I'm making some experiences with concrete and I'm looking for a way of making the ramps more resistent without adding metal pieces... so I heard about sand.
Is there any ratio I should follow in order of having THE PERFECT mix?

ANY HELP , WILL HELP

I don't know about sand, as from my experience, it takes the smoothness out of whatever You're making.

my personal fave is anchoring cement, like rockite. it drys fast, smooth, and is easy to work with after.
           
How ever, it might do you some good to look up gfrc, or glass fiber reinforced concrete, which gives you both strength and superior surface finish. also, you can imbed wire mesh like wire fencing into the crete to give it another boost strength wise.

Hoped that helped out

^^ rockite.... cures harder than concrete...