How much cement for reinstalling gate posts?

[Justin3]

Hi, I am reinstalling our 4.4m rural gate on powder-coated steel posts so I can automate the gate with an Automaton swing arm mechanism. Although the gate itself is not very heavy (~38kg) it spans over 4.4m so I want to make sure I use the right amount and sort of concrete mix. What sort of ratio should I mix please, and how many bags would I need given the posts are 2m tall and 10cmx10cm, of which about 900mm will be underground? I plan to screw some cross pieces to the bottom half of the posts to help reduce movement within the concrete, and I imagine I will need to dig holes 200-300mm diameter x 1m deep. Many thanks for your assistance

[JacobZ]

Hi @Justin3,

Thank you for your question.

Your dimensions and current plan sound excellent.

Start by digging a hole 300mm in diameter and 1 metre deep. Put 100mm of compacted drainage gravel at the base of the hole. This will act as a solid base for the p,ost to sit on and allow any water that might get down the hole to drain through instead of sitting at the base of the post causing issues.

I'd suggest using post mix concrete, which is specifically designed for setting posts. It will set up slightly quicker than a standard concrete mix so that work can continue faster.

With the 100mm of drainage gravel at the base of the hole, you would be left with a cylinder that is 900mm tall and 300mm in diameter to fill with concrete. This is 0.06m3 of space to fill. As a 20kg bag of concrete is roughly 0.01m3 in volume, you will need 6 bags.

When the concrete is in the hole, agitate it to remove any air pockets, then plumb the post in both directions using a spirit level. Brace the post in both directions using some spare timber and stakes to hold it in position while the concrete hardens. 

Check out How To Install Posts For a Picket Fence for some further guidance.

Let me know if you have any further questions.

Jacob

[Justin3]

Thanks Jacob but I'm not too worried about the setting time and would think mixing my own blue metal, sand and cement would be a lot cheaper wouldn't it, particularly because I already have the blue metal? I'd need 12 bags of the post mix concrete for the two posts which would be ~$102.

[JacobZ]

Hi @Justin3,

If you already have materials which you'd like to use, then you could mix 1 part cement with 2 parts sand and 3 parts aggregate (blue metal), which is a stock standard mix for concrete. 

Using this ratio, you would need 2 x 20kg bags of cement and 4 x 20kg bags of sand, which certainly works out cheaper.

Let me know if you have any further questions.

Jacob

[Justin3]

Great, thank you! So that's 2 cement bags per post isn't it? Many thanks

[JacobZ]

Hi @Justin3,

It would be 1 bag of cement per post.

Each post hole will need 0.06m3 of concrete -

• 0.01m3 of cement (1 x 20kg bag)

• 0.02m3 of sand (2 x 20kg bags)

• 0.03m3 of aggregate (roughly 60kg of blue metal)

If you were using bags of each material, it is really easy to keep track of the ratio, but considering you will be using bags as well as your own materials, which might not be packaged according to its weight, the best thing to do is use a shovel to control your ratio while mixing. 

Add 1 shovel of cement, 2 shovels of sand, then 3 shovels of aggregate, keeping each shovel approximately the same size to ensure you keep a consistent mix.

Jacob

[Justin3]

So good. Thanks for all your help, I really appreciate it!