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The full scale pile group load test would be quite difficult to execute + due to defective piles there are greater chances of settlement and thus making the entire pile group sink as a result extra loss to the contractor and a complete failure of project. So I would rather opt for concrete core tests. In addition GPR will be useful in giving idea about the condition of pile. So any pile with weak concrete mix will possibly be identified using GPR as per my observations.

There are different way depending upon the risk category of the structure. Pile capacity of concrete is one things and geotechnical capacity is another. The best way would be to do a pile load test on this pile group. Geotechnical consultants normally do such test and it helps verify capacity of existing piles both structurally and geotechnically. For concrete strength, like you said core testing is one way. I am not familiar with GPR for strength testing of concrete piles. I have seen a lot of results of GPR surverys to find buried lines but nothing to determine strength. Hope this helps.

I haven't done any slab retrofits myself, but looking at the picture, the following solution comes to my mind. You can consider the additional thickness of slab + original one as one slab. Do your analysis and find the compressive and tension force in a unit width and design the studs for that width. That force will tell you how many studs you need. Also you need to check stud capacity and Hilti adhesive capacity. Stud capacity should be easy as it is made of steel. For Hilti, yo can use their software Profis.

Thanks bro for your reply. Exactly what I have been thinking to adopt. If there is any pile that is not meeting the standards will definitely have a changed profile and will be helpful in indicating the defective pile. In addition destructive tests on extracted cores will aide to the investigation.

Why don't you go with CFRP?