The Brew Cabin

Posted: **Mon Jul 29, 2019 12:52 pm**

Hi all,
I am planning on overlaying some existing concrete to the rear of my house. Existing concrete is a bit patchy, but I don't want to replace it all for a 'proper' job as we are hoping to have the money to build an extension in 3 or so years time. So this overlay is just to make it look a bit better for a while, and doesn't have to last a lifetime.
There is currently an exaggerated slope on the drive and I also want to reduce this by taking up approx. 4" over about a 8 foot distance.
I was planning on isolating from the existing concrete with visqueen and using a sub base to take up the excess fall.

I am after some advise on the following please (all welcome)
- What would the sub base need to be, would sharp sand be enough ?
- Should the order be visqueen, sub base new concrete, or sub base, visqueen - new concrete ?
- How thin could I go with the new concrete layer. It will have cars driving over it
- What would be the best mix to use for the new layer ?
- At some points it will be between a garden wall & house (about 16' apart), will it need expansion gaps at each end (old concrete currently butts up to both)

I plan to put a colour into the mix, and maybe do some bands of decorative aggregate floated into the surface to break it up a bit.

Not sure yet if I'll mix myself, or get ready mix delivered ( I have a hand me down full bag diesel mixer my Dad bought in 1965 !)

Thanks in advance for any replies
Mark

Posted: **Thu Aug 01, 2019 11:49 am**

Sand is NOT a sub-base material. It is too easily washed away. It must be crushed rock or crushed, clean hardcore (brick/concrete). It would require a minimum depth of 80mm compacted.

The minimum depth of concrete suitable for light vehicular traffic is 100mm - see Concrete Hardstandings

The build-up would be: existing overlain by sub-base, then a DPM (visqueen or similar) then min 100mm concrete with reinforcement (mesh or fibres)

The concrete should be a C20 equivalent as a minimum.

Movement joints and isolation/expansion joints should be incorporated against any vertical structures, such as garden walls, buildings, fences, etc.

Posted: **Thu Aug 01, 2019 9:36 pm**

Thank you Tony, much appreciated.

So is the 100mm needed even over the existing concrete ?
Would I be able to go thinner if I did away with any sub base ( that I was only going to use to take up the excess fall, as it looks a bit OTT) and just applied direct to existing concrete (with a DPM in between ) ?

I am assuming here that it wouldn't be good practice to have a slab addressing the excess fall ie going from 25mm to 75mm ?

I only want this to last 2-3 years , so I'm not looking for something that will last forever .

thanks again
Mark

Posted: **Sat Aug 03, 2019 12:28 pm**

Anything less than 100mm of concrete is very likely to break-up more-or-less traightaway if trafficked by vehicles, regardless of what lies beneath.

You could take a chance with, say 60 or 80mm, but IMO, it's just not worth the gamble, even if it is only for the short term.

The Brew Cabin

Overlaying existing concrete - Looking for any advice & guidance