Please see image for deshuttering time as laid out in IS codes.
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Concrete gains strength with time after casting. It takes much time for concrete to gain 100% strength and the time for same is still unknown. The rate of gain of concrete compressive strength in higher during the first 28 days of casting and then it slows down. If you see the table then at 7 days concrete gains around 65% strength of fck.
Now the IS code allows us to deshutter the slab (see table) in 3 days, however, the condition being we need to keep the props in place. the code allows us to remove and then refix. However, I am of the opinion that we can remove the shuttering but by designing the shuttering in such a way that we can remove the shutters by keeping the props in place similar to what we do in case of aluminum shuttering ( generally referred to as Mivan ).
Further another critical aspect which most of the time is overlooked while deshuttering is the absence of engineers supervision while deshuttering, just as erection of shuttering is important equally critical is at deshuttering.
Engineers tend to neglect this part here in India and that's the reason for some of the collapses or deflection of slabs/beams after deshuttering. I would give similar attention while deshuttering to avoid untoward incidents.
Before deshuttering ensure the strength of concrete by testing the cubes on day you want to deshutter so take extra cubes for say day 4 and see what is the concrete strength and if it has gained the desired strength only then you can proceed with the deshuttering. However keeping props in place after deshuttering is mandatory till the concrete gains its strength as per the design.
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