Suspended Concrete Slab - No Hardcore!?

[Newman21]

Morning

We have just commenced the build of an orangery off the back of our property.

The plan and quote was to have a 7m x 4m beam and block base with an air gap of around 150mm, so a fair bit of excavation. However when the builder turned up he suggested a slab as it worked better with the existing levels and to be honest didn’t make much difference to mead long as it was done properly.

The trenches have been dug and inspected by BC, however when he visited he told the builder that the slab would need to be reinforced with a steel mesh that sits on the inner skin and is knitted into the existing house footings - creating a suspended floor.

The builder is now not going to excavate any soil and is simply going to pour onto the existing crumbled up old patio. Does this reinforcement do away with any need for hardcore and blinding?

Seems very odd leaving everything in place and just building up from a knackered patio. This seems like a very cheap option given hardly any materials are needed compared to B&B - would anyone expect this saving to be passed on?

Any help would be greatly appreciated.

Thanks

 

[^woody^]

The key word is "suspended".

Your builder will need to replace hardcore with ventilation.

 

[Notch7]

he told the builder that the slab would need to be reinforced with a steel mesh that sits on the inner skin and is knitted into the existing house footings - creating a suspended floor

can a slab be suspended?
how does wet concrete stay up in the air?

Im not saying its wrong, just curious to know

 

[Newman21]

Thanks Woody. I appreciate the key word is suspended - but a 7m span with 100mm of concrete, with a steel mesh within, just doesn't sound very substantial.

How would you create ventilation - is this with physical ducts, as surely the concrete would pour through and fill any available voids?

 

[^woody^]

can a slab be suspended?
how does wet concrete stay up in the air?

Im not saying its wrong, just curious to know

Mixing for long enough puts lots of bubbles in it.

 

[^woody^]

Thanks Woody. I appreciate the key word is suspended - but a 7m span with 100mm of concrete, with a steel mesh within, just doesn't sound very substantial.

How would you create ventilation - is this with physical ducts, as surely the concrete would pour through and fill any available voids?

Yes, sorry, 7m is long span and would need to be looked at by a suitable person and designed. But if it spans the 4m width that should be better, but even so 100mm needs to be confirmed and the mesh properly specified and placed and not just dropped in.

 

[blup]

I believe broken bricks and concrete can be used as hardcore. The builder should be able to judge if the materials are appropriate and of the right size, then there is the issue of proper compaction and blinding.

The project is your responsibility, not the builders, re the building regs, so perhaps worth confirming with BC on this and ventilation before proceeding. It is in your interest and the builders to be working to a design.

Blup

 

[geraldthehamster]

If it's being poured on top of a broken patio, it's not exactly suspended, is it? It will end up with a few voids is all.

 

[WLoverbd2]

To

Mixing for long enough puts lots of bubbles in it.

Lol.

@Notch - In response to your query about concrete being supported. To produce a suspended concrete floor, you can use a cellcore product (cordek) or a collapsible cardboard form. They are usually used to provide heave protection on suspended ground slab.