Thread: garage slab concrete - subbase

Originally Posted by untamed

I am pouring a 100mm slab on M class soil which is "moderately reactive" from memory with clay and all that other crap in there

I am going for 25mpa as my father in law owns a concrete truck and drives for [will not plug name here for certain reasons ]. I guess it will be a bit stronger in the long run. I think the reo he recommended was F52 or something?

Question for you smart blokes: how much subbase (crushed rock) should I be putting under the slab if any? The ground is very firm, I kid you not, it is like concrete at the moment! How do you concretors compact it, with a basher or just stamp on it?

What's going on the slab? Is it going to have vehicle traffic? How big is it? My advice is to be more concerned with footings than the subbase, just 100mm of clean sand or crusher dust will be fine for that. Yes, it should be compacted, although this is much easier with sand as you only need a hose and a vibrating plate compactor. The crusher dust should be done with a Wacker Packer or a roller, preferably vibrating. If the underlying soil is the natural material (not controlled fill compacted in layers), you may need some thickening beams to prevent the slab from cracking with soil movement. There is also the issue of subsurface drainage, to make sure that the soil under the slab doesn't get constantly wetted and dried.

crap. not the answer i was looking for :eek:

I have 100mm clearance now which is enough for the concrete, which means I need to go down another 100mm.

The size is approx 7.5 x 3.5 meters. It will have vehicles up to 2.5 tonne on it.

Originally Posted by untamed

crap. not the answer i was looking for :eek:

I have 100mm clearance now which is enough for the concrete, which means I need to go down another 100mm.

The size is approx 7.5 x 3.5 meters. It will have vehicles up to 2.5 tonne on it.

Make sure you use 25MPa and you might like to consider 2 layers of mesh, top and bottom, depending on how concerned you are about cracks. Is there a building going on it or is it just a driveway?

It is just the slab in the garage, no buildings on it.

100mm is a bit thin for double mesh if looking for strength just go up a bit in mesh size. Check your local council for their regs on footings and slab thickness, that will be the best place to start. Plenty of depth at 100mm to support 2.5 tonne but shed loads will have some bearing on slab design as will soil type. The local building inspector is really the best place to start, at least that gives you the minimum standard which will be sufficient for most purposes.

John.

Most carpark pavement are 150mm thick with 82 mesh, 62 or 72 mesh is heaps. !00mm thick is fine on good ground, i would have a thicknessing beam around the perimeter and where a dummy joint is to be trowelled eg 200mm thick and 300 mm wide.

This answer is based on having worked with my father (a concretor for over 30 years).

Never have we put 100mm of sand or gravel dust (what we generally use) below a slab. About 50mm is enough. Its generally used to level off as sand/gravel is cheaper than concrete. Do not be concerned if you have a 30mm cover somewhere and 60 elsewhere. Just get is close to level. Use a string line and measure the clearance you have.

Then the plastic and then use F72 mesh. F82 if you want to be sure. If its one of these colourbond sheds then just thicken the edges with a 200mm deep trench (overall). If its brick go to 450. Dont forget the trench mesh or bars.

A really good idea would also be a 900mm path right around the garage. It helps keep the water away from the garage

Originally Posted by boban

This answer is based on having worked with my father (a concretor for over 30 years).

Never have we put 100mm of sand or gravel dust (what we generally use) below a slab. About 50mm is enough.

Depends on the subsoil and the moisture likely to get under. The subbase is there to keep moisture away as well as act as a cushion over the clay.

Originally Posted by boban

Its generally used to level off as sand/gravel is cheaper than concrete. Do not be concerned if you have a 30mm cover somewhere and 60 elsewhere. Just get is close to level. Use a string line and measure the clearance you have.

Then the plastic and then use F72 mesh. F82 if you want to be sure. If its one of these colourbond sheds then just thicken the edges with a 200mm deep trench (overall). If its brick go to 450. Dont forget the trench mesh or bars.

A really good idea would also be a 900mm path right around the garage. It helps keep the water away from the garage

All good advice, but I strongly disagree with you about the importance of a good sand/crusher dust layer.

I agree exador! U normaly put sand down to stop the plastic membrane from being pierced from the subgrade, which u only need for a dwelling not pavement. U can wet the subgrade before pouring concrete to stop it drying out to much. u normaly put crushed rock down when the ground is no good or you have dug to deep.

how important is the plastic membrane? my concretor hasnt mentioned anything about it, so i am presuming he isnt going to be using it?

Originally Posted by untamed

how important is the plastic membrane? my concretor hasnt mentioned anything about it, so i am presuming he isnt going to be using it?

It's pretty important. He is going to use it, as pouring directly onto sand would be nearly unworkable due to the water in the concrete draining away, leaving dry cemented aggregate - not pretty.

Originally Posted by Exador

It's pretty important. He is going to use it, as pouring directly onto sand would be nearly unworkable due to the water in the concrete draining away, leaving dry cemented aggregate - not pretty.

got my subbase down, approx 60mm of crushed rock.

with regards to the plastic membrane, on the concrete.net.au website, it states it is 'optional' and is only to reduce friction under the slab. Doesnt really state that the concrete will be unworkable or leaving a dry cemented aggregate. However, what you have stated Exador makes a little sense.

Here is a link to the referenced information, http://www.concrete.net.au/viewpdf.p.../Driveways.pdf have a read of it and let me know what you think?

Originally Posted by untamed

got my subbase down, approx 60mm of crushed rock.

with regards to the plastic membrane, on the concrete.net.au website, it states it is 'optional' and is only to reduce friction under the slab. Doesnt really state that the concrete will be unworkable or leaving a dry cemented aggregate. However, what you have stated Exador makes a little sense.

Here is a link to the referenced information, http://www.concrete.net.au/viewpdf.p.../Driveways.pdf have a read of it and let me know what you think?

All good information, except that they're suggesting a very thin sand layer as it is only a driveway, therefore not much water drains away, therefore the concrete remains wokable. In your case, use the plastic - it's cheap and you'll have a better pour and a better slab for longer. It's always down to that cost tradeoff in the end.

The plastic membrane is simply a barrier to prevent ground moisture carrying up through the slab. When pouring the plastic serves as a barrier to prevent moisture getting away thus slowing down the rate of cure. In summer this is a help in winter a pain. Not sure about concrete becoming too dry for lack of a barrier, that would mean the ground beneath was capable of wicking away moisture very quickly and could be aleviated by drenching the ground before the truck arrived. I'd only use plastic if the area is covered such as a garage, for outside driveways and paths I have never used it myself nor ever seen it used by others. Incidently sub base when dug below ground level can become a sump holding water every bit as much as an aide to drainage when the surrounding ground acts to absorb or carry away moisture.

John.