Joints in screed 2

Is it a bonded topping? If so, putting joints in the topping may or may not have much effect as the topping will crack to match whatever cracks are in the raft.

If it isn't bonded, then our standard recommendation is twice the depth in feet, so a 4" thick (100mm) topping should have joints at 8ft (2440mm) o/c.

Is the topping reinforced? The amount of reinforcing can affect the joint spacing.

The topping is unreinforced slab and there is no reinforcement between the slab and the raft so can we say that is unbonded

You should consider bond the topping to the slab below.

Just because there is no reinforcing doesn't mean it won't be bonded. There are millions of square metres of toppings that are bonded to the substrate without reinforcement crossing the plane. For the topping to be truly unbonded, there needs to be a form of bond break to allow the topping to shrink without restraint from the slab below.

Intentionally bonding the topping, which generally requires roughening of the surface and adequate cleaning would restrain the topping and likely result in a randomized pattern of cracking in the topping. This would also result in any cracks in the raft to telegraph through the topping.

Thanks a lot I think we're should make it unbonded in this case what is the required joints and can we pour that whole area at once or we need to make joints during pouring

Generally the control joints are sawcuts that are done after the slab is poured but before it is cured. Realistically as soon as the slab can support the weight of the saw and operator. Around here we say that a maximum of 24 hours after pouring otherwise the sawcuts will be useless as the slab will have already had it's shrinkage cracks formed.

I did indicate what spacing I would recommend. I do not have a codified number to the ACI for that since I don't practice where the ACI code is used. But my local standard, CSA, is extremely similar. Crack control joints are not specifically laid out in the code for something like this. It is more of a recommendation and should be adjusted to each situation.

Should be 6 hrs to 8 hrs. 24 is way too long, specially for a thin topping. Microcracking has already determined the crack pattern and sawcutting it is nearly useless.

Dik

Agree with dik...sawcuts should be the same day. Microcracking occurs early on, reduces the effective cross section and develop into "random" cracks.