The first cracks occur vertically near the upper and lower contact points, and propagate along the vertical line which extends between these contact points. Also you can see a lot of actual videos in "YouTube" for this test.

Best regards.

Thank you for your answer.

In fact, I am interresting in modelling the traction strength of concrete. We already know that the cracks begin in the ITZ (interfacial transition zone between aggregate and the cement paste) which is the weaker phase of the concrete; And what I woul like to know is: how would the cracks propagate from the ITZ to the cement paste? are there a network of microcracks that spreads from the ITZ to the paste or the break is fragile?

Best Regards

There are two views in this regard. From a mechanical standpoint the ITZ represent the connection between the aggregates and the cement paste and must transfer the stresses between these two.

Since the aggregates are usually stiffer than the cement paste, severe stress concentrations are likely to occur at the ITZ. Thus ITZ is the region where micro and macro-cracks initiate and propagate, while the other regions are barely damaged or still in the elastic regime.

In another view, upon loading, micro cracks in ITZ propagates and coalesces with existing macro cracks resulting in failure of bond and do not allow transfer of stresses.

Best Regards.

Thanks Dr Elango for your answer

There is another reason to cracking. Its alkali silica reaction which occur due to the reaction between the reactive silica in Aggregate and alkali in cement paste in presence high humidity. This reaction produce gel surround the aggregate and cause pressure lead to cracks.

First crack is initiated at the tip of the largest void in concrete media. The crack is propagating into concrete until the tensile stress at crack tip drop to a value less than the concrete tensile strength (fct).

best regards

Cracks starts where they find a point of weakness. Where the highest stress concentrations occur, the structure is weakened, and this creates a path for micro cracks to start propagating. In the splitting test, the crack start at the top and bottom surfaces in contact with the load and the bottom support.

This is a nice video explaining ITZ:

https://www.youtube.com/watch?v=6o9QgLRiaBg

Thanks a lot Mr Messaoud Saïdani , Mr Alaa Hussein Al-Zuhairi and Mr Salhin Alaud For your answers.

The first visible crack may depend on a load level as a percentage around of the maximum when the sample started to fail.