Here in this concrete counter form image with reinforced steel or rebar,
we have used 4" lag screws to anchor the #3 rebar, staggered every 24"O.C. The counter or shelve here is formed to 2" in depth and 12" wide with the length being 10' long between the two end points of the adjacent walls. Because it sits atop a 2x4 framed wall, it will cantilever 2" to the kitchen area and roughly 6" pass the other side of the same wall into the living room. 
The image to the left here shows the importance of protecting the wood framing beneath with flashing paper to act as a moisture barrier and future dry-rotting adherent. The small spacers under our #3 rebar allows the new aggregate pour to completely surround the reinforcement steel for optimum strength.
DIY-Tools & Materials Needed for Your Counter Top Mix:
- plywood or melamine, plus screws to build the mold
- bagged countertop mix (or "scratch" mix below)
- dyes or pigment if you're coloring your countertop
- grinder / polisher with water feed (can be rented)
- diamond polishing discs of various grit
- Tupperware or similar sandwich containers to make concrete samples
- concrete mixer- concrete vibrating tool (can be rented)- dusts masks
- eye protection- ear protection
Pouring a concrete countertop is DIY project. Truthfully, it isn't like curing cancer to mix and pour concrete well, but... getting the mixture, the technique, and the finishing just right takes a little practice.
That's why it's a good idea to make sample pours first so that you can get an idea what you're working with and how small variations in the concrete mix can make a difference.
Many people start with a premixed concrete countertop mix. They are specially formulated to be flowable and high strength with plasticizer that makes it flow well even at a low water / cement ratio.
A low water / cement ratio is important for a good result that resists cracking. The Quikrete mix has high enough flow that the need for mechanical vibration is minimized, and it's made for minimal shrinkage.
Alternately, you could start with regular bagged concrete mix that is rated for a hardness of 5,000 psi or greater, available @ Home Depot or most of your home improvement centers.
Here is a counter top mix you can make "from scratch" that's enough to make a small test block and that can be scaled up by weight.
You can use a kitchen or postal scale to weigh out the sample measurements. It is recommended that you use white sand and white cement to maximize your color options:
Product / Weight in Grams
Portland Cement 210g
Sand 625g
Nycon G nylon fiber 1g
Metakaolin 30g
Fritz-Pak FR1 Water Reducer 5g
Water 80-90g
If you're using Quickrete Countertop Mix or Sackrete 5000, or some other pre-mixed countertop mix, start with about 875 grams of dry mix and about 85 grams of water to make a sample.
Whether you're using a bag concrete mix or my "scratch" concrete countertop recipe, use the smallest amount of water necessary to craft a mix that you can work with.
With color, try weighing out a small amount (a teaspoonful, for example) of the pigment and adding it to your sample.
Of course, you can only estimate how it's going to look once it's cured, but again, you have to start somewhere.
Write down how many grams of colorant you use in every batch so you'll have a basis for guessing how much more or less you need in subsequent samples and so you'll also know about how much you'll need for the final product once you get the color you want.
If you want a darker concrete countertop - all the way to a dark charcoal gray - start with Home Depot's Quickrete Countertop Mix, which is darker than the scratch recipe above, and add charcoal colorant.
Start with one bottle per bag of concrete countertop mix when mixing the final countertop.
Of course you can still experiment with the colorant in your sample slabs.
After you've poured your sample, allowed it to set and removed it, ask yourself what's right and what's wrong about it. Color will likely be the thing that determines what you should try next.
When you have a sample that pleases you, keep it and let it cure under conditions as close to the curing conditions of the final countertop as possible.
In fact, you should keep all of your samples so you can try out your concrete polishing and sealing techniques on them. Once you have your sample smoothed to the finish you want, you can seal it, according to the directions or you can seal it with beeswax (which may not be the best at standing up to what the average kitchen countertop endures) or even pure tung oil (like what is used on wood). It depends on the final look you want.
