I asked in the main forum about questions folks want covered in the emerging products panel at ICE, and some specific questions about concrete came up. I will attempt to answer each briefly, and if anyone wants further information please let me know.
Startup:
"So one question might be how much of an investment is it to become a concrete countertop shop?"
Not much, really. You need a concrete mixer, which is only about $2000 to $3000, and some casting tables, which you can make yourself. You should be able to use your existing grinders, but you need to purchase diamond pads specifically designed for concrete. Concrete is actually more abrasive than most natural stone, because it is hard rocks and sand in a soft binder (cement), not all hard. It will tear the diamonds out of the pads if you use stone pads.
Jeff (our president and instructor) is working on a longer article about this very topic. I can post a link to it once it's done if anyone wants further info.
Cast in place:
"if the customer wants the concrete seamless..should I charge more for on site pour?
How long does it take to cure if the project isnt climate controlled when pouring on site?
Should the top of cabinets have a layer of plastic down so the
condensation of the crete on the bottom doesnt moisten up the cabinet
top rails????"
There is more risk involved in cast in place than with precast, because you're working in someone's home instead of your shop, and you have one chance to get it right. So yes, I would charge more.
You still need to use proper curing practices, which means covering the tops with curing blankets and keeping moist.
Absolutely, you must have a layer of plastic, not only to protect the cabinets but also to prevent swelling of the plywood sub-base from cracking the concrete. It's actually easier to use cellular PVC as the sub-base instead of using plywood covered with plastic.
Cracking:"A quote from a customer... "why would anyone want a concrete
countertop, everyone knows concrete cracks, look at anyones driveway"
How are concrete tops designed to where they will not crack? We get a crack in solid surface we have a warranty claim??"
There are 3 certainties in life. Death, taxes and concrete cracking. However, there are different types of cracks. Structural cracks are large, very visible cracks and should never happen in high quality concrete countertops. Hairline cracks are tiny, barely visible cracks that do not affect the performance of the countertop. Those happen occasionally, and your contract should state this, along with a definition like "hairline cracks are generally smaller than ½ mm in width that do not threaten the integrity or stability of the slab nor prevent the continued and normal use of the countertop".
Casting techniques:"Concrete is always placed, never poured."
There are 2 basic types of mixes & casting techniques for precast, non-GFRC countertops. One I call the "pop a perfect slab" (PPS) method, and the other I call the "grind and grout" (GG) method.
For PPS, a very fluid mix is used in a perfect, watertight form, it is vibrated, and when you pop it out of the form it's supposed to be perfect and all you have to do is seal it. In practice, this is extremely difficult to achieve. Anyway, that type of concrete I would refer to as poured.
For GG, a very stiff mix is hand packed into a dimensionally perfect but not necessarily totally pristine form. It is not vibrated. When you remove the form, there are voids in the surface due to air, called bugholes (yes, that's the technical term). You then grind the surface a little or a lot, depending on how much aggregate you want to expose. Then you fill in the voids with a cement paste called grout or slurry. Then you seal. It sounds like more work, but it's really not and allows a wider range of looks. That concrete, I would refer to as placed.
Edges:"Do most people offer than same profile edges as other stone edges...I
got the plastic mold edges but I notice ogees are not too popular..is
this cause of cleaning issues or squared can chip easier?"
Precise, squared edges on concrete can chip. Concrete is brittle. Actually, I tend to see more types of edges on concrete, except bullnose on precast because that one is tricky to do. Floral edges like the one pictured in this article
http://www.concretecountertopinstit...p?itemid=8 are easy though.
Hope this helps!