Steel reinforcement etc.

Generally speaking I ask that your engineer specifies 2 layers of A393 welded mesh in the slab and 2 in the walls as well. If the slab needs to be larger to bear the weight of the structure above then it will have a heel and that might have trombone shape bars as well. The slab and the wall are connected, and the wall is prevented from being pushed over, by two rows of starter bars, 16mm or 12mm or one row of each depending on what they specify. I use U bars throughout the wall, rather than along the top, to make the wall steel straight and plumb.

The rebar should be clean before the concrete is poured. Here you see a compressed air line in use. Round the edges it can be difficult keeping mud out. But all steel reinforcement must be clear for 40mm in every direction so that every bar is surrounded in concrete.		I recommend that the two faces of mesh in the wall are controlled by these U bars on 1.2m centres throughout. You should start fixing them and all the steel upright in the middle of a wall and work toward corners. Corners have L bars to join 2 walls together.

Note:
• There should be safety caps on all uprights to prevent anyone falling on them being punctured.
• A smooth blinding of semi dry or C20 very very wet concrete to virtually self level. Minimum 75mm thick.
• steel neat, clean and securely tied.
• scaffold board used for edge formwork where it equals the slab depth required.
• This formwork is supported by steel road form pins banged through the concrete blinding and brought up to level with clods of earth underneath. Bits of hardcore underneath would be better or include some wooden pegs and nail the board at the right height. If you can put your pins or pegs in correctly before you blind, the blinding will make them stronger.
• Mesh supported on concrete spacers and chairs. (Links to some concreting accessory suppliers are at the bottom of the page).

1. Concrete spacer bar.
   
   These are 1m long and you break them up into pieces. I allow 0.4m per square metre.


2. Continuous wire chairs.
   
   These go between the layers of mesh. Tie each chair down twice both sides. I allow for continuous lines 800mm apart.


3. Tying wire. At only £18 a roll get extra.

You can recognise the bottoms of the bent starter bars in this photo because they are less rusty than the mesh. There is a lot of congestion to get them through. The worst is where they go through continuous chairs. They are tied on top of the bottom layer of mesh. You have to wriggle them through mesh then tie them to a bar, fixed to the top mesh, that positions them correctly and in a straight line. In the image below they lined up the mesh very precisely so that the starter bars could be tied to the squares. Every engineer is different. This one wanted 2 layers of mesh in the top yet his starter bars are short and easier to get in than most.

For instance, I always try to buy mesh with flying ends. On two sides of your basement, you can have flying ends and tie straight 10mm bars exactly where you want to fix the starter bars. On the opposite sides you can extend the mesh with bars tied to the mesh then bars tied to those. Perhaps I could visit and get you started. When you lap flying ends with a normal side you aren't wasting steel with identical bars side by side against each other, you aren't making a mountain at corners where 4 sheets need to overlap, and if you don't need full sheets down an edge you can plan to throw away the bit with 2 bars missing so less money thrown away. (I took this image from www.brcmcmahon.com, a supplier in Ireland)

I have sketched a corner to show how we actually get the bent starter bars in place. You need to imagine a top layer of mesh as well with a bar in the correct place to tie all the starter bars to. This is the idea .... You might use two people for the first bits. By fixing two bars to the bottom mesh back and front and once to your top layer bar, using a bar as a diagonal, the 3 bars '1' will stand up strong. Ditto for 3 bars '2'. Then one person can fix the upright corner starter bar '3' and a corner bar. Your whole corner is strong. You will need two people again to fix a long length (of wall steel '4') from a corner to a bar at its far end, and a few bars evenly spaced to stop the long bar sagging. Having got this far, it's easy for one person to fill in the gaps with bars. Try to keep your diagonals out of the first pour. Everything you've used above this pour is temporary and you will reclaim all that steel to use where it is designed to go once the slab concrete has set.

Note below: The rotating laser. One man using the laser detector properly, that is, upright. The vibrating poker. Goggles, gloves and steel toe-capped wellies in use.

and above the pale vertical rods. Top of which is top of concrete. These simple rods (cost £1.75 each) are on the slab levelling page.

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