Concrete mix. Can I use 10mm gravel from Driveway? (Ed.)

[Tom99999]

Hi, I've managed to get a load of 10mm decorative stone for free. It's mainly clean but will clean it before using.

Would it be advisable to use in the concrete base mix for the slab I need to pour for my garden office project along with sharp sand and cement?

If so, what mix to use?

Or should I avoid?

Thanks

 

[Tom99999]

This is a picture of the gravel I was hoping to use for concrete slab mix. It's mainly 10mm with smaller bits too

 

[Cheslea2010]

A traditional concrete mix would have 20mm stone in but as it is for a shed you shouldn't have any issues using 10mm. A suitable mix would be 3:2:1. (3 parts Gravel, 2 parts Sharp Sand, 1 part Cement)

 

[Tom99999]

Thanks, it's for a garden office, I'm using 100mm thermalite blocks on top of the slab if that has an influence on strength required in concrete mix

 

[yorksbuilder]

Will be fine. Note any organic matter (leaves etc) will reduce the strength of the concrete, so as you say, make sure it's cleaned first

 

[stevie888]

Crushed stone will tend to give a stronger concrete than shingle because the individual stones will tend to lock together, but it will not flow into place as easily. As said above get it clean

 

[Ivor Windybottom]

Put a bit of extra cement in to make up for any muck and twigs, all will be fine. It's only a shed, not a garage or suspension bridge.

I turned two shed bases, a garden wall and a load of old paths into ballast with a crusher then used it to make new concrete for a foundation and cavity fill for the new garden wall. There was definitely a fair bit of organic matter in, it all eventually decomposes to become voids but you've usually got surplus strength anyway.

10mm ballast is less common than 20mm but is widely available. I've used lots of it to make concrete, as it flows into voids much better, e.g. around rebar and into corners of formwork. Theoretically weaker, but in reality probably similar or possibly even better.

 

[Notch7]

This is a picture of the gravel I was hoping to use for concrete slab mix. It's mainly 10mm with smaller bits too
View attachment 370850

looks bigger than 10mm to me although theres nothing in the pic to reference a size against

concrete is usually made from aggregate which is graded with 60% -80% coarse particles and then various bits down to fine sand

if you only have gravel it will end up gap graded without any fines to fill up and bind

It will still probably be ok though

 

[Ivor Windybottom]

Agreed. Ballast contains everything from 20mm/10mm down to dust. This mix will have lego bricks and sand, with nothing sized between them to fill the voids.

I'd put a bit more sand/cement than normal in, to make a paste that can fill the voids. 3 gravel, 2 sand, 1 cement would normally be a reasonably strong mix, so more like typical strength with this stuff.

Alternatively mix it with sharp sand and compact it into the ground beneath the slab, then buy ballast for the concrete itself.

 

[Ivor Windybottom]

10mm means that all the stones have fitted through a 10mm sieve. They could be 10 x 30mm or any other length. This is normal, 20mm gravel or ballast contains stones that are much larger than 20mm in their longest dimension.

 

[Deluks]

looks bigger than 10mm to me although theres nothing in the pic to reference a size against

Looks like he got it for free from the train tracks!
He'll be chuffed with that.