Cement question.

[anthony]

I’ve started my new observatory. 
Hoping for advice regarding concrete. In the images you see my pier. I’m going to fix it directly to the concrete in the image. As it is, it rocks because the concrete does not have a smooth surface. 
I have some sand and cement. If I make a little square in wood to constrain it, is there a simple sand/cement mix I can do that could be flat but also stand up to the compressive load of a hefty pier, losmandy G11 and imaging scopes?

Main aim is a flat surface. If not perfectly level I can readily correct small discrepancies with the pier top plate. 
 

If it’s a “yes!”, what ratio sand:cement:water?

What thickness?

PS Build Thread

https://stargazerslounge.com/topic/423693-my-observatory-build/

 

 

Edited July 11 by anthony
Add build thread.

[Paul M]

A thin bed of mortar or shallow plinth in compression is going to crack and break away eventually. I'd fix the concrete surface and bolt direct. Don't worry about level. You can always get the mount base level with those standoff bolts but, really, it doesn't matter. the pier can be at any jaunty angle you desire!

A nice flat surface to bolt the pier to is desirable to avoid any nodules eventually crumbling and causing the fixings to loosen.

So I'd just get one of these and remove the lumps in the concrete slab. 10 minute job:

https://www.amazon.co.uk/Draper-DRA71269-Scutch-Holding-Chisel/dp/B000MPO8IS/ref=asc_df_B000MPO8IS/?tag=googshopuk-21&linkCode=df0&hvadid=697277231999&hvpos=&hvnetw=g&hvrand=8598900267523537066&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9189584&hvtargid=pla-456244629806&mcid=f0d8982852e13c69ae3fce6237768ce2&gad_source=1&th=1

 

[Swoop1]

Anthony,

When imaging, a pier bolted into a concrete slab will pick up the vibrations from walking on it etc and ruin your subs.

Not quite so important if for visual only. if imaging remotely- not so much of a risk but a lrge slab may vibrate more than a block below ground level- even if only from passing HGV's nearby.

I would recommend concrete as opposed to just mortar. The aggregate content of concrete helps with the strength.

The usual mix for concrete is 1:2:3 or 4 -1 part cement, 2 parts sand, 3 or 4 parts aggregate depending on the strength you want.

If you are going to leave the slab as is and mount to it, you will need to provide a key between the slab and the new concrete block.

In construction this is done with a scabbler, a pneumatic device that basically hammers away at the surface to roughen it.

You could do this with a club hammer and bolster due to the small area you will be working on. I would also be tempted to drill the threaded rod into the slab, put a couple of nuts on each length above the slab (tightened against each other, perhaps with a large washer between them) but below the top level of the new block, before you pour. This will provide a more secure setting for the rod/ bolts to tighten against.

 

 

[carastro]

Oh dear concrete already in place.  It is normally recommended that the pier concrete be separated from the surrounding area to avoid vibration.  I wonder whether there is a way around this?  The guys on here will have far more DIY "know how" than me.  

[JeremyS]

My pier is bolted directly to the concrete pad and never had a problem. Mind you I observe alone and don’t move about much in the dome.

[anthony]

Cheers all. Much appreciated.
Although it wasn't an afterthought, the pad was poured when doing another job at home and there were too many things going on in life for me to avoid the little (😑) error of forgetting to make an independent pad. In my old obsy build I did the typical 1m3 concrete with a mini scaffold within with protruding threaded bar.

I hope it'll be OK though I do plan to do some enthusiastic jumping during an imaging run to see what happens. I'm luck in that I will have a data cable running to the observatory and my warm room for remote imaging will be the house about 20m away. Once set up, I won't have to be in the obs.

Thanks for the input regarding the concrete. Given me a good starting point on deciding the best fix for me in this situation. 👍

[M40]

Just my thought's, rather than trying to smooth off the base, use studs in your existing concrete base, leaving them long, go to a good electrical wholesaler and buy a length (3M) of 41x41mm slotted unistrut (galvanised steel), cut two pieces to length, drop them over a pair of studs and fix your pier. This will leave a gap of 41mm give or take between pier base and existing concrete.

When ready, fill between pier base and existing concrete base with a concrete mix, just make a temporary wooden bund wall around the base where you are going to fill with wet cement. As long as your stud fixings are good, that's not going anywhere. All the best 

[saac]

2 hours ago, anthony said:

Cheers all. Much appreciated.
Although it wasn't an afterthought, the pad was poured when doing another job at home and there were too many things going on in life for me to avoid the little (😑) error of forgetting to make an independent pad. In my old obsy build I did the typical 1m3 concrete with a mini scaffold within with protruding threaded bar.

I hope it'll be OK though I do plan to do some enthusiastic jumping during an imaging run to see what happens. I'm luck in that I will have a data cable running to the observatory and my warm room for remote imaging will be the house about 20m away. Once set up, I won't have to be in the obs.

Thanks for the input regarding the concrete. Given me a good starting point on deciding the best fix for me in this situation. 👍

If you are remote observing I doubt there will be any transmission of vibration to worry about. Regarding the unevenness of the concrete you could try using a levelling compound or, if you want a quick solution, place a square of compressible rubber sheet (type of mat found in tool chests or thin yoga mat) between your pier ground plate and the concrete sub base. This will compress down as you tighten the tie down bolts and as a bonus will also provide a measure of dampening. At the end of the day, in the unlikely event that you do find your pier suffers from ground transmitted vibration, it would be an easy enough task to cut through the slab with a concrete (Stihl) saw, excavate some earth and pour an isolated pier foundation.  However, given the stage you are at at the moment, I'd only go to that effort if I knew there was a confirmed problem. 

Jim 

Edited July 11 by saac

[carastro]

Just thinking laterally, whether putting foam interlocking flooring which  many of us have, would help with the vibration issue, so what you are walking on is soft (and more confortable too).   This is a photo of my flooring where i was demonstrating my spider picker upper.  I also have a separated pier (you can see where I have put the gaffer tape over the gap).  

 

 

Edited July 11 by carastro

[anthony]

Guys, thanks you again. All this is super helpful. 👍👍👍