Concrete

[Wendelspanswick]

I am in the process of sorting out the foundation pads for my barn/shed (10m x 5m). I've dug my 6 holes, 600mm x 600mm and 750mm deep which I now need to fill with concrete.
I am working from an Irish Goverment PDF on erecting agricultural buildings which is really handy:
http://www.agriculture.gov.ie/media...rmbuildingspecifications/pdfversions/S101.pdf
The guide specifies C28/35 concrete for the pads. Ideally I would get readymix in but the logistics of getting a concrete mixer or a trailer of readymix nearly a mile up a steep and rough forestry trail is out of the question.
So the plan is to collect 2 ton of 20mm to dust aggregate from the local quarry in my Fergie type 3 ton trailer and mix it up with my 3 point linkage mixer mounted on the John Deere.
It shouldn't be too bad as I will be shovelling the aggregate from the trailer straight down into the mixer which I can then tip straight into the holes. (Mixer has 1/6 of a cu.M capacity and I need 1.5 cu. M so 9 or 10 mixes).

So after all that rambling am I right in thinking that for each cubic meter of C28/35 concrete I need 350kg of cement? So for 1.5 cubic meters 525Kg of cement which seems a lot!

There are online calculators but none seem to specify what grade of concrete they are calculating for.

 

[AndyStobbs]

I'm no builder but that sounds like a stiff mix. 52kg in 1.5 cube sounds more likely

 

[the snooper]

it states here http://www.concretecentre.com/codes...te_standards/european_standards_concrete.aspx

Typically C28/35 or C32/40 concretes with minimum binder contents of 325 kg/m3 and maximum w/c ratio of 0.55 are used to provide for a good surface finish and ensure adequate abrasion resistance.

 

[mike os]

You should not use 20 to dust as it contains too much fines

Use 4 parts 20 mm ballast ( graded clean 20 to about 8mm) and 2 parts sharp sand

For 1.5m3 550kg cement is about right.. you need a lot of cement to achieve a proper concrete mix

2 tonnes of sand and ballast will make about 1m3 of concrete with cement added

this will achieve something approaching a c28 mix but getting spot on requires some science

 

[AndyStobbs]

Well I'm really surprised about that! Wouldn't of thought surface abrasion to be a big factor in a footing but non the less I'm amazed anything made with 3-2-1 is still upright.

 

[Sid]

I'm no builder but that sounds like a stiff mix. 52kg in 1.5 cube sounds more likely

That would be unbelievably lean! That's just over two bags of cement in 1.5 cubic metres! 52kg in about 3.6 tons of concrete.

See https://en.wikipedia.org/wiki/Types_of_concrete#Modern_concrete

 

[pedrobedro]

There's a calculator here for a 1:2:4 mix I think which is what I would use for a general purpose job like yours. I've just tried it and for 1.5 cube it comes out at 480kg of cement which sounds reasonable to me.

 

[AndyStobbs]

That would be unbelievably lean! That's just over two bags of cement in 1.5 cubic metres! 52kg in about 3.6 tons of concrete.

See https://en.wikipedia.org/wiki/Types_of_concrete#Modern_concrete

Well I did say I'm not a builder. So out of interest, at 500kg of cement to the cube, what percentage does that work out too by weight? - or tell me the density of concrete and I will work it out.

 

[AndyStobbs]

Looked it up, and going on 2400kg/cube its sounds stiff foe 500kg but a lot more normal than the 50kg as I said before - my brain must be taking a day off today.

 

[Wendelspanswick]

Off to Wickes then for 22 bags of cement, not too bad at £4.06 per bag if you buy more than 5.
I think I will ring them beforehand and get them wrapped onto a pallet so they can fork it straight onto the trailer.
Thanks for confirming my thoughts.

 

[Tigman]

get 24 bags and put 4 in each hole , seems like a lot of cement to me . but then i only use the readymix stuff or postfix