Fence post mix ratio?

[Tony K]

Evening. I'm going to be installing 2.7m concrete fence posts, and rather than buying the postcrete, I've got a load of bags of cement to use up. 

 

Question is, what mix ratio is best for the job? I have heard I can use aggregate and sand, but what ratio? 

 

[Barryscotland]

1 cement 2 sand 4 aggregate

[Tony K]

Cheers Barry. 

Any idea how much cement I'll get through per post? The posts are 10cm wide, so the hole will be 30cm2 and 65-70cm deep. 

[tonyshouse]

More than you think 🙂

 

i cheat and for a 600deep hole I prop the post in a perfect position on a string line at the right height then fill to 150mm deep with concrete and ram then fill with arisings a further 200mmamd ram then more concrete to within 50mm of surface and ram and flaunch , makes them easier to get out again. 

[Temp]

Volume works out at 0.05 cubic meters per post. The calculator here..

 

https://everydaycalculation.com/concrete.php

 

Says for each post/0.05m^3 you need...

 

Cement 16kg

Sand 0.02 m^3

Stone 0.04 m^3

 

It can also work out how many 50kg bags you need or double up if you have 25kg bags.

 

[PeterW]

How many have you got to do..? You may struggle getting a neat 300mm hole that deep but if you work on it being roughly 2 cubic feet that’s going to take 4 buckets ballast, 2 sand and 1 cement so you’ll probably only get 2 posts to the bag if you’re lucky. 

[Tony K]

Thanks all. 

I've got nine holes to do. I'm trying to compare the cost of getting enough sand and stone with the cost of just biting the bullet and getting postcrete. 

 

A 25kg bag of sand yeilds about 0.015m3, so based on the calculations above I'll need at least one bag of sand and two or three of stones per post. Call it ten bags of sand and thirty of stone altogether. If I'm lucky I'll get the bags for £3 each, so £120 in materials, as I have the cement already. 

 

Alternatively, I reckon I'll need three bags of postcrete per post, which means about £160 if I go down that route. It will of course be quicker and easier with postcrete. 

[Iceverge]

Like @tonyshousesays. Just do the bottom ~150mm in concrete. Fill in to just below the surface with whatever. Tamp firm. Then do the 150mm just below the surface with more concrete. 

 

BTW if you have the patience adding soil in 50mm layers and really compacting well will result in a rock solid fence. No concrete needed, but it is time consuming. 

[Conor]

Any rubble? Fire in a few broken bricks and leave good gaps and you'll have the amount of concrete needed.

[Gone West]

7 hours ago, Iceverge said:

BTW if you have the patience adding soil in 50mm layers and really compacting well will result in a rock solid fence. No concrete needed, but it is time consuming. 

+1