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a question concerning molding technique,
planning counter top for my laundry room and haven’t decided whether to pour and finish in place or mold and pour top side down: would like to know more about the difference in finished appearance. also wondering about difficulty in avoiding bubbles and insuring clean release of edge from mold vs. difficulty in hand finishing top and edge detail. in anticipation of a perfect job my first time 🙂 any experience anyone had to share would be appreciated.
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Mark,
If you're not experienced with a trowel (and in some cases, even if you are) I'd recommend that you cast inverted.
Line the bottom and sides of the form with a cheesy, cheap, but GLOSSY formica and you will get a glass-smooth top with a mirror-like finish when you uncrate your slab.
Bubbles are certainly more common when casting than when troweling. There are methods and techniques to minimize or eliminate bubbles, vibrating being the most common. Should you have any residual pinhole voids they can either be filled with a bit of portland/sand slurry or with colored epoxy.
The most important thing is to seal all form corners well so no liquids leak out and leave voids or rough corners. If you do use a wooden edge molding, treat it with a gloss urethane so the water in the concrete mixture doesn't raise the grain of the molding, creating a "fuzzy" edge.
Casting also allows you to add a decorative edge detail using molding.
I've never had any problems with form release, your biggest potential problem would occur with an intricately detailed edge molding. I doubt it would be required, but for peace-of-mind you could use a form release agent in addition to the gloss urethane.
*I am planning on doing a concrete counter one of these days-- I did some experiments with some test sections. My advice: definitely pour upside-down. I used melamine laminated 3/4" ply and got a perfect smooth finish that had no dust on it. I vibrated by tapping a hammer (sparingly) on the underside, and the voids were barely perceptable-- in fact, I wanted them to be larger so I could speckle the surface with another color. I have yet to find a way of getting a good edge. I tried caulk that I pressed into the corners with my thumb, but it didn't come out nice. It took me a while to find a piece of cove wood moulding with a thin enough edge to hopefully not show up on the finished concrete-- but I have yet to try it out.dave
*Use some of that cheep plastic wraped molding for the edge . The simpler the profile the better . I used it on a pour just because I had it laying around . It came out nice except for some of the thiner parts cracked off of the edge.
*How do you guys lift and flip the finished counter? I figure a 2' x 8' x 3.5" counter to weigh in at about 720 pounds. I'm a pretty big guy, but that is a back breaker. Do you reinforce more to accomodate the moving of the slab? Maybe #4s at 6" or something like that? Do you use perlite or something in the concrete mix to make it a bit lighter? Any additional support in the cabinetry?
*I've got a rig where I screw dolly wheels into the long edge of the form that the countertop was formed in. After curing, the top, form and all, are stood on edge (on the wheels) and wheeled up to the cabinets. The entire rig is leaned against the cabinets and then lifted/tilted up and slid into place. I do use #4, spacing depends on the design of the countertop. I've used perlite/vermiculite/expanded shale to reduce slab weight, but not where strength is needed and I think slab integrity could be compromised. Slabs are generally 2" thick.The biggest one I've set myself was about 600 pounds. I'm 6'4, about 250-260ish or so, in average shape. It's the mechanical leverage (tilt/slide)that helps a bit.I build cabinet carcass boxes out of 3/4" birch ply, so adjacent boxes will have two 3/4" ply sides separates by a 1/2" spacer. This allows a 2" wide face frame stiles to be applied to the fronts of the boxes. Plenty sturdy.
*A Fine Homebuilding article a while back went through the concrete countertop make and install process. I recall it being done not in place, and formed and poured face up. I recall the setup was done in the garage. Fiber mesh additive was a feature of the mix, and some wire mesh reinforcing was used. Steel trowel finishing was done, and the finish coating was an industrial quality moisture-cured urethane. Good luck!
*Thanks, Mongo - the tip on the wheels is excellent. I am actually just a bit bigger than you, so I am confident I can do the "tilt and flip" method you described. I am still a couple of months away from countertops in the house I am building, but I sure appreciate the help!Gene- the fibermesh concerns me because my experience with it has been the fizzy finish everyone talks about. Okay for a house slab but not cool for a finished counter. I'll stick to rebar for this, I think. The FHB article I have seen has the counter cast in place, face up. I think the upside down casting will produce better results, regardless of how good of a finisher you are (the edges and corners can be controlled better).Thanks, again, all - I know this is one of those topics that people say "look in the archives" all of the time and I appreciate the responses.SamiSami
*Sami, There was an additional cast-inverted concrete countertop article that preceeded the pour-in-place one that your are referring to. Maybe a few years back? If you don't subscribe or can't find the back issue, let me know and I'll see if I can hunt it down and scan and send it to you.You're right (in my opinion) to be wary of letting the mesh fibers get near the surface of the slab. One technique to avoid this is to only mix the fibers into about 2/3rds of your mix. When casting inverted, first place the fiber-less mix, then your support structure (rebar/hardware cloth/mesh, etc), then place the mix with the fibers. Just a technique.
*That would be issue 90, pp 86-91.Good luck,Andy
*Mark, You are right about the fiber mesh leaving a fuzzy finish. However it can be overcome with the methoddescribed above, or you can use it in the entire mixand burn it off with a propane torch after youremove your forms. The only counter tops I have done have been castin place. We used the fiber in the whole mix,burned it off when everything was cured, and it looked great. I do think though I would try the inverted method of pouring if I ever have to do anymore. Hope this helps. Tim
*I am looking for the FHM issue 90 that features the making and installing of concrete countertops. The back issue does not seem to be available anymore. Does anyone have one they would like to sell. Better yet a copy of the article is all I need. We are thinking of making them for our kitchen. Thank you.
*this month's JLC ( just came in the mail today ) has a GREAT conc. countertop article..they haven't updated their site.. so the mail was faster than the net...
*JLC? wazzat?
*rein....JLC.. journal of light construction...http://www.jlconline.com/forums/
*In the above mentioned FHB article on concrete pouring in place...the author recommended finishing first with a mag trowel, then a steel trowel. Can someone tell me why?
*.. i don't have the article... but we always used a mag cause it had more drag... as it went across the surface it rolled the aggregate and pushed it down into the substrate... this brought more cream to the surface and then the steel trowel could give you a slick finishif you wanted a finish half way between a ""broom"" and a ""steel"".. the mag would be the trowel to use...... i'm not a cement finisher... i've just done 'em and supervised 'em... so ... someone who does it 8 days a week might have a different take.....
*Hey, thanks, Mike! That's what I wanted to know.
*Hey Guys ! Just did a counter the other day ! poured it upside down on high pressure form plywood and formed it with 1 1/2 thich 3/4 stock and I taped edge of 3/4 stock with packing tape and once formed ,used silicone to calk the edge ! This seals in water and gives edge a nice smooth slight round !As far as weight goes - use light weight aggragate , it helps in the weight big time! A friend of mine does these counters all the time ! Used black dye , it looks like graphite colored soapstone! upside down is the only way to get glass finish! Grout air pockets with portland cement and dye in which you used! used crevalle 880 releasing agent! spelling on crevalle might be wrong ! don't have can in front of me !
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a question concerning molding technique,
planning counter top for my laundry room and haven't decided whether to pour and finish in place or mold and pour top side down: would like to know more about the difference in finished appearance. also wondering about difficulty in avoiding bubbles and insuring clean release of edge from mold vs. difficulty in hand finishing top and edge detail. in anticipation of a perfect job my first time :) any experience anyone had to share would be appreciated.