Observatory Progress

[MarkS]

Hoping to finish the digging this weekend (I've been away the last 2 weekends) and then lay the concrete over the Bank Holiday weekend - first the piers then the shed base.  The shed itself will have a wooden floor with airflow underneath.

I've just calculated the amount of concrete I need - 4 cubic meters - that's getting on for 10 tons.  A cubic meter for each of the 2 pier foundations and another 2 cubic meters for a concrete raft as the shed base.  
No plans to challenge Chris's record:
Is this a record?

I'll be careful not to make this mistake:
No isolation between pier and base!

Mark

[MarkS]

The prospect of shovelling 10 tons of ballast and cement through a cement mixer and moving it by barrow makes me think a delivery of ready mixed concrete will be a much better idea.

I'm not afraid of hard work but discretion is the better part of valour.

Readymix is probably cheaper as well - even if they have to pump it down to the garden.  Travis Perkins and Jewsons were both quoting around £650 for the materials to make 4 cubic meters of concrete.

Mark

[mickw]

I think I would be inclined to cast the piers myself - buying/selling or hiring a mixer

Then make the choice of ready mix or DIY for the pad.

Ready mix will mean 2 visits - piers then pad

[MarkS]

The pier foundations and the pad could be laid done in one visit (I think).  I just need to make sure there is isolation in place between them.  The cylindrical piers can then be cast on top of the foundations later by me (there will be reinforcing bar already cast in place awaiting the piers).

Mark

[mickw]

You would need to prefabricate the "isolation" possibly in the shape of a top hat so they could just be laid on top of the piers - lest you suffer the ire of the concrete man 

[MarkS]

Let's clarify this.

The shed base is (I mean will be) a big rectangle with shuttering around it.  Within the rectangle are two big squares which are the big holes for the pier foundations.  These squares have their own shuttering (wood or other material) around them to prevent the concrete that is poured down the pier foundation holes from touching the concrete that is poured into the observatory base.

That shouldn't irk a concrete man as far as I can see.  He just pours it into 3 separate locations.  Just need to ensure that the level of concrete in the pier foundation holes remains an inch or 2 below the level of concrete forming the observatory base so that, later, the wooden flooring can span straight over the pier foundations without touching them.  At some later time I cast the concrete pillars onto the pier foundations - the flooring won't touch them either.

Mark

[mickw]

I forgot you were flooring over the lot.

[MarkS]

I have replanned this project.

New (accelerated) schedule is this:

Finish digging this weekend.
Readymix concrete delivery one day next week.
Begin wooden shed construction Bank Holiday weekend - so I get 3 days of shed building instead of 3 days of mixing concrete

[Mac]

Just a question.

If you have the shuttering separating the pier base and the foundation base,
how will you remove the shuttering once the concrete is poured?
only the bottom of the shuttering will be surrounded by concrete on both sides, and will be extremely hard to remove.
removing the shuttering on my pier base was hard enough and that only had concrete inside of it.

Mac.

[The Thing]

The usual thing I've read is to use an isolating material e.g closed cell foam AKA camping mat where you would think you would put the shuttering separating the pier base and the floor, it will have concrete on both sides to keep it in place. Trim to height afterwards. Alternatively you need a three layer shutter (plywood?) so you can pull out the centre section and then more easliy remove the pieces in contact with the concrete, then stuff the gap with closed cell foam or similar. If you cover the ply in clingfilm, vaseline,, axle grease or similar it should come away more easily (that last bits for Tony's entertainment )

[Tony G]

Mark,

You could use polystyrene sheet against the shuttering (about 25mm thick) and once your concrete has gone off you can then pour petrol slowly over the polystyrene and it will melt away, leaving a small void to allow you to remove the shuttering.
Don't smoke whilst doing this task.

Another one of 'TONY'S TECHNICAL TIPS'

Tony G

[MarkS]

Levelled the ground and finished the shuttering today.  Just got to smash up the hardcore and lay it in place and then also put the rebar in place for what will be the concrete piers themselves (they will be cast later on).

In answer to Mac's question I have an inner and outer shell of shuttering in place around the pier foundation as follows:

XXXXXXXXXX
X   i       i   X
X  XXXXX   X
X  X     X   X
X  X     X   X
X  XXXXX   X
X  i       i    X
XXXXXXXXXX


The links represent by "i" are weak links that I can easily break apart once the concrete has hardened.


I was a bit lax when laying the shuttering - forgot to double check the orientation - so the shuttering ended up being oriented incorrectly meaning the concrete slab will end up half a degree off the N-S axis

[mickw]

Does that mean the piers are now outside the observatory 

[MarkS]

No.  What makes you think that?

[mickw]