How many bags of cement for fenceposts?

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meekzone

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#277237 2-Oct-2020 19:23

Need approx half cube of concrete for fenceposts. Best option seems to be mixing it myself with a concrete mixer. Based on google research, recommended ratio is 1-2-3 (cement-sand-aggregate).

The problem is, builders mix is sold per cubic meter, cement is sold in kilos. So just how many 20kg bags of cement do I need for 0.5 m3 of concrete?

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#2578297 2-Oct-2020 19:43

See 'recommended mixes and working out quantities for some more useful ratios here https://www.goldenbay.co.nz/applications/diy/

8x 20kg bags for 0.5m3, assuming 1:5 (3:2:1). That guide suggests you could get away with less cement for fenceposts - 1:8

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timmmay

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#2578298 2-Oct-2020 19:44

According to my back of the envelope maths that's ten thousand bags of cement.

blackjack17

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#2578309 2-Oct-2020 20:35

nickb800:

See 'recommended mixes and working out quantities for some more useful ratios here https://www.goldenbay.co.nz/applications/diy/

8x 20kg bags for 0.5m3, assuming 1:5 (3:2:1). That guide suggests you could get away with less cement for fenceposts - 1:8

according to that link

5 bags of 40kg cement to 1.25m3 of builders mix

so 1 bag per 0.25m3

for 0.5m3 2 x 40kg of cement which = 4 x 20kg

OP check the price difference of hiring the mixer, getting the builders mix delivered going and getting the cement vs just buying ready mix

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mentalinc

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#2578325 2-Oct-2020 20:54

Buy a few bags of the ready mix stuff and pour it in the hole and add water per the guide on the back!

Lifes too short to be mucking around with making the same thing yourself.

Plus the stuff in the bag sets hard in like 10 minutes (but takes a while to cure to strength), why mess around with the sloppy stuff.

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meekzone

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#2578342 2-Oct-2020 22:07

nickb800:

See 'recommended mixes and working out quantities for some more useful ratios here https://www.goldenbay.co.nz/applications/diy/

8x 20kg bags for 0.5m3, assuming 1:5 (3:2:1). That guide suggests you could get away with less cement for fenceposts - 1:8

LEGEND mate, that's exactly the type of info I was looking for, lot harder to find than you'd expect.

meekzone

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#2578343 2-Oct-2020 22:08

blackjack17:

OP check the price difference of hiring the mixer, getting the builders mix delivered going and getting the cement vs just buying ready mix

I'm going with 1:6 cement:mix. From Recommended concrete mixes and raw material quantities, for 0.5m3, that's 120kg cement and 0.63m3 mix. That's approx $100 + fuel for towing. That's significantly cheaper than 26x2 bags of premix (~ $450).

meekzone

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#2578344 2-Oct-2020 22:09

mentalinc: why mess around with the sloppy stuff.

It's a lot of post holes to fill unfortunately. $200-300 saved is a LOT of steaks on the barbie, nomsayin.

djtOtago

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#2578346 2-Oct-2020 22:16

mentalinc: Buy a few bags of the ready mix stuff and pour it in the hole and add water per the guide on the back! Lifes too short to be mucking around with making the same thing yourself. Plus the stuff in the bag sets hard in like 10 minutes (but takes a while to cure to strength), why mess around with the sloppy stuff.

The OP requires 0.5m3 of concrete. That would require about 50 20kg bags of ready mix quick set concrete. 20kg bag of ready mix only makes about 0.01m3 of concrete

About $9.00 a bag at M10 so about $450.00 worth.

meekzone

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#2578347 2-Oct-2020 22:22

djtOtago:

The OP requires 0.5m3 of concrete. That would require about 50 20kg bags of ready mix quick set concrete. 20kg bag of ready mix only makes about 0.01m3 of concrete

About $9.00 a bag at M10 so about $450.00 worth.

That's a bingo.

panther2

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#2578367 2-Oct-2020 23:35

Used about 25 20kg bags on our picket fence. Just used ready mic huge time saver as opposed to mixing your own

MrAmerica

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#2580095 6-Oct-2020 14:13

mentalinc:
Plus the stuff in the bag sets hard in like 10 minutes (but takes a while to cure to strength), why mess around with the sloppy stuff.

Nothing like a few beers after all the sweat, noise and cement dust, and waiting for the concrete to set. 10 minutes, that's only one beer, what's the point of making concrete if there's only one beer in it?

sidefx

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#2580096 6-Oct-2020 14:17

MrAmerica:
Nothing like a few beers after all the sweat, noise and cement dust, and waiting for the concrete to set. 10 minutes, that's only one beer, what's the point of making concrete if there's only one beer in it?

Drink faster.

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mentalinc

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#2580101 6-Oct-2020 14:23

MrAmerica:
mentalinc:
Plus the stuff in the bag sets hard in like 10 minutes (but takes a while to cure to strength), why mess around with the sloppy stuff.

Nothing like a few beers after all the sweat, noise and cement dust, and waiting for the concrete to set. 10 minutes, that's only one beer, what's the point of making concrete if there's only one beer in it?

But there are 50 bags, so probably a good ratio of beer to posts and bags of pre mix could be found.

You've also forgotten to allow for the job done in 5 hours vs 4 days in the costs calculations.

Mates helping for an afternoon is a few beers and they're happy. two weekends back to back, much bigger BBQ required to say thanks!

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gregmcc

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#2580215 6-Oct-2020 17:17

Rather than concrete there is this 2 part stuff kind of like expander foam, but for fence posts

http://www.baxgroup.co.nz/Kongcrete.html

There may be others, not sure on how cost effective it is compared to concrete.

WinNZ90

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#2580252 6-Oct-2020 19:40

I was just wondering how long are your posts, I know at 1.8ms, post go .5 to .8 in the ground depending on where the fence is going to be and what it will be keeping out/keeping in/holding up.

At the base of those holes, you have a layer of gravel approx 50mm thick and then once the post is in another 50mm around the post. And then 1 and half bags of rapid set concrete. This would give you space leftover at the top, to cover over with dirt nicely.

Using the gravel also means that you won't be replacing the post in 10 to 15 years time.

Once post is in the ground, before adding concrete, take three boards, on 3 of the 4 faces (if your using square posts) nail a board on. Then make sure the post is straight and put something heavy on the ends of the boards to hold them in place on the ground. Check for straightness again, do adjustments if needed, then add concrete.

It pays to also stir/poke/mix the concrete around in the hole a bit to sometimes, just to make sure that air pockets have been removed.

This will cut down on the cost of beers and steaks too

If you have big posts, let me know and what sort of movement they are going to experience over time

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