Reinforcing in concrete slab raft?

[David WE Roberts]

Workshop question again:

As usual, every time I think I have something sorted someone else throws up questions.

I am (weather permitting) soon to be laying a concrete raft 7.8m * 3.5m

100mm deep with 200mm depth around the edge to form footings for a single block single storey wall.

I was going to lay it just as a concrete slab, as the interior will not have much weight on it (unlike a garage floor).

One of the Builders Merchants suggested that a slab that big should really have some reinforcement.

O.K. - 4 sheets of reinforcing mesh @ £16 a pop (+VAT) isn't a massive extra cost but is it necessary?

The sheet in stock (Travis Perkins and Jewsons) is "Steel Fabric A142M 3.6m

• 2.0m".

The steel used looks pretty puny - not like the reinforcing steel we used when I was a lad when the wire was as thick as a pencil. Will a single layer of this stuff make a massive difference to the structural strength of a raft?

Is 100mm thick enough (judging by the small floor from a previous shed I had to break up, this thickness can be pretty tough).

TIA

Dave R

[Harry Bloomfield]

David WE Roberts was thinking very hard :

I would suggest 4" might be a bit thin, for that area, but it depends on the sub soil. Yes rebar will make a lot of difference, but I would suggest not that much if only 2mm thick. I would suggest 3/8".

[Archie]

Another question. Have you considered putting in insulation under the raft to help keep your feet warm?

[David WE Roberts]

Not for an unheated workshop/store at the bottom of the garden. If I decide to have a 'person friendly' area later I can always add insulation above the floor.

[TMC]

Would the panel recommend a dpm?

regards

Tony

[Tim Watts]

On Mon, 18 Jan 2010 17:11:57 +0000, "TMC" wibbled:

Yes (we me anyway).

Pros: the floor stays dry reducing the rust potential to your stored items.

Tiny Cons: The price of a bit of heavy guage plastic DPM and a bit of (10mm) blinding sand over the ground or hardcore to prevent the DPM getting ripped to bits while you pour the concrete.

Big cons: None.

My inclination would be to size the pad such that the pad is a little higher than the ground and that the shed wall cladding can be hung over the side slightly - this will prevent ground runoff rainwater from getting into the shed. At least don't make the pad vastly too big and flat at the same time.

[David WE Roberts]

Sorry - full story to be found in previous posts.

Plan is to put down 100mm MOT, blinding sand if required and a DPM before laying the concrete raft. Also intended to match the concrete block wall to the edge of the raft to hopefully avoid a lip which chucks water under the wall. No external cladding planned - ATM it is just dense concrete block which will be pointed and painted. DPM layer in the block wall to reduce rising damp. Did consider rendering but again ATM can't be arsed. Sloping metal roof (will be insulated between rafters) Inside of walls lined at some point with insulation and board (I have a saved previous post from Andy Dingley about dry lining a shed). I have a wood burning stove and may install it at some point for occasional heating.

Designed to be under 30sqm internal floor area and of mainly inflammable materials to avoid involvement with planning and building regs.

I had to suspend work just before Christmas when the weather went bad.

[Tim Watts]

On Mon, 18 Jan 2010 17:44:22 +0000, "David WE Roberts" wibbled:

Sorry - didn't check.

Sounds excellent :) Good luck.

Cool. Piccies please (even if it's not all done!) :)

[David WE Roberts]

You into mud, then? That's all there is at the moment. I will try and record progress as I go along and post the pictures. :-)

[Steve Walker]

And just a general question while everyone is talking about slabs - the usual construction for conservatories appears to be MOT, sand, DPM, insulation, concrete, screed. Now when they widened the motorway around here, they actually built it on polystyrene blocks to spread the load and I have since read that it can be used under slabs, straight on the ground, with no MOT or sand, so why is this construction never mentioned?

SteveW

[js.b1]

- DPM

- Services pipe (hockey stick is ok)

- Design so any future cladding overhangs to avoids splashback/runback

- Price out glass fibre flakes v rebar, any rebar is better than none

- Insulation on the inside re ceiling, walls & floor

When you put insulation on the outside (above the ceiling, outside the walls under cladding, below the concrete raft) you are creating a thermal mass. That works fine IF you have sufficient kW inside continually to actually heat that thermal mass up (which by tonnage of blocks takes quite some time). When you put insulation on the inside of floors, ceiling & walls you have minimal thermal mass (open the door and the heat is gone), but it takes seconds to get warm and re-heat so warm again. For any occasionally heated room that is ideal.

A dehumidifier might actually give enough background heating and keep things dry. X-Dry are a silica gel based device which will outperform conventional units at low temperatures and provides heating. Stick on an energy meter for a year if you want to balance dryness v heating.

[David WE Roberts]

Do you have any references for this? Obviously of interest.

[js.b1]

Spread the load... I wonder if that is to change the impact profile from heavy goods. Essentially change the short duration high peak impulse into a long duration low peak impulse?

[Tony Bryer]

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.

[Les Desser]

In article , David WE Roberts Mon, 18 Jan 2010 16:39:15 writes

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.

[The Medway Handyman]

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.

[The Natural Philosopher]

Anothther thing worth doing - remember if its clay, it will eventually dry out a bit under the floor and subside -- is to put plenty of hardcore and possibly high density polystyrene down as insulation under it, that will take a bit of movement.

They don't call em concrete rafts for nowt, you know.

The aim is to have a solid base which may settle a bit, but wont twist or crack.

[The Natural Philosopher]

yes.

[David WE Roberts]

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

[Dave Osborne]

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.

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