Outdoor concrete slab - expansion/contraction slots or side insulation recommended?

[Dave Robinson]

I'm about to pour a 4m x 8m x 150mm slab between a C-shape plan reinforced block wall. I'll be using a DPM on top of the hardcore and reinforcement fabric within the slab. Should I be considering expansion or contraction slots for a slab this size, or would anything else at the side of the slab be recommended to 'decouple' it from the wall and its foundations (mechanical, thermal or moisture)?

 

[r896neo]

Your dpm folded up against the wall will be enough of a slip joint to seperate slab and wall.

As for an expansion joint I would put one in the centre. Your on the limit and might be alright without but I would, even if its just a saw cut after 48hrs.

 

[Dave Robinson]

I'll add a saw cut then! Just the reassurance I was after, many thanks!

 

[r896neo]

If the concrete is a finished surface just ne sure its hard enough before cutting so you don't spall aggregate out of the surface.

 

[chappers]

no need to cut all the way through, about half way will do, its contraction stress you need to guard against. the saw cut doesn't actually stop this but puts any crack where you want it.
As said you are probably over the top there for a 150mm slab the general rules are length to breadth ratio ideally 1.5:1 and certainly no more than 2:1 and at those ratios a joint approximately every 30 x the depth so for 150mm approximately every 4.5 metres. and certainly no area over 25sqm
You can also cut the joint in wet and in hot temperatures this is essential as the slab will probably crack in less than 12hrs, in this weather anytime within 24 hrs will do.
Though to be fair if the surface is going to be covered stress cracks in a 150mm slab won't matter so long as the base is solid and well compacted.