Thread: Concrete base advice

Hi Everyone,
Weekends almost here so back to my pond construction but was wondering if all those with a concrete bases put any plastic down underneath the concrete ? I have done the dig out and am on clay, I plan to put 4 inches of MOT1 down and 8 inches of concrete. Would I be better to put a blinding layer of sand on top of the hardcore then plastic before the concrete goes in or is plastic not needed ?

Hi Mike
when I built my pond I put plastic down also running it up the sides of the pond.
this will stop water coming through the walls while building and make it easier to fibreglass.

I would put plastic down but I wouldn’t bother with mot. I would just thicken the concrete and put remesh in

Hi,
I'm curious why such thicknesses are used.
I can understand a good depth under the walls - its like the foundations for house walls, they are deeper than the slab.
But for the base of the pond, I struggle to see why you would need more than 4-6" or 100-150mm if you prefer.
4-6" is perfectly adequate for a domestic drive, which is carrying point loads of, say, 35 psi under the wheels, but a pond base is only carrying an evenly spread load of about 2 psi.

Thanks Very much, appreciate the advice

Originally Posted by GDL

Hi,
I'm curious why such thicknesses are used.
I can understand a good depth under the walls - its like the foundations for house walls, they are deeper than the slab.
But for the base of the pond, I struggle to see why you would need more than 4-6" or 100-150mm if you prefer.
4-6" is perfectly adequate for a domestic drive, which is carrying point loads of, say, 35 psi under the wheels, but a pond base is only carrying an evenly spread load of about 2 psi.

You are probably right that its overkill, but it seems to be what everyone goes for, even building supplies websites and such like recommend 30cm (10") thick base for a 1.5m to 2m deep concrete/block built pond.

Most people settle for that because the absolute nightmare of a cracked base years down the line don't bear thinking about.

When I built a garage BC made us dig 1.2m footings, despite the garage being attached to the 100 year old house sitting on 30cm footings. More recently a friend was forced to dig 2 metre footings for a garage "due to soft clay and a tree in the vicinity" the tree was on the pavement around 25 feet away from the closest corner of the garage.

Still, my current pond has a circa 10" base, originally built circa 2002 by the previous owners. The garden slopes towards a stream and the adjoining filter pit which has a much shallower base had broken away from the pond and separated by about 3 inches as it gradually works its way towards the stream.

Left of the gap is the edge of the pond, right is the filter pit:




The back wall of the filter pit opposite the pond:





Had the pond base been less substantial I wonder if part of the base would have moved with the filter pit?

Either way, over engineering these things is not a bad idea, and for me has probably made the difference between having to start the build from scratch against being able to build on the existing pond base.

just in the process of having a pond built, the base is 8-9" thick and the filter base is 4" thick

Originally Posted by GDL

Hi,
I'm curious why such thicknesses are used.
I can understand a good depth under the walls - its like the foundations for house walls, they are deeper than the slab.
But for the base of the pond, I struggle to see why you would need more than 4-6" or 100-150mm if you prefer.
4-6" is perfectly adequate for a domestic drive, which is carrying point loads of, say, 35 psi under the wheels, but a pond base is only carrying an evenly spread load of about 2 psi.

all foundations vary dependant on the soil conditions.
as an example to build a small single story extension on a friends house,
he had to get 10 concrete piles driven 20 meters into the ground

Well he had to get them done after the extension was built...
as the surveyor did a bad job and the extension tilted away from the house by about 8"
his house was featured on a nightmare builders TV programme...

most of the concrete cost is haulage,so to overspec doesnt cost as much as you think.Its just a case of peace of mind for me.

Thanks for the replies.
I agree that it depends hugely on your ground. It was just that the figures as a general recommendation surprised me.
I looked at some swimming pool sites and they were talking about 4-6" for the base, though some talked about 8".
Mine is well over 8", possibly 12. Initially thicker because I am on heavy clay, but mainly because the idiot builder tried to stop ground water penetration from below by adding another 4".
Trouble is that he used a wet mix, which makes the concrete porous, so that didn't work.

My big recommendation if you have high ground water levels then make the mix as dry as you can, so you end up with waterproof concrete. It's harder to lay, but well worth the effort.
Otherwise you will never be able to fibreglass it.

Cheers

Who told you that drier concrete is waterproof bud

There's a lot of research on it, but I found good explanations here https://waterproofconcrete.co.uk/how...0concrete.html. if you can put up with the writing style.
The key quote for me was "The point is, UK university text books now teach students how to make completely waterproof concrete without admixture, and have been doing so since 2011. "
If it's been the case for that long, why don't more people know about it?
I looked at a lot of the source material and other sites too after the debacle that was my base.

Not wishing to get into an argument but my pal owns a concrete firm , with specialist equipment for industrial floors etc and he laughed. Adding cement wouldn’t make concrete waterproof as cement is porous You can add acrylic etc That would work. Reducing water strengthens concrete for sure

It does not suggest adding cement, it talks about changing the stone/sand/cement/water ratios.
I'm not a chemist, my degree is in maths, but I have read some of those papers and they are written by people a lot cleverer than me.
I'm just offering my two pennyworth to the debate.