View Full Version : concrete countertop thickness
01-16-2003, 11:15 AM
I am attempting to install concrete countertops and have looked at the postings in this forum as well as purchasing Fu-Tung Cheng's book on the subject. The standard thickness of a concrete countertop appears to be 1.5 inches. If I pour the countertop in place on 3/4 inch plywood, can I use 3/4 inches of concrete for the counter top with 1.5 inches on the edges?
01-16-2003, 11:48 AM
It sure sounds reasonable enough...I have to admit I wanted a beefier-looking counter, so I never thought about going thinner.
Someday I'm going to have a kitchen to make countertops for... :wtf:
I have a word doc on concrete countertops with some great info from a guy who's made quite a few countertops. If you'd like, I can e-mail it to you.
01-16-2003, 12:13 PM
Hi glensuna, Welcome. :) How about a first name? :)
I'm not a concrete countertop guy, but it seems to me that a 3/4 in. top with another 3/4 inches of edge overhand equals an inch and a half. Why not just pour the inch and a half?
01-16-2003, 12:22 PM
Are you trying to fit the 3/4" section onto an existing structure?
You might check out www.decorative-concrete.net they have forims there including one for counter tops, although I have heard it is not too active.
Do a search for counter top thickness. They don't seem to think too much of 3/4".
01-16-2003, 12:49 PM
Thanks for the replies. I refaced my kitchen cabinets, built new doors and rearranged the base cabinets to open up my kitchen. I temporarily put 3/4 inch plywood on the cabinets for a make-shift countertop (the old counter top was laminate and had the sink hole in the wrong place).
When my better half suggested installing a cement countertop, I thought that I would use the plywood that I have in place as a base for pouring the cement in place. The articles that I read on pouring the countertop in place, used either 5/8 inch CDX or 1/2 cement board as a base and poured 1 1/2 inches of cement for a total thickness of 2 to 2 1/8 inches. My counter space is small so I prefer a thinner counter-top just for the appearance. I am not a concrete person so I wondered if the 1 1/2 inch slab that others used was a minimum for structural (cracking) purposes.
01-16-2003, 01:00 PM
I have to think that the extra mass of the thicker top is going to be beneficial. Hang around. There will be others in on this. It's a topic that comes and goes around here. 'Bout time for another discussion. ;)
01-16-2003, 01:01 PM
I think you are right about the thickness being necessary to avoid cracking.
01-16-2003, 01:07 PM
I just remembered that Cami bought the book. Don't know if she ever read it, but she has it. ;)
01-16-2003, 01:10 PM
Yep, I read it. Aeons ago. ;)
Kinda sounds like Glen has the book already, but it's worth it. Nice pics. :D
01-16-2003, 06:37 PM
I too have considered doing concrete countertops and have purchased Cheng's book. I also have a couple of Word documents (possibly the same ones Cami mentioned) that offer various degrees of help with the process. I have also done some additional research and some test pours. If you are interested in anecdotal evidence (nothing really scientific);), I'll offer the following observations.
For strength reasons and to help control cracking, the concrete must be mixed with a minimum amount of water. This results in a "dry" mix that is relatively difficult to place, consolidate, and finish. There are additives available that will help the slump (make the concrete "softer" (???)) without adding water. These "water reducers" are somewhat difficult to locate and usually not readily available in smaller amounts (less than a large drum).
Because of the way a countertop is supported on the cabinets, and its size and relatively thin cross-section, additional reinforcement is absolutely necessary. I used welded wire mesh (a conventional slab reinforcement) and pencil rod (#3 rebar).
The amount, quality, and size of the aggregate in the mix will have an effect on the resultant strength of the concrete. I used washed pea gravel and clean sharp masonry sand.
My results were sort of a mixed bag. I discovered that with any pour less than 1 1/2 inches in thickness it was very difficult to place the reinforcement properly and keep it in place while placing and consolidating the concrete. While placing the concrete, it is extremely important to completely consolidate the pour to preclude voids or pockets in the finished product. I eventually rented a small concrete vibrator to help with the consolidation process.
Best results were obtained from 1 1/2 to 2 inches of thickness. One inch and less was almost a complete failure due to the inability to keep the reinforcement in place while placing the concrete in the forms and consolidating. My recommendation would be to pour to a minimum thickness of 1 1/2 inches with a 6 to 7 sack mix and minimum water, and use appropriate reinforcement.:)
01-16-2003, 07:02 PM
You're quite a guy, Howard. A man of many facets, as it were. :)
I'm sure it's a regional thing, but the "pencil rod" I'm familiar with is 1/4 in. in diameter and has no ridges or barbs like re-bar does. It's pretty slick stuff and containing about half the mass of the 3/8 re-bar, will bend in much straighter and tighter configurations. That might be one of the answers.
Do you pour your tops inverted or top side up? If inverted, I'm wondering how the reinforcement is kept up away from what will become the top surface.
Jim Buckley has stated many times that what makes SLC flow is polymers. I wonder if they are the same stuff you're talking about. And I wonder if slc and aggregate would work for a top.
And I remember last time around that CX had some interesting comments, also. I know he won't mind repeating them, and I for one won't mind re-hearing them. ;)
This is a very interesting topic.
P.S. And didn't we have an outside expert comment on the stuff that makes the concrete flow?
01-16-2003, 07:21 PM
Dang, John, it's gravity.
Ever'body wants something to 'spain something as simple as gravity. It's not just a good idea, it's the law!
Oh, you meant the concrete. Never mind!
01-16-2003, 08:29 PM
The SLC and aggregate would work John, the only problem is it will scratch easily. They are underlayments, although Ardex claims to have some stuff that can be used to resurface driveways.
Pencil rod as you described is what I know it as.
01-16-2003, 08:44 PM
I suspect you are probably correct about terminology John. We called #3 bar and smaller "pencil rod." Never did use any of the smooth bar--might help a little.
I hope I didn't give the impression that I had poured complete countertops.....I haven't. I am considering them for my kitchen reno but still undecided. What I have done is pour several small test pours to go along with my research.
Used four knockdown forms 2 ft. by 3 ft. that were framed with 2x4 (3 1/2 tall) and varied the thickness of the pour. Each set of four contained exactly the same mix. The only variation was the thickness of the pour.
Poured three different sets, varying cement ratio and aggregate to see exactly what was possible with my technique. I found the thin pours (less than 1 1/2 inches) were extremely difficult to consolidate even with the use of the vibrator. These usually had unacceptable voids in the surfaces. This was probably due to the placement of the reinforcement as you alluded.
Wet cured. Stripped the forms at 14 days. Examined the completed slabs at about 28 days. No noticeable cracking, but a lot of voids in the thinner pours.
Perhaps a different consolidation technique could at least partially eliminate the voids. I don't know. My biggest problem was the placing of the reinforcement. Every test of 1 inch or less had the mesh or bar showing somewhere. The vibrator seemed to work much better when there was more mud to work with. Incidentally, I'm not sure about the strength of the thinner pours either. Several of them broke a few days later during handling.
SLC--polymers--hmm. Interesting idea John. I may buy a few bags and try a couple of pours with some pea gravel. Wonder what else is in it.....??
01-17-2003, 04:16 AM
I guess you didn't see flatfloor's post above. He doesn't recommend it.
I've seen professionally done concrete tops that were poured inverted. I wish now I had asked the guy how the reinforcement is suspended during the pour so that it doesn't approach the surface. Hmm.
01-17-2003, 07:09 AM
I noticed in Cheng's book that he supports the wire mesh/rebar on styrofoam, then wires the mesh to drywall screws on the outside of the mold. Then he takes out the styrofoam and pours. After the concrete has set a bit, he cuts the exposed wire just below the top of the pour.
He suggests placing the rebar 1 1/2 inches below the exposed side of the rebar to avoid ghosting. This raises a bunch of questions for me:
Does the ghosting rule apply to the mesh too? What about if pencil rebar is used instead of 3/8ths?
If a pour is made in place with 3/4 inch plywood as a base, is rebar necessary?
01-17-2003, 07:53 AM
If you never done a concrete counter - top before I strongly suggest you dont pour it on site . my first few I was not happy with especially if its in your own house. A good place to start is to go to go to www.naturalhandyman.com go to natural handyman concrete countertop page and its free advice and it will walk you through the whole process, remember this is messy job but after you do a few you'll get it down
Greg, I edited your post only to fix the link to natural handyman. JB
01-17-2003, 12:14 PM
Here's the link to the article:
01-17-2003, 01:56 PM
He suggests placing the rebar 1 1/2 inches below the exposed side of the rebar to avoid ghosting.
I think I understand what you mean here. I'm not sure that you have it completely correct however.
Anyway, any reinforcement placed in too close a proximity to the exposed surface could show a shadow or "ghost." This is one of the reasons that what JB said about keeping the reinforcement away from the surface is so important.
Yes, I believe that you will need the reinforcement. It doesn't really matter what you use for a base. Concrete will shrink and crack to some extent as it cures. The reinforcement is necessary to hold it all together. The most dangerous areas are the small ones around cutouts (like a sink). The amount of "crazing" or cracking you get is dependent on a lot of factors, not the least of which is the amount of water in the mix. Keep the mix as dry as possible to help alleviate shrinkage cracking.
Will a 3/4 inch slab work??? Maybe. I sure wouldn't bet on it, but maybe. I haven't been able to figure out a way to do it but maybe you can.
Whatever you decide, read Cheng's book thoroughly, search for the available information on the web, and always do some test pours first.:)
01-17-2003, 03:51 PM
I only read the first handyman article on the site, and there is another one posted, but what struck me is that this guy makes no mention of reducers, which as we've noted, are hard to come by in small quantities. Instead he mixes the mud sort of like deck mud (not much water) and more or less pounds it into the form in two lifts. Water, of course, is the enemy in any type of concrete or portland cement operation. It's necessary to activate the cement, but after that it just screws things up.
Anyway, to the point. I was concerned with suspending the rebar in an "inverted" pour where the top is on the bottom of the form. The handyman guy is doing an upright pour and then mechanically finishing the top after the form has been removed. Much easier, and there's no worry about surface checking because you can patch it with fill.
01-17-2003, 10:41 PM
They carry super plastisizers (reducers) in small quantities. Also colors and a few other cool things. They cater to the hobbiest.
FritzPak has some of the best reducers available. Find a distributer in your area off thier site. The smallest amount your can buy is a box of 24, 2.5 lbs. packs. A lot of material, but much fun to play with, and the $130 or so is not that bad.
You do not pour countertops, you place them. The mud is rather stiff. For a rookie, cast it upside down using a melamine form. This gives a rather smooth surface (better than a rookie can get finishing the top). Chengs' pic showing the rebar suspended by wire tied to screws is the best way I have found to prevent sinking. Use silicone to caulk the joints where the melamine meets. Believe it or not, acrylic seems to suck up some water from the crete, and creates a weak corner. Concrete does not like 90 degree corners to begin with. Something to think about. Get creative. You can cut strips of styrofoam the thickness of the top, spray with xylene to texture one side of the foam, and glue to the inside of the uprights of the form. This will give your top a split face block (broken off clean) look. Just one tip;)
Most cabinets can support concrete tops, just like granite pretty much. 1.5" thick tops average around 18.75 lbs per sq. ft. 2" tops are around 25 lbs per sq. ft. Thicker is always more desirable for customers, but for a DIYer, is more forgiving. Reduce ghosting. Little longer dry times. If casting, the weight is something to think about. How long is each run? You want to minimize seams I am sure.
I use a palm sander without sandpaper on it to vibrate the sides and bottom of the form after placement. This, along with the plastisizer, will prevent honeycombing.
What colors? How are you going to seal? Do you want inserts for hot pans? Any crazy designs?
I can help you on your mix and methods, if you like. I have done quite a few tops. I am working on a large project right now. Casting panels, 4'x4'-4'x6'-4'x8', and placing them on the side of a building, 40'x25'. Light grey, medium grey and black. The designer wants a random "ashlar" pattern. Just a side gig:bang:
01-18-2003, 12:53 PM
John B. when you get a chance read the 2nd article on handy-man concrete counter tops
01-18-2003, 09:52 PM
Yeah, I will, Greg. Time. Time. :D
"You do not pour countertops, you place them."
Thank you, Tim. ;)
06-25-2003, 03:21 PM
Well folks, I finally finished the project. I used deep black coloring, 3/4" plywood as the base and 3/4" concrete. To hold the mix together I used a combination of mesh and stainless steel screws. Thanks for the tips.
As a sidebar, people have asked me if it was slate and were surprised when I told them that it was concrete.
06-25-2003, 03:51 PM
They look veeery nice, Glen. I love black in a kitchen... I've trimmed down the pic to slightly less than lifesize, and I'm uploading it for everyone to see.
06-25-2003, 05:12 PM
Wow, thats nice!Just what I want to do in my fixer-upper.Cami,I would love to read those docs if you dont mind.Bill
06-25-2003, 07:00 PM
Wow, that looks great. Just reading this thread has ideas floating around in my head. I must try this concrete counter stuff.
06-26-2003, 06:13 AM
Sure, Bill- just email me whatever address you'd like me to send it to. Same goes for anyone else that wants a copy.
Yep, looks nice, Glen. :)
Those of us who haven't yet done our first top, but have one or more in our immediate future, would like to know if yours starts to crack or has any other problems with a pour that thin. Everything I've read (and all my experience with pouring concrete and making concrete repairs) says you can't get away with only three quarters of an inch in that application. It'd be real handy to know that ain't necessarily the case.
I've also read that getting a good black color is difficult, too, but it looks like you did just fine. The first one I'm supposed to do wants to be black, so I'd be real interested in knowing 'zackly what went into your top.
Also like to know how you finished it: just trowel finish after the pour, or did you have to do more work after it cured? Etc. Did you pour with the sink in place, or install it after? My first one wants a stainless double sink undermounted.
Do tell all. :)
06-26-2003, 09:00 AM
The coloring came from concretecoloring.net. I used Deep Black and the price was very reasonable. The concrete was a gray sandmix poured over plaster lathe, which was screwed to the plywood (lined with 6 mil plastic). If you omit the screws, the concrete will pull away from the plastic covered plywood. The plastic keeps the plywood from warping from the wet concrete. The plywood underlayment also made the installation of the undercounter mounted sink pretty easy. I rolled the lath over the edges and screwed it to the edges so that the concrete would cover the edges.
The mix was poured in place and trowelled smooth. After sanding, I sealed and then waxed it with a food safe sealer and wax. I sanded instead of grinding, because it did not create as big of a mess. This was the final phase of the kitchen remodel, where I tore down walls, put up new walls, refaced the cabinets in oak, built and replaced the cabinet doors, installed and sanded oak flooring, so I wanted to keep the mess down.
Note to Cami: Thanks for posting the picture!:)
06-26-2003, 07:50 PM
Looks great, Glen. ;)
Did you block out for the faucet holes, or did you drill them?
06-26-2003, 08:31 PM
It does look nice. I know black is difficult to photograph. Would it be possible to post a close up?
Cami, I'd like to read those docs. :)
06-27-2003, 09:39 AM
I blocked out the holes for the faucet with pieces of an old broom handle wrapped with duct tape.
Art in Stone
06-27-2003, 11:51 AM
That looks great. ;)
I read the article also, interesting.
I've come across quite a few people that have wanted concrete counters. Looks like something you can get extremely creative with.
Is there a market for it?:confused: I would think there is. I don't know of anyone that does it, and I know there's people that want it. Sounds like something to think about. An alternate income source? Maybe? At $70 a foot why not.
06-27-2003, 02:14 PM
We just finished a kitchen island (1.5 inches thick but looks like 2.25"). Also did one back in January or so for a guest bath counter. We're happy with it.
Haven't ground the newest one yet. Think I may just sand as it is already pretty smooth but I did "grout" it with a slurry of portland and latex additive. We poured in place and I think the only place where we did have some rough places is where the fibers clumped. I torched them off so hopefully I won't have a problem when I really start finish sanding. Did you use fibers? Did they clump?
As for a market? There already is one. For this last pour, everyone and their pet duck wanted to come and see so they could do their own or have someone come and do it for them.
06-27-2003, 08:03 PM
Where IS Fauquier County? ;)
Hi Leigh. :)
06-28-2003, 07:18 AM
Fauquier is an hour west of D.C. and just south of Rob.
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