INTERFECIAL

TRANSITION

ZONE AND ITS

EFFECT ON

STRENGTH OF

CONCRETE

SUBHA GHOSH

MASTER OF CIVIL

ENGINEERING

001210402014

What is ITZ?

During mixing, casting, and consolidation (usually by vibration) of concrete, a layer of water accumulates around aggregate particles.

Additional bleed water

gathering mainly under large

aggregate

particles.

Wall Effectzone closest to the aggregate contains predominately small grains and has a significantly higher porosity,

while larger grains are found further out.

This region may extend some 50 m or more μout from the aggregate surface.

Thus, the aggregates are surrounded by a zone of relatively high water/cement ratio.

Due to the way of formation ITZ is not a definite zone, but a region of transition.

ITZ-Properties

A so-called ‘duplex film’ in direct

contact with the aggregate surface,

comprising CH preferentially oriented

with c-axis normal to the aggregate

surface, the outer part of the film

being a layer of C-S-H gel

Crumbie (1994)

The space around the aggregates is less effectively filled by hydration products, and at the same time there is greater tendency for CH (Ca(OH)2) and ettringite to develop in this space, since these two compounds form and deposit preferentially in large pores.

The weakest part of the interfacial zone lies not right at the physical interface, but rather 5 to 10 m within the paste μfraction, with the fracture path running along the cleavage planes of the oriented Ca(OH)2 crystals.

ITZ-Effect on Strength

Most studies of strength show that

increasing the cement-aggregate bond

strength increases the concrete strength,

whether in tension, compression, or flexure.

Alexander and Taplin (1962, 1964) Based on a regression analysis of the data then

available. It has the form

σ=b0+b1m1+b2m2

where σ =concrete strength (compression or flexure)

b0, b1, b2=linear regression coefficients

=480, 2.08, 1.02, respectively, for compression

=290, 0.318, 0.162, respectively, for flexure

m1, m2=modulus of rupture of the paste and of the cement-aggregate bond, respectively.

From previous expression, it may be seen that a

change in the flexural strength of the paste has

about twice as much effect as does a change in the

flexural bond strength.

ITZ-Modification

Bentur and Cohen (1987) were able to show that by improving the density of the interfacial zone, and hence presumably increasing the bond, by using silica fume, improvements in concrete strength of the order of 25% to 30% over plain cement concrete were realized. This occurred without any corresponding increase in the strength of the respective pastes, pointing to the improvements in the interfacial region as therefore being the most likely source of improved concrete strength.

O Densification is typically achieved by using a very fine mineral admixture such as silica fume in the mix.

effects:O (a) they improve the packing density in the interfacial

region, thereby largely eliminating the ‘wall effect’ mentioned earlier;

O (b) they act to reduce bleeding, thus also reducing the size of the transition zone (Carles-Gibergues et al);

O (c) they act as growth nuclei for multiple generation of CH crystals which therefore have smaller size (i.e. grain refinement); and

O (d) they participate in longer-term pozzolanic reactions which continue to densify the interfacial zone with time.

Densification

O Grain refinement can be achieved in different ways. Monteiro and Mehta report on two processes

: one using an ASTM Type K (expansive) cement, the other using carbonate aggregates.

By this large CH crystals are replaced by small and random oriented CH crystals.

GRAIN REFINEMENT PROCESSES

O These processes often involve pre-treating or pre-activating aggregates with chemical solutions, or low water/cement ratio pastes, thus supposedly giving them a greater affinity for reacting or interacting with the cement paste.

PHYSICO-CHEMICAL INTERACTION PROCESSES

O Interfacial Transition Zone in Concrete by J.C.Maso

O Cement and Concrete Research journal.

REFERENCES

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