I'm going to cast a countertop that is 3 cm thick and very worried that dark marks where the reinforcement mesh is will show in the finished result as I've seen in many pictures online.
And I'd like tips on technique during the casting rather than products that need to be bought because all the products are already purchased, so that can't be changed. Using fine concrete.
And I'd like tips on technique during the casting rather than products that need to be bought because all the products are already purchased, so that can't be changed. Using fine concrete.
Know-It-All
· Västra Götaland
· 12 300 posts
No idea but:
Why do dark marks appear?
Is it because the mesh is rusting? If so, clear coat or replace with fiberglass?
Can you pour in concrete first, then wait a bit and after a while set the mesh in?
Why do dark marks appear?
Is it because the mesh is rusting? If so, clear coat or replace with fiberglass?
Can you pour in concrete first, then wait a bit and after a while set the mesh in?
Yes, this is what I want to know. I haven't started casting yet myself but want to find out how to prevent that phenomenon before I start.
Use stainless reinforcing mesh. Typically used in bathroom construction.
The marks are not from the reinforcement rusting. It does not rust as the concrete protects it. However, there can be marks from the concrete being "disturbed" when the reinforcement is lowered. I don't have a good answer on how to solve this, but extreme precision with where the mesh is placed (should be in the middle) should help.
But if you have stainless reinforcement, the "discoloration" doesn't happen.V vectrex said:
It is not the rust that causes color variations in the concrete after the mesh has been lowered. It is because the consistency, or whatever you might call it, changes where the mesh has been pressed down. The best thing is probably to fix the mesh at the right height before pouring concrete. Alternatively, fill halfway, lay in the mesh, and fill the rest.
Then you should thoroughly mix the surface layer so that the concrete is completely homogeneous.
Then you should thoroughly mix the surface layer so that the concrete is completely homogeneous.
I have indeed used expanding concrete for the slabs I have cast, but I have made a rig for the reinforcement so I can fill just under half the height first without reinforcement. Then I hang the reinforcement construction and fill the rest of the concrete. I have not gotten any marks from either the reinforcement mesh or the reinforcement bars.
Stainless steel is not necessary and just unnecessary money.
Stainless steel is not necessary and just unnecessary money.
The question is whether or not to compact after you have laid down the reinforcement? There seem to be different theories on that. Hanging the reinforcement becomes difficult since I need to draw a straightedge over the frame.
But it feels like you want the reinforcement as far away from the bench surface as possible, and then it's almost better to place it as close to the top as possible when casting so it ends up as far down as possible when the slab is in place. If reinforcement tracks are visible on the underside of the slab, it doesn't matter.
But it feels like you want the reinforcement as far away from the bench surface as possible, and then it's almost better to place it as close to the top as possible when casting so it ends up as far down as possible when the slab is in place. If reinforcement tracks are visible on the underside of the slab, it doesn't matter.
Had no major issues using the straightedge, it was just a matter of angling "around" the hangings.
The risk you take by placing the reinforcement close to the surface is that it may start to rust and then the concrete will explode. If, as mentioned, you fill half of the mold first, you won't get any marks, and if you also vibrate the first half first, you'll have both belt and braces.
The risk you take by placing the reinforcement close to the surface is that it may start to rust and then the concrete will explode. If, as mentioned, you fill half of the mold first, you won't get any marks, and if you also vibrate the first half first, you'll have both belt and braces.
3 cm thick is a challenge, if you make it thicker it helps a lot, it won't be as sensitive to moving afterwards as well. I have followed this guide from Finja with good results https://bit.ly/2LQXmJm. A change I made, which mexitegel also mentions, is that it's best to pour in half of the concrete without reinforcement and tap out all the air bubbles; this step is what otherwise causes marks on the surface from the reinforcement. Then you add the reinforcement and pour the rest of the concrete, and then you don't touch the concrete except gently at the front to remove the bubbles there.
Trying to find some reinforcement where the bars aren't lying on top of each other so the mesh doesn't become twice as thick. Hard to find...
By the way, another question, if I want a slightly matte surface, not so shiny. Should I then NOT treat the plywood or do I have to do it anyway? Or is the material of the plywood enough to release the slab?
By the way, another question, if I want a slightly matte surface, not so shiny. Should I then NOT treat the plywood or do I have to do it anyway? Or is the material of the plywood enough to release the slab?
You have to treat the form, otherwise, there's a high risk that pieces of concrete will stick to it, not very nice. But it's not the surface of the form plywood that gives the concrete slab its smooth surface, you need to sand the slab's surface once the concrete has hardened to get it smooth (a hell of a hard job). Then again, concrete is porous so depending on what you're going to use it for, you might need to seal the surface with water glass to prevent it from absorbing liquids. There are examples of surface treatment in Finja Betong's guide. I used beeswax, but then the surface becomes a bit shinier.peterwesterlund said:
I was thinking of buying floor reinforcement mesh, but it's too short. Can I buy two and overlap them or is it pointless? Won't it be as stable in that area?
Reinforcement mesh is for crack reinforcement, for "stability" you should use regular reinforcement steel.
To make it durable, use 10 mm or preferably 12 mm reinforcement steel along each long side. Tie down rebar along the short sides and every 20-25 cm. Here you surely realize that 30 mm thickness is too thin
To make it durable, use 10 mm or preferably 12 mm reinforcement steel along each long side. Tie down rebar along the short sides and every 20-25 cm. Here you surely realize that 30 mm thickness is too thin
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