if concrete cubes fail, we don't want to go with core cutting procedure,can we proceed to Rebound test? is it acceptable or not. because as per standard this test result is not accurate, it have 25% variation.
If NDT is your only option (where coring & compression testing are not available), using either a rebound/Swiss hammer or UPV testing will give you a relatively-reliable indication of your concrete properties, but they will never be as accurate as results from performing a standard cylinder break will. Even coring will not provide an exact f'c since the process of cutting & extracting the core sample can introduce anomalies that can effect the test results....
Keep in mind that any testing method relies on the statistical methods used to analyze the data - You cannot simply take a Swiss hammer onto the site, run a couple of tests and get your f'c: You need to develop a procedural and analytical plan that will provide you with enough proper data to make a proper assessment.
There is an excellent piece of research that compares these 3 methods - Here's a link to it:
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Doing NDT test and acceptance of results is purely dependent on knowledge of structural engineer and his assessment base on the results got in test.
Yes Rebound hammer results comes with a huge variation possibilities as its only measure of surface hardness and indicated results may have variation of 25% with that of actual concrete strength in situ.
UPV could help you in understanding the internal structure of the concreted portion is dense or having voids in it by measuring the velocity. if it has voids, it will take more time to travel through it, if its dense and having lessor or no voids, it will travel faster.
Core test - As the possibility of over vibration and method of core preparation, in most of the cases the results of core test does not represent the actual concrete strength.
There is no test so far developed which could indicate the strength of the concrete directly by assessing sample from in situ either destructive or in non destructive way.
Nor the cube prepared and tested at lab represent the concrete in situ fully and is just an indication of strength under the standard controlled conditions.
It is most important to ensure cube casting, curing and testing is done correctly, as there are 'n' number of parameters which can make impact on what we see as a result of cube testing.
Failure in cube testing results should be analyzed for its causes to better understand the reasons for low strength or failure in results of test.
Due to various condition affecting the what we has as a result or concrete in situ, it is the responsibility of structural engineer to take decision on it as he knows the structural aspect better than us.