diy concrete (table / cabinet ) advice ??

tajanes

I’m looking into building an A/V cabinet.   A couple of sections 18 in wide for equipment, and a third section I’d like to put in my older sub mounted downward firing (and out of view behind sliding shelf doors).  Overall ~5 1/2 feet wide, a 20 in. shelf height box supported by the sides to the floor ~ 28 in. overall height.  It’s got to meet ~WAF standards, as it will be in the living room.

Thinking a nice finished solid wood top, and looking into ‘pouring’ a couple of concrete sidepieces (rough cal of 28x18x1.5 inches each side will weigh in around 60 lbs, so the sub should be stable).  

So I’ve been looking at YouTube diy concrete tabletops, and thought I’d check in with the builders here.  Has anyone given the concrete tabletop a go?  Any ‘learned’ advice?

thx

R-Carpenter

I haven't build one myself but concrete tabletop or countertops have been done for many high end residential apartments. Also, if I remember correctly, someone on Parts Express forum printed matrix structure and filled it with concrete. I think it was a set of overnight sensations.

rjj45

I have poured a lot of concrete in my youth. Make sure that your forms are strong. For your purpose, probably a 3/4 inch baltic birch form should be good enough, but make sure that it won't bow on some of the longer dimensions. Sit the form on a solid bench or on a couple of 2/4s and you will be good. Depending on the viscosity of your mix, you may/probably will get some bubbles. Bubbles in the interior will be no problem for your use, but cosmetically, a bubble on a face detracts. (And there will always some small bubbles on the surface. Don't worry about them) On a professional level we used big-ass vibrators when we poured concrete foundation walls. For your build, probably a couple of PE bass shakers should work well.
Not sure how that will work in practice. You will need to use them on the underside of the form. Maybe you should plan on using the top face of the pier on the outside of the cabinet, because you can see any bubbles there while the crete is curing, and the bottom face on the inside of the cabinet. Hmm, just realized that you can/should put some kind of steel bracket (small L bracket) in the crete so you can easily attach your top and shelves to the pier. Should be pretty easy to figure out.  Patching bubbles or voids later never looks good, so don't even think about it cuz of WAF. You can leave the concrete raw, which sometimes looks good, or you can polish it up with a grinder and a stone.In most cases you want to seal it, because it will tend to shed little bits of girt over the years, and that's not cool for WAF. You can paint crete with an epoxy paint to different colors if needed. The grey stone contrast with the wood would be my preference. Nice conception.

But it should end up looking great, and be totally functional. That's about all I have on this subject.

ani_101

rjj45 said:

 Maybe you should plan on using the top face of the pier on the outside of the cabinet, because you can see any bubbles there while the crete is curing, and the bottom face on the inside of the cabinet. 

How do you get rid of bubbles on the surface?

rjj45

Oh, forgot about that aspect of the trade. So concrete is a mixture of portland cement, sand and aggregate, which is small pebbles. You buy a bag of this stuff at Big Red and add water and stir it to create a slurry. For driveways, for instance, if you add too much water you will weaken it, but for your use, you can mix it fairly thin, perhaps as thin as a stiff potato soup. 
Probably use a 5 gal bucket. It will be heavy.
Forgot to add, suggest you seal the wood forms with 2 coats of shellac, then cover them with some oil so the concrete doesn't stick to the form when you separate them. Probably canola kitchen oil will work for you (or maybe mineral oil) And when you separate them, don't try to use a putty knife between the form and the concrete, you will pull out chunks and cause surface/edge damage. Put together the forms with screws, wait 2 days after the pour, then remove the screws and use a hammer or 1 lb "knocker" to strike the wood at right angles to the crete. Oh, and probably best practice to put small 45 degree strips on the edges all the way around, even the top. Look at most concrete walls or parking garage beams and they always to that because the edges are the most vulnerable to tear out when you remove the form, and besides, it adds a little bit of cosmetic detail. 
Ok, now to the pour and finishing. 
Pour in the slurry, stir it a bit on the edges and bottom to make sure it's hitting the whole form (otherwise you can get voids), do your vibrator thing on the sides and maybe bottom if you can reach down there, wait 30 minutes to one hour, and the crete will start to thicken up. when it gets like mashed potatoes cooling down, get a sponge and start stroking the surface smooth. If there are rocks on the surface, whack 'em down a bit with a piece of wood, then follow with the sponge. You will have some small bubbles. Obsess on them if you want, doesn't matter. The sponge may also create sort of pleasing surface texture. You may get that or not. Depends. If you want to try to finish the face totally effing smooth, you can round off the edges of a 6x8 piece of wood (maybe smaller) and work with it a bit to get total smoothness. By this time, the concrete may be pretty stiff. If any problems develop at this stage (say a rock), you can wet the sponge (just  a little) and smooth it over.
Oh, and I suggest your favorite libation while you sit there and watch the crete setup. Works for me!
Never realized it was so complicated!

ani_101

Awesome - thanks for the tips and the topic - this is also on my list to try... want to start with small concrete bases and then move on to larger pieces for side tables, etc.... want to try colored concrete...

tajanes

Thx for the advice - yes I was thinking of embedding L brackets 4 each end (two to attach/support the top, and two for the bottom- thinking maybe best to hold off on the internal verticals and measure after the fact.... easier to trim to fit

joeybutts

I did a bartop in place, used some rebar and mesh, high strength, used a piece of crown molding for the front edge for some character.  It was difficult to screed with no back edge to screed off of but it came out real nice, grinding and polishing brought a nice texture and some nice stone artifact came up and through.  If you don't want that look it will be much easier with what you are doing as you'l have forms and can get that cream to the top.

PWRRYD

This is what you want to use.  Lots of videos online how to do it.

https://www.homedepot.com/p/Quikrete-80-lb-Commercial-Grade-Countertop-Mix-1106-80/202832660

tajanes

perfect- thx 

Kornbread

Maybe add some glass marbles, granite, bits of shale, whatever, to add some character?