Concrete pad for external water tank

[Pord67]

Hi all

I'm about to put in a concrete pad for an external water tank that will weigh 3 tons when full. This will be the biggest concrete job I've tackled and I'd appreciate your input please.

 

The top soil has been dug away and the pad will sit on the surface of very heavy clay sub soil (currently also supporting the foundations of our self-build house). Dimensions of the pad are 2700mm square and 125mm thick.  A C40 mix seems to be best for septic tanks so I'm assuming the same for this. I'll be mixing it myself, and I plan to add grid rebar. 

 

My questions are:

 

1. should I put down membrane and gravel layer first? If so, can I use a random mix of rocks and stones (we have plenty on site)?

2. there's currently a layer of soupy churned up clay on the surface, is it best to remove this first?

3. I've found variations online for a C40 mix, can anyone tell me the correct mix?

4. If necessary could I pour the slab in two sittings? 

 

TIA, any other suggestions welcome.

[markc]

Thats a big pad for a 3t tank. do you really need a full slab?

no need to clean off the clay, plenty of hardcore and compact it.

do not pour it in two haves, no need for an expansion joint etc. so a double pour just adds a potential failure/break point.

 

PS 2700x2700 with 3 ton tank is only 411kg/m2 - very low loading

Edited January 20, 2021 by markc

[Conor]

That's basically a cube. Might be worth speaking to local concrete suppliered and see if you can get a mini mix delivers? You don't want to do it on two pours.

 

You can pour straight on to the clay, just scrape away any sludge.

Edited January 20, 2021 by Conor

[Pord67]

Thanks, appreciate the replies.

 

The pad is sized to keep to the tank manufacturer's recommendations, ie 600mm wider than the tank diameter (2135mm) and 100mm thick for tanks up to 2500l or 150mm for tanks of 4000l plus. I've plumped for 120mm for our 3000l tank. They say their warranty is invalid if concrete isn't used, and presumably also if these dimensions aren't roughly adhered to. They don't specify the mix, I just assumed our tank to be similar to a septic tank in weight and size.

 

Having said all that, I'd very happily grab any opportunity to reduce the size/cost/work involved. Our site is difficult to access up a potholed track and even then the mixer truck wouldn't get closer than about 30m to the pad location, which is why I've resigned myself to mixing myself.

[dpmiller]

No digger or dumper on site to ferry it?

[Pord67]

28 minutes ago, dpmiller said:

No digger or dumper on site to ferry it?

No, just me and a wheelbarrow. Getting the mixer up our track is unlikely anyway.

[TonyT]

Put a sheet of plastic on a path/road, few cones etc 

 

get ready mix dumped onto plastic and then barrow away, much easier way of doing it

[Pord67]

On 20/01/2021 at 14:55, markc said:

Thats a big pad for a 3t tank. do you really need a full slab?

no need to clean off the clay, plenty of hardcore and compact it.

do not pour it in two haves, no need for an expansion joint etc. so a double pour just adds a potential failure/break point.

 

PS 2700x2700 with 3 ton tank is only 411kg/m2 - very low loading

Thanks for that Mark. Is a C40 mix overkill? 

[markc]

28 minutes ago, Pord67 said:

Thanks for that Mark. Is a C40 mix overkill? 

Hi, Yes C40 is overkill - used for high strength beams etc. C20 would be adequate  for your use.

 

[markc]

@Pord67, just a thought - your tank is a flat bottom variety i assume, or is in on legs? - concrete strength and slab design will be very different

[Pord67]

Thanks Mark. Yes, flat bottomed tank. Presumably still worth using rebar with a C20 mix?