Concrete Pier - Concrete Mix

[Martin Farmer]

Hello all,

I'm currently in the process of building an observatory and I would loke to canvass thoughts on the type of concrete mix or ready mix (with or without gravel) you have used on your concrete pier.

I have a carboard tube hats 14" diameter that I intend to use 4 16mm rebar in.

My gut tells me to use a high strength concrete ready mix without gravel but i would like confirmation that I'm following the right path.

 

Thanks

Martin

[Owmuchonomy]

Standard 3:1 mix, no gravel.  The block should be a minimum of 600mm cubed. You will be disappointed with the outcome of a cylinder only 14" in diameter. I used 700mm cubed  and that was nice and solid.

[Martin Farmer]

Thanks for the reply,

Further clarification on my previous post.

I have already cast a concrete base that's quite thick, I intend to drill into this @ 200mm deep and use chemical resin to fix 4 M16 studs 1 meter long.

Over the top of this i'm placing a carboard tube 14" in diameter and 900mm tall - this is the pier section I am referring to.

But it was the concrete itself that I wanted confirmation on.

 

Regards,

Martin

[malc-c]

I just used a 3:1 mix of sand/stone mix to cement (including the stones as a sand/cement mix is technically a mortar mix) when filling the tube with rebar for my mount.  

 

[whipdry]

I used bagged 20mm ballast it's a mix of sharp sand with stones upto 20mm, 3:1 mix... poured in a bucket full at a time, every 2 to 3 buckets a pumping action with piece of wood in the concrete will help release air bubbles. Don't know why I only used 3 lengths of threaded s/s rod for fixing the top plate but would use 4 if I ever made another concrete pier.

 

Peter

[iapa]

On 26/05/2024 at 10:12, Owmuchonomy said:

Standard 3:1 mix, no gravel.  The block should be a minimum of 600mm cubed. You will be disappointed with the outcome of a cylinder only 14" in diameter. I used 700mm cubed  and that was nice and solid.

Over the years, I’ve seen a number of articles that suggest a smaller base than is recommended in the Northern States of the US ( where ground frost is more of an issue long terms), and where the 3’ x 3’ came from as far as I can determine.

I put in 2’ x 2’ at ly location (Central Belt of Scotland) and that hasn’t budged as far as I can tell.

Certainly one PA set up, it didn’t shift for several months - until I did a complete tear down for dome installation, haven’t set up again yet