Steel post installation design. Help needed

[Marvin]

I have to fit two 3mm bare steel 80mm by 80mm box posts into the ground. There is no room for a base plate and bolts to fix them to a lump of concrete so I am going to treat them like fence posts.  As the first 400mm down in the hole is unstable due to decking etc I have dug a hole 1200mm deep to take the posts. I am going to use Postcrete 

 

The posts will stand about 2400mm above the hole and have a horizontal force of about 60kg at about 2700mm above the solid ground.

 

What are the best options to avoid the post rusting when its in the ground.

 

Obviously painting the outside and covering the top.

Paint the inside?

Foam fill the inside? 

Concrete the inside?

Cap the top and bottom?

Vent the top to let humidity out but no rain in?

Wrap the bottom in plastic but make sure no water can seep between the plastic and the metal?

 

Suggestions please.

 

 

 

 

 

 

 

[Conor]

Have you ordered them yet? You really want galvanised posts.

Failing that, bitimous based waterproofing paint is appropriate. Cap it and don't worry about the inside.

[markc]

Hi, cap the top, no need to paint inside or fill.

Do not wrap with plastic, let the concrete get hold of the steel. When concrete is set, mastic or silicon where post enters concrete and also slope top of concrete away to shed water.

im guessing these are for a hammock or similar so not structural

 

Edited August 5, 2022 by markc

[Marvin]

2 minutes ago, markc said:

Hi, cap the top, no need to paint inside or fill.

Do not wrap with plastic, let the concrete get hold of the steel. When concrete is set, mastic or silicon where post enters concrete and also slope top of concrete away to shed water.

im guessing these are for a hammock or similar so not structural

 

Thanks yes not structural. Has Postcrete the same density as concrete (i.e. not porous)?

Would it be best to let the concrete rise up in the box steel or cap the bottom off?

[markc]

Just now, Marvin said:

Thanks yes not structural. Has Postcrete the same density as concrete (i.e. not porous)?

Would it be best to let the concrete rise up in the box steel or cap the bottom off?

No need to cap the bottom, the concrete will no rise very much.

postcrete is great, I usually throw 1 bag in to get a hold of a post them when it’s firm use more to fill and poke/mix to behave more like wet pour

[Marvin]

6 minutes ago, Conor said:

Have you ordered them yet? You really want galvanised posts.

Failing that, bitimous based waterproofing paint is appropriate. Cap it and don't worry about the inside.

Hi @Conor, Thanks.  External metal all to be painted. Below ground. Good idea about the bitimous based paint. I will use it below ground.

 

 

[Marvin]

2 minutes ago, markc said:

No need to cap the bottom, the concrete will no rise very much.

postcrete is great, I usually throw 1 bag in to get a hold of a post them when it’s firm use more to fill and poke/mix to behave more like wet pour

Yes I do the same. 👍

[Carrerahill]

34 minutes ago, Marvin said:

I have to fit two 3mm bare steel 80mm by 80mm box posts into the ground. There is no room for a base plate and bolts to fix them to a lump of concrete so I am going to treat them like fence posts.  As the first 400mm down in the hole is unstable due to decking etc I have dug a hole 1200mm deep to take the posts. I am going to use Postcrete 

 

The posts will stand about 2400mm above the hole and have a horizontal force of about 60kg at about 2700mm above the solid ground.

 

What are the best options to avoid the post rusting when its in the ground.

 

Obviously painting the outside and covering the top.

Paint the inside?

Foam fill the inside? 

Concrete the inside?

Cap the top and bottom?

Vent the top to let humidity out but no rain in?

Wrap the bottom in plastic but make sure no water can seep between the plastic and the metal?

 

Suggestions please.

 

 

 

 

 

 

 

 

First, I would get them galvanised or cancel and reorder galvanised steel posts. 

 

 

If it is all too late for that I would weld a cap onto them, then pour some thinners into it and swish around to degrease, then use something like Galvafroid paint or a good zinc chromate primer tipped into them, thinned a bit and swished, or get Dinitrol type stuff, swish it around until fully coated and tip out the excess. Let that dry. Then drill a vent hole, about 12mm (bigger on any side you wont see) just below the cap on each side, recoat with paint. Drill vent holes around the post about 100mm above what will be ground level, paint the holes. Sand and degrease the post and paint it properly, I would personally spray it with Etch primer and then primer it then 2k paint it, but DIY store products applied well will work too.

 

First fill your hole with clean gravel and set your post, then backfill for the first 100mm of the post with gravel, this will stop the post just rotting out in damp concrete and let it breath. Then fill with concrete. That should last well. 

 

Thing to consider, when this post is cold and air temps rise condensation will form inside and out, this cycling over time can lead to the inside slowly rotting away, so internal protection from good paint products is a must. You do not want to seal the post at the bottom by concrete or a cap as moisture will run down inside and pool and the rot will start. 

 

Essentially you want to do the opposite of what may seem logical at first, you want it to breath. I have seen raw steel sit outside for decades and never come to any real harm past surface rust because the steel has been able to breath internally and dry out quickly.

Edited August 5, 2022 by Carrerahill

[Marvin]

1 hour ago, Carrerahill said:

First, I would get them galvanised or cancel and reorder galvanised steel posts. 

 

 

If it is all too late for that I would weld a cap onto them, then pour some thinners into it and swish around to degrease, then use something like Galvafroid paint or a good zinc chromate primer tipped into them, thinned a bit and swished, or get Dinitrol type stuff, swish it around until fully coated and tip out the excess. Let that dry. Then drill a vent hole, about 12mm (bigger on any side you wont see) just below the cap on each side, recoat with paint. Drill vent holes around the post about 100mm above what will be ground level, paint the holes. Sand and degrease the post and paint it properly, I would personally spray it with Etch primer and then primer it then 2k paint it, but DIY store products applied well will work too.

 

First fill your hole with clean gravel and set your post, then backfill for the first 100mm of the post with gravel, this will stop the post just rotting out in damp concrete and let it breath. Then fill with concrete. That should last well. 

 

Thing to consider, when this post is cold and air temps rise condensation will form inside and out, this cycling over time can lead to the inside slowly rotting away, so internal protection from good paint products is a must. You do not want to seal the post at the bottom by concrete or a cap as moisture will run down inside and pool and the rot will start. 

 

Essentially you want to do the opposite of what may seem logical at first, you want it to breath. I have seen raw steel sit outside for decades and never come to any real harm past surface rust because the steel has been able to breath internally and dry out quickly.

Hi @CarrerahillThanks for your info. I did think of moisture problems, that's why I was thinking of filling using a foam gun. No air inside no moisture inside?

I did think of drainage at the bottom, However I was concerned with the water table.- No rain here for 7 weeks and the soil at 1.2m down is very  damp. Would it rise up the steel box when the water table rose again....?

 

 

[JohnMo]

I was told by materials engineer that it is best seal tubes.  Any atmosphere (oxygen) inside the tube is used up and cause a small amount corrosion, once that process has happened the atmosphere is inert, so no further corrosion occurs.

 

An open vent tube gets replenished with new oxygen so corrosion can continue.

[Marvin]

51 minutes ago, JohnMo said:

I was told by materials engineer that it is best seal tubes.  Any atmosphere (oxygen) inside the tube is used up and cause a small amount corrosion, once that process has happened the atmosphere is inert, so no further corrosion occurs.

 

An open vent tube gets replenished with new oxygen so corrosion can continue.

Yes this was my thinking....