Concrete Counters - Any Pros Here?
 

Today I laid v1 of my poured-in-place concrete counter . . . yes, that means there will be a v2, which will require me to shovel the 400 lbs of ruined Quickrete from v1into a bucket and carry it off somewhere . . . probably throw it over the fence to rest with the bunny. If the concrete had cost more than $20 I'd be really angry at myself.

I consider myself to be proficient with concrete work, but 'architectural concrete' is a new one. It seems as if there was WAY too much large aggregate in the mix (bagged 4000 psi Quickrete) and I had to beat it just to get any cream to float- and there wasn't much at all when it did; in some spots the piece resembles a rock pile. Ended up using 5 gallons of water for 5 bags, which is about 1.25 gallons more than the minumum recommendations. The counter is 37" x 84" with a depth of 2.5" at the front lip (1" all the way across the face) and an average depth of 2" all across the remainder. There is a 14.5" sq plug for the sink (not perfectly square due to the 1" radius corners but close enough).

I figured it would take 5 80 lbs. bags. I mixed 5 80 lbs. bags and ended up putting nearly all of it in due to some really low spots following the screed. One hour after mixing, the piece still hadn't set up but, like I said, I didn't use much water so I don't think it was over hydrated. So my question is:

Did I happen upon a bad batch of concrete, or did I just try to shove too much material into the form? Any suggestions would be appreciated.

I doubt you had a bad batch of concrete. What I would have done was use sand mix W/O aggregate and add some cement to get the strength. They get the strength/PSI cheaply by using large aggregate. You can use more cement to get same strength with small aggregate but you need to add more cement because the aggregate has to be surrounded with it and it takes more to surround with small aggregate by volume than with big. Also the more cement you add the more water you can add maintaing strength and increasing the "cream "your looking for. Another good thing to do would be add fiberglass to the mix this serves same purpose as wire mesh, rebar etc. Your concrete will set even if over hydrated the strength will just less.

I had considered using the glass fibers but, since the pour was so thin, I didn't want to do two pours to keep the fibers from showing on the surface. I hear the fibers will show even after grinding and polishing. I might try concocting my own mix with more sand and less aggregate, though. That might work best.

I have dealings with a few concrete countertop guys here. I finally found one that I like to work with. None of them would tell me the formula or methods they like to use ( like I really want to pour countertops for my clients?) After a few jobs with the same concrete top sub, he finally spilled his beans to me only because he knows that I will go back to him for more business. The same thing I was thinking all along, smaller rocks and a little more cement. His other solution is to pour in his shop then goes out to install like any solid surface materials. He makes the bottom form out of Melamine. He pours it up sidedown. He allows gravity to do the work to get rid of all the voids and air pockets. He uses an air hammer to beat it from the bottom upwards for this.

I don't think you got a a batch. It should be ok. After all, how much stress do people put on their tops? Is not like a driveway or anything.

how much do you think it might cost for a desk top 16'x3'?

he's usually 75 bucks per sq installed. 2" or so thick.

Why concrete desk top, you know that concrete is not really flat?

Jeff

You need a special concrete
Buddy Rhodes Concrete Countertop Mix | Concrete Training, Products, Kitchens, Baths, Furniture, Tiles

Concrete cracks so that's something you have to live with. Like granite, if you set a wine glass down the wrong way it will break it, get used to replacing china and glassware or use plastic dishes. Concrete is porous so you have to wax it or it will stain. If you have a prissy wife with an obsessive cleaning fetish it's great. If you have kids who cut stuff without a cutting board or don't clean-up after themselves forget it.
Look at your lifestyle and be honest about it before you buy.

My trade is cabinetmaking (although I messed with cars for a while), I owned a shop for a long time and I have two kids.
What do I have? Plastic laminate.

The trick with the fibers is once finished take a small propane type burner like the little ones you find at hardware stores and burn them off, doesn't take much. I imagine you are going to seal the top once sealed you can't see fibers. The fibers will go a long way mitigating cracking. Good luck.

I've never done it, but I was listening to radio talk show with a pro and a guy from Quickrete.

I assume you're using the Quickrete made for countertops.

They use rebar and recommended putting a diagonal piece across any inside corners. They use a mold and pour it upside down. They also use a shaker to get the airbubbles out, but they said you could probably get them out adequately with a rubber mallet on the sides of the mold.

A buddy of mine specializes in this stuff and has done some absolutely beautiful stuff including glass inlays.

I am also fortunate to work in the R&D department for an oilfield service company. We have 12 cement chemists on staff and a PhD who has spent her career specificaly on cement . If you have a specific cement chemistry question, email me and I will find out for you.

I am considering concrete counter tops for the house I am building as well.

That's the way most people recommend doing it, but I'd be stuck trying to flip it and place it with my wife- there would be much complaining and I imagine she would likely drop it at some point during the move.

I figured out what was wrong. The mix was entirely too dry, causing me to put too much mix in (rather than flowing smoothly, it was clumping and wouldn't spread evenly), and my initial material projection was a tad high even though I went through it about ten times. To illustrate how bad the mix was, in two hours I removed the concrete and reset the mold; I'll even be able to reuse the rebar. So I'm out $16 in concrete, $3 in caulk and about 4 hours of labor. Not too steep a price this time.

Assuming you have a desk that would hold the weight (and wouldn't have to reinforce it), the cost of the top would probably be about $90 (mold materials, rebar and concrete). There's a really good book on it by a man named Cheng. The total cost for my ~21 sq ft top was $74 at HD. The actual counter base cost was about $85 for lumber, hardware and Durock.

edit: mold materials might run it up a bit. I'd say a 16' x 3' mold might cost closer to $110 to make but the concrete wouldn't be more than $40.

Saw them do it on Dirty Jobs once. Poured the mix in a mold good side down.

That's what all of the books say to do but I like to learn the hard way. I have about four sample batches of concrete curing around the house right now just to see what the actual water ratio should be. I'll tell ya, it's nowhere close to what is suggested on the bag . . . don't know why I chose yesterday as the day to start reading directions.

there is a bag version of quickcrete for countertops..i think.

i seen a DIY show do it..poured in place. it had only sand as an aggregate. looked like grout.

But just like fiberglass work, your piece is only as good as your mold.

You're absolutely right, there is a concrete intended for architectural pieces; my problem is that I need a really high strength material to support what's going on top- an 8-head Cornelius soda dispenser with its own ice bank. The install manual recommends a surface capable of holding 750+ lbs. There's also going to be a multi-head beer dispenser, popcorn machine and bar sink . . . the good news is that the sink plug worked really well the first time around. It was designed to come out in pieces so it could be reused if necessary.

edit: The Quickrete product recommended for countertops is Non-Shrink Precision Grout. I never really thought of grout in this manner but look at the numbers:

Compressive Strength at 28 Days:
Plastic - 14000 psi (96.5 MPa)
Flowable - 12000 psi (86.5 MPa)
Fluid - 8000 psi (55.2 MPs)

Second edit: Okay, so Quickrete's newsletter recommended the above grout product for concrete counters. However, they also have a concrete product made specifically for concrete countertops called- wait for it- Countertop Mix. Why they'd push the grout over the mix is one of life's great mysteries.

Check this out. http://www.amazon.com/Concrete-Countertops-Design-Finishes-Kitchen/dp/1561584843 My dad has this book and covers everything very well. Can't get the Amazon link to work. Do search on Concrete Countertops by Fu-Tung Cheng

I've just about worn the book out; it's pretty informative but in my opinion his method of mixing concrete isn't the best course for a simple 21 sq. ft counter. If I were tackling my entire kitchen, say about 90+ sq ft of counter space, I could see ordering a batch from a concrete supplier.

This currect project is just a small bar area outside the theater.

He's the father of modern concrete countertops. he's the real deal according to some designers I have worked with.

It is not the compressive strength you need to hold up all that weight. It is your support down below. Is this going on a commerical setting or a home? do you have cabinets down below? If so, you need to beef up that area and do not go over 3 feet in length on that one cabinet. under normal condition, I almost never built my cabinets more then 3' in length. have your finish counter sitting on 3/4" plywood subtop should do the trick. If your cabinets are built and install correctly, it should hold up a LOT of weight. Make sure you shim both front and back of the side and not just bottom shelf. Build your toe kick out of wood or 3/4" plywood. try and not use those European leg levelers. They are great for installation, but...

Are your cabinets or what's going below in place already? Show me a picture, maybe I can help you a bit better. I have done lots of custom kitchens and baths. I also own a cabinet/wood shop.