making decorative concrete blocks

[Harry Davis]

I'm planning to design and make some decorative square concrete blocks with holes in.

Well-known patterns include a square suspended inside the outer square; and four quarter circles, one centred on each corner, with radii equal to the block's side-length.

What mix of concrete or screed should I use?

Thanks for any help with this!

Harry

[Harry Davis]

"Phil L" wrote in news:KCkWt.54713$ [e-mail]:

Yep.

I'm grateful for this advice. Sounds like a bit of a challenge, then! Might some plasticiser in the mix be useful? Or something other than a standard cement? I'm not sure whether Mastercrete already contains some.

Ah but I'll be designing my own! :-)

Harry

[Chris J Dixon]

I spotted this comment elsewhere:

"In the70s, it was screen block walling. You could buy moulds for screen blocks in Woolies (TruFact!) and every wannabe builder-gardener was churning out up to two-dozen blocks a week in their back yard, soon to be assembled into what could only be described as a "wall with character".

Rarely did any two blocks have the same colour or texture due to the vagaries of mixing concrete in a 2-gallon bucket (litres hadn't been invented back then), and even more rare was a block with actual 90° corners. Many had bulging sides due to the moulds deforming when filled with the mix, and on completion the whole thing usually resembled something dreamed up by Dali on one of his bad nights.

On my travels, I sometimes see surviving relics of these constructions, defying all laws of nature, and I really ought to photo-document them before they finall succumb to weathering and/or gravity. They are a thing of genuine amazement and awe! :D"

Chris

[Chris Hogg]

We have some of those, made by a previous owner. Fortunately only a short low wall screening the dustbins. From the texture on the outsides, it looks as though he used newspaper as some sort of mould release aid. Several thick coats of masonry paint hide the worst of the imperfections.

[harryagain]

I wouldn't bother. The bought ones are pressed into the mould using a fairly dry mix. Anything you could make would be crap (ie fall to bits pretty toot sweet.)

[Harry Davis]

"Phil L" wrote in news:J3nWt.51839$ [e-mail]:

Click here and tell me whether you still think all screen blocks are ugly:

Maybe you will, but it's a matter of opinion. I think a lot of mid- century modernism ('midmod') is fantastic!

Thanks for this info!

I've noticed in neighbour's walls that some of the blocks look surprisingly sandy, more so than most 1:3 screed.

Harry

[meow2222]

Its a long time since I did this stuff, so memory might be a bit off on som e details...

Think we used a wet 3:1 mix. You need strong & dense.

that surprises me, we did ok, and had no fancy equipment to work with.

There are 2 approaches, a wet mix as we used, that has to stay in the mould till set and cured for days. Factories use a dryish mix and eject from the mould immediately. Dryish mixes need strongly vibrating or pressing. Wet w ants vibration to remove air pockets, but dont overdo it or the mix separat es somewhat.

Cure is slow. You can speed cure a bit by covering with polythene. More hea t & more damp.

not IME. More intricate moulds for screen blocks do take time, so to get an y throughput you need to go with dryish mix and shaker table - not hard to set up but needs doing.

It's far cheaper than buying blocks, but it is work.

You can buy mixes that give a much finer finish, most slabs, blocks etc don t use them.

IIRC we did at least a dozen per day per person. With wet mix the number of moulds is the big limit. Pink ones sold for more back then - bet there are n't many of those still in use!

Any job can be screwed up by incompetence. There's no reason to though. Don 't underestimate the mould accuracy required, 1mm out does show in a final wall, 2mm looks lousy.

They look however you choose them to. You can make crap if you like, or mak e something special. I'd sugest taking time over choosing a block design, a nd making the mould with precision & style. The blocks are, if things go ok , just copies of the mould cavity.

with wet mix, yes. With dry you need to make a tool that presses it out wit hout putting the slightest amount of sideways force on it, as factories do it.

An old orbital sander to vibrate the mould is ok for a wet mix

Polythene is the best release agent, it gives a smoooth finish

One decision to make early on is whether you need 1 or 2 sided blocks.

These days I'd include some chopped mixed synthetic fibre to reduce crack r isk. Shred some carpet or scrap clothing.

The wiki article covers some of this. Look at industrial block/paviour maki ng on youtube, it'll explain much.

NT

[Harry Davis]

[e-mail] wrote in news: [e-mail]:

Many thanks for your very helpful post.

A few follow-up questions if I may!

Would that be sharp sand and mastercrete? (I've been doing a lot of concreting recently, and the main reasons I bought mastercrete were because it comes in plastic bags rather than paper ones, making lifting a bit less hassle, and because I thought that since I'm a bit of a newbie, buying something a bit away from the bottom of the range would compensate for my lack of experience. So far, all has gone well, and I was pleasantly surprised by the whiteness of the final colour, but I digress...)

Did you make your own moulds or buy them?

I might leave the dry mix until later, because at the moment a big throughput isn't required; but for future reference, how do you set up a shaker table?

Buying mixes seems like cheating! What is the difference in the components? White cement? Some kind of different sand?

Is timber the material to use? I've been wondering whether something might be doable with silicone, but it would need not to fall apart after the first few blocks.

Got it; strong in compression, weak in tension.

Thanks for this. Roughly how long do you need to do it for?

Thanks for this too!

And all of this! :-)

Harry

[The Natural Philosopher]

Not everybody is incapable of DIY harry.

Even if you are.

I've successfully cast concrete many times.

[harryagain]

TurNiP, my last DIY concrete job was nearly two hundred tons for my earth shielded house.

[polygonum]

Rather depends on which silicone! Standard bathroom sealer is unlikely to be up to it. This is a link to various silicone rubbers used by, among others, sculptors. The same company also offers other mouldmaking materials.

One of the standard casting techniques used with rubber moulds is to make an original item. Then cover the item with the rubber material to a usable thickness. Then cover that with a substantial layer of plaster - using all sorts of reinforcement such as metal bars and scrim. You therefore end up with a mould that has the fine details, easy release and flexibility of rubber on the inside, but the strength and rigidity of the plaster on the outside.

Provided the design is simple enough you would only need a simple one-part pour-in design - but the same basic technique can be used to make fully 3D objects by designing a multi-part mould - suitably trapped.

[meow2222]

just building sand & cement.

made from 2x4

motor, offset weight on shaft, bolt to underside of table. Table mounted on rubber lumps. It will get wet, so pick an LV motor.

Memory not good here, but iirc both sand & cement are different

its very available, cheap & workable. Tendency to warp is a problem. Do use heavy enough timber to eliminate even tiny tendency to bulge.

I've no experience with rubber moulds. I don't see how one would get sub 1mm accuracy with them

Theres no tensile or compressive strength when its cast. Its simply a case of not moving the sand granules, even slightly.

Pass. We didn't use any type of vibrator machinery.

NT

[Harry Davis]

[e-mail] wrote in news: [e-mail]:

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Many thanks for all of this very useful advice. I'm almost ready to start now!

One last question: am I right that the polythene used as a release agent should be cut into pieces of exactly the same shape as the surfaces inside the mould, with vertical pieces glued, otherwise the block will show folds from the polythene?

Thanks again!

Harry

[meow2222]

We didnt. Its a long time since I did this, I'm pretty sure we just arrang ed the folding to happen somewhere that would be out of sight. Expectations have increased since then. Ideally wrap the poly round each wood member be fore screwing the mould together. Pull it tight a little, but avoid creasin g.

NT

[Harry Davis]

[e-mail] wrote in news: [e-mail]:

Thanks for this. I hadn't even thought of putting the polythene there before screwing the mould together!

Harry