More concrete sub-base questions

[julianf]

Sub-base, so far as i know, is an interface between soil, and concrete.

As i understand it, soil moves, depending on moisture content. The degree of movement will also be dependant on soil make up - ie different soils perform in different ways.

Right?


Which brings me onto when people say "you need 100mm of subbase" or "150mm" etc. without ever asking about the soil conditions?


This is what im trying to avoid with this introduction! : )

As i understand it, the subbase is an inert layer that does not expand and contract with moisture, but will move, very slightly, to take up the movement of the soil?

Or is it just a cheaper substitution for concrete? Ie it would be better to do the whole job with concrete but just more expensive?



The soil here is stoney. Lot of irregular shards of stone. You cant dig the soil with a spade. With effort you can scrape at the surface, or you can use a pick to hack your way though it.


The concrete that ive taken up around the place (just paths, back yards, etc) has been 2" laid on top of about an inch of pea shingle, on top of soil. Some cracking, but no major movement.


I want to lay an internal floor in an old stable. Just domestic use - no lathes etc. Im proposing 4" of concrete, with mesh, on DPM.


My question -

Im finding it hard to see the point of taking out soil that is so hard that i need a pickaxe to work it.
Im tempted to copy what seems to have worked in the past - a layer of pea shingle (which is, to all intents and purposes, self compacting) and then the DPM (no blind, as not needed with shingle) and then the concrete.


What im looking for is someone to say "yep, that will probably be fine" or "no, you cant do that, because...." (without just quoting me specs for clay soil)


Any takers? : )

 

[cumbriasteve]

When I was building my garage I was having similar problems, it was not loose soil but was also not what I perceived as ideal sub base.

In the end I got a lorry load of road scrapings and a compactor, probably a good 6" of scrapings compacted very well.

I laid 4" of readymix on that and no problems at all with the car being driven in and out daily.

So my opinion is it would be fine, but I also live far enough away from you to not worry about a lump of concrete through the window if it isn't .

 

[julianf]

If i was ok raising the height, that would not be an issue but the finished surface depth is the issue.

As an indication, before i came in, i was scraping the surface with a shovel, and sparks were coming off. Ie the stone is solid enough that its not just rolling out of where its sitting.

I swept the top in the end, and its tough as you like.


...however, i guess it must still move a bit with differences in moisture content? Maybe not - i just dont know.

The room, although not square, is about 3m x 4m, so not a massive slab.

 

[Barking Mat]

Won't the pea shingle pierce the DPM? Which is why its best to put down sand to protect the dpm?
That's my understanding.....

 

[cumbriasteve]

As an indication, before i came in, i was scraping the surface with a shovel, and sparks were coming off. Ie the stone is solid enough that its not just rolling out of where its sitting.
.

I think it sounds ok to lay onto if it's coming across as that compacted, as a back up you could lay a mesh before pouring and if space allows an inch of course sand which will level off and protect the membrane.

 

[rcx132]

Sub-base, so far as i know, is an interface between soil, and concrete.

As i understand it, soil moves, depending on moisture content. The degree of movement will also be dependant on soil make up - ie different soils perform in different ways.

Right?


Which brings me onto when people say "you need 100mm of subbase" or "150mm" etc. without ever asking about the soil conditions?


This is what im trying to avoid with this introduction! : )

As i understand it, the subbase is an inert layer that does not expand and contract with moisture, but will move, very slightly, to take up the movement of the soil?

Or is it just a cheaper substitution for concrete? Ie it would be better to do the whole job with concrete but just more expensive?



The soil here is stoney. Lot of irregular shards of stone. You cant dig the soil with a spade. With effort you can scrape at the surface, or you can use a pick to hack your way though it.


The concrete that ive taken up around the place (just paths, back yards, etc) has been 2" laid on top of about an inch of pea shingle, on top of soil. Some cracking, but no major movement.


I want to lay an internal floor in an old stable. Just domestic use - no lathes etc. Im proposing 4" of concrete, with mesh, on DPM.


My question -

Im finding it hard to see the point of taking out soil that is so hard that i need a pickaxe to work it.
Im tempted to copy what seems to have worked in the past - a layer of pea shingle (which is, to all intents and purposes, self compacting) and then the DPM (no blind, as not needed with shingle) and then the concrete.


What im looking for is someone to say "yep, that will probably be fine" or "no, you cant do that, because...." (without just quoting me specs for clay soil)


Any takers? : )

Hey the design does depend on the soil conditions but there are standard prescriptions, that's what I was given by a practising civil engineer friend and it's what I followed. I tried to find it for you but the design is lost.

If you're just putting a pre-fab on it then you're not totally sc***d if the slab moves or breaks, so maybe then you can follow the rubbish advice people post on the internet, whatever it is, DPM + rubble + concrete. If you're putting a £20'000 brick garage on it that will weight much more and you'll have serious problems if the slab can't cope, so then I'd go for a proper slab which means lots of Type 1 sub base, sand blind, TWO laters of proper rebar, and C40 concrete.

The big mistake that I made was that I was advised not to pump the concrete to the back of the garden where it was needed, as it has to be more watery and hence weaker. So the concrete was wheel barrowed. Took so long that the conrete started setting which seriously weakened it. Bad advice.

 

[brightspark]

pumped concrete weaker how come . I think not how do they build tower blocks and bridges with it then

 

[julianf]

Ill take some photos - the floor will not support walls. The structure exists - an old stone stable.

So the slab will go up to the walls, but not support the walls, apart from on one side, where there will be glass (ill dig deeper here, i think)

All it needs to do is support standard domestic stuff.

 

[rcx132]

Ill take some photos - the floor will not support walls. The structure exists - an old stone stable.

So the slab will go up to the walls, but not support the walls, apart from on one side, where there will be glass (ill dig deeper here, i think)

All it needs to do is support standard domestic stuff.

So not much to worry about then!

pumped concrete weaker how come . I think not how do they build tower blocks and bridges with it then

Weaker doesn't mean weak. Just means weaker than non runny/non pumped.

 

[bigegg]

If the soil's that solid, I'd put an inch of sand, dpm, inch of sand and then 2-3 inch of concrete (or well grouted paving slabs) - especially if it's just for foot traffic, and no heavy loads.
There is a patch of concrete at the side of my house that is 40+ years old.
It's been occasionally run over by the car, used as a work area and generally abused and is still pretty solid (chipped at the corner where a skip was dropped on it)
It's only about an inch thick, on 3" of brick rubble straight over clay soil.

 

[Chillitt]

I'm very pleased you asked this question as I have exactly the same thing to do here. Just started clearing out the loose stuff, then there's a lime based floor over good old Cotswold brashy horribleness.

 

[sandy2210]

pumped concrete is not more "watery" it just contains plasticisers we are pumping over 500m3 daily to hold wind turbines up !!!!!!!!!!
what you are proposing will be fine if just domestic use remember mesh needs 50mm either side of it or you could use fibres but its a Language to get a polished finish on

Please adhere to the rules re language

 

[brightspark]

pumped concrete is not more "watery" it just contains plasticisers we are pumping over 500m3 daily to hold wind turbines up !!!!!!!!!!
what you are proposing will be fine if just domestic use remember mesh needs 50mm either side of it or you could use fibres but its a **** to get a polished finish on

don't cut corners with the concrete then. my daughter works on the offshore ones all the time

 

[gaz1]

The soil here is stoney. Lot of irregular shards of stone. You cant dig the soil with a spade. With effort you can scrape at the surface, or you can use a pick to hack your way though it.

I want to lay an internal floor in an old stable. Just domestic use - no lathes etc. Im proposing 4" of concrete, with mesh, on DPM.

My question -

Im finding it hard to see the point of taking out soil that is so hard that i need a pickaxe to work it.
Im tempted to copy what seems to have worked in the past - a layer of pea shingle (which is, to all intents and purposes, self compacting) and then the DPM (no blind, as not needed with shingle) and then the concrete.


What im looking for is someone to say "yep, that will probably be fine" or "no, you cant do that, because...." (without just quoting me specs for clay soil)

Any takers? : )

Ill take some photos - the floor will not support walls. The structure exists - an old stone stable.

So the slab will go up to the walls, but not support the walls, apart from on one side, where there will be glass (ill dig deeper here, i think)

All it needs to do is support standard domestic stuff.

if its solid enough not to be picked then just concrete straight over
some of the old stables used cobble stones compacted down over the years they are solid and dont move at all much better than the new way of doing them

 

[julianf]

If the soil's that solid, I'd put an inch of sand, dpm, inch of sand and then 2-3 inch of concrete (or well grouted paving slabs) - especially if it's just for foot traffic, and no heavy loads.
There is a patch of concrete at the side of my house that is 40+ years old.
It's been occasionally run over by the car, used as a work area and generally abused and is still pretty solid (chipped at the corner where a skip was dropped on it)
It's only about an inch thick, on 3" of brick rubble straight over clay soil.

How come you recommend sand on top of the DPM?

I doubt that i would be able to keep it in one place once the concrete started getting pushed about?

 

[bigegg]

How come you recommend sand on top of the DPM?

I doubt that i would be able to keep it in one place once the concrete started getting pushed about?

I used sharp gravel as aggregate and was recommended to use a layer of sand to prevent punctures.
Dunno how necessary it is?

 

[brightspark]

blind it with sand then dpm then concrete no point in putting sand in with the concrete

I used sharp gravel as aggregate and was recommended to use a layer of sand to prevent punctures.
Dunno how necessary it is?

yes sand over the aggregate but under the dpm

 

[CornishPete]

If it were me then a thin layer of compacted sand, dpm, a142 mesh sat on 2" chairs, then 4" concrete over (1.2m3 would give 12m2 @ 100mm thick). This will be a good general purpose slab. Anything heavier duty would be overkill and cost more than necessary Imo. You would be looking at around £50-60 for two sheets of mesh and £200-250 for 1.2m3 concrete delivered up my way. I tend to round up to the the nearest half cube if finished level isn't critical, for good measure! (Mixing even a relatively small quantity of 1.2m3 by your self ie buying ballast, cement and using a concrete mixer, is a false economy, it'll only save a small amount of money but take a lot of time and effort that would be better spent getting the slab level and nicely finished.) if you can get a barrow to it then volumetric is the best way by far. If nothing else they'll help you barrow it in!

 

[bigegg]

blind it with sand then dpm then concrete no point in putting sand in with the concrete
yes sand over the aggregate but under the dpm

Lol.
Sharps in the concrete!

 

[julianf]

The access makes ready mix problematic.

As it is, im planning on buying the aggregate in 25kg sacks, so as i can carry it, by hand, through our kitchen....

When i last got a quote on ready mix, it was £70 per cube, with a £70 delivery charge. So about the same cost as doing it by hand, i think.


That quote is not recent though - again, its just easier with this one to mix by hand (with a mixer...)