Rebar placement

[gavztheouch]

Hi I have 4 layers of rebar in my foundation. Two on bottom with 40 mm cover and two on top with 40mm cover. I searched the internet but cannot find any information as to wether the sheets that sit on top pf

each other need to be staggered so the longitudinal and lateral bar are not sitting g directly next to or on top of each other. The reason in my mind is the concrete cannot flow right round the bars if they are right next to each other. The downside of offsetting the sheets is it will make it harder to get both sheets up to the edge of the foundation with the same cover of 40mm.
 

 

 

[markc]

Reinforcing is tied together so it acts as a continuous piece, overlap and wire together.

[twice round the block]

As long as a vibrating poker is used while the pour is going on you shouldn't get any voids.

[Jenki]

Only issues I've run into, and it depends on the mesh size, is that when you get 4 layers of mesh getting a poker through all the mesh layers can be sometimes an issue.  Also 40mm cover on top needs lots of support for the mesh or you may find its a lot closer to the top due to the mesh bending and doing what mesh does.

[gavztheouch]

1 hour ago, Jenki said:

Only issues I've run into, and it depends on the mesh size, is that when you get 4 layers of mesh getting a poker through all the mesh layers can be sometimes an issue.  Also 40mm cover on top needs lots of support for the mesh or you may find its a lot closer to the top due to the mesh bending and doing what mesh does.

Yeah I’m worried about the mesh creeping up a bit due to bent sections not nesting nice or the isoquick not being exactly 250mm in places. It’s all the compounding errors that made me decide to put the heating pipes under the top two layers of mesh.

[saveasteading]

The double layer would seem to be for ease of handling rather than a single layer of heavier mesh. Tying them is best. It seems vastly overdesigned just in case there is a soft spot beneath so don't worry. Just try to stagger it, by halving a sheet or rotating some,   to reduce multiple overlaps of overlaps.

 

Can anyone explain why a slab sitting on eps needs to be so heavily reinforced? I'm thinking perhaps a supercautious standard detail.

Indeed, why do nearly all SE designs show top and bottom mesh for domestic loading?

[Tom]

3 hours ago, saveasteading said:

why do nearly all SE designs show top and bottom mesh for domestic loading?

 

Tell me about it @saveasteading, we had FOUR layers of A393 mesh in our slab

 

4 hours ago, gavztheouch said:

Yeah I’m worried about the mesh creeping up a bit due to bent sections not nesting nice or the isoquick not being exactly 250mm in places. It’s all the compounding errors that made me decide to put the heating pipes under the top two layers of mesh.

 

Are you using mesh panels with flying ends? Makes lapping them simple so you don't get a build up.

[gavztheouch]

4 minutes ago, Tom said:

 

Tell me about it @saveasteading, we had FOUR layers of A393 mesh in our slab

 

 

Are you using mesh panels with flying ends? Makes lapping them simple so you don't get a build up.

Yes I have flying ends Tom, do you have any pics of your rebar mesh going in?

[Tom]

9 minutes ago, gavztheouch said:

Yes I have flying ends Tom, do you have any pics of your rebar mesh going in?

 

 

This is the first two layers in, UFH pipes on top of that, then you might just be able to make out the deck chairs we then used to sit the remaining layers on. There were big sheets, which were an absolute nightmare to handle, but we just about got in to the swing of it by the end!

[gavztheouch]

12 minutes ago, Tom said:

 

 

This is the first two layers in, UFH pipes on top of that, then you might just be able to make out the deck chairs we then used to sit the remaining layers on. There were big sheets, which were an absolute nightmare to handle, but we just about got in to the swing of it by the end!

rebar mesh is horrible stuff to work with.

 

Did you tie your UFH to the lower layers or did you raise them up and attach to the underside of the upper layers after? 

[Tom]

16 minutes ago, gavztheouch said:

Did you tie your UFH to the lower layers or did you raise them up and attach to the underside of the upper layers after? 

 

Zip-tyed to the lower layers, all 1km of it, what fun. Not sure tying to the underside of the upper layers would have been possible anyway TBH.

[gavztheouch]

@Tom How thick is your slab? Do have any concerns by attaching the UFH to the lower mesh. If the concrete slab is nice and conductive to the heat I guess most of it will move up the way. 
 

thanks

Edited May 14 by gavztheouch

[Tom]

200mm thick slab, but the heat aint going anywhere but up, there's 400mm of insulation beneath it.

[ETC]

What on earth is your foundation supporting?

4 rows of mesh…..😳😳😳

 

Whoever specified the mesh should be able to tell you exactly where it should be located.

[Tom]

37 minutes ago, ETC said:

What on earth is your foundation supporting?

The walls and the slab - the slab is suspended

38 minutes ago, ETC said:

Whoever specified the mesh should be able to tell you exactly where it should be located.

They did 

[Gus Potter]

3 hours ago, ETC said:

Whoever specified the mesh should be able to tell you exactly where it should be located.

You've nailed it. Sometimes I make note on my drawings to say why I want a something to be the way it is. I made one today.. "we need this small timber strut to stop a big steel beam from twisting" it's £1.00 bit of wood but is essential to make the design economic.

 

Now for the purist I'm deviating from the BS method of detailing. But after 40 years in this game I feel.. why not just put a few extra notes on the drawings to help folk that may not be aware of the pitfalls? In the last few years I've just started writing this stuff on my drawings in plain English rather than being a smart arse and making folk jump though hoops. Contractors, Architect's et al and have a hard enough job as it is.

 

Why do I do this plain English thing.. short story. I had a dispute with the NHBC (not the first or last time) .. big claim several hundred thousand which I won on behalf of the Client. NHBC sent Professor Barry Hasseltine up to Scotland to investigate. Prof Hasseletine wrote a lot of the books that we as SE's use and also had a hand in the design codes we use. The Prof and I agreed and the claim was paid.

 

His report astounded me.. a teenager could have understood what he had written.. his English was very basic but very elegant.. he told a captivating story... evidence based... but at the end he included his qualifications and experience just in case a dafty though his plain English indicated a lack comprehension of the problem.. wow! What a skill and craftmanship in report writing he demonstrated.. that is one of the main reasons I try and write the way I do on this forum... keep it simple and use plain English.

 

Raft slabs, ground bearing slabs,  how we insulate them and move more towards making the Passive slab affordable are the things of the future. I got into this about 15 years ago.. it's exciting and I'm still learning.. I've got a grasp of the SE design but still trying to find a way to make it affordable for common constuction. I've got a few local builders getting into it now so the wagon seems to be rolling.

9 hours ago, saveasteading said:

Can anyone explain why a slab sitting on eps needs to be so heavily reinforced? I'm thinking perhaps a supercautious standard detail.

Indeed, why do nearly all SE designs show top and bottom mesh for domestic loading?

I can tell you when and where you need to reinforce slabs on EPS, but I've spent 15 years learning how to do this; firstly in a commercial environment, cold stores and in the very competetive cold formed steel industrial shed market (take this as 700 buildings a year all over the UK!)  and much of this happened long before the self build folk caught onto it.

 

Yes.. lots of SE's take fright when faced with this kind of thing.. but many Clients want to have their cake and eat it.. in terms of long term service and risk. SE's are an easy target as lots of folk know we carry high levels of PI insurance.

 

The easy way to explain this.. we as SE's that carry the PI insurance (the important bit) need to make things safe so they won't fall down and kill folk. If things start to bend a bit and your windows, doors, roof, basement starts leaking after say ten years then so long as you the Client accept this (serviceability) risk then we can progress. A big changed happened with the adoption of the Eurocodes which let us "negotiate" the serviceability so long as it does not compromise safety. Clever eh? but for the less "open" type of designer. a way to win your design brief!

 

But for the average self build on BH that needs lending it doesn't fly as you can't negotiate the serviceabilty. The SE needs to make the design compatible with the mortgage lender criteria.

 

@saveasteadingThat is why you may be correct on over designed slabs etc but it just does not fly for the average member on BH.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[saveasteading]

8 hours ago, Gus Potter said:

The SE needs to make the design compatible with the mortgage lender criteria.

Interesting. 

Is the underlying issue that the SE is not normally considering the whole design, but being given bits to do within someone else's concept?

 

And often chosen by cheapest fee rather than to be more involved and  encouraged to optimise?

[saveasteading]

8 hours ago, Gus Potter said:

his English was very basic but very elegant.. 

Wonderful. 

If we can understand it then we know he does too.

 

When an explanation is complex I start to wonder if it is bluff and fluff.

 

What a contrast to the excessive wordiness we come across in so many submissions, especially in planning submissions.

 

 

It brings to mind a tender submission I once did for an office block, d and b.

At the interview they criticised only providing a 32 page document while other tenderers had hundreds. I apologised but suggested that all the necessary information was there....and so it proved.

Got the job. I saw the competitors' documents and they each must have cost many tens of £k.

 

8 hours ago, Gus Potter said:

need this small timber strut to stop a big steel beam from twisting" 

 

I understand that German, and some other, codes do not allow timber as a support for steel. Eg a masonry wall can provide stiffness but a timber stud wall cannot. Ditto timber joists on steel beams, where steel cross bracing is still required.  Overdone? Yes but the logic is of shrinking timber.

 

Extra notes explaining the purpose of a component is an excellent habit. It's more likely to be done, and done well.

[gavztheouch]

17 hours ago, Gus Potter said:

You've nailed it. Sometimes I make note on my drawings to say why I want a something to be the way it is. I made one today.. "we need this small timber strut to stop a big steel beam from twisting" it's £1.00 bit of wood but is essential to make the design economic.

 

Now for the purist I'm deviating from the BS method of detailing. But after 40 years in this game I feel.. why not just put a few extra notes on the drawings to help folk that may not be aware of the pitfalls? In the last few years I've just started writing this stuff on my drawings in plain English rather than being a smart arse and making folk jump though hoops. Contractors, Architect's et al and have a hard enough job as it is.

 

Why do I do this plain English thing.. short story. I had a dispute with the NHBC (not the first or last time) .. big claim several hundred thousand which I won on behalf of the Client. NHBC sent Professor Barry Hasseltine up to Scotland to investigate. Prof Hasseletine wrote a lot of the books that we as SE's use and also had a hand in the design codes we use. The Prof and I agreed and the claim was paid.

 

His report astounded me.. a teenager could have understood what he had written.. his English was very basic but very elegant.. he told a captivating story... evidence based... but at the end he included his qualifications and experience just in case a dafty though his plain English indicated a lack comprehension of the problem.. wow! What a skill and craftmanship in report writing he demonstrated.. that is one of the main reasons I try and write the way I do on this forum... keep it simple and use plain English.

 

Raft slabs, ground bearing slabs,  how we insulate them and move more towards making the Passive slab affordable are the things of the future. I got into this about 15 years ago.. it's exciting and I'm still learning.. I've got a grasp of the SE design but still trying to find a way to make it affordable for common constuction. I've got a few local builders getting into it now so the wagon seems to be rolling.

I can tell you when and where you need to reinforce slabs on EPS, but I've spent 15 years learning how to do this; firstly in a commercial environment, cold stores and in the very competetive cold formed steel industrial shed market (take this as 700 buildings a year all over the UK!)  and much of this happened long before the self build folk caught onto it.

 

Yes.. lots of SE's take fright when faced with this kind of thing.. but many Clients want to have their cake and eat it.. in terms of long term service and risk. SE's are an easy target as lots of folk know we carry high levels of PI insurance.

 

The easy way to explain this.. we as SE's that carry the PI insurance (the important bit) need to make things safe so they won't fall down and kill folk. If things start to bend a bit and your windows, doors, roof, basement starts leaking after say ten years then so long as you the Client accept this (serviceability) risk then we can progress. A big changed happened with the adoption of the Eurocodes which let us "negotiate" the serviceability so long as it does not compromise safety. Clever eh? but for the less "open" type of designer. a way to win your design brief!

 

But for the average self build on BH that needs lending it doesn't fly as you can't negotiate the serviceabilty. The SE needs to make the design compatible with the mortgage lender criteria.

 

@saveasteadingThat is why you may be correct on over designed slabs etc but it just does not fly for the average member on BH.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Why does it not matter if sheets or rods are placed next to each other in such a way that the concrete cannot completely encase each rod. Is it because the yield strength of the rod is below the amount of grip the rod has inside the concrete, even if it is not complete encased. The rod will stretch before the concrete loses grip?

[saveasteading]

4 hours ago, gavztheouch said:

that the concrete cannot completely encase each rod. 

It does matter, depending on the significance of the structure. Considering this point is a part of quality design. 

For high performance the concrete should be uniform and the designed mix should surround all bars.

However for these floor slabs, the surface area of bar, by using 2 layers, is very high. The big stones might not get through but the sandy paste will, so it will all be encased.

And it's a floor slab, not a dam. Unless you drop a piano from a great height it should be ok.

Always assuming good quality control and processes. 

Touching the vibrating poker against the bars helps move the concrete into the spaces too.

 

 

[scottishjohn]

On 14/05/2024 at 18:32, gavztheouch said:

Yes I have flying ends Tom, do you have any pics of your rebar mesh going in?

that will be due the ground you are building on 

only one layer in mine as tis sitting on live roack and compacted granite 

[gavztheouch]

My engineer has specified I give 40mm cover to the rebar top, bottom and sides

 

I noticed my 105mm high chairs are more like 110mm. 

My isoquick is not 100% flat.

 

I cannot buy high chairs the exact size need to give me the correct cover top and bottom.

 

Some of the mesh is bent which will creat high and low spots.

 

These are just a few reasons why it is practically difficult to achieve 40mm cover without a massive tolerance of say ~20mm

I have called and emailed my engineer but I’m worried he is away on holiday as I have had no response. I can’t afford to wait around so I’m thinking about dropping my bottom cover to 30mm as this will be easier to achieve. Top cover will be aiming for 40mm but I know there are some high spots of at least 8mm so that will bring that area down to 30mm before we start thinking about bent rebar and other things.  
 

What are the downsides of reducing cover? I can think that at some point it will weaken the bond between the rebar and the concrete if it get too close to the surface. Also water ingress rusting rebar may be more

likely but the concrete sits on isoquick which is 200mm above ground level.

 

 

Edited May 23 by gavztheouch

[saveasteading]

3 hours ago, gavztheouch said:

What are the downsides of reducing cover?

If you are building an exposed concrete structure near the sea then be very worried, as the salt will attack the steel.

Otherwise, the occasional 5 or 10mm shouldn't matter.

 

as previously discussed I feel this is overdesigned but can't say for sure without seeing the principles of the design.

 

All construction professionals should have to spend time with a concrete gang, then designs would be practical. 

[gavztheouch]

On 14/05/2024 at 18:45, Tom said:

 

 

This is the first two layers in, UFH pipes on top of that, then you might just be able to make out the deck chairs we then used to sit the remaining layers on. There were big sheets, which were an absolute nightmare to handle, but we just about got in to the swing of it by the end!

Hi, what spacing are you using on your continuous chairs. Looks about 600mm? My first row I’ve done is 800mm thinking about reducing it to 600mm. I’m going to have 3 big lads standing on it pouring the concrete. Don’t want the chairs to spread/squash down and lose height.

[Tom]

9 hours ago, gavztheouch said:

Hi, what spacing are you using on your continuous chairs. Looks about 600mm? My first row I’ve done is 800mm thinking about reducing it to 600mm. I’m going to have 3 big lads standing on it pouring the concrete. Don’t want the chairs to spread/squash down and lose height.

 

I think 600mm was about right tbh. As long as you tie them down they shouldn't spread.