self build waterproof basement formwork membrane concrete rebar
 

Understanding steel reinforcement.

Generally speaking we 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.
  self build basement


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 he specifies.

The rebar should be clean before the concrete is poured. Here you see a compressed air line in use.

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.
basement slab rebar     basement wall rebar



To get steel this straight along a wall you MUST, first, fix the two sides together, upright, in the middle about 1.2m from the floor. Then you work your way toward the corners with more U bars on 1.2m centres. Finally fix corners upright both sides after both walls have been plumbed.

If you don't add the U bars described above you cannot be this neat.

Professional steel fixers will argue till the end of the earth you never use these U bars and you CANNOT fix steel this upright or this neat. Impossible. Utterly unreasonable of you to expect them to do this.

Yet all my self build clients who fix their own steel do so this upright and this neat. Your formwork is a lot easier if you haven't got to use tonnes of force to bend curved wall steel to where you need it to be.

self build basement waterproof concrete steel reinforcement rebar

    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).
Your engineer is unlikely to specify your accessories. You need to order
  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 both sides. At least 3 ties per m. I allow for continuous lines 600mm apart.

  3. Tying wire. At only £15 a roll get extra.
waterproof basement construction

waterproof basement construction I have sketched a corner to show how we actually get the bent starter bars in place. You need to imagine a string line between profile boards for the actual steel location. This is the idea ....

You might use two people for the first bits. By fixing two bars to the mesh back and front and using a bar as a diagonal, the 3 bars '1' will stand up. Ditto for 3 bars '2'. Then one person can fix the upright bar '3' and fix a corner bar '3' to it and to a bar '1' and to a bar '2'. Your corner is now 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. Having got this far, it's easy for one person to fill in the gaps with bars. Sometimes the bar '4' is moved along and pushed into the bank.

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 once the slab concrete has set.

If you can, tie the top mesh so that tying the starter bars to a straight bar in the mesh sets them in exactly the right place. Or else you can tie a straight bar to the mesh and fix the starters to that.

Or, I prefer, buying the mesh with flying ends and extra straight 10mm bar. In this way I fix extra bar to the mesh where bars are missing exactly where the starter bars need to be and I have maximum room to get my starter bars in.

    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.
concrete slab
self build concrete pour slab levelling system

and above the pale vertical rods top of which is top of concrete. These simple rods (cost £20) are the slab levelling system.


The mesh in this image has flying ends. When you lap sides 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.


waterproof basement construction (I took this image from www.brcmcmahon.com, a supplier in Ireland)


I always fix walls with A393 mesh 4.8m along the wall and 2.4m high with the flying end tails upwards. Therefore, when I am filling and pokering concrete only 1.9m high the poker is not obstructed by horizontal bars 2.3m high. The work of concreting and pokering is easier. And I buy plenty of straight 10mm bar to fix to the top of the wall by hand, which is pretty easy with all the steel I'm fixing to already held firm in set concrete.




In case you want to order your steel yourself, (under Concrete Reinforcement in Yellow Pages) you will need to know the language.

steel reinforcement shape bending codes

British Standard preferred meshes in stock size sheets 4.8m long 2.4m wide.

British Longitudinal Wires Cross Wires Mass
Standard Reference Size mm Pitch mm Area mm²/m Size mm Pitch mm Area mm²/m kg/m² kg/sheet
Square Mesh Fabric
A393 10 200 393 10 200 393 6.16 70.96
A252 8 200 252 8 200 252 3.95 45.5
A193 7 200 193 7 200 193 3.02 34.79
A142 6 200 142 6 200 142 2.22 24.57
A98 5 200 98 5 200 98 1.54 17.74
Structural Fabric
B1131 12 100 1131 8 200 252 10.9 125.57
B785 10 100 785 8 200 252 8.14 93.77
B503 8 100 503 8 200 252 5.93 68.31
B385 7 100 385 7 200 193 4.53 52.19
B283 6 100 283 7 200 193 3.73 42.97
B196 5 100 196 7 200 193 3.05 35.14
Long Mesh Fabric
C785 10 100 785 6 400 70.8 6.72 77.41
C636 9 100 636 6 400 70.8 5.55 63.94
C503 8 100 503 5 400 49 4.34 50.00
C385 7 100 385 5 400 49 3.41 39.28
C283 6 100 283 5 400 49 2.61 30.07
Wrapping Fabric (basementexpert note: very thin)
D49 2.5 100 49 2.5 100 49 0.77 8.87



Size 6mm 8mm 10mm 12mm 16mm 20mm 25mm 32mm 40mm
Area (mm²) 28.3 50.3 78.5 113.1 201.1 314.2 490.9 804.2 1256.6
Weight kg per m 0.222 0.395 0.616 0.888 1.579 2.466 3.854 6.313 9.864
Approx no. of 12m bars per tonne 375 211 135 94 53 34 22 13 8


Links

Major reinforcing suppliers are Hy-Ten   Express Reinforcements   BRC   Rainham Steel   Steel Reinforcement Suppliers

Accessories suppliers are

"Buildspan - the concreters warehouse"   Speedcrete   Max Frank   Lemon Groundworks   Formwork Direct   Siteright Construction Materials



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