Casting concrete ( about 2 square feet)

[Tigercubrider]

I want to cast a shape in concrete. It will be about 12" square and 24" long

My idea is to make a mold in (building?) sand - wetted or possibly mixed with a tiny bit of cement to help hold the shape. I am ok about the top being rough- just the sides and bottom (of the mold) need to be "ok". ..... line with tinfoil or polythene?

A decent finish would be preferable on the outside, I can add a rougher mix inside (or not) if it helps with strength. I'd add a bit of mesh or rebar into the centre of the mix, so mix a layer plastered on the sand surface and followed by the rougher mix while still wet?

Once full, I can leave the mold to dry for a week/2 ? covered with poly? Covered with wet sacking? I believe that spraying concrete after taking it from the mold would make it harder?
How long? wrap it in poly?

Thanks

 

[Burnerman]

Why not make your mould in timber? Priming and painting will give you an excellent finish and no need to line with polythene or foil (foil would tear and stick for sure).
Fixing the mould with screws makes for simple dismantling and reuse if necessary.
John

 

[JohnD]

you don't need to paint wooden formwork, but the grain and knots will show if bare. As it dries and shrinks, it will come loose. if it is just a slab or foot you are casting, you don't even have to cut the boards precisely. Formwork has to be strong enough to withstand your compaction. You can trowel the top for neatness. If it is outdoors, dome or slope it enough for rain to run off.

The concrete will gain most of its strength if you keep it literally wet for at least two weeks. For your slab, a watering can and some plastic sheet or a binliner weighted down around the perimeter will do fine.

if you can keep it wet for 28 days it will reach almost the maximum strength and hardness. The curing process gets slower over time. It never actually stops, but after 1 year is only 30% harder than after 28 days. Once it has dried out it will not restart. Underground concrete, in tunnels, fenceposts in wet ground, or around drains, that never dries out, will be noticeably hard and strong after being wet for 50 or a hundred years.

7-day concrete has very little strength, less than half its potential (see graph below)
In hot sunny weather, pour it after the sun has passed, so it will not be exposed to hot sunlight until the following day, when it will be covered. Drizzle is very good for new concrete.

if it is an object you intend to lift up, leave it for at least a couple of weeks to reduce the risk of it breaking.

 

[Burnerman]

Painting gives a much better finish - which is what Tigercub wants, I think? Maybe if he tells us what it's for we can advise more.
John

 

[JohnD]

Or a bit of scrap laminated chipboard, if it's to hand? Very smooth face.

I think it's for a headstone.

 

[Burnerman]

I did some experimenting with 6th form pupils when I was teaching - the design brief was to create bird baths (plus pedestal) in concrete.
We found that if we used OSB the marks of the strands remained visible, the same applied when we used newspaper. Polythene seemed to wrinkle up even it seemed to be flat when we laid it down
We didn't have any laminated materials available which I agree could have been better.
When we tried to give more interest to the pedestals by adding timber mouldings the cement mix often broke off even though the edges were tapered so we had to use a paint and primer to act as a release agent - even better when we applied a thin layer of oil brushed on.
By using a plastic pipe core we could make the structure considerably lighter and it was strong enough - PVA glue added to the mix, together with the inevitable Fairy Liquid was the answer there. We didn't use any rebar but chicken wire mesh was good - until it too came to the surface
John

 

[Tigercubrider]

Ok.
So.....
I want to "cast" a petrol tank in something solid enough to hit.
I'm not too worried about the finish as it will have a covering.
The bottom of the tank is flat, it's the curve of the top and back to front taper that I want to capture.

I have a tank (second hand) that is too big and want to cut the bottom off, fit it over the casting, and hit it so that it deforms to the shape. It's aluminium so should shape reasonably. Once it fits, I can get the bottom welded back
If I can't hit it on the concrete, at least I can fit/remove/bash/refit and so on

I could carve the shape out of wood. That would be a lot more time and effort to create.
Although I do know where I can get a massive log from.

 

[JohnD]

I've known metal shaped that way using hydraulic pressure

Some kind of roller might be better than hitting it.

You will need the concrete to have maximum strength and hardness then, as there is a fair risk it will break.

You want to shape it like the inside of the tank, and form sheet metal round it?

Is it an exposed tank, like on a Morgan or something? Or a 1920's motorbike? Could you do it copper, which can be soldered and is quite soft if annealed?

 

[Tigercubrider]

It's a triumph hurricane replica. I have the fibreglass, and a trials aluminium tank that's part way there.
It needs to fit inside the f-g

Normally timber bucks are made but for a one off (maybe a few) it's quicker to use something I can cast
I am doing this very low tech. You can dish aluminium by using a tree trunk with a depression cut into it. If you cut the hole "avocado" shaped, you can vary the curves.

Inside the fibreglass unit

 

[JohnD]

one of these tanks?

i can't visualise what you're doing.

 

[Burnerman]

I think concrete is the last material you should be considering - how would you sculpt the negative shape in it?
Maybe you'd get somewhere with an english wheel, but bossing aluminium is fraught with problems - constant annealing being necessary.
Perhaps you could show us what you have already? I'm pretty unclear about this one.
Triumph Trident motor above?
John

 

[lostinthelight]

If you want a lump of wood go on arbtalk and see if anyone can help.
https://arbtalk.co.uk/forums/

 

[Tigercubrider]

I have the fibreglass unit, but not the aluminium tank.
I have a similar tank, but it's wrong in a number of places.
So I need to reshape it.

To make the cast, I was going to press the fibreglass into something that would leave an impression.
Then I can add something that will allow for the thickness of the material (3-4mm)
I'm trying to this at home but if I can get back to work soon I have access to all sorts of sculpting tools and materials.

Another option is to scrap the aluminium idea and make the whole thing out of Ethanol safe fibreglass but that's not easy either and aluminium is my favourite.

 

[Tigercubrider]

This is a bike that was in a fire. Trying to replicate the tank that you see. The cover (melted) is what I have.
I have the "shoe", I'm trying to make the "foot"

 

[Tigercubrider]

Out of this