Intermediate Transition Zone (ITZ) in concrete

What is ITZ and how it is developed in concrete

ITZ in civil engineering terminology is known as interfacial transition zone or intermediate transition zone. This is the phase present in concrete where two different phases meet with each other. As you can see in the picture you can see clearly the layer of ITZ presence 

In concrete aggregate is considered as one phase and surrounding paste is considered as another. For concrete to bear load, it is imperative that there should be stress distribution between paste and aggregate.

When these two phases of concrete meet, there is always an ITZ layer present. This layer has properties which are neither of aggregate phase nor to the paste phase.

What happens is when that near aggregate slurry (water+cement) accumulates causing a layer which is shown in picture below. This layer is week because of its chemical composition.

Strength

In reality, ITZ has probably the lowest strength properties therefore under loadsstress in aggregate phase will be different than that of the paste for same strain level. This type of non-uniform stressdistribution will cause ineffective transmission of forces between paste and aggregate. If ITZ is not taken care of than this can lead to stress concentration which will ultimately cause cracking

Sources of Crack in Concrete

We all know cracking is one of the major problems in concrete and it should be considered while mix design, but for that, we have to know the sources which can cause cracking in concrete. Here is a list of sources of cracking in concrete

Use of low-grade materials:

One of the major sources is the use of lower quality materials which include both concrete and steel in reinforced concrete structures.

Shrinkage:

Shrinkage can cause cracking if not controlled during mix design and curing stage. Shrinkage can become critical in high strength concrete because of low water/cement ratio and also because of use of Mineral Admixtures.

Quality and Type Of Aggregate:

The quality of aggregate used in concrete determines the overall strength of concrete. If the aggregate is of poor quality it will not make a proper bond with cement.

Overloading of structure:

Overloading of structure especially at younger age is a common source of cracking. This can happen if formwork is removed before time or more construction load is present.

Mistakes at design stage in office:

If there are errors at design stage then it is obvious that problems will occur at site. concrete cracking is one of them.

Improper Curing:

Another major cause of concrete cracking. If curing is not done appropriately for given time span then one should expect cracking.

Early Formwork Removal:

If formwork is removed before concrete has achieved strength, there will be cracking.

Use of Congested Reinforcement in Lean Concrete:

If you use heavy reinforcement in average quality concrete then stress distribution between steel and concrete can become non-linear causing cracking.

Mistakes at Site or during erection:

Proper and trained labour and workmanship are necessary for any sitework. Lack of it during concreting can cause cracking.

 

Effects in concrete mix design

Concrete Mix design is complete science and it is based on a lot of research. Popular Mix Design method is highlighted in ACI 2011 committee report. It is a systematic approach in which user starts with defined slump and strength and some other parameters. End result gives quantities of different constituents.

Performance-based methods are alsoavailable in which engineer based on his experience selects initial proportions and modifies as needed. Both systematic and performance-based methods have certainadvantages and disadvantages which will be covered in later posts.

Though mix design is a comprehensive process still it is very useful for engineers to know certain mix design concrete thumb rules which will make his life easier. Here is the list,

By adding 1 litre of water in 1 cubic meter of concrete mix

Increase slump of about 25 mm is expected.It will decrease compressive strength of about 1.5 to 2.0 N/mm²Increase shrinkage potential of about 10%Waste as much as ¼ bag of cement

Effect of increasing concrete mix temperature by 1 celsius

About 4 liters of water per cubic metermaintains equal slumpAir content decreases about 1%Compressive strength decreases about 1.0 to 1.5 N/mm²

Effect of air content on concrete mix 

If air content increases 1%, it will result incompressive strength decrease of about 5%If air content decreases 1%, then it will cause yield to decrease about 0.03 cubic meter per 1 cubic meter of concrete mix.If air content decreases 1%, then slump willdecrease about 12.5 mmIf air content decreases 1%, it will result in durability decreases of about 10%.