Concrete Correct Size Of Aggregate

[gibbsy09]

Hi in order to obtain a nice tidy smooth slab of concrete, eg for a drive, a shed etc, should the mix contain, large and small aggregate, eg 20mm stones 10mm, or even 10mm down to smaller gravel, obviously sharp sand, I believe if you ordered a ready mix to your property, it would have a precise mix.

Reason for asking I just did a smallish slab with just 20mm ballast, and it was hard to get a smooth finish, for a novice, also from my research online, is it true to say when starting the finishing phase, straight after screeding should a wooden or resin float be used to smooth and push down the aggregate, as using a mag float would trap the bleed water, and mag float should be used for the final phase, I've more to do, however I only have a mag float, and resin are pricey, but if it's needed then it's needed, just want to be clear.

 

[Ivor Windybottom]

I don't understand your description, it all sounds needlessly complicated. All I know is that you've got to bash the surface repeatedly to push the chunks down and make the "fat", i.e. the sand/cement/water rise to the surface. It doesn't matter what you hit it with, it could be a wooden plank, a float, a brick or your head. Then you smooth this finer layer of slop with a trowel.

You can't just smooth it straight away, it needs tamping first, whatever the aggregate size.

20mm aggregate is standard, I use 10mm for filling around pipes, cavity fill etc - anywhere where bigger pebbles could clog and stop it flowing into small nooks and crannies. 10mm will be weaker than 20mm, if just a straight slab then there would be no good reason to use finer.

Whether 20mm or 10mm, this is the maximum size. All concrete should contain all sizes from the maximum down to dust. The tiniest particles are held together by the cement, surrounding the bigger bits, surrounding the pebbles. The best way for self-mixing is to order 20mm or 10mm Ballast, not separate gravel and sand. Ballast is basically concrete minus the cement and water, just chuck it in the mixer and it should be good. If it's really chunky then you may need to add a bit of sharp sand but this shouldn't normally be needed.

 

[gibbsy09]

I don't understand your description, it all sounds needlessly complicated. All I know is that you've got to bash the surface repeatedly to push the chunks down and make the "fat", i.e. the sand/cement/water rise to the surface. It doesn't matter what you hit it with, it could be a wooden plank, a float, a brick or your head. Then you smooth this finer layer of slop with a trowel.

You can't just smooth it straight away, it needs tamping first, whatever the aggregate size.

20mm aggregate is standard, I use 10mm for filling around pipes, cavity fill etc - anywhere where bigger pebbles could clog and stop it flowing into small nooks and crannies. 10mm will be weaker than 20mm, if just a straight slab then there would be no good reason to use finer.

Whether 20mm or 10mm, this is the maximum size. All concrete should contain all sizes from the maximum down to dust. The tiniest particles are held together by the cement, surrounding the bigger bits, surrounding the pebbles. The best way for self-mixing is to order 20mm or 10mm Ballast, not separate gravel and sand. Ballast is basically concrete minus the cement and water, just chuck it in the mixer and it should be good. If it's really chunky then you may need to add a bit of sharp sand but this shouldn't normally be needed.

Hi thanks for the reply, so when you see the experts on YouTube doing a huge slab, and you see the concrete getting pumped in, then two people or more screeding, after that they go over it with a bull float, are they not showing the part where the large aggregate is getting bashed Down eg tamped, Thanks.

 

[Ivor Windybottom]

If it's full of water then it won't need much to get a layer of slop. Readymix can be pretty runny, just so it flows. This will make it weaker, it's a compromise.

If you're making concrete yourself then it's usually mixed stiffer, in which case you need to work it to get the liquid to come up.

 

[jeds]

Hi thanks for the reply, so when you see the experts on YouTube doing a huge slab, and you see the concrete getting pumped in, then two people or more screeding, after that they go over it with a bull float, are they not showing the part where the large aggregate is getting bashed Down eg tamped, Thanks.

Pumped concrete is a completely different thing - it contains flow and plasticising additives. Tamping concrete does the opposite of pushing aggregate down - what it does is bring the fat to the surface, although I appreciate that is effectively the same thing.

From your description, your concrete may have been poorly graded rather than the aggregate being too big. Or it didn't have enough cement content or was too stiff. Properly mixed concrete will tamp easily and fat will come to the surface and shouldn't cause any issue with aggregate surfacing. A max 10mm aggregate should be ok although a lot of people make concrete from ballast which has larger aggregate. Whatever it is it should be well graded from the large down to sand and it should contain the right amount of cement. If you want it more workable, include an additive.

 

[Ivor Windybottom]

Yes, pushing the pebbles down displaces the slop and pushes it up. It rises to the top, hence the name "fat".

I've tried adding mortar plasticiser to concrete, to try to mimic readymix. It's nicer to work with and OK for underground work, but it's definitely quite porous after setting, as it has a more aerated surface. But I'd say it's quite similar. Probably not a good idea, but I like an experiment!

It may be possible to buy additives for concrete, but you'll probably need to buy it by the truckload.

Blue Circle used to state that its Mastercrete cement that already contains additives is LESS suitable for making concrete than its cheaper plain cement without additives. I've never really understood why it's seemingly fine for readymix companies to add stuff but not when mixing your own.