Delaying 2nd pour ? - To avoid stress on shuttering

[Rick]

I have a separate post on shuttering. 9m x 6.6m reinforced raft.
It is 150mm thick in centre and 300 deep for 500mm wide all around perimeter (with structurally calculated steel reinforcement)
The volume means I need 2 deliveries of C30 Readymix.

When it comes to pouring the raft, a friend has suggested that it would be worth putting in first pour, and aiming this at the thickened perimeter, and putting this in fairly stiff.
As I have to have 2 loads anyway, put a delay inbetween loads, this will allow the first pour to start setting, and then put the 2nd pour in topping up perimeter and filling in centre. He thinks this would save a lot of stress on the shuttering.

The question is two fold ... is this a good idea ? and secondly how much of a delay should I build in, too much would mean a non-consolidated raft I guess.

[Tony McC]

No - you want two more-or-less consecutive pours to prevent the formation of a day joint, which is a plane of weakness through the structure.

Even if you allowed 4 hours between pours, there would still be massive stresses on the shuttering because you are placing a second load of concrete on top of the first, which isn't fully hardened and so will inevitably transfer much of the loading laterally, ie, onto your shuttering.

Forwork has to be built to be strng enough to withstand the mass of the whole pour: you can't rely on a partially-stiffened previous pour to prevent laod transfer outwards.