Building a 'Concrete' Ramp, Concrete Mix
Some issues to consider: 1) steepness of ramp, 2) structural support, 3) design of bottom edge, 4) mix design. 

1) In the U.S., we can’t exceed a ratio of 1:12. That is, 1cm of rise requires 12cm of run. Also, it requires a handrail. 

2) Concrete weighs about 150 lbs. per cubic foot. Whatever supports the “concrete floor” will also need to support the concrete ramp. 

3) Concrete that comes to a point, will “chip off” over time, especially if wheelchairs roll over it. I’d chip down into the concrete floor (if it’s structurally capable) so you have a 4cm or so edge. (We often place a piece of rebar within 3/4” of the edge too.)

4) we like a “rich” mix for tapered concrete projects. That is to say, a 5 sack or more mix is important. Also, do not use too much water. If it’s too runny it will not keep it’s shape on the slope. 

There are many factors in the design and finishing. Here’s a site that may help. 

https://www.concreteconstruction.net/how-to/materials/top-10-myths-in-concrete-construction_o