Let's start again.

You wish to ASSESS an existing structure for the likelihood of cracking prior to coring operations.

Is the above what you wish to do? If so, we are going to need more information, but let's get the question understood first.

Is the purpose of coring to determine non-conforming material or to determine why something has happened. You need to locate the rebar prior to coring. I know of no qualitative manner of determining coring location, generally visual and based on experience.

With respect to the first item results of coring tests are often inconclusive, subjective and contentious. A clear understanding of the objective is required and any remediation. This should be established prior to coring, with all parties in agreement. My personal experience with coring for this item has been dismal. Coring may not be the panacea.

With respect to the second item, coring is always an interesting endeavour. You need to core where the damage has occurred, but also in an area of reduced stress to prevent damage to the structure. It is often that the area examined is a region of high stress. This where experience is required.

Dik

Yes Ingenuity/Dik, the question is;
I wish to ASSESS an existing structure for the likelihood of cracking prior to coring operations.
The purpose is to core the structure(manhole) to create hole for ducts.

Erasa

Precast manhole manufacturers often publish a guide to limits on coring their precast concrete structures. You might try looking up their guidelines to see where, and how close, you would be allowed to core for various openings in their products. This would give you a good starting point for your research and assessments.
Dave

Thaidavid

if you are just making a hole (I assume in reinforced concrete?) for a (small) duct to go through, than unless the structure is deteriorated or this is a very large hole, why would you expect any cracks? If this is a larger hole, than suggest you need to reinforce the hole after you cut it to prevent cracking.

...and select a low area of stress, and, locate any reinforcing. Check really carefully if the structure is per-stressed.

Dik

Usually we do this kind of operation prior to pouring concrete by embedding a pipe of the required diameter.

KARMA

It seems the question is unnecessary. If you are in a compression section of the concrete you might weaken some of the resistance to crushing, not cracking. If you are in the tension part, the concrete has no tensile stress contribution, so why worry about cracks? Normally reinforced concrete design does not depend on the ability to resist tension.