Concrete Base - Help Required!

[paulweller84]

Hi, I've come looking for some straight answers, my research online has left me with many different, confusing and generally contradicting answers. Can anyone give me the definitive answer to my question? I have a 2m x 2m x 30cm hole dug in heavy clay soil, inside of this I have built a wooden formwork frame, inside of that laid a damp proof membrane and filled the hole to about 3-6cm from the top with rubble. From here I'm assuming I need to pour concrete over this to the level of the top of the framework, but what mix do I need for the concrete for this? And can anyone hazard a guess at what quantities based on the measurements given and the photo provided? Any help would be massively appreciated, I'm nervous about getting this vital stage wrong! (Please note the photo shows the hole only partly filled)

 

[denso13]

1:2:4 mix will be ok for shed which works out at 0.3m³ (75mm thick). Individual quantities can be woeked out at http://www.source4me.co.uk/calculate_concrete_mix.php

 

[b3taman]

Its a shame your pic doesn't seem to have displayed - not on my browser anyway.
What is this base for, and why has it got a damp proof lining? If this is just a base for a timber shed then it seems you're using a sledgehammer to crack a nut. You could have just used council grade paving slabs onto compacted soil and that would have been good enough for a little pad 2m square.
Assuming you're doing something more complex, then your planned concrete depth is very thin indeed - 75mm would be more usual. Usually the minimum quantity of a bulk bagged all-in aggregate a builders merchant will supply is 0.5m2. This would need about 14x 25kg bags of cement to go with it. Use half a bag of cement to about 12 shovels of aggregate whether using a small mixer or mixing by hand.
Hope this helps.

 

[paulweller84]

It has a damp proof membrane as everywhere I looked for info it said to use one to stop damp rising through the concrete. Is this not right then? The base is for a hot tub to go on, and one of the requirements for the warranty to be valid is a very good base so I took no chances!

 

[b3taman]

Ah! - a hot tub - that makes sense - water is heavy stuff.
Sounds like what you're doing is fine.
Not sure about the need for a membrane, but bit late now
I would either remove some of your rubble or attach some extra battens on top of your existing formwork to increase the concrete depth.
Your 0.5m of all in ballast (aggregate) and 14 bags cement will allow you to concrete to a depth of around 125mm, which is way over the top, so you could buy just 8 cement to begin with, concrete to a depth of 75 - 100mm and save your left over ballast for another job (Don't buy too much cement, it has a very short shelf life).

 

[paulweller84]

So if used 8 bags of cement would I need about 3/5 of the 0.5sqm ballast? Think I've calculated that right. And I'd just need to chuck cement, ballast and water in a cement mixer? Thanks for the advice so far, as you can tell I'm no DIY expert!

 

[b3taman]

Hello Paul - yes, your calculation is right.
A typical small mixer will manage about half a 25kg bag of cement at a time, plus about 12 shovels of aggregate. Just run a sharp stanley knife around your cement bag halfway up to give you the two roughly equal portions per bag.
Most book advice says you're supposed to mix the cement and aggregate first in the mixer, then add the water, but in practice most people usually put the water in the drum first otherwise you'll quickly get a thick layer of sticky cement/aggregate building up on the inner wall of the drum. How much water you'll need will be a bit of trial and error - depends on how wet the aggregate was when delivered. I would suggest it will be somewhere between 6-8 litres per mix. Too dry-add a bit more water - too wet - add a bit more cement. As you're going into pre prepared formwork then I'd make it on the sloppy side so it flows down into your hardcore a bit and is sort of self levelling.
Don't worry too much about exact quantities - it'll sort itself out and be much easier than you're imagining!
Have fun.

 

[r896neo]

75mm is to thin for a hot tub.

If i was doing it i would put a sheet of steel mesh in and 5 (125mm) inches of concrete.

It may seem like overkill but an extra 100 quid on materials in much cheaper than a split in your tub base due to the slab cracking.

If you ask your hut tub mftr i'm sure this is what they would recommend.

 

[paulweller84]

Aargh this is confusing, I can't get a definitive answer anywhere online. I did one of those calculator things and it said I needed 1440kg of ballast. This seems ridiculous for a 2x2m base. Tearing my hair out with this, everyone seems to tell me a different way of doing it. Absolutely no idea how much stuff to buy and if I need to take any rubble out first!

 

[r896neo]

Get a recommendation from the hot tub vendor what the minimum spec is for your warranty to be valid.

Generally for quantities over half a cubic metre its almost as cheap to get readymix delivered.

Mixing that much will take half a day and cost about 70 quid in materials whereas getting half a cube readymix from a mini-mix type outfit may only be 80?

you could leave out the mesh if you want to take a chance but you need a minimum of 100mm depth of concrete. 75mm is the minimum you can pour concrete because thinner than that it will crack up but it really has little strength at 75mm thick

 

[paulweller84]

Thanks for the advice people. I'm going to play it safe and get a ready mix delivery instead, didn't realise you could get deliveries of such a small amount. Fingers crossed this should be easy now!