Concrete mix to make ornamental block.

Postby tonyjeffs » Tue Jul 22, 2008 7:18 am

I'm making some exposed aggregate concrete blocks to match and repair a 70 year old garden wall.

I'm not doing too badly getting there by trial and error.
-Using a 1-2-5 mix with a small amount of plasticiser seems best, but is grainy and doesn't flow, so it's not great to work with, and voids result if I'm not careful. Any suggestions on a better mix? Is it better relatively dry or should I keep it wetter to make it more workable and flowing?

I use a stiff brush after about 10 hours to brush the outside of the blocks , and then hose down to expose the aggregate finish. Does that technique and time seem about right, or is there a better way?

They arent very strong when I take them out of the formwork, but hopefully they'll acquire more strenght over a few days, or would a different mix be stronger?

I'm going to try adding a bit of soil (!) to the mix in the hope that they'll look a bit older. Might this work?

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Simply Build It

Postby Freeman » Wed Jul 23, 2008 8:18 pm

Are you vibrating the mix once in the former? tapping the sides should be sufficient, this will help you to avoid the voids.

Once they are laid, paint them with watered down yoghurt, this speeds up the ageing process.
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Postby tonyjeffs » Sun Aug 17, 2008 5:49 am

Thanks for tip. Sorry for delay - I lost track of the forum.

Your advice works well, and I seem at last to have got the knack. I've also gone for a wetter mix that'll flow a little.
I haven't got to the yoghurt part yet, though!

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Postby tonyjeffs » Wed Sep 10, 2008 10:42 am

That tapping technique works really well. Y' can see the level in the mould drop slightly and occasionally the bubbles of air escaping.

I have another question - the variable unpredictable quality of my concrete.
Sometimes the blocks I make turn out good and strong, but my latest batch are pretty chalky, and I don't know why. After 24 hours of a 1:2:4 mix, they're still soft. I appreciate it takes days for concrete to cure properly, but I thought they ought to be stronger than that. I've started adding a bit more water so that it flows more easily into the mould- perhaps that's the problem -would it weaken the product? Or is it just the cooler weather slowing the process down? Perhaps if I add pva it might help?

THe blocks have a 1cm square lip at the front which overhangs the block below - this is the bit that needs to be strong, else it'll snap off.


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