Hi, I plan to cast a thin concrete slab 1.5cmX140X60 where I will attach LED downlights over my kitchen counter. I intend for the concrete LED fixture to be mounted/hung about 3cm from the ceiling. The ceiling is essentially concrete, but has been framed with approximately 3.2cm (2cm wooden stud, 1.2cm drywall). I initially thought about placing the LED lights directly in the ceiling, but I think it looks nice both because I have a concrete counter and because it creates a nice shadow effect on the ceiling.
How do I attach this slab in the best way? I definitely don't want the slab to fall on my head while I'm cooking. Visually, I want to avoid fixtures on the underside of the slab, as the LED fixture will look cleaner and more attractive this way. But is that unavoidable?
How do I attach this slab in the best way? I definitely don't want the slab to fall on my head while I'm cooking. Visually, I want to avoid fixtures on the underside of the slab, as the LED fixture will look cleaner and more attractive this way. But is that unavoidable?
Concrete has a density of about 2300 g/dm3, so your slab will weigh 0.15 * 16 * 6 = 33 kg. So you won't need any massive fittings. I would have cast some form of hooks into the reinforcement in the slab when you cast it, and then hung the hooks in 6 concrete expanders. With more attachment points, you avoid having the slab deformed by its own weight.
Thank you for the quick response! Yes, plus the holes for the 6 LED lights, it will weigh even less... Hooks sound like the best solution. The best might be a hook with hooks on both sides, one that hooks into the reinforcement and the other to the concrete expanders? Should the expander have a loop like this or how should the expander look?
I think it would be excellent with 6 expander hooks, or the expander loops you linked to, in the ceiling and then S-hook which you clamp around the reinforcement with pliers and cast into the board. If you place everything correctly, you can then hang the S-hooks in the expander hooks.
Going to buy building materials and started thinking about what type of concrete might be best for such a thin slab? According to the manual from the LED lights, the slab can be a maximum of 15mm, so I will probably cast about 13mm.
Got a response from an expert at betong.se:
Common dry-bagged Finbetong often has stone size up to about 6 mm. It is usually recommended for this to have a thickness down to 20 mm, but it often works well down to 15 mm as well. For thinner cross-sections, one should choose a "concrete" with smaller stones, and possibly slightly higher strength. There is, for example, dry-bagged Reparationsbetong which has stone size up to 4 mm, allowing for a casting thickness of about 10 mm. Reparationsbetong generally also has slightly higher strength (about C40/50) than standard concrete (about C30/37).
Furthermore, I recommend that you reinforce your slab in some way. Either by embedding a lighter reinforcement mesh (or "chicken wire", plaster mesh, etc.) in the middle of the cross-section, or by using fibers in the concrete. There is Reparationsbetong that contains fibers, but not standard concrete as far as I know. If you can't get fiber-reinforced products, you can add the fibers yourself (suggested plastic fibers). However, it can be difficult as a private person to get hold of fibers for concrete, but you can check with a nearby factory concrete supplier.
The cover layer for the reinforcement should be at least equivalent to the diameter of the reinforcement, and preferably larger than the largest stone size. This is to ensure adhesion to the reinforcement and reduce the risk of the reinforcement "showing through" the surface.
Another option would be to use floor leveling/putty, which often comes in fiber-reinforced forms. However, I feel that you lose a bit of the "concrete feel" with these floor products.
Common dry-bagged Finbetong often has stone size up to about 6 mm. It is usually recommended for this to have a thickness down to 20 mm, but it often works well down to 15 mm as well. For thinner cross-sections, one should choose a "concrete" with smaller stones, and possibly slightly higher strength. There is, for example, dry-bagged Reparationsbetong which has stone size up to 4 mm, allowing for a casting thickness of about 10 mm. Reparationsbetong generally also has slightly higher strength (about C40/50) than standard concrete (about C30/37).
Furthermore, I recommend that you reinforce your slab in some way. Either by embedding a lighter reinforcement mesh (or "chicken wire", plaster mesh, etc.) in the middle of the cross-section, or by using fibers in the concrete. There is Reparationsbetong that contains fibers, but not standard concrete as far as I know. If you can't get fiber-reinforced products, you can add the fibers yourself (suggested plastic fibers). However, it can be difficult as a private person to get hold of fibers for concrete, but you can check with a nearby factory concrete supplier.
The cover layer for the reinforcement should be at least equivalent to the diameter of the reinforcement, and preferably larger than the largest stone size. This is to ensure adhesion to the reinforcement and reduce the risk of the reinforcement "showing through" the surface.
Another option would be to use floor leveling/putty, which often comes in fiber-reinforced forms. However, I feel that you lose a bit of the "concrete feel" with these floor products.
Finjas Reparationsbetong contains fiber and is easy to obtain. I agree with the expert that a mesh in the middle is not a bad idea, also consider making the mounting holes during casting as it can be a bit tricky to drill them afterward in such a thin slab. If you plan to embed fastening materials, attach them to the reinforcement mesh. Consider pouring a bit thicker if you plan to sand the slab later. If you don't intend to sand, choose material for the mold that corresponds to the surface finish you want.