Reinforcing in concrete slab raft?



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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@privacy.net wrote:

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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
SteveW
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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 whole area.
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.
--
Les Desser
(The Reply-to address IS valid)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mon, 18 Jan 2010 16:39:15 writes

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 shed/workshop base.
Cheers
Dave R
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
David WE Roberts wrote:

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:
http://www.pavingexpert.com
particularly these pages:
http://www.pavingexpert.com/concrete.htm
and
http://www.pavingexpert.com/reinfrc1.htm
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
David WE Roberts wrote:

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 subsequent impact.
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
www.medwayhandyman.co.uk
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 ...
--
Les Desser
(The Reply-to address IS valid)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 :-)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

<snip>
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
http://www.pavingexpert.com/images/concrete/xs150gb.gif
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.
Cheers
Dave R
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
Les Desser
(The Reply-to address IS valid)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Les Desser wrote:

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...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave Osborne wrote:

Not if you put a DPM down..that stops water coming up..
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
Les Desser
(The Reply-to address IS valid)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
David WE Roberts wrote:

Reading:
http://www.pavingexpert.com/reinfrc1.htm
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.

yup.
Sounds ok.
--
Cheers,

John.

/=================================================================\
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.