Why is Hardcore Placed Beneath a Raft Foundation?

Postby Tom Archer » Tue Feb 13, 2018 4:59 am

Greetings - I'm wondering whether someone can answer this question.

When trench foundations are concreted, the concrete is poured into the bare soil of the trench, but when raft floors are cast, all the diagrams show a layer of compacted hardcore underneath.

I'm looking at building a garden wine store, using a concrete raft base, thick Celcon block walls and a lot of insulation, to moderate the temperature.

The concrete I can get delivered via mixer and pump, but getting hardcore delivered is not so simple - it would be easier to use more concrete.

Is there an important reason why rafts are laid on hardcore? - or is it just because hardcore is usually cheaper?

The raft I have in mind will be 30cm thick and double reinforced to make the slab wholly self supporting. The site will be cleared of topsoil down to a level clay base and then shuttered.The top of the resulting slab will then be a few cm proud of the surrounding soil.
Tom Archer
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