Floor make up

[LSB]

10 hours ago, Iceverge said:

Unrelated to this but 100mm insulation looks a bit slim. Can you boost it at all?

quote is not to scale, we are using 150

also the Cellcore at the bottom helps with insulation

 

[LSB]

14 hours ago, saveasteading said:

Mesh is sensible, spanning the shorter dimension. How far does it span?

the whole build is 240 sqm

 

but, this has 3 sections each with its own roof.

 

So, section 1 is 6*4, which we are working on now, this is B785

Section 2 is 24*8 and B313 (or something like that, haven't got the email in front of me) is specified as there are steel pillars in the centre of the roof with padstones at the bottom.  All bar 2 of these are 'in' walls, but 2 are in the family room.

Section 3 is 24*4 and A mesh is specified.

 

We are DIY self building and doing section 1 first including roof and floor (up to concrete) so we can use for storage.  We still have the old roof on the rest supported by old walls and acrow props with all our supplies stored.

The supplies will be moved to the new bit so we can start on the 2nd section.

 

The build, like many on BH, will take years, hoping to get water tight by the end of next year.

[George]

13 hours ago, ETC said:

…………….Or two layers of A393 on top of each other with the mesh staggered at 100mm centres?

Could do but that'd be 25% more steel weight.

[LSB]

2 hours ago, George said:

Could do but that'd be 25% more steel weight.

we already have 2 layers of mesh 🙂

 

[LSB]

So, about to order the mesh and I realised that I need concrete spacers.

Do we have to use specialised ones or can we split some blocks to make our own, just another thought about trying to save costs.

We were provided with about 80 broken 7.3n blocks (since replaced) but wondered if we could make some spaces out of those as the ones we've been quoted for £8 each.

 

We have 2 layers, so first one needs to be 50mm up and then another 100mm above that.

 

[Mr Punter]

The first layer you could do with blocks but you won't have 50mm.  You can get concrete block spacers.  Second layer metal deck chairs tied down.

 

Shop around for prices.

[saveasteading]

They aren't £8 each. Maybe that is a bag of them.

I prefer plastic wheel spacers as they snap on easily and done weaken the concrete much.

[Gus Potter]

On 03/10/2023 at 12:42, George said:

Don't see B-meshes out in the wild very often. #

 

As it's a suspended slab it probably does need fairly heavy-ish reinforcement.

 

You could replicate the B785 spec with A393 with additional loose 10mm bars every 200mm (so slot between the A393 squares). This would be easier to get hold of.

 

However do make them check because a 200mm thick slab with 10mm main reinforcement is pretty hefty for domestic purposes. Not mad or necessarily over the top... but... hefty.

This is a clever solution from @George for common domestic applications, very practical (and once you see why so obvious) and cost effective. You'll be surprised how cheaply you can buy loose bars and how easy they are to cut them on site yourself to supplement an A type mesh.

 

For all here is a bit of an explanation and a bit of "but here is the catch" at times!

 

An A type mesh has a bar spacing of 200mm each way. The table below is a good start to look at.

 

 

"A" type meshes are usually used to control cracking in slabs, particularly the A142 and A193. These are "non structural mesh applications" which means that if your floor tiles crack then the building / floor will not fall down.. that keeps SE's awake at night if things could fall.. loss of life. Often we use A142 and A193 for small ground bearing slabs and slabs on insulation, sometimes these ground bearing slabs that have less (further apart) movement joints in them... the concrete shrinkage stresses are higher so we use a heavier mesh to give us more space between the crack control joints.

 

Now often we can use say A252 and A393 meshes for say ICF floor slabs, walls and suspended floors. Here the mesh suddenly becomes part of a reinforced concrete member... it no longer is there to just control cracking but forms an integral part of a main structural component.. which is a structurally reinforced concrete member.. like a reinforced concrete beam over a big bifold door opening.  Now the design codes require a minimum percentage area of steel reinforcement based on the cross section area of the concrete as this is now critical to safety.. won't fall down as opposed to cracking your floor tiles. This minimum percentage requirement often results is a big jump in the amount of reinforcement you need to use.

 

Often you'll see on BH folk (the ICF world is a case in point) wondering why they have lots of steel mesh and bars specified for what looks like an innocuous slab that others seem to get away with. I hope the above explains that the two different animals.

 

The B meshes are intended for reinforced concrete (structural members) so have closer and heavier bars in the span direction if spanning in one direction, the cross and smaller diameter bars work as crack control and to tie it all together.. so they are thinner and at larger (200mm )spacing. In the right application they are cost effective.. they would not make them if they were not.

 

For all and on BH we want to keep things simple stupid and get stuff off the shelf. Most SE's designing self builds will / should take into account that we don't want to complicate concrete design and rebar as the more complicated the more it will cost on site.. introduce risk that things may not get built quite the way we hope. We will often design the reinforced concrete members simply.. technically over design but in the round most economic for you folks on BH once you take into account labour and material availability.

 

Now if we were designing say a car park or multi story building with multiple continuous spanning reinforced concrete beams we use a technique called "moment re distribution" which means we appreciate the steel will stretch and when it does it sheds load to other parts that have spare capacity so we get more bang for our buck. Now an intrinsic difference between an A and B mesh is that the steel has a different amount of "stretchyness" called ductility.

 

On 03/10/2023 at 17:06, LSB said:

hey have agreed to A393 for part of the build (we are building in sections), but the part we are doing must be B785, he did tell me why

The above may have the info you did not hear which is why they don't want to swap out the more ductile B mesh for an A mesh. I suspect they may not have gone into that much detail.. so keep pushing and posting if you want some feedback.