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Concrete base question...


D33P

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Hi everybody peeps,

I am looking to put down a concrete base to fix a pier to in my lawn, roughly 1m3

My friend is going to weld up a pier for me, and another friend is going to powder coat it.

So all I have to do is put down a firm base to fix it to... Happy days!

Am I right in thinking that I can mix up to the usual ratio of 3,2,1 and just put it in the hole dry?

The idea being that moisture will be pulled from the surrounding ground to set the concrete. Thinking this will be better to tamp it down nice and tight and remove the chance of air pockets.

Am I on the right track, or barking up the wrong stick?

 

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Hmmmm, don't think I would do that, as it will work in time, but will take a good while to get rock hard all the way through, much better to do it wet me thinks.....:)

unless you are happy to wait a good few weeks.....or more

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9 hours ago, D33P said:

 

Am I on the right track, or barking up the wrong stick?

 

Sorry James but I'm afraid you are definitely barking up the wrong stick.

as mentioned before I've only ever seen this method used for garden fence posts and such, never a large quantity of concrete. The issue is you are likely to have pockets of the mix that have more concentrations of cement or sand and it is this that will likely cause you problems later.

unfortunately a wet mix would be the only solution to ensure you have a uniform and stable mix.

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Thanks for the replies guys.

I knew it was a common way of putting up fences, but wasn't sure about if it would work for such a big lump of concrete.

I'll hire a mixer and do the job properly :icon_biggrin:

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I doubt if you need 1m³ of concrete - a hole about 60cm square and deep is ample for UK conditions (unless you are on really deep clay?).  Should also be about a quarter of the work (and price!!).  The idea of a huge concrete block seems to have come from the USA where they have issues with frozen / thawing soil and / or extreme wet and dry seasonal cycles that lead to soil movement.

Another vote for a wet mix.  With a dry mix that size the outside of the "block" will harden and become waterproof - thus not allowing water to penetrate to the middle of the block, which will remain dry for a considerable time (maybe years!).

A tip for wet mix - when you pour the concrete get a stick (A broom handle, if you have nothing else, will do) and use that to "prod" the wet concrete vigorously - it will get rid of any voids and air pockets. If you do this after each load you can be sure of getting a truly "solid" block.  Don't forget to put the studs/bolts etc in to hold the pier :) !

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3 hours ago, Bizibilder said:

I doubt if you need 1m³ of concrete - a hole about 60cm square and deep is ample for UK conditions (unless you are on really deep clay?).  Should also be about a quarter of the work (and price!!).  The idea of a huge concrete block seems to have come from the USA where they have issues with frozen / thawing soil and / or extreme wet and dry seasonal cycles that lead to soil movement.

Another vote for a wet mix.  With a dry mix that size the outside of the "block" will harden and become waterproof - thus not allowing water to penetrate to the middle of the block, which will remain dry for a considerable time (maybe years!).

A tip for wet mix - when you pour the concrete get a stick (A broom handle, if you have nothing else, will do) and use that to "prod" the wet concrete vigorously - it will get rid of any voids and air pockets. If you do this after each load you can be sure of getting a truly "solid" block.  Don't forget to put the studs/bolts etc in to hold the pier :) !

Yup  - totally agree, my pier is about 60cm square  and it's rock solid!

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Great tips, thanks.

I'm all for over engineering, but I think you're probably right about 1m3 being a bit over the top. It's not like I'm mounting the Hubble!

At 60cm3, would you consider knocking up by hand? or still hire a mixer? I can hire a mixer for about £18

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I hand mixed for my pier block and it was very hard work (not used to hard graft!!) for £18 I would get the mixer.  No brainer really.  You still have to shovel the stuff into it and out again but it is the mixing that is the real back-breaker.

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That settles it then.... Mixer it is!

Hoping to get this done next Monday.

I've had a NEQ6 for a few months (bought cheap on astroboot) that I haven't used yet because I don't have anything to put it on.

Can't wait to get it up and running.

How long should I leave the base to cure before drilling it and fixing the pier?

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1 hour ago, D33P said:

That settles it then.... Mixer it is!

Hoping to get this done next Monday.

I've had a NEQ6 for a few months (bought cheap on astroboot) that I haven't used yet because I don't have anything to put it on.

Can't wait to get it up and running.

How long should I leave the base to cure before drilling it and fixing the pier?

You won't regret using a mixer. I poured an approximately 60 x 60 x 60 cm cube a couple of weeks ago and the mixing took about an hour.

I'm no expert, but curing can start as little as one hour after mixing depending upon conditions. I used mastercrete... the tech spec states that it reaches 25% final strength after one day, and something like 75% (from memory) after seven days. I would be wary about drilling and inserting expansion-style bolts too soon as I'd imagine the localised forces may be too much if it hasn't reached a reasonable strength. 

I'm sure others might have a better opinion.

 

 

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I'd give it at least a week if you can as a minimum.  As things turned out I left mine for about a month before I got around to installing the pier.

Good decision on the mixer, you won't regret it.

Jim

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Thanks for all the help everybody, I really appreciate it.

60x60x60cm, use a mixer, leave it at least a week before drilling. Got it :icon_biggrin:

I have one more question.... just one, I promise.

Did/would you use any rebar?

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10 hours ago, D33P said:

Great tips, thanks.

I'm all for over engineering, but I think you're probably right about 1m3 being a bit over the top. It's not like I'm mounting the Hubble!

At 60cm3, would you consider knocking up by hand? or still hire a mixer? I can hire a mixer for about £18

 

9 hours ago, Bizibilder said:

I hand mixed for my pier block and it was very hard work (not used to hard graft!!) for £18 I would get the mixer.  No brainer really.  You still have to shovel the stuff into it and out again but it is the mixing that is the real back-breaker.

I agree that the mixer is definitely the best method, properly mixed concrete is important  - tricky to achieve by hand in a wheelbarrow. Also I use concrete additive (dissolved in the water). However, like wood filler, be careful not to over mix; can weaken the set concrete. As you say, it's satisfying to do a job properly/over engineer it :)

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James, I didn't use any re bar in mine - some do some don't, some use a 1 cubic metre pad and some don't!  From my perspective, the load that the pad will experience is quite frankly trivial against the concrete's structural capacity.  Re bar will also rust after a time and this can lead to "spalling" (sometimes referred to as concrete cancer) - the expansion of the oxidising iron pushes the surrounding concrete to create voids. Having said that, I reckon a pier pad could sit 20 years without any deleterious effect.  I guess what re bar could do would be to help stabilise and set the pad if the re bar projects horizontally into the surrounding earth.  I think if you use it, it won't do any harm and if you don't use you will have a perfectly usable pad.  There is a tendency to overthink things like the pad and pier - at the end of the day it's a hole in the ground filled with concrete and levelled.  Good luck with it, have a few beers in the fridge ready for when you are done so you can sit back and marvel at your work, its a good feeling.

 

Jim

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Just one other thought James, have you decided how you are going to mount the pier to the pad.  Are you going to place studs in the pad and let the concrete set around them or are you going to drill the pad once set and use chemical anchor bolts?

 

Jim

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My base was about 800mm cube and I poured the concrete pier at the same time, all in one go.  I did use rebar and the pier head is fitted onto 4 16mm diameter pieces of studding in the concrete pier with a bent bottom end in them to stop them turning.  I agitated and prodded the wet concrete as I poured it with an old broom handle to get the air out and avoid voids.  Finished off with slightly wetter mix at the top so that it self levelled stopping about an inch or so below the tops of the studding.  It's solid as a rock :)  Originally made for an NEQ6 mount but now has an EQ8 on it and fine.

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Jim,

You're spot on about over thinking it. Thinking objectively, I'm building something that could probably safely support several tonnes! But in reality it's unlikely to ever see any more that 50kg. I have always subscribed to the idea that bigger is (usually) better. If M6 fixings will do the job, use M8. If 4mm cable will take the load, use 6mm etc..

Pier is made of 175mm steel tube with a 10mm wall, welded to a 1/2" steel plate, and a brake disc welded to the top. So suitably over engineered.

I'm still undecided about using rebar. I can certainly see the structural advantages, but never considered the potential problems of it rusting. I might be able to get some stainless studding for free, if I can I'll use that. Again overkill, but best of both worlds I suppose. 

I plan on drilling and fixing with 20mm anchor bolts, but chemi-bolts might be a better idea to reduce stress on the concrete.

 

Gina,

Great idea using a wetter mix to self level the top. I'll definitely be doing that.  

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James, I speak as an engineer, for over engineering read higher cost and more effort.  Seriously though, I know what you mean, when it's your project you wan't to make sure that that pier will be standing come the next extinction level event. It's all part of the enjoyment and we all do it.:icon_biggrin:  To give you an example, I've just run a buried power line to my observatory and I took the opportunity to lay a duct for data cable as well (no idea what to put in it, although the forum has helped).  I had 25 m spare so I thought, I know I'll lay two ducts lol.  I could become an internet provider with all that space but I thought what the heck you never know.  Now with that in mind, I'd make those bolts at least 40mm high tensile SS (pre stressed of course) secured all the way to the bedrock and then chemically anchored - just in case.:happy7:  Good luck with it, remember to upload some photographs.

 

Jim

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I know where you're coming from regarding costs and effort. I used to work in sheet metal, and if you can use a thinner gauge and get away with it there's fortunes to be saved on bigger production runs. Especially when using non ferrous.

These days I'm a 'maintenance engineer' (not really an engineer, more of a mechanical/electrical handy man). My tendency to go overkill comes from not wanting to go back to the same problem again and again. It never ceases to amaze me what the animals (operators) on the shop floor manage to bend or break. It seems they are always finding new and inventive ways to destroy the indestructible!

Your idea of 40mm bolts got me thinking though. Maybe we're all going about this the wrong way. 1x M100 bolt driven down to bedrock should do the trick. Plonk mount of choice on top. Job done:headbang: 

I'll take pics as I go along and post once its complete.

Thanks again to everybody who has posted, you guys are awesome!

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