how are stones added to polished concrete slab?

hi all,

id like to lay an outdoor slab that i want to polish - approx 40sq mts. decided to go white concrete with aggregate, f82, 32mpa, 140mm thick with road base and plastic on bottom, with 1 horizonal and 1 vertical saw cut.

just wondering how they scatter say 20-25mm white pebbles or quartz on surface - do they randomly throw them on top and the smooth trowel or do they only push them in say 3/4 way and then grind smooth afterwards?

also, what type of surface finish should i be asking the concretor to do?

cheers,

b

I havent done it myself but from what I have seen is they seed the top after screeding with the pebbles and float in, hen grind when set, or they get a pebble included into the mix which takes the place of the normal aggregate component. You can get quartz added to the mix, just might be extra $$

It is definitely added into the mix at the concrete plant and your concretor takes it from there. Have seen river stone thrown in the mix instead of quartz.

Never seen what you suggest done outdoors. And polished concrete would be inadvisable because slippery when wet. When done indoors, marble (not quartz) is added to the mix, and the product is called "terrazzo." Quartz would likely beat up the grinder too much, and wouldn't add anything to the effect. Around 1963, we did an outdoor job in precast architectural concrete with quartz for wall panels; used belt sanders and went through belts with large garnet(?) chips as if someone else was paying for them. Finish wasn't very smooth at that.

For outdoor use, exposed aggregate (including quartz) can be scattered shortly after initial set, or included in the mix with a surface retarder to enable brushing away the matrix. Your concretor should be familiar with both these processes; if not, better talk to someone else. INDOOR terrazzo is very much a specialty job, which the concretor may not include in their repertoire anyway.

Joe

You can also simply polish the top off already laid conrete mate did it a couple of years ago in his dining room slab was laid 20 years ago

Yeah i've had that done a few years ago, worked really well, Got a new house tho now with timber floors.

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just wondering how they scatter say 20-25mm white pebbles or quartz on surface - do they randomly throw them on top and the smooth trowel or do they only push them in say 3/4 way and then grind smooth afterwards?

also, what type of surface finish should i be asking the concretor to do?

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The aggregate is in the mix. You'll have to grind the surface back to the stone.

Theres more to it than but id end up in concrete shoes:oo:

the agrigrate is included in both polished floor and exposed agrigrate mixes.

exposed agrigrate is done verry simpley the slab is layed and the top is hosed off while still wet to expose the pebles.

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exposed agrigrate is done verry simpley the slab is layed and the top is hosed off while still wet to expose the pebles.

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Thats washed or exposed concrete :)

Autogenous has it right. The "pro" mixes just substitute small river pebbles or similar for blue metal. If you're doing it yourself don't want to do a thick pour though - mebbe only a couple of inches thick - as the larger stones tend to settle to the bottom and you only end up with small agg at the top. (There are tricks to minimise this, though.)

For exposed agg finishes, if you don't like the idea of blowing holes where you don't want 'em, try sprinkling sugar on the top while it's still wet. (no joke!)

The sugar delays the setting of the surface while the sub-surface sets normally. Don't ask me how or why. I dunno, but it works... it makes it a bit like licking the cream off a jelly - pretty damned simple, provided you get the timing right. :; Get the timing wrong and, well... :shrug: ...that's why it costs so much extra: knowing the when as well as the how. :U

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Th e sugar delays the setting of the surface while the sub-surface sets normally.

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Its a retarder.But I wouldn't applicate it in that manner. I think it also destroys to some extent the cement.

Agreed... I definitely wouldn't use it on anything structural.

However, we paved around a house about 3 or 4 years back with an exposed agg finish (all mixed by hand. [groan]) and it has stood the test of time fairly well. Only the occasional loose pebble, which I'd have expected anyway.

Mind you, the thing about that sort of paving is that it's usually bordered by lawn and, of course, the householder usually wants that lawn to stay nice and green and, well... growing. Now, the final step in an exposed finish is washing away the top surface, right? It doesn't take much to realise that this makes a serious mess of things like... lawns. :roll:

I've done it once. Never again. I'll pay some other silly bugger to do it next time. If there ever is a next time! :U

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Originally Posted by autogenous

Its a retarder.But I wouldn't applicate it in that manner. I think it also destroys to some extent the cement.

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I'm no concreter but we have laid 130m2 of exposed aggregate and have not had any wear/structural issues. We didn't sprinkle with sugar, rather disolve the sugar in warm water and use a weed sprayer to apply when you could walk on the concrete, just after final trowel. Exposed using garden hose and looking at other examples throughout my area who used chemical/high pressure and even sandblasting to expose and there are none better. Pic attached.

PS. As Skew said, it makes a massive mess removing the top layer of slurry, only advised if you haven't completed your garden!

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Buggermedumplings:: and have not had any wear/structural issues.

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My only concern was that people may see the term retarder then start mixing sugar with concrete thinking that they had a retardant suitable for slowing the total set of the concrete down.

The manner in which sugar is used for washed concrete is to retard the surface layer to remove that layer. The cement that sits at the surface to be removed not only retards, I think sugar renders the cement useless which is good because it is removed.

If for some reason someone were to mix the sugar with the concrete trying to slow the concrete down then there is a possibility they risk poor structural integrity of the product.:)

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However, in some cases when the dosage of admixtures exceeds a certain critical point, hydration of cement compounds will never proceed beyond a certain stage, and the cement paste will never set. Thus, it is important to avoid overdosing a concrete with a retarding admixture.

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Understood Autogenous- Can't say I've ever tried that or would want too.