Rebar in a Concrete Slab

nowImonboards

Hi all,

I'm pouring a 150mm thick concrete slab. I'm planning to add two layers of A142 mesh, 50mm from each surface. I was using one of AJ's excellent posts as a guide.

Reference: pavingexpert.com

However, it doesn't cover how to construct the rebar in the perimeter under the wall. Can anyone point me to some info on how I could go about that?

10-10-20

Rebar footing cages is what they are called. I thought you might be able to source them from builder's providers, but I can't find even one. They must be assembled on site in that case. Bit of a job to make-up and bend and then twist together that lot.

Anybody know of a supplier of pre-assembled cages?

nowImonboards

Hey 10-10-20,

Thanks for replying (and looking). Hopefully someone can suggest a source :)

Mellor

https://www.boards.ie/discussion/2058369573/rebar-in-a-concrete-slab

Not sure who AJ is. But that’s a paving website and not a great detail. I assume you’re building a shed or some other out building.

nowImonboards

Hi Mellor,

It's a foundation that will initially support a veranda but would like to future proof it to potentially take something heavier later down the line. Perhaps a real conservatory or similar. What are you thinking?

Mellor

https://www.boards.ie/discussion/comment/122632884#Comment_122632884

It will be adequate for a veranda or similar exposed structure. But I would not be trying to enclosed the space and extend indoors to the slab.

The DPM is protecting the concrete sand controlling cure time. But it’s not waterproof at the wall

Calahonda52

https://www.boards.ie/discussion/comment/122632292#Comment_122632292

This might work.

OP

as the foundation is fully supported along the ground you don't need too many of these as there is little enough shear

a few diagonal stirrups will keep the cage from tilting

https://www.youngcivilengineering.com/2018/01/types-of-shear-reinforcement.html

They are sometimes called shear stirrups

If you are thinking ahead then you will need good quality concrete properly vibrated and cured

https://www.amazon.co.uk/Concrete-Reinforcing-Stirrups-Diameter-Rectangular/dp/B0D1315F9D?th=1

nowImonboards

Thanks very much to all.

Pity the Amazon supplier is out of stock. I'll ring around and see if I can find any.

In the meantime, if anyone has anything else to add, feel free.

Lumen

Devil's advocate: I love a bit of DIY overengineering but I'm not sure this kind of attempted future proofing is a good idea. You can't be sure exactly what you'll need in the future since you don't have a specific design in mind, and you risk spending loads of time and effort on something that might have to be ripped up and redone anyway.

Even if it's strong enough you might decide to vary the dimensions the second time round and then all the loads will be in the wrong place.

For a proper conservatory you need proper footings, which is down to bearing subsoil or rock. A raft might do it but I would be seeking the advise of a structural engineer.

Lumen

Ah, hang on…

https://www.boards.ie/discussion/2058360909/foundations-in-sub-base-material

mickdw

You can order up links bent to your required dimensions from any rebar supplier or through many hardware stores.

You could also easily bend them yourself with a little bit of prep. Drill 4 steel pegs into concrete slab or wall that's not too important to create 4 corners and simply bend the bar around them. Leave 100mm or so overlap in each direction from a corner.

Also 50mm cover top and bottom means you are placing all your slab reinforcement in the middle third of the slab and get getting nowhere near full benefit from your concrete depth.

I'd reduce that cover specially on top surface and get your rebar to perhaps 80mm apart at least.

10-10-20

50mm might be too centered alright, but the point is to ensure that the rebar is well embedded to prevent spalling after 30 years or so in a harsh environment (seaside, etc).

nowImonboards

https://www.boards.ie/discussion/comment/122634252#Comment_122634252

Hi Lumen,

I get what your are saying and you are probably right. The thing is, I have already done 90% of the work and spent the time so haven't much to lose at this point. I've just been trying to do the best job I can with the knowledge gained from the research I have done.

I definitely won't be going bigger with the structure down the line. The length is 6m which is the exact length of the house so I don't see any reason I would change that dimension. The width goes 3m from the house which is plenty and I wouldn't go any further as it wouldn't leave me with much garden.

Since there is a minimum price for a concrete truck, I adjusted the widths and depth of the perimeter beam to use up the full amount of concrete I can get for the minimum price. Considering the floor area is 6m x 3m, that allowed for an additional 500mm wide x 150mm deep beam around the perimeter. Since the thickness of the slab is 150mm the perimeter beam would then be 500mm x 300mm around the front and side edges. It would have been the same price if I didn't do that so it only cost me a bit of extra digging, filling and whacking.

I didn't think it would be much more effort to put some 12mm rebar in that beam. Of course, I have underestimated the effort at every step of the way so far.

https://www.boards.ie/discussion/comment/122634398#Comment_122634398

Not sure what you are saying there. That post was about the suitability of the sub-base and the required depths. This one is about the reinforcement.

https://www.boards.ie/discussion/comment/122634476#Comment_122634476

Thanks Mickdw,

Good idea for getting the bends there. Thanks for that.

Regarding the coverage, I'm was going by the advice in AJ's post I linked to above. He recommended that there should be at least 50mm coverage above and below each layer of the mesh. He also has a post with a section titled 'Reinforcement - a view from inside the concrete industry' which states:
"When mesh is used, it should always be placed near the surface of a slab with a minimum 50mm cover if it is to minimise plastic shrinkage cracking."

As my slab is only 150mm there isn't much wiggle room. Are you thinking I should increase the thickness of the slab to 180mm? That would end up adding an additional EUR75 or there abouts (unless I reduced the width of the beam).

mickdw

50mm is overkill to Me.

Anyway, a typical raft slab will only have top steel other than at beam positions so I wouldn't worry either way.

nowImonboards

Thanks for the input.

Can anyone help me understand, what is the purpose of the stirrups/links? Are they just to hold the longer lengths of rebar in place until the concrete sets or to they perform a function themselves?

If they are just only required to hold the rebar then could I just cut square sections from the left-over mesh?
I'm preempting that is a no-no but would like to understand why for educational purposes. I realise the welds on the mesh are not very strong but if they are indeed just to frame the longer lengths then that shouldn't be an issue.

Calahonda52

https://www.boards.ie/discussion/comment/122635577#Comment_122635577

In slab, maybe but not on site as it will rarely be 100% exact.

This is based on fixing my first steel in '75 for McAlpine in England😀

Calahonda52

Please re-read my post 7 and re-read this link

Mesh

https://www.youngcivilengineering.com/2018/01/types-of-shear-reinforcement.html

nowImonboards

Thanks Calahonda52,

I've read it a few times and done a bit of additional reading. From what I understand the stirrups are mainly a concern when the beam not supported all the way along. As mine is uniformly supported in a trench (with 150mm of compressed hardcore underneath) then, if I understand correctly, I don't need to go overboard here. I might try to just put in what I need to hold the 4 main longitudinal lengths of rebar together, and a diagonal to stop it collapsing.

Wish me luck…

nowImonboards

One more thing - I was reading about adding reinforcement fibres to the concrete mix. Apparently these can be used in place of mesh / rebar. For the quantity I want they would cost EUR70-80 extra but I could (hopefully) return the rebar and get that back. It might save a load of messing around but could be harder to float.

Anyone have any thoughts on that?

Lumen

https://www.boards.ie/discussion/comment/122637139#Comment_122637139

I'm no expert but from what I dimly remember fibres help tensile strength a bit (couple of factors at best) but are nowhere near as effective as rebar. They make sense where it's impractical or expensive to put the rebar in place, which isn't the case here.

That said I don't really understand why you need all this rebar because this strip foundation is directly under an even longitudinal load. Concrete is good in compression. The purpose of footings is to transmit the loads to the soil underneath.

I am not an engineer and structural advice is against the forum charter, but in this design it seems to me that your biggest risk is frost heave under this shallow strip foundation causing differential movement w.r.t the house, i.e. cracks appearing at the house junction. But that's only an issue if you have masonry walls. With a steel or alu (presumably) verandah that's a non-issue.

That's why I made the point earlier about this being an odd halfway house. It's overengineered for a lightweight prefab verandah and underengineered for a proper masonry extension.

But that's just my unqualified opinion.

nowImonboards

https://www.boards.ie/discussion/comment/122637590#Comment_122637590

Hi Lumen,

You may be unqualified but you make a lot of sense.

I was wrong about the fibres. They help reduce cracks whereas the rebar helps protect from the tensile stress. While rebar doesn't stop cracking, it can hold a cracked slab together (assuming there isn't enough force to break the rebar). The crack however can allow water and air to get to the rebar, causing corrosion which can then weaken the rebar. The rebar can also corrode due to capillary action in the concrete which is why the coverage is important.

(If any of that is inaccurate please correct me)

With that in mind I will just stick to the rebar. It may be overkill but since I have it, I may as well try to use it. It may be over-engineered but I think it is relatively little additional effort for the additional strength. It may not be enough for a future conservatory but who knows what kinds of structures we will have in 15 years from now. I didn't see any of these verandas around 15 years ago. I'd hate for my future engineer to say: "If only you had thrown a bit of rebar in there you could reuse it".

One clarification. It isn't an even longitudinal load. The weight can shift around a lot due the the fact that the walls are glass sliding doors. If the panels from 2 sides are completely open to the same corner that would be 6 + 3 = 9 panels in total positioned in the same corner at once. That's about 450kg excluding the weight of the rest of the structure (plus glass roof) transferred to a post in the same corner. With my limited experience I find it difficult to relate that to what the slab may be able to take but I wouldn't say it is negligible. I would love if I was wrong though.