How to build concrete counter tops?
Sorry, but this is really not a DIY project. The mixture used is a "top secret" for those who do it for a living. Think about the mess you will enjoy if it is not done properly. I even saw one done on a home repair TV show and the installer refused to let somebody know the mix he used.
You'll stipulation to build a mold of the counter you want. If all you want is a flat surface, great - that's straightforward to do. If you're wanting to build-in some features, that's more involved and you're going to need to read up elsewhere.
If you want to shape the countertop in situ, that's a really involved, difficult situation that should be done in conjunction beside new construction or a central remodel - the space will be out of bounds for quite some time.
Build your mold sturdy ample to resist the pressure of the wet concrete. It have to have the exact inside dimensions - minus a tiny bit! - you want for your finished product, because trying to build concrete bigger is really hard, and sand it down, while not impossible, takes forever. When your mold is done, wax its inside surfaces really, really ably with auto or attach wax. Buff it smooth. Wax it again. Seriously, you don't want the concrete to stick to this thing, and wax is how you do it. You could use form release oil approaching the concrete contractors do, but that could cause staining or will an unpleasant odor on your finished countertop.
When you're building your mold, don't forget that cured concrete is HEAVY. If you're following my suggestion to build a slab that you install, you need to be capable of lift it and move it around. Think in the order of that when you're sizing the mold. Figure on it weighing going on for 150 lbs / cubic foot, or a 12"x12" slab, 2" thick, weigh about 25 lbs.
Add fillet of plasticine or molding clay into the corners so your finished concrete doesn't have sharp corners. Sharp corners crack sour really easily.
Examine your project. Are in that going to be any weak spots, resembling a narrow bridge from one place to another, or a projection? Get some reinforcing cable or bar arranged for insertion. You can place it now and suspend it (don't forget, your mold is in fact upside-down, so if the bar drops to the bottom, it could show through the top of your finished piece) near some slats reaching all the opening across your mold, and wires to hold the bar at the desired echelon.
Okay. Double-check everything. Satisfied? Great: you're ready to mix and pour. You want to pour the in one piece thing contained by one piece so there isn't a cold cohesive, a place where the concrete started to set up, and consequently you added more. That's weak and will crack.
If you plan to incorporate colorant, you can mix it into the wet concrete or you can attach it to the surface later. There're advantages to be have both ways. I prefer mixing it in, so the color go all the passageway through.
Okay, once it's cured you can strip away the forms and you've got a finished slab. Wait, finished? Not so much. But you can rub down the surface beside finishing stones to clean it up and sort a finer finish. You can use certain acids to etch the surface for ocular interest, you can add surface color.
My favorite is to use a bed of pea gravel contained by the bottom of the mold, then grind the surface after curing to expose that aggregate for a wonderful terrazo effect - thoroughly pretty, neutral from a distance, and it hide all comportment of stains. Tough as nails, too.
There are plenty of articles surrounded by back issues of Fine Homebuilding, Family Handyman and other, similar publications. Read up on it. Cast a couple of stepping stones to attain your eye in, and step for it.
Answers: With no offense meant, Why?
Certainly the concrete would enjoy to be on a substrate, framed to alllow shape, and screeded, floated, smoothed, to be used after.
You may get answers that also read out SEALED, which works well for warehouse floors, but for areas where on earth food prep takes place, or food items come contained by contact, I personally wouldn't use concrete.