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Rebar in concrete slab.
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kirstyfern



Joined: 03 Jan 2010
Posts: 1574
Location: Great Dunmow, Essex
PostPosted: Thu Jun 30, 11 8:46 pm    Post subject:  Reply with quote    

Acorns wrote:
If it's going to be driven on us the square mess, fill the area with concrete then drop the mess in and walk on it to make it sink.
I was a ground worker for many years and thats how we did it.



That sounds very dangerous!

We raised our mesh up 4" on stones in a few places and then put in 8" concrete making sure it went under and over the mesh.

dpack



Joined: 02 Jul 2005
Posts: 34285
Location: yes
PostPosted: Fri Jul 01, 11 10:13 am    Post subject: Reply with quote    

mesh and a few bars as pegs vertically

use a good concrete mix aggregate

granite 14 inch to dust with 20 % 3/4 inch cracked pebbles will give a super strong concrete but for a base slab nornal sharp sand /grits / pebbles should be ok

concrete snagger from feltam ordinance factory , some of that was very special concrete

Nick



Joined: 02 Nov 2004
Posts: 33857
Location: Hereford
PostPosted: Fri Jul 01, 11 10:20 am    Post subject: Reply with quote    

I've got some mesh in, it's 125mm deep, and about 1800mm square. Using 1:2:4 mix, and it's going to support a pizza oven, which is turning out quite heavy. More steel in the hearth slab, too.

dpack



Joined: 02 Jul 2005
Posts: 34285
Location: yes
PostPosted: Fri Jul 01, 11 10:35 am    Post subject: Reply with quote    

125 should be fine

keep the concrete damp to harden off for a while before topping it with the oven ,full strength takes decades but a week or so should be fine for a base slab .watching the corners crack off under the mass of oven would be sad .let the whole thing set for as long as poss before firing up

some extra lime in the oven mix will make it less likely to crack with the heat

Nick



Joined: 02 Nov 2004
Posts: 33857
Location: Hereford
PostPosted: Fri Jul 01, 11 10:52 am    Post subject: Reply with quote    

The oven I will come back to as that's a specialist job, for sure. Next step, after the base, is a three/four high U shape of block work (9 inch cavity blocks), with another slab on top, then an insulating layer, followed by the oven floor. How soon can I put the blocks on the slab?

dpack



Joined: 02 Jul 2005
Posts: 34285
Location: yes
PostPosted: Fri Jul 01, 11 11:22 am    Post subject: Reply with quote    

a week of warm and damp will take it up to 80% or so of final strength which will be hard enogh to work on and to hold the mass of blockwork

do any planishing after 24 hrs to give a nice even finish to bits that will show ,a foot of floorboard with a block handle will make an adequate tool to rub the surface smooth

some metal poking up into the blockwork will help tie it all together

keep the blockwork damp as you go along and try to let the mortar for the blocks set off enough before adding more layers ,

when blocklaying a stiff mortar holds the block against gravity better than a more runny bricklaying mix .make sure the blocks are well wetted and take your time with bubble ,string and rubber mallet to get each block set perfectly .if layer one is perfect it is easy to build up

if the oven is to be very heavey it may be worth filling the hollow blocks with concrete as you build up for extra commpressive strength

at a guess the oven will be heavey

Nick



Joined: 02 Nov 2004
Posts: 33857
Location: Hereford
PostPosted: Fri Jul 01, 11 11:36 am    Post subject: Reply with quote    

The oven will be heavy. Firebricks are not light.

I have around 70, and they must be 2-4kg each.

Then, allow for mortar, hearth, cooking floor, chimney and so on....

dpack



Joined: 02 Jul 2005
Posts: 34285
Location: yes
PostPosted: Fri Jul 01, 11 11:51 am    Post subject: Reply with quote    

fill the hollows might be sensible , make the hearth slab strong but not tied to the support walls with metal just resting on a smooth concrete wall top

thermal expansion of the slab could pop the walls if they were fastened together i recon it should hold together best if the three main elements are able to expand without cracking each other

does that make sense ?

Nick



Joined: 02 Nov 2004
Posts: 33857
Location: Hereford
PostPosted: Fri Jul 01, 11 11:53 am    Post subject: Reply with quote    

Yep, perfectly. I've a couple of support joists I was going to run across the open sides of the U, coupled with ply shuttering to pour the hearth on, and the blocks filled with more concrete. Probably over engineered, but, well...

dpack



Joined: 02 Jul 2005
Posts: 34285
Location: yes
PostPosted: Fri Jul 01, 11 12:10 pm    Post subject: Reply with quote    

a ton or so of oven a metre or so up need overengineering

it isnt just the thermal expansions and mass but it needs to last a long time in the weather

wilddcw



Joined: 26 Dec 2011
Posts: 1
Location: Torremanzanas, Spain
PostPosted: Mon Dec 26, 11 7:10 pm    Post subject: Reply with quote    

just out of interest - I've a neighbour who built the foundations for his (illegal) house using old metal bedsteads instead of re-bar. It has been there for 5 years and no cracks! I've done the same for sheds etc.

Vanessa



Joined: 08 May 2006
Posts: 8324

PostPosted: Tue Dec 27, 11 10:05 am    Post subject: Reply with quote    

Hahaha, bits of our house in France are built with poured concrete walls. Yes, honestly!! And if neighbours are to be believed, there's no rebar, just random bits of metal - empty oil cans, scraps of metal, wire, a bicycle frame

It's been standing for over 50 years like that, and no cracks!!

crofter



Joined: 11 Feb 2007
Posts: 2252

PostPosted: Tue Dec 27, 11 11:24 pm    Post subject: Reply with quote    

Vanessa wrote:
poured concrete walls


Known as "run concrete" here, very common building technique until relatively recently. Minimal reinforcing used, and whatever is to hand would suffice. Some houses have massive cracks where incorporated steel has corroded and burst the concrete apart - railway track lintels are a bad idea!

Run concrete outbuilding with porthole:


alice



Joined: 18 Feb 2006
Posts: 2820

PostPosted: Wed Dec 28, 11 8:38 am    Post subject: Reply with quote    

crofter wrote:
Vanessa wrote:
poured concrete walls


Known as "run concrete" here, very common building technique until relatively recently. Minimal reinforcing used, and whatever is to hand would suffice. Some houses have massive cracks where incorporated steel has corroded and burst the concrete apart - railway track lintels are a bad idea!



Living so remotely, I guess folks did the best they could with the materials avaialble. The beams in our roof were whole tree trunks with the bark still on.

Vanessa



Joined: 08 May 2006
Posts: 8324

PostPosted: Wed Dec 28, 11 9:09 am    Post subject: Reply with quote    

Ooooh, I feel so much happier about it now!

One of our outbuildings is of what can best be described as "non-standard construction". A mix of red blocks (the ones with lots of large holes through), concrete blocks and poured concrete, plus concrete EDF pylons and metal EDF pylons cut to length. The roof structure is a mix of concrete EDF pylons, telegraph poles and random bits of wood.

One estate agent looked at it and said "nowadays, this would be considered very eco-friendly, re-using everything, wouldn't it?"!!!

Again, it's been standing for over 40 years, so can't be "badly" built, for all the shoddy appearances!

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