Concrete coarse aggregate size

[gh483]

I have broken up a solid concrete ground floor and excavated the underlying material (to a depth of around 200mm, level with the footings), to install under-slab insulation. The excavated material appears similar to beach shingle consisting of slightly rounded, mainly flint stones. The largest ones are around 80mm (major axis) but most are 40 to 50mm. Since I have such a large amount of this material to get rid of, and a lot of concrete to pour, would it be advisable to use it as coarse aggregate for the new slab mixture? Or would the concrete be seriously weakened by stones this size?

I should mention that the slab won't have any major structural role but needs to be adequate for the screed, some internal single story walls, people, baths full of water, furniture etc. I guess the compressive strength wouldn't be an issue (compared with the insulation beneath it) but needs to be shear-resistant enough to spread the load over the insulation properly, which would be my main concern. I wasn't planning on using any steel reinforcement.

 

[JohnD]

How thick will your new concrete slab be? 35mm is the largest commonly used.

Rounded pebbles do not form such a strong mix as crushed stone, but I have seen quite large rocks (they call them "plums") in mass concrete in Australia. They must all be encased completely in regular mix.

For placement where strength is not critical, such as fencepost holes, I use any spare clean rubble or stone, placing it after a layer of mix and pouring more over it to cover and envelope the pieces.

 

[gh483]

It wouldn't want to have to make it any thicker than 150 mm. If I have to make it thicker than this just to re-use the 'free' aggregate then I would probably stick with purchased ballast and 100 mm slab.

The stuff I dug up has rounded edges, but mostly not to the extent of 'balls' or 'eggs'. Clearly, crushed stone with lots of sharp edges would be optimal but what I have is somewhere in between.

Incidentally, can anyone think of a clever, cheap DIY solution for sieving the aggregate (exclude stones above a certain size)?

 

[bsr]

Sieving through a sheet of expanded metal fencing would get you a 25mm aggregate but take forever. Commercial screens use an engine powered vibration of the screen to speed things up.

 

[JohnD]

Roll them down a trough with a slot in it.

 

[jeds]

I'd be more concerned with where they came from and possible contamination or sulphate. But then, I suppose it's been there a long time with no issues?

 

[Ivor Windybottom]

I've made concrete from crushed concrete (and bricks and other assorted rubble). I've just used it for garden wall foundations, cavity fill and for a sub-base under stone paving. In my case I had big chunks so hired a mini crusher and put the lot through it, on its finest setting. I added some sharp sand to make up for the lack of fine aggregate.

It's going to be of unknown strength and probably not frost proof unless covered with something else, which is probably why it's not generally done commercially. You'll end up with a slab of something, you've no idea how good it is. It's been fine when I've done it, but I haven't used it for anything demanding.

Also it's very tough work shovelling crushed rock from a heap into the mixer, it really doesn't want to be scooped up due to the jagged shapes.

If you're building walls on it then it may make sense to play it safe, get rid of it or use it elsewhere and buy aggregate. Also look at readymix, it's often around the same or less than buying aggregate and cement and obviously a lot less work.

 

[gh483]

Jeds and Ivor, perhaps I should clarify the stuff I was proposing to use as aggregate in the new slab were the loose stones originally underneath the broken-up slab, (actually underneath the DPM) not the remains of the slab itself. As I said these are mainly natural-looking flints similar in appearance to beach shingle.

Anyway, I've had an idea for a sieving mechanism involving a rolled up piece of wire mesh fence , two large planks and some bricks to prop them up.

 

[JohnD]

I had visualised it as an oversite slab. What will be under your insulation?

 

[gh483]

Not sure exactly what you mean by oversite slab. The old floor consisted of (top to bottom):

50 mm screed,
100-150mm concrete,
DPM,
approx 200 mm loose stones,
stony compacted earth (?) I havn't dug below this, level of the top of the footings.

The new floor was going to consist of (top to bottom):

50 mm screed,
UFH
100-150 mm concrete,
plastic sheet to separate concrete from insulation
approx 200 mm EPS or XPS insulation
DPM
sand blinding (if required)
thin layer of stones (maybe)
earth

 

[Ivor Windybottom]

I'd say that if you put a large pebble in a concrete slab where the pebble is about half of the slab thickness then you will have created a huge weakness at that point.

Concrete gets its strength from its random and overlapping pieces. If you chuck a rock in then it definitely won't crush under vertical load but it would be a perfect weak point for a shear crack to start from, e.g. at the point where you load it with a wall on top of the soft insulation.

I'd use them elsewhere, either crushed or just as rocks for some other thing. They're probably a fairly trivial amount in terms of concrete, which is usually around £120 +VAT per cubic metre for readymix, mixed on-site as needed.

That (maybe) layer of stones is essential, shouldn't be "thin" and must be machine compacted. You can buy an electric one, they're not as good as petrol but adequate if done in thin layers at a time, plus you won't die from the fumes if used indoors.

 

[gh483]

I have decided that I won't use these stones on the mix without passing them through a 25 mm sieve at least and if that turns out to be not worth it I will just stick with the ballast.

Ivor your point about the sub base has led to some more questions but these probably belong in another thread so I have started one.