Exposed aggregate concrete

[r896neo]

So i am going to have a wee go at some exposed agg at home. Just a few tests to start but need some advice.

Once tamped how do you finish the concrete before applying the retarder?

Is a once over with steel trowel as you tamp good enough or should it be trowelled heavily and polished up a bit?

Also i imagine directions will be on the retarder but any timing tips appreciated

[Tony McC]

It depends on the coarse agg. Some need just one or two strokes from the float to smooth off the surface but some of the coarser aggs (and I'm thinking of the slate waste we used in N.Wales) need more of a polish to force down the agg and bring enough fat to the surface so that, once it's exposed, there's sufficient contrast/relief.

The retarder is trial and error at first but very, very, very generally speaking, 250ml per m² is usually there or thereabouts.

[Pablo]

Hi Simon, Give Sam a call at RTU they have quite a few sample panels in their yard in Whiteabbey. They also sell all the sealants and retarders but Larsen do a few too. He's done a few special recipes for me over the years on top of their standard stuff. I would never mix it myself unless it's a tiny area it never looks the same as the batched stuff.

[r896neo]

Thanks, I spoke to sam just before christmas about it and he basically said they are concentrating on drives and larger volumes for a few months until it gets out there a bit more. Its more the techniques i wanted to try rather than the finished product.

If i was laying it i would certainly be buying it in.

He was a bit cagey about prices for the 'exposa' for some reason as i was asking about minimum quantities etc. the what sort of money is it?

[TheRockConcreting]

Once tamped how do you finish the concrete before applying the retarder?

If you were to tamp exposed agg you would end up with a very sandy finish. To lay exposed agg, you gonna first need to learn how to screed concrete, this will take years to master.

To clarify what screeding is, as it is very misunderstood in england, its better known as free hand screeding, where the concreter uses an alloy straight edge of various different lengths, to work the surface of the concrete without ridding on top of any form works.

How do you get the heights?
This is done by either a laser level or string line and chocs, and using a magie to make the height pad.

Once you've mastered that then your gonna need to bullfloat the surface with a magnesium float not steel!

As for when to retard the surface, the right time is once the surface is flat, any lines, bumps or dips will be reflected in the agg below, this is where most get it wrong, simply because they can't see the effect till its to late.

[mickavalon]

Jay
Really rate exposed aggregate as a product, we've done a few sections for clients over the years, mainly pathways in Gardens and steps etc, but would really like to add it too the companies repertoire. Did you undertake training in techniques or just previous experience? We're based in the West midlands, and well away from giving you any competition.

[Tony McC]

It used to be part of the apprenticeship scheme for Concrete Finishing. I learnt it from a mate who was serving his time as a CF when I was serving my time as a streemason. I would never consider meself an expert, but what limited skills I did develop, I did through practice, practice and more practice.

Sadly, too much concrete training nowadays comprises little more than watching a couple of videos, practising on one square metre, then buying 500 quid's worth of kit from the 'training provider'.

If there is a more fickle mistress than concrete in the building trade, I've yet to meet her, and handling her is not summat you can learn in a half-day course.

If you have a yard or some space you can use, retarding and exposing concrete is summat you *can* practice in your own time. Timing is everything - learning to judge just when to start the exposition treatment is part art and part science.

[TheRockConcreting]

Jay
Did you undertake training in techniques or just previous experience? We're based in the West midlands, and well away from giving you any competition.

I mastered my trade thanks to the six years i spent in Oz, they really know there concrete.
They constantly work to a high tolerance 1-2 mm finish because the houses are all timber frame. Unlike here where the brickys and tilers are left to get over the 20 mm variances. Because of the screeding techniques they use to place standard grey concrete, placing exposed agg becomes like second nature.

Its all well and good going on a training course or having a crack at home on a 2x2 slab, but on such a small sample you will inevitably end up just sticking it off ridding on the forms and this will be of no use to you in the real world where slabs are large and funny shapes.

What you need to know is how to free hand screed, i would go out and buy some alloy screeds, marshaltown have a range, stay away from speed cretes idea of the so called "oz combo screed" they must have been on same heavy drugs when they dreamed that one up, they have another screed that is good but for the price just go with the marshaltowns ones.

On your next grey slab pour start learning how to use your new screeds. There is a real art to using one, i will do my best to explain what your gonna need to know.

Start small use the 1.2m screed, its easier to control. In the concrete you've just poured use a mag float and laser to create 4 400x400mm flat pads on the surface of the concrete 1m apart. Line yourself up in between two of them, placing your screed down on the pads tilt the screed slightly and pull it straight back ridding on the pads. Balancing the force from both arms here is key. Then repeat but this time drop the angle of tilt of the screed. Then in a half figure of 8 motion tilt the screed the other way and float away from you returning back tilted the other way following the rest of the figure 8.
If you got this far your almost there, all thats left know is to check its flat place the screed at the back of the pad keeping the screed flat slowly pull it back towards you. If its flat you should the see a nice smooth 1.2x500 pad in front of you with two 1mm lines at either end of your screed, the line are very important its how you know your not ridding high in the guts of your nice new pad you just screeded. If the lines are not there or only there on one side repeat from step one.

Once you got the first pad made turn round and make the other one. Now you should be left with a train track. Start at the top and screed your way to the end using the same technique.

Master this and your on the road to mastering exposed agg.

Key points,

don't piss up the concrete keep it to a nice 100mm slump (not a ground works idea of 100 slump either), to wet and it will keep flowing back on its self after you've screeded it.

resit the temptation to tamp the concrete, this is not going to get you anywhere.

[lutonlagerlout]

i would love to see a time lapse of the rock in action
admittedly I have been part of the tamp or roller tamp everything generation
its just how we were taught
LLL

[TheRockConcreting]

The next slab i do i'll get a video, for now i found this on youtube he's abit of a messy screeder, but you get the idea.

https://www.youtube.com/watch?v=9aOIzcZljr8

[msh paving]

what about the compaction of the concrtete? MSH

[TheRockConcreting]

If your referring to the vibrating magic screeds all they do is paste up the surface and drop the agg down abit.

A magnesium bull float does the same thing.

I only ever use a poker on concrete thinker than 225mm, stiffer than 100 slump or wall/edges. You get plenty of compaction from walking in and racking concrete upto 225mm thick.

[msh paving]

I guess you must be right, not the way most concrete co's work is done in england, magic screeds are used all the time all over the place MSH

[Kuts]

I usually use those roller screeds, I've got a 500m2 factory floor to do in the next few weeks so ill have play screeding.

i love the exposed agg and im sure i could sell loads of it, its just i have no idea how to do it!

[lutonlagerlout]

The next slab i do i'll get a video, for now i found this on youtube he's abit of a messy screeder, but you get the idea.

https://www.youtube.com/watch?v=9aOIzcZljr8

that is the antithesis of the way i have learnt
which always involved some kind of tamping
so which way is right?
LLL