Casting concrete

Anybody have any experience of casting small items from concrete?

I'm not getting into gnomes, but I want to start making what I'd call a 'padstone' (not sure if thats the right name).

Essentially it would be about 10" square x 2" thick with a chamfer on all 4 sides to leave a 3" square at the top.

The application is for 3" posts to support decking joists. Can't find a ready made one anywhere.

I'm thinking an 18mm plywood mould to cast maybe 4 at a time, using silicone spray as a release agent.

I saw on The Salvager a while ago Rico making something out of cast concrete & he used an orbital sander to vibrate the moulds & get the air out of the mix - I guess this would make a stronger product?

Any idea what mix or just use pre mixed bags?

Reply to
The Medway Handyman
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Better making the widest part at the top and having the chamfer moulded into the bottom...also a 10 inch square chamfering to 3 inches is way OTT, I'd suggest 150mm to 75mm over a 100mm thickness

make one or two every day as they need a few days out of the mould to achieve full strength, and I'd use a strongish handmade mix, IE six gravel (10mm is best), 3 grit sand and 1 cement, quite wet and a few taps with a hammer on all sides of the mould is more than enough, but this is where the trouble starts - most woodedn moulds end up getting knocked to pieces after a few goes - I've heard of people using cling film as a barrier, but it does leave fold marks in the finished product, but if they are below decking, who cares?

Reply to
Phil L

I cant have the thing too thick, thatdestroys the object of the excercise. The two options I currently use are (1) a concrete block. Cheap & strong but too thick for many applications so a square hole has to be dug into the ground, right next to a joist. (2) a paving slab, three times the price and too big to level easily - also split easily if not flat.

Take your point I could make them smaller, but 2" is the maximum thickness.

Would that be what they call sharp sand?

Cling film sounds good, appearance doesn't matter.

Thanks

Reply to
The Medway Handyman

How about monoblocks?

Reply to
Jeremy Nicoll - news posts

Yes I know how to - and can do it Dave, it's that old apprenticeship I did that gave me the knowledge.

Simply think in reverse and how to make the shutter box easy to dismantle - if you can do that.

BTW - silicone is not a good release oil - there are specific oils for the job that don't react with the concrete.

Again, if you are going to make a re-usable shutter box (or boxes), think of how you can make the box[es] with a non-stick lining - it really is simple. I'll give you a clue - think of an everyday laminate!

Oh, and you don't need an orbital sander as a makeshift vibrator on boxes that size (you can become overly reliant on power tools), as simple hammer will get the 'fat' around (rather than get the air out) - and it is the dry mix of the concrete with the correct amount of added water that gives it its strength (too much or too little water makes the stuff weak) - along with steel reinforcing bars as necessary (not that you'll need bars here as long as the load is in compression rather than tension).

As for "pre-mixed bags" - if you got the money to burn then go ahead, but why not just hire a ready-mix wagon seeing as you can add the costs to the job for the customer to pay?.

That's the problem with some "handymen" Dave, if they can't buy it, they don't know how to make it - that's what 3 months of 'research' doesn't teach you.

BTW, would you know how to shutter up a sewer inspection chamber to fit the manhole cover on *and* make the shuttering easy to release and pull out through a limited opening?

Yes there is a great deal of sarcasm here Dave, along with a lot of 'tips' if you can rattle that brain of yours and work them out (not to difficult btw).

Tanner-'op

Reply to
Tanner-'op

Thickness is OK, but I want more surface area to spread the load. A 8" x 4" monoblock is 32 sq ins, wheras an 18 x 9 concrete block is 162 sq ins. My planned 10 x 10 pad is 100 sq ins - a large enogh area.

Thanks though.

Reply to
The Medway Handyman

I've always liked polythene as a release agent. There's no mess, no chance of failing to coat it somewhere, it gives a perfect smooth finish, it can be used repeatedly, and no matter where you are you can put your hands on some immediately.

Downside is it wrinkles and gets trapped in the concrete surface if you dont take the time/care to do it properly, ie to line each mould piece before assembly.

In your case though I dont see the wrinkling being an issue, so you could use a bit of binbag for each mould lining, treating it as disposable.

Simplest way to eject the green form is to give the mould slightly tapered sides. That way a very little movement sees the sides come away from the product, making clean release easy.

However, why make a bunch of moulds if you can buy them ready made. Rectangular polythene food storage containers come in the size you want, eliminating the mould work, long as you dont mind rounded corners. Cut the mould down to 2" height & fill to the brim. Now if you put a flat sheet on it and invert, and remove the mould as soon as it sets. I'd still line the mould for easy release, as any difficulty would see the mould bend and break the workpiece.

1:3:5 is the classic strong mix ratio.

Been some while since I did any of this, you've been warned :).

NT

Reply to
meow2222

Thank you!

Reply to
The Medway Handyman

Best ask the Mrs first though, before taking the hacksaw to her best Lock'n'Lock

Owain

Reply to
Owain

You really are a patronising prick.

Reply to
Andy Dingley

Agree 100%.

The most impressive thing about Dave is his willingness to learn.

The second most impressive thing is that he is prepared to admit that he doesn't know everything. This displays an element of realism - such modesty does not appear to afflict many time-served builders, who profess to know everything but often fall a very long way short.

Reply to
Bruce

Just one thing I didnt mention first time... these moulds are nowhere near rigid enough to use the 'normal' way up. If used that way they will bend the green form at ejection time, ruining it. They need to be filled and inverted, then lifted off the green form when its ready.

NT

Reply to
meow2222

In early 90's I made some concrete slabs to replace some sandstone flags for the entrance to a charity hut. Due to the amount of traffic the original sandstone flags wore out quite quickly. We cast our own as couldn't find anything in suitable finish, affordable cost and suitable size.

We made moulds out of scrap timber, placed a layer of sand in bottom, flattened in a wavy/lumpy/not flat pattern using a spoon, so cast surface wouldn't be flat, lined with 500 gauge polythene and used strong mix conventional cement mixed with small (5mm ?) aggregate. Used a Hilti SDS drill on "hammer lock" to vibrate out air and liberally spray cement everywhere before moving to a less powerfull Bosch SDS.

First passes weren't very good, we put sloping edges on allow easy mould release but the sloping edges causes issues with filling the joints, it was felt the cement would just knock out. Also despite being quite thick (inch odd) one broke when first laid down an jumped on. Also as stones were visible on the surface water penetration and frost damage was likely.

Anyway 2nd pass, which are still laid today the following changes were made:

- Sides were vertical and mould unclipped to release blocks.

- A layer of cement with no aggregate was poured into the mould first to provide a water resistant surface.

- This first layer was allowed to go off slightly before 4 primed steel reinforcing bars were added.

- Topped up with aggregate cement to suitable depth.

Was it all worth it, probably not, took ages as we only had 3 moulds and blocks were ruined by taking them out too early. Also a pain to lay as each block was a slightly different thickness and required extensive fiddling to get level. Also when they had to be cut (as someone decided to edge with a line of engineering bricks) there was the issue of steel reiniforcing to cut and protect from rust. Anyway had great fun and learnt a lot.

Reply to
Ian_m

We had a different issue using home made slabs. It turned out they werent quite perfectly square, very close but over several slabs it started to add up. With commercial ones the edges can be used for alignment, but you cant necessarily count on that with home mades. Hence the order of laying needs a little more thought, and aligning the slabs can be a bit slower.

There were none of the troubles you mentioned Ian, I wonder if it was perhaps because they'd been left standing a month or so to cure. They lasted well.

NT

Reply to
meow2222

messagenews:QXRwk.54440$ snipped-for-privacy@text.news.virginmedia.com...

I made a slab by "accident", when I was testing mixes for my thin concrete soundproofing wall. It was sharp sand, cement and glass-fibre strands. The result was virtually indestructable. It was poured onto polythene- lined cement board resulting in a perfectly flat surface. Wasn't vibrated more than just jiggling the mould by hand. Simon.

Reply to
sm_jamieson

Have you thought of using silicone rubber for the mould?

Or a vinyl rubber?

Reply to
Rod

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