Reinforcing in concrete slab raft?

On Mon, 18 Jan 2010 16:39:15 +0000, "David WE Roberts" wibbled:

Might be worth standing back a bit now. Lots of replies about major structural rafts etc.

How heavy is this all likely to be? Will the wall blocks be lightweight (eg celcon) or concrete? What's the roof - timber+felt or tiled or metal?

At one extreme, if it were a wooden "shed", you could quite frankly stand it on paving slabs laid on a bit of sand onto well compacted earth and it's unlikely to go anywhere. And if it did, the timber wouldn't care that much within limits.

At the other extreme, if there is a lot of load on the walls and the ground is crap and liable to move or wash away, you could go for a trench of concrete for the wall base and a separate floating floor slab.

It sounds like looking at conservatory build guides might be useful, and cut the floor slab down a notch or two. The size and load of the walls are likely to be in a similar range and it is the walls you want to ensure integrity. The floor cracking a bit over time will not be as serious if you under engineer it, compared to walls going.

Reply to
Tim Watts
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Mostly answered already near the top of the thread. Heavy concrete block - for painting. Not the rice crispies version for rendering. Unless this makes a major difference to the requirements for the base. Metal roof, on rafters, insulation between rafters. The ground is good - nice and flat and not clay.

My build seems to be in something of a gap between different standard solutions.

It is not a wooden shed on a simple flat concrete base.

It is closer to a garage, but doesn't need the extra strength in the floor to support a heavy vehicle moving in and out on a regular basis.

The design shown at

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for a keeled raft slab seems a little over the top - a big leap forward from a simple slab.

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does not give me a definitive answer. It shows a simple slab for a prefabricated concrete garage (less strength than I am aiming for) or the picture above for a brick built garage (more strength than I am aiming for).

My current feeling is that it might be sensible to add a layer of reinforcing sheet because the extra cost is not massive. It would sit in the middle of the 100mm slab - 50mm above and below. All I need to locate now is a supply of 50mm spacers. I don't feel inclined to add the T20 bars as well.

I am going to ring Building Control for advice - although I will not need approval the regulations are usually sensible.

Cheers

Dave R

Reply to
David WE Roberts

Reading:

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says: "A142 mesh is often used in 100mm thick light-use slabs such as paths, driveways and garage bases"

So that seems to be ideal for this application.

yup.

Sounds ok.

Reply to
John Rumm

I've checked into it and according to Vencel's (Jablite's) website: "Heavy structures, such as road embankments, can impose unacceptably high loading on the underlying soils. Lightweight Fillmaster blocks reduce the pressure on soils."

I was already pretty sure this was the case, as I seemed to remember that when the M60 was widened, the local papers referred to the construction method being one that is regularly used in Norway for constructing roads over poor ground.

SteveW

Reply to
Steve Walker

I can't find the original reference (I will try again), but I have found reference on Vencel's (Jablite's) website of using it for "hardcore replacement," where they show it still being placed on sand and a DPM, but without any other preparation.

I'm actually looking at options for a conservatory base at the moment as it happens and I've been considering whether to dig out properly or to use a DPM and Jablite (possibly with sand) directly over the existing patio - it's been there for decades and isn't really expected to move now. Two of the three dwarf walls would be positioned just off the existing flags and the third just means lifting a row to dig its foundation. I'll probably do it properly, though, even if it means digging out the area to avoid raising it too high.

SteveW

Reply to
Steve Walker

It is reputed that the local railway viaduct (constructed 1849-51 and still in use) has woolen bales as foundations, presumably for similar reasons (clay soil).

Reply to
<me9

In article , snipped-for-privacy@privacy.net scribeth thus

Wasn't the railway line across chat moss near Manchester done the same sort of way?...

Reply to
tony sayer

Leasowe Lighthouse in/on the Wirral is supposedly built on bales of cotton, based on the same principle - shame a ship had to founder and sink to give the foundations mind you... No details on a quick Google but it isn't the sort of thing you can mis-remember from school is it?

Reply to
Neil

In article , The Medway Handyman Tue, 19 Jan 2010 00:26:01 writes

Concrete is *very* weak under tension.

Any concrete is going to be subject to tension forces must be reinforced.

When a heavy weight hits the concrete with enough force to deform it, the opposite face is subject to tension and will very easily crack.

Do it a few more times and ...

Reply to
Les Desser

In article , David WE Roberts Tue, 19 Jan 2010 16:55:54 writes

From my limited experience in these matters I would say that is the worst place to put it. It will add very little to the strength of the slap, other than to keep it together in the event of it cracking.

The reinforcement has to go in the part of the concrete that is subject to expansion forces. So on a slap that is supporting walls, the steel has to go in the upper half.

Reply to
Les Desser

Except that conventional wisdom says that you need 50mm cover all round to prevent the steel corroding. So the only place to put it in a 100mm slab is in the middle...

Reply to
Dave Osborne

Rebar turns concrete into something very resilient - and would have helped Haiti.

House built on a hillside of clay & numerous springs, concrete raft with extensive rebar tubes in the keel & mesh across. The land behind rises appreciably, so there is a degree of frost-heave at play. In the worst 1980s winter the frost heave would lift 3x2 paving like an earthquake fault pattern, jamming a garage door which normally had

0.75in clearance. In the recent winter the second night we hit -13oC the frost heave actually creased the L-angle above the garage door (the door is 250lbs T&G wood) and tore upwards a heavyweight bolt striker like foil. 2009 had saturated the ground quite effectively.

On both occasions (1984? 2009) around 1am the big 11m steel beam which spans the rear rang like a bell and simultaneously the ground floor juddered. I suspect frost heave causes it to shunt slightly, a neighbours garage floor which was just poured without rebar cracked & shifted from frost heave this winter leaving a raised section. The strip foundations did have rebar and were unaffected (indeed probably interacted with the floor.

Hit a thin paving slab, the reverse is in tension and thus breaks. Rebar makes all the difference.

If you are doing a conservatory, worth designing in a heat pump (B&Q Airforce to Ebay) - you can use it to cool in summer, but also heat in winter very effectively. Ideal for when the conservatory becomes a utility room :-)

Reply to
js.b1

Not if you put a DPM down..that stops water coming up..

Reply to
The Natural Philosopher

In article , Dave Osborne Thu, 21 Jan 2010 00:20:58 writes

Sorry - correction: read "tension" in place of "expansion" above.

  1. Then it has to be made thicker - if the steel has to go in the bottom half
  2. In this case I think it needs to go into the top half where there should be no water penetration.
Reply to
Les Desser

tony sayer has brought this to us :

Yes, I remember something of the sort.

Reply to
Harry Bloomfield

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember tony sayer saying something like:

It's been done in many places - long before the railway came, too.

Reply to
Grimly Curmudgeon

The message from Grimly Curmudgeon contains these words:

The idea of the woollen bales was to let the embankment consolidate and knit together. The wool would eventually rot, but by that time the embankment would be knitted together enough to distrbute the load.

Bishop weed and similar plants were planted on embankments to start the binding process.

Reply to
Appin

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