First of all, our ramp-molds work with every kind of concrete! But we recommend a special kind which is called "self leveling concrete". This kind of concrete is usually used to build floors (also known as "self leveling screed", "leveling compound", "self leveling underlayment", "self smoothing compound" or "flowable concrete"). This concrete is very fluid so we can make sure it fills out the whole mold.
Every construction market have this kind of concrete on stock and the salesman will help you to choose the right concrete if you show him these text.
Here you'll find some concretes which we have already tested. Again: a lot of other concretes working as well. This is just a small summary to get an idea of what you can use:
Knauf - Fließspachtel ALLROUND / Faserflex
Available in most german construction stores or online - easy to use, perfect surface and cheap.
There are a lot of different kinds of "self leveling concretes" - different kind of brands as well as different kind of properties. In our opinion the best concrete to choose is a "fiber reinforced self leveling concrete". This sort of concrete has some small fibers in it and makes the concrete stiff and strong, even in small and thin areas. We use these concrete for our own obstacles.
QUICKRETE® Countertop Mix (No. 1106-80)
Is specially formulated flowable high-strength concrete mix for pre-cast and cast-om-plave concrete countertop applications. Available at LOWES hardware store: (other kind of QUICKRETE® compounds can be used, ask the salesman)
A lot of other concretes work as well...
And a lot of other "self leveling compounds" or "castable concrete" works good for ramps - a google search will show you which one is available in your country.