Wanting to make a concrete block, 140x100x30mm. Will be making a wooden mold (from old shelving so have plastic laminated coating), plus some veg oil as a ghetto release agent.

1. I would like a relatively good surface finish – so should I try and use a finer aggregate (ie no gravel/stones, just sand)?
Rather than getting a mix bag of sand/aggregate/cement, would it be better to get some cement, and then just mix with soft building sand such as;
http://www.homebase.co.uk/en/homebaseuk/building-sand—maxipack-921236
To get a finer mix? Or will that not be very strong?

2. Want to put 2 or 3 bolts/screws into said concrete block, going into the 30x100mm face (so not very tall). Bolts will probably be 1.5″ long or so. Not a huge amount of load going through them, but want to be able to do the bolts up pretty snug. Are concrete screws straight into the concrete going to be OK? Or plastic plugs? Or do I need to go the whole hog and get fancy concrete expander plugs?

Thanks,
Duane.

I think any kind of bolt or screw into your 30mm face is going to risk cracking in the shortest direction. Could you use some cast-in studding or suchlike.

If you can get the right consistency, then compacting concrete made with normal aggregate to give a smooth finish should work. It might need some practice.

Grano

I was wondering if the 30mm face would be too short. How about burying a block of wood in the concrete, which can be drilled through to and screwed into?

Buy a concrete slab and cut that to size after you’ve drilled your holes.

Trying to make as much myself as possible!

Is grano widely available?

what is the point of this? as in what will its use be?

30mm is really not alot, if you count a screw of dia 6-10mm that doesnt leave much material and concrete is somewhat fragile when that thin..

but it would help to know why you need this to suggest help..

I reckon a headstone place or worktop manufacturer would give you a granite offcut or sell you a small bit like that for about the cost of buying the material. Pay them to put the holes in it too.

Actually, pocketrocket’s suggestion is a good one if you have access to an angle grinder

Headstone manufacturer? Kitchen worktop offcut?

Trying to make as much myself as possible!

Why, concrete pavers are cheap.

Re: grano. Any builders merchant sells it. Mix with cemet and it goes off like bell metal!

It’s going to be used as a base for a desk lamp I have designed.

Get a piece of granite,marble or slate.

Build a little shutter, get a bag of C40 premix from a builders merchant, on your little shutter put supports for the screws / bolts you want to install, place them as required, mix the amount you need and pour.

Fixings will be cast in place, block will be proper heavy and a good finish as it has a very high cement content

the finish will depend on how you work it, for a smooth finish you need to float the surface to bring up the fat in the cement.

Burying the fixings in the cement is a good idea – not sure why I didn’t think of that – will be way simpler than burying a block of wood and screwing into that.

Any thoughts on the best kind of fixings to use? Should I bury them a mm or two into the surface incase they get pulled out any amount when doing the screws up?

Will something like this pull out – do I want something a bit more heavy duty?

http://www.wickes.co.uk/Wickes-Heavy-Duty-Fixers-12x100mm-Pack-6/p/510003

For fixings, I’d get some hex head bolts and cast them in, head down, with the right amount projecting, then put domed nuts on them.

Which surface of your block are you having as the open face? To get a good finish you’ll need to either vibrate the shutter of tamp it thoroughly.

Are you going to try to get sharp corners on the block? It’s hard to do as they are delicate at the stage you’ll want to strip the shutter, and you’ll always get a bit stuck in the corner. If you want a neat edge it’s usual to put a fillet in the shutter to get a chamfer.

Unless you coat the finished block it will always be a bit sandy for a desk.

Get a piece of granite,marble or slate.

Which will look a hell of a lot nicer.

How about casting it in a low temperate alloy instead? Give it a Google.

Will have a think about burying bolts, will be a bit trickier to get the right projection etc rather than just burying a fixture, but will have a think.

Currently bottom of the mold will be the top face of the base – unless its particularly advantageous to have the top face at the top, in which case I’ll make some kind of lid for the mold. I’m after rounded corners for the base, so will either file/sand these, or try put some fillet in the mold.

To seal it I’m thinking just a simple water/pva mix.

Try casting in a threaded insert, then just screw into that afterwards?

Something like these:

http://www.anchorinserts.co.uk/supertorq.php

1. The aggregate in concrete gives it it’s strength. Using only sand would mean it would be weak, not a good finish. Rough angular stones are better that rounded gravels, and ideally you want a mix of sizes, so that the smaller aggregate fits in the gaps between the bigger ones, right down to the fine sand. Think of the cement as glue, you want the thinnest layer possible between the aggregate, no voids with only cement, or worse, air.
To get a good a good finish you need to get all the air put of the concrete while it is wet. On a small piece like this, vibrating the whole of the shuttering is probably the way to go. I’m sure you could use a power tool to get it to vibrate. Once it is good you will see bubbles rising to the surface. Don’t vibrate for too long though, as you can cause the aggregate to separate out with the lightest material rising to the top.
The other thing to do is give it plenty of time to cure. A couple of days at least. Do not let it dry out or freeze while it is curing, or the chemical reaction between the cement and water will stop and it will have a powdery surface.

2. I think setting the bolts into the wet concrete may be best, if a bit of a faff rigging up a way of suspending them while the concrete is beginning to cure. Any sort of expanding bolt or rawl plug is likely to split open a block that small.

I wouldn’t be surprised if it took a few goes to get this to turn out the way you want it.
Please post pictures when it’s done.

Cheers for the info.

You say not to let it freeze or dry out – how do I stop it drying out? Do you recommend just using a standard concrete mix then? Can I use any of the quick drying variants to speed up the process?

Speaking to a mate and he thinks a simple plastic rawl plug in a hole drilled once the concrete is set will be fine – as it will not be taking much load (bit of wood and a light fitting). Do you think the drilling or the expansion of the plug will crack the concrete?

Ta,
Duane.

To stop it drying out while it cures, the easiest thing would be to put the whole thing (still in the mould) in a bucket of water as soon as it’s set solid. And put it back in once you’ve removed the mould. Concrete doesn’t dry, it cures, and the chemical reaction needs water.

If you’re only putting light loads on it, and you let it cure properly (4 weeks) before drilling it, a plastic plug is probably OK. But it’s a risk.

OK, maybe burying a couple bolts it is then!

Will sticking the mold+concrete in a sealed bin liner for the first few days help? When should it be removed from the mold? Would spraying it with water every now and then once it’s out of the mold help?

Will sticking the mold+concrete in a sealed bin liner for the first few days help?

Yes, it will help stop water evaporating.

When should it be removed from the mold?

3 to 7 days, depending how strong the mix is and how warm it’s been kept (warmer is better).

Would spraying it with water every now and then once it’s out of the mold help?

Yes, but putting it in a bucketfull would be better.

While it’s good to keep it wet once it’s set, it’s important not to have too much water in the mix, or it will separate out and be weak and porous. But too little, and it won’t compact properly. Plasticisers like Febmix (washing up liquid is better than nothing) will help reduce the amount of water needed to make it workable, so making it better overall – the lowest water/cement ratio you can get good compaction with is the key to good concrete. More cement gives a stronger mix but provided it’s kept wet while curing is unlikely to have much downside on a small block like yours.

Form work can be removed between 24 & 36 hours after casting. Wrapping it in a damp cloth and bin bag will keep it damp. Spraying and wrapping after the for form work is removed is also a good thing.
Concrete will still be curing after 28 days in the right conditions, but will have most of it’s strength after 7 days.

You might try jesmonite instead of concrete. Look up ac730. Its often used instead of concrete now for decorative and reproduction work.

Let’s be fair here, we ain’t talking concrete 😉