Scratch Build – In Progress "Xposure" polished concrete desk....update 03.04.16

While I am unsure whether you an talking about me or the concrete...hehe. I can tell you concrete doesn't smell. mucky yes but smelly no. but its only really messy if you screw up...like I did by over dosing the superplastiser in my first mix. its of the wall are still black from the dye. to be fair though its only really the dye that stains.

Im hoping the next update will show more prettiness.....

its water you want to be my friend....https://www.youtube.com/watch?v=7ijCSu87I9k

Love the top comment on that like.. is the internet all over

May look cavernous but I want me god damn it......to be fair I'm moving **** around all the time. I have had to put my shapeoko 2 away while i do all this due to the dust. really can not be bother to take it to pieces to clean the bloody thing.

Ok ok.....

 

So..Getting on with it

last time i posted my slab looked like this


after a quick grind it went to this













I let the slab dry out at this point so I could feel the rough patches and then regrind.





I then very slow went up through the grind pad all the way to 3000grit. And is this slab smooth...hell yes its like a sheet of glass. Granted these are wet but once I wax the slab this is how it will look.







Before I can do anything else I have to dry the slab fully. So i moved it inside and will wax it as one of the last things.

This is how it current loks having been dry for over a week now. looks a little dull now. I have been sanding the same area so is dusty. I have faith that it will look awesome once waxed!!!









Oh yeah sort of near done the front leg now. more wood updates tomorrow.

So whats left?

the wood work is coming on nice have a small update on that tomorrow.
and the concrete only need waxing.

so one more weekend and the desk will be done. then on the wall mounted computer case.

So tomorrow I hoping to finish the designs on the computer ideas.

that's it for tonight...good night

as usual let me know your thoughts.

 

Concrete... Hmmm...

 

The exposed aggregate finish looks really nice - I considered trying something similar at my old house, for kitchen worktops, but ended up sticking the idea in the too difficult box and chickened out.

Couple of questions, if you don't mind - as much for my education as anything else:

I know WRA can improve the surface finish but you were grinding it back anyway, so were you just using it for cement reduction or did you need to retard the mix?

Also, did you use any mesh? I know you can't really reinforce with fibres when you're surface grinding, but I'd be crapping it worrying about it breaking/cracking.

Not a criticism - I think it's a fascinating project.

 

Hi Spreadie,

I actually used 2 admixtures.

The first was the WRA (water reducing agent for those not in the lingo) this type of admixture is not just for surface finish. The one i ended up using can reduce the water by up to 25% with just a 0.6% dose (the dose in relation to the cement content). I used a dose of 1.5% so I could reduce the water content by a massive degree (something in the region of 30-40%) while also maintain a really low water/cement ratio (WCR). Cement only needs a WCR of around 0.25 to full hydrate. Any extra water above this only be there to make the concrete flowable, and will need to come out of the slab one way or another. This leaves pour and channels through the structure. These pours reduce the strength of the concrete to quite a significant degree. By the way it is impossible to get to 0.25WCR, well quite difficult. The lowest I have seen is 0.34 but that was in a very special precast set up use all manor of compaction kit. I used around 0.40WCR (maybe a little lower the mix design is at home) for this project, which was only possible due to the WRA.....

When you talk about retarding the mix..It will retard it a little and early age strength will be reduced but by 28days the strengths will on average be a minimum of 160% stronger in compression (this will also mean high flexural strength) when compared to a mix of equal consistence with no WRA. There are other factor to consider if your interested but enough information for now.

(I only know these figure because this is the type of testing I do for my job)



So as you can see the WCR is a balancing act between strength and workability.



Simple representation of concrete structure and packing. The higher the WCR the more space there is between cement particles. Its also a worth while point to make that if you can keep the WRC low the concrete is much much less porous. This can be good and bad depending on your application. If this was to be out side in the rain and freezing and thawing it would crack due to the water expansion. where as with a high WCR it would be less prone to.

Now for my application its going to be inside no freeze thaw and I need the strength as high as I can get it.

For the application for worktops. They would normally be cast in place so surface finish is really important. I would keep the WCR higher to be able to finish the slab. Also make it easier to cast as you can make it self compacting concrete so need less compaction and work in general to place. you wouldn't really grind as much as I have to be fair either. I have really gone to town on exposing the aggregate. for worktop I would suggest a more salt and pepper look. Though if you wanted a more deep aggregate look you can spray the surface with a retarder and then brush the top layer off. this is quite rough though. I have a sample of this at home will up load it later. To take the sharpness of this a resin top could them be poured on. (can you tell I have been doing all these things to test what I can and cannot do)

The second admixture I used was nonosilica. Normally its microsilica used but I have access to material not found in shops due to my job. the silica is use to increase strength and also increase packing of the concrete thus filling pours. It make the mix "sticky" and has a more cohesive feel.

Question 2.....I did use mesh. there is rebar in around the top, and chicken wire runing all through it. Thought to be honest I dont see the concrete breaking under normal use. It will take me to drop something dam heavy on it or drop the slab its self to break it (well I hope, no I know it will ish)

The main reason for making it as strong as possible is so the material takes the final grind better.


I hope this answers your questions? all be in a little long winded. Need anything else let me know.

 

Brilliant, thank you for the informative response - I've never considered the effect WCR has on workability - every day is a school day.

Clearly, I missed the reinforcing - shoulda gone to specsavers.

Going to bookmark this, just in case the missus starts talking about the worktops again.

Thanks again!

 

Out of curiosity, is curing under a high vacuum (or after a vacuum pull) something done for special castings? That would allow excess water to be 'boiled' off down to a preferred level (as long as you get your time/evap model right) after pouring.

 

I actually have no picture of the reinforcement. see if I have any.

By the way fibers are a massive subject. Which is you intend on doing kitchen worktop with a salt and pepper could be important. I would use a Glass fiber type easy to sand down and will no pull chunks of material of your project than if you used steel fibers.

I intend on doing a concrete worktop in the future, so i have done a large amount of research on the matter. Currently not sure on the mix design so cant answer that I would have to know the materials been used so I can work out the best mix otherwise the packing will be off. It would be some mixture of 4mm marine sand, 5-10mm stones and crush glass up to 10mm.

This website is fantastic different aggregates and things. I used just standard B&Q pea gravel and sharp sand. However if you want something very pretty picking the sands and materials will impact on the colour of the concrete and its final ground look.

http://www.boud.com/

The cement will obviously have an impact on the colour. your limited to grey grey or white. The grey though can vary massively depending on the other things in the mix.

Anyway. I would cast the slab with a % of glass fibers and chicken wire. the mix would be fairly workable to be able to flow naturally. and trowel finished. Finally the concrete would be covered by a plastic sheet to keep the moisture in the slab. It help ensure the slab is fully cured.

http://www.atsdiamondtools.co.uk/node/16

Then after a couple of days to a week, I would start to grind. start low and work upwards. Its best to grind wet but can be done dry. The easiest way inside would be to use hand pads like below with small amount of water (link above are good examples, always been helpful with me). Will take longer but less messy. You wouldn't need to go any higher than 800grit to 1500grit. You would need to stop at 400grit to grout the slab to remove pits and dents. (the grout will be just cement and water, mix wise depend on if you need it colour and the cement you are using and applied with a scapper trying to remove as much of the excess as possible). This is left again for a couple of day before going back over with 400grit and then 800grit.

I would then finishes the slab with a resin of some sort. you could also acid stain it at this point. the website below is great for this stuff. Though this tents to be more for floors. I have to do more research on finishing for concrete in food preparation areas.

http://www.concrete-sealants.co.uk/

I hope these site are happy im giving free advertising out.....hehe. Hope it helps you too. Message me here if you ever need a mix design for it....I would always start with a small slab first like I did. This helped me avoid so many mistakes.

 

Actually its the opposite. The water is kept low. Material vibrated into place and them the slab steam cured. The issue with using a vacuum is you would pull all of the water out and hence the cement would not fully react. the extra water isnt really an issue to be fair but depend on your project to how low you need it.

Vacuum curing is better or resin based system where you are needing to pull the air out. I have played with this but is a massive ball ach to get right

 

I admire your material choices....not so much the leading with a sponsorship beg....

If you want kit,mail manufacturers and link the log....dont start it and hope sponsors will come,because they wont....

I think that if you pull this off,it will look spectacular......and im not given to gushing praise either.

You 'could' of gone with a Corian surface instead and saved yourself a massive job,I think you know that already however,You wouldnt of got such an 'organic' looking mix tho. Did you plasticize your cement mix?


Also,as a side note,your informative and straight forward answering of questions is most refreshing.Im glad you took the time to share your methodology.

 

Yes I know shameless plug for sponsors....I fully intend to message people for kit though to be fair I have brought most of it. Basically come down to whether its air or watercooled. I wanted to get the desk bit finished before started. have work to show. Think its always nicer that way people can see what your work is like.

There are many ways I could have done this desk, though I work with concrete I know how to make a good mix. So really it was the only real choice. Plus if I have pick another way it wouldn't have had the same look.

Yes I plasticized my own mix. Like I said I work for a large admixture company as have access to all these wonderful things.

Thanks for the praise gushing or not. I just wanted to do something original and different. which Im hoping when its finished it will be.

I think we should all share information because everyone has there own take on an idea, it my be similar but never the same and it how true innovation happens.

 

I like you.

I will watch the rest of this log with avid interest.

If its WC kit you are looking for,I may be able to help you on a few bits and pieces.

 

busy busy..... seems that I have found it hard to find the time to get working on this. Also some issues with how to attach the concrete to the wood. (some issue....quite big issues)

So a little design work.
-I have extended the wood that the concrete sit on at the back.
-Added plug sockets to the wood
-Added metal plates surrounding the plugs, to sever as a point to store charging phones or tablets and headset when not in use
-Aluminum center channel to hid wires
-Aluminum siding



So the wood work.....pre-sanding.


post sanding, glue all removed.


channel for desk to sit in.


plug socket




joined together and time to clean up the back and channels added for wires to plug sockets to sit in.





This is where I ran in to big issues. I tried to attach the concrete to the wood but I only crack the surface of the desk......YES i said cracked. Well I mean more of a chip but god I was annoyed. I cracked it because I thought I would be able to drill and plug it. It drilled fine and got the plug in. Sadly when i screw it the pressure on the concrete was clearly to much. In fact I was so annoyed I took a week off to think how I was going to fix the issue.

I did save the bits that chipped off and I have attached it with a structural resin. picture below. Thankfully its in the back corner that the monitor will cover not ideal but it will be fine.



The solution was to order some aluminum channel to run along the back edge. Well to lots to be honest. The first to epoxy onto the concrete and the second to glue onto the first aluminum to screw onto the bit on the concrete. Pictures show it better......

Aluminum glued onto the concrete. I use the weight of the slab to get a good bound.



Next was t glue the 2nd channel onto the 1st. I sanded each surface to try and get the best bound. The glue used is meant to have a strength of 150kg/cm2. So was more than enough.




The final setup....



Now I thought this is where my issue would end......strength of 150kg/cm2.... yes in tensile.... however flexural strength not so much..... It just snapped off. BALLS......

So 2nd solution.....






This is finally going to work.... dry run assembly.




Going to need to buy some more Aluminum channel to go down the center (pictured below) and also a small strip to hid the over cut on the channel I cut for the concrete to sit in. You can also see the stripes that can are going to be glued to the sides.






So that it for tonight...... been a trying month..... But fingers crossed I am on the home stretch. Only really sanding and staining left.

Tomorrow I will detail some of the other project I have worked on in the last year.... One that the wife requested I finish last weekend.

Anyway that's it from me. Let me know your thoughts, please ask any questions you have.

 

In my town (a very old one) there's a noble palace with the sidewalk under the portico made like your desk. The people does not call the palace with the noble name but we call it "Aching feet palace" because in time (centuries) the concrete of the sidewalk has been consumed by the walking on it but the stones, that are harder, are still there protruding and causing pain to the soles...
Google maps: street wiew
Strada Maggiore, 38
40125 Bologna
Maybe your descendants will call your desk "Aching elbow desk"...
Mod on!!!

 

Yay for update !

Really glad you've overcome all the minor stumbling blocks, (no pun intended), good luck with remaining steps

Don't tell me your 'other' build is a concrete case

 

Excuse me Sir but I was out for some time, but now I'm back and I see NOW PROGRESS here. Explain yourself!

 

In dec i broke my wrist. Not been able to fully weight load on it yet so currently out of the game a little. Should be back on it in feb/march depending how my review goes.

In the mean time I have been considering the case design. I should have something soon on for this..... working out what I can actually make with the equipment I have.

 

Gotcha. Get well before doing anything to screw your arm up.

 

Sucks about the crack/chip but glad to see it making progress any way

 

Sad news.... Like Cheapskate said first get well, then get back to work