Thread: Concrete floor for water tank. Some questions.

I'm going to be doing a concrete floor for a water tank to sit on. The size for this will be 2.1 x 2.1 metres and 100mm thick. I will be boxing it all up and using F72 reo.

I've calculated the amount of concrete required and it came to .44 cubic feet. That is if my calculations are correct.

Just a few questions.

Will treated pine be suitable for the boxing, we have some at work and I think it's around 25mm thick. I was just planing to nail it all together and then remove the lot when the concrete is finished.

Is that reo suitable, we just had a slab done for a steel shed and that was what was used.

Should I use concrete pre mix bags or cement bags and I think you add crushed rock, sand etc?

How do you work out how many bags are required.

Pretty new to this but am willing to learn.

Thanks

Originally Posted by Com_VC

I've calculated the amount of concrete required and it came to .44 cubic feet. That is if my calculations are correct.

No, it's 0.44 cubic metres

Will treated pine be suitable for the boxing, we have some at work and I think it's around 25mm thick.

Just use plenty of pegs to support the boxing - concrete is surprisingly heavy.

Is that reo suitable, we just had a slab done for a steel shed and that was what was used.

Yep should be fine

Should I use concrete pre mix bags or cement bags and I think you add crushed rock, sand etc?

I would buy the cement and all-in (sand/aggregate) and then mix on site, but it is personal preference (doing it this way is normally cheaper).

How do you work out how many bags are required.

8 x 20kg bags of cement (16 per m3) and 0.6m3 of all-in (1.3 per m3)

Premixed bags (I think) work out to about 44 x 20kg bags

Hi,

Can you give me an a rough idea on the cost?

Also will it be the same strength as using pre mix which I believe is 20mpa?

I bought 5x premix bags the other day at $8.50 each. I'm guessing this way would work out much cheaper?

Thanks

Originally Posted by Com_VC

Hi,

Can you give me an a rough idea on the cost?

Also will it be the same strength as using pre mix which I believe is 20mpa?

I bought 5x premix bags the other day at $8.50 each. I'm guessing this way would work out much cheaper?

Thanks

real rough cost for .5m3 is about $40 for sand & stone, and about $50 for cement, 20mpa is 12 x20kg bags per m3, I am not 100% sure, but I don't think the bags of pre mix are 20mpa (been too long since I was in the game

What mpa would be required?

Also with the shed we had to have a 50mm sand bed underneath the concrete, would this be required?

Should I use a plastic membrane as well?

Hi CVC,
Terrian's estimates are probably pretty good.

I think the tank manufacturers specify 20mpa be used. Don't know about the sand bed, but when I did mine (I've done 3 in the last 6 months), I didn't use a sand bed but I did use plastic. One of the benefits of plastic is that it helps reduce water loss from the concrete during curing.

Just made a call for a quote.

They didn't have an all in one mix, but I assume this will be pretty much the same thing?

Sand $50m3
Stone $55m3

12x 20kg bags of cement $112.20

Does the price for the cement seem a bit excessive or is this the going rate these days?

Originally Posted by Com_VC

What mpa would be required?

20mpa will be fine, that the water tank is a static load.

Also with the shed we had to have a 50mm sand bed underneath the concrete, would this be required?

nope.

Should I use a plastic membrane as well?

only if you are concerned that you water tank might get a wet bottom (ie: no

Originally Posted by Vernonv

Hi CVC,
Terrian's estimates are probably pretty good.

One of the benefits of plastic is that it helps reduce water loss from the concrete during curing.

and that can be a problem....better to lightly wet the area (and I do mean lightly!), and really that is only if if it is a really hot day...

Originally Posted by Com_VC

Just made a call for a quote.
They didn't have an all in one mix, but I assume this will be pretty much the same thing?
Sand $50m3
Stone $55m3

sounds about right for the prices, but make sure it is concrete sand, not brick sand... and the stone should be 14mm agg.
keep in mind that 1/2m3 of sand wont be $25, more like $30

12x 20kg bags of cement $112.20

Does the price for the cement seem a bit excessive or is this the going rate these days?

yep, thats a little on the high side ($9.35 per bag) I would expect $7.50 - $8.50

Just where are you located ?

I'm located in Cranbourne, but can probably travel a bit. I'm often in the Dandenong and Frankston area.
I remember someone saying once that you can get slightly lumpy cement at significantly reduced prices, would be classed at seconds I guess. Seeing that I’m doing a water tank base I’m guessing it wouldn't cause any issues seeing that it will mostly be covered anyway.<o></o>

By the way, thanks for all the help.

Originally Posted by Com_VC

I'm located in Cranbourne, but can probably travel a bit. I'm often in the Dandenong and Frankston area.
I remember someone saying once that you can get slightly lumpy cement at significantly reduced prices, would be classed at seconds I guess. Seeing that I’m doing a water tank base I’m guessing it wouldn't cause any issues seeing that it will mostly be covered anyway.

<o></o>Cranbourne Sand & Soil would be the garden supplies to go to. Can't say I have ever seen 2nds bagged cement (I was in garden supplies retail for 12 or 13 years).

By the way, thanks for all the help.

thats what the forum is for

Yeah Cranbourne sand and soil is where I got those prices from.

Is concrete sand a different colour to brick sand?

Also what is the mixing ratio that I should use? cement, stone, gravel.

I'm guessing with the area that I have to concrete i'd be better off investing in a cement mixer rather than mixing it all up in a wheel barrow. Might look out for some second hand ones.

Thanks again

Originally Posted by Com_VC

Yeah Cranbourne sand and soil is where I got those prices from.

Is concrete sand a different colour to brick sand?

yep, the brick sand will be a yellowish / orange colour or white, the conce\rete sand looks more like beach sand.

Also what is the mixing ratio that I should use? cement, stone,gravel.

make that stone, sand, cement 3stone, 2 sand 1 cement, but don't just shovel it, use a 10lt bucket for the measurements...

I'm guessing with the area that I have to concrete i'd be better off investing in a cement mixer rather than mixing it all up in a wheel barrow. Might look out for some second hand ones.

It is only about 12 barrows per cubic meter

Bunnings had 2.2cubic foot mixers for about $400, but really, unless you have lots more to do, just hire one (there is a hire place just east of CSS I think)

Why on earth are you using concrete?

Most of the tank suppliers up here recommend a base of compacted crusher dust--or concrete. (And there's been a hell of a lot of tanks put in around Brisbane in the last 12 months)

I recently put in 3 tanks (2 x 2500L and 1 x 5000L). Simply made a box out of 100 x 50 treated pine, filled with crusher dust. Wet it down and compacted as I went.Added about half a bag of cement to the top 25 mm. Works just great.

Damn side easier and cheaper than concrete and does the job. Just make sure its all reasonably level.

I used concrete for all my tanks (3 so far, 2 to go) because they are not sitting on completely level terrain and (from experience) sand and/or gravel bases wash away and you end up with a tank on an angle or not properly supported.

I would only consider using a non concrete base if the surrounding terrain was perfectly flat and the likelyhood of water running across the site minimal.

If you have a bit of time keep an eye out for any concrete trucks in your area or formwork ready to get poured. When they have finished with a job one of the concrete truck driver's problems is what to do with any leftovers when they are cleaning up. I have gotten quite a few box trailers of leftover concrete to do small slabs, and sections of bigger ones, that way. It is important not to get in their way or make a nuisance of yourself as they are trying to get the job done as quick as they can, but if you are in the right place at the right time you could score the leftovers for a slab or gratis. Lay a plastic tarp in the trailer or back of the ute to make removal or cleaning up easier.

When I put a tank in I wanted it high enough off the ground to get a bucket/watering can under the tap so I put it on a concrete slab.

Originally Posted by Terrian

only if you are concerned that you water tank might get a wet bottom (ie: no

If you are putting reo you need a plastic membrane to prevent problems with ground moisture.

Originally Posted by DvdHntr

If you are putting reo you need a plastic membrane to prevent problems with ground moisture.

tell that to all the tens of thousands of people around Australia that have concrete driveways or garage floors )

Put it this way, it is *not* common practice to lay plastic unless the slab is for a house etc...

Originally Posted by Ronaldo451

If you have a bit of time keep an eye out for any concrete trucks in your area or formwork ready to get poured. When they have finished with a job one of the concrete truck driver's problems is what to do with any leftovers when they are cleaning up.

good point, but bear in mind that this is really only the end of the working day, common practice is to batch on the last load.

i'm with jack52, why go to that much trouble for a water tank, there are heaps of products that compact well, lillydale topings, crusher dust as stated.

Originally Posted by Dan574

i'm with jack52, why go to that much trouble for a water tank,

Because possibly it's your only source of water and you would rather not risk it.

there are heaps of products that compact well, lillydale topings, crusher dust as stated.

True but concrete will not wash away over time, whereas as "compacted" material will (except of a completly flat site).

Originally Posted by Vernonv

True but concrete will not wash away over time, whereas as "compacted" material will (except of a completly flat site).

Also, the likes of crushed rock / lilydale toppings has really nice, sharp stones that can (and will) make nice little holes in the bottom of your water tank )

My water tank is stainless steel so I cant see it getting any holes in a hurry

Although i'm still not too keen on a compacted material, crushed rock etc. I'd rather a solid concrete base that i'd know wont cause any issues in the long run.

F52 reo will suffice, but since you have the thicker F72 that will be fine, just make sure no parts of steel protrude the slab. Just lay a compacted crushed rock base about 75mm, and peg in your formwork on top

If you are getting in a concrete truck, you will have to get .6m3 as they work in multiples of .2m3 intervals. They will sting you for a low quantity fee however, plus wash out fee's etc so the cost will come out to around the $150-$170 mark. It will however, be the easiest, most convenient way of getting your concrete in. The faster you get it inside the boxing and screeded off, the easier it will be for you. Try a smaller concrete company, they usually are cheaper than the bigger ones.

Buying pre-mixed bags will be a pain, they dont go far. Plus will cost you a fortune, probably close to $200, to get enough to do the slab.

Getting the raw materials will save you a bit, but you'll be mixing about 7 barrows up, preferably with a mixer to get an accurately mixed batch.

I'de bite the bullet and order the conc...save your back and time.

Originally Posted by Com_VC

My water tank is stainless steel so I cant see it getting any holes in a hurry

oh y, sounds like 'famous last words'

I can't understand why everyone wants to put tanks on a concrete slab. A friend of mine is a corrugated iron tank maker and when he supplies tanks he advises installing them on corrugated iron. Reason - tanks sweat underneath and it stops water build up. OK important for iron tanks but same reasoning for plastic. When we installed 4 x 5000 gal tanks we laid a couple of rows of sleepers then put plain zinc coated corrugate iron down and installed the tanks on top - 20 years later and still good.

hire a compactor for a couple hours of an afternoon, and get [1m3 or 2m3] of crusher dust delivered fill the area and compact it level and theres your spot for water tank or tanks.
A hell of a lot cheaper just make sure its level. Try and keep the crusher dust at least 50mm thick minimum and exceed the width on both sides of the tank by 300mm

Just got a price on the crusher dust and it's $56 for 1m3 so not so bad.

He also suggested red road gravel, it's $38m3 but doesn't compact as easily apparently?

The red road gravel might look a little better?

Also would I get away with making my own compactor. Piece of board with a wooden handle?

Thanks

20kg bag of concrete at bunnings $6.70 per bag