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. 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 want your steel tying to be as simple as possible, and I want your concrete to be completely protected from cracks. That's why I explain what is likely to be specified and how and why I change it.

This engineer described starter bars as L shaped, which they are. However, the correct terminology is 'shape code 11' A straight bar is usually either 'shape code 00' or 'shape code 1'. All the shape codes are on a chart lower down. When you get your final design, every different bar will have its own unique Bar Mark, or BM. For example: BM1 H16 150 2000 11 600. Which would mean that all the BM1s are 16mm diam steel, there are 150 of them, each is 2000mm long, shape 11 so L shaped, and bent at 600mm. If we accept that the lines and the dots represent welded A393 mesh, then the difficulties here are The starter bars have to be tied to the lower layer of mesh while you are kneeling on very uncomfortable steel bars and reaching down through the top layer, lower than where you are kneeling, to do the tying. Those two layers of mesh have to be separated. The top layer supported on 'things' the correct height and frequent enough. Those 'things' are a nuisance preventing the starter bars getting threaded into position correctly or easily. The starter bars have to come out of the floor slab very accurately positioned to go up the wall with 40mm of concrete to the front face and 40mm of concrete to the far face.

This isn't exactly a basement diagram. But you can see that the soil has been backfilled on the right. Imagine that the left is your basement floor slab. Therefore, the heel may be on your design or it might not. If you have clay, you are likely to have a heel. If that wall is particularly heavy and the weight needs to be spread, you are likely to have a heel. Even if you don't have a heel specified, an extra 50mm of concrete would give your wall formwork somewhere to sit, making life easier for you. The toe will be your basement floor slab, continuing to the stem and any heel the other side. Note that the side of the stem (basement wall) to your basement accommodation is the front face, or near face. The other side of the stem, or basement wall, on the backfilled side is the rear face, or far face.

This photo popped up on facebook. It is here because it is both simple and useful. The little spacer blocks are accessories that your engineer won't specify. You need to know that you need to order them. The concrete blinding is nice and flat and level. You won't waste more expensive waterproof concrete filling holes later. Neither will the spacer blocks sink when there is more weight and people walking over it as they pour the concrete. And, because the blinding is neat, you can mark it up, should you wish. We can see A continuous yellow paint line representing the near face of the wall that will go on top of the floor slab after it is poured. A dashed yellow line representing the far face of the wall concrete. Another dashed yellow line representing where the starter bars need to be tied. red lines representing every starter bar. Accurately set nails for corners of concrete and starter bar lines. Probably put there by the survey engineer who first drew up a topographical plan of the land, who you paid half a day to come back and set out.

Both these teams, both made up of experienced labour but who had never built a basement before, saved a bit of money not blinding with concrete all the way to where the walls would go. That caused both their corner pins banged into the soil to move when they pulled their string lines tight. That caused their starter bars to get tied in a curved line. Many were wrong and some were done again. And their spacer blocks sank where they weren't on concrete, making their starter bars lean away from the vertical. They had to crank them with a scaffold pole and a lot of hard work. Some walls had to be extra thick concrete to include the starter bars in the wrong place. Very wasteful of time and money.

1. The mesh is flying end. More about that lower down.


2. At this stage, only the bottom layer of mesh is laid down on spacer blocks, and the starter bars are being tied at the same level where you are kneeling. Not lower making you bend more and be unable to breathe.


3. U bars are being tied at the same time.


4. Two straight 10mm bars have been tied down to the U bars exactly where the starter bars need to be, to be straight, upright, and correct in the walls.


5. The U bars and the straight bars have replaced the top mesh around the edge. The top mesh needs to be tied to the U bars with 400mm of overlap, not all the way to the edge. The top mesh doesn't need to get in the way of the starter bars being in the right position.


6. The U bars are the correct height to support the top mesh around the edge. Continuous wire chairs are tied on next. Then the top mesh.

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.
• Mesh supported on concrete spacers and chairs. (Links to some concreting accessory suppliers are at the bottom of the page).

1. Concrete spacer bar.
   
   Often called Mars Bars on site. These are 1m long and you break them up into pieces. I allow 0.4m per square metre.


2. Continuous wire chairs.
   
   Often called Toblerones on site. These go between the layers of mesh. Tie each chair down twice both sides. Then twice on top. I allow for continuous lines 800mm apart.


3. Tying wire. At only £18 a roll get extra. And, perhaps, get a cordless tying gun. The Makita works well. the others I have seen have all been rubbish, constantly blocking up. I don't rate potato sack wire because they snap. You need a roll of proper tying wire and proper nips where you need the tie to be very tight and strong. For instance, when you set up starter bars at corners.

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 the top mesh then tie them to a bar in the bottom. it is very difficult to also tie them in a straight line where they need to come up inside the wall. 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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