Floor slab

[sblewer]

Hi guys

I'm hoping someone can give me some advise.

I have put down my DPM, ready for concrete, but have an issue regarding tamping the surface.

One side of the slab is up against my house, so I can't get to it to tamp in the old traditional way, and length ways it is too long, as it is over 10 metres long.

My other issue is that I am building a raft foundation, and the Structural Engineer has specified 2 layers of A252 mesh, 1 layer is 25 mm from the bottom of the slab and the other is 25mm from the top, therefore I can't walk on it once I have put the second layer in, for obvious reasons.

Now I dont have to get the finish perfect, as the insulation and screed will be going on top, but I need to get a reasonable level and even finish, so has anyone got any ideas?

The concrete will be barrowed round by a Supply and Barrow company, so no worries there, but it will be a C35 mix, and so I wont have much time to mess around with it, for obvious reasons?

I would really appreciate some help.

Thanks

Steve

[msh paving]

are you shure 25mm cover top and bottom,50mm is normal if you lap the mesh on the joints you will lose the 25mm cover 252 is 8mmX2=16mm -25mm cover does not give you any room to play with mesh will poke through,you need to chair the steel apart and tie it then you can walk on it as you pour concrete, to get the level flat you need to fix 2 rails to the top mesh layer to screed the concrete off the remove rails and fill the strip and trowel in ,it will be hard work barrowing concrete over steel you will need to board it with scaffold board to run on,have you though about pumping it?raft barrowing is gonna be slow,need to be nice and wet to flow around the mesh and vibrated in,as its a raft the building control will need to see all the mesh tied and fixed with correct lap,so you will have to walk on it which is normal practice anyway,i usually use a magic screed on concrete pours and a lazer lever to get it flat

[sblewer]

It definitely says 25mm coverage top and bottom, I must admit I thought that was strange. I might contact the Structural Engineer to check that.

Pumping isn't really an option, it would have to be pumped about 60 yards, as my house isn't directly situated on a road or culdesac, and the cost for pumping that far is ridiculous. I have found a company that barrow for free and as they have already been to my house twice, they know the routine.

As far as the top layer of mesh is concerned, I have intended to lay the top layer of mesh, once the concrete was almost at the correct height, and the push down, but from what you are saying Building Control want to see both layers in before signing off, so that's out.

If I purchased the steel chairs to support the top mesh, how many per square metre would I need to support a man and barrow full of concrete, just so I know how many to order/

Cheers

[msh paving]

you need to know the slab thickness, take of the cover then you need chairs to suit,you steel supply company will have them in stock deck chairs are best as they will take the load,one every 2/3 feet, dont forget spacers for the bottom layer,what sort off raft is it is there any beams in it?to tie mesh too?as a structal raft you cant tred mesh in as you wont get correct lap joints

[sblewer]

The slab thickness is 150mm.

The raft is a 750mm footing with 8 No. 16mm rebard running horizontally tied together with boot shaped 12mm rebar. The mesh can be tied to the tops of this, as I have only poured the footing part so far, in order to build the outer skin of brick up to damp, then pour final concrete up to slab thickness, up to brick work.

Sorry what do you mean by 'as a structal raft you cant tred mesh in as you wont get correct lap joints' I don't understand, sorry.

[msh paving]

thats ok you answered my question with the 16mm and 12mm bars,dont forget to put some jablite or styrofoam around the brick work to form a cavity,take carefull note of the steel levels ontop of each other ,you need to check cover on steel to decide chair size, will be 50mm chairs at 50mm cover

[sblewer]

I am going to contact my Structual Engineer tomorrow and confirm the 25mm cover, seems a bit thin to me also, and as you said where they lap the coverage will be less, so that cant be right.

Can I just ask you a couple more questions if you don't mind?

I am now going to lay my bottom layer of mesh on 25mm blocks (assuming this is the right depth), do I then place the Deck Chair spacers on top of that layer of mesh, and then put my top layer on those (the chairs seem to be sold in lengths of 2m if that is the right one).

Will this then be able to support me standing on it?

Also, so the concrete guys dont have to walk on the mesh, do you think it will be ok for them to pour over the side from the brickwork, and with a vibrating poker, allow the concrete to flow by itself over the slab area?

Thanks a lot once again for your help on this.

[msh paving]

I am now going to lay my bottom layer of mesh on 25mm blocks (assuming this is the right depth), do I then place the Deck Chair spacers on top of that layer of mesh, and then put my top layer on those (the chairs seem to be sold in lengths of 2m if that is the right one).

are the deck chairs triangular if so they sit flat ok but if they are flat you need to bend them in to a circle and tie off to hold shape then place them

yes thats the way, wire them all togher tight, lap needs to be 10x bar diameter

Will this then be able to support me standing on it?

no problems at all

Also, so the concrete guys dont have to walk on the mesh, do you think it will be ok for them to pour over the side from the brickwork, and with a vibrating poker, allow the concrete to flow by itself over the slab area?

not a chance, concrete wont flow through the mesh,the only way is to run on boards and tip where its needed,i would not dream off barrowing but its up to you,

[seanandruby]

i think you will find 25ml to be the minimum cover. you can open the chairs so you have 50ml cover. if the round ones you can fix the steel and tap it all down a putlog, or sledge hammer so that the rebar/ mesh bends the chairs to your level. hire a lazer level so you can tamp the concrete and finish it on your way out. scaffold boards over the mesh will do as a barrow run.

[sblewer]

Further developments. I have spoken to my Structural Engineer and he has said the coverage is as follows:

Bottom 40mm
Top 25mm

These are minimum's

So if I am using A252 mesh (8mm bar), am I right in thinking that I need 75mm high Deck Chairs?

I have worked it as follows:

40mm coverage bottom
Mesh
75mm Deck Chairs
Mesh
35mm Coverage top

Does this seem ok?

Cheers?

[lutonlagerlout]

IME the mesh tends to spring about a bit when you walk on it,albeit with planks,
personally i would make the raft say 200 thick and have the mesh 50 mm in top and bottom to allow a bit of tolerance

if you are building off this raft ,there must be a toe to the external edge ,surely?

LLL

[sblewer]

Thanks for the reply, but i have a couple of problems with that suggestion.

I can't make my slab 200 now, everything except the mesh is in place now, and all depths sorted for 150 slab thickness.

My other problem is unfortunately I don't understand what you mean by a tow to the external edge, is there any chance you could explain what you mean, dealing with a novice here I'm afraid!!!!

[msh paving]

its the bit the brick work sits on

[sblewer]

I'm hoping the above link works, and will show the raft design.

I am now at the stage where the main rebar and boot shape is in and concreted up to main horizontal, and also outer skin of engineering bricks are laid up to damp.

The oversite is prepare with min 150mm hardcore, 25 mm sand blinding and 300mu DPM.

As you can see above I have to lay bottoma layer of mesh so that it laps through the tops of the boots, which I have done, and I will be tying them together, so all I need to do now is lay the top layer of mesh.

The mesh is 8mm thick, so if I have to have coverage of 40mm on the bottom, and 25mm on top, what size chair should I order?

I thought if I ordered 60mm high ones, then I would be safe for coverage taking into account laps of the mesh. Also I intended to lay them about every 1metre of so, for added stability.

Cheers

Steve

[msh paving]

50mm chairs 150 slab,more cover is easer other wise you will be fighting with the mesh poping up dont forget jablite around brick work to form cavity