence of the interfacial transition zone on properties of concrete – Strength
- The interfacial transition zone, the weakest link of the chain, is considered as the strength-limiting phase in concrete.
- Because of the presence of the interfacial transition zone that concrete fails at a considerably lower stress level than the strength of either of the two main components. Aggregate and hydrated cement paste or mortar usually remain elastic until fracture in a uniaxial compression test, whereas concrete itself shows inelastic behavior
- With increasing stress, the matrix cracks gradually spread until they join the cracks originating from the interfacial transition zone. When the crack system becomes continuous, the material ruptures.
- Considerable energy is needed for the formation and extension of matrix cracks under a compressive load. On the other hand, under tensile loading, cracks propagate rapidly and at a much lower stress level. This is why concrete fails in a brittle manner in tension but is relatively tough in compression. This is the reason why the tensile strength is much lower than the compressive strength of concrete.
- The microstructure of the interfacial transition zone, especially the volume of voids and micro cracks present, has a great influence on the stiffness or the elastic modulus of concrete.
- In concrete, the interfacial transition zone serves as a bridge between the two components: the mortar matrix and the coarse aggregate particles. Even when the individual components are of high stiffness, the stiffness of the composite is reduced because of the broken bridges (i.e., voids and micro cracks in the interfacial transition zone), which do not permit stress transfer. Thus, due to micro cracking on exposure to fire, the elastic modulus of concrete drops faster than the compressive strength.
Influence of ITZ on durability
- The existence of micro cracks in the interfacial transition zone at the interface with steel and coarse aggregate is the primary reason that concrete is more permeable than the corresponding hydrated cement paste or mortar.
- Everything else remaining the same, the larger the aggregate size the higher the local water-cement ratio in the interfacial transition zone and, consequently, the weaker and more permeable would be the concrete.
- This is the reason why at a given cement content, water-cement ratio, and age of hydration, cement mortar will always be stronger than the corresponding concrete. Also, the strength of concrete goes down as the coarse aggregate size is increased.