thickness of concrete

[belu]

We are planning to extend our small terrace; the size of the new part, including two steps, will be 20 square metres.  Because the site is sloping our French builder will need to construct a supporting wall along the longest edge which will be rendered once finished. He estimates that we will need 18 cubic metres of cement to fill in the void.  The price for this cement alone has come in at over 1600 euros.  Can anyone tell us if there is a cheaper way of doing this (except hand mixing please!) - maybe it would be possible to use more hardcore (we think the depth of the concrete at its deepest point will be about 1 metre)?  Any suggestions would be much appreciated.

[Proxima]

WOW.. 1 mtr thick concrete floor.. I've never ever heard of anything that thick for a building such as yours.

Are you sure you want a concrete floor? That extra meter of space could be a small tool shed or something if you had a suspended wooden floor. Just a thought.

1 mtr of concrete will take years to dry out (but concrete does get stronger as it ages)

The average thickness is 6 inches and I've never heard of anything domestic being over 12 inches.

Has the outside wall been built already? You could reinforce the wall (as you do with reinforced concrete) then you could reinforce the concete floor too and tie that into the wall.

To me, that makes a bit more sense than a mtr of crete.

What's the wall being built out of? block and stone? stone alone? or that terracotta rubbish they tend to use on some houses? or just block work?

It's the block work that can be easily reinforced. If you use blocks that have an opening top and bottom, you bed some upright steel rods into the foundations then build upwith the rods inside the block.. Every third row you put in horizontal steel bars and tie them onto the uprights (welded joints are best) but you must also fill up all the cavities in the blocks with crete as you go up. A reinforced wall.

A vast majortiy of all manner of buildings are completely built this way in parts of Asia to withstand earthquakes and typhoons.

More hardcore but tie in a reinfroced crete floor to the wall is my opiniion.

Go have a chat with the local builders merchants or ask another builder what they might do. Even the Mairie might have an opinion in the matter.

Sounds odd to me..will be interesting to see what others say here..or any further info you get elsewhere.

[belu]

Proxima

Thank you so much for this helpful email.  I must admit we did think that 1 metre of concrete was exceptionally thick therefore we will do as you suggest and speak to another builder and/or builders merchant.  The wall hasn't been constructed yet but will be made out of block, probably the open type so we could reinforce them as you suggest.  Am I right in thinking therefore that we could fill the void with hardcore (reinforcing with the steel rods mentioned) and then just put a layer of cement of between six and twelve inches on top?  If so, this would reduce the amount of cement required to between 4 and 6 cubic metres - quite a cost saving.

We can't use the underneath as a store as its right at the front of the property and it will need to have the same wall finish as the house itself.  Good idea though!

 

[powerdesal]

Belu, If I read the job correctly you are having an area of (say) 4m x 5m, which presently slopes down away from your house. The retaining wall will be at the lower part of the slope and will be, as suggested, a reinforced block wall. It has to be reinforced to take the pressure.The wall should also IMHO have drainage pipes through it at the bottom edge.The resulting void will be wedge shaped in section.

May I suggest that your builder puts in a wall along both sides between the existing and the reaining wall, this will obviously be 'tapered' and will create the void. As you correctly say, this void should be backfilled with rammed hardcore leaving a thickness of 200mm. The concrete slab would then be a uniform 200mm slab. (ps, its a concrete slab not a 'cement' slab - cement is part of the mixture). You may want to put a re-inforceing mesh horizontally in the concrete slab.

Backfilling a void of the total size with 'mass' concrete is IMHO totally unnecessary, unrealistic and, quite frankly, a silly idea. In my opinion your builder is taking the p***. Its a terrace extension not a bomb shelter [8-)]

The 200mm slab will thus need 4 m cu of premix.

[Pierre ZFP]

I think some builders just love to use lots and lots of concrete.  My Bro' in Law is one of them.  He built a patio base for me, about 12 sq yards and on completely flat ground.  I wanted a hole filled with around 4 inches hardcore (of which I had plenty) and max 6 inches of 'crete with a steel mesh in it. 

What I got was a hole with 6 inches or more of hardcore with nearly a foot of 'crete (so it cost nearly double) When asked why, BiL said 'Well I didn't want it to crack'

I'm going to sit on it and drink G&T for heaven's sake, not park a Chieftan tank on it.

BTW, the sizes above are in Imperial coz  BiL  'Don't do metric'  which is odd coz I 'Don't do Imperial' anymore so one day we're going to come a cropper over sizes.