I’m doing another concrete countertop (they’re addicting). All the other tops I’ve poure have been pretty verbatim the Cheng book. Now that I’m past getting my feet wet and doing my fourth one I want to experiment with the thickness a bit. All the other tops I’ve done have been 2″ – 2 1/4″ thick. this one I want to pour at 1 1/2″ thick. I don’t think I’ll be using rebar because of the thinness but will be using road mesh. Are there any special considerations as far as mix I should be thinking of or any other issues I need to know about before making the mold?
Replies
I've made counters 'a la Cheng down to 1.5" Most of the ones I've done have been 1 3/4" b/c then I can use 1" and 3/4" melamine for easy sizing and two-layered sink knockouts and whatnot.
I think with the thin counters you really lose strength when there's metal of any thickness taking up space in the depth of the countertop. Another guy i met locally with experience recc. only pencil rod, and only to reinforce the length of the counter, not the width, or depth... I used pencil rod in my last counter--it's been in for 6 mos or so and no crazing/cracking/settling.
Also, the ghosting problem--I used pigfence, which is about 1/4 or 3/16 thick wire, in a 1.5" counter and it ghosted badly.
You asked specifically about mix and mold. If you go Cheng mix from scratch, the fibers recc. have to be really dry, loose and integrated to do any good at all. I've used a bagged mix, "Crack Resistant 5000" in the yellow bag from home despot and got a much more plastic and apparently stronger mix than I'd ever gotten trying to mix it myself. Has fiber, water reducer in already. Just add color. It's a crush agg mix tho, but the rocks were colorful upon grinding.
The mold. Here's where if the mold has any twist or bow, you're hosed. Picture a little bow in the middle of your mold--now flip it over and there's a sliver of daylight under the finished piece while it touches down on your perfectly flat counter substrate. A few months later, it's going to break right down the middle across the width of the counter (don't ask how I know this is fact). Or if there's any twist, look out. I think the thicker tops can hold their own with whatever theyre sitting on. The thin ones will break if the mold and finished product are not same as the counter substrate as far as flat and twist goes.
I've made flat cribbing--like 2x6 frame to level and shim on the ground or over sturdy sawhorses to make a strong flat surface for the mold. This way you can get under it to vibrate with whatever. Found my rotary hammer works better than anything for vibing.
Hope helpful.
Ted,
I'll still use the same mix, the equivalent of a 7-sack or so.
I use half-inch hardware cloth in 1-1/2" thick tops, no bar unless there's a potential weak point in the design.
I rarely use fiber, but if I do, I do a two-stage mix. The first layer that goes into the mold (that will eventually be the surface and edges) gets no fibers. The remainder of the mix gets the fibers.
You get partial benefit from the fibers but none of the downside of hair on the surface. I've never had a problem.
No changes to the mold.
Mongo
I don't know what I am talking about here but could you use fibremesh in the mix? The same stuff I put in my sidewalks.
Dave
If you've been following cheng or others it has probably already dawned on you to let it harden up a bit longer to reduce the chance of cracking the works.
Cheng isn't big on keeping the concrete damp during the cure, but he's working in a town with high humidity. If it were me I'd keep it damp regardless of location. In sidewalks and whatnot that can amount to doubling the strength over pours that are allowed to dry out.
Ghosting was so hard to avoid with the 'tops I helped cast that I'd be temped to go with a fine mesh.
Beer was created so carpenters wouldn't rule the world.