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

-------------------------------------

 

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.

--
______________________________
Keep the whole world singing . . . .
DanG  (remove the sevens)
dgriff237@7cox.net

 



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..

 

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.


-------------------------------------

 

saka had written this in response to
http://www.thestuccocompany.com/construction/Re-Concrete-Garage-Slab-Help-18316-.htm
 :

Cwatters wrote:


-------------------------------------

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