Ready mix mortar for sail posts?

[Min]

I've bought a 4m x 3m garden sail with 2 galvanised steel posts (the other two corners will be attached to the house). The instructions say the poles should be set in holes 400mm x 400m x 800mm deep. The bottom 100mm should filled with gravel.
I'm trying to work out how much ready mixed mortar it'll take (and the price).
Does this seem right:
Volume needed is 2 x .4m x .4m x .7 = 0.224 cu metres.
I found an online calculator that suggests a yield of 0.0089285, so I calculate I'll need 25 bags of these 20kg bags of Blue Circle Ready Mix mortar. At just over £10 per bag, that works out at £252. It's made my eyes water!
Is that right?? I had no idea mortar was so expensive. Or is my calculation way out? The online calculator *seems* to give the answer in number of bags, but is that correct?
Thank you!

 

[Charlie George]

Mortar is the wrong material to use, postcrete or concrete is what you need, postcrete will be roughly half the price of your mortar, or if you mix your own concrete even cheaper again, size of the holes does seems big but there will be a lot of wind force with the sails.

 

[JohnD]

I'd also paint the galvanised legs first, at least the buried part and preferably 150mm above. They will rust quite quickly when buried, despite the galvanizing. Use an oil-based paint system or bituminous paint. Not hammerite.

To make the concrete look neat, make square wooden formwork, just at ground level, plus a few inches above and below to allow for movement. This visible part need not be 400x400, just the size of the legs plus 100 all round, and the top angled to shed rain.

With the money you save by mixing your own, buy a plastic spot board/mixing tray with raised edges.

Active Plastic Mixing tray (L)950mm (W)950mm | DIY at B&Q

This Mixing tray from Active helps to protect the surface around where you're working. It can be used to mix paint, cement or plaster. It is also safe for children's sand play.

www.diy.com

 

[Mr Chibs]

Mate did a job a few weeks ago, a slab, I think he wanted 1m³ (cube) of concrete, he ordered actual two tonnes (not jumbo bags they are 850Kg) of ballast and from memory around 15 bags of cement... so by your calculations, you need around a fifth/quarter of this.

You could sieve out some of the ballast, and use the larger gravel as your bottom layer.

Good luck

 

[Min]

Wow - brilliant tips. Thank you so much guys
Just so I'm clear . . . . a few more questions.

JohnD
1. I'm sinking a pole holder into the concrete. Although the seller says "Sleeve is heavy duty 5mm galvanised steel - it will last a lifetime" I will paint its outside. Will Thompson's Emergency Roof Seal be ok? It seems just like the last tin of bituminous paint I had!
2. Once installed, should I spray WD40 into it to keep it from rusting?
3. For the formwork do you mean something like this cardboard mock up (though I've made it a bit steep)? (Hoping to make the top of the concrete and sleeve just below level of the turf, so not a trip hazard when the pole is removed during the winter.)
4. Should I make the top couple of inches just cement, no ballast, so it's smooth?
5. Found a decent looking mixing board. Good idea - long long time since I mixed concrete.

Charlie George
I know, seems like big footings but I live next to Shirley Moor (in Kent) and daren't go against the sellers' recommendation.
5. They also say don't use postcrete. Is it much weaker than regular concrete?

Mr Chibs
Helpful to see your mates' quantities. Looks like if I order about half a tonne of ballast plus 4 bags of cement, that should be enough.
7. This ballast comes in approx 22kg bags. So that's 23 bags to make the half tonne. Does that seem right: 23 bags of ballast to 4 bags of cement? When I mixed concrete (long time ago) it was 3 shovels of ballast to 1 of cement. (I know volume is different to weight.)
8. I'll get this cement 25kg bag, Blue Circle General Purpose Concrete
9. For the 100mm bottom layer, does the shape of the ballast matter? I have a load of stuff I'm getting rid of from the front garden. It's flat edged, not smooth and pebbley. Pic attached

Thank you!

 

[Mr Chibs]

I think he was using 5:1 on the mix front, i.e. one cement for every 5 of ballast, the more cement the stronger the mix... but doubt you'd need 3:1 for what you are proposing.

I'd buy a jumbo bag of ballast from a logo builders merchants, they will deliver, cement too.

Doesn't matter what gravel you put in... if you have loads of that, why not get grit sand and make your own concrete using the 'stuff' you have, that's all ballast is, a mix of grit sand and pebbles.

Not sure If I'd paint the posts, probably not, but have done so on wooden ones.

Good luck

 

[JohnD]

JohnD
1. I'm sinking a pole holder into the concrete. Although the seller says "Sleeve is heavy duty 5mm galvanised steel - it will last a lifetime" I will paint its outside. Will Thompson's Emergency Roof Seal be ok? It seems just like the last tin of bituminous paint I had!
2. Once installed, should I spray WD40 into it to keep it from rusting?
3. For the formwork do you mean something like this cardboard mock up (though I've made it a bit steep)? (Hoping to make the top of the concrete and sleeve just below level of the turf, so not a trip hazard when the pole is removed during the winter.)
4. Should I make the top couple of inches just cement, no ballast, so it's smooth?

1 I should think so. The hole will fill up with water and mud and the steel will rust, inside and out. You can make a plug for it with a mushroom top. There might be a plastic plumbing part that will fit. Otherwise use your wet vac to suck it out each year

2 no. I don't think it will do any good and may degrade the paint.

3 do a pyramid if you want. I meant just a square, which can be small and neat. Or a round one if the pole is round. If you fill up the hole to the top it will look awful. If it is below the turf it will fill up with mud

4 no. Fresh concrete can be trowelled smooth before it sets. Don't overwork it.

 

[Min]

Mr Chibs, John D - thank you so much. Great advice.