Concrete Garage Slab Help!


Hi all,
Just need some advice on vapour sheets as well as laying the sub-grade.
Anyway it has been raining a lot here in Melbourne (Australia) and the dirt was like rock before all this rain. The dirt underneath the gravel has become very wet and soft, it's more like Clay than dirt and this has in-turn made the gravel very wet - Pictures in link below.
My question now is does this pose a problem when laying concrete? Will a vapour sheet be needed? or can the concrete simply be poured on top of the gravel once it has been compacted.
If a vapour sheet is necessary can the concrete be poured directly onto the vapour sheet or should gravel be put on top? It will be easiest if we can just pour on top of the vapour sheet.
My friend who is doing the concreting said that a vapour sheet isn't necessary but he hasn't been in the concreting business all that long and I am a bit worried. The shed will be used as a living quarters eventually.
Photo's here: http://home.sakatumi.operaunite.com/photo_sharing /
Any advice is much appreciated! Had some great advice in a previous thread so thank you to all that contributed there as well
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If you are planning to use a non permeable floor material, you absolutely want the vapor barrier. I would think you want the VB no matter what for occupied space. You can pour directly on the VB - your finisher really needs to know how to work the concrete with a VB as all the excess water of hydration needs to go up only, none will be going down. It is essential that the finishers stay off the slab until the bleed water has finished coming up. It is your summer, so this may require wetting the top, covering with visqueen or other means of keeping the top alive. Wood or magnesium floats should be the only tools on the slab until the last moment. If this is to be hard troweled (smooth) make sure NOT to use air entrainment.
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Gravel?
I couldn't see your pictures but...
The normal subgrade used here in the UK is crushed limestone or possibly recycled road planings which are cheaper.
Typical construction would be:
Concrete Plastic membrane Sand blind Subgrade (called "Hardcore" in the UK!)
The proceedure is to excavate down to around 300mm below finished level. Compact the ground using a vibrating plate. Then lay 150mm of hardcore in three layers. Each 50mm layer is raked level and compacted the same way. Doesn't take long. If your ground is very soft/wet you should really excavate down to firmer soil first.
Once that's done a thin layer of sand (also compacted) is used to protect the plastic membrane from the possibly sharp subgrade/hardcore. The membrane has two purposes. 1) to stop the wet concrete loosing water into the subgrade/hardcore 2) It also reduces damp penetration once the concrete is dry.
The subgrade/hardcore and the concrete would each be around 150mm thick (6"). If this is for a brick garage the a trench foundation would be dug around the edge. Possibly 300-400mm deep. Depends on your attitude to risk, soil type and proximity to trees..
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saka had written this in response to http://www.thestuccocompany.com/construction/Re-Concrete-Garage-Slab-Help-18316-.htm :
DanG wrote:

------------------------------------- Thanks for the reply DanG.
At the moment it is more like Winter with all this rain. We were planning to do the concrete on a day that around 20 - 25 C, the slab is going to be laid in a pre-existing shed - will covering with a visqueen still be necessary if it is a hot day? Thanks for the info about the wood/magnesium tools will look into what my friends use. It will be hard troweled but I don't think they have any special machinery.
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saka had written this in response to http://www.thestuccocompany.com/construction/Re-Concrete-Garage-Slab-Help-18316-.htm :
Cwatters wrote:

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Thanks you for the detailed reply Cwatters, much appreciated!
Sorry about the pictures the link went down with my internet last night.
Pictures here:
http://img410.imageshack.us/img410/5820/img3305j.jpg
http://img410.imageshack.us/img410/5820/img3305j.jpg
http://img214.imageshack.us/img214/9162/img3307f.jpg
We excavated about 250-300mm, however we didn't compact at 50mm intervals when laying the subgrade, we just poured in all the crushed rock and spread it around as even as possible. From the pics you can tell it hasn't been compacted at all yet, that is going to be done on the same day prior to pouring the concrete.
Will not compacting at 50mm intervals cause a problem?
Cheers
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Do I read this right ? Someone is suggesting compacting every 50 MM ? Isnt that 5 centimeters or roughly TWO INCHES ?
When I put gravel down under my slabs, I place a minimum of 6" gravel, then hose it down til damp and run the plate compactor. I've never dug much deeper than that but my subsoil was always pretty solid. I usually set my slab/driveway forms on top of the 6".
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There is absolutely nothing wrong with compacting the stone in 6" or 150 mm lifts. If the floor will eventually have a finish other than toweled concrete, a vapor barrier is a must. We typically use a 15 mil ( about 0.04 cm product with tape made for the system on the joints.
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I do it in two passes if using a plate. Perhaps one if you have a roller.
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A thin layer of sand can compact? req
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Into what's under it.
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*If this is eventually going to be used as living quarters then you should install a moisture barrier as well as some insulation in the slab. I think it usually goes moisture barrier first in contact with earth then insulation on top of that then your stones or wire mesh.
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