Concrete mold for countertop that bulges

[billyby]

Hi! I'm casting a kitchen countertop and was planning to cast tomorrow. But I notice now that the entire mold is bending a bit in the middle. Does anyone have tips on how to straighten it out? The formply was quite warped when I bought it, but the seller said it would even out when I brace it. The problem is that since the countertop is exactly 250, I had to brace outside the plywood.

 

[epw]

[image] [image] [image] Hi! I'm casting a kitchen countertop and was planning to cast tomorrow. But now I notice that the entire mold is bowing a bit in the middle. Does anyone have tips on how to straighten it out? The form ply was quite warped when I bought it, but the seller said it would even out when I braced it. The problem is that since the countertop is exactly 250, I've had to brace outside the ply.

Try screwing a brace to the ply from underneath to straighten the bottom? This should be complemented with more braces before casting so it doesn't start to sag from the weight of the concrete.

 

[myrstack]

What happens to the warping when you load the shape in the same way that the concrete will do?

 

[epw]

What happens to the bending when you load the mold in the same way as the concrete will?

Well, that's just a guess. Generally, you want everything to be perfect when casting a countertop. Are you planning to flip the mold when it's done?

 

[atomlab]

Of course, you must have a solid base. I would have screwed battens on the underside at cc40 and then placed it on the floor.

 

[billyby]

Test to screw a batten into the plywood from underneath to straighten the bottom? This should be supplemented with more battens before casting so it doesn't start to bow from the weight of the concrete.

Thanks for the response! How do I do that without making holes in the form itself if I screw battens from underneath? There won't be much grip if I only screw in one centimeter (the plywood is 12mm).

 

[billyby]

You obviously need a sturdy bottom. I would have screwed battens on the underside at cc40 and then placed it on the floor.

Thank you! Not entirely sure about cc40. Is it 4cm between each screw? And same question as above. What screws should I use to get a good grip on the mere 12mm I have on the plywood so I don't puncture the surface I'm going to cast on?

 

[billyby]

Yes, that's just a guess. Generally, you want everything to be perfect when casting a countertop. Are you going to flip the mold when it's finished?

Yes, exactly, it will be flipped.

 

[epw]

Thanks for the response! How do I do it so that I don't make a hole in the actual form if I screw beams from underneath? There won't be much grip if I only screw in a centimeter (the plywood is 12mm).

If, for example, you screw in a beam that is 45 mm, you can use a 55 mm screw and make sure not to drive it into the wood but stop just before so the plywood doesn't crack or bulge out. Cc40 means you should place beams with a 40 cm distance from the center of one beam to the center of the other. So, 35.5 cm between the beams if you use 45mm wood.

 

[EddieHansson]

But isn't it enough to screw in the edges? It should support the board, so it's enough if the rule is at the edge.

 

[billyby]

If, for example, you screw a beam that is 45 mm, you can use a screw that is 55 mm and make sure not to drive it into the wood but stop just before so the plywood doesn't crack or bulge. Cc40 means that you should place beams with 40 cm from the center of one beam to the center of the other. So it's 35.5 cm between the beams if you are using 45 mm wood.

Thanks for the great explanation! Yes, then I really think it should be enough. I'll update tomorrow on how it goes.

 

[billyby]

But isn't it enough to screw in the edges? It's supposed to support the board, so it's enough if the rule is at the edge.

Do you mean on the underside? Because on the topside, I already have rules at the edges as you can see.

 

[Anders_sbj]

How thick will the slab be? Reinforcement?

What will you treat it with? I have used stenfix 50 with very good results, coffee or wine does not stain the slab. However, I have sanded down the slabs so you can see all the different stones, there's a thin layer of concrete that easily absorbs everything, but that's often what people perceive as the concrete look. So taste is like the backside. Regards, Anders

 

[billyby]

How thick will the slab be? Reinforcement?

What will you treat it with? I have used stone fix 50 with very good results, coffee or wine doesn't set into the slab. However, I have sanded down the slabs so you can see all the different stones, there's a thin layer of concrete that easily absorbs everything, but that's often what people perceive as the concrete look. So taste is like a backside. Regards Anders

Thanks for the tips! The slab will be 3cm, the plan is to lay double reinforcement at the sink/range to prevent it from cracking. I will then sand down with a wet concrete grinder to reveal the stones as you say. Therefore, I'm using coarse concrete.

 

[billyby]

What do you think about this screw? Someone I talked to at Bauhaus didn't believe in the idea of screwing because 12mm is too little to go on. He suggested I use pl400 on the beams and put weights on. Opinions?