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
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?
On Mon, 18 Jan 2010 20:00:01 -0000, David WE Roberts wrote:
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.
On Mon, 18 Jan 2010 16:39:15 -0000 David WE Roberts wrote :
A142 is 142mm2 of steel per m width. As you surmise, it won't add a
huge amount of strength to the slab. What it does do in slabs is to
distribute concentrated loads (e.g. wheel loads in car parks) and
may stop cracking.
If you do incorporate it, be sure to space it so that there is 50mm
concrete cover. If less, it may rust and that will cause problems.
Tony Bryer, Greentram: 'Software to build on' Melbourne, Australia
www.superbeam.co.uk www.superbeam.com www.greentram.com
You refer to it as a "raft" while you then talk about "footings".
As I understand it, a raft is there to spread the edge loads over the
If there is no reinforcement then there is no raft. The weight of the
walls on the outside edge will just crack the "footings" off the rest.
The purpose of the steel is to stop the concrete cracking under tension.
In this case the steel would have to be laid near the top rather than
the middle or the bottom of the concrete. It may also need to be turned
down round the edges.
I have been involved in two raft structures so know the principles
involved, but have no idea as to the maths involved.
From my reading so far there are 'raft foundations' which are for use on
very poor ground where as you say the point load of a wall or column is
spread over a large area and reinforcement is used to ensure that the load
is spread without trying to bend the raft like a banana.
Here the anticipated load is not large and the ground is reasonably good, so
'raft foundations' would not be called for.
I use the term 'raft' for a concrete floor which is deeper round the edge to
provide extra support directly under the walls.
My intention is to pour in one piece, instead of pouring footings below
ground, building up the base of the wall, then pouring a floor slab in the
centre (as is the general practice for larger/spankier/habitable buildings).
I an not expecting the raft to be needed to support the walls - it is
'footings and floor' all poured in one go as a simple solution to a
With respect, you are getting your ideas slightly confused.
Firstly, be careful about thinking of primary cast in place concrete as
a floor. In many cases its primary purpose is not to *be* a floor at all
and the fact that it is flat and level is a happy coincidence.
Secondly, almost any form of "foundation" you use (other than a simple
slab for a wooden shed) is going to be deeper round the edges than it is
in the centre, so using the term "raft" for this idea is inappropriate
as well as incorrect.
A raft is a foundation which is designed so that the loading of the
building is spread over a large area. The deeper edges (aka lip, apron
or keel) act in two ways. Firstly, they provide additional structural
strength where the main loading is (i.e. the external walls are built on
the edges), but secondly they provide a lip which stops the raft from
sliding over the underlying ground (sliding as in slipping, as in the
whole building moves without collapsing, possibly as a result of a minor
landslide or mining subsidence).
I am not a civil or structural engineer, but I would have thought that a
raft foundation is almost always going to be reinforced with steel mesh
sheets. It is also likely that there may be a requirement for rebar in
the apron as well, depending on whether the apron is contributing to the
structural integrity or not.
As far as I am aware, normally, if you had strip foundations and then
in-filled the enclosed area to make a "floor", you would make make the
infill out of weaker concrete than the foundations, so that any movement
in the infill would not affect the integrity of the foundations. So
effectively, if you do decide to cast foundations and infill at the same
time, then you will be building a de facto raft which will need to be
reinforced to ensure its own structural integrity.
Finally, as Tony Bryer said, you need 50mm cover all round for
reinforcing mesh to prevent corrosion of the steel.
I recommend you read this website:
particularly these pages:
Only experience I have is the damage to concrete at the bottom of a set of
stairs leading to a beer cellar. Kegs of beer seem to 'miss' the big rubber
mat & have hit the slabs at the bottom.
First attempt to make good was a concrete paving slab, destroyed on first
Second attempt was to use 'heavy duty' concrete mix. Lasted nearly 3 weeks.
Keg of Fosters destroyed it.
Final solution - same mix with 6mm ish rebar. Seems completely
indestructible after 4 months & numerous impacts.
I don't know what this proves exactly, but rebar seems to add an incredible
amount of strength to concrete.
Dave - The Medway Handyman
Concrete is *very* weak under tension.
Any concrete is going to be subject to tension forces must be
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 ...
Rebar turns concrete into something very resilient - and would have
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 :-)
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.
Managers, politicians and environmentalists: Nature's carbon buffer.
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
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
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
for a keeled raft slab seems a little over the top - a big leap forward from
a simple slab.
http://www.pavingexpert.com 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.
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.
it 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.
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