Hello there Me. Chamarhy! I read the complete thread and I understand that you haven’t exactly got the answer you are looking for. Well, that’s partly because you are slightly misinterpreting cl. 11.3 of IS 456 (2000). It says that formwork must not be removed until concrete gains at least twice the strength it needs to stand the stresses it will experience during formwork removal. Now, if you look carefully, this doesn’t mean that the code has given this advice with concrete’s final compressive strength in view. It is solely there for the safety of the structural member during formwork removal. 

The table within that clause, says that for ordinary conditions (OPC is used, ambient temperatures >= 15°C, adequate curing is done), these stripping times for the concrete to reach that above mentioned strength will be so and so for so and so types of formwork. It is not talking about the compressive strength of concrete.

Apparently there is indeed no specific mathematical relationship between the formwork stripping time and the final (28 days) compressive strength. This guideline is solely for the protection of concrete during formwork removal. Obviously concrete gains strength the better the better it’s cured. So, if someone wishes to strip early and begin curing early, the code is trying to advice against it. Furthermore, concrete indeed attains strength in direct proportionality with the quality control in production, the quality of the materials and the adequacy of curing. As far as I think, there could be only a little implication that concrete gains strength well if it retains heat in very early age and formwork does insulate the concrete. But that’s not much important because you can’t wail too long to strip formwork so that you can start curing when it’s the right time.

Now, the next point is the theoretical or experimental evidence that you are seeking. I’d like to humbly tell you that the table in 11.3 which claims that these are the times for achieving double the strength requirement for formwork stripping must indeed be based on experimental data. It is easy to model the stresses induced at the time of stripping. Now, the strength of concrete within formwork can also be estimated via pull out test and a few non destructive tests. You can Google ‘concrete strength tests during maturity period’ and it’ll pop up with a number of papers and articles. The pull out test for instance, corellates the compressive strength to the force required to pull out a core of specific shape and size, in a specific way from the maturing concrete.

To conclude, the code is not talking about a relationship between removal time and the compressive strength. It’s only talking about the corelation between removal time and the strength at that very time of formwork removal, for which, experimental evidence has long been found.

Sorry for a long explanation. But, I’ll be more than pleased if it clears your doubt. Thank you!