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. 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.

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 Toblerones. Toblerones are tied on top of the bottom layer of mesh (either side every metre at least 300mm from the end of a toberone). Then the top layer of mesh is tied down to them once every metre. You have to wriggle starter bars through the top mesh, sometimes through the toblerones, then tie them to a bar in the bottom. It is extremely difficult to also tie them in a straight line where they need to come up through mesh and emerge from the floor slab concrete in the correct position inside the wall that you build next. Please read on.

Some people avoid the difficulty of getting the outer starter bars down through the tiny gap between mesh and edge shutter by deliberately enlarging their floor slab to the edge of their excavation. It does make getting the outer starter bars in easier, but it also costs extra concrete and waterproofing admixture. And the inner starter bars are still tangled in welded mesh. If you tie your starter bars in before you put up your edge shutter, you will find it much more difficult to tie them correctly and in straight lines. You will probably go back and cut some ties to move some of the bars.

A few years ago, I made this job easier by laying the bottom mesh to the edge, then U bars around the edge to keep the first toblerones far enough away to be out of the way of starter bars, Then two lines of straight 10mm bar. These 10mm bars are set and tied exactly where the starter bars tied to them will be exactly correct. A string line to set the 10mm bars then take the string away so no one trips over it any more. The top of the U bar added to the 10mm straight bars have replaced the top layer of mesh under the wall. the extension of the U bar will overlap satisfactorily with the top layer of mesh when it arrives later. Then, and only then, fix the starter bars. You can see that you now have enough space to use the tying machine. Recently, I specified the U bars to be 500mm long underneath to overlap with mesh already there, and 800mm long on top to overlap with the mesh yet to arrive. All 12mm diameter. This photo is very much during setting up a corner. I prefer to tie my setup bars with tying wire from the rebar supplier and nips. Because I can tie tighter than a machine. Setting up corners needs to include a corner bar at the top. And diagonals, that need not be in the floor concrete when it arrives, as well. 3 pairs of hands can be helpful at this time. Then a straight bar at the top connecting corners with a few starters along the way to prevent sagging. The remainder of this wall steel can now be fixed by machine.

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.

Flying End welded mesh. Usually A393, which has 10mm bars in both directions on 200mm centres in both directions. 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)

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. 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. Shape 1 is also known as shape 00. 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 Some engineers are fond of B1131. Where I see it, I try to get it changed. 126kgs is too heavy to carry around walking on mud then treacherous mesh not tied down yet. In one direction you have 1131 square mms of steel per metre. But in the other direction only 252. Many engineers who specify B1131 don't say which direction they want the stronger bars. I seriously think they don't know that B1131 is different one way to the other. The 8mm wire in one direction is never enough to control cracking on its own. If 12mm bars on 100mm centres are genuinely necessary - which I seriously doubt - then buy loose bar and mark the floor like in the photo at the top. Tie a few bottom bars as they have. Put the other bottom bars you need in between on the floor. Tie some top bars the other way. Pick the bottom bars up and tie them all to top bars. Tie the rest of the top bars. You need a tie at least every 600mm. When you get the hang of what you are doing, make your ties along diagonal lines 600mm apart. 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 Back to the Basement Building Questions Answered menu. Back to the Basement Building Construction Manual menu. For a fixed fee of £199 I will answer all your questions by email. More details here. Back one page Forward to next page Forward two pages Clay heave board Which concrete pump Buy waterproof concrete