What is the concrete mixture to make garden slabs? ?

I have some mold forms to make garden slabs, and need to know what the correct mixture is (sand/gravel/cement, to make the slabs. I have tried both 3:1 and 4:1 with sharp sand first then with 20mm pebbles to try and strengthen the concrete, but every time I take out the finished slab it crumbles, and breaks in half. Help?!