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Behavior of Concrete Microcracking Under Loading (Part-1)

Cracking of any form can lead to failure of concrete members. So in concrete engineering, this term is studied and handled with care. We have provided a basic idea about microcracking in the last post; here in this post we will discuss about the cracking pattern, elongation of cracking and appearing new cracks under loading. We have already learnt that this types of cracking remains in the concrete even in preloading state. Let’s learn about what happen when concrete becomes stressed to its ultimate loading.

Microcracking concrete under lightmicroscope
Microcracking in concrete under
lightmicroscope (Green llines)
Microcracking is determined by optical microscope as it is often not visible under human eyes. But a upper limit of cracking (0.1 mm) that can be detected by eyes without any visual instrument, is found in some findings. For engineering applications a lower limit of cracking can be set.

We will discuss about behavior of microcracking under cyclic loading in the next post. In this post we will discuss about gradually application of loads. The microcracks show stability under increasing load up to around 30 percent of ultimate load. Beyond this stress the cracks expanded with their number, length and width. As we discuss in the last post the cracking weaken concrete under its tensile stress.

Microcracking in concrete under uniaxial tension

The white lines are mortar cracks and black lines stand for combined cracking in concrete under uniaxial tension
Now the stress level depicted above is not fixed, it’s have sensitivity of W/C (water to cement) ratio in the paste. But in this stage cracks propagates slowly.

So far we are talking about cracking between interface of cement paste and coarse aggregates. When stress reached near ultimate capacity of concrete, cracks formed in the mortar i.e. bond between fine aggregate and cement paste are destroyed with the gradual increment. Dear reader in the next portion of this post we will finish this topic. Please follow the link to switch next part (Part-2).

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