Slab/Raft Foundation - Steel Reinforcement Question

[TonyE]

Hi everyone,

 

Ive got a 250mm deep slab foundation which has been specced by my SE to have 2 layers of A393 mesh Top and Bottom.

 

Ive also got 170mm U bars around the perimeter and its at this stage that I'm struggling to understand how I install the U bars to ensure I keep the concrete coverage correct.

 

Slab make-up is:

 

30mm Bottom Cover (Spacer)

A393 Bottom Layer:  20mm (2 bars in sheet:  long and lat)

U Bar:  H12 170mm  (150mm High Chair inside this)

A393 Top Layer:  20mm (2 bars in sheet)

30mm Top Cover

 

SO if I add up the math:  30mm+20mm+170+20+30= 270mm   too high.... 

 

Therefore I assume that the U bar must sit alongside one bar of the A393, thereby reducing the total height by that one bar of mesh (both on top and bottom layers).  So, 30mm+10mm (1 bar on bottom mesh)+170+10(1 bar on top mesh)+30= 250  and the 150mm high chair would span the 150mm between the 2 sheets......  This seems logical...

 

HOWEVER if i have U bars around both sides of the mesh (perimeter of the foundation) the U bars on the x/y axis will then hit the horizontal bar...  So the u ar can sit alongside on the one axis but not on the other....???  Does this mean I have to place the u-bar that fouls the mesh below the A393 mesh....

 

Ive resorted to getting lots of pencils and pens and trying to work out how I place the U-bars along both axis (length and width of slab) to work out how the u bar placements change the total height and also impact on the High Chair height.....

 

I'm due to start laying the steel tonight and would appreciate any advice.......

 

[George]

Therefore I assume that the U bar must sit alongside one bar of the A393

 

Yes

 

HOWEVER if i have U bars around both sides of the mesh (perimeter of the foundation) the U bars on the x/y axis will then hit the horizontal bar...  So the u ar can sit alongside on the one axis but not on the other....???

 

Really they should have been 150mm U-bars so they can fit within the mesh layers. However, if the mesh are the same way up you should be able to slot them in on both orientations. 

 

It's also not the end of the world if you angle them so they can fit in.

 

Edited May 9 by George

[TonyE]

My sincere thanks George, I’ve been worrying about this day and night!

 

I assume the High Chairs would normally be placed right up to the edge of the mesh around the perimeter?  With the slightly off-set U bar heights (where it turns the corner of the slab, one higher than the other) if I set back the high chairs a little it’d ease the difference.  It was just a thought.

 

 

[George]

The high chairs are for within the mesh. At the very edge the u-bars will do the job of keeping the top mesh supported.

[BotusBuild]

I had this problem, and there was a lot of "squashing" of high chairs and "fettling" of U bars. Alongside all of that I was fitting the UFH pipes to the mesh and trying to keep the whole lot with a 50mm coverage.

 

It can be done!! Take time to think through the laying of the mesh.

 

FYI - 2 layers of A393 is 30mm in thickness if you overlap them, not 20mm. If you get the overlapping at 90 degrees to each other that goes up to 40mm

[George]

On 10/05/2024 at 09:11, BotusBuild said:

I had this problem, and there was a lot of "squashing" of high chairs and "fettling" of U bars. Alongside all of that I was fitting the UFH pipes to the mesh and trying to keep the whole lot with a 50mm coverage.

 

It can be done!! Take time to think through the laying of the mesh.

 

FYI - 2 layers of A393 is 30mm in thickness if you overlap them, not 20mm. If you get the overlapping at 90 degrees to each other that goes up to 40mm

Yeah although that's always an issue. If cover is tight you can do flying ends or use loose bars to lap the mesh layers.

[BotusBuild]

On 13/05/2024 at 07:18, George said:

If cover is tight you can do flying ends or use loose bars to lap the mesh layers.

Next time.

 

NOOOOOOOOOOOOOOOOOOOOOOOOOOOOO - one house build has been enough 🙂 

 

Hang on - I have a garage to build and an outer porch floor to pour

[Gus Potter]

On 09/05/2024 at 14:03, TonyE said:

Hi everyone,

And a Hi from me too!

 

Hope this helps a bit although a bit long winded.

On 09/05/2024 at 14:03, TonyE said:

Ive got a 250mm deep slab foundation which has been specced by my SE to have 2 layers of A393 mesh Top and Bottom.

I'll assume this is some kind of reinforced concrete raft rather than a basic ground bearing slab ... yes.. may be stating the obvious and that we are talking about a self build rather than say a tightly controlled and site managed design where you have professional staff on site most of the time.

 

My starting point when designing anything like this is to look at what resources the Client has availabe to "manage" the construction and control the quality of the work. Do they want to use general builders and thus say be able to obtain more quotes or do they want to go for specialists for each work package and thus limit the spread. There is no free lunch here.. it's about getting the right balance that will work on your project for you and that takes a bit of work.. but get it right and you can save a lot of money.

 

Ok say we have identified they way you are set up.. the time you have available and your budget.

 

If I have this information I start by looking at how flat and level we can get the medium under the concrete slab and what that will cost in terms of labour and expertise on behalf of the Contractor.

 

This could be type one with a bit of blinding or your insulation. Remember the sheets of insulation are not perfectly flat.. and can take a fair bit of weight to make them so! Don't believe me.. go and ask the insulation supplier how flat their insulation is... not the thickness variation!

 

I'll often specify what is directly under the slab with a level tolerance of say +0.0mm to -15mm (half an inch in old money). What I don't want to have is a lot of really low spots.. the slab needs to shrink as it cures / dries out so massive thick bits of slab are not good as they lock the slab to the ground and promote cracking. @saveasteading et al have a more nuanced take on this slab shrinkage which we have discussed but stick to the basics for now. By setting a limit of +0.0mm I can then be sure that the slab is not going to be thinned out from below and compromise the cover to the reinforcement and what we call the effective depth. The effective depth is used when we design the slab for stength for example. But if you are a contractor any low spots .. needs extra concrete and that has to be factored into the cost.

 

Next I look at what is going to go on top of the slab.. how flat ( low and high spots) and level (measured over set datums) do we need it to be? There are common industry standards for this.. I'll not post now as it's lengthy although have posted info on this in the past. Anyway we need to know the tolerances of the top of the slab. Say you get your slab laid an you have low spots.. now your cover could be 10mm?

 

Once I know the top and bottom limits / tolerances I can get on with designing the slab itself so it will carry the loads require of it..

 

There tends to be some intial criteria here..for small DIY self build designs and I make some comment:

 

1/ Ok don't be clever at this stage.. there will be plenty time to get smart later.. fix problems that will arise during construction..when the rain is pouring down say and folk don't turn up.. go simple stupid for now.

 

2/ You want to be able to get everything off the shelf. Your A393 mesh is.. harder to get flying end mesh in small quantities. You can convert a mesh to a flying end on site by cutting it with a Sthil saw / angle grinder. For another day but plenty of folks on BH have done this and can tell you how to do it as do the builders I work with.

 

3/ It looks like your slab has some loading that causes a downwards and upwards bending force as you move along the slab.. hence the reinforcement in the top and bottom. Your SE has identified this and maybe made a judgement to go for a thicker slab and stuck to a standard off the shelf mesh. Ok some folk may disagree with the thicker looking slab.. but in the round I'm guessing it looks a good play if you take into account the buildability and top / bottom tolerances as above.

 

4/ There are load of folk on BH that say.. hey we didn't have to use this or that. There is a fundamental misunderstanding that can arise here. A true ground bearing slab is NOT a reinforced concrete slab but what you have looks like a reinforced concrete member.. just like you would see in a multi storey carpark. When we design reinforced concrete there is a bit in the design codes that require a minimum percentage of steel..  often this minimum percentage steel works for most of the raft type things.

 

Summary.. It looks to me that this a slab that could be justified on cost based on a DIY job.. but ask ten folk like me and you'll get ten different answers!

 

But here is the where I think you will struggle and find it very difficult to set out the mesh and get it compliant with the design.

 

yIt may look great on the drawing but any bent rebar comes with a bending tolerance as does the mesh. I don't think your designer has taken this into account and the site circumstances. I feel your 30mm cover is too ambitious for this type of project.. you are making a rod for your own back and I would not design it this way for that reason.

 

The U bars don't in real life come like this.. there are bend radius tolerances and length tolerances. You can find this on the rebar suppliers web sites and also in BS 8666, etc for those that want to check. In other words I think you cover is going to make this very hard for you and you will struggle to execute this.

 

Recomendations:

 

1/ List how you are going to do this part of the work.. the sequencing and what the skills are of the Contractors you have in mind. This will help as if you can explain this to yourself then you can explain to others.

 

2/ Go back to your SE and ask if you can increase the cover. They may say yes but there may be some local areas of the slab where we need some extra lose rebar.. say under walls or columns. Fine.. Travis Perkins et al do 3.0m bars off the shelf and you can chuck them in where the stress in the concrete is higher.. but your SE may need to do a bit extra work.

 

3/ If your SE protests then ask.. hey I told you this was a self build.. you tell me how I can reasonable achieve a 30mm cover without breaking the bank!

 

@TonyE hope this helps and all the best..